Process Design Considerations

Factors that govern the process design of conventional aerobic digesters include feed sludge characteristics, temperature, volatile solids reduction, oxygen requirements, and mixing. Other system design considerations and operational considerations are discussed later in this chapter.

Feed Sludge Characteristics Because the aerobic digestion process is similar to the activated sludge process, the same concerns, such as variations in influent characteristics and materials that are toxic to biological activities, are important. The aerobic digestion process is best suited for stabilizing biological solids such as WAS, because as discussed earlier, the process keeps the system predominantly in the endogenous respiration phase. If a mixture of primary sludge and biological sludge is digested, a longer detention time is required to oxidize the excess organic matter in primary sludge before the endogenous respiration can be achieved.

The concentration of solids in feed sludge to an aerobic digester is important in the design and operation of a digester. Advantages of higher feed solids concentration include longer SRTs, smaller digester volume requirements, easier process control (less or no decanting in batch-operated systems), and increased levels of volatile solids destruction. However, higher solids concentrations require higher oxygen input levels per digester volume. When the feed solids concentration is greater than 3%, care should be taken in designing the aeration and mixing system such that the system keeps the tank contents well mixed with adequate dissolved oxygen levels necessary for the digestion process.

Temperature The liquid temperatures in open-tank digesters depend on weather conditions and can fluctuate extensively. A major disadvantage of the aerobic digestion process is the variation in process efficiency as a result of the changes in operating temperatures. As with all biological systems, lower temperatures retard the process whereas higher temperatures speed it up. Because aerobic digestion is a biological process, the effects of temperature variations can be estimated by the equation

Temperature of Liquid in Aerobic Digestor, °C

Figure 4.4 Reaction rate Kd versus digester liquid temperature.

Temperature of Liquid in Aerobic Digestor, °C

Figure 4.4 Reaction rate Kd versus digester liquid temperature.

where

Kd = reaction rate constant, time

(Kd)20 = reaction rate constant at 20°C

q = temperature coefficient (ranges from 1.02 to 1.10, with an average of 1.05)

Figure 4.4 represents the change in reaction rate constant versus increasing operating temperature. An increase in the temperature of the system results in an increase in the reaction rate constant and implies an increase in digestion rate. In designing a digester, consideration should be given to minimizing heat losses by using concrete instead of steel, placing the tank below rather than above grade, and using subsurface instead of surface aeration. In extremely cold climates, consideration should be given to covering the tanks, heating the sludge, or both. The design should allow for the necessary degree of sludge stabilization at the lowest liquid operating temperature and should meet the maximum oxygen requirements at the maximum expected liquid operating temperature.

Volatile Solids Reduction The major objectives of aerobic digestion of sludge are to stabilize sludge, reduce pathogens, and reduce the mass of solids for disposal. The reduction in mass is possible only with the destruction of the biodegradable organic content of the sludge, although some studies (Randall, 1975; Benefield, 1978) have shown that there may be some destruction of nonorganics as well. Volatile solids reductions of 35 to 50% are attainable by aerobic digestion. To obtain the vector attraction reduction requirement of 40 CFR Part 503, a minimum of 38% volatile solids reduction or less than a specific oxygen uptake rate (SOUR) of 1.5 mg of O2 per hour per gram of total sludge solids at 20°C has to be achieved.

The change in biodegradable volatile solids in a completely mixed aerobic digester can be represented by a first-order biochemical reaction as follows:

dt where dM = rate of change of biodegradable volatile solids (M) per unit of time dt (A mass/ time), MT-1 Kd = reaction rate constant, T -1

M = mass of biodegradable volatile solids remaining at time t in the aerobic digester

The time t in equation (4.9) is the sludge age or solids retention time (SRT) in the digester. Depending on how the aerobic digester is being operated, time t can be equal to or considerably greater than the theoretical hydraulic residence time. When there is no recycle and no decanting from the digester, SRT is equal to the hydraulic detention time. Use of biodegradable portion of the volatile solids in the equation recognizes that approximately 20 to 35% of the WAS from treatment plants with primary treatment systems, and 25 to 35% of the WAS from contact stabilization processes (no primary clarification), is nonbiodegradable.

The reaction rate constant Kd is a function of sludge type, temperature, and solids concentration. The reaction rate constant for WAS versus temperature may range from 0.06 d-1 at 15°C to 0.14 d-1 at 25°C, as represented in Figure 4.4. Because the reaction rate is influenced by several factors, it may be necessary to confirm decay coefficient values by bench- or pilot-scale studies.

Oxygen Requirements Oxygen requirements for aerobic digestion were discussed earlier in Section 4.1.1. Equation (4.2) indicates that, theoretically, 1.45 kg of oxygen is required to oxidize 1 kg (1.45 lb/lb) of cell mass with no nitrification. Similarly, according to equation (4.4), 1.98 kg of oxygen is theoretically required to oxidize 1 kg (1.98 lb/lb) of cell mass. The results of pilot-and full-scale studies indicate that actual oxygen requirement range from 1.74 to 2.07 kg of oxygen per kilogram of applied organic cell mass (1.74 to 2.07 lb/ lb). Design experience indicates that a value of 2.0 is recommended. In a system with complete nitrification-denitrification, as discussed earlier, 17% less oxygen is required. For autothermal systems, which have temperatures above 45°C, nitrification does not occur and a value of 1.45 kg/kg (1.45 lb/lb) is recommended. Field studies have indicated that a minimum of 1 mg/L of oxygen be maintained in the digester under all operating conditions.

The inclusion of primary sludge in the digestion process requires an additional 1.6 to 1.9 kg of oxygen per kilogram (1.6 to 1.9 lb/lb) of volatile solids destroyed to convert the organic matter in primary sludge to cell tissue and to satisfy the endogenous respiration demand of the resulting cell mass.

Mixing Mixing is required in an aerobic digester to keep the solids in suspension. After the requirements for adequate mixing and oxygen transfer have been computed separately, the larger of the two requirements will govern the overall system design. Power levels of 20 to 40 kW per 103 m3 (0.75 to 1.5 hp per 103 ft3) of tank volume have been reported to be satisfactory for mechanical mixers. In diffused air mixing, air supply rates of 1.2 to 2.4 m3/ m3-h (20 to 40 cfm per 103 ft3) have been reported; the higher values are recommended for sludges of high solids concentrations. If polymers are used in the prethickening process, especially for centrifugal thickening, a greater amount of unit energy may be required for mixing. In cases where the air-mixing requirement exceeds the oxygen transfer requirement (e.g., when high-efficiency fine-bubble diffusers are used for aeration) supplemental mechanical mixing should be considered rather than overdesigning the oxygen transfer system. The increased capital cost for supplemental mixing should be balanced against power costs for more aeration to determine the optimum configuration.

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