Other Wastewater Residuals

In addition to sludge, three other residuals are removed in wastewater treatment process: screenings, grit, and scum. Although their quantities are significantly less than those of sludge in volume and weight, their removal and disposal are very important.

Screenings include relatively large debris, such as rags, plastics, cans, leaves, and similar items that are typically removed by bar screens. Quantities of screenings vary from 4 to 40 mL/m3 (0.5 to 5 ft3/MG) of wastewater. The higher quantities are attributable to wastes from correctional institutions, restaurants, and some food-processing industries. Screenings are normally hauled to a landfill. Some treatment plants return the screenings to the liquid stream after marcerating or comminuting. This is not recommended because many of the downstream pieces of equipment, such as mixers, air diffusers, and electronic probes, are subject to fouling from reconstituted rags and strings.

Grit consists of heavy and coarse materials, such as sand, cinders, and similar inorganic matter. It also contains organic materials, such as corn, seeds, and coffee grinds. If not removed from wastewater, grit can wear out pump impellers and piping. Grit is typically removed in grit chambers. In some treatment plants, grit is settled in primary clarifiers along with primary sludge and then separated from the sludge in vortex-type grit separators. The volume of grit removed varies from 4 to 200 mL/m3 (0.5 to 27 ft3/MG) of wastewater. The higher quantities are typical of municipalities with combined sewer systems and sewers that contribute excessive infiltration and inflow. Grit is almost always landfilled.

Scum is the product that is skimmed from clarifiers. Primary scum consists of fats, oils, grease, and floating debris such as plastic and rubber products. It can build up in piping, thereby restricting flow and increasing pumping costs, and can foul probes, flow elements, and other instruments in the waste stream. Secondary scum tends to be mostly floating activated sludge or biofilm, depending on the type of secondary treatment used. The quantity and moisture content of scum typically are not measured. It may be disposed of by pumping to sludge digesters, concentrating, and then incinerating with other residuals, or drying and then landfilling.

2.2 SLUDGE QUANTITY

Determining the quantity of sludge produced in the treatment of wastewater is required for the sizing of sludge processing units and equipment such as sludge pumps, storage tanks, thickeners, digesters, and incinerators. The best approach in estimating solids production is to base the estimate on historical data from similar facilities and the anticipated influent strength. Generally, solids production rates range between 0.2 and 0.3 kg/m3 (0.8 to 1.2 dry tons/ MG) of wastewater treated. In the absence of historic or plant-specific data, a rule-of-thumb approximation for solids produced in a typical wastewater treatment plant is 0.24 kg/m3 (1 dry ton/MG) of wastewater treated (WEF, 1998). It is also important to take into account the effects of industrial contribution, stormwater flows, and temperature on the influent wastewater characteristics and therefore on solids production.

2.2.1 Primary Sludge

Primary sludge solids production can vary typically from 0.1 to 0.3 kg/m3 (800 to 2500 lb/MG) of wastewater. A rule-of-thumb approximation is 0.05 kg/ capita (0.12 lb/capita) per day of primary sludge solids production. The most common approach in estimating primary sludge production is by computing the quantity of suspended solids entering the treatment plant and assuming a removal rate. The removal rate is usually in the range 50 to 65%. A removal rate of 60% is commonly used for estimating purposes, provided that the effects of industrial contribution are minimal and no major sidestreams from the sludge processing units are discharged to the primary clarifier influent.

Total solids in wastewater comprise dissolved solids + suspended solids (settleable and colloidal) + floatable solids (scum). The solids removal rate can be correlated to either the hydraulic detention time or the surface overflow rate of the primary clarifier. The following equations (Koch et al., 1990) demonstrate how solids production relates to detention time:

solids production = plant flow x influent suspended solids x removal rate (2.1)

(a + bT ) v y where removal rate = removal of suspended solids, % T = detention time, minutes a = constant, 0.406 minute b = constant, 0.0152

The relation of the solids removal rate to the surface overflow rate is presented in Figure 2.2. The figure also shows the BOD removal rate. The typical BOD removal rate is 50% of suspended solids removed. Several factors

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Surface Overflow Rate (meters/day) (m/d x 24.55 = gpd/sft)

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Surface Overflow Rate (meters/day) (m/d x 24.55 = gpd/sft)

Figure 2.2 Relationship between surface overflow rate and solids removal in primary treatment.

Duration of Peak Load, days

Figure 2.3 Relationship between peak solids loading and duration of peak load. (From U.S. EPA, 1979.)

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can affect suspended solids removal efficiency, chief among them being industrial contribution, sludge treatment process sidestreams, and mechanical factors of primary settling tanks, such as poor flow distribution and density currents.

Daily variations in primary sludge production can occur, usually in proportion to the quantity of solids entering a wastewater treatment plant. Peak rates of sludge production can be several times the average. Figure 2.3 shows the relationship between peak suspended solids entering a plant and the duration of time that the peak persists, which is based on a study of several large wastewater treatment plants. This variation in daily quantity is appropriate for large cities that have large areas with combined sewers on flat grades. The peaks occur when the solids deposited in the combined sewers are resuspended and carried to the treatment plant by storm flows.

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