Performance of a combined biofilm-suspended growth system for wastewater treatment

Abstract

This paper summarizes the results obtained using the hybrid aerated submerged fixed-film (HASFF) reactor which utilizes suspended as well as attached biomass for the treatment of wastewater. The HASFF reactor consists of four compartments with an array of fixed ceramic plates maintained under diffused aeration to support attached biomass with suspended biomass recycle to promote suspended growth in the reactor. Wastewater from a municipal treatment plant was fed to the reactor and its activated sludge was used for recycling in the hybrid system. Four pilot units were operated in the plant at various HRTs (2, 4, 6 and 8 hrs) with flow rates in the range of 0.32 to 1.2 m3/d, using primary-settled wastewater under organic loading rates up to 0.7 g BOD/g BVS.d. The data obtained showed that the overall BOD removal efficiencies were consistently above 94.0% at all HRTs, while the COD removal efficiencies ranged between 65.7–76%. The effluents' mean filtered BOD concentrations were between 4.5–7.5 mg/l whereas the mean filtered COD concentrations ranged between 70.0–89.6 mg/l. Increasing the hydraulic loading rate four-fold from 0.08 to 0.32 m3/m2.d had minor effect on the reactor's BOD and COD removal efficiencies indicating a robust biological process that is resilient to flow variation, thereby offering a viable treatment option for small plants. The HASFF process achieved organic removal and nitrification in a single reactor, with minimal sludge production. The organic removal rate was equally high at all HRTs whereas nitrification was directly proportional to the HRT, being higher at longer HRTs. The suspended biomass activity exceeded that of the attached biomass at shorter HRTs while the opposite was true at longer HRTs.