| A large quantity of waste activated sludge(WAS)is inevitably generated in municipal wastewater treatment plants(WWTPs),which has been a severe and important problem in environmental fields.Anaerobic digestion is considered as an efficient and environmentally benign strategy,by which the sludge reduction and recovery of valuable resources VFAs are simultaneously realized.However,the low sludge anaerobic digestion rate,dewaterability deterioration and disposal of sludge residue derived from digestion process hinder the development of WAS anaerobic digestion.Thus,this study focuses on the development of efficient and low-cost pretreatment,simultaneous enhancement of VFAs production and dewaterability of WAS and the utilization of anaerobically digested sludge residue in three aspects:Firstly,the effects of two novel pretreatment(ZVI-acid and the combined use of nitrite pretreatment and alkaline pH)on WAS hydrolysis and acidification were assessed.Then the combination treatment of NaOH and Mg(OH)2 was applied to anaerobic digestion of WAS for simultaneously enhancement of VFAs production and sludge dewaterability.Moreover,a new strategy of sludge management through the pyrolysis of anaerobically digested sludge to produce biochar was established,in which the prepared biochar was applied as the adsorbent for adsorption of phosphate and soil amendment.In addition,the,impacts of the types of commercial biochar and fertilizer applied to soil on the leaching and retention of nutrients were explored,which provided a theoretical basis and technical support for the utilization of WAS biochar.The detailed research contents are presented as follow:(1)Two innovative pretreatment(ZVI-acid and the combined use of nitrite pretreatment and alkaline pH)were employed to enhance WAS anaerobic digestion.The experimental results showed that the SCOD concentration increased from 388(raw sludge)to 1803 mg/L with pH 3.0 and ZVI dosage of 1g/g.Furthermore,the maximum total VFAs was 900mg/L,which was 1.53-fold higher than the blank test.Mechanism investigations showed that the hydrolysis of soluble substrate as well as acidification of hydrolysate was apparently enhanced by ZVI dosing.Nitrite pretreatment was demonstrated as an efficient strategy to enhance sludge solubilization and hydrolysis processes,which was attributed to the disintegration of sludge particle and dispersion of extracellular polymeric substances(EPS)in the presence of nitrite.Then,anaerobic digestion tests showed thatthe maximal VFAs accumulation(527.9 mg COD/g VSS)was achieved on the nitrite pretreatment&alkaline condition,which indicated that nitrite pretreatment and alkaline digestion achieve good synergistic effect on VFAs production.Mechanism investigations showed the alkali dosing could inhibit the concentration of FNA,protecting microbial community(especially acid-forming bacteria)during sludge digestion.(2)The VFAs production and sludge dewaterability were simultaneously enhanced with the combined of NaOH and Mg(OH)2.The maximum VFAs production(461 mg COD/g VSS)was obtained at the NaOH/Mg(OH)2 ratio of 75:25,which was much higher than that of the blank or soleNaOH.Moreover,nutrients removal and sludge dewaterability were improved by the combined using of NaOH and Mg(OH)2.Mechanism investigations revealed that the presence of Mg(OH)2 could maintain alkaline environment,which contributed to inhibit the activity of methanogens.Also,the bridging between Mg2+and EPS plays an important role in the solubilization and dewatering of sludge.High-throughput sequencing analysis demonstrated that the abundance of bacteria involved in sludge hydrolysis and VFAs accumulation such as Bacteroidetes,Proteobacteria and Firmicuteswas was greatly enriched with the mixtures of NaOH and Mg(OH)2.(3)The anaerobically digested sludge residue was used as feedstock to prepare biochar(ADBC).The characterization of biochar in different pyrolysis termpreture was studied and then the batch experiments were conducted to evaluate the sorption performance and mechanism of obtained adsorbents for phosphate in aqueous solution.The experimental results showed that the maximum adsorption capacity(21 mg/g)was achieved by ADBC obtained at 600?(ADBC-600),which was much higher in comparison with the biochar derived from raw sludge.The experimental data were analyzed with various sorption kinetic and isotherm models.The results showed that the pseudo-second order kinetic model and Langmuir isotherm model could best describethe adsorption of P on ADBC-600,which indicated thatthe adsorption process was controlled by chemisorption and the surface of ADBC-600 was homogeneous.Mechanism investigation suggested thatthe P removal process was mainly achieved by the precipitation of P through chemical reaction with Mg in the biochar.(4)The feasibility of reusing the P loaded biochar(ADBC-P)as an amendment for improving soil fertility was explored.The effect of ADBC-P on the chemical and physical quality of soil was evaluated by pH,available nutrients andorganic matter contents.The results showed that the application of ADBC-P had little effect for pH of soil,while significantly increased the available P,K and Mg contents and improved wheat grass growth.The enhancement of soil fertility with ADBC-P addition was most likely due to the presence of nutrients in the biochar itself.(5)The impacts of the types of commercial biochar(wheat straw biochar and Jarrah wood biochar)and fertiliser(chemical fertiliser and rock mineral-based fertiliser)applied to soil on the leaching of nutrients,wheat growth and uptake of nutrients was investigated by leahicng and pot experiments.Results showed that WSB decreased NH4+and NO3-leaching from CF-treated soil however improved cumulative leaching of P and K.In contrast,JWB addition reduced NH4+leaching from MF-treated soil,and NO3-and P leaching regardless of fertiliser type.Biochar addition can improve nutrients retention and plant growth though the reduction of nutrients leaching and increase of retention which are largely depend on the biochar and fertilizer types as well as specific nutrients,which may be because of direct nutrients supply of biochar and higher nutrients availability. |
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