Comparative evaluation of continuous piggery wastewater treatment in open and closed purple phototrophic bacteria-based photobioreactors

Abstract

Purple phototrophic bacteria (PPB) represent an innovative approach for wastewater treatment with a high metabolic plasticity, able to grow under aerobic and anaerobic conditions. This study comparatively assessed the long-term performance (450 days of operation) of an open and closed PPB-based photobioreactor treating of piggery wastewater (PWW). The influence of wastewater dilution, illuminated area to volume ratio, biomass settling and recirculation, and infrared light intensity on wastewater treatment was evaluated at 7 days of hydraulic retention time. An increase in PWW dilution from 4 to 8 folds did not entail higher TOC removal efficiencies (REs) in the open photobioreactor (87% versus 89%), but a significant increase in the closed photobioreactor (from 73% to 80%). The increase in the illuminated area to volume ratio increased TN-REs up to 99% and 49% in the open and closed photobioreactor, respectively, with a concomitant increase in the temperature of both systems. However, temperature control did not mediate a significant enhancement in PWW treatment. Biomass settling and recirculation resulted in higher TN-REs (80%) and TOC-REs (90%) in the closed photobioreactor. The increase in infrared radiation from 100 to 300 W m-2 fostered PPB growth. High water evaporation losses (deteriorating effluent quality) were recorded in the open photobioreactor, where carbon dioxide and ammonia stripping were identified as the main pathways supporting carbon and nitrogen removal.