Enhanced Anaerobic Digestion Of Refinery Waste Activated Sludge By Refinery Spend Caustic: Mechanism And Efficiency

Waste activated sludge（WAS）is a by-product in the biochemical treatment of wastewater.WAS is mainly comprised of microbial celles,contains high concentrations of organic matter.Anaerobic digestion may recover energy（e.g.,biogas）from WAS by biomass conversion,simultaneously recovering resource and reducing the WAS volume.The properties of refinery WAS（RWAS）are similar to those of WAS.However,RWAS adsorbs large number of refractory organics and toxic components（e.g.,petroleum hydrocarbon,gelatine and asphalt）,which makes the sludge floc structure more stable,and the hydrolysis of cell wall more difficult.Therefore,the methane yield is limited from anaerobic digestion of RWAS.Meanwhile,refinery spent caustic（RSC）is a hazardous waste produced from desulfurization of hydrocarbon products,with a high p H value（p H>13）,high salinity（1-4%）,and high pollutant content（COD at 20-40 g/L;Sulfides at 1-4%,Volatile phenol>200 mg/L）,which seriously threatened the stable operation of refinery wastewater treatment plant.RWAS has the dual characteristics of resource utilization and pollution.The composition characteristics（high alkalinity）of RSC makes it an attractive option for breaking the adsorption of petroleum on the surface of RWAS,disintegrating sludge floc structure and cell wall,and releasing intracellular organic matter,which relieves the restriction of limited hydrolysis,and increases recovery energy from anaerobic digestion of RWAS.It also provideds a way for RSC treatment.Therefore,this study comprehensively considered resource natures of RWAS and RSC,and based on the concept of“wastes-treat-wastes”,investigated the potential of refinery spent caustic（RSC）for the enhanced anaerobic digestion of RWAS.Thermal-RSC,electrolysis-RSC and RSC-alkaline fermentation were developed to improve RWAS solubilization and methane yield,and their enhanced mechanism were also explored.（1）Gelatine and asphalt adsorbed on the RWAS surface,and formed a layer of interface film with strong elasticity,which limited the action of hydrolase.Moreover,the activity of anaerobic microorganisms was inhibited by some toxic petroleum pollutants inhibit.All of those caused a 17.5 m L/g-VS of methane yield from anaerobic digestion of RWAS,which was far lower than that of municipal WAS（192.7 m L/g-VS）.The OH^-of RSC can remove the barrier of oil film on the RWAS surface,and enhance the disintergration of RWAS.The free ammonia of RSC destroyed the microbial cell wall and promoted the release of intracellular substances,which caused the effect of RSC pretreatment RWAS was better than that of Na OH.The maximal methane yield（66.1m L/g-VS）was obtained at RSC dose of 3%and reaction time of 2 h,which was about 3times that of unpretreated RWAS（20.3 m L/g-VS）.This study demonstrated that the enhanced anaerobic digestion of RWAS by RSC pretreatment was feasible.（2）Thermal-RSC pretreatment can break the adsorption bond between petroleum and sludge particles,accelerate the OH-mass transfer rate,and significantly promote the solubilization of RWAS.The concentrations of proteins,polysaccharides and volatile fatty acids（VFA）raised with the increase of temperature and RSC dosage.And their concentrations reached 1338.7,510.9 and 651.2 mg/L,respectively,under the temperature of 90℃and RSC dosage of 4%.Person correlation analysis showed that RSC dosage was positively correlated with humic acids and melanoid production,and the stagnation period of anaerobic digestion,indicating high RSC dosage could inhibit the anaerobic digestion of RWAS.The optimal conditions of thermal-RSC pretreatment were 90℃and 1%RSC dosage.After anaerobic digestion of the RWAS,the SCOD removal efficiency and VFA production were 66%and 736.2 mg/L,respectively,which was 5-fold and 3-fold higher than that without pretreatment.（3）Electrolysis-RSC pretreatment can oxidize petroleum,effectively disintergrate the RWAS floc structure,and improve the molecular characteristics of dissolved organic matter contributing to enhanced anaerobic digestion performance.The optimized conditions for electrolysis-RSC pretreatment were determined using response surface method,which were RSC dosage of 2.75%,voltage of 5 V,and reaction time of 2 h.The highest methane yield was 93.06 m L/g-VS.The molecular weight of the released DOM was mainly distributed at 200-400 Da with carbon number of C₁₂-C₁₉.The lignin or carboxyl aliphicyclic compounds were the main component with relative abundance of61.5%,having a high double bond equivalent（6.98）and aromatic index（0.56）.The correlation analysis between microbial speciese and organic compoents showed norank＿f＿Anaerolineaceae,norank＿f＿＿Bacteroidetes＿vadin HA17,norank＿f＿norank＿o＿Aminicenantales,Hyphomicrobium,and JGI-0000079-D21 played an important role in the decomposition and bioconversion of lignin/carboxylic acyclic compounds.（4）Alkaline fermentation can enhance the process of hydrolytic acidification with little inhibited by petroleum substances,and produce more volatile fatty acids by the transform of dissolved organic matter,which provides easily utilizable substrate for methanogenes.The maximum VFA yield reached 196.28 mg/g-VS at sludge retention time of 6 days.The main mechanism of alkaline fermentation is to enhance hydrolysis（6.9%）,acidogenesis（10.7%）,and acetogenesis（28.5%）,whereas methanogenesis（28.87%）was inhibited to achieve VFA accumulation.In alkaline fermentation system mediated by RSC,the high sulfate promoted the syntrophic relationship between sulfate-reducing bacteria and hydrogen-acetic acid producing bacteria,which facilitated the formation of acetic acid.Meanwhile,high content of free ammonia inhibited the activity of acetotrophic methanogens and reduced the consumption of acetic acid.Therefore,the proportion of acetic acid was more than 80%.And the ultimate methane yield reached to115.3 m L/g-VS during methanogesis after alkaline fermentation pretreatment of RWAS.（5）The performance of anaerobic digestion of RWAS using the above four pretreatment methods were compared in this study.The methane yield followed the order of alkaline fermentation（115.3 m L/g-VS）>electrolysis-RSC（104.87 m L/g-VS）>thermal-RSC（94.3 m L/g-VS）>RSC pretreatment（58.3 m L/g-VS）.Comprehensively considering energy input in sludge pretreatment and anaerobic digestion,as well as energy recovery from biogas,and the benefit from sludge reduction,alkaline fermentation had the highest net benefit.Moreover,a green economic treatment process for RWAS resource utilization was also proposed.