Study On The Efficiency Of Methane Production From Anaerobic Co-Digestion Of Dewatered Sludge And Waste Leachate

Anaerobic digestion is the main technical measure for sludge recovery and stabilisation.The dewatered sludge produced from the residual sludge of wastewater treatment plants after dosing has characteristics such as low carbon to nitrogen（C/N）ratio and low organic matter content（VS/TS=30%-50%）,which leads to low efficiency of methane production from its anaerobic digestion and poor economic efficiency.Leachate from municipal solid waste incinerators is characterised by high organic matter content and high C/N,but too high concentrations of ammonia nitrogen and chloride ions etc.can inhibit the anaerobic digestion process.Anaerobic co-digestion of dewatered sludge with waste leachate can improve the C/N of the digestion system,alleviate the inhibition of the system by ammonia nitrogen etc.and improve the efficiency of methane production.Based on this,the thesis investigates the methane production efficiency of anaerobic co-digest ion of dewatered sludge and waste leachate,determines the optimum ratio of the two wastes and investigates the mechanism of co-digestion to improve the methane production efficiency.The main findings of the thesis are as follows:（1）The methane production efficiency of anaerobic digestion of dewatered sludge can be improved by adding an appropriate amount of waste leachate.The highest methane production efficiency of the system is achieved when the volume of leachate and dewatered sludge（95%water content after dilution）is 50%,When the leachate dosage was between 10%and 50%,the cumulative methane production from the anaerobic co-digestion of the two wastes was positively correlated with the leachate dosage;when the leachate dosage was 80%and 100%,the anaerobic digestion process was inhibited due to the high Cl’ concentration in the supernatant（>3000mg/L）,which disturbed the osmotic pressure balance of the microbial cells.When the leachate dosage ratio was 50%,the cumulative methane production（4564.59 mL）and organic methane yield（351.16 mLCH₄/g-COD）of the co-digestion system were both maximum,and the highest SCOD removal rate（83.11%）was achieved,which were 180.40%,51.86%and 60.84%higher than the single digestion of dewatered sludge,respectively.（2）Anaerobic co-digestion of leachate and dewatered sludge can promote sludge reduction and stabilisation,and improve organic matter utilisation efficiency.The results showed that the particle size d（0.9）of the co-digestion system decreased from 154.036μm to 91.08μm when the volume input ratio of leachate to dewatered sludge was 50%,which was much lower than that of dewatered sludge alone（106.428μm）and leachate alone（114.233μm）;the organic matter conversion（45.29%）was 24.54%and 29.71%higher than that of dewatered sludge and leachate alone,respectively.The organic matter conversion（45.29%）was 24.54%and 29.71%higher than that of dewatered sludge and leachate alone,respectively,and the SP（soluble protein）and SC（soluble polysaccharide）conversions were 26.77%and 47.09%,31.64%and 44.70%higher,respectively,with significant reduction effects.The SP,SC and HA（humic acid）conversions in the residue were 43.03%,32.22%and 32.42%respectively,which were 24.72%,13.15%and 14.84%higher than those of the dewatered sludge digestion alone,and the stability of the residue was improved.（3）Anaerobic co-digestion improved microbial diversity and promoted the enrichment of methanogenic bacteria.(Methanosarciniales（Methanosarciniales）increased by 1.95%compared to dewatered sludge mono-digestion and Methanomicrobiales（Methanomicrobiales）increased by 4.30%compared to leachate mono-digestion;the archaea associated with hydrolytic acidification in the 50%leachate dosing group The highest percentage of Cloacimonadales（30.71%）was found in the 50%leachate dosing group,with an increase of 22.11%and 30.61%compared to the dewatered sludge mono-digestion and leachate mono-digestion,respectively.（4）The optimum operating parameters for anaerobic co-digestion of dewatered sludge with leachate were defined through semi-continuous experiments.The experiments were divided into Phase Ⅰ（5%material dosing rate）and Phase Ⅱ（8%material dosing rate）,and the cumulative methane production in the two phases was ranked as follows:50%leachate dosing group（31537.33 mL）>single digestion of dewatered sludge（7852.69 mL）>single digestion of leachate（751.61 mL）.The cumulative methane production in stage Ⅱ（17016.36mL）was 17.18%higher than that in stage Ⅰ（14520.97mL）,the peak daily methane production（1905mL/d）was 13.06%higher than that in stage Ⅰ（1685mL/d）,the peak cycle was shortened by 5d and the coenzyme F420 concentration ranged from 0.26 to 0.80 μmmol/L,with stronger methanogenic activity than dewatered sludge and leachate monodigestion.The peak concentrations of SP and SC in the 50%leachate dosing group in stage Ⅱ were 1.90 and 2.65 times higher than those in the dewatered sludge monodigestion,respectively.The optimum operating parameters for anaerobic co-digestion（stage Ⅱ）were determined as follows:digestion temperature 37℃,sludge moisture content 95%,material dosing rate 8%,digestion time 12.5 d,organic load 1.84 kg vs/（m³-d）,total gas volume 0.68 m³/（m³-d）.