Potentials of using a rotating biological contactor (RBC) for post-treatment of anaerobically pre-treated domestic wastewater

Abstract

This study has been carried out in order to assess the impact of COD biod. in an UASB effluent applied to a single stage, a two stage RBC system and an anoxic up-flow submerged bio-filter followed by a segmental two stage aerobic RBC on the removal efficiency of different COD fractions, Escherichia coli, ammonia and partially nitrate removal. The two (single stage) RBC's were operated at a constant HRT of 2.5 h and temperature of 21 °C but at different OLR's, viz. of 10 and 14 g COD biod./m 2 day due to the highly different UASB effluent qualities. The results clearly show that the residual values of COD total, ammonia and E. coli in the final effluent are significantly lower at the lower imposed OLR of 10 g COD biod./m 2 day. In view of the results obtained we recommend to use a single stage RBC system at OLR of 10 g COD biod./m 2 day and at HRT of 2.5 h for post-treatment of the effluent of UASB reactor operated at high temperature of 30 °C as generally prevails in tropical countries. The performance of a single stage versus two stage RBC system for post-treatment of the effluent of an UASB reactor operated at a low temperature of 12 °C has been evaluated. The single stage and a two stage RBC system were operated at the same OLR of 18 g COD biod./m 2 day and at HRT of 2.5 h. The results show that the COD fractions, ammonia and E. coli content in the final effluent of a two stage RBC system were significantly lower than the effluent of the single stage RBC system. Accordingly, we recommend a two stage RBC system at an HRT of 2.5 h and OLR of 18 g COD biod./m 2 day for post-treatment of the effluent of a conventional UASB reactor operating at a low temperature of 12 °C. The nitrogen removal from the nitrified effluent was investigated using a biofilm system consisting of three stages, viz. an anoxic up-flow submerged bio-filter followed by a segmental two stage aerobic RBC. The nitrified effluent of the second stage RBC was recycled to the anoxic up-flow submerged bio-filter reactor. The results obtained reveal that the introduction of an anoxic reactor as a first stage combined with recirculation of the nitrified effluent of the second stage RBC is accompanied with a conversion of nitrate into ammonia, at least in case the content of COD biod. in the UASB effluent is low. In such a situation the ammonia needs to be nitrified two times, which obviously should be avoided. Therefore in such situations of a too high quality anaerobic effluent in terms of biodegradable COD content, the introduction of a separate anoxic reactor for denitrification as final post-treatment step can not be recommended.