EFFECT OF ORGANIC SHOCK LOADS ON ACTIVATED SLUDGE-BIOFILM REACTOR

Abstract

Activated Sludge-Biofilm process is an aerobic hybrid biological treatment process which combines suspended and attached growth processes, formed by inserting biofilm carriers in to the aeration tank of activated sludge process. Hence it has the advantages of both the suspended as well as attached growth systems, with a high treatment efficiency and nitrification obtained in relatively short HRT as compared to ASP. Designing the multi-stage biological reactors has some advantages over a single unit with the same volume. Municipal wastewater treatment plants are often subjected to short-term organic shock loads of the order of 2 to 4 times the normal loading rate. Literature survey indicates that no work has been done to evaluate the performance of AS-Biofilm reactor under shock loads. Hence the effect of short-term organic shock loads on a two-stage AS-Biofilm reactor treating medium strength domestic wastewater was studied using laboratory scale model. The reactor was operated at an HRT of 6h and organic shock loads were applied by increasing the influent COD concentration to 2-4 times the normal values, each shock load applied for a duration of 6h; after which normal loading conditions were resumed. The reactor performance was evaluated by monitoring the effluent COD, NH4+-N, NO3--N and pH each hour until the reactor attained PSS after removing the shock load. Also in order to study the effect of organic shock loads on the microbial characteristics of the reactor, samples were collected from mixed liquor as well as from biofilm in boili the reactors at PSS and at the end of shock load. The samples were inoculated on media and the isolated bacterial colonies were subjected to differential staining to determine the gram character, cell shape and cell arrangement of each colony. The PSS COD removal efficiency of the first reactor at 6h HRT was about 72-73 % while that of the second was about 12-15%, with an overall COD removal efficiency of the two-stage AS-Biofilm system of about 76%. The maximum overall effluent COD concentrations obtained were in the range of 170-250 mg/L under the shock loads of 808, 1170 and 1358 mg COD/L and that at a shock load of 1900 mg COD/L was 617 mg/L. No nitrification was observed in the system after the shock load application of 1170 mg COD/L and beyond. The performance of individual reactors (R1 and R2) and the whole system was expressed in terms of effective OLR at shock load conditions. It was found that the efficiency of R1, R2 as well as the whole system increases with the increase in effective OLR which iii implies that under the normal operating conditions the reactor was under loaded. The optimum loading condition for R1, R2 and the whole system has been found at effective OLR of 320, 180 and 170 mg COD/m3/h respectively. It was observed at all stages of the experiment that the R1 had more types of colonies in both the MLSS and fixed biomass as compared to R2. Shock loads changed the bacterial foims of the reactor from gram positive rods to gram negative cocco-bacilli. It was found that certain gram negative cocco-bacilli can withstand organic shock load conditions. It has been concluded that though the reactor regained its treatment efficiency quickly after removing the shock load, yet there was significant change in the biodiversity of the reactor. Overall, the AS-Biofilm system is a resilient system and recovers very fast from organic shock loads; recovery time proportional to the magnitude of shock loads.