Community Succession And Control Of Invertebrate In Drinking Water Treatment System

The growth and propagation of invertebrates has become a big concern in ozone– biological activated carbon (BAC) advanced water treatment system in the south of China., which may not only affect water sense organ, result in taste and odour problems, but also have potential danger on causing disease. Therefore, how to control invertebrates and reduce the risk effectively in water treatment systems is a burning question.In this research, methods sampling survey and pilot experiment were conducted to analyze invertebrates community structure and the influence factors on breeding between each water treating procedure of two typical ozone -BAC water treatment plants in southern China, namely A and B. Based on the study, effective measures were put forward for invertebrates control in conventional and advanced treatment precesses. The results showed as follow:In water source reservoir of southern China, the diversity and density of invertebrates were high. 49 species (identified) of invertebrates were found in raw water and rotifer, cladoceran and copepod were dominating species. After conventional treatment, the species number of invertebrates sharply decreased. 29 and 15 species were found in plant A and B respectively. However, the species after BAC filter almost remained the same as after conventional treatment. The species in raw water were mainly the common euryecious species of tropical and subtropical habitats. However, benthic species, such as Harpacticoida, Monostyla and Lecane etc, dominatd after the treatment. Invertebrate species composition and density change after conventional treatment had an important impact on the growth of invertebrates in advanced treatment system (BAC filter). Benthic organisms such as Harpacticoida may grow and propagate readily in BAC filters. Conventional treatment system with imperfect facility and management may result in a mass production of invertebrates. Settling tank was the key process unit for the organisms breeding.There was a rapid propagation of invertebrates in BAC filters. After BAC process, the abundance of copepod in water increased by 1.85 times in water plant A, and the abundance of rotifer increased by 22.4 times in water plant B. The visible macro-invertebrate density in BAC filtered water of plant A, which was as high as 286ind/m3, exceeded the crisis limit for drinking water in 250ind/m3.The invertebrate growth obeyed Lorentz distribution model. Abundance peak time and frequencies were related to the species and activated carbon types. At the same time, a evident seasonal variation was showed in water treatment system. The peak-arrival order of six invertebrate groups indicated that the community succession might have the following trend: filter-feeding invertebrates (cladocerans)→small invertebrates(rotifers)→large invertebrates (copepod adults and copepod nauplii) and resisting invertebrates(oligochaetes and nematodes). The finding is significant to the control of inveterbrates in BAC filter. Invertebrate abundances correlated significantly with water temperature(P<0.05), except cladocerans (P>0.05).The results of lab tests and application tests showed that inactivation by pre-oxidation was an effective measure for control inveterbrates growth and reproduction in conventional treatment system. Chlorine dioxide and chlorine were suitable chemicals for inactivation. In pure water, when exposed to 2.0mg/L of chlorine and 1.0mg/L chlorine dioxide for up to 1 hour, Cyclopoid and Bosinina were completely inactivated. Backwash of BAC filter with chlorine water was the simplest and the most effective way to remove living organisms. 3mg/L chlorine was recommended for backwashing. However, when number of invertebrates in BAC filters were too high to be controlled by backwashing, it is suggested to soak the filters with NaCl solution with concentration as high as 20g/L. A 300mm sand layer under activated carbon layer could reduce inveterbrate penetration in BAC filter.The control method provides great theory guidance and technical support for security of drinking water.