Wastewater treatment in a dual biological system

Abstract

A new process for wastewater treatment has been developed by using the combination of CSTR and PBR. The system consisted of a circular tank (14 cm ID, 25 cm height) with a draft (8.8 cm ID, 20 cm height) concentrically mounted inside for the first stage and a packed column (8.8 cm ID, 41 cm height) for the second stage. As a support media for the formation of biomass layer, polypropylene Rashing rings (12.7 mm OD, 9.7 mm ID, 12.7 mm long) were used in PBR. The process was tested over a range of hydraulic and organic loadings typical of industrial scale operation. The process was operated various combinations of four retention times (4, 7, 10, 15 hour) and three organic concentrations (292,675,1300 mg/l COD) with synthetic sewage. Return-sludge of a local activated sludge plants (Chung Gye Chun) was employed as seeds for the process start up. It was found that the process was capable of function under organic loadings 1.6 to 1.9 kg COD/㎥/d and appeared similar or superior in performance to other biological wastewater treatment processes. Under the above mentioned loadings, the sludge yield was 0.12 to 0.31 g sludge/g COD removed and SVI of waste sludge was 26.2 to 36.6. Effluent COD was reduced to 29 (90.1% removal efficiency), 25 (96.3%), and 33 mg/l (97.5%) for organic loadings of 1.8, 1.6, 1.9 kg COD/㎥/d, respectively. Experimental results for polishing in PBR showed that approximately 60% of the SS and 70% of the COD were removed. A new critical equation relating the overall biochemical reaction rate in activated sludge process on activated sludge load was used for modeling. The concept used here was shown to be valid for conventional activated sludge loading range. The PBR, the second stage in the system, was modeled with the series of stirred tanks. Simple plug flow model could not adequately fit the experimental results. The axial mixing length was about 41 cm when the air flow rate was 13.15 cm/min and the liquid flow rate was 0.11 to 0.41 cm/m.