• Pergamon

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Wat. Sci. Tech. Vol. 31, No. 3-4, pp. 137-147, 1995.Copyright © 1995 lAWQ

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EVALUATION OF DISSOLVED AIRFLOTATION PROCESS FOR WATERCLARIFICATION AND SLUDGETHICKENING

Harish Arora*, James R. DeWolfe**, Ramon G. Lee***and Thomas P. Grubbj

* American Water Works Service Co., Inc. 1025 Laurel Oak Road, Voorhees,NJ 08080, USA** Gannett Fleming Inc., P.O. Box 67100, Harrisburg, PA 17106, USA*** Illinois-American Water Co., P.O. Box 24040, Belleville, IL 62223, USAt Purac Engineering Inc., 5301 Limestone Road, Ste 126 Wilmington, DE 19808, USA

ABSTRACT

Dissolved air flotation was evaluated using pilot and bench tests for water clarification and sludgethickening. A OAF pilot study was conducted on one water source with low turbidity, high color, highorganic content and algal blooms. For this water, coagulation with alum, cationic polymer and pH around 6.1resulted in removal of turbidity, color and algae. THMFP and TOC were removed by approximately 25 and50 percent, respectively. A river water source was used in another pilot study to identify limits on sourcewater turbidity for the OAF process. Tests conducted with induced high turbidity values resulted in filteredwater quality with low turbidity and complete removal of color. TOC was removed by approximately 35percent, however, inconclusive results were obtained for THMFP removal. Based on these pilot test, OAF isa viable clarification process, especially for source waters with low turbidity (infrequent spikes upto 100NTU), high algal blooms and high color. Bench-scale OAF assisted sludge thickening resulted inperformance similar to gravity thickeners. High recycle ratios (around or greater than 100 percent) wererequired for effective sludge thickening.

KEYWORDS

Dissolved air flotation; water clarification; sludge thickening; high algae content; high turbidity; organicsremoval.

INfRODUCTION

A typical potable water treatment plant (WTP) consists of coagulation, flocculation, sedimentation, filtrationand disinfection. However, raw water sources with low turbidities, high organic and color contents requirelarge sedimentation basins because of low density floes. Further, treatment is difficult during winter monthswhen settling characteristics are impacted by slower chemical reaction rates and greater water densities.Frequent carry over of unsettled floc to filters results in lower filter run times and high backwashrequirements. Dissolved air flotation (OAF) is a clarification process in which floes are floated and

137

138 H. ARORA et al.

skimmed. Suspended particles in raw water are flocculated and floated with the aids of air bubbles which areintroduced into a reactor.

Numerous published articles (Packham and Richards, 1972a, 1972b; Zabel, 1984; Hyde, 1984; Edzwald andMalley, 1989; Edzwald and Wingler, 1990; Kollajtis, 1991; Malley and Edzwald, 1991a, 1991b) havedocumented impacts of various operational and design parameters, such as mixing times, flocculation times,hydraulic loading rates, saturation vessel types, saturator pressures, percent recycle streams, bubblediameters and concentrations on the efficiency of OAF process. Other studies have compared OAF processperformance with other clarification process. Schneider et al. (1991) compared OAF and direct filtration fortreating a reservoir water consisting of low turbidity, low alkalinity, moderate TOC, moderate color andseasonal algal blooms. Based on pilot testing, OAF and direct filtration resulted in similar solids andorganics removal , however , OAF produced almost twice as much filtered water as direct filtration. OAF wasfound to be more cost effective based on higher water production and lower waste disposal costs. Gong et al.(1991) reported results of pilot testing which compared ozonation , coagulation, in-line and direct filtration,contact adsorption clarification, OAF and biologically enhanced filtration for a reservoir water with lowturbidity, low organic content and seasonal algal blooms. Based on different treatment strategies, includingwith and without ozonation, OAF consistently performed better than direct filtration both in terms of filteredwater quality and water production . Kaminski et al. (1991) compared pilot scale performance of OAF withother high rate clarification processes such as plate sedimentation and contact adsorption clarifiers (CAC).Raw water from this study was also derived from a reservoir with low turbidity, moderate organic content,moderate color and seasonal algal blooms. Pilot testing revealed similar OAF and CAC performance duringgood raw water quality periods, however, OAF outperformed CAC during poor raw water quality periodsboth in terms of filtered water quality and quantity. Nickols (1989) compared pilot-scale performances ofplate settlers, CAC and OAF for water received from a series of reservoirs. Based on pilot testing waterquality results, OAF was the selected full-scale process even though a preliminary design analysis showedsimilar costs for CAC process operated at 10 gpmlft2 and OAF process operated at 2.4 gpm/ft2.

While most of above mentioned studies were conducted on similar raw water qualities favoring OAFprocess, the intent of this study was to also identify limitations of OAF process when tested for a surfacewater with low to high turbidity, moderate organic content , moderate color and absence of algae. TheAmerican Water Works (American, 1991) owns and operates 67 surface water treatment plants in the UnitedStates of America. The raw water characteristics at some of these locations are favorable for OAFapplication . However, OAF is a clarification process which could also be utilized to treat other surfacewaters. Therefore, OAF was pilot tested at two selected sites, one with favorable raw water quality for theOAF process, and the other with river supplied surface water to identify limitations of OAF process. Further,application of OAF was also evaluated as a sludge thickening process .

WATER CLARIFICATION

Two pilot-scale studies were conducted to evaluate OAF assisted water clarification. The objectives forthese studies are summarized below:

- compare overall OAF performance with existing clarification treatment for the selected sites;- effectiveness of OAF for remov al of algae, color and suspended solids ;- determine upper limit of turbidity for OAF;- estimate filter run times for optimized doses of selected chemicals; and- compare sludge produced from OAF and existing clarification processes for each site.

The expected performance for the OAF pilot testing is described in Table I.

Dissolved air flotation

Table 1. Treatment Objectives For DAF Pilot Testing

139

Parameter

Floated Water TurbidityFiltered Water TurbidityFiltered Water ColorFilter Run LengthFilter Headloss

value

< 5NTU< 0.3 NTU 95 percent of Time5CU> 24 hours4 in/hr to 8 feet terminal

Raw In-lineWater Mixer

Mixer Mixer

Two-Sta2eFlocculafion

FloatScraper

FlotationUnit

AirCompressor

SaturationVessel

FilterColumns

'---'---r-~ TreatedWater

Fig. I. Process diagram for DAF pilot plant.

Table 2. Technical Specifications for the DAF Pilot Plant

Parameter

Flow RateNominal Flow RateRecycle Flow RateFlotation AreaNumber of Flocculation TanksFlocculation Tank CapacityFlocculation Time @ 36 gpmFlocculation Paddles Speed

Tank #1Tank #2

Description of Pilot EQuipment

Value

27-54 gpm36 gpm5- 10 percent9 fe2360 gallons each10 minutes each

2.5 to 25 rpm1.5 to 15 rpm

The pilot equipment was leased from Purac Engineering, Inc. (PURAC), a manufacturer of DAF equipment.The DAF pilot plant was mounted in a 28 foot long by 8 foot wide mobile trailer. A process diagram isshown in Figure I with technical specifications summarized in Table 2. Raw water for the pilot plant issupplied through a 3-inch pipe with in-line flowmeter, turbiditimeter, pH probe, chemical injection pointsand static mixers. Flocculation of chemically treated water is performed in a 2-stage tank with a total

140 H. ARORA et al.

detention time of 20 minutes at 36 gpm. After flocculation, water enters a dispersion chamber along with apressurized recycle stream. Air bubbles are produced upon release of pressure of the recycled stream. Airbubbles attach themselves to the flocculated solids and float to the surface where they form stable sludgelayer called "float". Float is intermittently skimmed by mechanical scraper, and collected in a sludge hopperfor its disposal. Floated water, which is collected from the bottom of the unit, is filtered using two 4-inchcolumns. Filters are equipped with air scour and water backwash. Floated water also flows by gravitythrough a bellows adjustable weir into recycle holding tanks from where 6 to 10 percent of the flow recyclesinto air saturation tank and then to dispersion chamber to maintain flotation process.

Case StudV#1 - Canoe Brook. NJ

The first study was conducted at the Canoe Brook water treatment plant (WTP) in New Jersey. Raw waterfor the Canoe Brook WTP is obtained from a system of shallow reservoirs fed by the Passaic River.Shallowness of the reservoirs and presence of nutrients promotes seasonal algae growth and very highconcentration of microorganisms. Typical raw water quality, as summarized in Table 3, consists of lowturbidity, moderate alkalinity, moderate hardness, high color and high algal count.

The existing water treatment at the Canoe Brook WTP consists of prechlorination, coagulation using alum,intermittent use of PAC, flocculation, sedimentation, pH adjustment using lime, filtration, chlorination fordisinfection and corrosion control. Typical monthly average alum doses, which vary from 30 mg/l duringwinter to 65 mg/l during summer, indicate difficulties treating raw water using conventional sedimentationprocess.

Table 3. Raw Water Quality For the Canoe Brook WTP

PARAMETER

TurbiditypHAlkalinityHardnessColorAlgal Count

VALUE

0.7 to 24 NTU6.5 to 9.930 to 100 mg/L as CaC03

34 to 148 mg/L as CaC03

11 to 70 CU2000 to 4000 chains/ml

Physical and Inorganic Water Quality. The pilot study consisted of 23 separate runs conducted duringAugust of 1991. Coagulants utilized for the study included alum, ferric chloride and polyaluminium chloride(PACL). A cationic polymer was used to evaluate its effectiveness in floc formation. Furthermore, impactsof pH adjustment were evaluated using sulfuric acid.

Alum, at a comparable full-scale dose of 55 mg/l, produced floated and filtered water turbidities of less than0.6 NTU and 0.5 NTU, respectively. Algae was completely removed, whereas filtered water had a color of 2standard color units (CU). The corresponding dual-media filter run was estimated to be around 122 hours fora terminal head of 8 ft and a unit filter run volume (UFRV) of 29,000 gal/ft2. In two separate runs, alum at25 mg/l, cationic polymer at 1.5 mg/l and pH around 6.2 resulted in floated and filtered water turbidities lessthan 0.4 NTU and 0.08 NTU, respectively. Corresponding filter runs and UFRVs were estimated to be from55 to 110 hours and from 13,000 to 26,000 gal/ft2, respectively.

Ferric chloride, up to a dose of 55 mg/l, did not result in consistent floated water quality. However, ferricchloride at 55 mg/l, cationic polymer of 0.7 mg/l and pH adjusted to 6.1 resulted in floated and filtered waterturbidities around 0.3 NTU and 0.08 NTU, respectively. Floated water color averaged from 3 to 6 CU,however, filtered water color was less than 3 CU. Algae was removed by filtration following flotation, butnot by flotation alone. The estimated filter run and UFRV were 110 hours and 26,000 gal/ft2, respectively.

Dissolved air flotation 14 1

Pilot runs were conducted with PACL up to a dose of 25 mg/1. Removal of turbidity increased with anincrease in PACL dose, however, color and algae were not effectively removed. PACL doses of above 25mg/l are high for water treatm ent, and therefore, further application of PACL was not evaluated.

Samples were collected for particle counting analyses for selected pilot runs, as summarized in Table 4.Analysis of the data indic ates that under similar operating conditions, a cationic coagulant dose of 1.5 mg/las compared to a dose of 1 mg/l resul ted in better particles removal of all sizes. Furthermore, remov al ofparticles increased as a function of their size. A comparison of OAF surface loading also indicated similarparticles removal of all sizes. indicating that OAF could be operated at higher surface loadings. Further,particle counting data indicates a 2-log particles removal, and chemical disinfection will be required underthe current Surface Water Treatment Rule.

Float samples consisted of approximately 0.8 percent solids, similar to those which are observed in thesettling basins for the existing treatment scheme.

Table 4. Particle Count Data for the Canoe Brook OAF Pilot Study

Particle Size , IlmParameters Location

% Reduction2 3 5 7 10 > 15 Total

Raw 17003 26790 28902 92362 115765 3687 3 317695Alum = 25 mglL

Floated 15647 21291 16403 10588 1224 162 65316 79 .4Polymer = 1 mglLLR = 4 gpmlff Filtered 14259 16335 8949 4712 583 82 44934 85 .9

Raw 8967 14942 18740 70364 83547 27766 224327Alum = 25 mg/L

Floated 1463 1174 588 710 2 17 33 4186 98.1Polymer = 1.5 mg/LLR = 4 gpmlff Filtered 432 336 155 105 25 14 1067 99.5

Raw 9459 15111 16585 51672 73806 23279 190362Alum = 30 mg/L

Floated 1664 1271 659 564 245 68 4471 97 .7Polymer = 1.5 mgILLR = 6 gpmlff Filtered 2728 2546 1385 794 142 49 7644 96.0

Notes: LR = Flotation Loading Rate

Organic Water Quality. Samples were collected from selected pilot runs for the organic analyses, assummarized in Table 5. Raw water total organic carbon (TOe) varied from 7.4 to 9 mg/l during the pilotstudy. After flotation and filtrati on, an average TOC removal of 50 percent was observed for the selectedpilot runs. Samples with 20 to 25 mg/l chlorine were incubated for 7 days at 250C for selected pilot runs toevaluate their trihalomethane formation potential (THMFP) values. An average THMFP removal of 23percent was observed in floated and filtered samples. As part of another American Water Works (1993)study, disinfection by-products were analyzed through various locations at the Canoe Brook WTP. This dataindicates that the exist ing treatment reduced raw water TOC by 25 percent . The filtered water THMs andhaloacetic acids (HAAs) were analyzed at 102 Ilg/l and 115 Ilg/l during the summer season, indicating asignificant presence of precursor material in the raw water. Another interesting observation was higherHAAs values than the corresponding THM values. Similar observations were made for other WTPs whichreceive their raw water from reservoirs.

Comparison with Existing WTP Operations. Pilot testing results were compared with full-scale operations atthe Canoe Brook WTP . Thi s comparison indicated that an alum dose ranging between 30 to 85 mg/l and apH of around 6.2 resulted in filtered water quality with turbidity ranging between 0.11 and 0.65 NTU andcolor ranging between 2 and 5 CU. Each filter was backwashed once every day. Based on a compariso n ofthe physical water quali ty, the existing treatment and the DAF process result in similar eftl uent water

142 H. ARORA et at.

quality. Further, full-scale settling basin sludge samples consistently contain I percent solids. Therefore , thechoice between DAF and other suitable clarification processes should be made on an economic analysis.

Table 5. Organic Data for the Canoe Brook DAF Pilot Study

p y , spformation potential; TOC - total organic carbon.

Location Coagulant TIlMFP Percent Toe Percent(}tglL) Reduction (mglL) Reduction

Raw - PH 246 7.68

Floated - PH Alum 186 24.4 4.62 39.8

Effluent 195 20.7 4.23 44.9

Raw - RES 240 7.4

Floated - RES Alum 176 26.7 3.76 49.2

Effluent - RES 185 22.9 4.67 36.9

Raw 250 8.61

Floated Alum 215 14.0 3.89 54.8

Effluent 193 22.8 3.97 53.9

Raw 230 9.01

Floated Ferric 168 27 3.81 57.7

Effluent 168 27 3.53 60.8

Notes: PH - ohotos nthesis: RE: -re iration; l filVU- s: - trihaIomethane

Case Study#2 - Norristown. PA

A second pilot study was conducted on a river water to identify limitations of DAF process for a highturbidity source water. Raw water for the Norristown WTP is obtained from the Schuylkill River, a waterwhich consists of low turbidity with infrequent spikes up to 100 NTU, moderate alkalinity and highhardness.

Currently, raw water at the Norristown WTP can be chemically treated with powdered activated carbon ,alum or ferric chloride, coagulant aid, chlorine dioxide or chlorine, and lime. Chemically treated water flowsthrough mixing basins via in-line mixers, flocculation basins and sedimentation basins. Lime is added tosettled water for pH adjustment prior to filtration. Filtered water is disinfected by applying chlorine andammonia. Typical ferric chloride doses range from 5 to 50 mg/l, depending on raw water quality.

Physical and Inorganic Water Quality. A DAF pilot study was conducted during May 1993, and the rawwater quality for this period is summarized in Table 6. Ferric chloride and PACL were the coagulants usedfor the study. Furthermore, usage of coagulant aids and pH adjustments were not performed during thestudy. At DAF process loading rates varying between 2 to 4 gpmlft2, a minimum ferric chloride dose of 15mg/l was required to produce a filter water quality consistently less than 0.3 NTU from the raw waterturbidity of 5 NTU. Corresponding floated water turbidities averaged 0.9 NTU. Filter run length and UFRVwere estimated to be 114 hours and 27,000 gaVft2, respectively, at a filter loading rate of 4 gpm/ft 2. At 6gpmlft2, the estimated filter run length and UFRV were 27 hours and 9,600 gaUft2. The corresponding full­scale ferric chloride dose varied from 25 to 45 mg/l, which was significantly higher than those observed forDAF pilot operations. During the pilot testing, turbidities higher than 10 NTU were not observed, therefore,pilot runs at higher turbidities were conducted by addition of artificial turbidities using mud. At raw water

Dissolved air flotation 143

turbidities ranging between 30 to 90 NTU, the DAF process produced acceptable floated and filtered waterqualities, however, flocculation mixing intensity and ferric chloride dose had to be appropriately increased.

Table 6. Raw Water Quality During the Norristown Pilot Study

PARAMETER

TurbiditypHAlkalinityHardnessColor

VALUE

3 to 100 NTU7.560 mg/L as CaC03

125 mg/L as CaC03

80 CU

Pilot testing with PACL at a dose of around 15 mg/l and at filter loading rates of 4 and 6 gpm/ft2 alsoresulted in filtered water turbidities and color values consistently less than 0.3 NTU and zero, respectively.The estimated filter run lengths were around 200 hours with corresponding UFRV of 50,000 gal/ft2.

Float samples collected during testing were found to contain 2.0 percent solids, indicating that this type ofsludge could be dewatered without a sludge thickening step at a similar treatment facility. Sludge sampleswere sent to a vendor to evaluate its dewatering characteristics utilizing a belt filter press. Based on testingresults, the floated sludge could be easily dewatered to 25 percent solids with the aid of a cationic polymer.These tests indicate that floated sludge can be easily dewatered, however, sludge characteristics can differfrom one site to another and should be accordingly evaluated.

Table 7. Organic Data for the Norristown DAF Pilot Study

Location Coagulant TIlMFP TOC Percent(;tg/L) (mg/L) Reduction

Raw 96 4.25

Floated Ferric 263 2.37 44.2

Effluent 107 2.76 35.1

Raw 249 2.81

Floated PACL 197 2.39 14.9

Effluent 179 2.20 21.7

Raw 143 2.50

Floated Ferric 159 1.63 34.8

Effluent 215 1.75 30.0

Raw 249 2.78

Floated Ferric 166 1.82 34.5

Effluent 160 1.83 34.2

Notes : PACL - Polyaluminum chloride.

Organic Water Quality. Similar to the Canoe Brook pilot testing, samples were analyzed for organicsanalyses and the results are summarized in Table 7. Raw water TOC ranged from 2.5 mg/l to 4.25 mg/Iduring the pilot study. DAF treatment with ferric chloride resulted in an average TOC reduction of 35percent, whereas, DAF with PACL resulted around 20 percent TOC reduction. THMFP tests were alsooerformed similar to Pilot Studv I. however. the trends in the data were inconclusive.

144 H. ARORA et al.

COMPARISON OF DAF WITH OTHER CLARIFICATIONTECHNOLOGIES

Based on results of this and other published studies, DAF is a viable water clarification process, specially forsource waters with turbidities less than 100 NTU, high color and algal blooms. High rate clarification canalso be achieved in commercially available upflow contact adsorption clarifiers (CAC). Previous Americanstudies (1991a and 1991b) have evaluated CAC for a variety of source waters, some of which had similarraw water characteristics as for the Canoe Brook WTP. One advantage of CAC is that the clarificationloading rates can be as high as 10 gpm/ft-', almost twice those practiced for DAF. On the other hand, thewaste stream generated during backwashing of the CAC is more dilute than the float produced for DAF.

In order to compare feasibility of the two mentioned high rate clarification technologies, an economiccomparison was performed for water production rates of I MGD and 10 MGD . A flow rate of I MGD wasselected because complete treatment systems are available as package units from the manufacturers. At aflow rate of 10 MGD, on-site fabrication of these process will be more economical. In the economiccomparison, common equipment, such as chemical feed systems, filtration for 10 MGD systems and controlsystems, were not included. It is also assumed that filtered backwash wastewater facilities can also handlethe CAC backwash water. Tables 8 and 9 display the technical specifications and estimated constructioncosts for the DAF and CAC processes, respectively, for design flow rates of I MGD and 10 MGD. Based ona comparison of the estimated capital costs for the I MGD treatment systems, the CAC system wasapproximately 60 percent of the costs for the DAF system. For the 10 MGD systems, flocculation andclarification costs for the CAC system were approximately 40 percent of those for the DAF system. It shouldbe noted here that one manufacturer was contacted for obtaining costs for each of the mentioned systems,and a complete design including various manufacturers and economic analyses may result in a differentcomparison. Operations and maintenance costs were not compared, however, anticipated O&M costs for aDAF system might be higher than those for a CAC system primarily because of pressurized air saturationsystem. On the other hand, CAC system requires frequent backwashing and this can result in lower waterproduction.

Table 8. Design Criteria for the Dissolved Air Flotation Systems

System Size : 1 MGD

Two package units of 0.5 MGD eachFlotation Area : 100 ft2 each unitHydraulic Loading Rate: 3.5 gpm/ft?Flocculation : two stage each unitFlocculation Time : 10 minutes each stageTanks : epoxy coated stainless steel tanksDual Media FiltrationFilter Media Depth 3 ft

Estimated Probable Construction Costs: US$9oo,000

System Size : 10 MGD

Flotation Area : 1800 ifNumber of Flotation Basins: 3Size of Flotation unit: 30 ft x 20 ft x 12 ft eachFlocculation basins : 3Number of Flocculation Stages : 2Size of each Flocculation Basin: 31 ft x 20 ft x 12 ftFlocculation Time, each stage : 10 minutes

Estimated Probable Construction Costs: US$2,loo,000

Dissolved air flotation

Table 9. Design Criteria for the Contact Clarification Systems

System Size: 1 MGD

Two package units of 0.5 MOD eachClarification Rate : 10 gpm/ffFilter Loading Rate: 4.0 gpm/ffDepth of ContacClarifier Media : 48 inchFilter Type : trimediaFilter Depth: 38 inchTanks : epoxy coated stainless steel tanks

Estimated Probable Construction Costs: US$550,OOO

System Size : 10 MGD

Number of Trains: 4 @ 2.5 MOD eachClarification Rate : 10 gpm/ffClarification Area : 174 ff eachDepth of ContacClarifier : 48inchDepth of ftlter media: 38 inch

Estimated Probable Construction Costs: US$71O,OOO

SLUDGE THICKENING

145

By its use in water clarification, DAF can also be used for thickening of sludge produced in a watertreatment plant. In order to evaluate DAF assisted sludge thickening, bench-scale experiments wereconducted by Komline-Sanderson, a manufacturer of DAF equipment. Furthermore, sludge samples werecollected from three water treatment plants which thicken their sludge currently in gravity thickeners. In thismanner, performance of DAF assisted thickening could be compared with present operations.

Description of Testinc Procedure

The DAF bench testing is performed with I I capacity flotation cylinders. Air is dissolved under pressure inprocess water, and air bubbles are produced when water is depressurized. In order to simulate differentrecycle rates, various volumes of chemical conditioned sludge is mixed with specified volumes of aeratedprocess water in the cylinders. After a specified time, particle rise rates, float solids and effluent clarity arenoted for each cylinder. Applicability of DAF for the sludge is then assessed based on data collected throughbench testing.

Case Study#l - Jumpinc Brook. NJ

At Jumping Brook WTP, raw water is derived from Shark River, Jumping brook and Manasquan Riveralong with ground water. Treatment chemicals that can be added to raw water include PACL (PACL),coagulant aids, chlorine and lime. Water is then mixed, flocculated and clarified in solids contact clarifiers,and filtered. Sludge from solids contact clarifiers through two holding tanks is thickened in two gravitythickeners. Thickened sludge is conditioned with a polymer prior to dewatering in belt press filters andhauled to a landfill for final disposal.

Based on current operations, sludge withdrawn from clarifiers at approximately 0.25 percent solids isthickened to approximately 2 percent in gravity thickeners. Bench DAF testing utilizing a cationic polymer(high molecular weight and high charge density) and an anionic polymer (high molecular weight and low

146 H. ARORA et al.

charge density) resulted in thickened sludge with 2 percent solids from a settled sample with 0.6 percentsolids. Both tests required 200 percent recycle rates at pressure values of 65 psi. The corresponding air tosolids ratio was 0.028. Other tests utilizing the same polymers but at 100 percent recycle ratio resulted inthickened sludge with approximately 1.9 percent solids. These results indicate similar performance of OAFassisted sludge thickening, however at greater than 100 percent recycle rates, with the existing gravitythickeners.

Case Study #2 - Hays Mine. PA

The raw water source for the Hays Mine WTP is Monongahelia River. The raw water has moderateturbidity, low alkalinity, moderate hardness and neutral pH. Treatment at the Hays Mine includesprechlorination, addition of potassium permanganate, addition of PAC (as required), coagulation with ferricchloride and cationic polymer, clarification in solids contact clarifiers, filtration through filter-adsorbers,addition of corrosion inhibitor, and disinfection with chlorine. Under present operations, sludge from solidscontact clarifiers at approximately 2 percent solids is thickened to 5 to 7 percent solids in gravity thickeners.

OAF bench testing was performed utilizing an anionic polymer. Seven tests were conducted with recycleratios varying between 10 to 300 percent. OAF resulted in unstable float blanket with recycle ratios of up to200 percent. At a recycle ratio of 300 percent, a float blanket with 3 percent solids was established, but itwas lost with minimal agitation . Inability of OAF to thicken ferric sludge could probably be attributed toheavier ferric solids andlor high solids contents of settled sludge sample .

Case Study #3 - Richmond Road. KY

At Richmond Road WTP, KY, raw water is obtained from Jacobson Reservoir and Kentucky River.Treatment at Richmond Road consists of chemical addition, including potassium permanganate, chlorine,alum, cationic polymer and lime; flocculation, sedimentation and filtration and disinfection usingchloramines. Sludge at approximate solids concentration of 0.3 percent is withdrawn from sedimentationbasins and thickened to around 2 percent solids in gravity thickeners.

OAF bench testing was conducted utilizing an anionic polymer and at recycle rates varying from 10 to 100percent. At recycle rates of 10, 25 and 33 percent, unstable float, which broke with minimal agitation, andbottom sludge were observed. At recycle rates of 75 and 100 percent , stable float with approximate solidsconcentration of 3 percent was observed. These results indicate that for similar WTPs OAF might be aviable sludge thickening process .

CONCLUSIONS

Water Clarification

OAF is a viable clarification process for raw waters with low turbidity, high color, high organic content andalgal blooms. OAF pilot testing results consistently met or exceeded all the treatment objectives summarizedin Table I. Turbidity spikes up to 80 to 90 NTUs did not affect filtered water quality. As with otherclarification processes, selection of chemicals and optimized pretreatment is the key to successful DAFoperations. Other high rate processes, such as CAC, should be also evaluated along with OAF for acandidate raw water source, especially where algal blooms are encountered, before a final selection of thebest process. The recommended OAF process parameters include flocculation for between 10 to 20 minutes,flotation hydraulic loading between 4 to 6 gpmlft2 and a recycle ratio between 8 to 12 percent.

Slud~e Thickenin~

Based on limited bench testing , OAF can be a feasible sludge thickening process on site specific basis,however, extensive process research on pilot-scale and design modifications are required before its full-scaleapplication. Process recycle ratios of more than 100 percent, which can result in significant energy

Dissol ved air flotation 147

consumption, are required for effective sludge thickening . Furthermore , bench-scale performances should beverified on pilot-scale, confirmed, and compared with other available options for each site.

CONVERSION OF UNITS

gpmlft2 = 0.0408 m3/m2/min

in =0.0254 mft =0.3048 mgallon = 0.003785 m3 = 3.7851gaUft2 = 0.0408 m3/m2

MGD = 2.63 m3/min = 3.785 MUD

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American Water Works Service Company (1991) . Evaluation and Pilot-Testing of the Adsorption Clarifier. Research Report,Voorhees, NJ, USA.

American Water Works Service Company (1993) . Disinfection By-Products- System Survey of Occurrence , Control and Fate.Research Report, Voorhees, NJ, USA.

Connecticut· American Water Company (1991) . Evaluation of Clarification and Filtration Alternatives for the Putnam FilterPlant. Research Report, Greenwich, CT, USA.

Edzwald, J. K. and Malley, J. P. (1989). Removal ofHumic Substances and Algae by Dissolved Air Flotation. USEPA Report No.60012-89/032, Cincinnati, OH, USA.

Edzwald, J. K. and Wingler, B. J. (1990). Chemical and Physical Aspects of Dissolved Air Flotauon for the Removal of Algae.Aqua, 39, 24-35 .

Gong, B., Edzwald, J. K and Tran , T. (1993). Pilot Plant Comparison of Dissolved Air Flotation and Direct Filtration. Proceedingsof the Annual AWWA Conference, San Antonio, TX, USA.

Hyde, R. A. (1975). Water Clarification by Flotation - 4. Technical Report No. 13, Water Research Center, MedmenbamLaboratory, Medmenbam, UK.

Kaminski, G. S., Dunn, H. J. and Edzwald, J. K. (1991). Comparison of Dissolved Air Flotation To Other High Rate ClarificationMethods. Proceedings of the Annual AWWA Conference - Water Research for the New Decade, Philadelphia. PA, USA.343·365.

Kollajtis, J. A. (1991). Dissolved Air Flotation Applied in Drinking Water Clarification. Proceedings of the Annual AWWAConference - Water Quality for the New Decade, Philadelphia. PA. USA. 433-448.

Malley. J. P. and Edzwald, J. K. (199la). Concepts for Dissolved Air Flotation of Drinking Water. Journal Water SRT-Aqua.40(1).7-17.

Malley, J. P. and Edzwald, J. K. (199Ib). Laboratory Comparison ofDAF with Conventional Treatment Journal of AWWA. 83(9 ).56-61.

Nickols, D. (1989). Pretreatment Alternatives for Catskill Water. Journal of New England Water Works. 103(4).175-187.Packham. R. F. and Richards. W. N. (I972a). Water Clarification by Flotation - J. Technical Paper No. 87., Water Research

Assoc., Medmenbam Lab .• Medmenham, UK.Packham. R. F. and Richards. W. N. (l972b). Water Clarification by Flotation - 2. Technical Paper No. 88.• Water Research

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JWST 3/4-K