| It is imperative to solve the serious environmental problem that the sludge produced by the municipal sewage treatment plants has stacked too much. People need to change the traditional solution program imminently. Therefore, it is paid more attention for the research field that how the sludge can be treated in order to realize the rational and efficient resource utilization. Microbial desalination cells (MDC) can convert the chemical energy in organic of the sludge into the electricity power directly, and the produced electricity power can be used to desaline the seawater in-situ, it can realize the organic remove and sludge stabilization simultaneously. This technology offers a new opportunity to achieve the energy-form and the reutilization in the sludge treatment. At the monment, the studies in MDC system performance are still in preliminary stage. For the available substrate studies, there were few reports that the dewatered sludge, produced in the municipal sewage plant with the normal treatment process, was directly used as an anodic substrate. It is worthy to make further research in the reutilization with the sludge residue after the electrisity generation.It was suggested that the dewatered sludge as the MDC anodic substrate can generate the electricity power, desalt the salt water. And the rest anodic sludge can be continually used as a soil conditioner to improve the saline-alkali soil. In order to improve the MDC property and efficiently, and aviod the desalting-formed exceeding accumulation in the anodic sludge salinity, the MDC construction was modified. The5-Chambers Microbial Desalination Cells (5c-MDC),5-Chambers Microbial Capacitance Desalting Cell (5c-MCDC) and7-Chambers Stacked Microbial Desalination Cells (7c-MDDC) were prepared. The desalination and electricity generation property, organic matter degradation and forms transformation of heavy metal, and plant nutrition characteristic in anodic sludge were studied respectively. The improvement effects of the dewatered sludge and anodic sludge on the saline-alkali soil and the potential ecological risk of heavy metal were also analysed.Through the study in the MDC with the dewatered sludge as an anodic substrate, the result indicated that this system possessed the started rapidly (5~6d) and stabilized in long-term operation (200~300d) characteristics. The electricity efficiencies were almost equal in the3type MDCs, and the highest one (Pmax=3.178W/m3) was achieved by5c-MDC. In the desalination aspect: the5c-MDC was the best in desalination efficiency, and the desalination efficiency decreased with the NaCl initial concentration increased, that was61.48±0.10% (2g/L)>52.50±0.26%(10g/L)>48.30±0.61%(35g/L). Meanwhile, the5c-MCDC and the7c-MDDC both can avoid the salinity accumulation in the anodic sludge. Moreover, the7c-MDDC sludge degradation effect was the best in these three type cells. Specially, in the7c-MDDC, the organic matter removal rate was the highest; the removal rate can get17.71%at the42nd day, and32.7±0.81%at the200th day; an obviously increase can be observed in the available nitrogen, phosphorus and potassium content with time. After42days operation, the seed germination index in the sludge leach liquor can reach66.63±3.02%in7c-MDDC. Under the long-term operation in5c-MDC and7c-MDDC, the results showed that, as the operation times passed, the anodic sludge pH value can be maintained between6.5and7.5, the humic acid fluorescence peaks in sludge dissolved organic matter were enhanced, the heavy metal stable residual state proportion increased, and the heavy metal bio-availability decreased greatly.The results of saline-alkali soil improvement by the dewatered sludge or the anodic sludge indicated that, the dewatered sludge and the anodic sludge both can decrease the soil bulk density, pH value and exchange sodium percent, and increase the soil organic matter, total nitrogen, total phosphorus, available nitrogen, phosphorus and potassium content. Meanwhile, the anodic sludge improvement effect on the saline-alkali soil was much better than the dewatered sludge effect. When the sludge proportion exceed30%, the soil bulk density was below1g/cm3, pH was decreased by1~2, exchange sodium percent was below30%, the soil organic matter, total nitrogen, total phosphorus, available nitrogen, phosphorus and potassium content were increased by2.0~5.9,1.5~4.6,1.5~4.5,3.7~10.5,4.1~19.7,1.1~1.8times, respectively. As the incubating time extension, the sludge-treated soil properties, such as the soil bulk density, pH value, exchange sodium percent, organic matter content and total nitrogen content decreased continually, but the available nitrogen, phosphorus and potassium content gradually increased during the incubating period. Based on the seed germination and crop plants experiment results, the reasonal addition proportion of the dewatered sludge and the anodic sludge were both about30%~50%. After the long incubation and plant growth, the proportion of the0.05~1mm microaggregate could increase in the saline-alkali soil treated by sludge. Through adding sludge in the saline-alkali soil, soluble microorganism product and fulvic acid content in the soil dissolved organic matter increased, the humic acid fluorescence intensity enhanced. All these results indicated that sludge treatment can accelerate the formation of the soil aggregate, increase the microorganism amount and activity in the soil, and improve the soil humification degree. Nevertheless, two kinds of sludge addition can both increase the heavy metal total content in the soil. In the preliminary stage, the heavy metal bio-availabilities can be enhanced partly, but the total heavy metal amount was obviously lower than the soil environmental quality standards, and the residual state proportion was the highest among them. After long-term incubation, except As element, all the other element bio-availabilities decreased, and tended toward the more stable residual state. The enrichment factors of the heavy metal or similar heavy metal, which were detected between soil and wheat or soil and alfalfa, can be ordered like this: Ni, Zn, B, Cu, and Cr. The same element enrichment factor was higher in alfalfa than that in wheat.This study offered a system method to exploit the comprehensive utilization of the dewatered sludge. This work was made some theoretical foundation in MDC performance optimization, function expansion and saline-alkali soil improvement by the sludge. |
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