Study On The Disintegration Mechanism Of Activated Sludge By ClO₂-ultrasonic Combined Process And Sludge Reduction Based On The Cryptic Growth Of Microorganism

With accelerating of urbanization process and increasing of public attention on the water pollution, the number of urban wastewater treatment plant and wastewater treatment capacity are undergoing a fast develop. Total 2299 of municipal wastewater treatment plants which have a treat capacity of 1.1246 billion m³/d were constructed in China up to June, 2010. The number of wastewater treatment plants increased 1.2 fold and the treat capacity increased by 75.3% when compared to that of the end of the 10th Five-year plan. However, the wide use of an activated sludge process for wastewater treatment result in production of a considerable amount of excess sludge, which must be safely disposed of due to its potential environmental risk. The excess sludge could up to tens of millions based on 80% of water content. However, disposal of these solid wastes is extremely costly, which may account for up to 60%～70% of the infrastructure costs and 30%⁴0% （even 60% for some case ）of the total operating costs of wastewater treatment plant. Therefore, excess sludge disposal represents an increasing challenge for wastewater treatment plants. It would be an ideal method to solve the excess sludge-associated problems through reducing sludge production during wastewater treatment rather than during post-treatment of the sludge produced.Recently, the sludge reduction technologies which can be summarized as uncoupling metabolism, maintenance metabolism, enhanced microorganism cryptic growth and bioaugmentation have attracted a wide attention. These technologies have their own advantages and drawbacks. The enhanced microorganism cryptic growth technology is defined as the microorganism growth using died microorganism as substrate in which both the process of cell disintegration and cell growth are contained. Some technologies can be incorporated into this process for assisted sludge reduction because these technologies can kill the microorganism in a few minutes and died microorganisms is subsequently served as substrate for the microorganism in wastewater treatment system. In other word, this process can be regard as enhanced self-oxidation rate and increased attenuation coefficient of activated sludge in traditional wastewater treatment model. As a result, reduced sludge production could be achieved.Disintegration pretreatment of sludge is the first step of sludge reduction based on enhanced microorganism cryptic growth. Several technologies, which involve mechanical, chemical, thermal and biological methods, have been explored for sludge disintegration. Ultrasonic disintegration and ozonation has been successfully applied for sludge disintegration. But high cost of ozone generation and high energy requirements limit ultrasonic application while chlorination-generated potential harmful byproducts, such as trihalomethanes （THMs）, pose a serious challenge to the full scale application of this technology.In this study, the ClO_2- ultrasonic combined sludge disintegration technology was proposed according to the enhanced microorganism cryptic growth theory and was incorporated into the activated sludge process based on comparison different methods used for sludge reduction. The excess sludge was disintegrated and was then returned to the system for further reduction through enhanced microorganism cryptic growth.This study concentrate on investigating the mechanism and technical aspects of the ClO_2- ultrasonic combined sludge disintegration technology which will enrich the theory aspects of the technology.The sludge disintegrability, factors and mechanism of the ultrasonic and ClO₂ based sludge disintegration were first investigated individually through lab experiment, respectively and optimum conditions were obtained. Then, the similar studies were conducted under ClO_2-ultrasonic combination condition. The sludge disintegration performance, factors and optimum combination conditions were investigated and biological effectiveness related synergistic effect mechanism was elaborated. At last, the model of sludge disintegration was proposed through orthogonal experiment and the effect of the order of the factors was determined.Two parallel operated SBRs, which have a treatment capacity of 3.6 m³/d were constructed and were evaluated according to their long-term operation performance. The effect of the treatment process on quality of the effluent was also investigated. Finally, the mechanisms of the sludge reduction based on enhanced microorganism cryptic growth were clarified and the regulation strategy for performance improvement of the system was found. Major conclusions were drawn as follows:Effect of energy density and reaction time on disintegration of the sludge with different solid content was investigated using ultrasonic alone. The results showed that the SCOD presented a linear increase with the ultrasonic time when ultrasonic energy density and sludge solid content were in the range of 0.5W/mL³.0 W/mL and 2.5g/L¹5g/L, respectively. Higher ultrasonic energy density resulted in a higher increase in SCOD. However, slowly increase in SCOD was observed for the sludge with a low solid content after ultrasonic disintegration of 6 min. The SCOD increase, MLSS decrease and the concentration of NH4+-N, TN and TP were observed positive correlation with the ultrasonic energy density. The sludge floc was disintegrated after 6 min of ultrasonic treatment. The energy requirements for ultrasonic treatment based on 300 mg/L SCOD follow the order: Es （3.0W/ml）> Es （2.0W/ml） > Es （0.5W/ml）> Es （1.0W/ml）. The optimum performance can be achieved at ultrasonic energy density of 1.0w/ml and reaction time of 6 min.The sludge disintegration using ClO₂ oxidation alone was studied. The results showed that the sludge disintegrability of ClO₂ was excellent. The optimum ClO₂ dose and concentration can be determined as 8 mg ClO₂/ g dry sludge and 1000mg/L, respectively. Treating sludge with ClO₂ for 60 min resulted in 110.3mg/L increase in SCOD while the concentration of polysaccharide, protein TP and TN increased by25.2、55.1、13.1and 15.23mg/L, respectively. And thus, 92.5% sludge breakage was achieved. The SVI of the sludge was decreased from 51 to 33 and the settleability of the sludge was improved.The sludge disintegration under ClO_2-ultrasonic combination condition was investigated. The results showed that the optimum operational condition were 4 mg ClO₂/g dry sludge of ClO₂ for 60 min and 0.5 W/ml of ultrasonic treatment for 6 min. Sludge disintegration resulted in 920, 420, 36.2, 7.9, 50.3 and 81.7mg/L increase in SCOD, BOD5, TN, TP, polysaccharide and protein, respectively. The ratio of BOD,TN and TP was calculated as 78.2﹕4.58﹕1 which indicated a high biodegradability.The ratio ofΔSCOD,ΔTN andΔTP in the disintegrated sludge liquid was calculated according to the experimental data. 121.95﹕6.7﹕1, 78.1﹕1.16﹕1 and 116.46﹕4.58﹕1 were obtained under condition of ultrasonic alone, ClO₂ alone and ultrasonic-ClO₂ combination, respectively. Among these, the ratio ofΔSCOD,ΔTN andΔTP under ClO₂ treatment alone is much lower than that of the theoretical value which is 122﹕7.7﹕1. We inferred that most of nitrogen-containing compounds was oxidized by ClO₂ and escaped from the liquid phase as N2 gas while part of organic carbon compounds was oxidized directly to be CO₂. In comparison, the ratio ofΔSCOD,ΔTN andΔTP under ultrasonic treatment alone is approach to the theoretical value. Under combined ultrasonic-ClO₂ treatment, low dose of ClO₂ was firstly applied to break the cell wall. It would be better that the dose of ClO₂ did not result in significant leakage of cell content to avoid extra consumption of ClO₂. Then, the sludge can be easily disintegrated at low ultrasonic energy density（0.5W/ml）and short reaction time（6 min）.The mathematical model of the ultrasonic-ClO₂ combined sludge disintegration was proposed according to the ternary regression method. The SCOD, TN and TP in disintegrated sludge liquid showed a exponential relation with ultrasonic treatment time, ultrasonic energy density and ClO₂ doses and follow the order: ultrasonic treatment time > ClO₂ doses > ultrasonic energy density. A ClO_2-ultrasonic-activated sludge process combined system was developed and used for sludge reduction during treatment of municipal sewage. The system was operated continuously for 30d to investigate its long-term performance. The results showed that 70% of excess sludge was subjected to combined ClO_2-ultrasonic treatment and was then returned to the system resulted in 54.86% reduction in sludge production. The system performs well and the SVI of the sludge is between 40⁷0mL/g. The quality of the effluent was not deteriorated.The concentration of SS、COD、TN and NH4+-N was totally not affected as compared with control system. Only a slight increase in TP was observed （average 0.37mg/L vs. 0.26mg/L）. The sludge load in the tested and control system were 0.159 and 0.126 kgCOD/（kgMLSS·d） which corresponding to the sludge age of 12.86 and 19.28d, respectively, based on the calculation of the sludge production rate and microorganism attenuation coefficient. Notable decrease in sludge production rate from 0.41 to 0.186kgSS/kgCOD without changing the operational parameters of the system indicated that the endogenous metabolism in the system was greatly enhanced and thus the self-oxidation rate of the sludge was accelerated （The attenuation coefficient increased from 0.036 to 0.0602d-1）.