Time, biological solids retention

S = effluent substrate concentration, mg/liter So = influent substrate concentration, mg/liter SRT = biological solids retention time, mg/liter... 

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. 

There is an apparent upper limit to the strength of waste a biological system ean handle in a liquid system. In some cases the constraint is oxygen transfer. In other eases it is the solids concentration in the mixed liquor and the shear whieh is neeessary to keep it in suspension. Recent work with membranes replaeing the elarifiers have indicated that the upper limit to solids is between 2% and 4% total solids, but not all of that is active biomass. The solids retention time in the system is often well over 40 days. The apparent upper limit on waste expressed as BOD is about 600mg/l -800 mg/1. Stronger wastes ean be treated, but they comprise a portion of the total flow and in effect are diluted. 

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. 

Figure 2. Steady-state relationships between solids retention time, effluent waste concentration, and specific treatment efficiency for completely mixed, continuous flow biological treatment processes...

In general, aeration time is handled in the solids retention time studies as suggested by Lawrence and McCarty. There are some indications that a combined activated carbon-biological process performs better on some wastewaters at a longer residence time. 

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). 

Cicek N, Macomber J, Davel J, Suidan M T, Audio J and Genestet P (2001), Effect of solids retention time on the performance and biological characteristics of membrane bioreactor , Water Sci Technol, 43,43-50. 

---