Role of microalgae-bacteria symbiosis in the disinfection and removal of emerging contaminants in domestic wastewater

Abstract

Population growth and industrial development put pressure on water resources by demanding large amounts of drinking water and requiring effluent treatment systems to be expanded in volume and treatment efficiency. The adequate treatment of effluents generated in different activities should maintain an adequate quality of surface waters, decreasing the cost of treatment for potabilization and reducing the impact of the presence of pathogens and emerging pollutants on the environment. The inadequate treatment and disposal of domestic effluents increases the risks of transmission of water-borne diseases, degrades surface and groundwater, and is considered a factor in the development, selection and distribution of antibiotic- and multidrug-resistant bacteria and genes. Emerging pollutants are a new class of pollutants that includes drugs, personal care products, hormones, preservatives and plasticizers, found in low concentrations (ng L-1 to mg L-1) in effluents and surface waters, and which can impact the environment and human health. Conventional wastewater treatment systems are not removing pathogens and emerging pollutants to a sufficient degree, either because of the high costs of more advanced processes necessary, or because of the generation of toxic by-products, more persistent and with greater risk to the environment and human health. Thus, the demand for the development of new technologies for the treatment of domestic effluents arises. The microalgae bacteria systems are one of the most promising options for the wastewater treatment, being able to remove nutrients, organic matter, pathogens and emerging pollutants, with sunlight as the main source of energy and producing biomass that can be used as fertilizer or substrate for biogas production. Among the most used configurations for treatment and bioremediation of effluents are the High Rate Algal Pond (HRAP) type reactors and variants, because they have low construction and maintenance costs, have a lower hydraulic retention time and less space requirements than conventional lagoons and are capable of performing primary, secondary and tertiary treatment simultaneously. The most recent development is the addition of an anoxic unit (AX) before the HRAP, a system denominated AX-HRAP, which has improved productivity, bioremediation capacity and system efficiency. In order to determine the capacity and mechanisms acting on the removal of pathogenic bacteria and emerging pollutants in microalgae bacteria systems, tests were conducted in batches and in pilot reactors. In the batch tests, the influence of biomass (microalgae and activated sludge) was evaluated, as well as the effect of addition of CO2 or O2, type of photoperiod and type of microalgae present, on the removal of different pathogenic bacteria (Escherichia coli, Pseudomonas aeruginosas, Clostridium perfringens, Staphylococcus sp. and Enterococcus sp.). The symbiosis microalga- bacteria offered the best condition for removal of these pathogenic bacteria. The type of microalgae present influenced significantly the efficiency of disinfection, and using a 24 h photoperiod, which would require constant artificial illumination, is not advantageous for disinfection. In pilot tests, the removal efficiency of pathogenic bacteria was tested under different hydraulic retention times, feeding regimes and CO2 supplementation, concluding that the addition of CO2 and different hydraulic detection times do not influence the removal of Escherichia coli, Pseudomonas aeruginosas, Clostridium perfringens, Staphylococcus sp. and Enterococcus sp,, but the feeding regime can negatively affect the removal of Escherichia coli, while shading can improve the removal of Staphylococcus sp. The main mechanism acting in the removal of these pathogenic bacteria are the production of toxins and antibiotic substances by the microalgae. On the other hand, the removal of emerging pollutants was evaluated in an AX-HRAP followed by an anaerobic system under different hydraulic detention times. Removal of these compounds ocurred, mainly by biodegradation, and with efficiencies of > 40% for all 16 emerging pollutants tested in the AX-HRAP reactor under all hydraulic retention times tested. Biomass production in the AX-HRAP system should permit for subsequential biogas (bioenergy) production. Thus, the microalgae bacteria system, in both HRAP and AX-HRAP configurations, proved viable for treatment of domestic effluents, achieving high efficiency in the removal of pathogenic bacteria and emerging pollutants, and can operate as a combined primary, secondary and tertiary system in waste water treatment plants (WWTPs).