Urban wastewater treatment at ambient conditions using microalgae-bacteria consortia in a membrane high-rate algal pond (MHRAP): The effect of hydraulic retention time and influent strength

The effects of hydraulic retention time (HRT) and influent wastewater strength on microalgae-bacteria performance were studied in two sets of experiments in a 6-day biomass retention time membrane high-rate algal pond (MHRAP) pilot plant to assess its pollutants removal potential. In the first set of experiments, the MHRAP was continuously fed with effluent from primary settling and operated at 6-, 4- and 2-days HRT and in the second set, the MHRAP was operated at 4-, 2-, and 1-day HRT using pre-treatment effluent. Results showed a complex interaction between algae-bacteria consortium, ambient conditions and MHRAP operation. In set 1, the highest total nitrogen (T-N) and total phosphorus (T-P) removal efficiencies were reached with 6-day HRT (23.6±0.6% and 32±4%, respectively), partially due to free ammonia nitrogen stripping and phosphorus precipitation, because of abiotic factors, which reduces the activity of nitrite-oxidizing bacteria and led to partial-nitrification. On the other hand, the 4 and 2-day HRT T-N and T-P removal efficiencies were reduced (below 10% and 5%, respectively) because the abiotic factors limited the physicochemical processes, and less biomass was taken up. In set 2, a suitable equilibrium between the microalgae and bacterial communities at 2-days HRT achieved high T-N (91±5%) and T-P (71±8%) removal efficiencies. Mass balances indicated that the nitrogen was mostly removed by nitrification-denitrification (87% of T-N) and T-P predominant removal mechanism was precipitation. The COD removal efficiencies were higher than 86% in both sets, since the particulate COD and colloids were removed by ultrafiltration. The obtained results showed that MHRAP technology would allow reducing COD and nutrient loads in the effluent under certain operating conditions, demonstrating the potential for resource recovery and wastewater treatment of microalgae-bacteria consortia. • The performance of a membrane high-rate algal pond pilot plant is evaluated. • Primary settling effluent treatment is hindered by its lack of BOD for denitrification. • Treatment of pre-treated wastewater at 2-day HRT accomplished discharge limits. • Mass balances indicated that N is mostly removed by nitrification-denitrification. • Mass balances indicated that P is mostly removed precipitation. • Using ultrafiltration membranes allowed minimum COD removal efficiencies of 86%.