Characteristics of Polysaccharides Variation in Sludge in Sludge Drying Reed Beds

Frontier of Environmental Science March 2015, Volume 4, Issue 1, PP.20-24

Characteristics of Polysaccharides Variation in Sludge in Sludge Drying Reed Beds Rui Chen 1, Yubo Cui 1#, Wei Gao 1,2, Xinnan Deng 1, Jiangang Li 1 1. College of Environment and Resources, Dalian Nationalities University, Dalian 116600, China 2. School of Materials Science & Chemical Engineering, Harbin Engineering University, Harbin 150001, China #

Email: cyb@dlnu.edu.cn

Abstract The research mainly investigated characteristics of polysaccharides variation in sludge during the process of sludge stabilization in sludge drying reed beds (SDRBs). The experiment was designed to have 3 units. Unit 1 (conventional sludge drying bed) was set as contrast with no plant cultivated thereupon; Units 2 and 3 (SDRBs) were planted with reeds. Units 1 and 2 were connected to the atmosphere through an aeration apparatus installed underneath. The three units were run according to the following cycle: seven days for a cycle, about 5 minutes feeding with 300 L per unit on the first day of each cycle. The results show that polysaccharides variation in stabilized sludge has a trend of decrease with time in three units. Comparing polysaccharides variation, the conventional drying bed is significantly higher than SDRBs. At the end of the first year, polysaccharides in the two SDRBs were 1.96 mg/g and 1.29 mg/g dry sludge, and in the conventional drying bed is 4.18 mg/g, the removal efficiencies are 57.58%, 57% and 42.29%, respectively. The results suggest that reed growth in the SDRBs is helpful for the decomposition and transformation of polysaccharide.

Keywords: Sludge, Phragmites Australis, EPS, Polysaccharide

1 INTRODUCTION Recent years, the surplus sludge amount has increased a lot with the improvement of wastewater treatment capacity in China. According to relevant research, 80% of the wastewater treatment plant sludge in China has not been handled properly. The pollution and re-contamination problems have already been prominent caused by sludge random stacking. Depending on the different wastewater treatment process, surplus sludge moisture content is generally between 99% to 97%. Due to the high moisture content of the sludge, the primary task is sludge volume reduction, namely sludge dewatering. In recent years, originated in Europe and the United States, the sludge drying reed beds (SDRBs) technology for small and medium sized wastewater treatment plant has been widely studied and applied (Weisner, et al., 1989; Zwara W and Obarska Pempkowiak H, 2000; Cui, et al., 2008; Stefanakis A.I., et al., 2009). SDRBs derived from the constructed wetland wastewater treatment technology, composed of shallow bed of filler and Phragmites australis (Cole and Stephen, 1998). In the SDRBs, sludge uniformly distributed on the surface of the SDRBs where its moisture content is lost by evapotranspiration by plants partly, while mostly is drained through the filler layer, then the solid matters are accumulated on the surface of the beds. Sludge leachate would be reflux or discharge after wetlands filler layer treatment, and solid matters further doing stabilizing treatment (Cui, et al., 2012).

Activated sludge extracellular polymeric substances (EPS) existing in activated sludge bacterial cells, mainly are the organic matters from microbial metabolism and adsorbed. EPS composition is complex, which mainly consist of polysaccharides and proteins, accounting for 70% ~ 80%, and the low content humus, nucleic acids, uronic acids, lipids and amino acids etc. are also EPS components (Dignac M, et al., 1998; Houghton, et al., 2002). The polysaccharide as a main component has been studied widely. EPS is the third largest category of activated sludge composition besides cells and water, belonging to the key substance for hindering sludge volume reduction and - 20 http://www.ivypub.org/fes

resources utilization. Thus EPS is one of the important research object in optimizing biochemical process for treating wastewater, controlling activated sludge pollution and promoting surplus sludge resource utilization. Experiment through either with or without plant and ventilation settings mainly studied stabilized sludge polysaccharide variation after sludge drying beds dehydration, thus reflecting the efficiency of sludge decrement, reaching rational utilization of SDRBs function and optimizing reed bed design.

2 EXPERIMENTAL SET UP AND METHODS 2.1 Experimental Set Up The SDRB system was located in the Wastewater Treatment Plant in Dalian Development Zone Liaoning Province, China, which composed of sludge pump, sludge influent tank and reed bed. The experiment was designed to have 3 units with a specification of 3.0 m×1.0 m×1.3 m. Unit 1 (conventional sludge drying bed) was set as contrast with no plant cultivated thereupon; Units 2 and 3 (SDRBs) planted with reeds. Units 1 and 2 were connected to the atmosphere through an aeration apparatus installed underneath for providing oxygen through the bed filler gap. Each unit ran independently.

Testbeds were set up in late April 2012 and had a 65cm media layer which was filled with slag (20 cm), gravel (20 cm), coarse sand (5 cm) and quartz sand (24.5 cm) from the bottom in turn. And the testbeds had another 65cm space for sludge accumulation.

2.2 Experimental Commissioning In early May, the sprouting reeds were transplanted to sludge bed with 5-8 cm insertion depth. Reeds grown to about 1.0m tall after adaptation period, the survival rate was about 80%. Then the experiment was started formally.

The sludge drying beds were fed with surplus sludge intermittently. One feeding cycle was seven days, about 5 minutes feeding with 300L per unit on the first day of each cycle. The sludge from a pipe located along the shorter side and spread on the surface. Here, the sludge fed is rapidly distributed over the bed and part of its water content is rapidly drained by gravity through the gravel layer and drainage. The solid matters were accumulated on the surface of the beds. The beds were fed with sludge during the vegetation period, rested during winter. Eight samples were collected in 2012, the first one was collected on July 7, 2012 and the test stopped at the end of November. At the end of this test, each bed was fed with 5.5 m3 surplus sludge.

The surplus sludge was taken from A/A/O process in wastewater treatment plant.

Samples were collected before feeding, as the accumulated sludge thickness varied under the sludge dewatering and digestion during the sludge stabilization. Sampling depth was selected at the ports: from the bottom-layer (2 cm from media surface).

2.3 Analytical Method The sample was dried for 2h in drying oven(105℃), 1g of each sample was placed in 250ml round bottom flask and was mixed with 80% ethanol - water 20ml refluxing(1h), filtrating, washing the residue with 80% ethanol - water 3 times, collecting the residue and refluxing(1h) with 30ml distilled water, filtrating, washing the residue with distilled water 3 times. Finally, the filtrate was qualified to 100ml and set aside for subsequent use. The contents of polysaccharide in accumulated sludge were measured by anthrone-H2SO4 colorimetry.

3 RESULTS AND DISCUSSION 3.1 Polysaccharide Contents in Sludge Figure 1 shows characteristics of polysaccharide variation in stabilized sludge in unit 1. Figure 1 indicates that the polysaccharide content had a decreasing trend during the operation in 2012, and the biggest drop was 2.03 mg/g in August. At the end of the experiment, the polysaccharide content changed slightly. Throughout the experiment operation stage, the polysaccharide content reduced 3.32 mg/g and the removal efficiency was 42.29%.

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Figure 2 shows characteristics of polysaccharide variation in stabilized sludge in unit 2. Figure 2 indicates that, during the operation, the polysaccharide content changed slightly in July, after August the polysaccharide content kept a declined tendency in course of time. Throughout the experiment operation stage, the polysaccharide content reduced 2.66 mg / g and the removal efficiency was 57.58%.

Figure 3 shows characteristics of polysaccharide variation in stabilized sludge in unit 3. As is shown in Figure 3, unit 3 had similar change trend with unit 1 and unit 2. The polysaccharide content had a little increase in late July, but changed slightly from July to August, and then the polysaccharide content had the trend of decreasing. During the operation, the polysaccharide content reduced 1.71mg/g and the removal efficiency was 57%.

Figure 1 - Figure 3 illustrate stabilized sludge polysaccharide content changing trend, characteristics and range of the three units clearly. Generally, in the stabilized sludge, all of three units polysaccharide content decreased obviously with the accumulated sludge depth increasing. The polysaccharide content of unit 1 showed the highest level on July 7, unit 3 on July 28 and unit 2 on August 11. Then the content showed a declined tendency to the lowest level until late November. Overall, the sludge moisture content in unit 2 was lower than unit 1. Comparing polysaccharide variation, the conventional drying bed was significantly higher than the two SDRBs.

FIG.1 POLYSACCHARIDE VARIATION IN STABILIZED SLUDGE IN UNIT 1

FIG.2 POLYSACCHARIDE VARIATION IN STABILIZED SLUDGE IN UNIT 2

FIG.3 POLYSACCHARIDE VARIATION IN STABILIZED SLUDGE IN UNIT 3

3.2 Discussion Extracellular polymeric substances (EPS) are complex mixture of high molecular polymers excreted by microorganisms, lysis and hydrolysis products and organic matter adsorbed from the substrate. Generally, the percentage of the EPS which are important component of surplus sludge in sludge total organic matters is 50%~90% (Urbain V., et al., 1993). It has been reported that the composition, content of EPS have an important effect on sludge flocculation, settleability and dewaterability (Yu, et al., 2009; He, et al., 2008). EPS is highly hydrated and can be - 22 http://www.ivypub.org/fes

combined with a lot of water, it can strengthen the activated sludge concentration and filtration performance, and therefore EPS have a major impact on sludge dewatering performance, considered to be a major factor affecting sludge dewatering performance besides particle size distribution (Higgins, et al., 1997).

Due to the importance and complexity of the EPS, the effect of EPS on the performance of activated sludge has been the focus of scholars. But according to previous studies, the result is inconsistent. Some scholars believe that the effect on activated sludge dewatering performance is closely related to the polysaccharides content, while some argue that the EPS protein content is more important than the polysaccharides content. Houghton observed that the relationship between EPS content and sludge dewatering performance is not a simple linear (Houghton, et al., 2002).

According to Zhu Rui’s report, water-soluble polysaccharides were critical for sludge dewatering performance (Zhu, et al., 2010). As can be seen from Figure 1 to Figure 3. At the beginning, the sludge polysaccharides content of unit 1 was significantly higher than the unit 2 and 3, which may be due to the reed adaptation training stage, the growth of reed conducive to transformation and decomposition of polysaccharides in sludge. Reeds in the growth process will release secretions to soil environment through rhizosphere, these secretions can promote the growth of microorganisms. Meanwhile, the rhizosphere enzymes together with microorganisms may effect on degradation and conversion of polysaccharides in sludge. As the experiment at early stage, the effect of aeration apparatus on sludge polysaccharides content is unclear, requiring further investigation. Based on the above results, contrast the characteristics of sludge moisture content during the same period of the three units, the variation of sludge polysaccharides content was similar with the variation of sludge moisture content with time, which is accorded with the results of water-soluble polysaccharides having a negative impact on the sludge dewatering (Luo, et al., 2005).

The conclusion could be obtained that the reduction of polysaccharides was beneficial to sludge dewatering, and sludge drying bed with reeds planted was favorable to degradation and transformation of carbohydrate.

4 CONCLUSIONS Polysaccharides content showed a common decreasing trend by comparing accumulation sludge polysaccharides in SDRBs and traditional sludge drying bed, but the two reed beds sludge had similar polysaccharide content, which is lower than traditional sludge drying bed. This was mainly attributed to the growth of plants beneficial to decomposition and transformation of accumulated sludge polysaccharides. Root exudates improve sludge microbial activity, the degradation and transformation of sludge polysaccharides through endogenous digestion without exogenous nutrition.

ACKNOWLEDGMENT The research was financed by the National Natural Science Foundation of China (51278088) and the Fundamental Research Funds for the Central Universities in China (DC12010205).

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AUTHORS 1

Taiyuan University of Technology, Taiyuan, Shanxi Province,

Province, China, Date of Birth: Aug. 08,

China

1989.

2002.03-2005.06 Doctor degree in Environmental Engineering

Educational background:

Harbin Institute of Technology, Harbin, Heilongjiang Province,

2009.09-2013.07 Undergraduate student in

China

Environmental Engineering Anhui Normal

Major field of study: Water pollution control.

University Wuhu, Anhui Province, China

He is the Professor in Dalian Nationalities University, Dalian,

2013.09-2014.09

China.

Rui Chen, Place of Birth: Heilongjiang

Graduate

student

in

Environmental Engineering Dalian Nationalities University Dalian, Liaoning Province, China 3

Xinnan Deng, Place of Birth: Heilongjiang Province, China

Major field of study: Environmental Engineering.

She is a graduate student in Dalian Nationalities University,

Date of Birth: May 29, 1991.

Dalian, China.

Educational background: 2

Yubo Cui, Place of Birth: Inner Mongolia,

China. Date of Birth: Sept. 10, 1968.

Engineering

Bachelor

Chengdu

Engineering, Anhui Normal University Wuhu, Anhui Province, China

Educational background: 1987.09-1991.07

2009.09-2013.07 Undergraduate student in Environmental

degree

in

University

of

Technology, Chengdu, Sichuan Province, China 1993.09-1996.05 Master degree in Municipal Engineering

2013.09-2014.09

Graduate

student

in

Environmental

Engineering, Dalian Nationalities University, Dalian, Liaoning Province, China Major field of study: Environmental Engineering.

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

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