| Developing efficient and low-consumption technologies for pig farm wastewater treatment is a grand challenge as the demand increases with intensive farms.The anoxic-aerobic?AO?process is a mainstream treatment for piggery wastewater,which has been limited by severe foaming issues of aerobic biological units.However,normal methods such as spraying,defoamer and so on are incapable of foam controls.To solve foaming issues in biological treatment units for piggery wastewater,this work took three piggery wastewater treatment stations in Hangzhou as research objects.Surface tension apparatus,high-throughput sequencing and other technical methods were used to analyze the foam characteristics from three perspectives,including chemical composition,extracellular polymeric substances?EPS?and microbial community.Furthermore,the potential strategies for foam control in biological treatment units were put forward.The major results are as follows:1. The foaming capacity and foam stability of the aerobic biological treatment units of piggery wastewater were evaluated by foaming volume and stability time per unit volume mixture.The results showed that the foaming volume?r=0.898?and stability time?r=0.884?of piggery wastewater per unit volume were positively correlated with their COD concentrations.The study of the mixture physical properties revealed that the surface tension of foam mixture supernatant(58.9-69.7m N·m-1)was lower than that of its sludge mixture supernatant significantly,while its viscosity?0.24-0.72 Pa·s?was higher than that of sludge mixture.Results of correlation analysis showed a clear correlation between the surface tension and foaming volume?r=-0.764?,viscosity and foam stability time?r=0.924?of mixtures,indicating that the foaming volume and stability time per unit volume mixture can characterize the foaming characteristics of biological units intuitively and quickly.Analysis of the mixture zeta potential revealed that the absolute value of the foam mixture zeta potential?10.4-15.3 m V?was lower than that of its sludge mixture?16.4-23.3 m V?in each treatment station,reflecting that the Van der Waals force of foam mixture was stronger and the liquid film strength was higher to some extent.Further analysis of the mixture organic composition revealed that the average proportion of long-chain fatty acids?LCFAs?in the foam mixture?32.6%?was significantly higher than that of sludge mixture?19.7%?,speculating that the high content of LCFAs decreased the surface tension of foam mixture?r=0.739?,which was the main reason for the foaming of aerobic biological treatment units of piggery wastewater.2. Analysis of the content and composition of EPS in the mixtures showed that the EPS content?calculated by proteins?PN?and polysaccharides?PS??in the foam mixture was as high as 13.8-30.2 mg·g VSS-1,which was 35.0%-97.2%higher than that in their sludge mixtures.Thereinto,the content of PN,PS and humic acid-like organics in the mixtures were correlated with the proportion of LCFAs positively,related to the emulsification metabolism of bacteria by secreting EPS presumably.Meanwhile,the content of PN,PS and humic acid-like organics in mixtures were correlated with the absolute value of zeta potential negatively,the content of humic acid-like organics was correlated with the foam stability time positively?r=0.785?.The results indicated that the EPS composition of mixtures,especially humic acid-like organics affect the foam stability.Analysis of the microbial structure of mixtures revealed that the dominant bacteria included AKYH767 spp.,Chitinophagaceae spp.,Limnobacter spp.,Thauera spp.,Truepera spp.,etc.Thereinto,Hydrogenophaga spp.and Pseudomonas spp.with functions of secreting EPS,emulsifying and metabolizing hydrophobic organics such as LCFAs,were significantly enriched in the foam mixture.RDA analysis showed that LCFAs were correlated with the foaming volume and the abundance of emulsifying metabolic bacteria positively,the proportion of humic acid-like organics in the mixtures were positively correlated with the foam stability time and their relevance increases in turn.It is speculated that the foaming problem was mainly caused by the influent LCFAs,its induction of dominant bacterial succession,and EPS secretion.Reducing the LCFAs content of influent is the key to alleviate the foaming problem in aerobic biological treatment units.3. The pretreatment strategies such as air flotation and bioelectrochemistry were studied to removal typical LCFAs in piggery wastewater and alleviate foaming problem.The results showed that traditional air flotation pretreatment had lower efficiency in removing palmitic acid and stearic acid,which were only 36.4%and37.5%,respectively.Bioelectrochemical enhanced anaerobic pretreatment removed87.2%of palmitic acid and 73.1%of stearic acid,increased by 10.5%and 13.9%compared with the anaerobic treatment control group.Sludge dehydrogenase activity?DHA?analysis found that the sludge DHA of the electrochemical enhanced group was as high as 262?g TF·g-1VSS·h-1,which was 1.2 times that of the anaerobic treatment control group.Results indicated that electrochemical enhanced strategy could improve the sludge activity and enhance the removal performance of typical pollutants in piggery wastewater effectively,providing new sights for alleviating the formation of biological foam and its subsequent impact. |
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