Primary Wastewater Treatment -- Clarifier Operating Principles

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The operating zones in a clarifier are a zone of clear water, the sedimentation zone (discrete and flocculant settling), and the thickening zone as illustrated in Ill. 7. The depth of the thickening zone (sludge bed) is dependent on the depth of the tank allocated for sludge thickening. The water inlet must always be above the sludge bed (thickening zone) for high efficiency solids capture. Solids can be carried to the outlet launder, if the feed goes through the top of the sludge bed. The objective is to operate the clarifier with as low a sludge bed depth as possible, yet still produce a high underflow sludge solids concentration, if needed.

The solids mass balance around the clarifier must be maintained at all times. The influent settleable dry solids less, the effluent settle able dry solids, plus the underflow dry solids, must be zero, to keep the continuous operation of the clarifier in balance. The required underflow sludge pumping volume can be estimated by the influent settleable solids, less the effluent settleable solids in mL/L times the influent flow.

The solids inventory (sludge bed) in the clarifier becomes greater with an increase in the influent solids, or a decrease in underflow pumping. If not corrected, this imbalance results in an increase in the effluent solids. For example, plants that store sludge in the clarifier to meet an 8 h per day sludge dewatering schedule classically have increased effluent solids when not dewatering. Likewise, trying to store sludge in the clarifier over a weekend can be disastrous, particularly if the settled solids contain organic material. The organic material can undergo anaerobic decomposition producing methane and hydrogen sulfide gases, which can float the settled solids. Organic solids should not be kept in the clarifier longer than 24 h.

Sludge Bed Depth Measurement

Regular measurement of the sludge bed depth is an easy way to maintain the clarifier solids balance. An increase in sludge bed depth indicates that the underflow blowdown needs to be increased, while a decreasing depth shows that the underflow pumping can be decreased.

The best operation is one that can pump the underflow continuously to subsequent sludge handling facilities, instead of an on/off pumping schedule.

The sludge bed depth can be measured manually using a sludge judge. This is a clear plastic tube having feet or meter graduations with a check valve at the bottom. A core sample of the sludge bed is taken with the sludge judge and the depth of the sludge read on the side of the tube. This measurement always has to be taken at the same place on the clarifier walkway or wall, since the floors of clarifiers are generally on a slope.

Automatic sludge bed indicators are also available. Some are a probe type based on the transmission and receiving of an infrared beam, while others are based on sonics. The sonic transmitter and receiver are located at the water surface, and the face of the instrument's control panel indicates the profile of the clarifier depth and sludge bed, much like a fish finder. The infrared types of indicators do not work in situations where the water is highly colored. Either type of indicator can be used to automatically control the underflow pumping, to continuously keep the sludge bed depth and thus the solids balance, in the optimum range.

The rake drive torque can be used as a surrogate for the sludge bed depth in large clarifiers handling settleable solids with a high specific gravity, such as in the steel and paper industry. The rake drive torque usually goes up as the sludge bed depth increases. The motor amps cannot be used for the bed depth indication, since the rake drive unit is geared down to such a low speed.

Influent Flow Control

Continuous measurement and recording of the influent flow to a clarifier, indicates the instantaneous hydraulic loading and changes with time. Flow rates greater than the design overflow rate, result in an increase in the effluent solids. Rapid, short-term, large increases in the flow rate need to be avoided, since the resulting hydraulic surge can wash solids out of the clarifier. It may take a clarifier as long as two hours to recover from such a surge.

Wastewater discharged to the primary clarifier is often quite variable, due to storm water and intermittent discharges from an industrial plant. An equalization basin is often provided to even out these variations in both flow and composition. The most efficient solids removal occurs when constant flow is treated within the clarifier's design rating.

On/off level switches may control the forwarding pumps from the equalization basin or the pump station wet well, which is a poor practice. The influent pumps need to be controlled to eliminate the clarifier surging created by this on/off control. One method is to use a modulating valve that is controlled by the basin or wet well level that regulates the pump discharge directly to the clarifier, or recycles a portion of the flow back to the basin or wet well in order to keep the clarifier influent close to a constant rate. Another method for multiple pumps is to have a baseline constant speed pump or pumps in con junction with a variable speed pump or a third pump, which has a modulating discharge throttling valve. The pump speed or discharge throttling valve is adjusted proportionally to the level in the wet well or equalization basin, so that the clarifier influent flow rate is gradually adjusted to eliminate high peak flows while maintaining the hydraulic loading within the design value.

Hydraulic devices such as overflow weirs and gates must divide the flow equally to multiple clarifiers. One clarifier in the group can become hydraulically overloaded with a poor division of the total flow. Dye or diagnostic TRASAR chemistry tests can be used to check and adjust flow distribution.

Chemical Treatment

Chemical treatment of the clarifier influent can be used to increase removal of suspended solids, by flocculating the colloidal non-settleable solids to convert them to settleable ones. Depending on the degree of overloading, problems associated with high hydraulic or solids loading can be alleviated by chemical treatment.

Coagulants and/or flocculants may be used, depending on the industry and the goals of the chemical treatment. The coagulants may be inorganic salts, such as iron or aluminum, organic polymers having various formulations, or a blend of organic and inorganic. Anionic, nonionic, or cationic polymer flocculants are used alone or following a coagulant. The polymer flocculants are available in different molecular weights for each charge family. Polymer flocculants, besides increasing the clarity of the effluent, may also increase the underflow sludge concentration, depending on the chemical dosage.

The appropriate chemical program for a specific installation is ascertained by jar testing. The coagulant if used, is generally added upstream from the clarifier at a point with high energy mixing, such as the suction side of the clarifier feed pump. The long molecular chain flocculants cannot withstand high energy mixing, since the molecular chain can be sheared. The flocculant is typically added just outside the clarifier or into the clarifier feed well.

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