Evaluating Biological Treatment Systems: (i) Moving Bed Biofilm Reactor versus Biological Aerated Filtration, and (ii) Sulfide-Induced corrosion in Anaerobic Digester Gas Piping
The research presented in this report is in two sections. Section I involved the performance of a moving bed biofilm reactor (MBBR) versus a biological aerated filtration (BAF) and Section II involved study on causes of deposition in anaerobic digester gas piping.
The first section evaluated and compared the performance of a laboratory-scale MBBR and BAF for organic carbon and suspended solids removal. A kinetic study was also performed on the MBBR to evaluate the system performance. The purpose was to recommend one of the systems for the Force Provider project, which provides a containerized city for the U.S. Army. The effluent criteria against which the systems were evaluated were total 5-day biochemical oxygen demand (TBOD5) and total suspended solids (TSS) of 30 mg/L each. The report is based on a 5-month laboratory -scale study of the two reactors.
The MBBR performance depended on the percent of media provided in the reactor and the organic loading. At a media volume, which displaced the reactor volume by 40 % (heretofore called 40 % media volume), and surface area loading rate (SALR) of 20 g BOD5/m2-d, the system performance deteriorated with time. At 40 % media volume and SALR below 15 g BOD5/m2-d, the system performance improved but still did not meet effluent criteria or average. TBOD5 reduction was generally poor (approximately 50 %). Soluble BOD5 (SBOD5) concentrations were frequently below 30 mg/L and TSS concentrations were often higher than influent TSS. Overall, TSS wastage from the system (both effluent TSS and intentional wastage) averaged 0.032 kg/d.
BAF system performance was excellent for TBOD5, CBOD5, SBOD5 and TSS removal, and were consistently less that 30 mg/L. Overall TSS wastage from the BAF (both via effluent and backwash) average 0.027 kg/d and was 16 % less than for the MBBR. Based on demonstrated performance, the BAF was the only viable reactor for the project.
Section II of the report focused on possible causes of deposition in an anaerobic digester gas piping at a local wastewater treatment facility (Peppers ferry regional wastewater treatment facility).
Industrial waste input to the treatment facility has increased lately and accounts for 40 % of the plants wastewater inflow. An industry in Pulaski, VA, Magnox Inc. generates and disposes highly concentrated sodium sulfate, (70,000 mg/L) which is a by-product of its activities, to PFRWTF wastewater influent stream. As a result of Magnox industrial waste input, a pilot study was carried out to determine the effect of its waste on the activated sludge treatment units. Results indicated that Magnox industrial waste input would not have adverse effect on the aeration basins. However production of H2S, which can have effect on the anaerobic digester was reported (Olver Inc., 1995). Field analysis of data reported by Olver Inc. (2000) showed that H2S concentration in PFRWTF anaerobic digester gas was rising. X-ray photoelectron spectroscopy analysis of deposits found in the digester pipe together with results obtained from the laboratory-scale study revealed that iron and sulfur played a role in the deposition in the digester gas pipe. The laboratory scale study revealed that ferrous ion in the digester feed possibly precipitated over 90 % of the hydrogen sulfide gas produced in the digester, thus protecting the digester from adverse effects caused by hydrogen sulfide.