Study On The Mechanism,and Trade-offs Between Energy Saving And Emission Reduction Of Anaerobic-anoxic/nitrifying Induced Crystallization Process

Currently,wastewater treatment technology should possess the functions of water quality improvement,low energy consumption and pollutants-recycling under the concept of sustainable development.In the conventional biological nutrient removal processes,there often exist several shortcomings including the insufficient carbon source in biological denitrification,sludge retention time conflict between phosphorus removal organisms and nitrifiers,and opposite targets of energy saving and emission reduction.Consequently,the effluent quality standard could not meet the Chinese Discharging Standard for Urban Wastewater Treatment Plants.Phosphorus is the main element of water body eutrophication,but it is also a scarce resource.The urban domestic sewage contains rich phosphorus resource,which has great potential for phosphorus recovery.If the phosphorus in wastewater is recycled,it can not only avoid the eutrophication of water body,but also realize the sustainable utilization of phosphorus resources.For the biological nutrient removal from urban sewage,the induced crystallization coupled with activated sludge system not only can meet the goals of nitrogen and phosphorus removal,but also can realize the recycling of phosphorus resources.However,there are some problems in the operation of the combined process:a lower phosphorus recovery efficiency and purity as a result of homogeneous precipitation in induced crystallization unit;the effects of chemical phosphorus recovery on biological nutrient removal system has not been clarified;the introduced induced crystallization unit increases the aeration and chemical consumption of the system,which makes the coupled process with higher energy consumption.To overcome above drawbacks,the research takes anaerobic-anoxic/nitrifying/induced crystallization process(A2N-IC)as model technology which was developed in our previous work.Firstly,a novel high-quality phosphate recycling strategy based on the supersaturation control is proposed.Secondly,the dynamic equilibrium relation between chemical phosphorus removal and biological phosphorus removal system,as well as its influence on key microorganism in the system will be investigated.Lastly,the evaluation and improvement of integrated process are also investigated under the instruction of multiple objective optimization theory.The above researches would provide theoretical guidance and technical support for sewage phosphorus recycling industrialization,and also lay a solid foundation for efficient and low energy consumption wastewater treatment process.The metastable zone of solution is the precondition of crystallization process and is also the key to obtain high quality crystal products.The effects of different factors on the metastable zone of HAP crystallization were investigated,which can provide theoretical guidance for the subsequent study on the recovery of phosphorus from wastewater based on HAP crystallization.The results indicated that metastable zone width(MZW)was increased with the rise of cooling rate.According to the Kubota equation,there is a linear relationship between the MZW and the cooling rates.The presence of crystal seeds delayed the occurrence time of critical conductivity thus increasing the MZW.Calcite as the crystal seed,exhibited the best performance on improving the MZW due to its specific surface area and composition among three tested seeds.MZW became narrow with the increase of the stirring intensity,and the acidic solution(pH 6.0)had a larger MZW than that in alkaline solution(pH 8.0).Several typical impurity ions could broaden the MZW of HAP,and the order of the impact on △MZW was:CO32->Fe3+>Cu2+>SO42-.The homogeneous nucleation of hydroxyapatite(HAP)crystallization in high levels of supersaturation solution has a negative effect on the phosphorus recovery efficiency due to the generated HAP microcrystalline with poor settleability.In this study,a series high-performance approaches for phosphorus recovery from anaerobic supernatant using multiple reagents feed ports,different recirculation ratio and three series-coupled air-agitated crystallization reactors were developed and characterized.Compared with conventional supersaturation control strategies(multiple reagent feed points and recirculation),multistage induced crystallization system showed a high recovery efficiency and low microcrystalline ratio.Furthermore,the new process was found to have a broad spectrum of handling ability for different concentrations of phosphorus-containing solution with the range of 5～350 mg L-1.Meanwhile,the crystallized products were detected and analyzed by scanning electron micrograph(SEM)with energy dispersive spectrometry(EDS)and X-ray diffraction(XRD),which were proved to be HAP with a high purity.Particle size analysis showed that microcrystalline size was successively increased with the sequence of series-coupled reactors,confirming the conjectural mechanism that multistage induced crystallization system provided an appropriate condition for crystallized products growth,aggregation and precipitation.The traditional acclimation method of DPAOs sludge hampers the fast start and steady operation of enhanced biological denitrifying phosphate removal process.An efficient one-step domestication method with mixed electron acceptors and short time post-aeration was developed for the enrichment culture of denitrifying phosphorus removal sludge.Results showed that the proposed method with optimal proportion of nitrite and nitrate(8/37)could significantly shorten domestication time(28 d)compared with the traditional two-step method(60 d).Compared with seed sludge,microbial community shifted obviously,and the dominant microbial population of Dechloromonas-related phosphorus removal bacteria increased significantly.Meanwhile,anaerobic/oxic condition was favorable for the enrichment of Candidatus Accumulibacter-related phosphorus removal organisms,and short time post-aeration in the proposed method could reduce the potential public health hazard.For better coupling of chemical phosphorus recovery systems and biological nutrient removal systems in A2N-IC system,this study investigated the comprehensive effect of chemical phosphorus recovery on the biological nutrient removal process reflected by process performance and activated sludge characteristics.The optimal SSR was 0.3,at which point the dual goals of resource recycling and meeting effluent discharge standards can be achieved.Variations of activated sludge characteristics at different SRR indicated that the polysaccharide were more important than protein in resisting the toxicity of the imported Ca2+ on microorganisms.The SVI and MLVSS/MLSS ratio were all increased with the increase of SSR,this might because the increase in Ca2+ favored the disintegration of zoogloea and released more biodegradable organics.Metagenomics and metabonomics were employed in profiling mode to reveal the changes of activated sludge system when introducing the chemical phosphorus recovery unit.Results showed that the microbial community structure,functional potentials,key enzymes,and intracellular metabolites were changed by the induced crystallization unit.The relative abundance of microorganisms and key enzymes related to nitrification were less changed,while the abundance of related microorganisms and key enzymes of phosphorus metabolism were changed greatly.The accumulation of intracellular amino acid is conducive to resisting adverse external environments.The lower 3-hydroxybutyrate in DS1 indicated that the stored energy and carbon source(i.e.,PHAs)in denitrifying phosphorus removal sludge was negatively affected by the introduced phosphorus recovery system.The overall variations in the metabolic profiling revealed by metabolomics analysis were in general agreement with the results of functional classifications based on metagenomic analysis.A systematic calibration and validation procedure should be developed for the complexity of mechanistic model of anaerobic-anoxic/nitrifying(A2N)two-sludge system.To this end,an efficient method based on phase experiments,sensitivity analysis and genetic algorithm was proposed for the model calibration.Phase experiments(anaerobic phosphorus release,aerobic nitrification and anoxic denitrifying phosphate-accumulating)of A2N-SBR were performed due to the fact that it can reflect the process conditions accurately and improve the model calibration efficiency.The calibrated model was further validated using thirty batch experiments and three months dynamic continuous flow experiments for A2N-SBR and CF-A2N,respectively.Several statistical criteria were conducted to evaluate the accuracy of model predications including ARD(average relative deviation),MAE(mean absolute error),RMSE(root mean square error)and Janus coefficient.Results of visual comparison and statistical analysis all showed that the calibrated model could provide accurate predictions for the effluent COD,NH4+-N,TN and TP with only one iteration.In A2N-IC process,the introduced induced crystallization unit increases the aeration and chemical consumption,which makes the coupled process with higher energy consumption.In this study,multi-objective optimization method was used to optimize the relationship between water quality,energy consumption and total volume of construction.It found that the effluent exceed the standard index was decreased with the increase of energy consumption.The effluent concentration of COD,TN,NH4+-N and TP under optimal strategy were better than the original strategy,and the average effluent concentration were decreased by 2.22 mg/L,0.47 mg/L,0.13 mg/L and 0.02 mg/L,respectively.After optimization,the total energy consumption was decreased from 0.152 ￥/m3 to 0.126 ￥/m3,and the total construction volume was increased from 0.316 m3 to 0.332 m3、Meanwhile,the cost of pump energy(PR),reagent consumption(RC)and sludge production(SP)showed a drereased trend,but the aeration energy(AE)was increased.