Abstract
Concern regarding the potential penetration of water treatment systems by pathogens such as Cryptosporidium parvum oocysts has highlighted problems with traditional (chemical) treatment systems. Slow sand filters are considered to be a robust option for water treatment both within industrialised and developing countries. The minimal use of chemicals (tertiary disinfection recommended) during the treatment process also makes the system an environmentally sound alternative. Although, historically, slow sand filters have been shown to be robust barriers to the penetration of pathogens, the mechanisms which bring about water treatment within the beds are poorly understood. Factors which influence the particle capture performance of biological water treatment systems are examined. The influence of operational practices and influent water characteristics on particle capture are investigated through pilot scale work and through surveys carried out on full-scale works in developing world countries. The pilot plants studied incorporated up-flow gravel prefilters and fabric enhanced slow sand filters. A novel gravel prefilter design, which incorporated an underdrain cavity to enhance the cleaning process, was developed and tested at pilot scale. A detailed investigation was carried out to further the understanding of organism and filter characteristics on particle capture. Different suspensions of an alga (Chlorella vulgaris) and a bacterium (Bacillus globigii var niger) were dosed onto sand beds and their influence on bed properties examined. This work demonstrates the influences organism and water properties have on plant performance. The biological maturity of a treatment system was highlighted as being of major importance. Diurnal particle fluctuations within the pilot beds were observed, and may have resulted from low nutrient conditions. Such characteristics have not been reported prior to this work. Traditional filtration theory and biofilm theory are reviewed. The relevance of filtration theory to biological filtration is questioned. Consideration should be given to the influence and interaction of physical, chemical and biological mechanisms on particle capture.