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Recently, however. the Polish government instituted an ambitious program to build domestic and industrial waste treatment plants and to clean up the river. There are also some encouraging pollution control stories. In 1997. Minamata Bay in Japan, long synonymous with mercury poisoning, was declared officially clean again. Another important success is found in Europe, where one of its most important rivers has been cleaned up significantly through international cooperation. The Rhine, which starts in the rugged Swiss Alps and winds 1,320 kIn through five countries before emptying through a Dutch delta into the North Sea, WATER POLLUTION 27 has long been a major commercial artery into the heart of Europe.

5 million underground chemical storage tanks. Many of these tanks were left behind at abandoned gasoline stations or industrial sites, their exact whereabouts and contents unknown. The EPA estimates that about 42 million 1 (II million gal) of gasoline are lost each year from leaking underground storage tanks (LUST). Considering that a single gallon of gasoline can make an aquifer unsuitable for drinking, these old, rusting, forgotten tanks represent a problem of tremendous proportions. The EPA now requires that all new tanks have double walls or be placed in concrete vaults to help prevent leaks into groundwater.

Secondary treatment consists of biological degradation of the dissolved organic compounds. The eftluent from primary treatment flows into a trickling filter bed, an aeration tank, or a sewage lagoon. The trickling filter is simply a bed of stones or corrugated plastic sheets through which water drips from a system of perforated pipes or a sweeping overhead sprayer. Bacteria and other microorganisms in the bed catch organic material as it trickles past and aerobically decompose it. Aeration tank digestion is also called the activated sludge process.