Your Clarifier & ByFilter Thomas H. Martin, Press CEF

There are methods that can be used option.1 The scale usually does not purposes, and to test new coagulants to optimize clarifier and filter press dissolve, however, and the acid may and polymers. There is a quick and efficiency. By optimizing settling damage the clarifier. Also, the plates easy method to check the settling rate. performance, solids loading and or plate pack unit should be removed Complete the coagulation and sludge withdrawal rates, the at least once a year to clean the sludge flocculation steps and pour the equipment can be fine-tuned to hopper. This will help prevent solution into an Imhoff settling cone optimize its fullest potential. The “channeling” where one side of the and record the time required for the ability to dewater sludge, however, clarifier is favored. Be sure to check floc to settle from 1,000 mL to 800 is usually another limiting factor. for tools that may have been dropped mL. This distance is usually about 1 Simple tests that are easily per- into the clarifier and block the sludge 1 /4 in. (31 mm) on most cones. Then, formed may be used to determine withdrawal ports. Special consider- measure the distance between the these factors, and optimize the ation should be given to the ceiling parallel plates in the clarifier. Typical 1 efficiency of the system. height above the clarifier. Removing spacing is between 1 and 1 /2 in. A the plates can be very difficult when wastewater with good settling The clarifier itself is not usually the there is not enough clearance. properties should drop one inch bottleneck in a solids/water separation Flow rates for optimum conditions during five minutes or less. Maximum system. Many parallel plate clarifiers are set by the manufacturer. For flow rates are only possible with a can be run at their rated capacity, or example, a 200 gal/min clarifier can fast-settling floc. even a little higher, when separating easily handle a hydraulic load of 200+ metal hydroxide solids. The highest gal/min without overflowing. It is also Solids Loading possible efficiency must be obtained easy to separate sand and water at Equipment manufacturers specify the from the equipment so that capacity high flow rates. Separating metal maximum load of solids that may be can be increased, without investing hydroxide “floc” with a specific applied to the clarifier. Typical more capital in additional equipment. gravity only slightly heavier than loading range is from 500 to 2,000 The clarification flow rate can be water, however, will be more chal- mg/L. Another important specifica- maximized by: lenging. Fortunately, high flow rates tion is the ft2 of settling area. This can usually be achieved if the settling factor increases in direct proportion to • Improving settling rates rate, solids loading and sludge the flow rate. • Determining solids loading withdrawal rate are all optimized. The solids loading may be calcu- • Optimizing sludge withdrawal lated in the laboratory by filtering a Settling Rate 1,000 mL sample of the flocculated There are five major factors that Perhaps the most important factor solution through a filter paper of affect the performance of parallel affecting clarifier efficiency is how known weight. The sample is then plate clarifiers, commonly called fast the solids or “floc” will settle. dried and weighed. The dry weight lamella settlers: The density of the precipitate, minus the weight of the filter paper is coagulant and polymer being used • Maintenance determines the • Flow rate settling rate. • Solids loading Usually “jar • Sludge withdrawal tests” are con- • Settling rate ducted in the lab to optimize the Maintenance is very important if coagulation and performance is to be optimized. The flocculation clarifier should be pumped down processes. These below the plates at least once a week. tests should be If possible, the plates should be pulled done on a regular out and cleaned to remove scale and basis for control restore the smooth, slippery surface. Clarifiers with a plate pack that must Fig 1.—When the be removed as a “unit” should be water is withdrawn cleaned with a high-pressure spray too fast, a washer. Acid washing with inhibited phenomenon called hydrochloric (muriatic) acid is also an "coning" may result.

36 PLATINGPLATINGPLATING &&& SURFACESURFACESURFACE FINISHINGFINISHINGFINISHING the mg/L of influent solids. Unfortu- Solids loading above 500 mL may known as coning. When the sludge is nately, this test is time consuming and result in floc being carried out of the withdrawn too rapidly from the the solids loading may fluctuate. clarifier. On the other hand, low bottom of the clarifier, a funnel- There is a quick and easy test that solids of 200 mL or less may contrib- shaped cone may develop.2 This will may be used as a guide however. Fill ute to “pin floc.” Pinpoint floc is a cause water instead of solids to be an Imhoff cone with flocculated very small particle of floc that does pumped out. The phenomenon is solution to the 1,000 mL level and not settle rapidly and is easily carried illustrated in Fig. 1. start a stopwatch. Check the clarifier out in the effluent. Since pin floc This problem may be minimized by specifications for the volume or contains metal hydroxides, permit using an interval timer to cycle the capacity in gallons. Divide this figure limitations may be exceeded. A pump on and off. The cone will fill up by the desired flow rate to get the minimum solids loading of 200 mL will with sludge again during the off cycle. retention time. For example, if the help minimize pin floc by trapping the Unfortunately, cycling the sludge clarifier will hold 3,000 gal of solution small particles and filtering them out as withdrawal pump makes it more and the desired flow rate is 200 gal/ they travel up through the sludge difficult to equalize the influent and min, then the retention time may be blanket in the clarifier. underflow rates. Usually the sludge is computed using the following pumped from the clarifier to a conical formula: Underflow bottom tank for thickening. In order The last factor that must be controlled to optimize the sludge withdrawal 3,000 gal of solution = 15 min is the underflow, more commonly rate, the volume pumped out must be 200 gal/min known as the sludge withdrawal rate. measured accurately. In addition, the This is important because clarifier underflow must be inspected to make This means that at a 200 gal/min manufacturers specify an optimum certain that sludge, not water, is flow rate, the solids will have 15 min level for the sludge blanket. If the pumped out at the end of the cycle. to “drop out” in the clarifier. By blanket is too high, then the floc will This may be easily accomplished by noting the solids level in the Imhoff be carried out in the effluent. When installing a “T” fitting with a valve cone after 15 min of settling, an the blanket is too low, pin floc will and hose on the discharge pipe from estimation of the solids loading can be not be filtered out of solution. the sludge withdrawal pump. The made. One can expect the solids level Optimum conditions exist when the hose and fittings must be the same to be in the range of 100 mL to 500 sludge blanket is at the ideal level and size as the discharge plumbing, and mL. This range is for metal hydrox- the solids removed (underflow) are head pressure is not taken into consider- ide precipitates, and not for heavy equal to the solids added (influent). ation. Measure the amount of time that solids from burnishing or tumbling This is difficult to obtain in practice, is required to fill a 55-gal drum with the operations. however, because of a phenomenon underflow. Then calculate the flow rate by using this formula:

55 gal Underflow, gal/min = Time in Min

A good underflow that is not diluted with water will show little settling (100 mL or less) in a five-min settling test. Now it is possible to balance the influent and underflow solids. The influent solids will be equal to the percent of settled solids from the solid loading test multiplied by the flow in gal/min.3 For example, if the 250 mL of solids settled during the 15 min settling test and the flow rate is 200 gal/min, then the influent solids are calculated as follows:

Influent Solids = % Settleable Solids x Flow/gal/min = .25 x 200 = 50 gal/min

A 55-gal drum of sludge would Fig. 2—A large almost have to be emptied every tank with at least a two-hour retention minute to balance the influent solids capability is loading. This presents another needed for challenge—what to do with 3,000 gal thickening and of sludge per hour! Additional jar dewatering. testing is required. Fill an Imhoff

January 1998 37 cone with underflow sludge and note average volume of thickened solids Equipment to thicken solids and the sludge level after 60, 120 and 180 produced during a typical working dewater the sludge, however, must be min intervals of settling. Typical day. Then, pilot tests should be run to properly designed and be large metal hydroxide sludge will reduce by optimize the production of filtercake enough in capacity to keep up with 50 to 75 percent in volume after one in the shortest possible time. Experi- solids removed by the clarifier.P&SF to two hours of additional settling. A ments should be conducted using a large capacity, well-designed sludge diatomaceous precoat to protect the References thickening tank may be required. filter cloths from blinding. Precoating 1. Parkson Corporation, Ft. Lauder- usually decreases the cycle time and dale, FL, “Calcium Scaling: Sludge Thickening produces a clear filtrate immediately. Formation and Removal from Thickening and dewatering the sludge The press is not usually cleaned Lamella Gravity Settler/Thickener is usually the limiting factor in until the filter feed pump slows to 20– Plates in Metal Hydroxide Applica- maximizing clarifier flow rates. 30 sec between strokes. Because the tions,” 1980. A large tank with a minimum of two goal is to dewater the largest volume 2. Tortorici, L.D. & B.L. Pastushenko, hours’ retention time is needed. of sludge in the shortest period of California State University, Sacra- Moreover, the tank should be de- time, try this experiment. Start the mento, “Residual Solids Manage- signed to promote fast settling and feed pump at 25 psi after precoating ment,” Industrial Waste Treatment, produce a clear, high-quality superna- the filter cloths. Measure the filtrate Vol. II, 2nd Edition, 1995. tant. Sludge thickeners usually are volume (in gal/min), and try to keep a 3. Lancy International, “Determining tall, cylindrical tanks with conical constant volume of flow by slowly the Sludge Withdrawal Rate for bottoms and are constructed much increasing the air pressure to the Lancy International, Inc., Solids like municipal clarifiers.4 The solids pump. Some filter press manufactur- Separators/Econo-treat Units,” enter a large 10 to 12 in. diameter ers have equipment that will do this 1988, Warrendale, PA. tube at the top of the tank and flow by automatically. Once the air pressure is 4. Cheremisinoff, Paul N., Columbus, gravity to the exit inside the tank raised to 100 psi (maximum), wait for GA, “Gravity Separation for about 24 in. above the bottom of the the filtrate flow to decrease to less Efficient Solids Removal,” The cone. This minimizes disturbances to than 50 percent of the original National Environmental Journal, the sludge blanket when solids are volume. This is usually the point of December, 1995. pumped in. The clear supernatant gently diminishing returns. The flow through overflows into a weir around the the press decreases dramatically as the About the Author circumference of the top of the tank. filtercake becomes thicker. Blow the Thomas H. Martin, This concept is illustrated in Fig. 2. press down with air for a few minutes CEF, is a consult- The effluent may require polishing until very little water is draining from ing chemist with in a sand filter to remove small the filtrate discharge pipe. Open the Delta Compliance suspended solids if very low permit press and examine the filtercake to Consultants, a limits must be met. determine if it is dry enough to Division of Delta Thickened sludge, approximately landfill. To qualify for landfilling, the Chemicals & one to two percent solids, is with- filtercake must pass the “paint filter” Equipment, Inc., drawn from the bottom of the cone test. The filtercake should be placed 12466 E. 62nd St., and dewatered in a filter press. in a paint filter and no free liquid is Indianapolis, IN 46236. He is a pub- Again, a proper balance must be allowed to drip-out within a five-min lished author and holds honor degrees achieved between the capacity to period. from both Purdue University and Indiana thicken solids and the ability of the Properly sizing the filter press to University. He is certified as a manufac- filterpress to dewater the slurry. keep up with sludge production can turing engineer, hazardous materials be accomplished through pilot testing. manager, environmental trainer and Sludge Dewatering For example, if a one ft3 press will registered hazardous substance The filter press should be operated at maintain an average flow rate of two professional. He also holds Class D maximum efficiency to keep up with gal/min over a six-hr filtration cycle, (industrial) and Class IV (municipal) the sludge production. Selecting a then a 10 ft3 press should flow 20 gal/ wastewater treatment licenses. press with a large enough capacity to do min. Therefore, a large press (25 ft3) Martin teaches regulatory compli- the job requires evaluating these factors: would be required to dewater the 25 ance at Indiana University, Purdue gal/min of thickened sludge generated University at Indianapolis, and • Volume of sludge to dewater from clarifying a 200 gal/min influent. wastewater treatment at Indiana • Average cycle time Vocational Technical State College • Down-time for cleaning Conclusion (IVY Tech). His firm consults with The complete solids/water separation industry on special problems concern- The capacity of the filter press is system must be evaluated to maxi- ing regulatory compliance with the difficult to estimate and usually too mize clarifier flow rates. More often Occupational Safety & Health small of a press is purchased. Select- than not, the ability to dewater sludge Administration (OSHA), Resource ing the proper size can be made easier will be the limiting factor in the Conservation & Recovery Act (RCRA) by renting a small portable unit (0.5 process. Settling rates can usually be and the Clean Water Act (CWA). to 1.0 ft3) for pilot testing. Based on improved and the solids loading Martin currently serves on the AESF the techniques presented in this paper, determined accurately. The sludge Board of Directors and is chairman of one should be able to estimate the withdrawal rate can also be optimized. the Society’s Environmental Section.

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