| For the past few years,the sludge production of municipal sewage treatment plants is growing in China,and an effective way of sludge treatment and disposal is urgently needed.Thermal hydrolyzation-anaerobic digestion treatment technology can effectively improve the sludge treatment load of municipal sewage treatment plants,and is recommended for sludge treatment in China.However,the way of thermal hydrolyzation-anaerobic digestion sludge resource disposal still needs to be clarified.Garden use is a national action plan for prevention and cure of soil pollution in support of a means of sludge disposal,therefore,high solid content in sludge thermal hydrolysis anaerobic digestion process and garden use of disposal of the sludge disposal route,can be effective in achieving sludge stabilization and harmless at the same time,in the form of plant nutrients in further organic matter and nitrogen and phosphorus in sewage sludge utilization.At the same time,plant fibrous waste in agriculture and landscaping production,has the characteristics of high organic matter content,large porosity,can overcome the long-term use of sludge garden greening matrix hardening,low content of organic matter.Therefore,this study takes the thermal hydrolyzation-anaerobic digestion sludge of urban sewage treatment plant as the object to explore the effects of thermal hydrolyzation-anaerobic digestion and plant fibrous waste combination on plant growth characteristics of urban sewage treatment plant sludge applied in landscaping.The potential ecological risk of heavy metals,nitrogen and phosphorus leaching risk and soil microbial community structure after garden utilization were further discussed.The main research is as follows:(1)The physical and chemical properties of thermal hydrolyzation-anaerobic digestion sludge were analyzed.The results showed that the basic physical and chemical properties of thermal hydrolyzation-anaerobic digestion sludge,such as nitrogen and phosphorus nutrients,heavy metals,water content and p H value,were all within the standard range of sludge garden utilization,but the content of organic matter was low(<25%).After the combination of plant fibrous wastes(decommissioned garden wastes or crushed corn stalks),the organic matter content in the thermal hydrolyzing-anaerobic digestion sludge could meet the standard,while the heavy metal content was further reduced,and the electrical conductivity was within the acceptable range of plants(<2ms/cm).Therefore,the physicochemical properties of thermal hydrolyzation-anaerobic digestion sludge combined with plant fibrous waste meet the standard of garden utilization.(2)On the thermal hydrolysis-anaerobic digestion sludge under different distribution of matrix of plant growth characteristics of research,the results show that under the same conditions,compound with rotten garden waste of plant root length,seedling height,chlorophyll content and the effect of tiller number was superior to that of compound with broken corn stover,garden waste this is due to the finished high humus content.The results of the analysis of the optimal blending ratio and the range of plants showed that the mature garden waste was the best blending matrix for landscaping under the blending ratio of 60%,and tall fescue was the best plant.Therefore,thermal hydrolyzation-anaerobic digestion sludge should be combined with plant fibrous wastes with high humus in garden utilization.(3)The microbial community structure of soil under suitable plant and garden soil was analyzed,and the results show that thermal hydrolysis-anaerobic digestion sludge compound with 60% rotten garden waste optimal distribution of the matrix in garden utilization of soil,soil microbial population structure and gardens all contains a lot of plants stimulates the growth of microbial flora,at the same time,The microbial abundance and diversity of compound matrix were better than that of garden soil.Therefore,the insurance effect,complementary effect and synergistic effect of microorganisms in the compound matrix formed after thermal hydrolysis and anaerobic digestion of sludge combined with 60% of decomposed garden waste are stronger and have higher stability.(4)The results showed that ryegrass and tall fescue showed different enrichment characteristics for different heavy metals.The enrichment ability of ryegrass for four heavy metals was in the order of Zn>Cu>Pb>Cr,while the enrichment ability of tall fescue was in the order of Cu>Zn>Pb>Cr.The two plants had strong enrichment ability for Zn and Cu.The proportion of heavy metals in the three migration pathways was analyzed.The results showed that about 40% of the heavy metals were absorbed by plants,9% were lost with irrigation water,and 51% remained in the compound matrix.Compared with garden soil,the proportion of heavy metal absorption in the optimal compound matrix increased during garden utilization,and the proportion of residual metal in the matrix was low.Under the condition that the heavy metal content reached the standard,the risk of water pollution caused by irrigation was very low.Therefore,in the process of garden utilization after thermal hydrolyzation-anaerobic digestion of sludge combined with decomposing garden waste,different plants show different characteristics of heavy metal enrichment,and the risk of heavy metal migration with irrigation water is low.(5)The risk of heavy metal pollution and nitrogen and phosphorus leaching in the utilization of thermal hydrolyzation-anaerobic digestion sludge garden was analyzed and evaluated.The results showed that the risk of heavy metal pollution in unpretreated sludge is medium grade,(MRI)was 165.33,while the environmental potential risk grade of sludge was reduced to low risk state(MRI was 118.35)after thermal hydrolysis and anaerobic digestion.Through the analysis of heavy metal forms in sludge,the exchangeable state and carbonate bonded state contents in sludge of municipal sewage treatment plant are higher,that is,the heavy metal activity is stronger.After thermal hydrolysis and anaerobic digestion,heavy metals are transformed from exchangeable state and carbonate bonded state with higher activity to residue state with lower activity.Therefore,thermal hydrolysis and anaerobic digestion treatment can reduce the activity of heavy metals and alleviate the risk of heavy metal pollution during the utilization of sludge in gardens.At the same time,the risk of nitrogen and phosphorus leaching during the garden utilization of thermal hydrolyzation-anaerobic digestion sludge was analyzed.The results showed that the concentration and amount of nitrogen and phosphorus leaching of different complex matrix were higher in the first three leaching processes.Some nitrate nitrogen concentration exceeds the limit of class IV of groundwater Environmental Quality Standard,and the leaching amount of the first three times exceeds 50% of the total leaching amount.Therefore,attention should be paid to the prevention and control of nitrogen and phosphorus leaching pollution in the early stage of garden utilization of thermal hydrolyzation-anaerobic digestion sludge. |
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