For over 150 years, slow sand filters have been an effective means of treating water for control of microbiological contaminants. Slow sand filters do not need constant operator attention, making them an appropriate technology for water systems that are small or that employ part-time operators. During the 1970s through the 1990s, research and field evaluations of slow sand filtration have demonstrated its efficacy for control of microbiological contaminants that were unknown in the 1800s. In addition, pretreatment processes such as roughing filters and pre-ozonation have been developed or adapted for use with slow sand filters, increasing the range of source waters that can be treated and the number of contaminants that can be removed in slow sand filters. Inclusion of a layer of granular-activated carbon in a slow sand filter bed has improved capability for control of synthetic organic chemicals. This paper reviews design concepts and process capabilities for slow sand filters and discusses recent innovations in slow sand filter design that now enable this technology to be applied more widely than would have been appropriate two decades ago. Key words: slow sand filter, design, operation and maintenance, microbiological contaminants, small systems, pretreatment.

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Careful planning can increase the likelihood that a pilot study will lead to reliable conclusions and recommendations. Pilot‐plant studies are often considered appropriate or necessary for effective planning, design, and operation of water treatment facilities. By carefully planning pilot‐plant investigations before they are undertaken and by diligently conducting the study, water utility staff and engineers can maximize the return on the resources invested in pilot‐plant programs. This article contains ideas and concepts on pilot‐plant testing that have been developed through two decades of work with pilot plants.

INTRODUCTIONOn-site sodium hypochlorite generation has been a proven technology since the 1930's. This process uses salt and electric power to generate chlorine. Salt is dissolved in a brine solution then passed across electrodes powered by a low voltage current. This process produces a dilute hypochlorite of 0.8% in solution. On-site hypochlorite generation requires the construction of a brine tank, rectifier, electrolytic cells, a product tank, metering pumps and controls. Recent improvements in system reliability combined with new regulatory requirements are forcing water and wastewater utilities to reconsider their reliance on chlorine gas. This paper summarizes two economic analyses of disinfection system alternatives that have been recently conducted.In order to with comply risk management and process safety requirements, several water treatment facilities have installed onsite generation. It is receiving increased interest for use replacing chlorine gas at remote or satellite locations in potable water distribution systems and in the pre-chlorination of source water from reservoirs. Wastewater treatment facilities have, for the most part, not currently adopted the use of onsite generation of hypochlorite. Future applications, such as break-point chlorination for ammonia control, disinfection of backwash water, and chlorination of nitrifying bacteria, are very promising. BACKGROUND