Microwave thermal pretreatment of sewage sludge

WANG Wei, QIAO Wei, YIN Keqing, XUN Rui

Department of environmental science and engineering, Tsinghua University, 100084, Beijing, CHINA, qw04@mails.tsinghua.edu.cn

Abstract Water could absorb microwave and increase temperature rapidly within short time. Municipal wastewater treatment plant sewage sludge was used to be heated by microwave digestion reactor. Sludge total solid content (TS) was 2%, 4% and 5%. Microwave heating temperature was 150 , 170 and 190 , reaction time was 5 minutes. Results show that temperature rising trend from ambient to three specified temperature was similar. Colloid structure was destroyed to release soluble organic components. Dewaterability and settling performance were improved. TS and suspended solid content (SS) reduced. High temperature resulted high SS dissolving ratio. For TS 2% sludge, at 190 40.0% of SS dissolved.

Keywords: microwave, thermal pretreatment, sewage sludge

INTRODUCTION Municipal and industrial wastewater treatment plants produce large amounts of sewage sludge, containing organic and mineral components. The disposal of sewage sludge produced by urban and industrial wastewater treatment plants is a matter of great concern. It is well known that sewage sludge is very difficult to dewater. Both the reduction of the amount of sludge and improving its dewaterability are of paramount importance. It has been known that thermal pre-treatment gives an improvement in the dewaterability of sludge. Experiments with municipal sewage sludge show that the highest yield of hydrolysis can be achieved at 165-180. The dissolved components are readily degradable in a digestion process. In addition the dewaterability is increased. Thermal pre-treatment prior to anaerobic digestion may result in net energy production from the system because of increased biodegradability and reduced digester heating requirements [1,2]. Conventional thermal energy source was electronic power. The heating time was comparatively long. The new and energy-efficient source was more important to improve thermal pretreatment economic advantages. It was realized that microwaves had the potential to provide rapid, energy-efficient heating of certain materials. The main applications of microwave heating today include food processing, wood drying, plastic and rubber treating as well as curing and preheating of ceramics. Water could absorb microwave energy efficiently. More than 90.0% (weight) of the sludge was water, so the microwave was a potential effective sludge thermal pretreatment energy source [3].

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Materials and Method Experiment reactor was MSD-6 microwave oven made by Shanghai sineo microwave chemical technology co., Ltd. The oven could operate below 200 temperature. The microwave reactor could provide sample digestion and extraction operation mode. System temperature was controlled and recorded by the reactor automatically. Our experiment choose sample extraction mode to heat sludge. 30mL sludge was put into 70 mL sample tube which was made of polytetrafluoroethylene (PTFE). The sludge heating temperature was 150, 170 and 190. At the pre-setted temperature sludge was heated 5 minutes. Sludge was colleted from an urban wastes water treatment plant. Colleted sludge was kept in a refrigerator at -4 before using. The original sludge contains about 5.0% TS (total solid). In order to review the effect of organic flocculant to the sludge dewaterability after microwave heating, polyacrylamide (PAM) was added into the sludge. Sludge containing 0.3% and 0.9% PAM (dry weight) was made by to adding 1.0g/L PAM. TS was measured by drying sludge at 105 for 24 ours. For SS analysis, before the same drying operation sludge was separated off the soluble solid by centrifuge at 3000r/min for 10 minutes. Sludge sedimentation property was measured by recording the dropping sludge solid interface at a glass column within 8 hours [4].

Results and Discussion Contrast with general electronic heating facilities, microwave oven use entirely different heating mechanism. Microwave could achieve shorter heating time, so the utilization efficiency of electronic energy was high. Sludge was heated at 150, 170 and 190 respectively. The temperature of sludge heating process was recorded. Figure 1 showed the increase of sludge temperature.

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Figure 1 Increase of the sludge temperature

From figure 1 we may see that three temperature increasing trend was similar. Sludge could increase to 150 within 5.0 minutes. For 170 and 190 the last time was 6.5 minutes and 7.0 minutes. The whole process could be divided into two stages. Before 2.0 minutes the sludge temperature increase rapidly and the temperature reach

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about 100. After this stage the increase of the temperature showed a mild trend.

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Figure 2 Sedimentation curve for TS 2% sludge Figure 3 Sedimentation curve for TS 4% sludge

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Figure 4 Sedimentation curve for TS 5% sludge Figure 5 Sedimentation curve for TS 4% sludge(0.3% PAM)

The effect of the microwave heating of sludge could be showed in the sedimentation curve directly from figure 2 to figure 5. For sedimentation test, the experiment sludge was TS 2%, TS 4% and TS 5%. Sludge (TS 4%) was added PAM. The mixture contains 0.3% PAM (dry weight). Figure 2 show that for TS 2% sludge, the heated sludge present obvious superiority to untreated sludge for settling property. From figure 2 to figure 3, the TS content of the sludge increased. The difference of the sedimentation characteristic between untreated sludge and treated sludge diminished. Differently, at 190 the heated sludge showed distinct advantage compared with 150 and 170. PAM could increase the treated sludge sedimentation performance shown in figure 5. When adding 0.3% PAM compared with the sludge with no PAM, at 170 sludge could settle rapidly. In microwave heating process, sludge solid structure was destroyed and organic components dissolve into soluble parts. So the solid content of the sludge decreased and the amount of sludge reduced. Part of solid components decomposed at high temperature and certain pressure condition. TS and SS were measured to present the volume reduction of sludge.

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Figure 6 TS/SS reduction for TS 2% sludge Figure 7 TS/SS reduction for TS 5% sludge

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Figure 8 TS/SS reduction for TS 4% Sludge (0.3% PAM)

Figure 9 TS/SS reduction for TS 4% sludge (0.9% PAM)

Sludge solid content reduction was presented from figure 6 to figure 9. TS and SS content decreased after the microwave heating at different microwave thermal temperature and for different feed sludge. Contrasted with TS 5% sludge, TS 2% sludge present more obvious TS and SS decompose trend. High microwave temperature would bring high TS and SS dissolving ratio. Figure 7 show that 190 was more effective for TS and SS dissolving. Compared with TS, SS more usually was used to analysis the effect of sludge reduction. Both in water oxidation (WO) and in microwave heating process the object was to reduce the amount of the sludge amount and improve sludge dewaterability. In microwave process, 5 minuets were enough to achieve high SS dissolving ratio. With the system temperature increasing, SS reduction effect was obvious better. The less sludge TS content leads to higher SS dissolving ratio. For 2% TS sludge, at 190 SS dissolving ratio was 40%. For 5% TS sludge, at 190 SS could dissolve 13%. PAM was effective to improve SS reduction. When adding 0.9% PAM to TS 4% sludge, 34.7% of SS dissolve.

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CONCLUSION

Microwave thermal pretreatment could improve sludge dewaterability and reduce sludge SS content. 5 minutes microwave heating could achieve considerable SS dissolving ratio. Temperature was the most important factor that affects sludge dewaterability. 190 was better than 150 and 170. For TS 2% sludge, 40% SS dissolve ratio could be achieved. Adding PAM could improve SS reduction and dewaterability.

ACKNOWLEDGEMNET The work was supported by Ministry of Education of China and National Nature Science Foundation of China.

REFERENCE [1] E. Neyens, J. Baeyens. A review of thermal sludge pre-treatment processes to improve dewaterability. Journal of Hazardous Materials, 2003(B98):51-67 [2] WANG Zhijun, WANG Wei, XIA Zhou, WU Shuxu. Experimental study on thermal hydrolysis and anaerobic digestion. CHINA WATER & WASTEWATER, 2003, 19(9):1-4 [3]D.A. Jones, T.P. Lelyveld, S.D. Mavrofidis, S.W. Kingman, N.J. Miles. Microwave heating applications in environmental engineering-a review. Resources, Conservation and Recycling, 2002, 34:75-90 [4] WANG Zhi-jun, WANG Wei. Thermal hydrolysis test of surplus sludge. China Environmental Science, 2005, 25(supply):56-60

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