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Cryptosporidium is a parasitic microorganism found in the environment (mainly in surface waters) as a robust oocyst. Cryptosporidial oocysts are generally between 4 and 6 microns in diameter. They are responsible for cryptosporidiosis, a disease in which patients suffer from acute diarrhea. The oocysts can survive for long periods before infecting another host, and the disease can be transmitted through the potable water supply.
During the operation of the filters, not only are removal efficiencies low at start-up, but later in the cycle flow surges can also cause sloughing off from the medium, resulting in a breakthrough of the infected particles. The practice of recycling sand filter backwashings (increasingly necessary because of current water shortages) greatly enhances the risks of contaminating the water supply. Moreover, oocysts are highly resistant to chlorine and other disinfectants at the concentrations normally used in municipal water treatment.
Slow sand filters are more efficient in retaining oocysts than rapid gravity units, largely because of the formation of a 'schmutzdecke' layer. This is a biologically active bed of bacterial and algal slime that forms an effective barrier for very small particles, including bacteria and oocysts. Nevertheless, oocysts can sometimes pass through this filter into the treated water before the schmutzdecke is fully established. Oocysts can also pass through the fissures that sometimes open up in aging sand filters as a result of poor operation and maintenance.
Challenge tests carried out using the AC fine test dust have shown that the spirally-wound backwashable depth filter has a removal efficiency of 98 percent for particles down to 2 microns. What is more important for oocyst removal, the filtration efficiency increases to 99.8 percent for 5 micron particles. The test results indicate that the spirally-wound backwashable depth filter has a filter rating that is an order of magnitude finer than that of sand filters. Furthermore, when fed with a surface water, the spirally-wound backwashable depth filter outperforms multimedia filters.
The exceptionally fine filtration achieved by this filter makes it particularly suitable for removing cryptosporidial oocysts from water supplies. In tests with sand filter backwashings containing 103 oocysts/ liter, no oocysts were detected in the filter filtrate, indicating removal efficiencies of better than 99.9 percent. Moreover, the operational characteristics of the spirally-wound backwashable depth filter ensures that it maintains a consistently high removal efficiency throughout the filtration cycle.
In some cases, this filter is used as a polisher for mainstream rapid gravity filters to prevent contamination of the treated water supply. A second application is to ensure that sand filter backwashings are free from oocysts before being recycled back to the start of the water treatment process.
The complete process is governed by the spirally-wound backwashable depth filter's Programicronsable Logic Controller, and it is designed to require minimal downtime and energy consumption. In-house evaluation of the VSP process has shown that under optimum operating conditions, a spirally-wound backwashable depth filter element can be heated to a uniform pasteurization temperature in 3.5 minutes. The element must then be held at the pasteurization temperatures for 5 minutes longer to ensure a 100 percent kill of the oocysts.
The steam can be turned off at least 2 minutes before the end of the holding time, because the temperature decay of the element is very gradual.
Each VSP cycle for a single element consumes approximately 5.5 kg of steam, and the total cycle time is shown in Table 1.
In-situ deactivation of the spirally wound backwashable depth filter element is a highly cost-effective means of destroying the cryptosporidial oocysts harvested from the feedwater. The costs are substantially lower than would be the case for sand filters because the bed volume of the spirally wound backwashable depth filter is smaller.
The spirally-wound backwashable depth filter mentioned in this article is known as the FIBROTEX, manufactured by Smith & Loveless, Inc. Smith & Loveless has a license agreement to sell and manufacture Fibrotex in North America.
Vessel drain 0.75 min.
Vessel evacuation 0.25 min.
Element heating to pasteurization temperature 3.5 min.
Element holding to pasteurization temperature 5.0 min.
Steam injection time 6.5 min.
Total VSP cycle downtime 9.5 min.