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2.7YY3 A COMPACT FLOTATION-FILTRATION TERTIARY TREATMENT UNIT FOR WASTEWATER REUSE Nazih K. Shammas Professor and Dean of Environmental Engineering Milos Krofta President Lenox Institute of Water Technology, Inc. 101 Yokun Avenue, P. 0. Box 1639 Lenox, Massachusetts, 01240 INTRODUCTION In many parts of the world the limited availability of both ground and fresh surface waters make it imperative to conserve water and to utilize every drop of available wastewater for reuse in beneficial purposes. The strategy for long term planning and management of water resources is more and more being based on the renovation and utilization of wastewater for use in agricultural and landscape irrigation as well as in industrial production.
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Water quality standards or guidelines have been initiated in many states and regions in order to protect public health, prevent nuisance conditions and preclude damage to crops, soils and groundwater (1-4). Risk-based wastewater reclamation criteria often require full tertiary treatment, especially in applications which have high potential exposure such as in using the reclaimed water Conventional full for unrestricted irrigation (1-4). tertiary treatment consists of a train of multiple processes which include rapid mixing, flocculation, sedimentation and granular filtration in addition to Such processinq, even when it is disinfection (1,2). economically 'feasible, is- costly- because of sludge handli-ng and tertiary sedimentation.._tanks (1, 5 ) Thus, research has been directed towards developins - - an innovative alternative capable of producing a comparatively highly clarified effluent (G-11).
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newly developed flotation-filtration unit (Sandfloat) is lant 1 1 ~ i n na an advanced water clarificati=n pacL,zg~ n r----r -----a combination of chemical flocculation, dissolved air flotation (DAF) and rapid granular filtration in one unit. The average processing time from start to finish is less than 15 minutes. The unique compact and efficient
A
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design is made possible by the use of the space (water head) above the filter for flotation, a space or water head, which in any case, is necessary for filtration. Two more features were used to reduce space requirements: 1 ) A static hydraulic flocculator was built into the central portion of the tank to combine a third process in the 2) A segmented continuous backwash filter sirigle tank. was used to eliminate requirements for large tanks for clearwell and backwash storage. Therefore, the end result replaces a conventional process requiring five separate tanks with a single, compact and cheaper unit (7). This paper will present results of five applications, using a pilot-scale Sandfloat, to demonstrate the performance of the compact plant in the tertiary treatment of various wastewater effluents and to illustrate the system's usefulness for wastewater reclamation, recycling and reuse. The five investigated cases are as follows: 1.
Treatment of primary municipal wastewater effluent at Hoboken, New Jersey.
2.
Tertiary treatment of activated sludge effluent at Oak Meadows Sewage Treatment Plant, Licking County, Ohio.
3.
Tertiary treatment of RBC Effluent at Jiminy Peak, Hancock, Massachusetts.
4.
Tertiary treatment of trickling filter effluent at. Norwalk, Ohio.
5.
Tertiary treatment of lagoons effluent at Arpin, Wisconsin.
SANDFLOAT PILOT PLANT
The Sandfloat pilot plant used in these investigations is shown -in-Figure 1 . The outside flotation-filtration tank is 1.5 m 'in diameter and has a depth of 1.8 m (total overall unit height is 2.10 m). The pilot plant has a design capacity of 300 m3/d of wastewater flow. The influent flow is m F x ~ dxitb, flozciilant and coagulant chemicals, then gently flocculated in the central zone of the tank. The backwash recycled from the filter is mixed with the inflowing water at the flocculator inlet. This eliminates the need to dispose of the backwash separately,
98
-57
b u t a l s o i n some c a s e s provides a "seed" of solids for better floc formation. The floc size required for removal of the solids in the flotation stage is smaller than that required for settling. This further reduce9 the space requirement for flocculation ( 7 ) .
When the flocculated solids reach the upper part of the flocculation zone, they are mixed with the recycle flow which contains millions of microscopic (20-100 micron diameter) air bubbles. The air bubbles are generated by injecting air into recirculated clarified water under pressure (5 Atms.), followed by rapid decompression under high shear conditions. The amount of recirculated water used varies depending on the amount and type of solids to be removed, but is generally 15 to 30% of the incoming flow. The air bubbles attach to the flocculated solids, or are entrapped in the floc to produce an airsolid agglomerate which rapidly rises to the surface of the tank. The accumulated float (thickened sludge) is removed by the spiral scoop at a solids content of 2 to 3% and discharged to the sludge handling system via the central sludge well. the sludge rises to the top, the clarified water flows downward through the filter bed. The bottom of the tank is composed of multi-sections of sand filter segments. Each of the 2 1 segments is individually isolated and backwashed while the remaining parts of the filter are on line. The filter bed consists of 2 8 0 mm high grade silica sand. Effective size and uniformity coefficient for the sand are 0.35 mm and 1.55, respectively. The backwashing, being uniformly extended over the complete filtering time, will in effect render the capacity of the backwash pump to be smaller, and minimize or even eliminate any overloading on the unit that may result from the recycling of decanted backwash water. - - - The backwash hood, pump and motor are mounted on a carriage which rotates on the upper rim of the main tank. The filter segments are set up f o r backwashing at a predetermined time interval which can be adjusted depending on head loss and the accumulation of solids. The backwash water containing the solids captured by the sand is recirculated back to the flocculator. The clearwell is located immediately below the sand bed. The clear water i s utilized directly for backwash as needed.
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The filter improves the reliability of the flocculation and flotation stages, f loccrilation is enhanced by the recirculated solids, and filtration is protected by the flotation stage. The combination of the three operations in a single tank reduces the head l o s s and turbulence to a minimum between stages, which increases the efficiency for removal of the fragile flocculated solids. No storage is required for clarified water or backwash. Backwash discharge is eliminated, and the only discharge is the float, a thickened sludge suitable for direct handling, thus eliminating the need for a sludge thickener. RESULTS AND DISCUSSION Treatment of Primary Effluent Hoboken is a city located on the west bank of the Hudson River, between Lincoln Tunnel and Holland Tunnel in New Jersey. It is an old city with an approximate population of 45,000. The primary treatment at the existing 68,000 m /d (18 mgd) Hoboken Wastewater Treatment Plant consists of screens and sedimentation clarifiers. The effluent was continuously treated by the Sandfloat pilot plant. At chemical dosages of 20 mg/L alum and 2 mg/L of nonionic polymer, the primary effluent was successfully treated, as is demonstrated in Table 1, by lowering the total suspended solids and BOD by 9 7 % and 8 8 % , respectively. The Hoboken primary effluent contents (COD = 2 6 0 mg/L; BOD = 103 mg/L; TSS = 5 7 mg/L) were reduced to 80 mg/L, 12 mg/L, and 2 mg/L, respectively. Turbidity and phosphates were also significantly removed; turbidity was lowered from 45 NTU to 3 . 3 NTU and phosphate-P was brought down from 5.6 mg/L to 0.04 mg/L. It is important to note that the Sandfloat was able to reduce the total coliforms from 240,000 per 100 ml to 5,000 per 100 ml without the use of any disiniectant
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Treatment .of Activated Sludqe Effluent
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Sandfloat pilot study was performed at the Oak Meadows Sewage Treatment Plant in Licking County, Ohio from August 26, 1991 to August 30, 1991. The objective of this study was tu demonstrate the Sandiioai’s performance in the treatment of their secondary clarifier effluent following an aeration basin. The Sandfloat was run for a five day period at various chemical dosages using up to 5.6 mg/L alum and 0.5 mg/L polymer. All results reported in Table 2 were based on daily composite samples. The Sandfloat
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was proven to be effective in the reduction of b o t h suspended solids (92'2, removal) and turbidity (91% removal). The secondary treatment plant effluent had a total suspended solids content in the range of 6 to 3 9 mg/L which was reduced to an average value of below 2 mg/L. In a similar performance, the treatment plant effluent had turbidities in the range of 2.6 NTU to 1 4 NTU, while the Sandfloat was able to attain an average The BOD values were effluent turbidity of 0 . 5 NTU. consistently at or below 1.0 mg/L. Treatment of RBC Effluent A Sandfloat unit was installed at the Jiminy Peak Wastewater Treatment Plant in Hancock, Massachusetts. The 400 m /d resort area domestic wastewater flows through an aerated equalization tank, two rotating biological (RBC's), one circular secondary sedimentation contactors clarifier (3.6 m diameter), one rectangular tertiary sand filter (3.1 m 2 area) and finally two ultraviolet disinfection units before being discharged to a leaching field. The Sandfloat was fed from the RBC's effluent just before the flow enters the secondary sedimentation clarifier.
Table 3 documents the chemical dosages applied to the Sandfloat influent, as well as the performance data for both the Sandfloat and the conventional combination of secondary clarifier and sand filter as they were run in parallel. The results demonstrate that with optimization of chemical dosages, it is possible to produce effluents that can satisfy the most strict standards or guidelines with BOD & TSS requirements of less than 5 mg/L. Another important conclusion can be drawn form the parallel performance of the Sandfloat unit and the conventional secondarv sedimentation olus filtration: the innovative ~. Sandfloa; is superior to-conventional tertiary treatment , not only. in removal of BOD, COD; lfSS and coliforms, but also in-land space requirement and consequently in capital cost. Treatment of Tricklinq Filter Effluent
of the Sandfloat in treating the The effectiveness secondary trickling filter effluent at the Norwalk Wastewater Treatment Plant in Ohio was investigated in this part of the study. Norwalk is located in northern Ohio, approximately 50 miles southwest of Cleveland and supports a population of 14,500 people. The plant treats combined domestic sewage and food processing waste.
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Secondary trlCklln9 1 I 1 1 . 1 c.1 fluent was pumped to the Sandfloat with a sublrlcI ::I 111 P sump pump at 164 m3 /d. Treatment chemicals weir .tc-lded directly to the feed line using masterflex VaI iabl e s1)cwd peristaltic dosing pumps. Alum was added appx-oximnt.c.ly 12 m ahead of the Sandfloat to allow for thoroligh mi x i rig. Anionic polymer (Nalco 7 7 6 9 ) was added in line just prior to the Sandfloat inlet compartment. TSS, BOD and phosphate-P tests were all performed on daily composite samples collected from the Experimental influent and effluent o f t h e Sandfloat. results are summarized in T a b l e 4 . It can be seen that on the average the Sandfloat met the tertiary effluent standards (TSS = 10 mg/L, NOD = 10 mg/L and P = 1 mg/L) considering the final three with removals above 9 0 % . testing periods ( # 6 , 7 and 8 in Table 4 ) when chemical dosages, 1 2 0 mg/L of alum and 0.25 mg/L of polymer, were optimized the Sandfloat effluent met the above mentioned effluent standards all. the time. In addition, the average sludge consistency was 3 . 2 % solids, which negates the need for sludge thickener. Treatment of LaqOOnS Effluent Arpin Wastewater Treatment I’lnnt in Arpin, Wisconsin, is a system of aerated lagoons treating a combination of dairy processing wastewater and domestic sewage with a high proportion (Over 75%) o f t.he flow coming from the dairy. The lagoons effluent contained a lot of colloidal substances, and had high color (green) and algae count. A sump pump was used t.0 feed the pilot plant from the third lagoon. Alum and polymer were added to the feed line at dosages of 6 to 40 mg/L of alum and 0.5 to 1 mg/L of anionic polymer. The Sandfloat proved itself capable of treating the lagoons effliinnt and producing a clarified effluent below 20 mg/L in UOI) and TSS.- The average total suspended solids and BOD values in th_e effluent were 6 mg/L and 1-2 mg/L, respcdjveiy (see Table 5 ) . The optimized chemical dosage W ~ S10 my/L for alum and 1 mg/L for this anionic polymer. At. all times during the study, a high consistency green !IoaLed sludge was removed by the spiral scoop. Consist.erlcicb.; of over 2% solids were obtained when the Sandflont.‘~ sludge SCOOP was operated intermittently allowing a t h ~ c ksludge layer to build up. Existing lagoons expericnc i n q operational problems can be improved or upgraded f o i effluent reuse by addition of a Sandfloat in series for tertiary treatment.
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CONCLUSION The technical feasibility of the innovative flotationfiltration Sandfloat system for producing a high quality effluent has been successfully demonstrated by continuous pilot plant operation. An existing secondary biological wastewater treatment plant can be easily upgraded by the addition of a Sandfloat to produce an effluent having a water quality compatible with water reuse requirements. The capital cost of such a system is low because of its short detention time and unique compact design REFERENCES
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1.
Asano, T., Richard, D., Crites, R. W., and Tchobanoglous, G., "Evolution of Tertiary Treatment Requirements in California", Water Environment and Technoloqy, 4, 2 , pp 36-41 (1992).
2.
Crook, J., "Regulatory Issues Associated with Reuse Practices Throughout the World", AWWA 1991 Annual Conference Proceedings, Resources, Engineering and Operations of the New Decade, Philadelphia, PA, June 23-27, pp 225-239 (1991).
3.
US-WPCF, "Water Reuse", Manual of Practice SM3, WPCF, Alexandria, VA (1989).
4.
Shammas, N., and El-Rehaili, A., "Tertiary Filtration of Wastewater for Use in Irrigation", Symposium on the Effect of water Quality on the Human Health and Agriculture in the G.C.C. States, Al-Khobar, Saudi Arabia (1986).
5.
Shammas, N., and DeWitt, N., "Flotation: A Viable Alternative to Sedimentation in Wastewater Treatment", Water-Environmental Federation, 65th Annual Conf., New Orleans, Louisiana, Sept. 20-24, pp-223-232 (1992).
6.
Krofta, M., and Wang, L., "Development Flotation-Filtration Systems for Water A: First Full-scale Sandfloat Plant in Water Reuse Symposium 111, Auq. 26-31, California pp. 1226-1237 (1984).
7.
GUSS, D., "Improving Water Quality with Flotation/ Filtration", 1991 Nonwoven Conference, TAPPI Proc., Technology Park/Atlanta, GA, pp 323-326 (1991).
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of Innovative Treatment, Part U.S.", Proc. of San Dieqo,
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8.
Zabel, T., "Flotation in Water Treatment", in "Innovations in Flotation Technology", Mavros, P. and Matis K., eds, Kluwer Academic P u b l i s h e r s , Netherlands pp 431-454 (1992).
9.
Krofta, M., and Wang, L . , "Tertiary Treatment of Secondary Effluent bv Dissolved Air Flotation and Filtration", Civil Enqineerinq for Practicinq and Desiqn Enqineers, 2, pp 253-272 (1984) (NTIS - PB83171165).
10.
Krofta, M., and Wang, L., "Wastewater Treatment by a Biological-Physicochemical Two-Stage Process System", Proc. of the 41st Industrial Waste Conference, May 1315 (1986).
11.
Uiuru, H., "Tertiarv Wastewater Treatment with Flotation.Filters" ,-Water Science and Technoloqy, 22, NO. 7/8, pp 139-144 (1990).
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Outside tank Inside flocculation tank Sandbed assembly with Screen Tank bottom Sludge collection tunnel Moveable carriage assembly Spiral scoop Scoop Variable speed drive ~ l e c t ~ i c s l - r o t a rcontact y PreSSUKe pump Air dissolving tube Compressed air addition point Aerated vater distribution pipes Rav vater inlet regulating valve Tank level control sensor Figure I.
16. 17. 18. 19. 20. 21.
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Raw vater inlet jet nozzle Coagulant addition point Polye~ectrolyte addition point Deflector ring into flotation tank Backwash hood assembly Clarified water pipeline Clarified water flov regulating valve Floated sludge discharge pipe Main carrcage drive Motor to lift backvash hood assembly Backwash suction pump Check valve (backflov preventor) Dirty backvash water recycle pipe Drain linc
Details o f the Sandfloat Pilot Plant
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Table 1.
Treatment of Primary Municipal Wastewater Effluent by Sandfloat at Hoboken, New Jersey
Parameter
Primary Effluent
Turbidity, NTU Phosphate-PI mg/L BOD, mg/L COD, mg/L TSS, mg/L Coliforms, No/100 ml
45 5.6 103 260 57 240,000
Sandfloat Effluent
Removal* %
3.3 0.04 12 80 2 5,000
93 99 88 69 97 98
*Chemicals addition: alum (as A1203) 20 mg/L and polymer 2 mg/L
Table 2 .
Tertiary Treatment of Activated Sludge Effluent by Sandfloat at Oak Meadows Sewage Treatment Plant, Licking County, Ohio Chemical Addition
Period*
SusDended Solids Turbiditv mqfL NTU In out In out
Chemical
mg/L
1
Alum (as Al2O3)
3.6
23
1.0
2.6
0.95
2
Alum (as A Polymer: lh:l?)
4.3 0.4
39
2.6
4.3
0.50
3
Alum (as Al2O3) Polymer: 1849A
4.9 0.4
5.9
1.0
4.2
0.29
4
Alum (as Al203) 5.3 Polymer: Percol LT-25 0.3
24
5
Alum (as Al2O3) Polymer: Percol LT-25
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5.6
14
0.46
3.7
0.34
5.e
0.50
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0.5
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nveiayc
Removal
2.6
-
1
L.J ?'
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0
92
%
*All tests were done an daily composite samples.
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Table 4.
Period
Tertiary Treatment of Trickling Filter Effluent by Sandfloat at Norwalk, Ohio
TSS, mq/L In Out
BOD, mq/L In Out
Phosphates-P, mq/L In out
1
104
11
54
5
8.8
0.8
2
136
6
51
3
9.2
0.4
3
122
12
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9.6
1.1
4
106
1
45
2
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0.3
5
72
4
43
3
6.7
0.3
6*
28
9
20
4
1.0
0.7
7*
132
7
32
4
4.5
0.2
8*
103
8
39
4
7.8
0.6
Range : min max.
28 136
1 12
20 54
2 5
4.5 9.6
0.2 1.1
Average
100
7.2
41
3.6
7 .I
0.6
.
Removal, %
93
91
92
Alum = 120 mg/L as &\rm and anionic polymer Nalco 7769 = 0.25 mg/L All tests were done on daily composite samples. *Optimized chemistry:
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