| In recent years,energy neutral or energy positive has been proposed as the important indicator of future sewage treatment in domestic and foreign countries.The main idea is first to recycle organic carbon(COD)which is converted into methane,and then utilize low energy consumption technology for biological nitrogen removal.Simultaneous partial nitrification,anammox and denitrification-integrated fixed film activated sludge(SNAD-IFAS)process has been listed as an important process for biological nitrogen removal in the future because the advantages of simultaneous removal nitrogen and carbon,small footprint,and concise process.Therefore,the aim of this study is to develop a combined algae anaerobic digestion with rumen fluid pretreatment technology with SNAD-PBR process treating sludge digestion liquor for enhancing productivity,aiming to achieve biological nitrogen,carbon and phosphorus removal while reducing energy consumption,utilize rumen fluid pretreatment and co-digestion with anaerobic digestion sludge(ADS)on algae and waste activated sludge(WAS)recovering organic carbon energy to increase productivity,and provide a new idea and process for energy self-sufficiency wastewater treatment.Firstly,based on the growth and metabolism characteristics of algae culture,this study explored the effects of different influencing conditions on nitrogen and phosphorus removal during the treatment of sludge digestion liquid by photobioreactor.Optimization results by Box-Behnken Design(BBD)response surface method showed that the maximum NH4+-N removal rate achieved to 29.9%with phosphorus concentration,CO2 concentration and light intensity at 40 mg/L,2.6%and 11847 lx,respectively.Further optimization results by Central Composite Rotatable Design(CCRD)response surface method showed that the maximum NH4+-N removal rate achieved to 18.6%with light intensity and CO2 concentration at 4.4%and 11220 lx,respectively.Under the optimum conditions,the effect of different initial ammonia nitrogen concentration on NH4+-N removal rate is positively correlated and the kinetic constants and saturation coefficients were 0.72 d'1 and 7.65 mg/L.With diluted actual sludge digestion liquid as influent,the optimal HRT and the harvest frequency for photobioreactor(PBR)was 4 and 2 days.Under the optimum conditions,TN removal rate and removal load were kept at 68.7-71.8%and 27.8-38.2 mg/L·d,respectively.Total phosphorus removal rate and removal load were 72.5-82.8%and 3.9-5.2 mg/L·d,respectively.Secondly,on the basis of PBR,long-term operation effect by SNAD-PBR coupling process was studied.The maximum removal rates of TN,TP and COD by SNAD-PBR process were 90.2%,100.0%and 70.5%under the optimal conditions(HRT:1+4 d,influent NH4+-N concentration:400 mg/L and reflux ratio:1:3).When the reflux ratio was 1:3,the lower C/N and higher removal rate indicated that the activity of ANAMMOX bacteria in the reactor would not be inhibited by heterotrophic denitrifying bacteria.Partial denitrification is another way to provide nitrite for ANAMMOX reaction.In the SNAD-PBR coupled process,SEM and FISH results showed that AOB,ANAMMOX bacteria and DNB existed in the SNAD system.The results of specific activity analysis showed that nitrification mainly occurred in suspended sludge while ANAMMOX and denitrification mainly occurred in biofilm.The results of high throughput sequencing analysis showed that the new environment was more suitable for Candidatus Brocadia with reflux system introduced.There might be two different nitrogen removal pathways in SNAD-PBR process.The nitrogen removal process on biofilm was mainly completed by Candidatus Kuenennia and Denitratisoma,while the nitrogen removal process in suspended sludge was co-operated by Nitrosomonas,Nitrospira,Candidatus Brocadia and Denitratisoma.In view of the problem that cellulose in the cell wall structure of green algae leads to insufficient methane production in anaerobic digestion,the effects of rumen fluid pretreatment and co-digestion with ADS on algae anaerobic digestion were investigated through batch experiments.The results showed that the introduction of rumen fluid increased the hydrolysis rate of algae anaerobic digestion,and excessive inoculation ratio of rumen fluid could inhibit the consumption of acetic acid and propionic acid.Compared with ADS digestion alone,rumen fluid pretreatment and co-digestion with ADS both enhanced the biochemical methane potential of algae anaerobic digestion.In addition,the pretreatment and co-digestion of rumen fluid achieved the highest biochemical methane potential with the condition of 48 hours pretreatment time and 10%inoculation ratio(w/v)and the kinetic parameters k and Bo were 0.44 and 130.1 L CH4Vkg VS,respectively.Microbial community analysis showed that the relative abundance of Ruminococcaceae and Methanobrevibacter increased firstly and then decreased gradually with the increase of rumen fluid pretreatment time,which indicated that excessive pretreatment time would inhibit the activity of bacterial community.In addition,rumen fluid can not only promote cell wall degradation by pretreatment,but also interact with ADS in anaerobic digestion process,thus improving the methane production potential of anaerobic process.On the basis of previous studies,batch experiments were conducted to investigate the effects of different substrate mixing ratios and initial pH conditions on the co-digestion of algae and waste activated sludge(WAS)with rumen fluid pretreatment and co-digestion with ADS.The results showed that rumen fluid pretreatment and co-digestion with ADS were conducive to the increase of cumulative methane production and the growth of algae was superior to that of WAS.Sludge mixing ratio between 50%and 100%is the optimum mixing ratio range on co-digestion.At this stage,the cumulative methane production and the decomposition and utilization efficiency of the system to the substrate had little differences.Excessive or low pH is not conducive to the co-digestion of algae and WAS by rumen fluid and ADS.Microbial community analysis showed that Sporanaerobacter,Ruminococcaceae and Ruminococcus were three main acid-producing bacteria in anaerobic digestion with pH at 8.1.At this time,the cumulative methane production reached the maximum and the values of k and B0 were 0.3 land 166.52 L CH4/kg VS.CCRD response surface method showed that the main and secondary effects of the influencing factors were as follows:WAS mixing ratio>pH.In addition,the difference between TVFA and cumulative methane production was not obvious in the range of WAS mixing ratio>50%and 9>pH>8.7.Finally,on the basis of small-scale experiments,simple process flow design,energy consumption analysis and operation cost estimation of combined algae anaerobic digestion with rumen fluid pretreatment technology with SNAD-PBR process were carried out to provide some reference for WWTPs requiring upgrading.Taking the water quality and quantity of Dalian Xiajiahe Sludge Treatment Plant as design parameters and SNAD-MBBR process as comparison,it is concluded that the energy consumption and estimated running cost of per ton of water in SNAD-PBR process are about 2.55 kWh/m3 and 2.23 yuan/m3,respectively with the designed water quantity of 400 m3/d,which has remarkable economic and environmental benefits.Through anaerobic digestion with rumen fluid pretreatment-SNAD-PBR coupling process to recover energy(CH4)from wastewater and optimize the operation of each treatment unit,the productivity/energy consumption ratio has reached 1.05,achieving the goal of complete energy neutralization and operation. |
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