oxidation pond guidelines 2005 final draft

Table of Contents

1. INTRODUCTION ...................................................................................................................................1

1.1 Overview......................................................................................................................................................... 1

1.2 What is an oxidation pond?........................................................................................................................... 1

1.3 Pond classification – based on oxygen levels................................................................................................ 1

1.4 Zones in oxidation ponds ............................................................................................................................... 2

1.5 Pond classification – relationship to other treatment ................................................................................. 2

1.6 Oxidation pond effluent quality.................................................................................................................... 3

2. DESIGN AND CONSTRUCTION ..........................................................................................................3

2.1 Pond sizing...................................................................................................................................................... 3

2.2 Hydraulic design ............................................................................................................................................ 4

2.3 Location and ground conditions ................................................................................................................... 4

2.4 Shape............................................................................................................................................................... 5

2.5 Depth............................................................................................................................................................... 5

2.6 Embankments, wavebands and freeboard ................................................................................................... 5

2.7 Construction ................................................................................................................................................... 6

2.8 Inlet structures ............................................................................................................................................... 6

2.9 Transfer structures ........................................................................................................................................ 7

2.10 Outlet structures ............................................................................................................................................ 7

2.11 Filling .............................................................................................................................................................. 7

2.12 Flows ............................................................................................................................................................... 7

2.13 Recording........................................................................................................................................................ 8

2.14 Seasonal variations ........................................................................................................................................ 8

2.15 Fencing............................................................................................................................................................ 8

2.16 Road access..................................................................................................................................................... 9

2.17 Warning notices ............................................................................................................................................. 9

2.18 Operator accommodation.............................................................................................................................. 9

2.19 Allowance for population growth ................................................................................................................. 9

3. OPERATION AND MAINTENANCE....................................................................................................10

3.1 General......................................................................................................................................................... 10

3.2 Sampling and analysis ................................................................................................................................. 11

3.3 Plants and vermin ........................................................................................................................................ 11

3.4 Odour control............................................................................................................................................... 11

3.5 Crash prevention.......................................................................................................................................... 12

3.6 Desludging .................................................................................................................................................... 12

3.7 Blue green algae ........................................................................................................................................... 13

3.8 Embankment erosion ................................................................................................................................... 14

3.9 Livestock....................................................................................................................................................... 14

3.10 Saltwater entry to ponds ............................................................................................................................. 14

3.11 Flies, fish and birds ...................................................................................................................................... 14

4. UPGRADES AND IMPROVEMENTS..................................................................................................14

4.1 Screening....................................................................................................................................................... 14

4.2 In Pond.......................................................................................................................................................... 154.2.1 Retention times.......................................................................................................................................... 154.2.2 Recirculation ............................................................................................................................................. 154.2.3 Baffling ..................................................................................................................................................... 154.2.4 Rock filters ................................................................................................................................................ 154.2.5 Rock groynes............................................................................................................................................. 15

4.3 Aerators (types and positioning)................................................................................................................. 154.3.1 Mechanical aerators .................................................................................................................................. 164.3.2 Diffused aeration....................................................................................................................................... 16

4.4 Post treatment .............................................................................................................................................. 164.4.1 Wetlands.................................................................................................................................................... 164.4.2 Disinfection ............................................................................................................................................... 174.4.3 Land treatment........................................................................................................................................... 174.4.4 Micro-filtration.......................................................................................................................................... 184.4.5 Advanced pond systems ............................................................................................................................ 184.4.6 Coagulation – sand filtration ..................................................................................................................... 18

5. FREQUENTLY ASKED QUESTIONS.................................................................................................19

6. REFERENCES AND FURTHER READING ........................................................................................19

7. WEB SITES .........................................................................................................................................19

8. INDICATIVE EFFLUENT QUALITY FROM IMPROVED POND SYSTEMS ......................................20

9. POND RECORD SHEET .....................................................................................................................21

Authors and Contributors.

The primary authors of this document were Stu Clark and Steve Cameron of NZET Ltd. Comments andcontributions were also received from; Les Collins, (Lakes Contract Services), Chris Tanner, (NIWA), StuCraibbe, (Fulton Hogan), Charles Willmot, (MfE), Rachel Ozanne, (Otago Regional Council), Ted Taylor,(Greater Wellington), John Harding, (MoH), Russell Winter, (Environment Southland), Chris Fowles,(Taranaki Regional Council), Humphrey Archer, (Beca), Shane Morgan and Peter Lawty, (WatercareServices Ltd), Graham Jamieson, (Works Filter Systems), and Peter Riddel, (Ecogent).

Status of This Draft:

This Guideline Document is in Draft form, we have termed this a “working draft”, in that it has beenchecked for significant factual errors and serious omissions. It is undoubtedly not in “perfect” form, orarguably in anything approaching this. Nevertheless, it is being released as a document which shouldalready serve its intended purpose, and, can be readily modified, updated and improved.

The proposed next stage in its development is to conduct a 1 day review workshop, under the auspices ofthe NZWWA, with input from representatives of the key stakeholders, TLA’s, MfE, MoH, Researchers,Consultants, Designers, and Regional Councils. This step will be subject to available funding, and, in theinterim, comments are welcomed at any time. Please provide comments and additional information,preferably by email to:

Feedback:

The Project Co-OrdinatorOxidation Pond Management GuidelinesNew Zealand Water and Wastes AssociationPO Box 1316WellingtonEmail: [email protected]

Attn: Jeff Page

Guidelines for the Design, Construction and Operation of Oxidation Ponds – Revised 23rd May 2005

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1. Introduction

1.1 Overview

This document is an update of the Ministry ofWorks Guidelines for Oxidation Ponds 1974. Itdraws from recent research and modern practices.It is written for use by pond operators anddesigners, planners, regulators and otherinterested persons. It is however, not intended tobe a primer on wastewater treatment, andassumes that the reader has a basicunderstanding of wastewater terms and practices.

These guidelines cover: how an oxidation pondworks, how it differs from other types of ponds;how to operate oxidation ponds; what to do whenthings go wrong; and how to add to and modifyexisting ponds to improve their performance. Theydo not cover consent requirements that relate toindividual ponds. Consents may include conditionsthat require specific actions that are not covered inthis document.

The oxidation pond is one of the most commonlyused methods for treating domestic sewage fromsmall to medium-sized communities in NewZealand. Fitzmaurice (1987) reported that of 160sewered communities with populations greaterthan 1,000, 100 used oxidation ponds as the mainmethod of sewage treatment.

Ponds are less costly to construct and easier tooperate than many other treatment systems.Modern ponds, with enhancements, have animportant role to play in wastewater treatment inNew Zealand. Ponds are robust, require lowenergy, are able to cope with hydraulic and organicloading peaks, and may provide buffer storage forland treatment systems.

In spite of their simplicity, however, they should notbe ignored. A good understanding of how theywork and attention to maintenance requirementswill make sure the pond delivers the best effluent itcan.

1.2 What is an oxidation pond?

Oxidation ponds are shallow earthen basins withinwhich wastewater is treated biologically. Ponds areable to reduce the level of many contaminants insewage including: Biochemical Oxygen Demand(BOD); suspended solids (SS); ammonia, and thenumber of microbes, including those which maycause disease (pathogens).

Oxidation ponds use algae and wind action tointroduce oxygen to the pond liquid. The wind andinlet flows will also create currents within the pondwhich help to mix the wastewater over the full pondarea.

The quality of outflow (effluent), from an oxidationpond, is very dependent on the action of thesecurrents, and therefore how long the wastewater isheld in the pond.

Microscopic animals in the pond graze on bacteria,reducing bacteria numbers considerably. Faecalcoliform bacteria, indicators of the possiblepresence of pathogens, also die off naturally withtime and exposure to sunlight. Therefore thelonger the sewage stays in the pond, the moremicrobes are killed.

Fig 1: The processes at work in an oxidation pond

Dissolved nutrients in the sewage, such asnitrogen and phosphorus, feed green algae whichare microscopic plants floating and living in thewater. The algae also use carbon dioxide (CO2)and bicarbonate as food much as land plants do.Like land plants, they release oxygen as a wasteproduct. The oxygen is used by helpful bacteria tobreakdown the incoming waste.

Algae produce oxygen from the process ofphotosynthesis, during the daytime. Pond oxygenlevels, and some other characteristics, like pH, willtherefore change throughout the day and from dayto night time.

1.3 Pond classification – based on oxygenlevels

These guidelines are about oxidation ponds, butthere are other types of ponds. Three differentpond systems are; oxidation (mechanically),aerated, and anaerobic; ponds. A term whichcovers all three types of ponds is “WasteStabilisation Ponds”. The following descriptionsand diagrams show how they differ from eachother.

Oxidation ponds (including primary and secondary(facultative), and tertiary (maturation), ponds),have a bottom layer of sludge with no oxygen(anaerobic), a liquid anaerobic layer above that,then a middle layer with small amounts of oxygen(anoxic); and a top layer with oxygen (aerobic)


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Fig 2: Oxidation pond

Some ponds have mechanical aerators on them.These range from facultative ponds with someextra aeration, to aerated lagoons that have somuch mechanical aeration the aerobic zoneincreases to nearly the whole depth. In someaerated lagoons much of the sludge is also kept insuspension.

Fig 3: Aerated pond

Anaerobic ponds are the opposite and have little, ifany, oxygen present. They are usually deep andoften are fed high strength dairy and meat wastes.

Fig 4: Anaerobic pond

1.4 Zones in oxidation ponds

Aerobic, anoxic, and anaerobic conditions exist inoxidation ponds. Different forms of wastewatertreatment take place in the four zones, shown inFigs 2-4 above. The features are described in thefollowing discussion:

The depth of the aerobic zone (the top zone) in anoxidation pond depends on: climate, the amount ofsunlight and wind, loading, currents and how muchalgae is in the pond water. The wastewater in thispart of the pond receives oxygen from the air, fromthe algae, and from the agitation of the watersurface for example from wind and rain.

In the pond’s aerobic zone, dissolved organicmatter and oxygen are used by bacteria for normalgrowth, producing more bacteria, and treating thewastewater. Carbon dioxide is also released,which is used by the algae, (being a type of plant).The long retention period in the pond ensures thatthe new bacterial cells eventually pass into the“endogenous respiration” phase (a wastingprocess). The end products of this are inert matterand more carbon dioxide. The aerobic zone alsotreats the odours from gases produced in the lowerlayers.

Below the aerobic zone, the anoxic zone providesa transition to the anaerobic layers below.

The interface between the sludge and the pondanaerobic zone is an area of intense biologicalactivity. Settled solids are washed around the pondfloor but tend to accumulate to greater thicknessesnear the inlet. Over the rest of the pond, there is auniform depth of liquid between the pond surfaceand the top of the sludge layer, regardless of pondfloor contours. Anaerobic decomposition reducesthe quantity of organic matter converting it tocarbon dioxide (CO2), methane (CH4) and residualmatter. The bacteria derive their energy from the“food’ they consume. The carbon dioxide andmethane released can be observed as gassing andsludge belching.

1.5 Pond classification – relationship toother treatment

A pond receiving raw sewage may be regarded asa complete (primary and secondary) treatment unit.Primary treatment (physical sedimentation)secondary treatment (the biological conversion ofunsettled organic matter which is responsible forthe greater part of the Biochemical OxygenDemand, into a settleable mass), and sludgedigestion (the slow further break down of thesettled solids) all take place in a natural manner.Some disinfection will also occur in such a pond. Itshould also be realised that some of the incomingdegradable organic matter not used for biologicalenergy within the pond, is passed out of the pondin a stable form as algae in the pond effluent.

Some typical combinations of ponds are shown inFigs 5 to 7. The terms primary, secondary, andtertiary oxidation ponds, are a New Zealandterminology. In other countries, primary andsecondary ponds are usually called facultative andtertiary ponds are called maturation ponds.


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A single pond that has raw sewage coming in iscalled a primary pond. In a primary pond, bothprimary and secondary treatment takes place.

Fig 5: Primary (facultative) pond: provides primaryand secondary treatment and sludgedigestion.

A pond that follows primary treatment e.g. aprimary clarifier or Imhoff tank is called asecondary pond. In it, mainly secondarytreatment occurs.

Fig 6: Secondary (facultative) pond: follows“conventional” primary treatment providessecondary treatment and sludge digestion.

A pond which follows another pond, either primaryor secondary, or follows a secondary treatmentplant, is called a tertiary pond. Such ponds are alsocalled maturation ponds. In tertiary (maturation)ponds, further treatment occurs. This is mainlydisinfection due to sunlight, cold, and predation byother organisms.

Fig 7: Tertiary (maturation) pond: following either aprimary pond or “conventional” primary andsecondary treatment

1.6 Oxidation pond effluent quality

Since the oxidation pond process is natural, andsubject to influences which are random – weather,and predominant microbiological species, etc - apond system cannot consistently produce effluent

of a precise standard. Pond performance isinfluenced by variations of season and climate, forexample ammonia removal is usually good insummer and poor in winter. Algal blooms aretypically a summertime occurrence, and impact onBOD, SS and bacteria levels as well as clarity ofthe receiving water, and potentially presence andlevels of algal toxins. Experience in New Zealand,has shown that as the load on a pond systemincreases, there is a deterioration of effluentquality.

Note that as most NZ wastewater systemsexperience high levels of inflow and infiltration ofstormwater, pond effluent concentrations ofcontaminants may be quite low due to dilutionrather than to particularly high levels of treatment.Typical results for primary ponds loaded up to therecommended rate of 84kg BOD/ha/day (1,200people/ha), are shown below

Typical effluent quality for other pond systems andpond modifications are given in section 8.

Table 1: Typical effluent results for a primaryoxidation pond

2. Design and construction

2.1 Pond sizing

In NZ, facultative pond sizing has typically beenbased on a population or organic loading rate. Theconventional design value for primary ponds hasbeen 84 kg BOD/ha/day or 1,200 people/ha. Theselevels have been proven to be conservative, withalmost all ponds loaded at these rates providingreasonable levels of treatment as long as theinfluent is from a mainly domestic source.

Secondary ponds (i.e. ponds which follow asuitable primary sedimentation process) are alsosized on the basis of 84 kg BOD/ha day. However,allowing for a 33% reduction of BOD in the primarytreatment unit, this equals 1,800 persons/ha.Ponds which are to treat septic tank effluenthowever, should be designed as primary ponds.

Contaminant Average Maximum Minimum

BOD5 (g/m3) 40 110 15

Suspended solids (g/m3) 50 150 10Faecal coliform bacteria(#/100 ml) 20 x 103 100 x 103 5 x 103

E coli bacteria (#/100 ml) 10 x 103 50 x 103 2 x 103

Total Phosphorus (g/m3) 8 16 4Dissolved ReactivePhosphorus (g/m3) 6 12 2Ammoniacal Nitrogen(g/m3–N) (winter time) 15 30 0.5Ammoniacal Nitrogen(g/m3–N) (summer time) 5 10 0.1

Total Nitrogen (g/m3–N) 30 50 10


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Tertiary pond design should be based on detentionperiod at average flow. Historically, in NewZealand, a detention period of 20 days has beenrecommended. Modern design trends, however,are towards smaller multiple ponds in series, (Maraand Pearson 1998). For example, two 5 daydetention ponds can theoretically achieve a similarperformance to one 20 day pond, with half thefootprint area. Ponds which follow a fullconventional (primary and secondary) treatmentplant are likely to increase the BOD and SScontent of the plant effluent due to the productionof algae within the pond.

Other methods of sizing ponds based on loadingrates and desired effluent quality can be used, butit is not possible to be very precise in designing asystem so influenced by uncontrollable factors.

A suggested limitation of primary pond area is 8-12ha. In some larger ponds, wind action generateswaves large enough to resuspend bottomsediments, which are then discharged in the pondeffluent. Also, as pond size increases well above12 ha, it is more difficult to distribute the inlet BODloading over the whole area, which can lead tooverloading in the inlet part of the pond.

2.2 Hydraulic design

Shilton (2001) presented an extensive study on thehydraulics of stabilization ponds. Twentyexperimental configurations were tested in thelaboratory and ten of these experimental caseswere mathematically modelled and had goodagreement with the experimental work. Shilton andHarrison (2003) then introduced guidelines forhydraulic design of oxidation ponds to "help fill theknowledge gap in the pond hydraulics area". Theyrecommend: Short-circuiting should be avoided as it results

in a large reduction in the discharge quality. Influent flows should be mixed into the main

body of the pond to avoid localised overloading,making sure at the same time, not to createshort-circuiting.

The solids deposition within the pond occurs asa result of the flow, rather than the flow beingredirected as a result of the solids.

A pond should maintain a similar andreasonably well defined flow pattern through therange of possible flow rates.

Baffles to shield the outlet.

Examples showing how to put some of theserecommendations into practice are given insections; 2.8, 2.10, and 4.2.3 of these Guidelines.

An important aspect of hydraulic design in ponds isthe hydraulic retention time (HRT). This is theaverage time that the incoming wastewater stays inthe pond. The HRT will affect the level of treatmentthe pond performs. Ideally, if there are no shortcuts– the pond can be considered a plug-flow system,

that is the flow comes in at one end, travels roundthe pond and having been everywhere, passesthrough the outlet. The HRT can then be calculatedby dividing the water volume of the pond by theflow.

If a primary pond serves 5000 people, and isloaded at 1200 people per hectare, then the pondwill be 5000/1200 = 4.16 ha in area. If the pond is1.5m deep, of which an average of 0.3m of thedepth is taken up with sludge, then the pondvolume is 4.16 x 10,000 (m

2/ha) x (1.5-0.3) =

49,920m3. If the average incoming flow is 300 litres

per person per day, or 0.3 x 5000 = 1,500m3/d,

then the HRT should be 49,920/1,500 = 33.3 days.

Now the actual pond HRT will not be 33.3 days. Itwill be a lot less. This is because of dead areas,where flow does not go, and currents, which causeshort circuiting. Estimating HRT is important sothat pond performance can be improved, usingbaffles and other methods. In order to find out whatthe actual HRT is, there are several methods, themost accurate of which is a tracer test.

Tracer testA tracer test involves releasing a substance whichcan be measured in very small concentrations intothe incoming sewage flow, and measuring howlong it takes to reach the outlet. Since it will spreadaround in the pond, the outlet measurements willneed to be taken over many days. The mostcommonly used tracer is Rhodamine WT, afluorescing dye. Measuring the amount of the dyepresent in a sample requires analysis using aspecial spectrophotometer called a fluorometer.

Sampling will need to take place several times perday, so this means using an automatic sampler,and a lot of samples to analyse. Rhodamine WT isalso sensitive to decay in sunlight, so a controlsample next to the pond to measure the rate atwhich its strength changes is needed, i.e.,reduction in concentration with time.

An alternative tracer is lithium, usually applied asthe compound lithium chloride.

Although a tracer test involves a lot of time andexpense. It still may be worthwhile, in that it willshow very accurately how long the influent spendsin the pond under the particular inflow, pond level,and wind and weather conditions which occurred atthe time the test was run. But what about resultsunder other conditions?

Other ways to estimate pond HRT are; computermodelling, physical modelling, use of drogues andfloats, visual observation of currents, and theconsent compliance test results.

2.3 Location and ground conditions

Ponds can be constructed in virtually any location;however it helps to reduce the cost if the site has


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suitable conditions and is located at a lowerelevation from the area serviced, so thewastewater can flow to the pond by gravity. Ideallyan area should be selected where the water tableis deep and the soil is heavy and impermeable. Siltor clay soils are ideal for pond foundations andconstruction. Building ponds over coarse sands,gravels, fractured rock or other materials, that willallow effluent to seep out of the pond or allowgroundwater to enter in, will require care andspecial engineering.

The siting of ponds is an issue which has receivedconsiderable attention, as various parties –designers and objectors for example, have tried tomake the “rules” fit their needs for particularcircumstances. Previously recommendeddistances have been “300m from built-up areas or150m from isolated dwellings. For populations ofless than 1,000 persons these restrictions may bereduced provided that there is adequate naturalscreening and that the prevailing wind blows awayfrom any housing area”.

In reality, however, there is no scientific basis forsetting these distances, and each site should beevaluated on its own merits. Having aeration, forexample will reduce the risk of odour problems, butthe same aerators increase aerosols possiblycarrying disease-causing organisms.

Conventional wisdom has been that siting in anopen area will to take advantage of the sun andwind, will assist the efficient operation of theoxidation pond and improve the quality of thedischarge. With more detailed consideration of,and attempts to control pond hydraulics, somedesigners prefer sheltered sites where high windexposure is avoided.

Avoid sites that are likely to flood, have steepslopes that run towards a waterway, springs orwater supply bore holes. The pond should beorientated with the longest diagonal dimension ofthe pond parallel to the direction of the prevailingwind, the inlet at the downwind end, and outlet atthe upwind end. Ponds should not be locatedwithin 2 km of airports, as any birds attracted to theponds may constitute a risk to aircraft.

The site should preferably be flat. Surface drainageshould be away from the site or should be divertedaway from the pond formation.

2.4 Shape

Wherever possible, and construction costs allow,ponds should be uniform in shape, preferablyapproaching square with rounded corners. Pondsmust not be too narrow in relation to length (pondlength should not exceed three times pond width),nor should they have sharp bays or re-entrants inwhich scum can lodge and weed growth develop.

2.5 Depth

Facultative, (primary and secondary), oxidationpond depths should usually be within the range1.5m to 2.0m. Maturation (tertiary), pond depthscan be less at 1.0-1.5m. The minimum acceptabledepth stops aquatic plants becoming established.Such plants restrict natural flow patterns withinponds. Ponds used for buffer storage havesuccessfully been operated at temporary depths of0.8m without any problems occurring.

Modern design trends are to work to depthsbetween 1.5 and 2.0m to allow for sludge storageand flow buffering capacity. No advantage isgained by making the pond any deeper. Greaterdepths can cause temperature stratification, whichcan result in the pond overturning when thesurface water temperature cools rapidly andbecomes colder than the temperature of the lowerwater. At this time (usually the first frost of winter) itis possible for the colder surface water to drop tothe bottom of the pond, causing the warmeranaerobic and sludge layers at the bottom to riseto the surface.

Ponds which have adequate mechanical aeration,(often used for mixing as much as for aeration),generally avoid thermal stratification and turnoverevents.

Slight variations in pond depth do not affect pondoperation. The natural movement of solids tendsto give a uniform depth of liquid layers.

2.6 Embankments, wavebands andfreeboard

Embankments form the sides of the pond. Theymust be well constructed to prevent seepage,settlement or erosion over time. Embankmentslopes are commonly 1 (vertical) to 3 (horizontal)internally and 1 to 2.5 to 4.0 externally, (if they areto be mowed). External embankments should beprotected from storm water erosion by providingadequate drainage. Internal embankments shouldbe protected from wave action erosion by usingconcrete wavebands or rock rip-rap. Where asynthetic liner is used, a device must be providedto allow safe entry and exit for maintenance staff.

Embankment tops should be wide enough topermit vehicular access for maintenance purposes,a minimum width of 4 m is recommended. Tracksshould be metalled to provide a good base forvehicle traction. Fill embankments should beconstructed on good foundations and becompacted according to earthworks constructionstandards for the soils involved. Although a wellconstructed embankment as shown in Figure 8will not be at risk from moving due to the weight ofthe pond water, good compaction will alsominimise settlement, form a good base forwavebands, and reduce the risk of erosion


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damage from floodwaters, or seepage flows fromwithin the pond.

Special care must be taken to locate any soft spotsor filled areas on the pond site. These should beexcavated and refilled with well compacted, goodfill material.

1–

1.5

m

0.5

m

0.5

m

Fig 8: Typical embankment and waveband design

Where pipes are laid through embankments caremust be taken with back filling around the pipe. Ifpipelines are to be laid through the base of thepond embankment it may be preferable to use PEor GRP pipes, laid at the same time that theembankment is built up. This will reduce theproblems associated with differential settlementand avoid the need to dig up the embankment torepair damaged pipework.

A wave band forms a clean edge to a pond,preventing erosion and making the pond easier tomaintain. Various materials have been used forwave band construction but to date, of the sheetmaterials, only concrete has been foundcompletely satisfactory. Polythene sheeting, whilea good option for small ponds, does not allowaccess onto the waveband for cleaning etc.;asbestos cement sheeting has broken up ormoved; corrugated asbestos cement sheeting isgenerally too narrow, allowing waves to pass belowthe sheets and lift them, and debris collects onstone pitching making cleaning difficult.

Concrete wave slabs must also be keyed into theembankment. The use of small precast slabs isnot recommended owing to the difficulty ofproviding an adequate key; unkeyed slabs havebeen known to slip. Joints between pre-cast slabsare also prone to weed growth.

If suitable larger rock sizes are available fromnearby rivers or quarries, and the pond has inletscreening, then rock rip-rap placed on a medium toheavy grade of geotextile can provide bankprotection at lower cost than concrete. Rock andgeotextile has the advantages of not being effectedby bank settlement and wave run-up is reduced.

Freeboard (the amount of waveband above thewater surface) and waveband width, must berelated to the size of waves which may form, whichdepends on the size and wind exposure of thepond. Typical wave band sizing for smaller ponds,(say up to 2 ha), is shown in Fig 8 above. Forlarger ponds specific design should be undertaken.

2.7 Construction

Certain site-related factors, such as the location ofthe water table and the composition of the soil,should always be considered when designing pondsystems. Ideally, ponds should be constructed inareas with clay or other soils that won't allow thewastewater to quickly percolate down through thepond bottom to the groundwater. Otherwise, pondsin sensitive areas must be artificially lined with clay,bentonite, plastic, rubber, concrete, or othermaterials to prevent groundwater pollution.Imported linings will increase construction costssignificantly.

When preparing a site for an oxidation pond allorganic material should be stripped from the pondarea. The subgrade is then compacted, any softspots filled; embankments formed, along with inletand outlet pipework; the base and sides sealed, ifthe soil used for construction is not fine enough tokeep the rate of seepage suitably low, and thewavebands and tracks constructed.

In cases where the ground water table can riseabove pond floor level the pond must be filled asquickly as is practicable and must be kept full toprevent the sealing layer from being lifted.

2.8 Inlet structures

The placement of the inlet pipe position, especiallyin relation to the outlet, predominant wind directionand pond baffling, will have a big impact ontreatment efficiency in ponds.

Fig 9: Modifications to pond inlet – to provide jetattachment to the embankment wall

The recommended design for pond inlets used toinvolve taking the pipes well out from theembankments on piers. Shilton, (2003), discussesthis and recommends for primary and secondaryponds, using a horizontal flow inlet where the inletflow is directed along an embankment and is thendeflected using stub baffles. Shilton argues thehorizontal inlet provides momentum to move thesolids deposits when the flow enters the pond, andthe “attachment” of the flow to an embankmentwall and subsequent deflection with stub baffles


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dissipates this energy in a controlled manner(which should be used to minimise short circuiting).For tertiary ponds with no settleable solids in theincoming flow, Shilton recommends more rapiddissipation of the inlet flow using a manifold orvertical jet in a corner with stub baffles each side.

Figure 9, shows how a primary pond inlet whichpreviously discharged into the centre of the pond,has been modified to flow along the pondembankment. In this case, the flow then passes toa surface aerator rather than a baffle.

2.9 Transfer structures

Pond to pond transfer structures should be kept assimple as possible. In many cases a pipe throughthe pond embankment at mid-depth is all that isrequired. Where it is necessary to control thedepths of each pond independently a weir box canbe set in the pond embankment. If the transferdraw-off is at pond surface level it should bebaffled to prevent floating debris from passing outof the pond.

With the growing use of pond buffer storage aspart of a treatment and disposal system, transferstructures may also be restricted, to allow a fixeddischarge rate between ponds. Fig 10 below showssuch a system in use between a secondary andtertiary pond.

Here, the small slot in the outlet chamber allowsonly a constant flow to pass with any excesscreating a build up in pond level. If storm flowsoccur, then the flow passes through the top of thechamber as it did prior to modification. At thisplant, pond levels have been dropped to 800mmdepth leading into the summer (no discharge)period, to allow for extra buffer storage. This depthreduction has not created any problems.

Fig 10: Transfer structure between secondary andtertiary ponds – allowing fixed outlet flowsuntil pond is full

2.10 Outlet structures

Outlets should be placed out of the main flow pathof the incoming wastewater. Final outlet positioning

can be selected after the inlet position/type andpond baffling has been designed.

Typically outlets have been either been from topwater level, surrounded by a scum baffle, orthrough a submerged pipe located approx 500mmbelow top water level, and feeding into a weir box.Outlet weir boxes for larger pond systems shouldhave facilities to allow the ponds to be completelydrained.

The use of screw-down penstocks should beavoided where possible. Ingress of grit topenstock seating has caused leakages. Wherepenstocks are used the screw threads should becovered with protective tape to minimize corrosion.

Outlet and transfer structures should always besited on the upwind side of the ponds, in prevailingconditions, to keep them clear of floating debrisand to reduce the likelihood of short-circuiting. Fig11 shows a baffled outlet structure to preventshort circuiting currents passing along theembankments , from flowing straight into the outlet.In this case, gabion baskets were used to createthe baffles.

Fig 11: Outlet baffling using gabion baskets – outletis between two stub baffles

2.11 Filling

Ponds should be completely filled and maintainedat operating level as soon after construction as ispossible. Rapid filling prevents the establishmentof weeds.

Ponds that are allowed to fill slowly generally sufferbank erosion until the liquid level rises above thewave band. Some slow filling ponds have becomeanaerobic, accumulating large areas of floatingsolids, because of the build-up of heavyconcentrations of organic matter in limited volumesof liquid.

2.12 Flows

In most cases the ability to accurately measurepond inflow is important. It provides information onthe condition of the sewerage system as well as


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information for future design purposes andconsents. Flow measurement of raw sewageusually involves a control device, such as a flume,which causes the flow to act in such a way thatmeasurement of upstream depth can be used tocalculate the flow rate. The increased upstreamdepth created by the flume can also be used toadvantage when screens are installed on the inletflow channel.

Measurement of the pond effluent flow rate iseasier and has more bearing on the effect of thedischarge on receiving waters. Effluentmeasurements do not usually agree with influentmeasurements as seepage and evaporation lossesoccur. A simple V notch weir is often used forpond effluent flow rate measurement. As with aflume, the application of a formula to the upstreamdepth measurement allows calculation of the flowrate.

Any flow measuring device, however, should beregularly checked and calibrated. In far too manycases such checking has found that highlyinaccurate measurements have been made, oftendue to; incorrect formulae being used , oldercontrol devices – (weirs or flumes), being too smallfor current flows, or poor or non calibration of depthmeasurement devices.

Fig 12: V notch weir with ultrasonic depthmeasurement on pond effluent

Terminal pumping stations should be fitted with“hours run” meters. This applies particularly tosmall pond systems where other means of flowmeasurement may not be necessary.

2.13 Recording

Most pond monitoring equipment comes withoutputs for recording and data logging of criticalinformation (like pond inflow rates and DOmeasurements). One option is for the data to bemanually entered during a site visit or, alternatively,it can be automatically logged onto a data logger orsent electronically through a telemetry system tobe logged at a base station.

Many discharge consents for pond systems nowrequire automatic measurement and logging of key

parameters such as; flow, temperature, anddissolved oxygen. Telemetry systems are alsobecoming more common for ponds. Bothautomatic measurement and telemetry aredesirable if there is the possibility of the pondsrequiring rapid intervention in response to lowoxygen levels for example.

2.14 Seasonal variations

Oxidation ponds will experience seasonal loadingvariations due to local weather conditions, rainfallintensity and stormwater infiltration. Some pondswill also have to cope with variable loading fromholiday populations or seasonal industry. Mostponds need more frequent checking in the springand summer when grass and weeds grow quicklyand when seasonal properties are occupiedcausing a higher loading.

In colder climates the rate of biological activityduring winter will be slower and could cause areduction in pond performance. The pondoperating level may need to be increased to helpincrease the operating temperature for the pondorganisms.

Pond life varies seasonally, with cyanobacteria,(commonly known as blue-green algal) bloomsoccurring in many ponds during peak summertemperatures. With the recently increasedrecognition of the possible impacts of blue-greenalgae and algal toxins on people, stock, and theenvironment. This is an important aspect of ponddischarges to surface waters.

2.15 Fencing

Fences are essential to keep livestock out of pondareas and to deter public access. The large areasof land usually involved, tend to make climb-prooffencing expensive, although from a health andsafety perspective, its use is desirable. In manycases, the “front entrance” to ponds is securityfenced in this manner, with the “back door” beingleft at stock proof fencing. Normal 7 or 8 wirestock-proof fences are usually all that is provided.

Fencing can be erected on top of the pondembankment immediately above the wave slab.This approach lessens the amount of land to bekept tidy but makes maintenance work such as theremoval of floating debris and repairing erosionmore difficult.

A second approach is to erect the fence so as toleave an access way around the top of the pondembankment. Pasture growth between the edgeof the pond and the fence must be controlled bymowing or by grazing. If grazing is practiced, asupply of drinking water must be supplied for thestock. Alternatively the area can be ‘mob stocked’to clean off all pasture growth in a day or so.


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2.16 Road access

The main access way to the ponds should be anall-weather vehicle track. The access way aroundthe pond embankment need not be an all-weathertrack since maintenance work can be planned inrelation to weather conditions, but it should ideallyhave a firm base.

2.17 Warning notices

Notices warning the public that access to the pondarea is prohibited should be placed so that anyperson approaching from any direction can see atleast one notice.

Signs should make it clear that no public access isallowed to pond areas and that there are water anddisease hazards with ponds. Where ponddischarges flow to a receiving water, it is alsobecoming common for signs to warn the public ofdisease risks from contact with affected zones ofthe receiving water. An example of the wordingused in an outfall to river sign is shown in Figure 13below.

Fig 13: Typical outfall signing

2.18 Operator accommodation

It is usually desirable to provide a building in whichthe operator can store equipment, carry out routinetests, keep plant records and wash after attendingthe ponds. The size of the building should berelated to the size of the ponds and to themonitoring requirements. Provision of a suitablewater supply is often a problem but this can beovercome by; collection, tank storage andtreatment of roof runoff water.

2.19 Allowance for population growth

Ponds should not be designed for more than a25% increase of the population at the time ofconstruction, except where there is good evidencethat a very high rate of growth or loading will occur.

In the past, a number of ponds have beenoversized and, because of evaporation andseepage, have never filled. In some cases thisproblem has been accentuated by the use oftemporary fences for dividing a raw sewage

section from a tertiary section. This practice simplyincreases the pond areas to be filledsimultaneously. Raw sewage and tertiary pondsshould be completely separate units.

Measurements taken at several pond systemshave shown that seepage and evaporation lossesare of the order of 100-150 m3/ha day during thesummer months. To illustrate the effect of thisorder of pond water loss, consider the followingcalculations:

Raw sewage pond area required for a designpopulation of 5000 people.

Area = 5,000 people / 1,200 people/ha = 4.2ha

Daily inflow from present population of 4,000 (at250 litres/head day) = 1,000 m3/day.

Losses due to seepage and evaporation

= 4.2ha x 150m3/ha/day = 630 m3/day.

Therefore during the summer periods more thanhalf the inflow to the raw sewage pond is lost byseepage and evaporation.

If a large, (20 day detention), tertiary treatmentpond were included in the system a further 2 ha ofpond surface area would be required to provide anadditional 20 days’ retention. Total loss due toevaporation and seepage would then be:

6.2ha x 150m3/ha/day = 930 m3/day.

Now the major portion of the pond daily inflowwould be lost. Virtually a “nil discharge” conditionwould result.

It has been shown that the average open-waterevaporation rate is between 650 and 800 mm peryear in most areas where oxidation ponds areconstructed in New Zealand. January monthlyaverage open-water evaporation is generallybetween 100 and 225 mm in these areas(Finklestein, 1973). The effect of such lossesmust not be ignored when a pond system isdesigned.

The build-up of heavy concentrations of organicmaterial and the lack of natural “flushing” of algaecan result in the development of excessively largealgal crops and in a reduction of the quantity ofoxygen that can be dissolved into the pond liquid.Under these conditions night time algal respirationmay absorb much of the pond’s dissolved oxygenand the pond could become anaerobic. It istherefore important to ensure that ponds are notover-sized. It is also under such conditions thatblue-green algal blooms often form with theresulting potential problem of algal toxins.


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3. Operation and Maintenance

3.1 General

Frequency of inspection and maintenance workshould be related to the size of the pond systemand the requirements of the monitoring program forthe discharge consent. Even the smallest of pondsshould be visited at least once a week. Recordsheets of pond operation must be filled in regularlyto maintain a full summary of pond operationalhistory. An example record sheet is given insection 9 of these Guidelines.

Systems with more than one pond operated inparallel or series may need operators to check andadjust flow levels or divert flows to and from certainponds to optimize performance.

Most inspections/visits should include brief checksof the banks, channels, grounds around the ponds,inlet and outlet structures, pond appearance, level,monitoring for dissolved oxygen, pH, temperatureand odour (if any). Records should be kept ofevery visit and all observations, includinginformation about the weather or other factors thatmay be influencing pond conditions. Moreextended inspections and formal sampling andtesting are also periodically necessary.

A simple and useful indicator of pond operation isthe dissolved oxygen, (DO), level in the pondliquor. The system must remain aerobic duringdaylight hours, from 10 am say, until night time.Note that DO content varies through the day andeven hourly; to obtain meaningful information,sampling times and locations must bestandardised and any low values should beconsidered in the context of what has happenedbefore and after, time of day etc. A good indicationof daytime DO is gained from readings takenbetween 11:00 and 14:00 hours.

To maintain facultative conditions there must alsobe an algal community in the surface layer. Auseful indicator of the number of algae present isthe chlorophyll-a concentration. Pearson (1996)suggested that to ensure stable facultativeconditions, a minimum in-pond chlorophyll-aconcentration of 300 µg/l was required. Tomeasure chlorophyll-a, take a fresh sample fromthe top waters (say 300mm below the surface), intoa clean container and despatch to an analyticallaboratory.

A simpler test for the level of algae present is the“stone test”. This involves picking up a rock,throwing it into the pond, and observing the colourof the resulting splashed water. Although hardlyscientific, this is easy to do, and is often a verygood rough indicator of pond conditions. A brightrich green colour indicates that the pond is in goodcondition and that there are plenty of algaepresent. A dull green, turquoise blue or yellowish

colour could mean that a less desirable type ofalgae is becoming dominant in the pond. A grey,brown or black colour often indicates there are notenough algae present. In calm weather and highloadings, the test may reveal a green layer overblack depths. This is a bad sign and can mean thepond is about to crash. Red colouring can comfrom extensive growths of Daphnia or Moina,especially on under loaded ponds. These willgenerally recede to normal levels in a matter ofdays and do not seem to cause any problems.

With regular inspections, testing, and recordkeeping, operators become familiar with thenatural cycles and in particular requirements of thesystem, as well as what factors tend to influence itsperformance.

Floating debris should be periodically removedfrom the pond surface (This work is best carriedout when wind action concentrates floating materialinto a corner of the pond). Sump suckers, (vacuumtrucks), are often used for this purpose. All pondstructures, including inlet, outlet and transferboxes, should be kept clean and free fromgrowths.

Debris washed up onto the wave band must beremoved. The wave bands should be brushed orwater blasted regularly to discourage slimegrowths. Pond banks must be checked regularlyand any erosion must be repaired immediately.Perimeter fences must be checked and kept ingood repair. Warning signs must be maintained ingood condition.

Floating weeds can become a nuisance foroxidation ponds as they have the potential to blockout sunlight which is needed for algalphotosynthesis. Removal should be in earlysummer before these weeds become too wellestablished. It is very important to ensure thatpond embankments are kept free of excess weedgrowth. Clumps of weeds can harbour insect lifeand may dangle into the pond, trapping floatingdebris and causing odour problems. Spraying ofweeds should be with appropriate sprays approvedfor use around water courses.

Buildings at the pond site should be maintained ata good standard.

The point of discharge of pond effluent to thereceiving water must be inspected and cleared ofweed growth to ensure free flow and rapid mixingof the discharge.

Electrical and mechanical equipment such asaerators and flow recorders need to be checkedand serviced as per the manufacturer’srecommendations. Aerators require daily greasingif not provided with an automatic greaser. In anycase, it is advisable to check aerators daily.


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3.2 Sampling and analysis

Samples should always be collected from thesame point or location. Raw wastewater samplesfor pond influent tests may be collected either atthe wet well of the influent pumping station or atthe inlet control structure. Samples of pond effluentshould be collected from the outlet controlstructure or from a well mixed point in the outfallchannel. When outlet pipes are not accessible,effluent samples can be taken from within thepond, near the outlet. These, should be collectedfrom a point 2 meters out from the pond edge and300mm below the surface. When sampling, careshould be taken not to stir up sludge from the pondbottom. Try to avoid collecting samples during orimmediately after high winds or storm eventsbecause solids will be stirred up after such activity.

Probably the most important sampling that can beaccomplished easily by an operator is routine pH,temperature, and dissolved oxygen (DO) testsseveral times a week, and occasionally during thenight. These values should be recorded becausethey will serve as a valuable record ofperformance. The time of day should be variedoccasionally for the tests so that the operatorbecomes familiar with the pond’s characteristics atvarious times of the day. Usually the pH and DOwill be the lowest at sunrise. Both will getprogressively higher as the day goes on, reachingtheir highest level in late afternoon.

A record of samples collected should be kept andinclude location, time, date, person sampling and aunique code number. This information should alsobe included on the label of each bottle (there maybe more than one bottle needed for each samplingsite).

The laboratory will usually provide the correctbottles (with any necessary preservatives), chain ofcustody sheets and sometimes an outer pack (e.g.chilli bin). It is usual to pack chilli pads or icearound the samples when transporting sampleseither locally or to a remote laboratory via courier.Samples must arrive at the laboratory as soon aspossible so the quickest transport method shouldbe used which may be overnight courier.

Generally consent conditions that requirelaboratory testing, specify that this be done at anaccredited laboratory.

A useful sampling tool is the “sludge judge”. This isa clear tube, with a non return valve in the base,which allows a stratified sample to be takenthrough the water and sludge layer.

3.3 Plants and vermin

Plant and vermin control is an important part ofgood house keeping for oxidation ponds. Weedsaround the edge of the pond are the mostobjectionable because they allow a sheltered area

for mosquito breeding and scum accumulation.Weeds can also hinder air circulation on a pond.Some weeds will grow in depths shallower than 1metre so an operating level of at least this depth isnecessary. Weeds (including roots) are bestpulled out by hand or sprayed as soon as theyemerge and before any germinate, becomeestablished and damage the embankments.

Any rat and rabbit infestation must be controlled,(rat and rabbit burrows can damage pondembankments). These vermin can be removed bya number of methods including poisoning andtrapping. Care should be taken not to leave rawscreenings lying around so as not to create a foodsource for vermin.

3.4 Odour control

Many NZ ponds will experience odour problems atsome stage during their life. Odours are usuallycaused by overloading the pond, a temporarybiological imbalance, such as an algal bloom, orpoor housekeeping practices and can be remediedby taking corrective measures. During highloading, when the entire dissolved oxygen hasbeen used up, odours are produced because theaerobic layer no longer exists at the top of thepond.

Oxygen levels in ponds are affected by watertemperature. This is for several reasons; colderwater is able to hold more dissolved oxygen beforeit is saturated, but warmer water temperatures willspeed up the rate of decomposition of waste andpromote the growth of organisms in the pond,including algae, which add oxygen byphotosynthesis. Figure 14 below shows howoxygen levels and temperature relate for a specific(non-mechanically aerated) oxidation pond.

Fig 14: Seasonal dissolved oxygen and temperaturevariability

There are several ways to reduce odours in ponds.These include; recirculation of supernatant fromaerobic ponds, the use of floating aerators, a rakeor long poles can be used to break up floatingmats, a boat with outboard motor can be used toadd oxygen, as can a land based air compressorwith a drilled pipe to produce air bubbles within thepond.

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3.5 Crash prevention

Ponds develop their own unique biologicalcommunity which varies with time. Apparentlyidentical adjacent ponds receiving the sameinfluent in the same volume often have a differentpH and a different dissolved oxygen content at anygiven time. One pond may generate considerablescum while the neighbour doesn’t have any scum.For this reason, each pond should be routinelytested for pH and DO. Such testing may indicatean unequal loading because of the internalclogging of influent or transfer pipes that might notbe apparent from visual inspections.

Tests may also indicate differences or problemsthat are being created by buildup of solids or solidsrecycling. pH and DO samples are usuallycollected at the outlet side of the pond unless acomplete profile is being done.

If it is suspected that the pond is going to “crash”ie, go anaerobic throughout it’s full depth, or thishas already occurred, then a full inspection of thepond should be immediately undertaken to assessthe pond’s condition and suspected reasons for thepond’s deterioration.

Some of the most common reasons for the pond“crashing” are:

Event Cause Remedial Action

LowOxygen

High strengthwaste

Remove waste, reducewaste influent strengthby pre-treatment, or addaeration

Aeratorfailure

Reinstate aeration

Poor mixing Modify inlet and baffling,possibly install mixers

Decay ofalgal bloom

Temporarily aerate withoutboard motor

SludgeBelching

Excessivesludge depth

Spread or removesludge

Overturn Anaerobicand sludgelayers moveto surface

Aeration or sodiumnitrate dosing, (notethere are concernsabout the effectivenessand health and safetyissues with sodiumnitrate dosing).

Death ofPondBiomass

Toxin ininfluent

Identify and neutralizetoxin, identify sourceand remove

Table 2: Remedies for pond crashing

3.6 Desludging

When oxidation ponds were first introduced in NewZealand, it was thought by many that they wouldnever have to be desludged. This has proved

incorrect. Even though some ponds have beenoperating for 25 years or more without having to bedesludged, many of these are now suffering fromexcessive sludge depths. These ponds areproducing poor effluent quality partly due toexcessive sludge being resuspended and thenwashed towards the outlet. The increased sludgevolume will also reduce the pond’s working depthand retention time, causing a further reduction ofeffluent quality.

Sludge storage in ponds is continuous with smallamounts being stored during warm weather andlarge amounts when it is cold. At coldertemperatures, below about 15 degrees C, thebacteria are not active enough to break down thestored sludge. As a general rule, sludge depth in apond should not be deeper than 600 mm inisolated pockets or 500 mm, or one third of theponds depth, which ever is greater, as an averagedepth in the pond.

Ideally, the pond’s sludge profile should bemeasured annually to gain an appreciation of thefill rate and pond’s projected life expectancy beforedesludging is required. If this is done, the cost ofdesludging required can be budgeted for inadvance. In some cases, desludging can bedelayed by directing unnecessary solids loads,such as septic tank clean out wastes, to otherpoints of disposal such as dedicated septagedewatering facilities. Sludge depth measurementto the top of the sludge layer can be made with asimple tool as shown below.

Flat board – approx300 X 300 mm

Weight for tension –approximately 0.6 -1 kg

Graduated stringline to surface

Fig 15: Sludge depth measuring tool

Measurement of the bottom of the sludge layer canbe made with a rod with graduations on it.Accessing the pond is best by boat (as opposed towading), however, health and safety issues need tobe well addressed.

There are a number of proven methods to removethe sludge from an oxidation pond. In NZ the mostcommonly used methods are;


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- If the pond can be taken “off line” an option is todrain the pond level down to the sludge layer. Thendewater the sludge using wind and sunlight. (Thisis best done in the summer time). It is possible todry the sludge to 50% dry solids content. At thislevel, the sludge can be removed by excavatorsand trucks.

- A temporary partition can also be made and thepond desludged into this, as was done atInglewood. Once dewatered, the sludge can beremoved using a digger or bobcat (care should betaken not to damage the pond floor orembankment with heavy machinery).

-Install a dredge on the pond to suck up the sludgeand pump it to either a dewatering vessel (like aGeotube), an evaporation tank/pond, or dewateron site using a mechanical plant such as acentrifuge or press.

Because the mechanical dewatering methodsinvolve equipment hire, polymer and power, andthe dewatered sludge may be 15-25% dry solids,(meaning higher cartage costs), it can be morecost effective to construct a new pond if spaceallows, and dewater the pond by drainage andevaporation.

Fig 16: Sludge dredge at work on oxidation pond

The Geotube works by filling a permeablegeotextile bag with sludge which allows filteredwater to drain through small pores in the geotextile.This filtered water can be redirected back into theoxidation pond for further treatment. The remainingsludge is trapped in the Geotube bladder and overtime the sludge volume is reduced. This systemallows repeated ‘topping up’ of the Geotube (sothat one tube can be used over an extendedperiod). When the final cycle of filling anddewatering has been completed, the bag is left forconsolidation. Dewatering levels reported are up tothe mid 20 % dry solids for initial fast cycles, andup to high 30% for final consolidation.

The advantages of this product are that you candewater onsite without interrupting the oxidationpond process, and all decanted water is trappedand returned to the pond. The bags are relatively

small and light (when empty) and take up a smallfootprint while on site.

Fig 16: Geotube pod being filled with pond sludge

Oxidation pond sludges, because they contain asmall proportion of fresh settled material, representa bio hazard. They also normally containsignificant levels of heavy metals which mayinfluence their disposal. They cannot be classifiedas Biosolids under the Biosolids Guidelines(NZWWAssn 2003), without testing, evaluation andusually, further processing.

When ponds are being desludged, care must betaken that any lining on the base is not removed ordamaged.

3.7 Blue green algae

Blue-green algae, (more correctly known ascyanobacteria), are naturally occurring organismsin most freshwater environments. They occur assingle microscopic cells or in colonies that formslimy strands visible to the naked eye. Blue greenalgae are of concern both in the oxidation pondand in any water body which receives the pondeffluent.

Nutrients from oxidation pond effluent can alsoencourage the growth of blue-green algae in thereceiving water. The process of nutrientenrichment in a waterway is called eutrophication.The main nutrients contributing to eutrophicationare phosphorus and nitrogen. When conditions arefavourable, blue-green algae populations can'bloom', multiplying at such a rate that theydominate the local aquatic environment. At thispoint, problems for other organisms start to occur.The water becomes malodorous and a brightgreen scum may appear on the surface. Somespecies of blue-green algae produce toxins whichare dangerous, sometimes fatal to livestock,wildlife, marine animals and humans.

The decomposition of dead blue-green algal cellsby bacteria consumes oxygen. When billions ofsuch cells die during a bloom, the water becomesoxygen-depleted. This can lead to the death of


Page 14

other marine organisms, including fish, which needoxygen to survive.

In ponds, blue green algal control is sometimespossible by; modifying pond loading, use oftemporary mixers and aerators, or dosing analgaecide such as copper sulphate. Oftenhowever, a rampant bloom will remain until colderweather conditions occur. Care must be takenwhen considering the use of an algaecide as deathof the cyanobacteria can result in high levels oftoxin release.

Under bloom conditions, it may be necessary totest for algal species and the presence of toxins,especially where the pond discharges into sensitivereceiving waters.

3.8 Embankment erosion

The action of waves, especially in larger pondswith high levels of wind exposure, can, over time,cause significant erosion to the internal pondembankments, even when they are protected bywave bands. Waves can overtop wave bandssaturating the soil behind and the pounding actioncan then weaken the saturated soil, causing it toslump. This problem has been addressed by usingcast in-situ wave bands, with the footing located atthe top rather than at the bottom, refer figure 18.

75m

m

300m

m

200mm

Fig 18: Recommended concrete wave bandconstruction details

Alternatively, rock on geotextile can be used,preferably over the full slope with a toe of rock atthe base.

3.9 Livestock

Cattle should not be permitted to graze aroundpond embankments. The cysts of the beeftapeworm (taenia saginata), which is parasitic toman and causes beef measles in cattle, can betransferred to cattle from domestic sewage.Grazing by cattle can also result in erosion of pondembankments through hoof damage.

There are no restrictions on grazing by sheep andgoats, although it would be prudent to place these

animals onto clean pasture for several weeksbefore culling.

3.10 Saltwater entry to ponds

Saltwater must be prevented from entering pondsand sewerage systems serving ponds.Combinations of sewage and saltwater whichcontain more than 5% saltwater cause sulphates tobreak down to sulphides, releasing hydrogensulphide. Where ponds are constructed in saltyareas on coastal land the top layers of salty soilshould be stripped from the pond site.

Salt water can also be contained in some industrialdischarges such as from ion exchange watertreatment systems and may occur in wastewaterreticulated in coastal areas due to infiltration.

3.11 Flies, fish and birds

There is a popular theory that pond insects are asource of nuisance and that insect-eating fishshould be introduced to control their numbers. Infact, properly loaded ponds do not have insectproblems. Such problems have only beenexperienced in the early lives of one or two initiallyunder loaded ponds, or following ponddecommissioning. Fish have no effect oncontrolling nuisance levels of midge and should notbe introduced without prior consultation with DOC,because of the possibility of them spreading to thereceiving environment. Birds on the other hand,especially swallows, can keep fly and midgenumbers down.

4. Upgrades and Improvements

4.1 Screening

Manually cleaned bar screens with 20-50mm gapsused to be the standard screening arrangements.These however, required regular cleaning and stillallowed a lot of solids to pass. There are now anumber of different automatic screens available forremoving solids from raw sewage. These includestep screens, drum screens and bar screens.

Fig 19: Step screen and press.


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Modern screens are also becoming much finer,typically with apertures around 3-6mm and with jetsto break up faecal material from the screenings.They also typically incorporate automatic presseswhich dewater the screenings and discharge themto sealed plastic bags; figure 19 shows a stepscreen and press, a popular screening option forraw wastewater.

4.2 In Pond

4.2.1 Retention times

Any pond treatment system works best with evenflow to encourage the rapid and continuous growthof bacteria involved in the biological breakdown ofthe wastewater. It is essential that the daily loadinginto the ponds is kept to the design standards ofthe pond system. A very large load may flush outimportant bacteria, eventually leading to systemfailure. Variation in loads will alter the retentiontime. Any extension of the time that the wastewaterremains within the pond system will increase thedisease-causing microorganism die-off. Theconcentration of microorganisms within the effluentwill be reduced and the effluent will be of higherquality before discharge into a waterway.

4.2.2 Recirculation

Whilst mechanical recirculation of pond liquor hasbeen attempted in this country and overseas, thelimited amount of reliable data available gives littletheoretical or practical support to the process.Only in one case in New Zealand where a strongsewage is treated in very large ponds has theprocess proved beneficial in rapidly diluting anddistributing the sewage into the body of ponds.Recirculation by simple pumping has provedsuccessful in alleviating odour and weed growthproblems in small badly sited ponds, where there isinsufficient wind action to provide natural mixing.

Some aerators, however, provide a strongdirectional current, and these can be used toimprove pond mixing.

4.2.3 Baffling

Shilton and Harrison (2003) have undertakenextensive research into pond hydraulics. In thiswork they suggest that baffling of oxidation pondscan produce significant improvements in effluentquality and make the following recommendations: A minimum of 2 baffles and maximum of 4 is

recommended. Even baffle spacing across the pond is the

most effective configuration. Short stub baffles can be as effective as much

longer baffles. Baffles are useful as an outlet shield.

Baffles can be made from a variety of materials;tipped rock, gabion baskets, or plastic sheets withweights and floats fitted. To be effective they do

not have to be totally impermeable, just sufficientto stop high speed currents.

4.2.4 Rock filters

A rock filter operates by allowing the wastewater totravel through a submerged porous rock bed,causing algae to settle out on the rock surfaces asthe liquid flows through the void spaces. Theaccumulated algae are then biologically degraded.The principal advantages of rock filters are theirrelatively low construction cost and simpleoperation. Odour problems can occur and thedesign life for the filters and the cleaningprocedures have not been firmly established.However, several units have operated successfullyfor years.

Rock filters have shown to be a low cost methodthat can be added to pond systems with goodresults.

4.2.5 Rock groynes

Rock groynes have become a very quick andsimple option to baffle one pond into multiplecompartments in series. The compartments use arock filter as a baffle which helps to filter out algaeand may promote nitrification and de-nitrification toremove nitrogen. There is limited but growingexperience with this system in NZ to date. Plantswhich have been upgraded in this way have almostall achieved reductions in pathogens and somehave achieved more stable suspended solids andnitrogen removal. However ammonia removal(nitrification) in winter is still unreliable. Some rockgroynes have also clogged.

One to two years operation is required to achievefull performance.

Fig 20: Rock groyne, partitioning tertiary (maturation),oxidation pond

4.3 Aerators (types and positioning)

Oxidation ponds rely on wind action to introducemuch of the required oxygen to the pond liquid andto develop flow patterns and mixing within thepond. Artificial aeration may be necessary whenthe pond is overloaded or during long calm windperiods. Care with aerator location can also help toimprove mixing and reduce short-circuiting.


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Mixing is important as non-motile algae tend tosink to the pond floor, and it is important that theyare returned to the surface to be in the effectivezone of light penetration.

4.3.1 Mechanical aerators

Motor driven, mechanical aerators provide acombination of liquid aeration and mixing.

Mechanical aerators can be classified as; impellortype aerators, which disturb the water and bring itinto contact with air, and aspirator type aeratorswhich pass air through the water. Mechanicalaerators can be further classified based onwhether the axis for the impellor is horizontal orvertical and whether the impellor is geared orrotates at the motor speed.

Figure 21 shows a typical vertical shaft surfaceaerator. The impellor sucks water up through thedraft tube and sprays it out in a circular patternradiating out from the aerator. These are usuallydirect drive from the motor to the impellor, and aretherefore less expensive than aerators involvinggearboxes. Some designers prefer caged rotor oraspirator type aerators to the vertical shaft type, asthey are reported to give better oxygen transferrates for the power used, are able to providedirectional currents which can be directed toenhance mixing and reduce short circuiting, and donot entrain as much bottom sludge.

Fig 21: Vertical axis surface aerator

A very rough rule of thumb for aerator sizing is that1 kW of installed aerator capacity will add 1 kg ofoxygen per hour to be pond water. For example, a1 Ha pond is loaded at 150kg/day BOD. Theadditional aeration required is thereforeapproximately (150-84)/24 or 3kW.

Aerators can be operated on timers, manuallycontrolled, or automatically controlled utilising DOmeters to switch them on and off. Continuousaeration can assist with pond nitrification, (theoxidation of ammonia to nitrate), thus reducing theimpact of the pond effluent on aquatic organisms inthe receiving waters.

Fig 22: Horizontal axis caged rotor aerator

4.3.2 Diffused aeration

Coarse bubble systems are useful for providingshort term aeration in pond systems, especially insmall ponds. Such systems use an air blowerlocated on the pond embankment or within abuilding, feeding air into a drilled pipe placed intothe pond.

The aeration pipe may be located on piers used tocarry the inlet pipeline, or weighted and suspendedfrom floats or cables. They should be placed belowthe surface but kept high enough so as not todisturb the lower layers.

There is also some limited use of fine bubbleaeration. Figure 23 shows a multi celled pondsystem with fine bubble aeration and suspendedsessil media.

Fig 23: Membrane fine bubble aeration.

4.4 Post treatment

4.4.1 Wetlands

The use of constructed wetlands in NZ hasincreased steadily since the 1980’s and currentlythere are more than 80 non-agricultural wetlandwastewater treatment systems in NZ, (Tanner C2002). Constructed wetlands can supplementpond treatment systems, providing advancedsecondary/tertiary treatment and polishing.Wetlands are also considered to be a practical


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means of addressing cultural issues and spiritualvalues for Maori.

Constructed wetlands in NZ can be most simplysubdivided into two categories: Surface flow wherewastewater flows through a shallow bed plantedwith emergent aquatic plants, and subsurface-flowwetlands where wastewater flows through plantedgravel-filled beds.

Figure 24 shows a surface flow wetland treatingoxidation pond effluent at Kaiwaka, Northland,(photo courtesy C Tanner, NIWA).

Figure 24: Surface flow wetland

Constructed wetlands can effectively reduce andstabilise SS and BOD concentrations in ponddischarges. Faecal bacteria levels can commonlybe reduced by around 1 log unit or more, but inputsfrom wildlife generally preclude achievement of E.coli levels below ~300-500 cfu/100 mls. Properlydesigned and sized wetlands can also reducenitrogen levels in pond discharges.

4.4.2 Disinfection

Multiple tertiary ponds in series are able to providegood reduction in pathogens and indicatororganisms down to levels often associated withdisinfection processes.

Disinfection of effluents which are treated throughpond systems is quite common and couldtheoretically be achieved using chlorine, UV light,or ozone. In practice, however, only multiple pondsin series with and without UV are used in NZ.

Chlorination has been used historically inwastewater disinfection in NZ due to its simplicityand long term history of effectiveness. It hashowever, fallen into disuse because of concernswith environmental effects of chlorinated organicsand residual chlorine, and operational costscompared to alternatives such as UV. Becauseresidual chlorine is toxic to aquatic species, aneffluent dosed with adequate chlorineconcentration to kill most microbes should bechemically de-chlorinated, adding to the complexityand cost of treatment.

UV Light is becoming the most common means ofdisinfection of wastewater effluents in NZ. UV

treatment systems use high intensity lamps thatare submerged in the wastewater or have lampsthat are surrounded by tubes that carry thewastewater. Disinfection occurs when theultraviolet light damages the genetic material of thebacterial or viral cell walls so that replication can nolonger occur. Care must be taken to keep thesurface of the lamps clean because surfacedeposits can shield the microbes from theradiation, thus reducing the performance of thesystem. Lamp fouling can be addressed by autowiping mechanisms, but the transmittance of thewastewater must be above 20% for UV to be aneffective disinfectant. Although there is somecorrelation between transmittance and suspendedsolids, this has been found to be quite variablebetween different ponds and specific monitoring fortransmittance or pilot trailing of UV plants isrecommended. As a general rule, pond effluentsrequire additional treatment, (multiple tertiaryponds, wetlands, etc), before UV light becomeseffective. Such a system is shown in figure 25.

Fig 25: Pressure UV system treating effluent with twostage ponds, surface flow wetlands, andirrigation filter pre-treatment.

Ozone is generated by passing oxygen (O2)through a high voltage potential resulting in a thirdoxygen atom becoming attached and forming O3.Ozone is very unstable and reactive and oxidizesany organic material it comes in contact with,destroying many disease-causing microorganisms,with the added bonus of removing otherwastewater components such as colour. As withchlorination, there is currently no use of ozonationfor pond effluents in NZ.

4.4.3 Land treatment

Land treatment of the pond effluent is becomingmore and more common in NZ, especially in areaswhere discharging effluent directly to water bodieshas become environmentally and culturallyunacceptable. Most land treatment systems takethe pond effluent, and then treat it to a betterquality. Pre-treatment options usually include somesort of filtration (to remove or reduce the algal


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levels) and sometimes disinfection (to reduce thepathogen levels), before the effluent is dischargedto land by; surface sprinklers, surface irrigation orsubsurface irrigation.

All of the above land application options have beenused with success but performance of landtreatment systems depends greatly on the quantityand quality of effluent, and the amount and thetype of land used. A good guide for furtherinformation and design of land treatment systemsis the NZ Guidelines for Utilisation of SewageEffluent on Land, (NZ Land Treatment Collective,2000). As operational experience with differentsystems increases, issues such as long termperformance in fine grained soils are becomingbetter evaluated.

Land treatment of pond effluent during summer lowflow periods in the receiving water, can be a goodoption. This provides land treatment when the landis best able to receive the effluent, and dischargeto river during the higher flow periods, whendilution and lower temperatures reduces theimpact. Ponds are particularly suited for pre-treatment for land treatment systems because theycan provide buffer storage to accommodate shortterm high inflows or periods when the landtreatment system is not able to operate.

A test pit dug next to a subsurface drip line isshown in figure 26.

Fig 26: Subsurface drip line

4.4.4 Micro-filtration

Micro-filtration of oxidation pond effluent involvespassing the flow through very fine holes insynthetic membranes which physically removemost particles and microbes. Currently only onepond system, at Dannevirke is being treated bymicro-filtration. The Dannevirke system wasinstalled in 2004 and is still undergoingcommissioning. It is preceded by three oxidationponds in series with the final pond divided in twoparts to provide buffer storage. Although theeffluent quality is very good, the installed cost ofmicro-filtration systems are at the top end forupgrade options.

Fig 27: Wastewater micro-filtration plant

4.4.5 Advanced pond systems

Advanced pond systems comprise 3-5 ponds inseries, with each one specifically designed for aparticular function. The different ponds typicallycomprise; a fermentation pond, a high rate algalpond, algal settling ponds, and a maturation pond.

NZ installations to date of high rate ponds forMunicipal Wastewater treatment have been limitedto pilot scale, but the concept appears to showpromise of achieving high quality results.

Fig 28: Advanced pond system pond layout

4.4.6 Coagulation – sand filtration

Sand filtration, using filters and processes similarto water treatment plants is another effluentupgrade option. Sand filters without coagulantaddition, (plain filtration), are not very effective onpond effluents. With coagulant addition, however,high quality results can be obtained, (refer section8 for typical values). The filter backwash isdischarged back to the pond system. NZ has suchfiltration plants at: Pauanui, Coromandel, andOmaha, in the Thames – Coromandel area. One ofthese uses a clarifier and sand filter, one asubsurface gravel wetland, and the third a tertiarypond, prior to the sand filters. Significant operatingcosts are involved due to the cost of coagulant.


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Fig 29: Sand filtration plant. Tanks are flocculation,backwash storage, and filters.

5. Frequently Asked QuestionsIf you are using an electronic copy of theseguidelines, holding down the control key plus a leftmouse button click on any of the questions belowwill take you to the relevant section.

How often should I measure the sludge depth ?My ponds have an odour problem !Is the colour of the pond important ?What are Blue-Green Algae?How do I design a pond?How do I build a pond?What operational records should I keep?What effluent quality should a pond produce ?What effluent quality can an upgraded pondproduce ?How can I improve the discharge quality?Can I treat the pond effluent with UV light?I’m confused by the different names used todescribe ponds.

6. References and Further ReadingArcher, H. and Donaldson, S. (1998) Low CostUpgrading of Oxidation Ponds for Public HealthProtection with Nutrient Removal Benefits.

Brockett O.D. Phenomenon of Biological Instabilityin Sewage lagoons at Mangere. BiotechnologyConference Prac. May 1975, Massey University.

Craggs R.J., Davies-Colley R.J., Tanner C.C.and Sukias J.P.S. (2003). Advanced pondssystems: performance with high rate ponds ofdifferent depths and areas. Water Science andTechnology 48(2): 259-267.

Fitzmaurice, JR. (1987). Municipal wastewaterdisposal in New Zealand. In Transaction of theInstitution of Professional Engineers of NewZealand. Vol. 14, No 1/CE, March 1987.

Mara, D. D. and Pearson, H. (1998). DesignManual for Waste Stabilization Ponds in

Mediterranean Countries, Leeds: LagoonTechnology International Ltd.

Marias, G. Dynamic Behaviour of Oxidation Ponds,Second Int Symp for Waste Traetement Lagoons,Kansas City, USA, 1970.

Middlebrooks EJ, Upgrading Pond Effluents: AnOverview. (1995) Watt Sic Tech Volt 31 No 12,

Ministry of Works and Development (1974)Guidelines for the Design, Construction andOperation of Oxidation Ponds.

NZWW Assn (2003) Guidelines for the SafeApplication of Biosolids to land in New Zealand.

New Zealand Land Treatment Collective, (2000).New Zealand Guidelines for Utilisation of SewageEffluent on Land. ISBN 0 478 11005 7.

Pearson, H. W. (1996). Expanding the horizons ofpond technology and application in anenvironmentally conscious world. Water Scienceand Technology 33(7): 1-9.

Shilton Andy (2001). Studies into the Hydraulics ofWaste Stabilization ponds. Ph.D. Thesis. MasseyUniversity, Palmerston North, New Zealand.

Shilton A. and Harrison J. (2003). Guidelines forthe Hydraulic Design of the Waste Stabilizationponds. Institute of Technology and Engineering,Massey University, Palmerston North, NewZealand.

Tanner, C.C.; Sukias, J.P.S. (2003). Linkingpond and wetland treatment:performance of domestic and farm systems inNew Zealand. Water Science and Technology48(2): 331–339.

Tanner, C.C.; Sukias, J.P.S.; Dall, C. (2000).Constructed wetlands in New Zealand–Evaluation of an emerging “natural” wastewatertreatment technology. Proceedings of Water2000: Guarding the Global ResourceConference, Auckland, March 19-23. CD ROMISBN 1-877134-30-9, NZWWA.

USEPA. Design Manual for MunicipalWastewater Stabilisation Ponds, 1983.

7. Web Siteshttp://www.lagoonsonline.com

http://www.epa.gov

http://www.usace.army.mil

http://encyclopedia.laborlawtalk.com


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8. Indicative Effluent Quality from Improved Pond Systems

BOD5 SS TN NH3-N TP DRP FC E Coli

Contaminant

g/m3

g/m3

g/m3

g/m3

g/m3

g/m3

cfu/100ml

cfu/100ml

Facultative (Primary) Pond 40 50 40 15 8 6 20x 103

10x103

Maturation (Tertiary), Pond 30 40 35 13 8 6 10x103

5x103

Multiple Maturation(Tertiary), Ponds in Series

30 40 25 10 8 6 2 x 103

1 x 103

Micro-Filtration 5 1 5 10 4 2 1 1

Rock Groynes 30 35 30 10 8 6 5x103

2x103

Coagulation and SandFiltration

5 5 20 10 5 3 50 10

Wetlands 15 15 25 5 6 4 5x103

2x103

Wetlands and UV Light 15 15 25 5 6 4 200 100

Table 1: Typical effluent quality from pond improvements (median values)


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9. Pond Record Sheet

PumpsSewage

Flow Oxidation PondsMeter

Reading(Hr orKwH)

pHDissolvedOxygen

Temp ofPonds(oC)

Appearanceof Ponds

Odour

Da

yo

fm

on

th

Pla

ntin

sp

ecte

d

No

.1

No

.2

Pu

mp

Ho

urs

Da

ilyF

low

(M3

)

Tim

eo

fT

est

No

.1

No

.2

No

.1

No

.2

No

.1

No

.2

No

.1

No

.2

No

.1

No

.2

Weather Comments

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