Evaluation of Ballasted Sand Flocculation (BSF) and UV-Disinfection Technologies for Combined Sewer Overflows (CSOs)
Combining High Rate Clarification (HRC) and Ultra Violet (UV) Disinfection has the potential to provide wastewater effluent quality that meets regulatory requirements at significantly lower costs and with fewer by-products than conventional treatment. In order to achieve maximum performance in an economical way, a study was performed to investigate the efficiency of this process in benchscale studies. This was achieved through testing the use of ballasted sand flocculation (BSF) and UV disinfection technologies in treating combined sewer overflows (CSOs), as well as regular wastewater during dry events. The performance of BSF and UV Technologies were studied using different types and concentrations of coagulants under varying influent water compositions (obtained from Bay View Wastewater Treatment Plant, Toledo) for both dry weather (wastewater during periods when there are no storm events) and wet weather (includes stormwater along with regular wastewater) conditions. The coagulants tested were ferric chloride, ferrous chloride, aluminum chlorohydrate (ACH) and aluminum sulphate (Alum). It was found that for both dry and wet weather treatments, alum was the best coagulant. Alum, when used with addition of Microsand (which is a ballasting agent that increases the weight of the formed flocs and makes them settle at rapid rate), was the best for wet weather treatment, while, for dry weather treatment, alum alone was the best among all the coagulants. Even though alum proved to be the most effective coagulant, due to its handling and disposal complications such as its storage, making up solutions, its use is often avoided in preference to iron – based coagulants. Also iron coagulants can be used over a wider range of pH as compared to alum. However, these have been previously determined to foul on UV lamps, which decrease disinfection efficiency. These try to accumulate on the lamps and form a layer around them, thus blocking the UV light from reaching the target. Also, they are efficient absorbers of the UV radiation thereby decreasing the disinfection efficiency. Thus, additional studies were completed using ferric chloride and ferrous chloride to investigate their tendencies to foul on the UV lamps. These studies determined that concentrations lower than 100 mg/l did not cause much fouling on the UV lamp, while, 50 mg/l ferrous chloride provided the best performance among the coagulants tested. The use of Microsand should be avoided for treatment in dry weather conditions while, for wet weather, alum with the Microsand should be employed to efficiently remove the solids, turbidity, UV-254 and nitrate. The use of BSF technology for CSOs would definitely prove better owing to its advantages such as rapid startup times, smaller retention periods, rapid settling of flocs, and highly clarified final effluent.
School:University of Toledo
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:combined sewer overflows ballasted sand flocculation bsf actiflo process uv disinfection wastewater treatment
Date of Publication:01/01/2004