Characterization of residual soluble Cod in entrapped-mixed-microbial-cell and activated sludge process

Abstract

To meet more strict standards than before for wastewater discharge and reuse criteria, a post-treatment for the secondarily treated effluent needs to be followed. This means not only additional cost, but a more complicated design and operation for a tertiary treatment is involved. Therefore, an evaluation of the characteristics, such as concentration, molecular weight distribution (MWD) and composition of residual soluble COD from a secondary treatment process is required. In this work, a conventional activated sludge process (ASP) and entrapped-mixed-microbial cell (EMMC) process (fluidized-bed and packed) were investigated. The investigation of the system performance shows: at the operation of 1 hr aeration/1 hr non-aeration with HRT 16.5 hr, all of the reactors had> 95% of COD removal and > 66% of nitrogen removal for treating high-strength synthetic wastewater (COD ~ 700 mg/l); at the operation of 1 hr aeration/1 hr non-aeration with HRT 6 hr, the EMMC reactors had> 93% COD removal and > 45% nitrogen removal for low strength synthetic wastewater (COD ~ 200 mg/l), while the ASP reactor had > 93% COD removal and 26.6% nitrogen removal at continuous aeration with HRT 6hr. Due to the longer SRT (solid retention time) for EMMC process, the MWD patterns and composition of the residual soluble COD (RSCOD) for the treated effluent of the EMMC are different from those of ASP when operated under the same HRT and aeration time. The study on the MWD shows the change of HRT, aeration and OLR cause different patterns for the RSCOD of the treated effluents from the same reactor. The experiment indicates that the effluent of the EMMC is more suitable to be treated physical-chemically than biologically in tertiary treatment, because it has more biologically stabilized components produced than that of the ASP process. Further biological oxygen demand for this stabilized effluent is apparently not required. The accomplishment of this work will provide a guideline for the requirement and design of a unit process for tertiary treatment.