Filter plant sludge disposal Robert M. Gruninger Garret P. Westerhoff
Malcolm Pirnie, Inc., White Plains, N. Y. 10602
The magnitude of water treatment plant wastes in the U.S. has been estimated at 1,000,000 tons per year, mostly from plants using filter alum-commercial grade aluminum sulfate-as the principal coagulant. The average water treatment using alum produces approximately 50 rngd water and 3 tpd or 1000 tpy solid wastes. Available data on the chemical and physical characteristics of alum sludges are limited. The main constituents of the sludges are aluminum hydroxide, material removed from the raw water, and residues of other chemicals added during the treatment processes. Most references classify alum sludge as a bulky and gelatinous material with a relatively low solids content that is difficult to dewater. Knowledge of the characteristics of alum sludge is essential to permit the engineering design of systems and facilities required to contain, treat, and dispose of the wastes. Equally important is the establishment of design parameters based on experience from successful installations. Generally speaking, there is little available information or few data on sludge characteristics, practical design parameters, or treatment costs. To date, there appears to be little potential for receiving state or federal grants for the elimination of alum sludge discharges from water treatment plants. Although governmental opinion recognizes the pollution potential of this waste, the problem is considered to be low priority compared to discharge of domestic sanitary sewage and other industrial wastes. Nevertheless, existing water treatment olants are now beins ordered to discontinue waste water discharges and &w plants are being required to provide facilities to prevent such discharges. The lack of state and federal research and development and construction funding has been, and will continue to be, a deterrent to the development of new technology to treat and dispose alum sludges.
Alum sludge producing plants A number of water treatment plants in New York have found it necessary to undertake programs for proper alum sludge disposal, following New York Department of Environmental Conservation's order to stop discharging alum sludge and to filter backwash water to lakes. Experience from four of these plants-60 mgd Sturgeon Point (Erie County), 100 mgd Shoremont (Monroe County), 43 mgd Eastman Kodak (Monroe County), and 40 mgd Dewey Avenue (Monroe County)-is reported. At the Sturgeon Point Plant, raw water is pumped from Lake Erie, coagulated with alum, flocculated, clarified in 122
Environmental Science & Technology
FEATURE Technology and money are needed to deal with alum from treatment plants which use it as principal coagulant gravity settling basins, processed on high-rate mixed media gravity filters, chlorinated, fluoridated, and pumped to distribution mains. At each of the other plants, raw water is pumped from Lake Ontario, coagulated with alum and clay in upflow clarifiers, processed on high-rate gravity filters, chlorinated, and pumped to distribution mains. The Shoremont Treatment Plant and the Dewey Avenue Treatment Plant share a common raw water intake system. The Kodak Water Treatment Plant has a separate intake facility, and contains a number of slow sand filters that receive and process raw water for use in manufacturing processes. At all these plants, alum is used either in dry form or as a concentrated liquid. In waters having sufficient natural alkalinity, alum reacts to form aluminum hydroxide, a precipitate, and one of the major components of alum sludge. Throughout the pH range of natural surface water supplies, the materials in alum sludge remain nearly insolu bI e. Both quantity and characteristics of the alum sludge are affected by the raw water quality and the dosage of alum and other chemicals or material used in the treatment process. Raw water quality may be measured by its turbidity, microscopic analysis, or other physical and chemical parameters. Matter removed from the raw water may include inert substances (sand, silt, or clay particles) and suspended or dissolved organic substances-algae, bacteria, and other microorganisms. These waste solids are the principal constituents in alum sludge. Waste solids from the water treatment plants are contained as suspended solids in two waste streams: coagulation-sedimentation basin waste water and filter backwash. With good pretreatment, approximately 80-90% of the suspended solids will be removed in the coagulationsedimentation basin waste water. The concentration of suspended waste solids in each stream is important in the design of waste water treatment facilities at these plants. These concentrations are dependent on method of desludging coagulation basins
TABLE 1
Estimated 1972 Alum Sludge Quantities" Roch-
esterb
Average 1972 plant flow, mgd Average suspended solids content of clarifier wastewater, % Average quantity of clarifier wastewater, m gd Average suspended solids discharge, Ib/ day of dry solids Computed suspended solids per mg plant flow. Ib/ma I
I
MCWAb Kodakb
ECWAc
15
38
30
28.6
0.13
0.5
0.22
0.3
0.3
0.1
0.2
0.12
3200
4400
4300
3000
210
116
143
100
I
0 S u s p e n d e d solids on dry weight basis. b Based on ,data rep o r t e d b y , p l a n t operating personnel. Based on design estim a t e s prior t o modification of coagulation-sedimentation basins.
or clarifiers, and on quantity of backwash water used. Experience at the four plants indicates the following range of concentrations: upflow clarifiers-0.1-0.3%; hydraulic removal from basins-0.1-0.5%; backwashing100-200 mg/l. Estimated quantities of waste water resulting from the coagulation and clarification process (Table 1) differ at each plant because of differences in chemical dosage, overflow rate, and detention time of the clarifiers and settling tanks used. Sludge thickening
Graduated-cylinder settling characteristics of wastes produced at the three Monroe County treatment plants (Figure 1) show that filter backwash solids settle rapidly, effecting a good separation of the liquid portion from the
Figure 1
Graduated cylinder settling curves 1000
14.0 Filter backwash wastewater Monroe County
14.0 Upflow clarifier wastewater Monroe County
E
d,
6
10.5
.-c d,
7.0
.n
c)
v)
3.5
750
10.5
'= 500
7.O
s v)
u
a
c)
Y
250
v)
0
20
40
60 Time-min
80
100
0 120
0 0
20
40
60
80
100
0 120
Time-min
V o l u m e 8, N u m b e r 2, F e b r u a r y 1974
123
TABLE 2
Alum Sludge Settling Tests Volume ot remaining concentrated solids, % ' Time hr- ' Sample Sample Sample Sample Sample Sample B C D E F min A
:oo :05
:30 4:OO
21:OO SamDle
ion 5 4 4 4
loo 13 8 6.5 6
ion 32
!l
ion 10.5.
12
i.5 7
10.5
6.5
100 9.5 7.5 7 6.5
100 8.5 7 6.5 6
S a m p l e description 103 niL'/I : o t a l solids i I soids a1 solids a1 solids + j m . ' I p d y r n e i
concentrated solids. For these plants, the liquid portion, which is not vastly different in quality from raw water, can be recycled to the water treatment plant raw water influent. The concentrated solids may be blended with the alum sludge for final dewatering inasmuch as the characteristics of these concentrated solids are somewhat similar to those of the alum sludge. Several series of laboratory scale alum sludge settling tests were run at Sturgeon Point. Data from a representative test (Table 2) indicate that as the concentration of solids in the sample increased, the settling times also increased. The addition of a nonionic polymer did not significantly affect the settling rate or ultimate concentration under various mixing and flocculating conditions. Additional testing with other polymers is continuing at Sturgeon Point. I t was determined early in the studies at the Erie and Monroe County plants that graduated cylinders or column-settling tests under static conditions would not yield all the design information needed to size alum sludgethickening facilities. The tests did, however, provide general indications of what might be expected under dynamic settling conditions. To simulate dynamic conditions more properly, therefore, a pilot clarifier/thickener tank (Figure 2) was designed and constructed to evaluate separation characteristics that might be anticipated under actual operating conditions. The rectangular pilot tank was 5 ft wide by 15 ft long and had a side water depth of 8 ft. A flow-straightening baffle was installed at the front of the tank and tube clarifier modules were installed in the final two thirds of the tank. Five sludge drawoff points were spaced at equal intervals along the length of the tank. The results of the pilot clarifier/thickener tank experiment at the Kodak plant indicated that concentrations of approximately 0.3% solids in alum-clay sludge could be thickened to the 3-6% range when the influent rate was approximately 35 gpm and sludge withdrawal rate 1.02.5 gpm. The detention time in the tank was approximately 2 hr and the rate through the tube clarifiers was 0.23 gpm/ft2 of horizontal area. During the period of thickening testing, the type of clay used for water treatment was changed, causing a significant alteration in alum sludge characteristics-the influent sludge concentration was reduced to approximately 0.1% in solids. With the changed characteristics, the inflow rate was reduced to 25 gpm, and the maximum sludge concentration that could be obtained was in the range of 1.5-2.5% solids. The thickened sludge withdrawal rate had to be held at about 1 gpm to achieve this sludge concentration in the thickener underflow. 124
Environmental Science & Technology
A polymer feed system was added to the pilot thickener influent. The addition of an anionic polymer was effective in increasing the settleability of the alum sludge and permitted operation of the pilot thickener at approximately 40-60 gpm, with good quality overflow and sludge solids concentrations of approximately 1.5-3.0%. At times during the polymer treatment program, plant operators noted that the sometimes lumpy sludge caused problems in the small-diameter sludge withdrawal piping system of the pilot plant. This experience illustrates some of the problems with alum sludge and the dependence of its settling characteristics on the chemicals used in water treatment. I t is likely that the amount and characteristics of raw water suspended solids would also produce significantly different sludges. The following conclusions were drawn three months after the thickener tank went into operation at the Kodak plant: 0 Alum sludge containing clay produced by upflow clarifiers can be thickened in a gravity thickening tank. 0 Alum sludge at solids concentrations ranging from 0.1-0.3% can be successfully thickened to a solids concentration of 2% or more. 0 Alum sludge-settling characteristics can be enhanced by polymer addition prior to gravity thickening. Smaller thickening tanks can thus be used.
Sludge dewatering The concentration of solids in the waste water from coagulation/sedimentation basins depends on the removal mechanism employed. Generally, removal is achieved hydraulically, via an underdrain or blowdown system; or mechanically, via a chain or scraper in a horizontal tank or a rotary collector mechanism in a circular tank. Sludge removed hydraulically is usually less concentrated than mechanically removed sludge. The concentration of solids in filter backwash is often considerably lower than that in the coagulation/sedimentation basin waste water. The usual goal in waste water treatment is dewatering to a concentration which can be handled easily by earth moving equipment. Observations at Sturgeon Point indicated that at a 20% solids concentration by weight, alum sludge can be compared to the consistency of a soft clay. At a concentration of 40-50%, it becomes comparable to a very stiff clay. The extent of dewatering neces-
Figure 2
Clarifierlthickener pilot plant
4
k Sludge Draw-off
Head
box
Baffle
Sludge draw-off pipes
-,
1
f
Section
Effluent weirs
Figure 3
Two-stage alum sludge dewatering process
sary for any given plant will depend on the method of ultimate disposal. However, it appears that the minimum concentration normally acceptable will be 20%. Most investigators conclude that where land is limited, mechanical processes Tor dewatering of thickened alum sludge should be used. This hypothesis has been borne out at the four water treatment plants. Several alternatives were investigated at each plant and it was concluded that separation of the solids from alum sludge is best accomplished in a two-stage system: primary thickening-a gravity liquid/solids separation to achieve a solids concentration compatible with the second stage separation (usually 2-6% total solids by weight) mechanical dewatering-a further liquid/solids separation to achieve a solids concentration compatible with the ultimate intended disposal of the solids (usually 20% or more total solids by weight) A typical two-stage alum sludge dewatering process (Figure 3) consists of a closed system with zero waste water discharge from the plant. Filter backwash is fed into a surge tank (or surge lagoon). After complete or partial liquid/soiids separation, the liquid portion is recycled back to the plant r a w water supply. At other plants, more studies may be necessary 'to determine the suitability of this liquid portion for recycle. The underflow containing most of the solids is mixed with the waste stream from the settling basins. The mixed stream, generally containing less than 0.5% solids, is fed at nearly a constant rate to a gravity-thickening system. Concentration of solids to 2-6% Is accomplished in the thickener prior to mechanical dewatering. The concentration achieved in the thickener has a significant effect on the economics of the mechanical dewatering system. In some systems, it may be feasible to include a secondary thickening step, such as a centrifuge, ahead of the mechanical dewatering system to achieve a solids concentration up to 12%. A series of pressure filtration tests was conducted on separate and combined sludges from the three Monroe County plants. It was found that with proper conditioning using lime, the sludges exhibited similar characteristics and could readily be dewatered with a pressure filter. Filter cake concentfations of 40-50% solids were obtained in laboratory experiments and in a truck-mounted pilot plant. Further investigations with pressure filtration, vacu-
um filtration, and other mechanical dewatering systems are currently under way at the Sturgeon Point Plant. Much-needed technology Experiences at Monroe County indicate that processing of water treatment piant waste water by gravity settling and pressure filtration was best fitted to meet the client's needs. A plan has been proposed, and is under consideration, to dewater the waste alum sludges from all three Monroe County plants at a single processing facility. Basic data that would enable rapid advancement in the technology of treating water treatment plant sludges are not readily available. Experience in treating this type of sludge at four water treatment plants in western New York indicates that the characteristics of the sludges are affected by the substances used in the water treatment process and may be highly variable. At the three plants of similar design in Monroe County, similar raw water sources produce sludges of similar quality. Sludges that were investigated could readily be thickened by gravity to about 2% solids, and then dewatered mechanically to a range of 40-50% solids. These concentrations appear to be suitable for landfill disposal in a properly operated sanitary landfill.
Robert M. Gruninger is project manager for the soiid waste program a t Malcolm Pirnie. He has extensive experience in the design, coordination of construction, and costing of water and sewage treatment facilities.
Garret P. Westerhoff is vice president of Malcolm Plrnie, and has held key engineering assignments prior to assuming his present position.
Volume 8. Number 2, February 1974
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