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Dr. Cluff's Patented Slowsand/Nanofiltration Method
The Powerful Combination of Slowsand/NanofiltrationThe patented combination of slowsand with nanofiltration offers the significant benefit of water treatment without chemicals. This combination successfully was piloted by the University of Arizona beginning in 1987. It was later patented. Other pilot projects include a recent study by the American Water Works Research Foundation in Tucson, Ariz. Though the final report has not yet been published, it has been written and is in the final review process. Slowsand filtration is a great treatment in and of itself. If combined with a sedimentation tank and roughing filter, slowsand filtration would meet most U.S. Environmental Protection Agency (EPA) requirements for drinking water. When a nanofilter is added all requirements can be easily met. Slowsand filtration, as used on the Colorado River directly from Lake Mead without a roughing filter, produced results comparable to those produced by micro and ultrafiltration membranes. After 18 months, the turbidity from the slowsand filter was found to be 0.05 NTU. The silt density index was 2.5 after two years in a study in Marana, Ariz. Slowsand reduces the total organic carbon (TOC) by at least 25 percent. (To produce a similar result using micro or ultrafiltration would require preparatory flocculation and sedimentation). Slowsand filtration can remove iron when coupled with aeration. Slowsand as a pretreatment to nanofiltration should not be dismissed on the basis that it is too labor intensive or that it takes up too much land area. Slowsand filters are cleaned by removing the top 3/8 inch of sand after dropping the water level slightly below the level of the sand. A small system, less than 40,000 gallons per day (GPD), can be cleaned in about an hour with one person. A 400,000 GPD system requiring 2,000 square feet can be cleaned in one hour or less, using a laser controlled tractor scraper and a single operator, or just over two hours, with 4 men to do the job. The dirty sand in a small system can be returned to the source where it may be rewashed and reused. On large systems, the sand may be washed on site with sand washing equipment. The run time between filter cleanings, on Colorado River water, for example, averages three to six months. When the Colorado River water was drawn directly from Lake Mead and then filtered, the run time was more than one year. When the water was first sent through the Central Arizona Project Canal, and then filtered, the run time was three to six months, from a two-year pilot study in Marana, Arizona. A slowsand filter requires just one square foot of filter per person using the water. For a population of 10,000, only 10,000 square feet is needed, or about one quarter acre of land. Another quarter acre would be needed for the storage tank and building to house the nanofilter. This is only a fraction of the farmland needed to grow the food for the same population. Most small drinking water systems should be able to find a half-acre of land, even in the most expensive neighborhoods. The alternative microfilter or rapid sand filter may take up less area, but would require additional holding ponds for the pre-filtered water and sludge handling facilities for the back wash material. These are not always needed in a slowsand system. In small systems, the dirty sand may be returned to the sand and gravel company and washed for reuse. The slowsand filter can be cleaned several times before the sand needs to be replenished. The slowsand filter improves with age as the microorganism colonies mature. It keeps getting better and better. The capital cost of building an in-ground slowsand filter using a high density polyethylene (HDPE) plastic liner, plastic pipe drain systems and plastic separating screens is inexpensive, particularly in those areas where the sand and gravel are readily available. The amortized capital and operating cost of a slowsand filter is only 10 cents per 1,000 gallons according to a recent U.S. Bureau of Reclamation report. This is one fifth of the cost using micro and ultrafiltration, as presented in the same report. If nanofiltration is added to the slowsand treatment train, then present and future EPA standards will be met. This membrane treatment can be added for about 30 to 50 cents per 1,000 gallons. The cost is dependent on water quality, water temperature and use of reject water. There is not much difference between the costs of nanofiltration and micro and ultrafiltration, particularly if nanofiltration is preceded by a slowsand filter. However, there are maximal benefits to nanofiltration that are not available when using micro or ultrafiltration. Nanofiltration offers soft water, fewer dissolved organics and therefore fewer disinfectant byproducts, no color problems, reduced lead, arsenic and nitrates. Nanofiltration also removes uranium, lead and manmade chemicals such as pesticides, herbicides, etc. It is an excellent treatment for municipal wastewater. Nanofiltration is sometimes overlooked because of concern with the reject stream. There are innumerable potential uses for the reject stream when ingenuity is applied. In arid and semiarid regions, it can be blended with irrigation water. In more populous areas, reject water can also be blended back into the wastewater plant effluent stream. This is where the minerals and most impurities would have been even if nanofiltration or reverse osmosis was not used. This process is working in a new RO plant in Julesburg, Colorado. Because nanofiltration/RO will reduce the use of salt water softeners, the salinity of the blended wastewater effluent might even be less than it was before nanofiltration/RO is used. If nanofiltration is used primarily to eliminate nitrates and organic materials in the water, a large percentage of the reject can be recirculated through the slowsand filter for added treatment before nanofiltration. If it is used mainly for softening, added treatment on the reject stream can recover calcite, which can be used by power plants that burn high-sulfur coal. Once the calcite is removed, the water can be run again into a reverse osmosis (RO) system to recover more water. Using these two methods, the waste stream would be reduced to an amount similar to the 5 percent waste produced by a micro or ultrafilter. Another use of the reject water is to grow shrimp while concentrating the mineral levels to that of seawater. It takes seawater concentrations to grow large shrimp. The potential profits from shrimp farming would approach $25,000 per acre per year. Nanofiltration can be used with most groundwater without using a slowsand
filter. However, slowsand can be a very effective pre-treatment for problem
groundwater, including shallow water containing iron or organic matter.
It is particularly important treatment for municipal wastewater. |
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