Studies On Biological Digestion With Earthworms And Resource Reutilization Of City Domestic Sludge

City domestic sludge mainly comes from sewage treatment, in which contains some toxic substances such as heavy metals, and easily causes secondary pollution problems for improper disposal. On the other hand, in domestic sludge exist abundant nutrients for plant growth. In this thesis, the differences of sludge characteristics before and after earthworm digestion were analyzed, and plant growth and uptake of heavy metals from the sludge and earthworm casts were also investigated.The main conclusions we obtained are as follows:(1) Water content in the domestic sludge decreased after digestion by earthworms, compared with that in the original sludge.(2) Organic content reduced 48.57% after the original sludge was transformed to the vermicompost. The decrease caused not only from the earthworm digestion and from the cooperation of microorganism in sludge.(3) Compared with the original sludge, the proportions of NH₄⁺—N and NO₃^-—N in the total nitrogen and the ratio of C/N increased after earthworms'digestion.(4) The proportions of available phosphorus in total phosphorus was significantly higher in the vermicompost than in the original sludge, while that of organic phosphorus in total phosphorus was relatively lower.(5) The proportion of available potassium in total potassium seemed lower in the vermicompost than in the original sludge, but no significant differences was found.(6) After the digestion of earthworms, pH in the sludge changed from 5.79 to 4.85, which probably resulted from the activities of earthworm.(7) Cation exchange capacity (CEC) in sludge increased while exchangeable base decreased after earthworm's digestion. Phosphatase activity was stimulated by earthworm's activity.(8) After the digestion of earthworms, the bacterial population had little change, while the population of actinomycetes and fungi remarkably increased.(9) Digestion of earthworms obviously reduced, the contents of the available Fe, Mn and Zn in sludge, but had no significant effects on available Cu content. The contents of total Fe, Mn and Cr were higher in the vermicompost than in the original sludge. Cd was not detected in the vermicompost, which indicated that earthworms had strong ability to accumulate Cd.(10) Solutions of the original sludge and the earthworms casts had certain inhibitory effects on the germination of perennial ryegrass seeds and wheat seeds, probably for the harmful substances in the domestic sludge.(11) Through analysis of the element compositions in the shoots of perennial ryegrass which grew in the original sludge and the vermicompost, it was found that contents of K, Mn and Zn in shoots had significance differences, which indicated that vermicompost promoted the nutrient absorption of perennial ryegrass.(12) Through the growth of perennial ryegrass in original sludge and in the vermicompost, into which Cu and Zn were respectively added, it was found that Zn content in the aerial parts of perennial ryegrass increased with the increase of Zn addition, while Cu was not detected in the aerial parts, probably due to the inactivation of Cu in sludge and vermicompost, or the reduction of transport from roots to the aerial parts.(13) Using the original domestic sludge and the vermicompost to cultivate wheat seedlings increased the contents of chlorophyll a, chlorophyll b and the total chlorophyll in wheat leaves, but the differences caused by the original sludge and the vermicompost was not significant. The contents of soluble sugar in aerial parts of wheat seedlings growing in the original sludge showed insignificant difference from that growing in the vermicompost, but that growing in the original sludge or in the vermicompost was significantly higher than that growing the control soil.