Solid and hydraulic retention times

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. 

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... 

Due to inadequate mixing by biogas, solid buildup at the bottom of the anaerobic pond poses another maintenance problem to the palm oil mills. Excessive solid buildup at the bottom of the ponds will reduce the effective digester capacity and consequently shorten the hydraulic retention time. Thus the treatment efficiency will be adversely affected. 

Biological solids retention time (Oc) has been suggested in this paper as the kinetic based parameter of choice for use in design and control of fiuidized culture continuous fiow biological processes. The value of Oc selected for design of the process, (Oc ), directly determines the volume of reactor needed for a conventional digester system and a given waste fiow (Q) since the value of Oc is equal to the hydraulic retention time (0). The relationship between Oc and the reactor volume for the system with recycle is more complex and involves consideration of the effects of solids recycle rate and recycle solids concentration. 

The concentrations of suspended solids and volatiles in the permeate are depicted in Figure 41.4, where it can be seen that levels stabilized with the same hydraulic retention time of 19 h, already considered as suitable. The concentrations reached were approximately 6500 mg/L for the suspended solids and 4600 mg/L for the volatile solids. 

High Rate This process is characterized by a shorter hydraulic retention time of the mixed liquor in the aeration tank and a higher loading rate than the conventional process. Consequently, the effluent leaving the process is not as high a quality as in the conventional process, i.e., it has a higher BOD and suspended solids. It often precedes a second-stage nitriflcation process. 

The hydraulic retention times (HRTs) of the activated sludge range from 4 hours to as high as 24 hours. With recirculation of sludge and wasting in the sedimentation tank, the suspended solids in the aeration basin are maintained at a concentration of 2,500 to 4,000 mg/L, while the sludge-retention times (SRTs) are between 10 and 30 days. 

The objective of this research was to assess the biotreatability of PW from Qatari gas fields during the March to November summer season under varying conditions of hydraulic retention time (HRT 16-32 hours), solids retention time (SRT 60-120 days) and temperature (22-38°C). Biotreatability was assessed based organics removal (% COD removal, % TOC removal) and biological activity (oxygen uptake rate, OUR). 

Factors affecting MBR performances and economics.TMP transmembrane pressure CF cross filtration SRT solids retention time HRT hydraulic retention time. 

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. 

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. 

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. 

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