PRIMARY EFFLUENT: CHLORINE OR UV RADIATION DISINFECTION?

Disinfection needs are based on receiving water requirements, with inland freshwater bodies typically having a 200 fecal coliform per 100 ml criteria and marine waters having a 35 enterococci per 100 ml criteria. These impacts can dilute the treated wastewater by several hundred to several thousand times signifying that the fecal indicator density leaving the facility can be several orders of magnitude greater than the receiving water criteria. This than can be translated into a two or two and a half log reduction for primary treated wastewater discharged through an ocean outfall.When disinfection is being considered for primary effluent discharging through a ocean outfall, which disinfectant will achieve the targeted fecal indicator density while maintaining process controllability. Keeping this in mind, there are two fundamental wastewater disinfection methods commonly used today: chemical (chlorine, halogen compounds, and ozone) and physical (ultraviolet radiation – UV). All disinfection methods have various factors influencing their effectiveness (WERF 1995). One important water quality parameter relative to disinfection dose (concentration/intensity and time) and organism survival is wastewater particles (suspended solids – both mass and size distribution) that reduce disinfectant (chemical or physical) effectiveness by interfering with the disinfection mechanism or by microbe shielding (organism entrapped in a particle). Chlorine is still a very commonly used disinfectant and UV is increasing in its use.An additional parameter (salt water infiltration) influenced the results of this study. Morning and afternoon testing showed high variation in the chlorine disinfection, while this impact on the UV disinfection was minimal. Bench-scale chlorine disinfection evaluation, chlorine demand evaluation, collimated-beam UV disinfection evaluation, and diurnal characteristics were performed for the primary effluent at the Sand Island Wastewater Treatment Plant. Chlorine work analyzed chlorine residual, and ORP along with enterococci and fecal coliform. UV work analyzed UVT254 along with enterococci and fecal coliform. Diurnal characteristics analysis included conductivity, total and volatile suspended solids, turbidity, UVT254, total and soluble BOD5, Clostridium perfringens, enterococci, fecal coliform, and FRNA bacteriophage. Not all of the diurnal characteristics are illustrated in this paper.