ROLE OF LIQUID RECIRCULATION AT BIOLOGICAL WASTEWATER TREATMENT PLANTS

Introduction. At biological wastewater treatment plants recirculation of purified liquid or activated sludge is used to reduce the high initial concentration of contaminants, improve the performance of the plants and the transportation of returned activated sludge in aeration tanks, as well as for other purposes. The use of recirculation or recirculation ratio increase at biological wastewater treatment plants leads (together with the dilution of untreated wastewater with treated liquid) to an increase in the hydraulic load in the plants as well as desludging facilities, changes the structure of liquid flow at the plants, and affects their performance. Thus, liquid recirculation under certain conditions can lead to both positive (improvement of wastewater treatment efficiency) and negative consequences (increase in volume regarding final clarifiers and growth of energy consumption for recirculation flow pumping). Methods. The purpose of the study was to assess the integrated effect of liquid recirculation on the performance of biological wastewater treatment plants of various types. The assessment was carried out based on the analysis of data on the performance of production facilities as well as standards, specifications, and guidelines. Results. The paper evaluates the role of recirculation during the operation of wet biological filters and aeration tanks with external and internal (longitudinal) recirculation. The author considers the influence of recirculation in wet biofilters on essential technological parameters of their work, conditions of mass transfer processes, and wastewater treatment efficiency, as well as the influence of the mixed liquor and returned sludge recirculation rate in aeration tanks on the amount of the dose and wastewater treatment efficiency. Conclusion. The recirculation ratio at biological treatment plants should be linked to the achieved wastewater treatment efficiency and based on technical and economic estimates. An increase in the recirculation rate is more efficient at a low degree of wastewater treatment and not economically feasible in case of sufficiently high treatment efficiency.