| In recent years, the amount of sewage sludge increased dramatically, to seek a reasonable disposal for sludge has become a challenge in urgent need to capture and research. The composted sludge for soil utilization was identified as the most promising way of sludge disposal. PAHs are common organic pollutants in sewage sludge, and studies on PAHs removal and its migration in soil was deficient. Therefore, we studied the variation characteristics of 14 kinds of PAHs in sludge under different simulated composting conditions, and optimized PAHs removal conditions, revealed the combining characteristics of PAHs and different forms of humus, and studied the organic matters’maturity and PAHs’variation characteristics through outdoor composting experiment. Additionally, we researched the migration of PAHs between soil and plant with the application of composted sewage sludge through field experiment and pot experiment. The main conclusions were as follows:1. There were plenty of OM, N, Pand K in four kinds of sewage sludges of Shenyang city. The Cu, Cd, Pb, Zn, Ni and Cr were detected in all sludges, and was mainly composed of Zn and Cu. Cu and Cd would be the first limiting factors while applicating in soil. Calculated with the application dosage 0.75 kg m-2 per year (CJ/T 309-2009), one kind of sludge’s safty application years in acid soil (pH<6.5) was 10 years, the othe three kinds of sludge’safty application years were ranging from 28-47 years. The concentrations of PAHs in four sewage sludge ranged from 2.63 to 18.3 mg kg-1, mainly composed of aromatics with 3-4 rings, and were mainly from burning source. Compared of CJ/T 309-2009 (PAHs≤5 mg kg-1), the were three kinds of sludges’ contents of PAHs exceeding 266.7%,48.4% and 14.5%. Therefore, heavy meatals wouldn’t be the limiting factors for the application of sewage sludge in soil if it was applicated reasonable.2. The simulated composting experiment (30 d) indicated, the effects of different bulking agents on PAHs removal was peat-straw>peat-peanut>single peat>single peanut>single peat. With the increase of water content, the removal rate of PAHs increased first then declined, and the best water content was 55%. With the increase of temperature, the removal rate of PAHs increased first then declined, and the optimal temperature was 35 ℃. With the increase of oxygen content, the removal rate of PAHs increased first then declined, and the optimal oxygen content was 8.64 L h-1 kg-1, the removal effects of low-molecular weight PAHs was unsatisfactory. As a while, the removal effects of 3-4 ring PAHs of all combining forms was better than 5 ring,6 ring, and 2 ring significantly. The results of orthogonal experiment indicated that, the influence size of different factors on PAHs removal was in the order:bulking agent kinds>water content>oxygen content>temperature. Under the condition of sewage sludge:peat:straw=3:1:2,55% water content,38℃, and 7.92 L h-1kg-1 oxygen content, the ∑PAHs removal rate was 79.6%.3. The influence of different simulate composting conditions on TOC degradation was consistent with PAHs. The increse of humic acid’s carbon contents and H/F ratio with peat-straw bulking agent was higher than other bulking agents. The increse of humic acid’s carbon contents and H/F ratio under 35℃ was higher than other temperature. There was an increase of humic acid’s carbon contents with the increasing water content, but the decrease of TOC was the biggest under the condition of 55% water content. Additionally, there was a decrease of humic acid’s carbon content and H/F ratio with the increasing oxygen content, but the decrease of TOC was the biggest under the condition of 8.64 L h-1 kg-1. Therefore, it was necessary to increase oxygen content in earlier composting for improving PAHs removal rate, and reduce oxygen content in later composting for improving humification degree.4. Under different simulated composting condition, there was a significant positive correlation between humus acid and PAHs, there was a significant negative correlation between humic acid and PAHs (except for 45% water content and 10.8 L h-1 kg"’oxygen content), there was a significant positive correlation between fulvic acid and PAHs, there was a significant negative correlation between H1 ratio and PAHs. The concentrations of PAHs with different combining form of humus was residual-state>alcoli-state>water soluble-state in the earlier 15 days during composting, while it was alcoli-state>residual-state>water soluble-state in the later 15 days during composting. The content of residual-state PAHs declined from 2508.0 ng g-1 to 198.8 ng g-1, the content of alcoli-state PAHs declined from 1125.4 ng g-1 to 569.7 ng g-1, The content of water soluble-state PAHs declined from 137.5 ng g-1 to 27.0 ng g-1. The results indicated that, with the conducting of sewage sludge composting, the combination of PAHs and humic substances turned more close, and the bio-availability of PAHs declined.5. The outdoor composting experiment indicated, the organic carton decreased 44.9%, C/N ratio declined from 20.7 to 14.9, H/F ratio increased from 0.51 to 2.47. The composted products had loose structure with brown color and earth breath, and had no undesirable odor. The DOM’s conjugate structure and aromatic structure increased, and different functional group structure changed during composting, which could be found through three dimensional fluorescence spectrum and infrared spectrum. The aromatization and humificaiton degree of humic acid and fulvic acid substances improved, which could be found through ultraviolet spectrum and infrared spectrum. The PAHs degradation rate was 81.4%, and the degradation effects of 3-4 ring compounds was superior to 5-6 ring and 2 ring compounds. The distribution characteristics of PAHs before composting was 4 rings>3 rings>5 rings>6 rings>2 rings, which turned to be 4 rings>5 rings>3 rings>6 rings>2 rings.6. The field experiment results indicated that the composted sewage sludge (2-25 kg m-2) improved aeolian sandy soil’s mechanical composition, the soil texture was nearly to with the application dosage of 25 kg m-2, the TOC, TN and TP fertility levels increased from six class to 3-5 class. The application of composted sewage sludge improved the soil’s enzyme’s activity, but the SA decreased when the dosage was over 20 kg m-2. With the increase of composted sewage sludge dosage, the soil’s PAHs contents increased, the contents of PAHs in T6 treatment soil was 137.6 ng g-1, were lower than the Soil Environment Quality Standards of Holand, which could be considered without soil pollution hazard risk (< 200 ng g-1). Not considering of PAHs degradation factors, calculated by the dosage of 2 kg m-2 per year, the contents of PAHs in soil belongs to pollution-free level (<200 ng g-1) if applying composted sewage sludge for 19 years continuously. The application of composted sewage sludge promoted the growth of Mongolia pine seedlings and Festuca a., but the growth effects appeared inhibited when the dosage was over 20 kg m-2and 10 kg m-2 respectively, so the dosage per year shouldn’t surpass 20 kg m-2 and 10 kg m-2 respectively. The migration coefficient of PAHs between plant’s aboveground part and underground part were>1, and were mainly composed of low molecular weight PAHs. The accumulation effect of Mongolia pine seedlings on PAHs was bigger than Festuca a., and the BCF of low molecular weight PAHs were>1, while the BCF of high molecular weight PAHs were<1. |
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