Slaughterhouse wastewater treatment using purple phototrophic bacteria: A comparison between photoheterotrophic and chemoheterotrophic conditions

This study aimed to evaluate the effectiveness of purple phototrophic bacteria (PPB) biomass, dominated by Rhodopseudomonas, for treating slaughterhouse wastewater (SWW) under photoheterotrophic and chemoheterotrophic conditions in batch bioreactors. Under both the photoheterotrophic and chemoheterotrophic conditions, PPB hydrolyzed SWW quickly, with both carbon (soluble COD: sCOD) and nutrients (soluble total nitrogen: sTN and phosphate: PO43-) reaching maximum concentrations within the first day. Active assimilation caused simultaneous decrease in carbon and nutrient concentrations within the first 8 days under the photoheterotrophic condition, resulting in high removal of sCOD (91.9%), sTN (70.1%), and PO43- (90.9%). Although removal rate constants (k) were almost five-fold lower than those under the photoheterotrophic condition, we observed substantial removal of sCOD (88.4%) and PO43- (78.2%) under the chemoheterotrophic condition, indicating the activation of versatile metabolic pathways (e.g. respiration and phosphorus accumulation). Biomass carbohydrate and protein content on the last day of the photoheterotrophic condition increased by 50% and 76%, respectively, compared with inoculated PPB biomass; the chemoheterotrophic condition saw 13% and 36% increases. Our results confirmed the effectiveness of the photoheterotrophic pathway in SWW treatment. The bacteria were also able to remove carbon and phosphorus from SWW during the prolonged dark period.