HOW TO CONTROL INVERTEBRATES IN DISTRIBUTION SYSTEMS: BY STARVATION OR BY FLUSHING?

摘要

History shows that in the U.S. and in Europe midge fly larvae, asellids, oligochaete worms, snails orrnother invertebrates occasionally become a nuisance to drinking water customers and therefor to waterrnsupply companies. Publicity the Netherlands in 1988 and 1989 concerned invertebrates in therndistribution systems of treatment plants that use ground water. Quantitative data about invertebrates inrnthis type of distribution systems were not available. To collect more data on invertebrates in this type ofrndrinking water, a two-year survey in 36 drinking water distribution systems was conducted by 17 waterrnsupply companies and Kiwa from 1993 to 1995. Invertebrates were found in every system (5 samplernsites, 8 series), some in large numbers. Microscopic organisms like protozoans (non-parasitic!) and rotifersrnwere found to dominate (P90 = approx. 200/m~3) in drinking water samples from treatment plantsrn(261) and house-taps (1297). Virtually the only other invertebrates found were the tiny nematodes andrncopepods (P90 = 20/m~3). In 261 drinking water samples from plants and 1297 from taps, each 200 l,rnasellids and snails were never found and midge fly larvae only in 2 tap water samples. Problems probablyrnoccur when the flow in the mains increases or changes direction when mains are broken, valves arernshut or mains are flushed. In these cases invertebrates may be flushed into smaller mains and pipesrnleading to the taps. In 1302 samples of 400 l water flushed out of mains via hydrants the copepods,rnwater fleas and oligochaete worms were dominant in numbers (P90 = 250/m~3 to 800/m~3), up to morernthan 10,000/m~3. Asellids were less dominant in numbers (P90 = 100/m~3, max. 1,500/m~3) but on averagernformed 86% of the biomass (= 365 mg/m~3 w.w.) in the samples. Water mites, turbellarians and nematodesrnwere also found frequently (P90 = 20/m~3 to 130/m~3), but midge fly larvae and snails were seldomlyrnfound.rnContinued research (1996-1997) aims to assess the effects and costs of two strategies for control:rndistribution of biologically-stable drinking water (controlling food levels) and periodic cleaningrnprograms to remove the invertebrates. The 1993-1995 survey was continued in 1996 in 14 distributionrnsystems (10 water works) to assess the effects and costs of control measures. The continued survey willrnfollow effects on invertebrates, sediments and bacteria. In a first evaluation the investigated controlrnmeasures largely perform as expected: Optimising treatment seems to gradually lower invertebraternbiomass in the distribution system, flushing only a small area of mains only has short-term effects,rncleaning a larger area has longer lasting effects. Theoretically intensive cleaning methods like piggingrnand air-scouring seem to have roughly the same effect as flushing with water. A more detailed statisticalrnevaluation of all results (last samples will be collected in December 1997) is necessary however to bernable to balance the cost/effect ratios of all types of control measures. This will enable water companiesrnto balance investments in water treatment plants and management of distribution systems.