The Mechanism And Control Of Ecological Risks On Land Applications Of Organic Solid Wastes

The development of our society and public living standard brings a large number of solidwastes, which pose the serious problems and challenges to our environment. Reusing organic solidwastes to agricultural land after composting may take the most considerably economic andenvironmental benefits. However, the presence of heavy metals and much higher contents ofnutrients bring potential risks on soil healthy, plant growth, and even underground water. Thisresearch investigated the effects of composting on heavy metal distributions and phytotoxicity oforganic solid wastes. Using pot experiments, we studied the evolutions of soil carbon, nitrogen,microbial population and diversity in short-term after addition of manure compost, and assessedthe impacts of sludge compost and sewage sludge on soil-plant system. Moreover, the activationof heavy metals and their downward movements in the compost-amended soil were investigatedby soil column experiment.Aerobic composting of sewage sludge (SS) and swine manure (SM) showed that, bothsystems decreased the total amounts and mobility of Cu, Zn and Pb, especially during thethermophilc and mesophilic phases. Furthermore, the germination rate (RSG), root growth (RRG)and germination index (GI) of pakchoi (Brassica Chinensis L.) and ryegrass (Lolium perenne L.)increased during the composting process, indicating that phytotoxicity of sewage sludge and swinemanure diminished with composting age. Linear regression analysis demonstrated that GI, whichrepresented phytotoxic behavior to the plants, could be poorly predicted by the sum of mobilemetal fractions (exchangeable +carbonate +reducible iron and manganese+ organic matter bound,EXCH+CAR+FeMnOX+OMB), i.e., Cu (EXCH+CAR+FeMnOX+OMB) in SS, Pb (EXCH+CAR+FeMnOX+OMB) in SM. However, the inclusion of other physicochemical parameters (pH,OM and DOC) could enhance the linear regression significances (R), suggesting that thetransformenation of heavy metal speciation and phytotoxicity of organic solid wastes duringaerobic composting was rather strongly dependent on multiple components than a single element.The pot experiments revealed that the population of Copiotrophic bacteria (colony-formingunits, CFUs), microbial activity and diversities fluctuated over time, and be demonstrated byHarmonics analysis in remarkable wave-like patterns with significant harmonics afterincorporations of cattle manure compost into soils. However, the pH, DOC, NH₄⁺-N and NO₃^--N only oscillated within the first week after addition of compost, and correlated with microbialpopulations and activities. The Pythium ultimum (soil-borne pathogen) damping-off, which wasmeasured by AUDPC (area under disease process curve), oscillated with significant harmonicsand similar periods to Copiotrophic CFUs after nutrient impulses. Moreover, the soil fromconventionally managed farm had the larger amplitude in fluctuations of microbial and chemicalparameters than organically managed farm after addition of compost, indicating that the responsesof conventional soil system were much stronger and organic soil ecosystem was steadier andhealthier.Lower addition rates of sewage sludge or composted sludge could significantly increase theheight and yield of pakchoi, and the growth of ryegrass enhanced with addition rates. Landapplication of sewage sludge or compost also increased the Cu and Zn concentrations in plants.Plant uptake of Cu and Zn significantly correlated with variations of their EXCH and CARfractions in the soil, and the coefficients increased with time. This revealed that organic wastesmight represent stronger impacts on the plants in long-term land application. Based on growth andmetal uptake of pakchoi and ryegrass, sludge compost posed less harm to plants and its optimumland application rate should be control within 20% and 60%, respectively.60cm long soil column experiment showed that, the most significant variations of soil pHappeared in 5～10cm soil profiles below compost application. Organic matter also had thedownward movements. Surface application (SA) of sludge compost significantly increasedconcentrations of Cu, Zn and Pb in upper soil leachate (0～10cm) in short-term, and accumulatedin the deeper soil sections. In the mixed applied treatment (MA), Cu, Zn and Pb in soil solutionsof 0～20cm sections reached the maximum on day 5 after addition of sludge compost, and metalcontents in the lower soil solutions (30～50cm) peaked on day 10. The results indicated thatcompost with high metal contents would bring risks on the underground water. Cu and Zn contentsin soil leachate were in logarithmic correlations with DOC contents, and Pb linearly related toDOC, indicating DOC played an important role on activation of heavy metals, especially under thesoil with high metal conditions. The mobile fractions of heavy metals, namely EXCH, CAR,FeMnOX, and OMB, also influenced their leaching concentrations. Especially the EXCH andFeMnOX of Cu, Zn and Pb were negtively correlated with their contents in soil solution.The bioleaching with Thiobacillus ferrooxidans (T.fe) was used for decreasing concentrations of heavy metals in sewage sludge. The removal rate of Zn reached 97.8% after 2 days. Theefficiency of Cu removing was 37% on the 4^th day and increasing persistently. For Zn, the removalrates were high when pH3.0～4.0, and when pH＜3.0 the removal rate of Cu was better. However,the losses of nutrition (TN, TP, TK and organic materials) were serious under the acidenvironment. The passivating agents fly ash (FA), lime (LM) and phosphorus rock (PR) were usedin the composting process to reduce the mobility of heavy metals. Three passivating agents couldsignificantly decrease total Cu, Zn and Pb in sludge. FA and LM reduced the percentages ofmobile fractions in Cu, Zn and Pb. However, PR could not work on immobility of Cu and Zn. Theresults from seed germination test showed that, the addition of phosphorus rock to sludgecornposting could significantly enhance seed germination rate and root growth, but lime-sludgecompost affected germination of pakchoi seeds and root growth of ryegrass. The coexistence ofhigh concentrations of Ca and high pH value might be not benefit for seed germination.