Kinetics Of Anaerobic Co-digestion Of Thermal Hydrolysis Sludge And Food Waste

During the rapid progress of municipal and industrial wastewater treatment in China,the activated sludge process plays an irreplaceable role in wastewater treatment process.Anaerobic digestion is an applicable technological option of the reduction,stabilization and utilization of waste activated sludge.However,two problems caused by limited organic in the waste activated sludge strained the application of anaerobic digestion in China:(1)the overall efficiency of the anaerobic digestion was limited by sludge hydrolysis;(2)the low methane potential caused by its limited organics restrains the economic feasibility of the sludge treatment and the biogas utilization.Therefore,the combination of thermal hydrolysis and anaerobic co-digestion were investigated in this study to solve the above problems.Thermal pre-treatment led to sludge defloculation,intracellular organics release and hydrolysis of waste activated sludge,thus bypass the extracellular hydrolysis,the rate-limit step of anaerobic degradation of particulate organic.The sequential enhancement on methane production and the kinetics acceleration were attributed to the readily biodegradable organics generated during thermal process.On the other hand,On the other hand,food waste,as co-substrate with high biodegradability and methane potential,was introduced to optimise the methane production and the energy balance of anaerobic digestion system.The influence of thermal pre-treatment(including temperature and time)on the cumulative methane yield and the methanogenesis kinetics were discussed.Temperature was the determinant parameter of thermal hydrolysis process and the optimum methane production was achieved at 165? and 30 min.The variation of cumulative methane yield revealed the primary effect of thermal treatment,which fundamentally modified the anaerobic biodegradability of waste activated sludge,was attributed to the flocs size reduction and the surface area growth,which allowed the extra opportunity for the extracellular enzyme adsorption and thus accelerated the sludge degradation.The 3-day Methane yield rate(Y3)of thermal hydrolyzed sludge increased with pre-treatment temperature(100?-200?),accounting for 45.5%to 62.6%of the cumulative methane yield.Using the First-order kinetics,the apparent hydrolysis constant,kh,increased from 0.06 d-1 of raw sludge to 0.25-0.34 d-1 of the treated sludge showed the acceleration of methane production rate with temperature.First-order kinetics was to describe the degradation features of thermal pre-treated sludge.However,the reaction curve model and the modified Gompertz model fit the profile of the lag phase observed in the anaerobic digestion.The results of semi-continuous experiment suggested that thermal sludge digestion can operate under the organic loading rate of 1.0-4.0 kgVS/m3-d,but the variations of ammonia and pH needed close inspection to avoid the free-ammonia inhibition.Heavy metal was a critical factor for the environment impact assessment of sludge management.Therefore,the impacts of temperature of thermal pre-treatment on the total content and the chemical speciation of heavy metal were studied.Referred to the standard of"sludge management of municipal wastewater treatment plant-garden application",the principal contaminants of the sludge were lead,cadmium and zinc.Those elements excessed the threshold content for the alkaline solid,and zinc almost exceeded the limitation for the acidic solid.Therefore,the utilization of this sludge as fertilizer in land application shall be restricted.The fact that the total contents of the heavy metals were almost impregnable during thermal pre-treatment,suggested that the major fraction of heavy metals was bonded to the solid phase of the sludge.Sequential extraction scheme was used to fractionalize the heavy metal in waste activated sludge.The result showed that the distribution pattern of As,mainly bonded in the residual fraction was unaffect by thermal pre-treatment.Cd and Pb were mainly existed in the acid soluble and the residual fraction.The proportion of Cd in acid soluble and oxidable fraction increased during the thermal process while Pb was converted into the residual form.Cr was partially converted into the oxidable fraction and Cu remiand unaffected,although both elements distributed evenly in the oxidale and the residual fractions.Ni,which boned to the reducible and the oxidable forms by 64.5%in total,loses its 14.5%of total content during the pre-treatment.The chemical speciation of Zn was insensitive to thermal pre-treatment and its share of the reducible and oxidable fractions excessed 53%in total.Based on the results of thermal hydrolyzed sludge,the influents of anaerobic co-digestion of thermal hydrolyzed sludge and food waste on the cumulative methane yield,methanogensis kinetics and stability were discussed.The results of the anaerobic biodegradability tests proved that:anaerobic co-digestion exerted a neglectable effect on the ultimate cumulative methane yield,and the theoretical methane yield can be estimated by the compositions of the substrates.The inverted V-type trend of the relative deviatione(RD)between the theoretical estimation of cumulative methane yield reached the maximum values of 32.0%-34.1%at day 10 and fall below 1.5%.That means the synergetic effect between thermal hydrolyzed sludge and food waste were the acceleration of methane production in the early phase of anaerobic digestion.By comparison,the modified Gompertz model was a better function to fit the cumulative methane yield of food waste,which experienced a lag phase(?)at the start of the anaerobic biodegradability test.As seemed in the continuous experiment,the addition of food waste in the anaerobic digester treating thermal ydrolyzed sludge did improve the overall methane yield;however the increasing organic loading rate exerted the negative effect on the stability.Therefore,it was critical to watch out the parameters(pH,alkalinity and volatile fatty acids)of the digester.Based on the principle of centralized organic waste management,the feasibility of energy neutral wastewater treatment plant with the capacity of 150 million person equivalent,?accounting for 4.8X106m3/d of wastewater,450 t/d of sludge and 195 t/d of food waste,was assessed in this study.From the view of energy structure,90%of the electricity and all the steam were consumed by wastewater treatment and sludge treatment,respectively.Combined heat and power lead biogas boiler in economic benefits by 0.88 RMB/m3 of methane,due to the energy demand and energy price.The integration of sludge hydrolysis process led to a steam deficit of 3.73×105 kWh/d,which increased the total cost of sludge treatment although the net electricity output and income were achieved.Sludge treatment system achieved energy self-sustain and an electricity output of 4.32×105 kWh/d,fulfilling 30%of electricity demand of wastewater treatment,with the addition of 105 t/d of food waste in the existing anaerobic digestion system.The integration of the 90 t/d food waste anaerobic digestion system with the wastewater and sludge treatment system was able to supply 3.32×105 kWh/d of electricity and 9.60×105 kWh/d of heat,achieving an overal electricity self-sufficiency ratio of 37.5%,and a heat self-sufficiency from 42.7%to 62.3%for the digestate drying process(from 80%to 50%,water content),respectively.The premise behind the energy neutral wastewater treatment plant were:(l)reduce electricity demand of wastewater treament process;(2)and improve the organic content of waste activated sludge to increase the methane production of the anaerobic digestion system.