Enhancement Of Sewage Sludge Composting By Phosphate Buffer Solution-Mg²⁺ Amendments And Its Mechanism Research

Composting is a low-cost technology for waste active sludge(WAS)treatment,in which organic matters are biologically converted into nutrient-rich,humus-like material for cropland application to improve soil fertility.However,massive nitrogen is lost via ammonia emission during the sludge composting process due to the low C/N ratio of WAS,which influenced sludge composting popularization.The main objective of this paper was to promote sludge maturation and reduce nitrogen loss.The effects of phosphate buffer solution(PBS)on sludge compost maturation and nitrogen loss were investigated.Then,Mg²⁺was added to further investigate the influence of sludge compost maturation and nitrogen loss.The changes of nitrogen,carbon source,microbial population structure and ammonia assimilation enzyme in various forms during compost were studied,and the related mechanism of PBS-Mg²⁺promoting sludge maturation and reducing nitrogen loss were clarified.Finally,a pilot project of sludge compost with a daily treatment capacity of 1 ton(moisture content:80%)was designed to verify the effectiveness of PBS-Mg²⁺on the pilot scale of sludge compost.(1)Effects of PBS on maturation and nitrogen loss in sludge compostingPBS addition was proved as an effective strategy to promote sludge maturation and reduce nitrogen loss.When the dosage of PBS was 0.6mmol·g^-1DS,compared with the Control,the organic matter loss(OML)increased by 25.7%,the cumulative NH₃-N emission decreased by 45.4%,and the total nitrogen loss decreased by 41.5%.When the dosage of PBS was 0.6mmol·g^-1DS,the effect of different adding time on sludge maturation is not significant,but the effect on nitrogen loss is significant.After 34 days of composting,compared with Control,the cumulative NH₃-N emission and total nitrogen loss of the experiment group treated with PBS on 4^th day decreased by 51.9%and 48.3%,respectively.(2)Effects of PBS-Mg²⁺on maturation and nitrogen loss in sludge compostingThe results showed that different types of magnesium compounds have different effects on sludge maturation and nitrogen loss(N_loss).The experiment group with MgSO₄ had the lowest nitrogen loss(N_loss/OML)for every 1%of organic matter degraded,which was 68.9%lower than that of Control.The addition of MgSO₄ has no obvious promoting effect on OML,but it can further reduce NH₃-N emission during composting.Compared with Control,after 34 days of composting with 0.4 mmol·g^-1DS MgSO₄addition,OML increased by 26.1%,cumulative NH₃-N emission decreased by 73.1%,and N_loss decreased by 73.4%.(3)The optimization of process parameter for PBS and MgSO₄The response surface method(RSM)is used for optimization of technological parameter,which were the PBS dosage,PBS dosage time and MgSO₄ dosage.According to RSM model,it's determined that the optimal dosage of PBS was 0.582 mmol·g^-1DS,optimal addition time of PBS was2.85 d,and the optimal dosage of MgSO₄ was 0.428 mmol·g^-1DS.Under this process parameter,after 34 days of composting,OML and N_loss were31.6%,and 11.7%,respectively.(4)The mechanism of PBS-MgSO₄ to promote sludge maturation and reduce nitrogen lossThe addition of PBS facilitated extracellular polymeric substances(EPS)solubilization in sludge,releasing carbon resources internally for microbial utilization.The results showed that K⁺in PBS could promote the transfer of polysaccharide in loosely bound EPS(LB-EPS)to dissolved organic matters(DOM).Compared with the Control group,during the entire composting process,the average content of polysaccharide in the compost with PBS increased by more than 68%.The effect of MgSO₄ on EPS secretion was not obvious.Moreover,the addition of PBS significantly increased the content of?-ketoglutaric acid during sludge composting,which enhanced the activity of assimilation enzyme and promoted the growth of Actinobacteria,Bacillus and Pseudomonas with ammonia assimilation function.The additional MgSO₄ caused struvite crystallization in the compost,which made part of the water soluble NH₄⁺-N in the compost convert to acid soluble NH₄⁺-N,reduced the content of water soluble NH₄⁺-N,thus further reduced the emission of NH₃-N and the nitrogen loss.(5)Evaluation of sludge composting in pilot-scale bioreactorsA pilot project of sludge composting with a daily treatment capacity of1 ton was carried out under the optimal technological parameters.After 34days of composting,the OML increased by 28.6%.The content of Org-N in composting products increased by 55.5%,the emission of NH₃-N decreased by 76.8%,and nitrogen loss decreased by 68.4%.Due to the addition of PBS-MgSO₄,the content of total nutrients(N+P₂O₅+K₂O)of composting products was higher(8.95%)than the limit value of organic fertilizer standard(?5%),while the total nutrient mass of control group was 3.82%,which did not meet the standard of organic fertilizer.