The Dewatering Performance Study of Frozen Pretreatment of Excess Sludge

Frontier of Environmental Science March 2015, Volume 4, Issue 1, PP.25-28

The Dewatering Performance Study of Frozen Pretreatment of Excess Sludge Hongxin Li1, Yubo Cui*2, TingtingYu3, Wenjing Yang4, Shuchang Zhang5 College of Environment and Resources, Dalian Nationalities University, Dalian 116600, China Email: 11006741848@qq.com; *2cyb@dlnu.edu.cn; 3 2273604875@qq.com; 4515929240@qq.com; 5 903173567@qq.com

Abstract At present, the technology of the traditional sludge pretreatment is mechanical method, chemical method, biological method and thermodynamics method, etc. and cold melt processing is a new and effective means of sludge conditioning, not only can it improve sludge dewatering performance, make the sludge particles structure more closely, can also reduce the bound water content, and to achieve the sludge cracked and accelerate the subsequent process of anaerobic digestion, etc. This paper analyzes the excess sludge is in about -20 ℃ in the refrigerator frozen at different times (0 h, 6 h, 12 h, 18 h, 24 h), recorded a total freeze on time. After freezing, under the condition of room temperature (22 ℃) to thaw, record the thawing time and then analyze its physical and chemical properties, dehydration and settling performance. Results show that when it is in the - 24 ℃, completely freezing time is 3.5 h, completely frozen at room temperature was 22 ℃, frozen time is 1.83 h. Results also show that with the increase of the freezing time, excess sludge concentration of COD, ammonia nitrogen concentration increasing, pH is declining.

Keywords: Frozen; Sludge Treatment; Dewatering Performance; Sedimentation Performance

1 INTRODUCTION Sludge is produced by precipitation in the process of sewage treatment and sewage out of the surface of the froth of the proceeds of the residue, it is the second product in the process of sewage treatment. At present, the problems of environmental pollution is increasingly serious, the sludge treatment problems is to be solved. Melting and freezing method is one of the most common sludge physical treatment method, its use began in the late 1940 s Britain Clements, etc.

Compared with the traditional sludge pretreatment technology, Frozen pretreatment does not spend a lot of energy, also doesn't need to add chemicals, its cost is relatively low, and the treatment effect is more obvious. The mechanism of freezing technology is loose and fluffy flocs is rejected by the gradual growth of the ice, a lot becomes into a more compact and round floc structure. With the increase of freezing time, the sludge flocs is more clear, the number of filaments is less , the sludge settling characteristics is improved. In addition, the freezing and thawing process will easily transform dewatered sludge into granule, easily dewatered sludge. After freezing treatment, sludge dehydration performance and sedimentation performance will be improved significantly.

Aiming at the frozen pretreatment for sludge, cold region in north China has natural advantages, it can take advantage of the natural conditions of cold melt processing to realize sludge dewatering. It is cold in winter in north China, sludge becomes freezing because of natural climate, ice crystal growth process can separate the solid and liquid. When spring or temperature rising, reuse the rise of temperature and solar radiation, etc, to make sludge dewatering and get dry melting to become filter cake, remaining solids removed from the traditional equipment. In the 19th century, in south-central Canada, there people use natural freezing method for municipal sewage plant sludge pretreatment, the result shows that the processing costs narrow 10 times compared with ordinary processing methods .So you can see, natural freezing treatment of sludge can not only effectively improve sludge dewatering performance, but also save the processing cost, simple operation, energy saving and environmental protection.

At present, the study of the physical and chemical properties of sludge after frozen pretreatment is still in its error - 25 http://www.ivypub.org/fes

state ,the freezing and thawing techniques can open up a new path for the treatment of urban and rural sewage sludge in the future .Through the study can also enrich theory of freezing and thawing, widen the crystallization process in the application of environmental engineering discipline, facilitate heat transfer processing technology in sludge treatment and disposal technology and is widely used in the field of environmental pollution and so on, and open up new field of freeze-thaw conditioning technology.

2 MATERIALS AND METHODS Excess sludge in the laboratory is collected from Henderson water sewage treatment plants in Dalian, sludge collected settle for 18 h, discard clear fluid, its moisture content is down to 98%;Puts excess sludge in - 20 ℃ or so to be frozen in the refrigerator, recorded a total freeze on time.

Experimental instrument used is 5 b - type 1 COD multi-function meter, METTLER TOLEDO analytical balance, pH S-3c acidity meter, electrothermal blowing GZX - 9070, temperature adjustable cryogenic freezer, high-speed centrifuge, island by gas chromatography GC - 2010, Daojin, etc

3 RESULTS AND DISCUSSION Sludge is frozen by different time, then to be fully thawed, take samples at regular intervals and detect the pH (See Figure 1).Cold melt treatment is an effective pretreatment technology of sludge, after dealing with the cold melt, the physical and chemical properties of the sludge change, and with the increase of freezing time, it can be seen that after freeze-thaw treatment sludge pH is falling, when the pH dropped to a certain value and then leveled off. The decline of Sludge pH shows that the content of volatile fatty acid (VFA) in sludge is growing, this is mainly because of the intracellular and extracellular ice in formation in the process of cell rupture and release caused by inclusions (Li and Li., 2012). Confirming organic matter is dissolved .As shown in Fig. 1, it can be seen that pH final steady at around 6.5, it shows that flocculent bacteria is growing, sludge settling dehydration effect is better.

FIG. 1 PH CHANGE OF SLUDGE

FIG. 2 COD CONCENTRATION CHANGES OF SLUDGE

In experiment we also measured the change with different freezing time of COD concentration in sewage sludge, it was shown in Figure 2.The concentration of COD of excess sludge increased after frozen .This suggests that as the effect of freezing on the sludge solid cracked and the organic matter is gradually been dissolved in sludge, As a result, the sludge solubility in the liquid phase gradually reduce pollutants. The reducing agents, most are the content of organic matter reducing, the quality of the sludge has been greatly improved, it is beneficial to the subsequent anaerobic digestion process.

The concentration changes of NH4-N of sludge after the frozen can be seen in Figure 3.From the chart we can see, with the concentration of NH4-N samples increases the freezing time is slowly rising, the rising rate reached the maximum when the concentration of NH4-N in the 3.0 hours, with the extension of time of freezing, the concentration of NH4-N increased slowly, but the concentration of NH4-N is not higher than it before. The increased concentration of ammonia nitrogen is related to the hydrolysis acidification process of degradable organic wastes anaerobic digestion in substrates. In the hydrolysis stage, proteins are hydrolyzed into peptides and amino - 26 http://www.ivypub.org/fes

acids under the action of microorganisms, one part of the amino acid will continue to be hydrolyzed into ammonia, nitrogen and hydrogen sulfide, etc, however, in the acidogenic stage, organic matter degradation also will produce a certain amount of ammonia, hydrogen sulfide, etc. It is because most of the organic nitrogen in the anaerobic fermentation of raw materials by oxidation reduction reaction of ammonia nitrogen removal, the concentration of ammonia nitrogen increased. While ammonia nitrogen is decline in the medium-term concentrations occurred at the peak stage of hydrocarbon generation. It main relates to the free ammonia volatilization in the system.

FIG. 3 NH4 -N CONCENTRATION CHANGES OF SLUDGE

FIG. 4 CHANGE OF SETTLE ABILITY

Sludge is frozen at different time, and then natural melts, the settlement changes are shown in Figure 4.

From Figure 4 we can see: residual sludge sedimentation is very slow, the volume decreased is almost 0 after setting 1 hour space. And after 3 hours melting of the frozen, it dropped by 5ml, the sludge settling velocity increased. This is because the frozen thawed sludge particle binding water and interstitial water is drained, the sludge particle agglutination settlement. When the sludge is freezing after 4.5 hours, the settling velocity in 1 hour is much faster than frozen in 3.0hours, suggesting that increased freezing time can improve sludge settling performance, but when the freezing time is further increased to 7.5 hours, the settling velocity is not further increased, therefore, 4.5 hours is a better time to improve the refrigeration performance of sludge settlement.

The specific resistance of sludge is the main parameter that reflects the dewatering performance of sludge. Sludge is frozen at different time, and then thawed, the change of resistance is shown in Figure 5.

FIG. 5 CHANGES OF SPECIFIC RESISTANCE

With the increase of freezing time, the specific resistance of sludge is increasing, after 4.5h of frozen, specific resistance of sludge decreases with the increase of freezing time. It shows that the dewatering performance of sludge which is frozen for 4.5h is improved, the analysis of the reason may be the sludge particles gathered themselves together, and particle increase, homogenize, head loss decreases when filtrate is through the pores, which reduces the specific resistance of sludge and improve the dewartering properties of sludge.

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4 CONCLUSIONS Frozen (freezing and thawing) preprocessing is a very effective way of sludge treatment. Freezing and thawing is a kind of disposal method of sludge which change sludge colloid properties by dissolving, and then change coagulation sedimentation performance of sludge. After freezing and thawing, the sludge sedimentation velocity and filtration velocity is higher than before freezing, etc. Freezing and melting processing can crack of sludge, change the solid material into a liquid composition. With the increase of freezing time, the concentration of pH and COD of sludge decreased, and the concentration of NH4 -n increased. The physical properties changes of frozen sludge not only reflect the dissolution of the sludge, but also reflects the improvement of sedimentation and dehydration performance of sludge .Frozen pretreatment methods of sludge will be to explore a new road in the field of sludge treatment in the future.

ACKNOWLEDGMENT The research was financed by the National undergraduates' innovative undertaking project of China (Dalian Nationalities University), the National Natural Science Foundation of China (51278088), and the Fundamental Research Funds for the Central Universities in China (DC12010205).

REFERENCES [1]

Li Y Y, Li B. Cold melting technology of excess sludge conditioning research. Industrial water treatment, 2012, 32(8): 56-58

AUTHORS Hongxin Li, Place of Birth: Jinlin

Taiyuan University of Technology, Taiyuan, Shanxi Province,

province, China, Date of Birth: March. 16,

China

1992. Educational background:

2002.03-2005.06 Doctor degree in Environmental Engineering,

2011.09-2014.09 Undergraduate student

Harbin Institute of Technology, Harbin, Heilongjiang Province,

in Environmental Engineering, Dalian

China

Nationalities University, Dalian, Liaoning Province, China

Major field of study: Water pollution control.

He is the Professor in Dalian Nationalities University, Dalian,

Major field of study: Environmental Engineering.

China.

She is a senior student in Dalian Nationalities University, Dalian, China.

Tingting Yu, Place of Birth: LiaoNing province, China, Date of Birth: May. 15, 1992.

Yubo Cui, Place of Birth: Inner Mongolia, China, Date of Birth: Sept. 10, 1968.

2011.09-2014.09 Undergraduate student in Environmental

Educational background: 1987.09-1991.07 Engineering,

Engineering, Dalian Nationalities University,Dalian, Liaoning

Bachelor

Chengdu

Educational background:

degree

in

University

of

Technology, Chengdu, Sichuan Province,

Province, China Major field of study: Environmental Engineering.

She is a senior student in Dalian Nationalities University, Dalian,

China 1993.09-1996.05 Master degree in Municipal Engineering,

China.

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