| Activated sludge process is the most effective biological wastewater treatment technology implemented by wastewater treatment plants in China,and wasted activated sludge is the main by-product of this treatment process.Since the wasted activated sludge contains some potentially toxic organic or inorganic pollutants,improper treatment and disposal will cause secondary pollution to the natural environment and threaten public health.Anaerobic digestion can lead to the reduction and stability of WAS,the killing of pathogenic microorganisms,and the energy recovery in the form of methane,it is widely used in wastewater treatment plants around the world.The efficiency of traditional anaerobic digestion is limited by the biochemical methane potential and hydrolysis rate of wasted activated sludge.In order to obtain higher methane production,sludge needs to be pretreated before anaerobic digestion.However,the existing effective technologies require a large amount of energy or chemical input,which increases the cost of treatment and is not conducive to the sustainable operation of wastewater treatment plants.It has been reported that free ammonia,which can be obtained in situ from the digestion liquid,not only accelerated the decomposition of wasted activated sludge,but also enhanced the biodegradability of wasted activated sludge at appropriate levels,resulting in high methane production.Therefore,the sludge pretreatment technology based on free ammonia is considered to have significant economic and practical advantages in practical applications.Although there have been several efforts to advance the application of free ammonia technology,free ammonia used in all the previous studies was derived from pure chemicals rather than real digestion liquid.The complex and variable composition of real digestion liquid makes the application of the free ammonia-based method in real-world situations full of uncertainties.To date,however,there is no research to show whether the reflow of actual digestion liquid can improve the methane production of sludge anaerobic digestion,which needs to be clarified urgently.Therefore,on the basis of the existing research results of free ammonia regulation of sludge treatment,this paper combined multidisciplinary research methods and means to study the effect of digestion liquid and its main components on methane production from anaerobic digestion of sludge.The influence of the digestion liquid on the key biochemical processes of sludge digestion,the changes in the physiological state,community succession and composition,key enzyme activities,key gene expression and metabolic pathways of microbial cells in different digestion systems,and the relationship between these characteristics and reflux ratio of digestion liquid was systematically analyzed to reveal the mechanism of the reflux of the digestion liquid to strengthen the anaerobic digestion of sludge.On this basis,the effect of digestion liquid on the dewatering performance of digested sludge was explored,and the mechanism insights were provided for sludge dewatering by refluxing digestion liquid from the aspects of sludge physicochemical properties,liquid composition composition and solid phase microstructure changes.The purpose of this paper was to clarified the mechanism of the enhanced anaerobic digestion of sludge by refluxing digestion liquid,and to provide direct technical support for the process application of this technology.In order to evaluate the effect of different reflux ratios of digestion liquid on sludge anaerobic digestion,a series of short-term batch experiments and long-term semi-continuous anaerobic digestion experiments were set up in this study.By refluxing different proportions(0%,5%,10%,15%,20% and 25% v/v)of the digestion liquid during the alkaline(p H 9.5)pretreatment stage of the WAS,the changes of typical intermediate metabolites(such as protein,polysaccharides and short-chain fatty acids),volatile suspended solids(VSS)content,and target product methane during the digestion process,and combined with the improved kinetic model analysis to study the effect of the reflux ratio of the digestion liquid on the anaerobic digestion performance of sludge.The results showed that the reflux digestion liquid can enhance the anaerobic digestion performance of the WAS,and the strengthening degree increased with the increase of the reflux ratio in the range of 0%-20% of the reflux ratio.The digestion liquid increased the biochemical methane potential of the sludge,but decreased the hydrolysis rate.The reflux of the digestion liquid accelerated the conversion rate of protein,carbohydrates and other organic substances in the digestion reactor,increased the content of short-chain fatty acids,promotes the release of ammonia and the VSS reduction of sludge,and effectively achieved high levels of free ammonia and sludge reduction.After the digestion liquid entered the sludge digestion reactor,the in-depth analysis and comparison of the sludge dissolution,the biodegradability of the dissolved substrate,and the reaction rate of each biochemical process in different reactors were the basis for understanding the effect of the digestion liquid on the anaerobic sludge of the sludge.A series of anaerobic digestion experiments with different model compounds as substrates were carried out to study the effect of reflux ratio and different components on the degradation and degradation rate of each substrate,so as to reflect the effect of digestion liquid on reaction rate of typical biochemical process of digestion.It turned out that the digestion liquid reflux promoted the migration of organics in sludge from solid phase to liquid phase and improved the biodegradability of released organics,but significantly inhibited the hydrolysis and methanogenesis processes.Ammonium and metal ions in the digestion liquid were the main factors for the improvement of methane production.The reason may be that the ammonium in the digestion liquid was converted into free ammonia in the alkaline pretreatment stage,which effectively promoted the decomposition of sludge and the release of organics,and metal ions may be involved in the expression and activation of key biological enzymes,promoting the reaction rate biochemical processes of digestion.Recalcitrant organics in the digestion liquid reduced methane production through suppressing the bioprocesses,especially methanogenesis,which may be related to the fact that humic acids hinder the regeneration of ferredoxin in the acetoclastic methanogenesis.The digestion rate and performance of sludge were the interaction of several key enzymes and functional microorganisms.The reflux ratio and different components of the digestion liquid may lead to changes in macroscopic characteristics such as the number and composition of microbial communities,and even induced changes in the key enzymes,functional genes,metabolic pathways and other microscopic characteristics of functional microorganisms,thus affecting the anaerobic digestion performance of sludge.It was important to understand the mechanism of digestion liquid affecting the anaerobic digestion system of sludge to analyze the changes of microbial characteristics of each system by using a variety of biological detection techniques.The results showed that the digestion liquid caused more and more severe damage to the physiological state of microbial cells.Digestion liquid reduced the activity of key enzymes involved in hydrolysis and methanogenesis processes.The presence of digestion liquid decreased the relative abundance of microbial genera responsible for decomposing organic polymers in the sludge,but increased the number of methanogens.Ammonium in the digestion liquid induced evolution of the microbial community structure and diversity in the reactor.The key genes involved in organics hydrolysis metabolism,primary substrate metabolism and methane synthesis were all highly expressed.Digestion liquid also upregulated the contribution of microbial variation induced by digestive fluid to methanogenesis.The simultaneous stimulation of functional microbial abundance and metabolic activity may be the main reason for the increase of methane production in digestion liquid reactors.At the end of the digestion process,the dewaterability of digested sludge was the key and decisive reference index for for the selection of subsequent treatment and disposal methods.Generally speaking,the water content of digested sludge was very high.In order to save the operation cost of subsequent links,dewatering measures such as pressure filtration and centrifugation were required for sludge.Dewatering was also beneficial to the further reduction of sludge,so this paper continues to investigate the effect of digestion liquid on the dewaterability of digested sludge.The change of physicochemical properties,liquid component content and solid microstructure caused by different reflux ratio of digestion liquid during anaerobic digestion was the key to analyze the influence of digestion liquid on dewaterability of digested sludge.The results showed that capillary suction time(CST)and specific resistance to filtration(SRF)of sludge were significantly reduced by reflux of digestiin liquid.Physical and chemical properties showed that the digestion liquid reduced the interaction force,adhesion and bound water content,increased the relative hydrophobicity,median particle size and contact angle,promoted the release of water inside the digested sludge,improve the dewatering efficiency of digested sludge.In addition,the change of solid phase structure by digestion liquid was beneficial to the dewaterability of digested sludge,but the increase of protein,carbohydrate and humic acid contents in liquid phase worsened the dewatering performance of digested sludge.Finally,the significance of the "closed loop" treatment mode of enhanced sludge anaerobic digestion with digestion liquid in scientific research and engineering application was expounded.Economic analysis showed that digestion liquid reflux can save ～$ 0.3 million cost in sludge transport and disposal,and get ～$ 0.8 million benefit in energy production from methane annually in a wastewater treatment plants with a treatment scale of 105 m3/day,as compared with traditional alkaline treatment digestion system.This study systematically elucidated the effect of reflux ratio and different components of digestion liquid on sludge anaerobic digestion.The mechanism of digestion liquid to enhance the anaerobic digestion and dewaterability of sludge has been deeply revealed,including the effect of digestion liquid on the various biochemical sludge digestion process,the evolution of macro-microscopic characteristics of the microbial community,the difference of physical and chemical properties of sludge,the change of solid phase structure and liquid phase composition.It provided a new idea for the development of economical and efficient sludge anaerobic digestion intensification methods,and also provided a Theoretical assistance for the expansion of related technical methods based on the digestion liquid intensification of sludge anaerobic digestion systems. |
PDF Full Text Request