Study On Coagulation Sedimentation-ultrafiltration Process For Improving Sensory Quality Of River Water And Its Mechanism

Urban river water is the city’s environmental carrier and an important resource.Suzhou City has implemented a series of actions to improve the water quality of rivers in urban areas since 2011.At present,the nitrogen and phosphorus indicators of water in Suzhou urban rivers can basically reach the surface water level IV standards,but the sensory quality(such as turbidity and chromaticity)of river water is still far from the public expectations and needs to be further improved.Ultrafiltration membrane is a new type of water treatment technology and has been proved that it is able to effectively purify pollutants in water.But there is less research on ultrafiltration membrane process to treat river water.Considering the specific needs of high-quality landscape water in Suzhou Pingjiang historical area,the coagulation sedimentation-ultrafiltration membrane combined process was used in this study to treat the Waicheng water around the city in Suzhou and replenish it into the river in the historical area of Pingjiang.The process effect and mechanism、operating parameters of improving the river water sensory quality were explored.The composition of membrane pollution and the optimal chemical cleaning cycle were also studied in more than one year’s experiment.The research results can provide technical parameters for subsequent project design and operation,and can also provide references for creating high-quality river water in other similar regions.The research shows that:(1)Suzhou river water has been treated by a coagulation sedimentation-ultrafiltration combined process,and its turbidity,SS,chlorophyll a,chromaticity,and TP all significantly decreased.The average removal rate was:Turbidity is 95.37±1.21%,SS is about 100%;chlorophyll a is 87.62±4.8%;chroma is 92.44±1.42%;TP is 70.27±6.64%,the removal effect is stable.The process has a certain removal effect on dissolved organic matter(DOM)in water,especially fulvic acid and humic acid.The purified water transparency is increased.At the same time,the aluminum content in the effluent is reduced,so that reducing the possibility of negative impacts on the safety of the river’s aquatic ecosystem;the nitrogen and phosphorus ratio in the water is increased,thereby reducing the potential for alage proliferation in the water,which is beneficial to reducing the risk of eutrophication of the receiving water.(2)The mechanism of the coagulation sedimentation-ultrafiltration combined process to improve the sensory quality is divided into turbidity removal mechanism and chromaticity removal mechanism.The turbidity removal mechanism is that suspended particles and colloids in the water are aggregated into large particles to precipitate during the coagulation and sedimentation stage,the unprecipitated particle diameter is larger than the the ultrafiltration membrane pore size,they will be trapped when the water passes through the ultrafiltration membrane.The mechanism of chromaticity removal is divided into surface chromaticity removal and true chromaticity removal.Surface chromaticity removal is because of the suspended particulate removal,and true color removal mainly comes from the removal of humic acid and chlorophyll a.(3)The optimal process operating parameters for the combined process in the non-low temperature period(March to November,water temperature is about 12℃-33℃)are:PAC dosage is 20 mg/L,ultrafiltration membrane membrane flux is 33 L/(m~2?h),the water production cycle is 40 min,and the air washing intensity is 120 L/(m~2?h).Under such process combination parameters,the process effluent water quality is good and stable,and the water production rate is high.In the low temperature period(December to next year February,water temperature 6℃-12℃),the best operating parameters are selected as follows:PAC dosage is 25 mg/L,ultrafiltration membrane flux is 33 L/(m~2?h),water production cycle is 40 minutes,and the gas washing intensity is133 L/(m~2?h).Under these process parameter,the ultrafiltration membrane can achieve a high water production rate,and at the same time,it can effectively alleviate the phenomenon of accelerating membrane pollution faster caused by low temperature.(4)After the membrane begins to filter,the contamination process is considered to be dominated by pore plugging resistance,followed by gel layer resistance and filter cake layer resistance.By numerical calculation,reversible pollution resistance accounts for about12%of the total resistance,and remains stable.The proportion of irreversible membrane fouling will gradually rise,and the rising rate will be slower and slower.At the same time,after researching 4 times of membrane chemical cleaning,it is considered that the irreversible membrane pollution ratio of about 28.6%may be the cleaning threshold of the ultrafiltration membrane.The optimal chemical cleaning cycle of the membrane is 40 d.Based on the analysis of the system long-term operation results,the coagulation sedimentation-ultrafiltration device can maintain stable removal effect to treat the turbidity and chromaticity in low sensory quality river water.If in the appropriate process parameters and cleaning cycle,the system can maintain long-term stable operation.