~ Pergamon

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War. Sci. Tech. Vol. 31, No. 3-4, pp. 327-340.1995.Copyright © 1995 IAWQ

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RECENT APPLICATIONS OF DISSOLVEDAIR FLOTATION PILOT STUDIES ANDFULL SCALE DESIGN

Steven R. Arnold*, Thomas P. Grubb* andPeter J. Harvey**

* Purac Engineering, lnc., 5301 Limestone Road, Suite 126, Wilmington,Delaware 19808, USAPurac Rosewater Ltd, Purac House, Birmingham Road, Kidderminster,Worcestershire DYIO 2LH, UK

ABSTRACT

The use of Dissolved Air Flotation (DAF) as a solids/liquid separation process in water treatment has been aneffective alternative to sedimentation for 70 years. The process was initially applied for removal of materialswhich had a specific gravity less than water, such as fats, oils, fibers, and grease. DAF installations expandedin the late 1960s to wastewater and potable water treatment. Today, Dissolved Air Flotation is utilized for awide variety of water and wastewater applications. DAF is applied extensively for wastewater sludgethickening and it is widely accepted in Scandinavia and the United Kingdom for potable water treatment. Ithas also gained a foothold in the United States with the stan up of a 7.5 mgd (28.4 Ml/d) potable waterflotation plant at New Castle, New York. The goal of this paper is to present recent applications of DissolvedAir Flotation technology on a variety of raw water sources. Descriptions and general design parameters of atypical flotation and a proprietary combined flotation and filtratrion process will be discussed.

KEYWORDS

Dissolved Air Flotation (OAF); industrial wastewater; potable water treatment; retrofits.

PROCESS DESCRIPTION

Dissolved Air Flotation

The modern pressure OAF process is based on the same principles as those patented by Peterson and Sveenin 1924. Figure I presents a cross-section of a typical DAF treatment train. Raw water enters the plantthrough a rapid mixer where dosing of selected chemical coagulant occurs. Treated water flows to theflocculation tanks which are designed with a hydraulic residence time of 20 minutes, split evenly betweentwo tanks to minimize short circuiting. Flocculation energy is produced by mechanical agitation, typically avertically mounted, paddle-type mixer. The design should emphasize control of velocities and properbaffling to prevent floc shear or short circuiting through the flocculation tanks. Energy input is variable fordifferent raw water qualities, but some studies have shown a G-value of 70 s-1 to be optimum (Gregory andZabel, 1990). Flocculated water overflows into a recycle dispersion chamber where the air saturated recyclestream is continuously introduced through a system of proprietary nozzles, valves, or orifices. Instantaneous

327

328 S. R. ARNOLD et al.

Ffcceutaror

decrease of the recycle flow pressure from 70-90 psi (485-620 kPa) to atmospheric pressure causesformation of air microbubbles ranging in size from 10 to 100 urn, with mean value of 40 urn (Zabel, 1984).Microbubbles attach to flocculated particles and form an air/solids aggregate which displays a low apparentdensity relative to the density of water. Charge destabilization is necessary for the attachment of bubbles toflocculated particles and is dependant upon proper pH, water quality, and addition of coagulants (Malley andEdzwald, 1991). Bubble size is also of importance, since large air bubbles result in high rise rates which mayexceed laminar flow requirements and cause poor performance due to floc breakup. Larger bubbles alsohave less surface area per unit volume, which decreases the number of bubbles and the chance for randombubblelfloc collision. Consequently, proper design of the air dispersion system is an important factor forsuccessful DAF clarification.

,_I- J Compreuor

S"".."on Tonk f -~l (_. C._\)_ __ -- Edue tor

- - J SUflace Skimmer

'~~~~;r~~ffil;~sb~-~-= / Bccycte Pumpn / .: t n ,

SlUdge

Fig. 1. Cross section of a typical DAF unit.

The bubblelfloc aggregates rise to the surface. where they form a stable layer of float (sludge). On thesurface, float will continuously thicken and is mechanically skimmed onto a sludge beach where itoverflows into a sludge trough or hopper for further dewatering or disposal. Mechanical skimmers may beoperated either intermittently or continuously, depending on the volume of sludge generated . Dry solidscontent of the sludge varies between 2 and 6%. Raw water quality, the type and dose of coagulant and/orpolymer used, and the method of float skimming are key factors that determine the percentage of dry solids .Purac's experience indicates that the replacement of rubber skimmer squeegees with specially designednylon brushes improves dewatering of float on the sludge beach, which may result in up to a I% increase indry solids content.

Clarified water is collected at the bottom of the unit by a series of slotted pipes which connect to anadjustable weir. The weir is used to adjust operating water level on the sludge beach for optimization ofsludge removal and sludge solids content.

A portion of the clarified water, typically 5 to 10% of the throughput flow, is recycled into an air saturationsystem by recycle pumps. The system is designed to operate at a pressure range of 70-90 psi (485-620 kPa).Recycle water is pumped through an eductor, which introduces air into the water stream from the air cushionof the separation tank. A float switch in the separation tank maintains level by controlling addition ofcompressed air. Tank design increases air/water contact time and separated undissolved air to maximize airsaturation. Operation efficiency of the eductor/saturation vessel combination has been measured at 75 to80%. Air saturated recycle flows from the separation tank to the recycle dispersion system. Microbubbles ofair are released from solution, when pressure is dropped to atmospheric across the dispersion valve.

Dissolved air flotation

Combined Flotation and Filtration - Flotation over filtration

329

The FLOFILTER® is a proprietary system combining two processes (flotation and filtration) into one unit.The flotation process is operated identically to that described previously with a constant level filter placed inthe flotation tank. Figure 2 presents a cross section of a typical treatment train. The flow is regulated by amodulating filter effluent valve controlled by a level switch on the flotation tank surface.

I . ICompressor

S.t"'.'·on Yon. r: ~. ( c. .=0_. , - - Educ lor

--- --.1 Surf dce Skimmer

"'i~~~!~~~iTIE;:;~~~'~'-:~ / Recycle Pump11 _~ / _.,~ n

\·/.l~Il,i'

~.Ic ...,.. .l ~h

Air

/Fil le r Med lil

S ludge

Fig . 2. Cross section of a typical combined flotation and filtration unit.

The standard design will accept up to 3 ft (0.9m) of filter media. Any type of filter media. including GAC.may be used to match the site-specific needs. If conditions require. modifications of the design canaccommodate increased filter media depth. Filter media rests on a nozzle-type underdrain system . Filteredwater then flows to a clearwell where process pumps recycle flow through the air saturation system.

A typical filter run lasts between I and 3 days. Filter backwashing may be initiated manually orautomatically by timer, headloss measurement. or by filtrate quality . Prior to the backwash cycle. float ingsludge is skimmed and water level is dropped below the backwash trough . Backwashing can be achieved byseparate or combined air scour and water wash. Depending on the application. the backwash may bereturned to the head of the plant for reuse.

SURFACE WATER APPLICATIONS

DAF technology may be successfully applied to treatment of most surface waters with average turbid ities upto 100 NTU. Occasional turbidities above 100 NTU do not disrupt the process or cause significantdeterioration of effluent quality. A DAF system can exhibit significant advantages over other technologieson a variety of water qualities including:

- Algae laden waters;- High colored waters ;- Low turbidity-low alkalinity waters ;- Waters supersaturated with air;- Cold waters.

The advantages DAF offers for treatment of the above raw water qualities can be explained through aninvestigation of coagulated particles characteristics. Sedimentation removes particles which exhibit settlingvelocities greater than the settling basin overflow rate. A particle which is large and has a high density willproduce a higher settling velocity than the overflow rate of the basin and will be subsequently removed bysedimentation. Low density particles with settling velocities less than the overflow rate of the basin will not

330 S.R. ARNOLD et ai.

be removed. Chemical coagulation of waters with algae, low temperatures, low turbidities, and high colortypically produce small particles which exhibit a low density, making them difficult to remove bysedimentation. OAF removes these particles by forming bubble/particle aggregates which rise to the surfaceof the flotation tank.

The authors have been involved in many pilot scale testing projects which demonstrate advantages of theOAF process. Examples of selected pilot plant results will be discussed below to emphasize the use of OAFon natural and simulated water conditions.

Table I demonstrates the effectiveness of OAF during a pilot study of paper mill process makeup water.Treatment goals require a turbidity less than I NTU and apparent color less than 10 Pt/Co units 80% of thetime. The raw water source exhibits high color, low turbidity and low alkalinity. Raw water temperaturesranging from 0-3.5°C were present during the study period. The formation of low density particles due tolow turbidity and high color, combined with increased cold water viscosity, produce a particle difficult toremove by conventional sedimentation. Treatment utilized alum for coagulation, a cationic polymer as aflocculation aid, and NaOH for pH control. The test met all treatment requirements and yielded a filter runtime of approximately 24 hours. Color removal was ,=96%.

Table I. Low Temperature, Low Turbidity Pilot Run

RAW FLOATED FILTEREDDATE TIME LOADING RECYCLE TEMP TURB APP TURB APP TURB APP HEADLOSS

RATE RATE COLOR COLOR COLORhours gpm/ft? % Recycle °C NTU PlICo units NTU PlICo units NTU PT/Co units ft/hour

1-21 0 5.0 6.6 1.0 2.06 67 0.83 27 0.27 2

1-21 1.5 5.0 6.6 1.0 2.06 67 0.83 27 0.27 2 0.13

1-21 3.5 5.0 7.1 1.0 2.41 81 2.20 33 0.23 4 0.19

1-21 5.5 5.0 7.1 1.0 4.76 76 1.47 22 0.14 4 0.15

1-22 19.5 5.0 6.7 l.l 2.69 63 1.98 25 0.12 0 0.31

1-22 21.5 5.0 6.7 I.I 4.42 63 2.15 26 0.15 I 0.32

River water color occasionally reaches 200 Pt/Co units and an effort to simulate this was undertaken duringthe study. A commercially prepared humic acid solution was fed into the raw water to achieve an averageapparent color of 184 Pt/Co units. Alum with a cationic polymer and NaOH for pH control was also used forthis run. Process goals were met and color removal reached 99%. Table 2 summarizes the results.

Table 2. High Color Pilot Run

RAW FLOATED FILTEREDDATE TIME LOADING RECYCLE TEMP TURB APP TURB APP TURB APP HEADLOSS

RATE RATE COLOR COLOR COLORhours gpm/ft? % Recycle °C NTU PlICo units NTU PlICo units NTU PT/Co units ft/hour

1-23 0 5.1 6.0 I.I 3.0 139 1.l0 31 0.18 3 0.11-23 2 5.0 6.2 1.4 159 1.50 33 0.19 2 0.11-23 4 5.0 6.2 I.8 * 176 1.59 40 0.15 3 0.11-24 18 4.8 6.9 2.3 243 3.34 50 0.23 I 0.11-24 22 5.0 6.4 2.6 264 4.45 59 0.15 I 0.11-24 26 5.0 6.6 3.5 * 223 2.45 45 0.12 2 0.11-25 34 5.0 6.6 3.2 10.3 197 2.19 51 0.10 2 0.2

Algae, which adversely affects other water treatment processes, can be effectively removed by OAFclarification. Studies preformed by the South Central Connecticut Regional Water Authority (SCCRWA) in

Dissolved air flotation 331

New Haven, Connecticut (Kaminski, et al. 1991) investigated algae removal. The results indicate excellentalgae removal during a moderate bloom. Table 3 summarizes algal data produced during several SCCR WAtreatment runs.

Table 3. DAF Algae Removal

Date Dominant Raw Water DAF Removal %Plankton Count Effluent

(#/ml) (#/ml)

9-13-90 Asterionella 2616 1 99.969-14-90 Asterionella 1868 0 1009-20-90 Asterionella 2012 1 99.959-27-90 Asterionella 3304 1 99.9710-1-90 Asterionella 1374 2 99.8510-3-90 Aphanizomenon 686 1 00.8510-4-90 Chlamydomonas 364 2 99.45

Table 4 shows the effect of DAF treatment on a typical low turbidity, low alkalinity, and low color NewEngland water. Flotation tank and filter surface loading was 5 gprn/ft (12m/h). Recycle was set at 8% of thethroughput flow. Alum was dosed at 15 mg/l during the entire run. The test run produced a consistentclarified water quality and a filter run of approximately 60 hours in duration.

DAF systems have been shown (Schneider, et al. 1991) to produce a consistent effluent during episodes ofchanging raw water quality and variations in flow. Investigations by Kaminski (1991) demonstrate thatwithout changing any other process parameters surface loading rate increases from 3 gpm/ft to 5 gpm/ft (7 to12 m/h) did not significantly affect effluent water quality. Recent studies (Gong, et at 1993) have used an 8gprn/ft (19.5 m/h) DAF loading rate for pilot testing on the Wachusett Reservoir, the Boston Massachusettsarea water supply.

Data obtained (Figure 3) during a pilot study performed by Purac Engineering for the Carmel Water District#8 (Lake Mahopac, Mahopac, New York) presents the systems response to a simulated raw water turbidityincrease at constant process parameters.

Turbidity EventOAFTreatment

Turbidity (NTU)

10

8

6

4

2

oe::::~::::~===:===:~=2 3 456

Time (Hours)

Raw Floated Filtered

---- - ----Fig. 3. Turbidity event DAF treatment.

332 S. R. ARNOLD et at.

Table 4. DAF pilot, New England

DATE TIME RUN FLOAT TURBIDITY APP COLOR FILTERTIME pH RAW FLOAT FILTER RAW FLOAT FILTER HEADLOSS(hours) (units)

NTU PtICo Units ft

10-4 21:30 0.0 6.7 0.87 0.58 0.17 0.5

10-5 7:30 10.0 6.7 0.81 0.61 0.05 27 11 3 1.2

10-5 8:30 11.0 6.7 0.79 0.57 0.04 2 1.4

10-5 9:30 12.0 6.7 0.83 0.56 0.04 9 1.4

10-5 10:30 13.0 6.7 0.80 0.54 0.04 11 2 1.6

10-5 11:30 14.0 6.7 0.81 0.56 0.04 10 2 1.6

10-5 12:30 15.0 6.7 0.81 0.54 0.04 11 2 1.8

10-5 13:30 16.0 6.7 0.89 0.53 0.04 9 2 1.9

10-5 14:40 17.2 6.7 0.77 0.54 0.04 2.0

10-5 15:37 18.1 6.7 0.74 0.54 0.05 8 2 2.1

10-5 16:35 19.1 6.7 0.74 0.53 0.04 2.2

10-5 17:30 20.0 6.7 0.78 0.56 0.04 8 2 2.4

10-6 8:00 34.5 6.6 0.85 0.65 0.05 26 11 3 4.3

10-6 9:00 35.5 6.7 0.84 0.63 0.05 4.4

10-6 10:00 36.5 6.7 0.84 0.55 0.07 10 2 4.5

10-6 11:00 37.5 6.7 0.84 0.61 0.06 4.7

10-6 12:00 38.5 6.7 0.85 0.59 0.07 9 2 4.9

10-6 13:00 39.5 6.8 0.84 0.55 0.06 5.1

10-6 14:00 40.5 6.7 0.84 0.56 0.07 11 2 5.2

10-6 15:00 41.5 6.8 0.83 0.57 0.08 5.4

10-6 16:00 42.5 6.7 0.83 0.59 0.09 8 2 5.6

10-6 17:00 43.5 6.8 0.82 0.56 0.09 5.8

10-6 18:00 44.5 6.7 0.85 0.55 0.10 9 3 5.9

10-6 19:00 45.5 6.7 0.89 0.59 0.10 6.1

10-7 7:00 57.5 6.6 0.84 0.63 0.19 8.1

10-7 8:00 58.5 6.6 0.85 0.53 0.17 26 7 2 8.3

10-7 9:00 59.5 6.6 0.85 0.54 0.15 8.4

10-7 9:30 60.0 6.6 0.85 0.55 0.17 8.5

Locally obtained clay was added to the raw water stream before the pilot headworks to simulate a naturalturbidity spike. No adjustments to chemical dosing, flocculation, recycle rate, or surface loading rates weremade. Despite an increase in raw water turbidity, floated water quality remained below the typical treatmentgoal of 0.5 NTU and filtered water turbidity remained below 0.05 NTU. This exemplifies a general tendencyobserved at other locations, including full-scale plants, that changes in raw water characteristics are handledwell by DAF systems. In other words, a DAF system provides ample time for an operator to adjust theprocess to changing conditions without loss of effluent quality.

MUNICIPAL WASTEWATER

Flotation is also used in the treatment of municipal wastewater and sewage. Chemical treatment of sewagewith flotation has been used in Sweden since construction of the first plant in 1961. Since then numerousplants have been built including a 40 mgd combined flotation and filtration plant in Hoboken New Jersey fortreatment of trickling filter effluent.

Dissolved air flotation 333

The DAF process offers a number of advantages compared to sedimentation. High solids removal rates, lowvolumetric sludge content, low sensitivity to variations in water quality and flow, and space savings are themain features. High oxygen content in the effluent is another important factor. In cases of increased solidsloadings the flotation stage is capable of removing a large part of the impurities, which would otherwise bedischarged untreated. In this way, the flotation system acts as a safety barrier to discharge permit violations.

In a conventional municipal sewage treatment plant, sludge is obtained from the different stages of theprocess. Sludge treatment is most economical if the dry solids content of the sludge is the highest attainable.Higher dry solids content equates to smaller sludge volumes for stabilization and dewatering.

Flotation will provide sludge with a dry solids content of about 3 to 6%, allowing direct dewatering withoutthe thickening step. This provides cost savings in equipment capital expenditures, space, and reducedchemical consumption required in the thickening process.

For biological sludge, a gravity thickener provides a dry solids content of about 2%. For this type of sludge,a flotation type thickener can raise the dry solids content to 4 to 6% with high operational reliability. Thus aflotation thickener will reduce sludge volumes one half to one third that of a gravity thickener. This resultsin substantial savings in digester volume and reduced dewatering or disposal costs.

The separation efficiency of a flotation type thickener yields a suspended solids content of less than 100rng/l in the subnatant. The retention time for a given volume of sludge in the flotation thickener is only onetwentieth of a sedimentation thickener. This, together with the input of oxygen by recirculation, means thereis less risk of odors. The capacity of a flotation thickener is 0.8-1.2 lbs/ft2/hr without polymer addition. Withsuch additives, capacities of up to 2 lbs/ft2/hr are possible.

RECENT FULL SCALE APPLICATIONS

Millwood Water Treatment Plant

The town of New Castle, New York obtains water from New York City's Catskill and Croton Aqueduct. TheUSEPA's Surface Water Treatment Rule (SWTR) required these sources to be filtered by June 29, 1993. Tocomply, the town performed detailed pilot studies (Nickols, 1989) to select the most effective technologyfrom the following processes; conventional horizontal sedimentation, inclined plate sedimentation, roughingfilters, DAF, sludge blanket and solids contact clarifiers. Due to wide variations in raw water turbidity andoccasional supersaturated dissolved oxygen in the water supplies, DAF was the recommended process. Thisled to construction of the 7.5 mgd Millwood Water Treatment Plant.

Raw water is drawn by gravity from the Catskill Aqueduct. Two rapid mix basins equipped with variablespeed mixers are placed in series to provide one minute of rapid mixing. Each basin contains chemicalinjection points for polyaluminium chloride, alum, chlorine, caustic soda, polymer, and potassiumpermanganate.

Chemically dosed water is distributed to five identical treatment trains containing flocculators and DAFclarifiers. Three flocculation basins placed in series provide 30 minutes of detention time. Each basincontains a variable speed mixer for control of flocculation energy.

Each DAF clarifier is 29 feet long by 14'11" wide (8.8 x 4.5 metres), resulting in approximately 2160 ft2

(200 m2) of surface area. The design surface loading rate is 2.4 gpm/ft? (6 m/h) at 7.5 rngd and a maximumof 4.0 gpm/ft2 (10 m/h) at 12.5 mgd. Detention time in the DAF clarifier is about 30 minutes at design flow.The recycle system is designed for 8 to 12% recycle flows but also allows for the selection of recycle ratesoutside of that range. The flow variation needed for an adjustable recycle system is achieved by varying thenumber of recycle pumps in operation. Five 25% duty recycle pumps are controlled by the system computer.The computer monitors plant flow and computes the required recycle by utilizing a recycle rate valueJWST 3/4-W

334 S. R. ARNOLD et al.

entered in its memory. Recycle pumps are turned on/off automatically according to the required recycleflow. Each pump has its own dedicated eductor to introduce and mix air prior to entering the air separationtank. There are two air separation tanks supplied with the system, one duty and one standby. Air saturatedrecycle leaves the separation tank through a header pipe for distribution to the OAF trains. Each OAF basinis connected to the main header by two vertical downleg. The primary vertical pipe has a manual stainlesssteel butterfly valve, and the secondary pipe has a motorized butterfly valve to be put into service whenrecycle demands are high. Each downleg pipe has a bottom lateral supplying evenly spaced stainless steeldispersion valves, each with an extended stainless steel stem and handwheel for manual adjustment

Each clarifier includes an adjustable speed chain and flight skimmer controlled by an on/off timer for theremoval and thickening of sludge. Each skimmer flight is equipped with nylon brushes to assist sludgedewatering on the sludge beach and to push sludge into a sludge hopper . Sludge is collected from each trainin the sludge tank and pumped to two drying lagoons for storage . Each clarifier is equipped with slottedpipes on the tank bottom to draw clarified water to an adjustable effluent weir. The weir is used to adjustclarifier water level to an optimum position on the sludge beach. Water passing over the weir is combinedwith water from other treatment trains and flows to the ozone contractors.

Ozone is generated on site and injected into the flotation clarified water in two ozone contact chambers. Thechambers provide about 4.5 minutes of detention time before water enters the filters.

Six declining rate, rapid gravity type filters contain 18" of anthracite over 12" of sand. Ozonated water flowsdown through the media and exits via a plastic nozzle underdrain system . Backwashing utilizes.an air scourand separate water wash. Filtered water receives a low dose of chlorine before entering the town'sdistribution system.

During start-up in August 1993, the raw water turbidity was approximately 2 NTU in the Catskill aqueduct .An 8% recycle rate and 4 mg/I dose of Westchior FA 700S (polyaluminum hydroxychlorosulfate) resulted inclarified water turbidities < 0.5 NTU and filtered effluent turbidities < 0.1 NTU. Flotation surface loadingrates were varied during the start-up/shakedown process. Loadings up to 4 gprnlft2 produced no deteriorationof effluent quality.

OAF - Retrofits & Plant Conversions

OAF was introduced into the UK for potable water applications in the early 1970s. Initial interest was for thetreatment of impounded water especially those with algal problems or water which produced a lightchemical floc. Early schemes were awarded for both new sites and also for bolt-on pretreatment facilities onexisting overloaded works or works where an increased throughput was required. The ability of OAF toprovide a compact bolt-on pretreatment stage was enhanced by retrofitting existing sedimentation basins toprovide DAF pretreatment Since the mid 1970s, several works have been retrofitted by incorporating DAFinto existing structures. Future retrofit designs are currently underway for several UK and Far Easternschemes.

The first retrofit plant for Wrexham Water in the UK upgraded the works from 5 to 10 mgd by retrofittingtwo of the existing horizontal flow sedimentation tanks with OAF. This conversion has run very successfullyfor some 16 years treating River Dee water with turbidities in the range 5 to 30 generally to less than I NTU.Floated sludge from the units is dewatered directly by centrifugation. The established success of thisconversion and experience uprating a 9 mgd works at EI Bustan in Egypt, laid the foundation for the morerecent work undertaken in Malaysia.

Metropolitan Utilities Corporation. Sultan Idris Shah II (SIS II) Water Treatment Works

The Metropolitan Utilities Corporation holds the concession to supply water to the Ipoh area of the centralMalaysian state of Perak. MUC is a joint venture operation comprising a number of companies includingNorth West Water International of the UK who are responsible for operations and maintenance.

Dissolved air flotation 335

The SIS II works was originally constructed in 1978 and comprised six horizontal flow sedimentation tanksand 6 rapid gravity filters designed for a 24 mgd throughput. The plant was designed and laid out such that amirror stage extension could be provided to double the throughput at some future date. The plant treats waterdirect from the River Parit with a quality as indicated in Table 5. Chemical treatment was with aluminiumsulfate.

Part of the MUC concession strategy was to increase the output of SIS II to 72 mgd by the year 2000, atrebling of the initial design figure. The plant extension was to be phased to match projected demand asshown in Table 6. In 1989, Purac Limited was asked to make proposals for the stagewise uprating of theworks using a OAF retrofit of the existing sedimentation units. OAF was selected primarily because of thenature of the raw water and the hydraulic constraints of the works. The existing rapid gravity filters wouldbe refurbished and additional new filters added to meet the plant output requirements.

Table 5. River Parit Raw Water Quality

pH

Temperature (C)

Total Dissolved Solids (mg/I)

Colour (Hazen)

Turbidity (NTU)

Table 6. Phasing of Works Extension

6.0 to 7.9

24 to 31

30 to 50

25 to 440

10 to 290

Phase Date Sedimentation Flotation Total

# Streams mgd # Streams mgd mgd

1992 1, 2, 3, 4 & 5 23 6 12 35

2 1994 1,2 & 3 14 4,5 & 6 36 50

3 1999 * 0 1 to 6 72 72

Table 7. UK Design Guidelines

Flocculation # Stages - 2

Flocculator Retention Time (mins) - IO/stage

Flocculator G Values (sec")

Surface Loading

Recycle

Air (gms/m")

- 10-50 variable

- 4 gpm/ft" max

- 8-10%

- 6.7 to 12

The existing sedimentation basins were 52' width x 142' length x 13' liquor depth. An unmixed flocculationzone was provided in the first 20' of basin. The original tank design is shown schematically in Fig. 4. Thehead loss across the basin was negligible thus affecting the method of sludge removal. The retrofit processdesign was based upon conventional UK practice as noted in Table 7. Sludge was removed from the units by"floating off' over submerged channels into a sludge collection trough. The original retrofit design utilized anumber of sludge channels along the length of the basin. The original retrofit design is shown schematicallyin Fig. 5. The overall effect on plant footprint is shown in Fig. 6.

336 S. R. ARNOLD et al.

Fig. 4. Original sedimentation tank design.

SlCTlOfoW. £L[VATIOH

.. , ... ..1--1 - - - -

~ ~

~ - - - - -i su.cc:r. Ctl.l.'H\ i

H - -I I I~ - - - - . -I ! !

-+ - - -i i..... - -.. , ... ..

PI.J.N ON SLUDGE COLUCTIQN

Fig. 5. Original retrofit design.

SO' ,.. SO' ISS'

WACHI~(RY DISSOlVED AIRSUllOlNG VESSELS

D DO~ ._ ._._ ._. _ .- ._._ .~ . _ . -

~~!!I!Ii i FilTERS~ . _ . _ . _ . _ . _ . _ ._ . _.L . _ . _ •i i S( OIW[ NUHON BASIN~._ ._ ._ ._ ._ ._ ._ ._ .~ . _ .- s sI I SPINE

i i . .~j ' -'- '-'-' -'-' - ' -',' -' -

PIP[ INUT

i i tABU rnerrc. , ,

l/CH,I,NNElt '-'-'- '-'-'-'- '-'r' -' -! ! 2 2

,r ' - ' -' - ' - ' - '- ' - '-'~'-' -

! ! I I~ . _. _._. _. _. _. _ . _.~ . _. -

ORIGINAL " IRROR UIAGE I I I I I I I I I I I I I IEXTENSIOIl

WIKlhG CHANNEL

ADwlN I COWired lloe( ulolionbotins4: CH[WICAlS

OSl en lIocculot,on lo n ~ s

Fig. 6. Plant footprint.

t:::::::::la

Dissolved air flotation 337

The recycle was provided from a pretreated water take off point in the filter inlet channel. Air saturation wasachieved using air eductors and recycle saturation vessels operating at 5-6 barg. The saturators were fed byinverter driven centrifugal pumps the required recycle flow being set on a series of injection valves withinthe flotation basin.

During initial site survey work, limited jar tests and laboratory flotation tests had been undertaken. Also, itwas apparent during the survey that floc tended to be well formed in the feed channel to the sedimentationtanks. However, to provide flexibility and cater for different conditions with the final increased throughputthe first stream converted to OAF had two flocculator stages. Tests on this first stream have shown thatflocculation times and/or stages could have been reduced.

In operation, the floated sludge from the units was thin, typically 0.2 to 0.5% dry solids. Desludging wascarried out simply on a time basis. The flotation zone was covered to protect the sludge carpet from rainstorms which would effect floated water quality.

The thin floated sludge concentration was not unexpected and in line with the original design calculations.However, MUC wanted to minimize losses and also reduced discharges from the plant. As a result a singletransverse sludge channel was designed to replace the initial lengthwise units. This system was a significantimprovement with dry solids in the rate 0.7 to 1.5 w/w. The desludging operation was completed in 5minutes and carried out every 4 to 8 hours depending upon raw water conditions. The commissioning staffconsidered that higher sludge concentrations could be possible though perhaps undesirable withoutmodifying other aspects of downstream sludge pipework design. Floated water quality was similar duringeither method of sludge removal.

Figure 7 displays average floated water turbidity as a function of raw water turbidity for basin loadings of9.6 to 12 mgd (2.75 to 3.4 gpm/ft2) and 8 to 10% recycle. Data points are average daily values with samplesbeing collected typically 6 times per day. During the same time periods clarified water from the existingsedimentation basins was typically 1.0 to 21.0 NTU (ave 3.5 NTU) at basin loadings of the order 0.47gpm/ft2.

June 1993 to November 1993

'. : I.:

10

8

~=

6

~

~

~ 4

"..I 2u,

00

. ..t-:..;.:~.: .50

.-

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150 200

Fig. 7. Performance of DAF System-SIS II WTW M.D.C.S.B.

Results cover normal plant operation and practices on the works. It should be noted that the chemistryapplied is the same for OAF and sedimentation streams. Pre-lime has been added and found to be moreimportant for OAF performance at higher aluminium sulfate doses for raw water turbidities> 50 NTU.Polymer is now also applied mainly to improve the performance of the sedimentation basin. Both Purac and

338 s.R. ARNOLD et al.

MUC consider that further optimization will improve on the current DAF removal performance ofapproximately 95%.

The conversion for the next stage is currently underway, utilizing only one mixed flocculation stage and thetransverse channel arrangement for sludge removal.

It is now accepted that the DAF process has broader applications than just for algal laden, thin waters. Inaddition, early concerns about the apparent complexity of the process have largely been overcome. Theprocess is easy to retrofit and capable of achieving large throughput increases within existing structures. It istolerant to load variations and can be started up and shutdown quickly. The headloss across the process islow and this is an advantage compared with certain other retrofit systems. The sludge removal system hasbeen designed within a plant's hydraulic constraints and basin geometry.

Industrial Wastewater

Dissolved air flotation is a well accepted treatment process for paper, refinery, food processing and textileindustrial waste discharges. A full scale DAF clarifier was installed in December 1993 for treatment oftextile dye wastewater in Shoemakersville, Pennsylvania USA.

Textile dyeing facilities have traditionally used large settling lagoons and aeration basins to reduce BOD,TSS, and color in wastewater discharges. Increasing regulations on direct discharge permits and morestringent pre-treatment standards by local POTWs force the industry to respond by upgrading to moreefficient and sophisticated means of treatment. Suspended solids and BOD impose a burden of removal anddisposal on the POTW. Dark colors associated with the industrial discharge prevent penetration of sunlightand thereby reduce bacterial degradation of BOD during aeration. High color discharges are aestheticallyunappealing while a significant number of dyes have been shown to exhibit varying degrees of toxicity onfish, algae and aerobic processes (Stahr, et al., 1981).

Chemical coagulation is a viable alternative for color reduction, however the amount of coagulant requiredfor optimum treatment is often exceedingly high with dosages of 500-1000 mg/I not uncommon (Stahr, etal., 1981). A DAF pilot study was undertaken in July 1992 to evaluate process performance, chemicaldosages and full scale conceptual design parameters at Wolfe Dye and Bleach Works in Shoemakersville,Pennsylvania.

Table 8. Wastewater Characteristics

TEMP. pH TRUE TSS BOD5COLOR

°c units Pt/Co units mg/I mg/I

Historical Averages 15-32 8.0-9.0 2000-2500 650 <100

Pilot Study, July 1992 28 8.5 1740-2015 200-340 44

Full Scale Startup, Dec, 1993 17-30 8.5-8.7 1600-1900 375-410

The Subject facility is a commission dye house employing 89 persons. The plant uses both fiber reactive anddirect dyeing processes in a variety of colors to serve regional clients as well as southern and midwestern USaccounts. At full production the operation produces 120,000 pounds per day (54,430 kg/d) of dyeing and80,000 pounds per day (36290 kg/d) of bleaching for cotton and polycotton blends. Mill discharges areestimated at 200,000 gpd (760 m-/day) during peak production. The plant currently operates at less thancapacity 5 days per week. Present treatment utilizes two aeration lagoons in series prior to discharge to thelocal POTW. Wastewater parameters vary due to daily operational changes and weekend shutdowns.Wastewater characteristics are presented in Table 8.

Dissol ved air flotation 339

Industrial discharge goals were contingent upon the capabilities and permit limits of the ShoemakersvilleWWTP. Piloting effluent goals were set at 200 mg/I TSS, 300 PtlCo color units and 6 to 9 pH. BOD was nota factor in process evaluation.

The pilot unit's 9 ft2 flotation tank was operated at loading rates of 1.6 to 3.0 gpmlft2 and recycle rates of 8%to 50% at 80 psi. Detention time within the two stage flocculators varied from 13 to 20 minutes. A cationiccoagulant of medium molecular weight was tested at 200 to 400 mg/I in combination with several highcharge cationic flocculants which looked promising in jar tests. Flocculant addition rates of 5 to 10 mg/Iwere investigated with and without pH adjustment. Sodium hypochlorite (NaOCI) dosages of 1200 to 1800mg!l were also briefly examined.

Optimum pilot treatability was achieved without pH adjustment using 200 mg/l of coagulant and JO mg/I offlocculant at 2gpmlft2, 12% recycle and 20 minutes flocculation time . Sludge generation amounted to 0.8%of treatment flow. The air to raw water solids ratio was 0.037. Effluent color and suspended solids were lessthan treatment goals at removal rates of 87% and 74% respectively. NaOCI results did not meet effluentcolor targets despite 75% solids removal rates. This was disappointing since NaOCI generated a sludge thatappeared easier to dewater in volumes of half those experienced with polymer treatment.

Design of a full scale DAF clarifier proceeded as a result of successful piloting. A design flow of 250,000gpd (946m3/d) was chosen. An in-line static mixer was used prior to two stage flocculation of 20 minutestotal detention time. Historical wastewater solids levels nearly twice those experienced in the pilot studydictated increasing full scale recycle flow to 20% of plant throughput. The air saturation system wasdesigned for 85 psi using an air eduction device and pressurized separation vessel. Air supply compressorand recycle pump were each supplied with one standby unit as backup. Tankage and piping were constructedof 304 stainless steel materials. A 2.0 gpm/ft2 (4.9 m/h) design loading rate was chosen as indicated by pilottests .

Plant commissioning took place in December 1993. A medium molecular weight organic cationic coagulantwas chosen for addition pre static mixer. Although more expensive than its inorganic counterpart, organiccoagulants typically offer the advantage of lower dosages, less sludge production and a broader pH operatingrange (Davis, 1991). Flocculant was dosed through a dispersing header after the second stage flocculator.

Equipment startup utilized 200 mg/I of coagulant and 10 mg/I of flocculant at design flow. DAF clarifiereffluents less than 300 PtlCo units and 100 mg/I TSS were observed in the initial weeks of operation.Removal rates were approximately 84% and 82% respectively. Tests of second stage flocculator solidscontent revealed an air to solids ratio of 0.036. Sludge solids and volumes have yet to be accuratelymeasured . These preliminary results support the need for full scale design based upon pilot treatabilitystudies which incorporate historical water quality data.

ACKNOWLEDGMENTS

The authors with to acknowledge the following persons for the contribution and assistance in this paper . MrF. Beaumont, Operations Manager, North West Water International (MUC's Operation and MaintenanceManagers) and Dr S. Tibke, Water Supply Manager, North West Water International (MUC's Operation andMaintenance Managers).

REFERENCES

Davis, J. (1991). Improving Dye Waste Water Treatment. American DyestuffReporter. March 1991.Gong , B., Edzwald, J. K. and Tran, T. (1993) . Pilot Plant Comparison of Dissolved Air Flotation and Direct Filtration.

Proceedings AWW A Annual Conference, San Antonio Texas.Gregory, R. and Zabel, T. (1990). Sedimentation and Flotation. In: Wmer Quality and Treatment. AWW A, 4th ed, McGraw-Hili,

Inc. New YorkKaminski, G. S., Dunn, H. J. and Edzwald, J. K. (1991). Comparison of DAF to Other High-Rate Clarification Methods.

Proceedings AWW A Annual Conference, Philadelphia PA.

340 S. R. ARNOLD et al.

Malley. Jr.• J. P. and Edzwald, J. K. (1991). Concepts for Dissolved Air Flotation Treatment of Drinking Waters . J. of WaterSupply and Technology : Aqua. 40. 1-7.

Nickols, D. (1989). Pretreatment Alternati ves for Catskill water . J. New England Water Works Association . 4. 175-187.Schneider. O. D.• Nickols, D. and Lehan. E. R. (1991). Dissolved Air Flotation and Polyaluminum Chloride-An Effective.

Economical Combination. Paper presented at AWWA Conference. Philadelphia. PA.Stahr. R. W.• Boepple, C. P. and Knocke, W. R. (1981) . Textile Waste Treaunent: Color removal and solids handling

characteristics. Virginia Poly technic Institute and State University. Blacksburg Virgina,Zabel, T. (1984) . Flotation in Water Treatment . In: The Scientific Basis of Flotation. (Ed. KJ. Ives), NATO ASI Series . Boston:

Martinus Nijhoff.