Study on the Effects of Contaminants on Enzymatic Hydrolysis of Excess Sludge and It’s Migration and Transformation Law

Author:Li Yu Long

Supervisor:li deng xin

Database:Doctor

Degree Year:2019

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Pages:124

Size:4359K

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Population growth,economic development,rapid urbanisation,rising environmental and public health standards,and improved sanitary systems have resulted in a vast number of wastewater treatment plants all over the world.As a by-product of sewage treatment plant,the output of excess sludge is increasing year by year.At the same time,because of the complex components of excess sludge,it contains not only available resources such as microorganism single cell protein,polysaccharide and so on,but also inorganic and organic pollutants.If untreated or improperly treated,it may enter into surface water or groundwater,which will cause serious pollution and damage to the water environment and ecosystem,and finally may endanger human health.Therefore,development of harmless,reduction and resource treatment and disposal of excess sludge becomes very urgent and necessary.There are various research methods of extracting protein and amino acids from excess sludge,such as acid hydrolysis,alkaline hydrolysis and ultrasonic hydrolysis.However,there still has many defects,such as great investment of equipment,strict extract conditions,reaction process is difficult to control,a large number of acid and alkali are needed in the process of hydrolysis,the subsequent process is very difficult and the secondary pollution to the environment is serious.In response to the above problems,on the basis of single enzyme hydrolysis of excess sludge in our laboratory,protein was extracted from excess sludge by enzymatic hydrolysis in this research.The suitable experiment conditions for enzymatic hydrolysis of excess sludge to extract protein were determined at first,and then the effects of metal ions and sulfadimethylpyrimidine on the enzymatic hydrolysis of excess sludge were analyzed,and finally the migration and transformation mechanisms of metal ions and sulfadimethylpyrimidine during the process of enzymatic hydrolysis of excess sludge were discussed.The main conclusions of this study were summarized as follows:(1)The optimum experimental conditions for extract protein from excess sludge by enzymatic hydrolysis method are as follows:with the change of various investigated factors,there was a strong positive correlation between protein extraction rate,organic matter content and SCOD/TCOD in the process of hydrolysis of excess sludge with enzymes,and there had a similar trends.The optimum experimental conditions for enzymatic hydrolysis of excess sludge were:initial pH value of 9.0,reaction time of 4 h,enzyme amount of 6%and reaction temperature of55 o C.Under the above experimental conditions,the extraction rate of protein,value of SCODR/TCOD and amount of organic matter were 65.89,61.43 and 81.05%,respectively.(2)The experiments were introduced to evaluate the effects of heavy metal ions on the enzymatic hydrolysis of excess sludge process.The results showed that excess sludge protease hydrolysis showed different roles with adding different concentrations of Cu2+,Zn2+,Mn2+and Pb2+during enzymatic hydrolysis of excess sludge.Each heavy metal ion,included Cu2+,Zn2+,Mn2+and Pb2+,exhibited Hormesis phenomenon,that is low dose promotion,in the contrary,high dose inhibition.Additionally,in the investigated concentration,Pb2+had always possessed inhibition.When the concentration of Pb2+was low,inhibitory effect was very small.However,with the increase of it’s concentration,Pb2+inhibition was more obvious.This is principally because Pb2+occupied the active center of protease,as a result,hydrolysis effect of excess sludge was inhibition.(3)The law of transformation of heavy metal during the enzymatic hydrolysis of excess sludge process was as follows:the sum of F1 and F2 of Cu,Zn,Mn,Ni,Pb,Cr and As is more than 25%in the raw sludge,and the content of F1 and F2 of Mn and Zn is more than 45%,illustrating that both Cr and As have potential migration risks,and Mn and Zn have the highest mobility.After extraction of Protein by Enzymatic Hydrolysis of Sludge,the general trend was that the metal in F1,F2 state with the highest mobility could be transformed into stable F4,F5state,and most of the metals existed in solid phase,in which F4 and F5 existed,in which Cu(85.9%)was used.Pb(89.9%),As(83.3%)and Cr(77.3%)had the best effect.One explanation could be that the changes of sludge moisture content,metal binding environment,sulfide oxidation,pyrolysis of organic matter and evaporation of volatile substances might caused the transformation of heavy metals.Furthermore,the process of enzymatic hydrolysis of excess sludge can promoted the stabilization of heavy metal ions.(4)The experiments were introduced to evaluate the effects of sulfadimethylpyrimidine pollutant on the enzymatic hydrolysis of excess sludge process.The results indicated that when the concentration of sulfadimethylpyrimidine was less than 1×10-44 mg/g Tss,the enzymatic hydrolysis efficiency of excess sludge was promoted.When the concentration of sulfadimethylpyrimidine was more than 1×10-44 mg/g Tss,the enzyme hydrolysis efficiency was inhibited obviously.When the concentration of sulfadimethylpyrimidine was 1×10-44 mg/g Tss,with the increased of enzyme dosage,the hydrolysis effect of protease became better and better.At the same time,the optimum reaction temperature of enzymatic hydrolysis decreased from55°C to 40°C.Meanwhile,with the addition of sulfadimethylpyrimidine,and the optimum pH value increased from 8.0 to 9.5.(5)The laws of transformation of sulfadimethylpyrimidine during the enzymatic hydrolysis of excess sludge process were carried out.The results determined that the percentage of sulfadimethylpyrimidine during the hydrolysis of excess sludge protease was significantly affected by the amount of protease added,reaction temperature,hydrolysis time and initial pH value.When sulfadimethylpyrimidine combined with protease fluorescence,the fluorescence intensity of protease decreased,may be due to the formation of non-fluorescent complex.Nevertheless,the stability of the combination between the above was weakened,and the interaction force is hydrogen bond or van der Waals force.Furthermore,the addition of sulfadimidine did not change the microenvironment around tyrosine residues in protease,but changed the microenvironment around tryptophan residues,and thus changing the secondary structure of protease.In conclusion,on the basis of our previous study,the influence of heavy metal ions and sulfadimethylpyrimidine on the enzyme hydrolysis of excess sludge was analyzed,and the laws of migration and transformation of the aforementioned pollutants during the process of enzyme hydrolysis of excess sludge were revealed.This research maybe plays an examplary role in the promotion and application of extraction protein from excess sludge by enzyme hydrolysis technology.