Investigation Of T. Tubifex's Growth And Reproduction And Its Migration Law In Source Water And Water Treatment Systems

T. tubifex propagating excessively in water sources can enter into water plant by its migration, which results in worm pollution in drinking water distribution system for its high stretching and disinfectant resistance capability. Since most people associate the organisms with low hygiene, it is necessary for water plant to forecast and control the worm pollution.The main objective of present research was to supply the theoretic proof for water plant to forecast and control the worm pollution. The study first aimed at the worm migration activity. The effects of temperature, DO and sediment property on worm migration were investigated by stimulating worm migration activity in lab. Second, the worm growth and reproduction regularity were studied by assessment the influence of temperature and DO. At last, the worm migration and distribution in sand filter of water plant were investigated by adopting pilot-scale test. The present major results were showed as follows:(1) migration is one of the primary means for T. tubifex to expand population distribution, and the migration modes can be divided into surface, deep and drift migrations that represented migrations in superficial sediment (0-2cm), deep sediment (deeper than 2cm) and from sediment to overlying water respectively. The worm migration in sediment can be effected not only by temperature and DO but also sediment properties. The suitable temperature and DO content for worm growth were 22℃and 8mg/L respectively since lowest total migration efficiencies were observed in such conditions. Increase of migration efficiency could be resulted in by the decrease of DO content and the increase or decrease of temperature. In addition, the influence of sediment property on the worm migration was realized by changing the worm food source;(2) Drift from sediment to overlying water, which induced mainly by water flow, is the direct reason that resulting in worm pollution in water plant. Moreover, the worm drift can be influenced by both temperature and DO. In present test, the worm drift was observed provide the water flow velocity exceeds 5.55mm/s. Moreover, drift rate increased with the increase of water flow velocity and the decrease of temperature and DO content;(3) The results of T. tubifex migration and distribution in sand filter showed that the sand filter can effectively intercept T. tubifex and the interception efficiency can be effected by particle size of sand and length of T. tubifex. The larger particle size and the longer worm length, the lower intercept effiency. Meanwhile, the operating conditions of sand filter will impact the worm migration and distribution in filter. To control the worm leakage risk effectively, filtration rate and cycle should be lower than 11.lm/h and 12h, respectively and backwashing intensity should be controlled about 25L/s-m2, at which 97.7% T. tubifex removal efficiency was obtained(4) The increase of temperature can significantly improve T. tubifex innate growth capacity and reduce the population doubling time. In present test, population doubling time of T. tubifex at 10℃,20℃and 30℃were 38.6d,18.0d and 8.6d respectively, which meant that the increase of temperature can promote the worm growth and reproduction. When DO content was decreased from 8mg/L to 5mg/L, population doubling time increased from 11.2d to 21.7d. However, population doubling time increased from 21.7d only to 25.8d if DO content continued to reduce to 1mg/L, which showed T. tubife has high capability of growth and reproduction even in low DO content.The present results showed that due to its special living habits, the worm can propagate excessively in water sources even in low DO content and enter water plant by water floating, which resulting in its pollution risk eventually. Both temperature and DO had significant impact on the above process. Full interception could not be realized by sand filter and some worms would penetrate sand filter and enter drinking water distribution system eventually. Moreover, the worm interception efficiency could be influenced by operating conditions of sand filter. The present results can provide a theoretical basis and technical support for water plant to predict and control T. tubifex pollution.