~ Pergamon

PH: S0273-l223(99)OOI33-X

Wal. &,. Teelr. Vol. 39, No.6, pp. 143-150,1999<C 1999IAWQ

Published by Elsev,er SCIence LtdPnnted In Great Bnrain. All rights reserved

0273-1223/99 520 00 + 0.00

RETROFITTING AN EXISTINGTRICKLING FILTER PLANT TO BNRSTANDARD - SELFS POINT,TASMANIA'S FIRST

Scott Morgan·, Ray Farley· and Rob Pearson••

• Hoban City Council, GPO Box 503E. Hobart, Tasmania. 700/. Australia.. ANI-Kroger, Level 3. 23/ Miller Street. North Sydney. NSW, 2060, Australia

ABSTRACT

Hobart City Council has three separate wastewater catchments. Prior to 1997 one catchment dischargeduntreated sewage to the Derwent River estuary. The State environmental authority required this wastewaterto be treated to nutrient removal standard. Following extensIve investigations to determine options, Councildecided to transfer this wastewater to the eXIsting Selfs Point trickling filter plant. Augmentation of thisfacility to bIological nubient removal standard was selected to meet the more Slnngent licence condIttons.Council required the new treatment process to utilise current best mdustry practice, to have provenperformance and to make best use of the existmg infrastructure, whtle mamtaimng cost effectiveness.

This paper discusses the rattonale for incorporating trickling filters mto the process speCIfication. It alsodescribes the maxunal use of previously existing plant, with over 90"10 mcorporated into the design. Asignificant feature is that the anaerobic digesters have been retained to ensure good biosolids stablhsatton.

Selfs Pomt has shown that retrofitting a trickling filter plant to BNR performance can be very cost effectivewhere eXisting faCIlities are m good conditIon. ~ 1999 lAWQ Published by Elsevier Science Ltd. All nghtsreserved

KEYWORDS

Anaerobic digestion; biological nutrient removal; nitrogen removal; phosphorus removal; sludgefermentation; trickling filter; ultraviolet light disinfection.

INTRODUCTION

The Hobart City Council area has three sewage catchments. Prior to 1997 the northern catchment area wasserved by a trickling filter plant at Selfs Point, the central catchment by high rate biofilters at MacquariePoint and the sewage from southern catchment including the suburbs of Sandy Bay and Mount Nelson, wasdischarged to the Derwent Estuary at Blinking Billy Point after only maceration.

A number ofsites in the Sandy Bay area had been assessed for locating a wastewater treatment plant to treatthe sewage from this catchment. None were deemed acceptable and the decision was taken to pump thesewage from Sandy Bay and augment the plant at Selfs Point to a capacity of 10.4 MI/day.

The Tasmanian State environment department had concerns about the nutrient loading on the DerwentEstuary and reqUired the augmented plant at Selfs Point to meet significantly tighter discharge limits than

143

144 S. MORGAN et at.

had previously prevailed for estuarine discharge. The environmental licence limits (given in Table I under"New Licence") are "never to exceed" measured once every six days.

Table I. Influent characteristics and effiuent requirements

Biochemical Oxygen Demand mgll 265 40 ISSuspended Solids mgll 200 60 20Ammonia-Nitrogen mgll 35 2Total Nitrogen mgll 41 10Total Phosphorus mgtl 8.5 3Oil and grease mgll 22 5Faecal Coliforms cfu/lOOmI 107 1000 750

Parameter Units TypicalInfluent

PreviousLicence

NewLicence

ContractGuarantee(95%ile)

IS20

2102-*

10**• No value guaranteed due to insuffiCient influent data•• This value is a 50%ile at two times the average dry weather flowrate

Following analysis by Council officers of available technologies, the anticipated combined influentcharacteristics and the condition of the existing Selfs Point plant, it was decided that the best option wouldbe a hybrid biological nutrient removal process incorporating the existing trickling filters. The relativelyhigh BOD:N and BOD:P ratios made this a viable option without the need to provide an external carbonsource.

After a detailed tendering process, based on a design and construct contract in accordance with theAustralian Standard AS4300, ANI-Kruger was selected to undertake the retrofit ofSelfs Point.

A major feature of the contract was a performance guarantee of 20% of the contract value, based on 95%compliance with concentrations given in Table 1. The phosphorus and faecal coliform values were reducedbelow the environmental licence limits to enhance the potential for effiuent reuse.

Retrofit. Most of the existing Selfs Point process plant was over twenty years old, but generally in goodcondition. Council determined that maximising use of existing structures would be very cost effective. Allthree of the selected tenderers produced innovative designs incorporating over 900!o of existing plant. Apartfrom the grit channel, bar screen, screenings incinerator and sludge drying sheds, which had all beentechnologically superseded, every process unit was retained with only minor refurbishment. It is estimatedthat the capital cost saved by using this philosophy was in excess of$1.5 million.

PROCESS DESCRIPTION

The augmented Selfs Point Plant has a design capacity of 45,000 EP at an average dry weather flow of 10.4MI/day. The influent wastewater is principally domestic in nature with a milk processing plant providingabout 12% of the organic load. The flowrate has a strong diurnal pattern with the flow peaking at 180-240I/sec mid-morning and dropping to 40-45 LIsee overnight. The organic concentrations also show a five toone diurnal variation at similar times to the flow and this results in a diurnal peak to minimum organic loadratio of greater than 20. The raw wastewater temperature varies from occasional peak temperatures of 24°Cto a minimum of 10.5°C in late winter. A simplified flowsheet of the process, without sidestreams isprovided in Figure I and a plant layout in Figure 2.

Inlet works and trickling filters. After receiving screening and grit removal treatment the influent undergoesprimary settling in a rectangular sedimentation tank. The flow is then split with a proportion passing to thetwo trickling filters and the remainder pumped directly to the inlet ofthe BNR anaerobic zone. The effiuentfrom the trickling filters is transferred without settling to the BNR system, while a portion can berecirculated to mainlain wetting of the rock media, when the inlet flowrate is low.

Rc·trolittmg an eXIsting tnckling tilter plant to I3NR standard 145

Bio - Reactors Clarifiers

Bio - P selector

Figure I Process flow diagram for the Selfs Point I3NR plant.

Main BNR system. The activated sludge part of the BNR system is a BioDcnipho enhanced triple ditchconfiguration. At the inlet of the BNR system (the Bio-P selector in Figure I) settled sludge from theclarifiers is mixed with primary effluent. Filtrate from the dewatering processes and prefermenter product isalso added into this zone. The trickling filter effluent is added after the anaerobic zone prior to the flowbeing introduced to one of the first two biological reactors. Overflow from the first stage reactors flows tothe third reactor. The system is different to the normal BioDenipho triple ditch configuration in that the thirdaeration reactor is in series with the first two reactors rather than in parallel.

Aeration is performed by surface rotors that also provide mixing. Submersed propellers provide mixingduring denitrification. The proportions of denitrification and nitrification within the BNR system arccontrolled by dissolved oxygen concentrations and varying of thc phase lengths. Denitrification is usuallyundertaken only in the reactor being fed from the anaerobic zone, though it can be extended to other reactorsif there is surflcient nitrilication capacity. Carbon source that has not been consumed in the anaerobic zoneis utilised in the denitrification rcaction. Alter passing through the third reactor the mixed liquor flows totwo circular clarilicrs.

Legend

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,,,"",,

ICM.EN~

GAIT N«J OR!AIE OWleEA

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n I.N D1$lW£C1lOH

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Figure 2. Plant layout at elfs Point.

IJisillji'clioll.The clarifier effluent undergoes I IV disinlcction prior to heing pumped 10 Blinking Rilly Pointfor dischar~c to the Derwent Estuary. The existing chlorine disinfection syslem IS currently hem); rctamedin a reduce:' capacity until State govenlmcnt requirements for effluent reuse ha\'C heen Iinalised. Some ofthe effluent is recycled as process water within the plant.

146 S. MORGAN et al.

Dewatering and digestion. The sludge collected by primary settling is pumped to a prefermenter with a twoto three day hydraulic residence time. After prefermentation the sludge is fed to a rotary drum thickener withthe filtrate being pumped to the inlet of the BNR anaerobic zone to provide readily biodegradable COD. Thesl~dge of 6-7% solids content is transferred to the anaerobic digesters. The excess biological sludge is alsothIckened by rotary drum thickener and pumped to the digesters.

The two digesters are to be operated in parallel at 5.5% solids content and thus provide over twenty daysretention time, meeting USEPA Part 503 Class B biosolids stabilisation requirements (WEF, 1995).Cogeneration with a total of 30kW electrical power capacity is to be installed to generate electricity from theexcess digester gas. This would not have been possible if the excess biological sludge was dewatereddirectly without further stabilisation.

The digested sludge is dewatered using a belt filter press, which produces a sludge cake of between 22 and26 % dry solids. Biosolids are transported from site for beneficial reuse. There is a small degree ofphosphorus release in the digesters and precipitant dosing has been included to dose the belt press filtrate, ifrequired. The ammonia released during digestion is removed in the main biological process.

Wet weather flow treatment. The catchments treated at Selfs Point have high wet weather flows and theplant has been designed to receive four times average dry weather flow (ADWF). The BNR system isdesigned to treat flowrates of up to 2.5 ADWF. Flowrates in excess of this figure are transferred to aretention basin for later reintroduction to the process.

RATIONALE FOR RETENTION OF THE TRICKLING FILTERS

In assessing the major options for the augmentation of Selfs Point the good condition of the existing processplant and particularly the trickling filters was considered important. The plant influent BOD:N and BOD:Pratios were greater than those required to achieve the required BNR standard and the retention of thetrickling filters was seen to provide a number of advantages, as follows:

1. reduction in size of aeration reactors in the activated sludge BNR system;2. reduced aeration costs;3. improved low temperature nitrification performance;4. increased ability of the plant to handle shock organic loads;5. possible reduction in sludge production;6. potential improvement in sludge settling compared to solely suspended growth processes;7. capability of treating sustained high flows to at least secondary level prior to discharge; and8. increased flexibility of plant to handle future changes of influent characteristics.

With regard to these potential advantages, the following experiences have occurred at Selfs Point.

1. At an influent temperature of 13°C the trickling filters achieved approximately 65-70% nitrificationwhen treating 60% of the influent flow and this appears to be alkalinity limited. It has been estimatedthat the aeration reactors are about 30% smaller than if the trickling filters had not been utilised.

2. The power costs of operating the BNR system portion of the plant are estimated to be 20-25% less thanan activated sludge system sized to treat the full flow.

3. The improved low temperature nitrification performance ofbiofilms compared to suspended growth hasbeen reported in Swedish plants using trickling filters (Parker et al., 1995), but no work to date has beencarried out at Self's Point to confirm this.

4. The plant has received significant quantities of milk products spilt at the dairy processor. One recentspill of 5000 Iitres of cream was estimated to have increased the influent BOD concentration from280mg/l to over 3000mg/1 for a period of several hours. A grab sample taken at the plant inlet during theevent was measured as having a COD of 15,000 mg/l. By increasing the proportion of the flow to thetrickling filters and diverting a portion of the flow to the detention basin the BNR system was protectedfrom organic overload and the daily composite effluent sample showed an ammonia nitrogen of 0.8 mg/land a suspended solids concentration of 2.3 mg/l.

Retrofitting an existing trickling /iller planlto BNR standard 147

5. While proce~s optimisation is sti~1 undelWay and thus operations are not yet fully stable, there doesappear to be In the ord~r o~ 10-15 Yo less waste sludge than would be expected from an activated sludgeonly system, though thIS WIll need confirmation with further measurements.

6. The efflu~nt from the plant is presently achieving flow proportioned composite suspended solidsconcentratIons of less than 2 mg/I without tertiary filtration, but further work would need to beundertaken to d~termine if this is primarily due to using trickling filters or some other reason.

7. N~ extended hIgh flow events have occurred since start up on the BNR process, but simple plantadJus~me~tscan be made to treat excess flow above 2.5 ADWF by following primary sedimentation withthe tncklmg filter process and secondary settling prior to discharge.

8. Modeling of the plant based on performance to date indicates that there is potential to treat more flowand load with the use of flow equalisation or upgrading ofthe trickling filter media.

METHODS

All analyses described within this paper were performed at the Selfs Point site laboratory. The methods usedwere in accordance with Standard Methods for the Examination of Water and Wastewater, (APHNWEF,1995), with the exception of the total nitrogen analyses, which were performed using a chemiluminescenttechnique. Daily composites were produced by flow proportioning 24 discrete samples.

RESULTS AND DEVELOPMENTS DURING COMMISSIONING

Plant performance during commissioning. Wastewater was first fed to the BNR system on March S 1997.The effluent BOD and suspended solids concentrations both soon fell to levels of less than 5mgll.Nitrification improved as the mixed liquor suspended solids (MLSS) built up and ammonia concentrations inthe effluent were complying with the environmental licence limit of 2 mgll by April 2 (as can be seen inFigure 3). Once ammonia nitrogen had reached the target concentration the proportion of denitrificationwithin the plant was increased and the total nitrogen licence limit of 10 mg/l was met by mid-April. Thereduction in nitrate concentration allowed the commissioning personnel to focus on optimising the enhancedbiological phosphorus removal processes.

Phosphorus concentrations of greater than 20 mg/l were consistently being achieved at the exit of theanaerobic zone by mid May indicating a high level of biological phosphorus activity. Microbiologicalanalyses indicated that there were good numbers of phosphorus accumulating bacteria present. Thephosphorus concentration in the effluent was generally decreasing and attained environmental licence limitof 3 mg/l in early June. This level was not sustained due to dewatering system problems which forced a shiftin process control priorities.

Electrical and control problems with some of the dewatering system equipment resulted in a build up ofexcess biomass in the BNR system during June. The control of the plant had to be modified to ensure solidswere retained in the plant until these problems could be rectified. This resulted in a drop off in theperformance of the nutrient removal processes in late June and July.

The biomass inventory was reduced to a suitable level and the focus returned to optimising nutrient removallate in July. The effluent phosphorus concentration dropped during the period of reduction in biomass, whenammonia concentration was elevated but low nitrate levels were being experienced. Following thereestablishment of full nitrification at the beginning of August, increasing the proportion of denitrificationhas resulted in the total nitrogen concentration in the effluent dropping to the range of 5-8 mg/1. Phosphorusremoval improved with the reduction in nitrate concentration and results at the time of writing are headingtowards the required limits.

Primary sedimentation tank performance. During commissioning of the augmented plant it became apparentthat the primary sedimentation tanks were removing more solids and organics than predicted in the processdesign. Results were verified over several days and at 2 hours retention at ADWF the tanks were removing60-70% of suspended solids and 40-50"10 of BOD. This resulted in insufficient carbon source reaching theBNR system and one of the two sedimentation tanks was taken offline. The single primary sedimentationtank is performing in accordance with the process design requirements.

148 S. MORGAN et a/.

Ammonia Nitrogen (mglL)3025 •20 #

15 .' '\10 ~

5 .. ........~...... .' . <\ ......0 . .' .

Mar Apr May Jun Jul Aug Sep

30Total Nitrogen (mglL)

252015 .

~"'II':#.10 '. . • fI6 •. '. ., . .;. A'5 • . ..

'. • '" . .#. . .0Mar Apr May Jun Jul Aug Sep

12Total Phosphorus (mglL)

9

8 '. t. . ..' . • . . ..~\~.. . '. • . 1-- ...3 .. ·. ..

." ·. ~ -0 ·Mar May Jun Jul Aug Sep

BNR System Solids (toones)40

30 #.~ '~ .'.~.. .. ...... ••,'¥..~;20

................. •~J ".'...10 . ...-0

~b •

Mar Apr May Jun Jul Aug Sep

Figure 3. Ammonia nitrogen, total nitrogen, total phosphorus, and BNR system solids content dunng thecomnussioning penod of the Self's Point Plant

Low alkalinity ofthe influent. The potable water supply in Hobart has a very low alkalinity and is usually inthe range of 10-15 mg/I CaC03. This leads to wastewater with a low alkalinity and the typical concentrationof raw wastewater at Selfs Point is 180 mg/1. Alkalinity is consumed in the nitrification process and manyplants with low alkalinity wastewater require carbonate dosing.

During July it became apparent that full nitrification was not being achieved after having been performingwell in earlier months. The partial nitrification occurred when the BNR system had been operating on 17%denitrification and four hour cycles. and low pH was measured in the third aeration reactor. The cycle timewas reduced to two hours and denitrification to 25% of the cycle. Measurements were taken during theaeration cycles in the BNR system and the results are shown in Figure 4. As can be seen the alkalinity roseand fell with ammonia concentration. In Tank C (the third bio-reactor) the pH was 6.5 during the four hourperiod with an alkalinity of 45-50 mg/I CaC03 yet virtually full nitrification was maintained. Reducedalkalinity will increase the likelihood of nitrification inhibition due to a lowering of pH. The BioDenipho

Retrofitting an existing trickling filter plant tn BNR standard 149

process allowed this adjustment to be made easily. This flexibility is not available with most plant desiother systems may have had to overcome this problem with alkalinity dosing. gns.

Ammonia Nitrogen (mglL)8 r---------=---.-:.--=-~----_,6

..

10:00 11:00 12:00 1300 14:00

Alkalinity (mglL CaC03)

13.00 14.00

Figure 4. Alkalinity and ammonia nitrogen concentrations in the bin-reactors on II August 1997.

Addition ofhydrolysate from the prefermenter. The prefermenter thickener can be operated unattended onautomatic startup. This gives a high degree of flexibility to enable the operational staff to determine the mostsuitable regime for adding hydrolysate to the BNR system. A number of modes have been trialed includingaddition of the hydrolysate in the low load period of the day and at the peak load period. Further work needsto be carried out in this area to optimise the system performance.

Effluent oxygen demand and suspended solids. Both the organic strength and the solids load of the effluenthas exceeded design expectations. During the first two weeks of August the effluent composite samplesaveraged concentrations of 1.6 mg/l for suspended solids. 16 mg/I for COD and less than 5mg/l for BOD(i.e. below the concentration for accurate determination). If maintained this level of quality will significantlyenhance the potential for effluent reuse.

Ultraviolet light disinfection and high transmissivity. The UV disinfection system incoJl'Orates flow andtransmissivity pacing. The UV lamp output is adjusted in accordance with both of these variables through asoftware algorithm. The performance guarantee is to meet less than 10cfuJIOOml at two times ADWF.Preliminary testing prior to the contractual performance evaluation has shown measurements in the rangefrom <10 to 40 cfuJIOOmI. The plant effluent transmissivity has varied in the range from 65·85% with atypical value of 75%. The supplier has reviewed the data and concluded that as the software is configuredfor a transmissivity of 55-65% the system is reducing lamp power below that required to achieve thedisinfection standard. The detailed evaluation will be performed after the software has been modified to suitthe higher than expected transmissivities.

Control system. The plant is monitored and controlled through a Supervisory Control and Data AcquisitionSystem (SCADA). The system has two onsite control computers and two remote access computers whichallow full control of the plant. The control system was designed to provide detailed process information tooperational personnel along with a high degree of controllability. Comprehensive online instrumentation isinstalled at Selfs Point to monitor nutrients within the process to facilitate decision making and processoptimisation.

CONCLUSION

The retention and incoJl'Oration of the existing trickling filters into the BNR upgrade at Selfs Point hasproven to be very cost effective. The trickling filters are a critical part of the nutrient removal process and

150 S. MORGAN et al.

contribute to the process flexibility and stability. The operational cost of the plant is significantly below thatwhich would be expected for a BNR system based solely on suspended growth processes.

The robust plant design based on the BioDenipho technology has demonstrated a high degree ofcontrollability which has been important at the Selfs Point plant due to the low alkalinity of the rawwastewater and the trade waste component of the influent.

Whilst the plant is still undergoing process optimisation it is clearly evident that it will be able to meet theenvironmental licence requirements. A notable aspect of the effluent to date is the extremely low effluentsuspended solids and BOD concentrations. This will ensure that an extremely high standard of disinfectionis achieved by the UV system and opportunities for effluent reuse will be realised.

The incorporation ofanaerobic digestion into the process design has resulted in the plant producing a stableand environmentally acceptable biosolids product. The odour and disposal problems often associated withBNR sludge have thereby been eliminated.

ACKNOWLEDGEMENTS

The authors wish to gratefully acknowledge the contributions made by all of the laboratory staff at the SelfsPoint plant and Emma Aisbett and Dennis McGrath of ANI-KrUger.

REFERENCES

APHAIWEF (1995). Standard Methods for the Examination of Water and Wastewater (191h edn.), Amencan Public HealthAssociation, Washmgton D.C., USA.

Parker. D., Lutz, M., Andersson, B. and Aspegren, H. (1995). Effect ofoperatmg vanables on nitnlication rates in trickling lilten.Wat. Env Res 67(7), 1111-1118.

WEF (1995). Wastewater Residuals Stabilization-MOP FD-9, WEF, Alexandria, Virginia, USA.