| It is in line with the long-term demand of sustainable development of China's sewage treatment industry,to explore"low carbon"oriented technologies based on the actual demand of energy saving and emission reduction in sewage treatment and sludge disposal.With the increasing prominence of eutrophication which promotes the continuous improvement of effluent standards for nitrogen and phosphorus in sewage treatment plants,the lack of denitrifying carbon source has been an important factor restricting the improvement of denitrification efficiency.In traditional activated sludge process,the utilizztion efficiency of denitrification remains bottlenecked on the its limited hydrolysis rate for the slowly-degradable particular carbon sources(X_s),which takes up almost 60%?75%of the influent organics.Part of X_s was metabolized in manner of aerobic mineralization,with additional aeration energy consumption and waste activated sludge(WAS)production.Volatile fatty acids(VFAs)production from WAS anaerobic fermentation is an important way to supplement carbon sources for denitrification.However,due to unbalanced nutrient with low C/N ratio and being wrapped by extracellular polymer substance(EPS),limited organics could be efficiently hydrolyzed by functional microbial,resulting in a low level conversion rate of carbon source,which was anticipated to be improved by co-fermentation with carbon-rich feedstock.In this study,a scheme was proposed to improve the denitrification efficiency of X_s in both sewage and sludge line,based on fine-sieving technology to collect part of X_s at the up-stage of the bio-reactor coupling with anaerobic fermentation of the fine-sieving fractions(FSF)and WAS.Properly collection of X_s was anticipated to save the part lost in mineralization and the corresponding aeration energy consumption.In addition,VFAs production from WAS could be significantly promoted conditioned by the collected FSF.Therefore,with the goal of low energy consumption and low environmental impact,the optimal allocation and efficient utilization of denitrifying carbon sources in sewage/sludge is realized,which provides a new idea for the sewage treatment plant to relieve the current economic and environmental pressure and meet the future"low carbon"development.(1)The carbon distribution and conversion characteristics of the resulting filtration through sieves with different pore size were analyzed,to explore the threshold value of the particulate carbon source that can be recovered.In addition,under the optimized interception conditions,the performance of carbon distribution and utilization in the anaerobic-anoxic-aerobic(A~2O)process was simulated.Finally,mass balance and energy consumption were preliminarily estimated based on the experiments results and design/operation parameters of A~2O process.The results show that the effect on denitrification efficiency can be controlled below 5%when the carbon source interception is less than 30%by fien-sieves with pore size of 100?150?m.The optimized conditions of fine-sieve collection are:pore size of 131?m,entrapment time of 40 minutes,flow rate of 105.0 L/(m~2·h).Under this condition,the denitrification efficiency was increased by 3.2%with more efficient carbon source.Also,the X_S lost by aerobic mineralization was decreased by 6.4%.Further analysis in mass balance and energy consumption revealed that the aeration energy could be reduced by about 16.1%with 27.9%less WAS yield.The activity of produced sludge was anticipated to be improved with 50.3%of suspended inorganic solids(ISS)reduced.(2)To explore more efficient carbon sources for denitrification,the performance of VFAs production from fine-sieving fractions(FSF)was investigated,with WAS fermentation as control.The hydrolysis of polysaccharides and proteins,VFAs accumulation and the corresponding dynamics were analysed.Then,microbial community structure wasanalyzed to reveal the microbial interaction mechanism in FSF fermentation.Furthermore,the feasibility of initial-alkaline motivated fermentation of FSF and its effect on cellulose properties was explored by fourier transform infrared spectroscopy(FTIR)and thermogravimetry analysis.Results showed that VFAs yieldsfrom FSF reached 525.8 mg/g VSS after 5-day fermentation,4.2-time higher than that of WAS.The correspongding VFAs conversion rate was 25%and 15%,respectively.The significant difference in hydrolysis rate constant made great contribute to this difference.The dissolution rates of polysaccharides and proteins in FSF were 2.78-time and 1.56-time higher than those in WAS.The relative abundance of Paraclostridium,a cellulose-hydrolyzing bacterium,reached 6.94%in FSF.Alkaline pretreatment of FSF fermentation promoted VFAs yields to 205mg/g VSS with initial p H value of 9.5,1.5 times higher than that in the control.The chemical structure of the amorphous and crystalline regions of cellulose was changed after alkali fermentation with the cellulose content decreased in limited level.After alkali pretreatment,the relative abundance of proteiniclasticum was significantly increased to 14.6%.Canonical correspondence analysis(CCA)revealed the mutual and interaction relationshipwas between microbials and environmental parameters.Mass balance and energy consumption suggested that the daily VFAs yields from FSF fermentation was 230.8 kg VSS/d,4.9-time higher than that of WAS.Alkali pretreatment could increase the yield by 5%,but it is still insufficient compared to the demand for denitrification.(3)In view of the bottleneck of limited conversation rate of organics in WAS fermentation due to unbalanced carbon to nitrogen ratio(C/N),the conditioning effect of FSF on enhancement of VFAs production from WAS was undertaken.Different VSS dosage ratios(F/W-1:6,2:3 and3:2)was investigated as well as thechanges in microbial community structure and the interaction mechanism between microorganisms and environmental factors.Results showed that the co-fermentation group with F/W-2:3 had the highest VFAs production with 432 mg/g VSS,3.5 times higher than that of WAS fermentation alone.The proportion of butyric acid and valeric acid were closely related with FSF and WAS composition,respectively,in the co-fermentation groups.Co-fermentation significantly promoted the hydrolysis of polysaccharides and proteins in WAS,with s COD dissolution rate increased by 40.8%in F/W-2:3 group.Microbial community structure and CCA analysis results showed that F/W-2:3 had the highest abundance of hydrolytic and acid-producing bacteria.WAS and FSF were respectively well correlated with the relative abundance of proteolytic bacteria Proteiniclasticum and scellulose-degrading bacteria Clostridium.(4)A model of utilization efficiency of organic matters for biological nutrient removal(BNR)was constructed.Some key parameters were evaluated,including C/N ratio,aerobic mineralization rate and VFAs conversion rate.Then,the model was used to evaluate the carbon utilization efficiency of the proposed process.Furthermore,the advantages of the new scheme over the traditional A~2O process in energy saving and emission reduction were evaluated by using a life cycle assessment model.Results showed that the carbon source supply rate increases by 1%when the mineralization rate decreases by 1%or the VFAs conversion rate increased by 2%.Under the optimized fine-sieving conditions,X_S lost in mineralization could be reduced by 15.1%with WAS reduced by 40.3%.Howerver,it also resulted in a 12.3%gap of organics demand for denitrification.FSFfermentation alone can compensate for 5.9%of the gap,alkali pretreatment can increase by 2.9%.Co-fermentation can compensate for 13.5%of the carbon source loss,and reduce the total energy consumption by 24.0%.After co-fermentation,the structure of denitrifying carbon source was significantly optimized,with SA/COD ratio as high as 0.5.The results of life cycle assessment showed that the proposed process could reduce the ecotoxicity,primary energy consumption and climate change potential by 21.9%,4.4%and 3.5%,respectively. |
PDF Full Text Request