| Aerobic composting is an effective method to harmless treat, reduce and reuse sewage sludge, and sewage sludge compost for land use is a promising means of disposal. However, there are many problems of composting technology at present, including adding excess amendment, low composting efficiency, high energy consumption, unstable quality of compost and unscientific application rate of sewage sludge compost. This paper studied variations of organic matters, plant nutrients, and heavy metals during sewage sludge composting process, and discussed rational aeration rate at different stages during composting and rational application rate of sewage sludge compost. The main conclusions and innovations were following.1. Aerobic co-composting of sewage sludge mixed with wheat straw and maize straw, lasting35d, was carried out. Wheat straw bran (d<5mm), wheat straw chip (3cm<d<5cm), maize straw bran (d<5mm) and maize straw chip (3cm<d<5cm) were amendments in treatment1～4respectively. The following parameters were measured at different composting stages:total organic carbon (TOC), dissolved organic carbon (DOC), humic substances content (humic and fulvic acid), H/F and fiber. The role and relevance of DOC, H/F and temperature, used as parameters of compost maturity, were analyzed. TOC decreased during composting process constantly, Dec of TOC reached to31.30%,27.09%,54.87%and48.97%respectively; DOC concentration declined to9.48g·kg-1,7.15g·kg-1,9.10g·kg-1and11.07g·kg-1gradually; humus content didn’t increase significantly; H/F increased to1.54,2.21,1.56and1.90respectively in the four treatments; Fiber was degraded slowly. The results showed maize straw bran was the best amendment in composting.2. The objective of this study was to investigate the influence of the proportion of maize straw on the quality of sewage sludge compost. Sewage sludge mixed with maize straw at three different ratios (0.5:1,1:1,2:1,W/W, dry weight) was aerobically composted for35d. Temperature, organic carbon (TOC), C/N, nitrogen (TN), rapidly available P, rapidly available K, humic acids (HAs) and germination index (GI) were examined and analysed during composting process. The pile temperature of all treatments could reach the national harmless treatment standards. TOC and C/N decreased gradually, and the decreases were positively correlated to TOC and C/N of the initial materials. At the end of composting, TN and rapidly available K increased, HAs didn’t increase significantly, GI of all treatments increased to more than60%. Rapidly available P fluctuated in the beginning, and then increased. The results showed that higher ratio of sewage sludge could produce better compost which had a higher lever of N, P, K, and HAs.3. BCR (European Community Bureau of Reference) sequential extraction method was used to investigate the effects of different ratios of maize straw to sewage sludge on speciation of Cu and Zn during composting process. The distribution ratios of different forms of Cu and Zn could reflect the transformation of heavy metals between the mobile fractions (acid-soluble fraction, reducible fraction, oxidizable fraction). The concentrations of Cu and Zn in the composted mixture increased, however, the absolute content of Cu and Zn remained stable after composting process. It was found that the heavy metals were only transformed between the mobile fractions, the distributions ratio of residual fractions remained stable. The concentration factors of Cu and Zn increased with the ratios of maize straw to sewage sludge. Higher proportion of maize straw might contribute to the transformation of acid-soluble fraction of Cu to oxidizable fraction. Cu was mainly found in reducible fraction and oxidizable fraction. The mobility and bioavailability of Zn increased during the composting process. Zn mainly existed in acid-soluble fraction and reducible fraction. The lower ratio of maize straw, the higher concentrations of Cu and Zn in the compost product, but Cu and Zn didn’t play a decisive role on seed germination (GI).4. The humification of organic matter has a significant effect on the physicochemical form of heavy metals during aerobic composting. In this study, influence of humic substances (HS) on bioavailability of Cu and Zn was characterized during120d co-composting of sewage sludge and maize straw. At the initial stage of composting, Cu and Zn in sewage sludge were released as organic matter was degraded, and water soluble Cu and Zn increased markedly. Water soluble Cu (primarily FA-Cu) and FA content decreased after21days whereas water soluble Zn (primarily Zn2+) increased during the whole process. Both HA-Cu and HA-Zn were significantly and positively correlated with HA and H/F, respectively. At the end of composting, the distribution coefficients of HA-Cu and HA-Zn reached27.50%and3.33%respectively with HA-Cu/HA-Zn ratio increased from1.29to2.73. The results suggest that Cu combined with HA more strongly than Zn, and composting treatment could decrease bioavailability of Cu markedly. Appropriate extension of composting time could contribute to the formation of HA and passivation of these heavy metals.5. FTIR, fluorescence, UV spectra and E4/E6of humic acids (HA) in sewage sludge compost were determined during120days composting process. The results showed a reduction in polysaccharides and aliphatic components, and a relative increase in aromatic components and oxygen element. The fluorescence spectra of HA exhibited a sharp reduction during composting process, and the primary peak didn’t shift significantly. The UV spectra of HA was simple, and it wasn’t suited to evaluate compost maturity. After63days composting process, spectroscopic characteristics of HA had a same variation tendency with pile temperature and GI, and they could be used as auxiliary indicators to evaluate compost maturity.6. Influence of air temperature on pile temperature variety during sewage sludge aerobic composting was discussed, and the relation of variety curve of pile temperature and aeration rate were analyzed. According to the research results published, rational aeration rate and control method of sewage sludge aerobic composting were put forward. The composting process could be divided into five stages:initiation stage, temperature slowly increasing stage, temperature fast increasing stage, high temperature stage and temperature decreasing stage. Aeration rate should be0.01m3·m-3·min-1,0.02m3·m3·min-1,0.04m3·m3·min-1,0.17m3·m-3·min-1and0.04m3·m-3·min-1at the five stages previously stated respectively. Time/temperature-based aeration control system was suitable, and ventilation interval was20mm.7. According to toxic response factor of different forms heavy metals, rational application rate of sewage sludge compost was determined. Pollution index and geoaccumulation index were used to evaluate contamination degree of sewage sludge compost. This method stressed the importance of the forms of heavy metals and made up for the shortage of controlling the total amount of heavy metals in the past. Rational application rate of sewage sludge compost in Yangling was3.69t·hm-2·a-1. The results showed Cd was the most important element to restrict application of sewage sludge compost in the area. |
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