Research On Variation Of Pollutants During Sewage Sludge Composting

Agricultural using of municipal sewage sludge is its resource utilization, and it is part of an important solution to their disposal. But there are still many disagreements in its utilization, which exist mainly due to municipal sewage sludge contains large amounts of heavy metals, organic pollutants and pathogenic micro-organisms and other toxic and hazardous compounds. Therefore, it is important to study environmental behavior, transport and transformation of heavy metals and their influence factors of migration and transformation, environmental behavior of organic pollutants in compost and transformation patterns in municipal sewage sludge composting process. Sludge agricultural resources utilization measures proposed for the sciences provide a basis for utilization of Municipal Sewage Sludge in agriculture to reduce contamination risk to the eco-environment in the process, give it a good eco-efficiency, environmental, economical and social benefits.(1) This study examined changes and their influencing factors of various forms of heavy metals at different stages during sewage sludge composting processes using BCR sequetail extraction method. Cu, Ni, Cr, and Pb, except Zn and Cd, in sewage sludge were mainly in an oxidation state and residual state, with low bioavailability. Composting processes significantly reduced the bioavailbility of heavy metals, and altered the distribution of heavy metals among varirous forms, transforming acid-soluble and reducible heavy metals into more stable forms, oxidation state (OXI) and residual (RES). Residual fraction of Cu, Zn, Ni, Cd, Cr and Pb at the end of the composting increased by6.3%,6.7%,22.0%,15.2%,11.0%, and40.5%, respectively, compared with those at the beginning of the process. Heavy mental forms were significantly correlated with DOC content, but not with OM. pH mainly affected distribution of reducible and bioavailable fractions of heavy metals.(2) With the sewage sludge as a raw material, the mushroom residue and straw as conditioners, four treatments of aerobic composting were designed which were A (sludge:mushroom residue: straw=1:0.4:0.025), B (sludge:mushroom residue:straw=1:0.3:0.025), C (sludge:straw=1:0.12) and D (sludge:straw=1:0.09). Various forms of heavy metals of the compost product were determined according to BCR sequential extraction method. The results showed that the Cu, Zn, Ni, Pb in sewage sludge were mainly in the oxidisable state and residual state, with lower biodegradable utilization. But the Zn and Cd were mainly in acid-soluble state and reducible state, with higher biodegrable utilization. Composting process significantly reduced the biological effectiveness of heavy metals content of Cu, Zn, Ni, Pb, and changed the formation distribution of Cr and Cd. The compost also changed the biodegradable utilization of heavy metals of Cu, Zn, Ni, Pb, Cd in the sludge to oxidation state or residual state which are more stable. After the composting, the relationship among the treatments of Cu, Ni, Zn biological effectiveness was A<B<C<D, and the treatment of A with more residual Pb and residual Cr increased compared to other treatments. It means that A treatment reduced heavy metals biological effectiveness most significantly, and had remarkable passivation effect to heavy metals.(3) Heavy metals in sewage sludge are the main limiting factor for agricultural use of sludge. How to eliminate the limiting effect of heavy metals has become hot spot in research. Humic acid(HA) and HA-bound heavy mentals in the sludge under composting were extracted using a sequential extraction method with H2O, Na4P2O7+NaOH, sequential extraction method, humic substances (HS) and heavy metals combined HS at different stages in the process of composting were determined to analyze their variation during the compost. The results showed that the heavy metals were mainly in humic acid (HA) complexes while heavy metals combined by fulvic acid (FA) were low. Heavy metals in compost were stable with low effectiveness and mobility. Heavy metals combined with FA were low while the heavy metals combined with HA (HA-Pb, HA-Ni, HA-Cu) which accounted for the total heavy metals were significantly increased. At the end of compost, HA-Pb, HA-Ni, HA-Cu increased by28.00%,17.34%,1.40%, respectively. The compost promoted the water soluble heavy metals into the metal complexes combined by HA. The capacity of HA to heavy metals fixation was Cu>Pb>Ni>Zn.(4) Through the research on the change of16polycyclic aromatic hydrocarbons (PAHs) of the sewage sludge and fungus mushroom composting process changes, we found that the total amount of16PAHs up to5.0819mg·kg-1at the beginning of sludge compost. After90days of composting, the content of PAHs reduced to3.9238mg·kg-1. At the end of the composting, the rate of degradation of PAHs was22.79%. The macromolecules solid humic acid was rising after decline, while the content of PAHs was increased first and then decreased. The degradation of PAHs occurred mainly in the composting treatment after20days. There were obvious differences of changes on PAHs whose number of benzene rings≤3and≥4in the composting process.In the whole composting process, the PAHs whose number of benzene rings≤3showed a downward trend while number of benzene rings≥4was in a fluctuating trend. We can see the fewer the number of benzene rings, the greater the relative degradation rate, and vice versa.