Effect Of Biochar On Transformation Of Organic Matter During Sewage Sludge Composting And Quality Of Compost

Biochar is a solid matter with porous structure produced by pyrogenation of biomass at the anaerobic or anaerobic conditions. The biochar is of rich carbon, stable property, high aromatic molecular structure and large surface area. Biochar, when used as amendments, can supply the carbon and energy sources to microbes, help the composting retain moisture and keep air flowing. The different sources of biochar such as charcoal, rice straw, wheat charcoal, or the same type of carbon with different pyrolysis temperature led to the different properties. However, researches on the usage of biochar are focusing on the loss of N and C during composting, information about the influences of the types of biochar on the transformation of organic matter during composting is scare. Therefore, we selected three biochars derived from wheat straws(WB), peanut shell (PB) and bamboo(BB), as conditioners, to proceed high temperature aerobic composting of sewage sludge. The impact of types and rates of biochars on the degradation and humification of organic matter during high temperature aerobic composting process were investigated and the quality of the final compost products was also evaluated. The results are as follows:(1) Temperature in all treatments reached50℃on the first day of composting, and met the hygiene requirements for the composting harmless standards. Addition of different origin biochar to the compost piles can increase the temperature rising rate but shorten the high temperature period, therefore reduce the loss of nitrogen and carbon, especially bamboo. Addition of biochar also improved the values of pH of compost piles especially the biochar derived from wheat straws. Though the values of electrical conductivity(EC) of compost piles were improved by biochar addition, they were lower than4.0mS·cm-1, showing no harm to plants when the compost products were used as soil amendments or fertilizers.(2)Total organic carbon decreased with composting time. Addition of biochar increased total organic carbon and the more the biochar addition, the higher the total organic carbon content. The difference in the TOC content resulted from biochar addition treatment occurred at the first three weeks, with the order of WB> PB> BB. Water-soluble organic carbon(WOSC) firstly increased and then decreased during the composting. The degradation rate of organic matter of compost piles with biochar increased was faster in the initial stage of composting, then was lower than CK. The biochar could decrease the content of WOSC effectively in the maturity stages. The rate of OM loss ranged from16.54%to46.06%, which occurred at high temperature period. Among the three types of biochar, increasing the rate of WB and BB addition did not increase PB is more different than other treatment with the increase of addition, the order was6%>9%>3%.(3)Biochar addition could reduce the content of HS, HA and FA, but increase the relative content of HA in HM, also increased the ratio of H/F. Among the three types of biochar, BB and WB were beneficial to increase the H/F ratio, and the highest H/F ratios were observed at the rate of6%. FTIR spectrum of HA from compost piles showed that applying biochar led to an increase in amides, but a decrease in polysaccharides and aliphatic components, especially at the rate of6%addition.(4) Composting could reduce the content of DTPA-Cu and DTPA-Zn in compost piles. This effect was enhanced by biochar addition. The highest efficiency was found at the rate of3%for BB and PB. As for WB, the inhibitory effect increased with the rate of addition. Addition of all types of biochar could not reduce the content of DTPA-Cd in compost piles.(5) The affinity of Cu to HA was strong, and the humification process increased the stabilization of Cu. compared with Cu, Zn had a lower affinity to HA, less Zn was complexed with HA. It was concluded that the humification process of organic matter was not the mainly way to reduce the DTPA-Zn during composting. Because HA formed in the humification process with lower hydroxyl and phenolic hydroxyl has lower affinity to Cd, the content of HA-Cd was significantly negative correlated with HA content.(6) Because of dilution effect, addition of biochar decreased the content of phosphorous and heavy metal in compost piles, but did not influence the total nitrogen content except BB.