Effects of Conventional Treatment, Tertiary Treatment and Disinfection Processes on Hygienic and Physico-Chemical Quality of Municipal Wastewaters

Conventional wastewater treatment, primary and secondary treatment processes with simultaneous phosphorus precipitation, is the most common process combination used in Finnish wastewater treatment plants (WWTPs). It typically eliminates most of the organic load and phosphorus, as well as part of the enteric microorganisms present in raw wastewater. Municipal secondary effluents still contain some organic matter and nutrients, which cause eutrophication and increase the oxygen demand on the natural waters. The presence of enteric microorganisms in the wastewater discharges decreases the hygienic quality of natural waters. Over-loading situations of WWTP can significantly decrease the efficiency of wastewater treatment and even force WWTP to by-pass untreated wastewaters directly into natural waters, causing adverse environmental effects. Conventionally treated wastewater may not meet the authority requirements set for wastewater discharges or wastewater reuse, especially in the future, as the regulation of wastewater discharges may become stricter in many locations. The efficiency of wastewater treatment can be improved by tertiary treatment and disinfection processes. The aim of this study was to evaluate the efficiency of different wastewater treatment processes on the removal of enteric microorganisms, phosphorus and organic matter from municipal wastewaters. The treatment efficiency of conventional biological-chemical wastewater treatment processes was studied in four Finnish municipal WWTPs. The effect of tertiary rapid sand filtration (RSF) and dissolved air flotation (DAF) processes, as well as chemical and biological-chemical contact filtration processes, on wastewater quality was studied in pilot-scale experiments. The applicability of the DAF process for treatment of primary wastewater effluents was also studied in pilot-scale experiments to assess the applicability of the process for treatment of WWTP by-passes. Some experiments were carried out in two full-scale tertiary DAF plants. The disinfection efficiencies of peracetic acid (PAA), hydrogen peroxide (H2O2), sodium hypochlorite (NaClO) and ultraviolet (UV) disinfection treatments as well as the synergistic effects of combined use of chemical disinfectant and UV were investigated in laboratory-scale experiments, followed by pilot-scale PAA disinfection experiments of municipal primary, secondary and tertiary effluents. Primary and secondary wastewater treatment with simultaneous phosphorus precipitation achieved around 95 % reductions of organic matter and phosphorus from the municipal wastewaters. The numbers of enteric microorganisms were typically reduced by between 90 and 99.9 %, but the secondary effluents still contained high microbial numbers, including pathogenic salmonellae. The tertiary RSF or DAF processes efficiently removed residual organic matter and phosphorus, and removed 90-99 % of enteric microorganisms from the secondary effluents. Increasing the coagulant dose (from 2 to 10 gAl/m) and the dispersion water recycle ratio (from 11 to 22 %) improved the purification results, whereas changing the flocculation conditions (G-value, retention time) or increasing the hydraulic surface load (from 5 m/h to 10 m/h) did not clearly affect the tertiary DAF process efficiency. The DAF process achieved significant reductions of enteric microorganisms, phosphorus and organic matter in the treatment of primary wastewater effluents, demonstrating that the process can tolerate high loads of suspended solids and could be used for the treatment of WWTP by-pass wastewaters during the WWTP over-loading situations. Peracetic acid was demonstrated to be an efficient disinfectant against enteric microorganisms in municipal primary, secondary and tertiary wastewater effluents. The combined PAA/UV treatments showed high disinfection efficiency and synergy benefits, while hydrogen peroxide and sodium hypochlorite showed low efficiencies in laboratory-scale disinfection experiments with organic matter rich synthetic wastewater. The results of the present study suggest that the combination of PAA and UV disinfection could increase the efficiency and reliability of wastewater disinfection processes. Universal Decimal Classification: 628.315.23, 628.315.3, 628.316.6, 628.345, 628.354 National Library of Medicine Classification: WA 785 CAB Thesaurus: waste water; waste water treatment; biological treatment; chemical treatment; disinfection; peracetic acid; sodium hypochlorite; hydrogen peroxide; ultraviolet radiation; synergism; filtration; flotation; coagulation; flocculation; faecal coliforms; Enterococcaceae; bacteriophages; Salmonella; phosphorus; quality