| With the growth of economy,increasing of population and development of society,the amount of waste activated sludge(WAS)has increased rapidly.WAS,as a by-product of wastewater pretreatment,has become a serious problem and a heavy burden of city operation.As a national environmental policy,“Water Pollution Control Action Plan” has pointed out that sludge treatment and disposition is an important issue and urgent problem.Besides,with abilities of transferring organic matter into biogas and reducing the amount of final solids for disposal,anaerobic digestion(AD)has become a valuable and priority technology for sludge treatment.Despite of these advantages,AD also has some inevitable limitations,such as a long solids retention time(20-30 days),a slow reaction rate,a low overall degradation efficiency of organic dry solids(30-50%),and the presence of other biogas constituents.Thus,various sludge disintegration technologies have been studied as a pretreatment.Supported by “863 Plan”,this paper studied three kinds of pretreatment technologies,ultrasonic,combined ultrasonic and thermal and combined ultrasonic,thermal and alkali,for sludge disintegration and AD efficiency enhancement.For the purpose of improving AD rate and biodegradability,the specific energy,temperature and alkali dosage were optimized and an optimal process of combined ultrasonic,thermal and alkali pretreatment was obtained.The efficiency and mechanism of combined ultrasonic,thermal and alkali pretreated sludge during AD were studied.All of the findings provided a practical and theoretical basis for understanding and using combined technology prior AD.We came to some conclusions as follows:Using specific energy(ranged from 0-24000 kJ/kg TS)and ultrasonic frequency(single frequency: 17,20,25,and 33kHz;double frequency: 20+25 kHz and 17+33 kHz)as parameters,the physical and chemical characters,dewaterability and AD efficiency of ultrasound pretreated sludge were investigated.The results showed that ultrasound pretreatment played a positive role for increasing sludge disintegration degree,soluble organic concentrations and nitrogen and phosphorus concentrations.With the increasing of specific energy,these positive effects also increased and the mean diameter of pretreated sludge decreased.It indicated that after ultrasonic pretreatment,sludge flocs were disrupted and organic matters were transferred into liquids,which was in coincidence with decreasing VSS concentration.Thus,from the point view of specific energy,sludge ultrasonication appeared to be three stage.Firstly,sludge floc structure was slightly disrupted and disintegration degrees were lower than 10% when specific energy lower than 400 kJ/kg TS,under which sludge particulate matters weren't released into aqueous phase.Secondly,when specific energy increased to 12000 kJ/kg TS,disintegration degrees and soluble organic concentrations increased rapidly.The former increased more than 20%,and the latter increased more than 200% comparing with untreated sludge.It showed that cavitation and mechanical effect of ultrasonic treatment promoted sludge disintegration,followed by the deteriorating of sludge dewaterability.Thirdly,when specific energy increased to 24000 kJ/kg TS,the correlation between sludge disintegration degree and specific energy disappeared.Thus,DD analysis indicated that 24000 kJ/kg TS was suboptimal for 100% cell disintegration.The SEM(Scanning Electron Microscope)results also came to the same conclusion.From the point view of ultrasonic frequency,double frequency obtained better sludge disintegration effects than single frequency.The disintegration degree,soluble protein and carbohydrate concentrations of double frequency were 7%,22% and 15% higher than single frequency.The reason for these phenomena was that cavitation and mechanical effect were improved under double frequency which increased the probability and intensity of cavitation.AD results showed that ultrasonic pretreatment did not always increase methane production.The optimal process of ultrasonic pretreatment was double frequency of 20+25 kHz and specific energy of 12000 kJ/kg TS.Pretreated by optimal process of ultrasonic,the cumulative methane production increased 26.64% than untreated sludge,VSS reduction increased to 26.49%,cake moisture content decreased 4.39% than untreated sludge.Optimization of combined thermal and ultrasound disintegration of WAS for enhanced organic releasing was carried out using response surface methodology(RSM).Sludge disintegration degree and soluble protein and carbohydrate concentrations indicated that specific energy of 12000 kJ/kg TS and temperature of 80? were optimal parameters of combined ultrasound and thermal pretreatment.Under the optimal condition,sludge disintegration degree increased to 42.20%,methane production increased 55.44% than untreated sludge,and total TSS and VSS reduction increased 21.28% and 20.87% than untreated sludge.Despite of that,the energy utilization efficiency decreased 44.17% than ultrasound pretreatment alone.To improve energy utilization efficiency and solids reduction,alkali treatment was included.RSM was also used to optimize parameters of specific energy,temperature and alkali dosage of combined ultrasound,thermal and alkali pretreatment.The optimum parameters of combined ultrasound,thermal and alkali pretreatment were obtained at specific energy of 8000 kJ/kg TS,temperature of 70?,and alkali dosage of 0.07 g NaOH/g TS.After pretreated by the optimum process,sludge disintegration degree,soluble protein and carbohydrate concentrations,mean particle diameter reduction,methane production,and total TSS and VSS reduction increased 54.61%,843.26%,1081.66%,85.40%,81.72%,30.15% and 41.18%,respectively,than untreated sludge.Compared with ultrasound pretreatment(12000 kJ/kg TS,20+25kHz),combined ultrasound and thermal pretreatment(12000 kJ/kg TS,20+25 kHz,80?),combined ultrasound,thermal and alkali pretreatment at optimum condition had advantages in organic release,solids reduction,methane production and energy utilization efficiency.In order to reveal mechanism of combined ultrasound,thermal and alkali pretreatment on AD improvement,the methane conversion efficiency of organic matters was calculated.It turned out that methane conversion efficiency increased 39.86% than untreated sludge.Considering volatile fatty acids concentration and total alkalinity,the acetic-alkali equilibrium during AD was more stable than untreated sludge.The proteinase activity and dehydrogenase activity improved 53.12% and 91.56%,and the time to reach the highest proteinase activity and dehydrogenase activity were shorted 6 to 9 days.Compared with untreated sludge,the hydrolysis,acidification and methane production percentage of TCOD of optimum treated sludge during AD increased 17.75%,17.78%,and 15.9%,respectively.The hydrolysis step was found to be the rate-limiting of AD process for untreated sludge,while rate-limiting step during AD was turned up to be methanogenesis step for sludge pretreated by combined ultrasound,thermal and alkali pretreatment.The results of continuous operation showed that the optimum pretreatment process had a stable treatment effect.The average TSS reduction of 70 sludge samples was 33.72% and the deviation of 70 values was in 5%.The average sludge disintegration degree was 56.3% and the deviation was in 10%.Moreover,the deviation of methane production values was in 5%.By analyzing the energy balance and solids balance,the economical evaluations of three different technologies were compared.For ultrasound pretreatment,12000 kJ/kg TS was a dividing point of economic feasibility.When specific energy was lower than 12000 kJ/kg TS,ultrasound pretreatment had a positive net energy income.While with the further increasing input energy,the economic feasibility was disappearing and extra investment was needed.Since methane production and solids reduction of double frequency were better than single frequency,the economic analysis showed that double frequency was better than single frequency.Combined ultrasound and thermal pretreatment improved economic feasibility than ultrasound pretreatment alone.Under the pretreatment condition of specific energy of 12000 kJ/kg TS,frequency of 20+25kHz,and temperature of 80?,the net income increased 29.29 ? per ton of sludge.For the optimum condition of combined ultrasound,thermal and alkali pretreatment,the net earnings reached to 44.56? per ton of sludge,which nearly saved 10% of operating cost of waste water treatment plant.Besides,the total solids reduction of pretreated sludge wad higher than 60%. |
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