Blended Wastewater, Disinfection
Flow to publicly owned treatment works (POTWs) increases during wet weather events, due to intrusion of storm water into sanitary collection systems and use of combined sewer systems. When such flows exceed the hydraulic capacity of a POTW alternate flow management techniques must be employed to ensure that the integrity of the
POTWs unit processes are not compromised. One technique is to treat influent flows through primary clarification and limit the flow to the secondary treatment units to design limits. Excess flow from the primary clarifiers, which by-passes secondary treatment, is then recombined or “blended” with secondary effluents and disinfected. During the
blending process particulate matter within primary effluent can reduce the effectiveness of disinfection. It is possible that the size of these particles may be related to the degree of protection observed for microbes during the disinfection step. In addition, ammonia and other related organic compounds in blended waste waters will react with chlorine producing chloramines, and other combined chlorine species which reduce the disinfectant's effectiveness for killing or inactivating pathogens. The objectives of this work are to determine the effectiveness of chlorine disinfection on a variety of microbial indicators within blended effluents, and to compare the disinfection results with specific water quality parameters for the purpose of developing a model for chlorine disinfection of blended effluents.
Materials and Methods
Samples of primary and undisinfected secondary effluents were collected from a local conventional activated sludge wastewater treatment plant (WWTP). These effluents were then mixed thoroughly but gently at three different ratios, 1:9, 3:7, and 5:5, to produce blended effluents. Samples of each of these blended products were assayed for indicator
organisms (fecal coliforms, E. coli, enterococci, total heterotrophs and somatic and male specific coliphage), suspended solids, turbidity, total organic carbon, ammonia, redox potential, pH and temperature (1, 2, 3). Blended effluents were then mixed with a dilute sodium hypochlorite solution to a final total chlorine concentration of approximately 2.0
mg/L and sampled after 30 minutes of contact time. Sodium thiosulfate was then added to the sterile sample vessel to immediately quench chlorine oxidation. Samples were then assayed for microorganisms as described above.