Acid Production From Cyanobacteria By Anaerobic Fermentation And Its Application As Additional Carbon Source For Denitrification

The eutrophication of water body led to the bloom of cyanobacteria,causing serious environmental pollution problems,it is necessary to find technically and economically feasible methods.The cyanobacteria had high organic matter content,and volatile fatty acids（VFAs）produced by anaerobic fermentation can be used as additional carbon source for biological denitrification of sewage,which can solve the problem of insufficient carbon sources in municipal wastewater treatment plants,and to achieve the purpose of treating“waste”with“waste”.Based on this,the paper studied the enhancement method of acid production from cyanobacteria by anaerobic fermentation,and the optimal dosage of magnetite was determined.The cyanobacterial fermentation liquid was purified,struvite precipitation combined with calcium phosphate precipitation was used to recover N and P in the cyanobacterial fermentation liquid and the optimal process parameters were determined.The experiment of using the puried cyanobacterial fermentation liquid as the additional carbon source for denitrification was carried out.Finally,the cyanobacteria fermentation liquid was applied to the denitrification biological filter（DNBF）for deep denitrification,the feasibility of the cyanobacteria fermentation liquid as carbon source for biological denitrification was investigated.The main research results of the paper were as follows:（1）The addition of magnetite can promote the acid generation from cyanobacteria by anaerobic fermentation.The optimal dosage of magnetite was 5 g·L^-1,and the cumulative amount of VFAs was 8.36 gCOD·L^-1,which was increased by 19.09%higher than that of the control.Among them,acetic acid was accounted for 57.66%of total VFAs,which can improve the utilization value of cyanobacteria fermentation liquid.At the same time,the maximal degradation rate of protein,carbohydrate and the activity of dehydrogenase were84.76%,72.49%and 89.86 TFμg·（mL·h）^-1,respectively.NH₄⁺-N and PO₄^3--P was released during the cyanobacteria anaerobic fermentation process.Magnetite addition changed the richness and diversity of microbial populations and community structure in the cyanobacteria anaerobic fermentation system,the genus of Macellibacteroides was significantly enriched in the magnetite group.（2）Struvite precipitation combined with calcium phosphate precipitation can effectively recover N and P in the cyanobacteria fermentation liquid.The optimal reaction conditions of struvite precipitation were:pH was 9,reaction time was 60 min,the molar ratio of Mg:P:N was1.2:1.1:1;the optimal reaction conditions of calcium phosphate precipitation were:pH was 10,reaction time was 15 min,the molar ratio of Ca:P was 6.68:1.The effluent NH₄⁺-N and PO₄^3--P concentrations were 22.83 mg·L^-1 and 2.7 mg·L^-1,and the recovery rate was 98.84%and94.62%,respectively.In the batch experiment of denitrification,when the cyanobacteria fermentation liquid was used as the exogenous carbon source,the removal rate of NO₃^--N reached 99.13%,and the denitrification rate was 1.98 mg·（g·h）^-1（calculated by mixed liquor suspended solids（MLSS））,the denitrification efficiency was better than the experimental group of adding ethanol and sodium acetate.（3）The operation performance of DNBF using the cyanobacteria fermentation liquid as carbon source was explored,the operation parameters of DNBF were determined,and the microbial community structure was analyzed.The DNBF with the synthetic cyanobacterial fermentation liquid as carbon source was successfully cultured after 13 days,while the reaction with sodium acetate and ethanol as the carbon source required 17 days.When hydraulic retention time（HRT）was 4 h and C/N was 5,the denitrification performance of DNBF with the synthetic cyanobacterial fermentation liquid as carbon source was rather to that of sodium acetate and ethanol,and the removal rate of NO₃^--N reached 98.05%,there was almost no accumulation of NO₂^--N.The analysis of DNBF was showed that at the 0-15 cm of the filler layer,more than 95%of NO₃^--N was removed,and NO₂^--N was accumulation.At the 30-45 cm of the filler layer,the accumulation of NO₂^--N disappeared.When the actual cyanobacteria fermentation liquid was used as carbon source of DNBF,the denitrification performance was worse than that of sodium acetate and ethanol,and the removal rates of NO₃^--N,TN,and COD were 63.99%,62.19%and 61.74%,respectively,the average effluent concentration of NO₂^--N was 0.5 mg·L^-1.However,the denitrification potential of the actual cyanobacterial fermentation liquid was similar to that of sodium acetate and ethanol,which was 0.2 gN·gCOD^-1.When the cyanobacterial fermentation liquid was used as carbon source of DNBF,it was beneficial to the richness and diversity of the microbial community in the biofilm.The dominate genus of bacteria with denitrification functions were Azospira,Thauera,Dechloromonas and Hydrogenphaga.