Enhancement Of Anaerobic Digestion Of Waste Activated Sludge By Electrochemical Pretreatment

The continuous increase of the sludge that derived from wastewater treatment plants?WWTPs?has attracted rising attentions of researchers and managers about its recycl and waste-to-energy issues.Anaerobic digestion is one of effective methods to treat the waste activated sludge?WAS?due to its excellent destruction and stabilization performance.Moreover,it is a cost-effective technology with energy recycled from the byproduct,such as methane.However,the anaerobic digestion rate and methane yield are inhibited owing to the complex and rigid floc structure of WAS.Disintegration is developed as the typical pretreatment process to accelerate the hydrolysis rate and improve the anaerobic digestion process.A number of pretreatment methods,such as microwave,ultrasonic irradiation,thermal,acidic,alkaline,and enzymatic have been previously studied.However,the physical pretreatment demands high-energy input when being applied to the field scale;for chemical pretreatment,the large amount of chemical addition might result in expensive operation cost and secondary pollution;biological pretreatment is a very slow process and requires careful control of operating conditions.Considering the drawbacks of the conventional pretreatment methods mentioned above,electrochemical?EC?technology is introduced as an alternative pretreatment strategy in this study.EC treatment can not only electro-adsorb pathogens on the electrodes and electrolyze them directly,but also generate free chlorine and energy rich intermediate products,such as hydroxyl radicals,which can completely degrade organic compounds.The reports of EC technology on anaerobic digestion enhancement are rare,though it is known to be effective in sewage sludge dewatering and inactivation.In this study,EC technology was employed as a pretreatment method to disrupt sludge floc structure and microbial cell walls,aiming to enhance the subsequent anaerobic digestion process.The main conclusions are as follows:?1?The enhancement of anaerobic sludge digestion using EC pretreatment was feasible.The EC pretreatment could achieve a highest SCOD of 3192mg/L,which was 12.32 times compared to the control,and the DD_COD of EC pretreated sludge could reach 9.72%.After anaerobic digestion for 43 days,the cumulative methane production and VS removal of EC pretreated sludge was 20.47%and 6.69%higher than that of the unpretreated sludge,respectively.A relatively superior process condition of EC pretreatment was obtain by electrolysis voltage of 20 V,electrode distance of 5 cm,reaction time of 30 min,initial sludge solid content was 30 g/L,and without adjusting pH value.?2?The effect of urea,NaCl and NaClO addition on sludge disintegration during EC pretreatment was compared.NaClO was proved to be the optimal agent of improving the EC pretreatment.The optimized condition of EC combined with NaClO?EC-NaClO?pretreatment was:NaClO dosage was0.3-0.4 g/L,electrolysis voltage was 20-25 V,reaction time was 45-60 min,electrode distance was 1-2 cm,initial sludge pH was 8-9,and initial solid content was 30-40 g/L.Under this condition,a DD_COD of 40.11%was achieved,and the level of VFA,protein and polysaccharide was 6.86,10.14and 4.83 times higher than that of the control,respectively.With the EC-NaClO pretreatment,the biodegradable composition in dissolved organic matter?DOM?increased significantly,thus enhancing the anaerobic digestion process.?3?Batch fermentation tests were carried out to compare the anaerobic digestion process between EC-NaClO pretreated sludge and unpretreated sludge.After anaerobic digestion for 48 days,the methane production of pretreated sludge reached 293 mL/gVS,which was 60.11%higher than that of the control.Besides,the TS and VS removal of pretreated sludge was32.52%and 42.72%,which was 11.37%and 11.95%higher than that of the unpretreated sludge,respectively.Moreover,anaerobic digested sludge with the EC-NaClO pretreatment shortened the stabilization period by 13 days.?4?Model-based analysis implied that the lag phase???of the control and EC-NaClO pretreated sludge was 3.5±0.4 d and 8.4±0.6 d,respectively.The hydrolysis rate constant?k?was both about 0.077 d^-1 for the two groups.However,the maximum methane production rate(R_max)increased from7.6±0.3 mL/gVS·d to 17.4±1.3 mL/gVS·d by EC-NaClO pretreatment.This implied that,EC-NaClO pretreatment mainly relied on enhancing the methane production rate to facilitate the anaerobic digestion process.?5?The relative abundance of phyla Chloroflexi,Firmicutes and Actinobacteria in EC-NaClO pretreated sludge,which were deemed as the common fermentative phyla and main contributor for VFA generation,was higher than that of the control.Meanwhile,the dominant genera of the control was Haliscomenobacter?belonging to phyla Bacteroidetes?,while it changed to Caldilinea?belonging to phyla Chloroflexi?in EC-NaClO pretreated sludge.This implied that,EC-NaClO pretreatment could enhance the organic degradability,improve the biogas production,and achieve a more stable anaerobic digestion system.With regarding to the archaeal distribution,the Methanosarcinaceae was enriched in EC-NaClO pretreated sludge.The dominant genera changed from Methanobacterium?belonging to class Methanobacteriaceae?to Methanosarcina?belonging to class Methanosarcinaceae?after pretreatment,indicating that EC-NaClO pretreated sludge has a higher biogas yield and a more stable microbial community.?6?The results of pilot-scale study revealed that EC-NaClO pretreatment significantly enhanced the biogas yield of anaerobic sludge digestion and shortened the stabilization period.After anaerobic digestion for 39 days,the biogas and methane production of EC-NaClO pretreated sludge were 1.83times and 1.89 times compared to the control,respectively.The EC-NaClO pretreated sludge could achive 48.9%of VS removal,which was only 42.64%for the control.