Sludge/solid retention time

The removal rate of pharmaceuticals by WWTPs depends on factors such as reactor configuration, redox conditions, temperature, hydraulic retention time (HRT) and solid retention time (SRT). SRT is the average retention time of sludge... 

Melcer and Bedford (1988) studied the fate of pentachlorophenol in municipal activated sludge reactor systems that were operated at solids retention times of 10 to 20 d and hydraulic retention times of 120 d. Under these conditions, pentachlorophenol concentrations decreased from 0.1 and 12 mg/L to <10 pg/L. At solids retention times of 5 d or less, pentachlorophenol degradation was incomplete. 

Estimate the food-microorganism ratio (F/M) and sludge age (solid retention time) of an activated sludge process designed to reduce the BOD5 of the spent stream generated from a biological production plant from 1500 mg/L to 50 mg/L. The wastewater flow is Q = 15,000 GPD, aeration tank volume = 45 m, MLVSS = 3000 mg/L, and net biomass yield coefficient (Tn) = 0.28 kg/kg. Also compute the performance efficiency of the plant. 

Kim, S., P. Eichhom, J.N. Jensen, et al. 2005. Removal of antibiotics in wastewater Effect of hydraulic and solid retention times on the fate of tetracycline in the activated sludge process. Environ. Sci. Technol. 39 5816-5823. 

Tests must be performed throughout implementation in order to optimize performance of the system. For example, indications are that RAC-activated sludge operates more efficiently on longer residence times and the same solids retention time. Temperature also has a major effect on reactor performance. 

The inverse of the term on the left-hand side of Eq. (1) is known as solid retention time (SRT) or sludge age, 0 ... 

The process conditions used to design the plants are summarized in Table 7.3. For the conventional activated sludge system, typical values for North American design were selected by using most of the default values suggested by CapdetWorks. The same sludge age [solids retention time (SRT)] was used for CAS and MBR, but hydraulic retention times and mixed liquor concentrations were significantly different. 

Clarmer-Thickener Clarifiers can serve as thickeners, achieving additional densification in a deep sludge sump adjacent to the center that extends a shoi t distance radi ly and provides adequate retention time and pulp depth to compact the solids to a high density. Drive mechanisms on this type of clarifier usually must have higher torque capability than would be supplied on a standard clarifier. 

Depth tends to be determined from the retention time and the surface overflow rate. As surface overflow rates were reduced, the depth of sedimentation tanks was reduced to keep retention time from being excessive. It was recognized that depth was a valid design parameter and was more critical in some systems than retention time. As mixed-liquor suspended-solids (MESS) concentrations increase, the depth should also be increased. Minimum sedimentation-tank depths for variable operations should be 3.0 m (10 ft) with depths to 4.5 m (15 ft) if 3000 mg/L MESS concentrations are to be maintained under variable hydraulic conditions. With MESS concentrations above 4000 mg/L, the depth of the sedimentation tank should be increased to 6.0 m (20 ft). The key is to keep a definite freeboard over the settled-sludge blanket so that variable hydraulic flows do not lift the solids over the effluent weir. 

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