| In 2015,Tibet issued "Ten Rules of Tibetan Water ",requiring that the effluent from Lhasa sewage treatment plant reaches the first class A and the rate of harmless disposal of sludge exceeds 90% by 2020.The key of raising the standard for wastewater is to realize the standard of nitrogen and phosphorus.The sewage treatment plant in Lhasa has low influent COD and insufficient carbon source,which greatly affects the effect of nitrogen and phosphorus removal.The excess sludge(especially the extracellular polymeric substances)in waste-water treatment plant contains a large amount of polysaccharides,proteins and a small account of DNA.If the organic matter in the sludge can be dissolved out and refluxed to the biological nitrogen removal treatment system,we can not only solve the problem of insufficient carbon source,but also realize sludge reduction.Therefore,how to crack sludge efficiently and make polymer organic matter dissolved has become the focus in current research.This paper takes excess sludge from a municipal sewage treatment plant in Lhasa as a research object,which analyzed the composition of the original sludge supernatant,and studied the influences of the separate ultrasonic cracking technology,the separate NaOH cracking technology and the ultrasonic cooperative NaOH cracking technology on the effect of the EPS sludge cracking;Meanwhile,theinfluence of ultrasonic power,pH value and reaction time on SCOD dissolution value in ultrasonic cooperative NaOH technology was examined.Through comparing these three kinds of cracking techniques,the best technology of sludge cracking of polluted water treatment plant in Lhasa was selected.This study provided a technical reference for plateau sewage treatment plant to achieve the goal of "sewage standard raising discharge" and "sludge low cost treatment and disposal ".(1)To facilitate the evaluation of the three cracking techniques,the basic properties of raw sludge were first determined,where the sludge concentration was 7103 mg/L and the SCOD of supernatant,protein,polysaccharide and DNA concentrations were 22.48,7.35,2.85 and1.71mg/L,respectively.(2)Taking the SCOD of the supernatant after sludge cracking as the evaluation index,the separate ultrasonic technology,the separate NaOH technology and the ultrasonic cooperative NaOH technology were evaluated.The results showed that ultrasonic technology,ultrasonic power and cracking time had a significant effect on the EPS cracking effect.Ultrasound could destroy a large number of sludge flocs structure and increase the dissolved organic matter in the supernatant.Under the condition of ultrasonic power was 100 w,cracking time was 20 min,SCOD concentration in sludge supernatant was 1401.09 mg/L,The protein concentration was 311.43 mg/L,the polysaccharide concentrationwas 109.23 mg/L,The DNA concentration was 66.25 mg/L;NaOH technology could destroy sludge structure and microbial cells and promoted the dissolution of organic matter,and the higher the pH value,the longer the cracking time were more conducive to sludge cracking.When the pH was 12,the reaction time was 20 min,sludge cracking and organic matter release effect were better;Ultrasound coordinated NaOH to crack sludge and the dissolution of organic matter increase with the increase of NaOH dosage and ultrasonic time.When the ultrasonic power was 100 w,pH was 12 and the reaction time was 20 min,the SCOD concentration was 4329.89 mg/L,among which the protein concentration was 962.86 mg/L,the polysaccharide concentration was 337.52 mg/L and the concentration of DNA was 202.5 mg/L.(3)The influence of ultrasonic power,p H value and reaction time on energy consumption per unit SCOD dissolution was investigated by using the response surface method.The results showed that the interaction between pH and reaction time of sludge was significant,but the interaction between ultrasonic power & reaction time and ultrasonic power & p H was not significant.It could be concluded that the optimum operating conditions were as follows: the ultrasonic power was 100 w,pH was 10 and reaction time was 30 min.Under these conditions,the actual energy consumption per unit SCOD was 27.910kwh/kg;Comparing with the optimal operating conditions determined by the influencing factors,we could find that the operating conditions obtained by response surface method were the best.(4)Through the analysis of sludge cracking degree,carbon-nitrogen ratio,carbon-phosphorus ratio and sludge morphology after cracking through three technologies,it was found that ultrasonic synergistic NaOH technology was better than the other two technologies to crack sludge,which could realize the complete dissolution of microbial filament structure and cell wall,and thus increasing the concentration of organic matter in sludge supernatant. |
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