| Regarding to low biogas yield and long digestion process of conventional anaerobic digestion (CAD) of the sludge with low organic content, high-solids anaerobic digestion may be applied in order to improve the volumetric biogas yield, and alkaline pretreatment may be used to accelerate hydrolysis of sludge particles and improve the release of intracellular organic matter. However, the effects of mono alkaline treatment on anaerobic digestion, especially high-solids anaerobic digestion are not clear. Therefore, sewage sludge with total solids content (TS)2%-15%was used as material for investigation of the effects of alkaline treatment on sludge disruption and anaerobic digestion, and the main results are as following:(1) NaOH treatment effectively disintegrated both low-solids sludge and high-solids sludge, and the treatment duration of30min was the most efficient. The disruption effect of the sludge with high organic content was more significant. For low-solids sludge, the optimum dose was0.04-0.1mol/L and sludge disintegration degree kept constant when sludge TS changed. For high-solid sludge, the optimum dose was0.2-0.3mol/L, but the required mass of NaOH actually decreased to reach the same disintegration level due to sludge thickening.(2) After alkaline treatment, the pH of th treated sludg increased. If the initial pH was higher than10, naerobic digestion process was obliterated; If the initial pH was between8-10, the start of biogasproduction was delayed and the cumulative biogas production decreased; When the initial pH was lower than8, the cumulative biogas production increased significantly; An initial pH of8.5did not weaken the anaerobic digestion, and did not improve it either. Considering the high salinity environment due to alkaline pretreatment and acidic neutralization, the dose of0.04-0.1mol/L NaOH and the neutralization agent of HC1was suitable considering the inhibition effects of Na+and SO42-. Under these optimized conditions, the biogas yield increased by25%-30%compared to the control because of the increased organic degradation rate.(3) During semi-continuous anaerobic digestion with pretreatment of0.1mol/L NaOH, the biogas production of mixed sludge (TS2%) with low organic content increased by3.6%-11.1%compared to the control. Alkaline posttreatment was tested considering that the influent sludge had a low organic content and was mainly composed of primary sludge. Between the10th and the12th hour of a24-h digestion cycle,5%of sludge was extracted from the semi-continuous digester with sludge retention time of20days, and then disintegrated with0.1mol/L NaOH and neutralized with HC1. The treated sludge was returned to the digester. This posttreatment resulted in a30%enhancement of biogas production in comparison with the control, and the dewaterability of digested sludge had almost no change.(4) During high-solids anaerobic digestion, the biogas production of sludge was close to that during low-solids process. After alkaine pretreatment, the organic degradation rate of high-solids sludge increased from26%to35%, but soluble chemical oxygen demand, total ammonia nitrogen and total phosphorus in the supernatant of digested sludge were increased significantly. Compared the costs of chemical agents and the benefits of more biogas and less digested sludge, the application of alkaline treatment to anaerobic digestion of high-solids sludge can produce a positive profit.This study systematically elaborated the optimum conditions and the effects of mono alkaline treatment when applied to anaerobic digestion of both low-solids and high-solids sludge with low organic content. The conclusions can provide reference for the economical and effective application of alkaline treatment to sludge anaerobic digestion. |
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