Effect of natural increase of pH and microalgae cyclical re-cultivation on biomass production and polishing of municipal secondary effluent

Abstract

A constant source of nutrient pollution that causes eutrophication is municipal wastewater treatment plant (WWTP) secondary effluent. This paper investigates the natural increase of pH, nutrient removal and biomass production from a cyclical re-cultivation system designed to favor the adaptation of microalgae consortia to WWTP secondary effluent. An outdoor experiment was performed with the microalgae consortia MC-10 under 3 sequential batch cycles. The natural pH increase (range 9–10) in the WWTP secondary effluent was established as an operation parameter because it certainly influenced microalgae biomass production (by maintenance and bioavailability of C and P), water treatment and pathogen removal. The increase of pH (9.7, 9.8 and 9.9) during the culture cycles of MC-10 probably generated conditions for fecal bacterial removal below detectable values (i.e., < 2 MPN/100 mL). The treatment of WWTP secondary effluent after batch cycles (cycles 1 to 3) using MC-10 indicated improvements in biomass productivity (66 to 167%) and a great potential for the reduction of total alkalinity (13.1 to 50.3%), NO3–-N (30.9 to 53.6%), PO43–- P (90.4 to 93.0%) and electrical conductivity (EC) (8.5 to 18.1%). Additionally, the harvested biomass showed a high mineral concentration (46.5%) suggesting its potential use as a bio-fertilizer.