Aerobic Fermentation Technology Of Sewage Sludge And Evaluation Of Safety For Products

The land utilization of sewage sludge was considered to be a positive, effective, promising way for sludge disposal. The treatment of sewage sludge compost could make sludge harmless and stable.The compost product contains numerous nutrient elements and rich organic matter which could improve the structure of soil and provide nutrients for the plants. It could achieve the resource utilization of the sludge, solving the problem of sludge. This paper aimed to study the aerobic fermentation technology of sewage sludge for Tangwang sewage treatment plant and the best application rate of sludge compost was also studied. The effects of sludge compost on soil and plants were studied with the pot experiment of Brassica chinensis and ryegrass. It aimed to provide theoretical and practical support for the application of city sewage sludge. This study included findings as the following aspects:(1) According to the test, it found that the aerobic compost with the ratio of sludge: mushroom residue:straw=1:0.4:0.025, while adding2%o microbial inoculants, could maintain the temperature above50℃for11d during the composting process. The composting process could achieve the sterilization for high temperature, which was satisfied with the standard of non-hazardous sanitary. The amount of nitrogen loss during composting was28.22%, lower than the traditional nitrogen loss30%-50%, which realized the protection of nitrogen, improving the quality of composting products and agricultural value. The maturity of compost products was tested according to the ammonium nitrogen, conductivity and seed germination index. Ammonium nitrogen content and the conductivity of the compost products and seed germination index was60.03mg/kg,4.11ms/cm,86.3%, respectively, which conform to the requirements of the maturity indices. The compost products were dark brown in color, presenting a loose crumb structure, with a taste of soil, so that the compost products had been rotted.(2) The results of the research showed that the treatment of sludge aerobic composting could not reduce the content of heavy metals in sludge, but it could change the distribution of heavy metal forms in sludge. The contents of Cr, Cu, Zn, Cd and Pb in compost products are consistent with the limit of the control standards for pollutants in agricultural sludge (GB4284-1984)(pH≥6.5).At the end of composting, the percentages for the contents of effective biological Cr, Cu, Zn, Cd and Pb accounts for the total contents decreased.The treatment of sludge aerobic composting was helpful to make the activity of heavy metal dulled and reduce its bioavailability.(3) The appropriate amount of sewage sludge compost could promote the growth of Brassica chinensis and ryegrass, improving their biomasses, increasing the absorption quantity on N, P, K. When sludge compost was applied with the rates of1%-15%, the fresh weight of Brassica chinensis increased with the rates. The sludge compost applied with the rate of10%had met the nutrients demand of Brassica chinensis, and N, P, K contents in the aboveground part reached to52.68,5.89,26.86g/kg, respectively. The biomass of Brassica chinensis significantly decreased with the increase of sludge compost application rate when the rates were higher than20%. The biomass of ryegrass at10%application rate of sludge compost was higher than the5%treatment significantly.(4) The accumulation of Cu, Cd, Zn, Cr and Pb in Brassica chinensis was observed and increased with the increase of sludge compost application rates, compared to the control treatment with no sludge compost applied. According to national food hygiene standard in China, the content of Cr in the aboveground part of Brassica chinensis exceeded the limit of0.5mg/kg when the application rate of sludge compost was80%, which was0.52mg/kg, while Cd content exceeded the limit of0.2mg/kg when the rate was60%, which was0.22mg/kg. The content of Pb in the aboveground part of Brassica chinensis exceeded the standard in all treatments. The effects of heavy metal on the growth of plant also depended on the species of heavy metal. The concentration of heavy metals in the aboveground part of Brassica chinensis was significantly higher than those in the underground part of Brassica chinensis. The contents of heavy metal in ryegrass were not only affected by the application rate of sludge compost, but also depended on the frequencies of cutting, there existing obvious differences for heavy metal content of ryegrass in different picking times.(5) During the process for the growth of ryegrass, it could change the root soil environment through the effect of rhizosphere, and had certain effects on the distribution of heavy metal forms. After planting ryegrass, the changes for biological effectiveness of Cr and Cu were not obvious. Pb transformed to oxidizable and residual fraction with low biological activity and its biological activity was dulled to a certain extent. Cd and Zn significantly by the transformation from residual fraction to the first three fractions, biological activity was activated to a certain extent.(6) Sludge compost was rich in nutrients, but excessive application made inhibition effect on plant growth, and it would lead to the pollution of the environment. According to the results of the test, it suggested that the appropriate application rate of sewage sludge compost was about10%, and the application rate of20%had become the upper limit of the application of sludge compost.