| With the increase of sewage sludge production year by year,sludge disposal has attracted more and more attention all over the world.Agricultural use of sludge is the most economically feasible and environmentally sustainable method of sludge recycling and reuse.The most serious problem for sludge agricultural use is that the heavy metal content exceeds the standard.Chemical leaching is a short-term and efficient method to remove heavy metals from sludge.In this paper,GLDA/EDTA-acid(citric acid,glutamic acid,aspartic acid)was used to remove heavy metals in sludge at the optimal concentration of 1: 1,1: 2,2: 1.The heavy metals(Cu,Pb,Zn,Ni,Cr,Cd)removal rate and retention of nutrient elements(total nitrogen,total phosphorus,Available-N,Olsen-P and organic matter)in sludge after treatment were analyzed.The treated sludge was evaluated by MATLAB fuzzy comprehensive evaluation method to determine the agricultural grade of sludge.The treated sludge was mixed with soil in different proportions,and then the Chinese white cabbage,cucumber and wheat were cultivated separately.The effects of sludge addition ratio on seed germination rate,root elongation inhibition rate and seed germination index were studied.The feasibility of sludge application in agriculture was evaluated by SPSS correlation analysis and survival analysis.Finally,the results are as follows:(1)GLDA/EDTA-acid can effectively remove heavy metals from sludge.The optimal ratio of EDTA to citric acid,glutamic acid and aspartic acid was 1:1,1:2 and 1:1 respectively.The optimal ratio of GLDA to citric acid,glutamic acid and aspartic acid was 1:2,1:2 and 1:1 respectively.(2)The results of fuzzy comprehensive evaluation by MATLAB showed that the sludge after GLDA/EDTA-acid treatment belonged to Grade A sludge according to the principle of maximum membership degree of fuzzy evaluation.(3)Considering the removal rate of heavy metals in sludge,the agricultural grade of the removed sludge and the retention degree of nutrients,EDTA-citric acid worked best at 1: 1.EDTA-glutamic acid worked best at 1: 2,and EDTA-aspartic acid worked best at 1: 1.The optimal ratio of GLDA to citric acid was 1:2;GLDA to glutamic acid was 1:2;GLDA to aspartic acid was 1:1.(4)In the seed germination experiment,when the amount of sludge added was 20%,it can not only improve the germination rate of Chinese white cabbage,cucumber and wheat,but also promote the growth of its roots and has the potential to replace inorganic fertilizer.The correlation analysis of seed germination index and nutrient elements in sludge soil by SPSS showed that there was a strong correlation,most of which had significant correlation.Moreover,based on the survival analysis of the Chinese white cabbage that was most affected by the amount of sludge added,the survival distribution of the Chinese white cabbage cultured with the added sludge was not different from that of the blank and soil,only the median germination time.(5)GLDA-acid was better than EDTA acid in the removal of heavy metals in sludge.The retention of total nitrogen and total phosphorus in sludge treated with GLDA-acid was better,while EDTA-acid had better retention of effective nitrogen,effective phosphorus and organic matter in sludge,but the difference was not significant.In addition,the proper amount of sludge after GLDA-acid composite treatment had less effect on seeds than EDTA-acid,and promoted seed germination and root growth. |
