Between January 12 and February 7, 1987, an outbreak of gastroenteritis affected an estimated 13,000 people in a county of 64,900 residents in western Georgia. Cryptosporidium oocysts were identified in the stools of 58 of 147 patients with gastroenteritis (39 percent) tested during the outbreak. Studies for bacterial, viral, and other parasitic pathogens failed to implicate any other agent. In a random telephone survey, 299 of 489 household members exposed to the public water supply (61 percent) reported gastrointestinal illness, as compared with 64 of 322 (20 percent) who were not exposed (relative risk, 3.1; 95 percent confidence interval, 2.4 to 3.9). The prevalence of IgG to cryptosporidium was significantly higher among exposed respondents to the survey who had become ill than among nonresident controls. Cryptosporidium oocysts were identified in samples of treated public water with use of a monoclonal-antibody test. Although the sand-filtered and chlorinated water system met all regulatory-agency quality standards, sub-optimal flocculation and filtration probably allowed the parasite to pass into the drinking-water supply. Low-level cryptosporidium infection in cattle in the watershed and a sewage overflow were considered as possible contributors to the contamination of the surface-water supply. We conclude that current standards for the treatment of public water supplies may not prevent the contamination of drinking water by cryptosporidium, with consequent outbreaks of cryptosporidiosis.

Biological activity within the sand bed had the strongest influence on removal efficiency of total coliform bacteria by slow sand filtration, as determined by six pilot filters (shown above). Temperature, sand bed depth, and sand size also had strong influence.

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.

This second article of a series outlines specific treatment technology for arsenic and selenium removal from potable water. The first article, which appeared in the February 1978 issue of the JOURNAL, discussed fluoride and nitrate removal. Subsequent articles will review the treatment technology for the six other NIPDWR‐regulated inorganic contaminants and the radionuclides.

The authors contend that sludge freezing can be a practical process for dewatering when done at the right rate with a sludge that has been adequately prepared.A MAJORITY of water plants now dispose of waste solids by return ing them to surface waters,1 and better treatment and disposal methods are needed. One method offering possible improvement, which was studied and used in England on a plant scale, is sludge freezing.Doe and others 2 found this process to be quite effective for dewatering alum sludge.