Pilot Study On Micro-polluted Raw Water Treatment By Enhanced Coagulation-DAF Process

With the development of industry and the improvement of people's subsistence level, water pollution became a common problem in the world. Water is necessary for human being in living and production, and no substance can substitute it. The fresh water in China is serious shortage, and temporal and spatial distributions of water are very heterogeneous.The water resource is very insufficient in Tianjin city, which has become an important influencing factor to social and economic development. The source water pollution is the major problem of water environment pollution control and water resource protect. The municipal water supply was confronted with source water deterioration and water standard improvement.Source water of Tianjin changes seasonally, the characteristics of typical micro-polluted source water in north China are annually demonstrated by the presence of low temperature low turbidity and high temperature algae laden coupled with high organic matter (OM). The conventional water treatment process must be improved to meet the requirement of the new standard of water supply, so we brought forward dissolved air flotation (DAF) process to replace sedimentation process in conventional treatment train. It is reported that the DAF process was effective to treat the low temperature and algal laden source water, and which was widely used in many countries. It is very significative to use DAF process to treat typical north micro-polluted water.This dissertation studied the pollutant removals efficiency by DAF and sedimentation process. By the experiment, the optimal process to treat the typical north China source water and the rules of pollutants removals were determined. A pilot plant with a capacity of 120 m3/d was constructed in a water works of Tianjin, China. Coagulating DAF is a reliable and robust process for removing various pollutants, it is available for DAF process to replace sedimentation process treating micro polluted water, coagulating DAF process has obvious predominant in water treatment compared with the sedimentation in north china.The operational parameters and pollutant removals were studied in different water periods. During the treatment of the normal temperature and normal turbidity source water, the coagulant dosage is between 10 and 15 mg/L, the recycle rate of is 9%, the turbidity removal ranges from 85% to 95%, CODMn and UV254 removals are 15~35%, the TOC removal is 10~30%, the bacteria removal is 80~95%, the THMs removal is 18%. During the treatment of low temperature and low turbidity source water, the coagulant dosage is between 15 and 20 mg/L, the recycle rate is 10%, the turbidity removal ranges from 80% to 90%, CODMn, UV254 and TOC removals are 15~25%, the bacteria removal is 60~90%, the THMs removal is 16%. During the treatment of high temperature and algal laden source water, the coagulant dosage is between 20 and 30 mg/L, the recycle rate is 12%, the turbidity removal ranges from 80% to 98.5%, CODMn, UV254 and TOC removals are 15~65%, the bacteria removal is 80~95%, the THMs removal is 20%.A new mathematical model for bubble-floc collision efficiency and its up flow velocity is presented, which has been designed for two cases: the floc particles are larger than air bubbles or air bubbles are larger than floc particles. Base on the collector of bubble and folc, the mathematical models are set up individually. It is found by the models that the collision of bubble-floc decrease with the diameter increasing, that is, if the bubble is the collector, the minor floc is better, and the minor bubble is better for the floc as the collector. The laminar flow is favorable for the bubble-floc collision, and the low temperature will reduce the bubble-floc collision efficiency. The bubble-floc up flow velocity model shows that the up flow velocity of bubble-floc is related to the size of floc and bobble and their number content, the bubble number which adheres to the floc is related to the size of floc and bubble The size control of little floc is very important for the DAF operation. The bubble number is so enough for the large floc, so the coagulation is not important. The back propagation neural network model for pollutant removal is also presented in the paper; the result shows it can good forecast turbidity and UV254 removal and the dosage of the coagulant.The scum of the DAF was researched by centrifugal dewatering experiments. The results showe that scum production is relaton to the coagulant dosage, raw water turbidity and suspended solid, effluent turbidity and suspended solid. The water content of scum can drop from 99.5% to 86% by centrifugal dewatering.