Study On Effect Of Electromagnetic Wave Pretreatment On Flocculation And Dewatering Performance Of Excess Sludge

With the increasing of municipal wastewater treatment plants, reduction of excess sludge becomes to be a serious problem now. The electromagnetic wave technology has been used to improve sludge dewaterability for decades, but it was confined to the static and oneness research which was unrealizable in real wastewater treatment plants. Research on the effect of electromagnetic wave on action on characteristic change and flocculation of excess sludge is important to practical application in wastewater treatment plant. A new integrated process, 2450 MHz or 5800 MHz electromagnetic wave combined with traditional flocculation conditioning technology, was established to pretreat excess sludge from Shahu wastewater treatment plant in Wuhan before dewatering. Firstly, excess sludge was pretreated by electromagnetic wave. Then PAM+ was added as flocculant before sludge was sent to churn-dasher. During the process, indexes of sludge characteristics such as moisture content, TN, TP, SCOD and EPS was detected to analysis flocculation and dewatering result of excess sludge.Test data and results is established as follows: 1 Single-factor experiments and response surface optimization experiments of integrated process are accomplished to show that dewaterability of sludge have the same variation tendency with intensity of 2450 MHz and 5800 MHz electromagnetic wave.Dewaterability of sludge is improved by lower intensity of electromagnetic wave, but became worse with moderate-intensity of electromagnetic wave, and then is promoted again by increasingly intensity of electromagnetic wave in different degrees. 5800 MHz electromagnetic wave’s interval of intensity which could be used to improve sludge dewaterability was more narrow and higher than 2450 MHz. The results of response surface optimization experiment illustrated that optimum of integrated process for 2450 MHz electromagnetic wave with excess sludge flocculation and dewatering was obtained when loading time was about 70 s, sludge load at 100 m L and dosage of PAM+ was about 5-6 g/kg-SS, power and p H had a variety of options; optimum of integrated process for 5800 MHz electromagnetic wave with excess sludge flocculation and dewatering can be obtained when power was about 200 W, time was about 40 s, sludge load was 70 mL, pH was 5 and dosage of PAM+ was about 9-10 g/kg-SS. On the eyes on promotion of dewaterbility of sludge, 2450 MHz electromagnetic wave was better than 5800 MHz, while its energy consumption was higher. 2 Effect of EPS on flocculation and dewatering characteristic of excess sludgeExperiments explained that lower sludge load, greater power and longer time of electromagnetic wave leaded to more dissolution of TB-EPS which multiplied content of LB-EPS at the same time. At initial stage of the integrated process, LB-EPS in outer layer of flocs peeled into liquid phase which improved sludge dewaterability. LB-EPS transformed from TB-EPS by continual increase of electromagnetic wave intensity adhered on surface of the flocs, then absorbed water in sludge increased greatly which deteriorated the sludge dewaterability. The proportion of protein in TB-EPS was an indicative evaluation index for sludge dewaterability: higher proportion of protein contributes to improvement of dewaterability. What’s more, there existed a linear relationship between content of LB-EPS and TN, TP and SCOD. 3 Influence mechanism of electromagnetic wave bio-effect for flocculation and dewatering characteristic of excess sludge.(1) Thermal effect of electromagnetic wave generated the solubilization of organics. A solubilization data model was modeled in integrated process for 2450 MHz electromagnetic wave with excess sludge flocculation and dewatering:CTN=0.007W-0.070 CSCOD=0.6×10-3W+0.458 CLB-EPS=0.019W+1.735CTN=0.052T+0.080 CSCOD=4.3×10-3T +0.470 CLB-EPS =0.137T+2.129CTN=-0.022V+5.436 CSCOD=1.81×10-3V+0.908 CLB-EPS =-0.059V+16.190(2) Power density window effect as a typical athermal effect was found in this study. In the integrated processes of electromagnetic wave with excess sludge flocculation and dewatering, 0-40 k Wh/m3 of 2450 MHz and 36.85-86.41 k Wh/m3 combined with 102.93-135.96 k Wh/m3 of 5800MHz were the density windows which could improve sludge dewaterability.(3) Microstructure changes of flocs was observed by SEM. As the connections for cells in flocs, content of EPS significantly decreased when sludge was pretreated by electromagnetic wave. And the decrement of EPS in 5800 MHz was lower than 2450 MHz. Due to the effect electromagnetic wave, sludge particles became tighter and bigger, margin also became smooth, which was useful to settling and dewatering of sludge. By comparing the SEM images of sludge in critical point of two power density window of 2450 MHz, the reason for deteriorate of dewaterability was that flocs were broken and difficult flocculation was caused by decrease of EPS.