Novel Filter Cleans Up Australian Sewage
A UNIQUE Australian-made technology will soon clean up sewage that the New South Wales beach town of Cronulla now dumps into the ocean with only primary treatment to screen out the solid waste. Noting that the technology can be exported, Nick Greiner, the premier of New South Wales, says, ``Australians would also have the satisfaction of knowing that Australian technology was helping to clean up the environment in other parts of the world.''
The new system, which the Cronulla treatment plant will start using in August to process waste for a population of 70,000, filters almost all solids, oils, greases, bacteria, and viruses from the sewage. No chemicals are used in the process, which cuts the waste-processing time from hours to six or eight minutes.
The key to the system is a patented filtration system developed by Memtec Limited, a Windsor, New South Wales, company that specializes in filters and water systems. Memtec makes a thin, flexible polypropylene filter that resembles cappelletti - thin pasta. The pores in the filter are only 0.2 microns in diameter, or 500 times smaller than a human hair. Seventy percent of the volume of the filter is air.
Suspended solids cannot get through the small holes. Since viruses and bacteria attach themselves to the minute particles in sewage, they are excluded from the cleansed product as well. Dr. Gary Grohmann, a virologist at the Westmead Hospital Research Institute, says tests he conducted on 26 separate runs showed no viruses passed through the Memtec membranes. ``It is probably the only method of physically restraining viruses that is quite effective. There are chemical methods and the use of ultraviolet rays, but then you are starting to add new parameters to the environment,'' he says.
The system cleans the filters by blowing compressed air back through the polypropylene filters. The cleaning process takes about one minute, barely disrupting the disposal process.
Normally, sewage treatment begins by screening out solids as large as diapers or as small as grit. Then the sewage flows to large settling pools, allowing a portion of the sediment to drop to the bottom of the pool. This is called primary treatment. In New South Wales, this is the only treatment mandated. The remaining sewage, including toxic chemicals and bacteria, is discharged directly into the ocean.
The next step in sewage treatment is secondary treatment, which usually involves settling beds and acres of lagoons where the sewage is aerated to accelerate a biological treatment. In the United States, most states now mandate secondary treatment of sewage.
Some European countries are now mandating tertiary treatment of sewage. This is further filtering and usually involves the addition of chemicals, such as chlorine, to destroy the bacteria.
According to Memtec, their filtration process produces an end product that is cleaner than any of the usual systems. Chris Tweedie, a spokesman for the Sydney Water Board, says, ``The result at the end is better than tertiary treatment.'' When announcing the contract last June, Mr. Greiner and Tim Moore, the state environment minister, said, ``Results of the membrane filtration method have to date been excellent.'' The process removes more than 99 percent of suspended solids, 97 percent of oils and greases, and more than 99 percent of bacteria and viruses. Some of its filters are now used by wine and fruit-juice producers.
But the process is not without problems. It will not work on raw sewage, for example, which clogs the filters. To get around this problem, Memtec will screen the sewage for large solids and then run it through a ``bio-reactor,'' a slurry of bacteria and anthracite coal fed with oxygen. The bacteria quickly break down the solids, making it easier for the filters to recover the water part of the waste. The sludge from the bio-reactors and the filter system is dried and may be recycled.
The Memtec process is being used at a small sewage plant serving 5,000 people living in the Blue Mountains, about 40 miles west of Sydney. The Water Board says the plant, which started up in August 1990, is operating without a hitch.