||The objective of this study was to investigate the effect of raw water turbidity on the performance of the high speed flocculation flat bottom type-sludge blanket clarifier in Pingjan water treatment plant. This study gathered the data of raw water quality of the plant during the periods from 2001 to 2005. The characteristics and distribution of raw water turbidity in relation to the achieving rate of effluent turbidity for the clarifier were discussed. On the other hands, to improve the achieving rate, the main factors which affect the performance of the clarifier, such as water quality, design parameters and operating conditions, were also investigated. Finally, the feasibility of improving effluent turbidity was evaluated by mixing with other influent source to form the sludge blanket when encountering low turbidity (5 to 10 NTU) in the raw water.|
According to the statistical data, the raw water turbidity was lower than 200 NTU with more than 90% of opportunity, except for the typhoon periods in summer and fall. However, the turbidity range in 5 to 15 NTU had the highest probability to occur (about 45%). The reason for low achieving rate of effluent turbidity, as was observed interestingly, had no concern with the season, though the variations of turbidity in the raw water was seasonal. Moreover, the upflow velocity and pH could maintain practically from 3.12 to 4.70 m/hr and 7.0 to 7.5, respectively, which were consistent with design parameters of the clarifier.
The raw water turbidity other than pH and coagulant dosage had directly found to be related to the formation of sludge blanket, which became the key factor for the performance of the clarifier. The sludge blanket could not be formed well and led the achieving rate of effluent turbidity to reach only 76% when the raw water turbidity was 10 NTU approximately. The sludge blanket could be formed normally between 15 and 200 NTU of raw water turbidity, but operators should take care of controlling water quantities accordance with variation of turbidity. As to the high turbidity of raw water for storm weather and typhoon periods (higher than 200 NTU), the strategy of clarifier operation was to decrease the influent flow rate at this time, and the most important consideration was to withdraw sludge instead of forming blanket.
In real-plant experiments of changing influent source, the raw water turbidity was increased to the range between 15 and 30 NTU by mixing 480,000 m3 of water from No. 195 water outlet of Shimen reservoir (about 5 to 10 NTU) and 86,000 m3 of that from the secondary pumping station (about 100 NTU). The stable sludge blanket in the clarifier was formed effectively. The concentration of blanket was about 200 mg/L at initial, but would gradually increase to the range of 400 to 800 mg/L with thickness of 2.5 to 3 m at average when reaching stable state. At this moment, the average removal of turbidity was about 87%, and effluent turbidity of the clarifier was lower than 5 NTU which meet the requirement of this plant.
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