Improving Dewaterability Of Sewage Sludge Through Bioleaching At Low Temperature And Its Mechanisms

With the accelerated progress of urbanization in our China,the number of municipal wastewater treatment plants and treatment capacity is also increasing.By 2012,the number of wastewater treatment plants has reached about 3340 with the amount of treated wastewater being 1.42×108 M3/d.According to statistic,the dewatered sludge(moisture content of 80%)generated exceeded 3.48×107 M3/a,and the sludge presents an increasing trend with the yield of 10%?15%annually.The excess sludge is characterized by high water content,heavy metals,and pathogens,which make the proper disposal of sewage sludge become more and more urgent.Therefore,to achieve high dryness of dewatered sludge has become a hotspot in the sludge field.Chemical or physical conditioning could improve sludge dewaterability significantly.However,organic matter in dewatered sludge cake is decrease dramatically,because of the introduction of large amounts of inorganic substances into sludge matrix,and it was unfavorable for utilization of sludge.In the last decade,bioleaching method has been developed as an attractive method to remove heavy metals,pathogens and stench from sludge.In addition,early experiments had confirmed that the sludge dewaterability could be improved after bioleaching,and the moisture content of the bioleached sludge could reduce to below 60%by chamber filter press.However,bioleaching technology is a microbiological process,and the factors that affect the microbial activity will impact the dewaterability of bioleached sludge.However,the monitoring results of practical engineering application showed that the dewaterability of bioleached sludge was obviously worse in winter and spring than in summer and autumn,and the moisture content of bioleached sludge cake reached to approximately 65%under the same mechanical dewatering condition.There is doubt that what is the main reason for the deterioration of bioleached sludge dewaterability at low temperature condition?How do sludge pH,EPS and bound water contents changes during bioleaching at low temperature compared with those at normal temperature?Whether bacteria can secrete large amounts of EPS for self-protection to adapt the low temperature environment which may cause the viscosity of bioleached sludge enhancement and dewaterability deterioration?In addition,how to take effective measures to improve the bioleached sludge dewaterability under the premise of low operating costs?This paper made a systematic study of these issues and the main results are as follows:The higher sludge concentration,the greater capillary suction time(CST)of sludge,and the rose of temperature was advantageous to the CST value decreased.In addition,the viscosity was increased with the sludge concentration rose at same temperatures.Fitting analysis revealed that the sludge viscosity and concentration consistent with the index model expression of ?=?o?exp(?·MLSS),and the sludge viscosity and temperature consistent with the andrade model expression of ?=KMLSSexp[E/(k·T)],respectively.The results showed that the low temperature significantly inhibited the oxidation activity of A.ferrooxidan and A.thiooxidans.The D500 value of A.ferrooxidan were rose to 0.230 and 1.300 in 9K medium at 10 ? and 28 ?,respectively.In addition,after 6 d incubation in SM medium,the number of A.thiooxidans colonies rose from 4.97×106 cells/mL to 4.87×107 cells/mL and 2.45×108 cells/mL,and the oxidation rates of S in the treatments of 10 ? and 28 ? were 6.63%and 35.29%(without SM medium),respectively.The mechanism of sludge dewaterability deteriorate at low temperature bioleaching has been discussed through studied on sludge pH value,EPS and bound water contents under a low temperature(10 ?)and a normal temperature(28 ?)bioleaching processes through flask experiments.The results showed that,the pH of sludge decreased gradually to 2.87?3.18 and 3.82?4.20 in 3?6 day from the initial 5.15 during the bioleaching process at a low temperature(10 ?)and a normal temperature(28 ?).In addition,the sludge EPS content gradually reduced from the initial 14.00 mg/g vss to 4.51 mg/g vss and 11.56 mg/g vss in 3-4 day and then rose slowly to 13.12 mg/g vss and 12.70 mg/g vss on day 6.At the same time,the bound water content of sludge decreased quickly and then rose rapidly not only at normal temperature bioleaching process but also at low temperature bioleaching process,and the lowest values were 1.57 mg/g ss and2.37 mg/g ss on day 3 and reincreased to 4.44 mg/g ss and 4.89 mg/g ss on day 6,respectively.It was found that the CST values of sludge were reached the minimum values with 18.3 s and 25.6 s on day 3 at the two choiced temperatures.The above results indirect proved that the slight variation trend of pH value,EPS and bound water contents lead to the dewaterability of sludge at low temperature was inferior to normal temperature.Further more,statistical results of this study showed that there were significant positive correlation between sludge CST and sludge pH,EPS content while positive correlation between sludge CST and sludge bound water content in sludge bioleaching system,and the correlation coefficients were 0.9484,0.8904 and 0.7951,respectively.Studies showed that the addition of accelerator A could improve the dewaterability of bioleaching sludge obtained at low temperature.When the dosage of accelerator A was 17%,the sludge pH value and bound water content was decreased from 3.82 and 4.89 g/g ss to 2.94 and 2.25 g/g ss,respectively,and the dewaterability of sludge improved 58.54%.It was found that,when the addition of accelerator A with the dosage of 17%,the infuse speed of bioleached sludge at low temperature(2 days)was rose from 0.31 L/min and 0.18 L/min to 1.20 L/min and 0.75 L/min at the treatment of inoculation and unvaccinated,respectively,and the dewatered cake moisture content of sludge decreased from 76.6%and 86%to 65.29%and 70.55%,respectively.In addition,pilot studies showed that the sludge specific resistance was decreased to about 1×1012 m/kg after the addition of accelerator A at different sludge retention times,and the infuse speed of sludge improved 43.16%.The average moisture content of sludges reduced from 77.3%to 68.6%by chamber filter press.The pH value decreased gradually at first and then maintained a stable state in treatments of different solid contents,and the nutrition was completely used by microbial after 2 days during bioleaching.It was found that the SRF of 2%,3%,4%,5%sludges decreased quickly and then rose gradually with the extension of bioleaching time.In addition,the higher solid content,the greater increase degree.It is determined that the optimum microbial nutritional dosage of solid content 2%,3%,4%and 5%sludge were 4 g/L,6 g/L,11g/L and 17 g/L,respectively.At this point,the lowest SRF of each solid content sludge were 0.61 × 1012 m/kg,1.22×1012 m/kg,3.09×1012 m/kg and 4.83×1012 m/kg,respectively.Through the engineering application,it was showed that dilutioning the solid content 5%sewage sludge to 3%before bioleaching was feasible.It could not only improve the dewaterability of bioleached sewage sludge,but also could shorten the sludge retention time and reduce operation costs.The results of this study have an important guiding significance for the engineering application of sewage sludge bioleaching at low temperature.