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A Study of Anaerobic Digestion of ExcessPetrochemical Plant SludgeL. Zhidong aa Dalian Municipal Design and Research Institute of EnvironmentalScience , Dalian , Liaoning , P. R. ChinaPublished online: 09 Mar 2011.

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Energy Sources, Part A, 33:899–907, 2011

Copyright © Research Institute of Environmental Science

ISSN: 1556-7036 print/1556-7230 online

DOI: 10.1080/15567030903289577

A Study of Anaerobic Digestion of ExcessPetrochemical Plant Sludge

L. ZHIDONG1

1Dalian Municipal Design and Research Institute of Environmental Science,

Dalian, Liaoning, P. R. China

Abstract Anaerobic digestion is an economically viable and environmentally friendly

process for the reduction of excess municipals sludge. Primary and activated sludgeare generated during wastewater treatment. This article adopted semi-continuous flow

experiments of an aerobic digestion in the middle temperature for excess sludge fromthe wastewater treatment plant of Jinzhou Petrochemical Plant of China Petroleum.

Fluctuation conditions of pH, volatile fatty acid (VFA), chemical oxygen demand(COD), volatile solid sludge (VSS), and gas production are all tested, respectively.

The result shows that: pH increased from 6.3–7.2, which agrees with the reported pH

changes in full-scale facilities. The removal ratio of COD and VSS are 70.3% and55.6%, respectively, which implies that the process is an effective treatment Maximum

gas production is 864 ml (CH4 content is 60.3%). The VFA had shown a sharpdecline under the low ratio of sludge dosage conditions. The concentration variation

of VFA was gradually stabilizing with advanced he ratio of sludge dosage. Anaerobicdigestion performance of municipal sludge was very good; however, further research

should take into account a balance between performance enhancement and economiccost.

Keywords anaerobic digestion, anaerobic solubilization, biogas generation, digestibil-ity, petrochemical plant sludge

Introduction

The most common biological process for wastewater treatment is generally based on

activated sludge. For several decades, final disposal of excess activated sludge was

accumulated into petrochemical plants. Wastewater plants were not primarily considered

when planning and designing systems for the treatment of wastewaters (Egemen et al.,

1999; Liu and Tay, 2001; Wei et al., 2003). During this time, most of the disposal methods

were related to landfill or sea dumping. The latter was banned in the United States and

in the European Community in 1991 and 1998, respectively (Page et al., 1985; Hill et al.,

1996; Davis and Hall, 1997; Camacho et al., 2002; Farrell et al., 1998; Goel et al., 2003;

Kearney et al., 1993; Wolny et al., 2008).

Since the 1970s, the alternative to dispose of sewage sludge into agricultural fields

as a low-grade N-P fertilizer and/or as a soil conditioner was received as an attractive

option for both economical and environmental reasons (García-Delgado et al., 1994).

Address correspondence to Dr. Li Zhidong, Dalian Municipal Design and Research Instituteof Environmental Science, Dalian, Liaoning 116023, P. R. China. E-mail: lzd_xx@163.com

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900 L. Zhidong

Despite the agronomic value of this residue verified in several studies (Snyman et al.,

1998; Pedreno et al., 1996; McLaughlin et al., 1987), the existence of metals in sewage

sludge has been a matter of great concern (Renner et al., 2000), due to the possibility of

metal accumulation in soils through the trophic chain, which has still been investigated

in recent studies (Bougrier and Carrère et al., 2005; Chua et al., 2002; Kunte et al., 2000;

Bhogal et al., 2003). The development of a biological process derived from bioleaching of

metal ores has been encouraged for metal solubilization from sludge due to its lower costs

compared to chemical solubilization using inorganic or organic acids, chelating agents,

among other chemicals (Sreekrishnan and Tyagi, 1996; Strasser et al., 1995; Hall, 1999;

Haug et al., 1978; Li and Noike, 1992).

Anaerobic digestion techniques are very favorable in terms of energy owing to the

fact that oxygen is not required, corresponding amounts of biogas are produced, and

microbial biomass is produced in lower amounts compared to aerobic processes (Lin

et al., 1995; Taherzadeh et al., 1997; Ligero et al., 2001). Wastewater treatment plant

sludges are gaining oxygen in the scope of processing and disposal. Thus, anaerobic

digestion techniques have been applied traditionally for the reduction of oxygen and for

the production of corresponding amounts of biogas (Dohanyos et al., 1997; Pinnekamp,

1989; Speece, 1983; Nashville et al., 2000). The stabilization of large volumes of organic

sludges is obtained at lower costs with a high destruction rate of pathogens (Liu, 1998;

Weemaes and Verstraete, 1998). Sand, food waste, and inorganic and organic materials,

including raw sewage, are the main ingredients found in primary sludge, while a settling

material that is produced at the secondary clarifier of the sewage treatment plant after

bio-treatment is called activated sludge (Watanabe et al., 1997; Fukushi et al., 2003; Choi

et al., 1997). It is reported that particulate organic compounds must compound before

compounds by microorganisms, so solubilization of organic compound is a major step in

acidogenic digestion because most of the substrates in primary sludge are in the particulate

form (Elefsiniotis et al., 1996). In the activated sludge, the main fraction of sludge

involving cellular materials withstands direct anaerobic degradation by opposing cell

walls as a physical and chemical barrier toward exoenzyme degradation and hydrolysis,

which makes anaerobic digestion of activated sludge difficult (Baier and Schmidheiny,

1997).

Although anaerobic digestion techniques provide many benefits, there are also some

major drawbacks, such as. Two major drawbacks of biogas systems are cost and mainte-

nance. In addition, the large amount of biogas required to generate energy is not looked

upon favorably. These factors have contributed to the concern that increased technology

on large dairy operations will lead to a further decline in small wastewater treatment

plants.

The aims of this study were to investigate the technological parameters of the

anaerobic digestion of excess activated sludge of the Refinery Wastewater Treatment

Plant in the second pond in FSWTP in China.

Materials and Methods

Seed

The seed was taken from the UASBR of Luzhou Starch Group in China having a VSS

(volatile suspended solids)/SS (suspended solids) ratio of 57.36%; SS and VSS were

31.9 g/L and 18.3 g/ L, respectively. Seed sludge as an inoculum sludge was 10% of all

reactors.

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Digestion of Excess Petrochemical Plant Sludge 901

Analytical Methods

Temperature and pH were measured daily. Samples were taken fresh for the determination

of total suspended solids (TSS), volatile suspended solids (VSS), soluble chemical oxygen

demand (SCOD), and volatile fatty acids (VFAs). All parameters were measured in 50

mL aliquots, which were centrifuged 20 min for the separation of soluble and suspended

solids. The supernatants were used for SCOD and VFA analysis, whereas the pellets were

used for TSS and VSS analysis. The supernatant of all primary, biological, and mixed

sludge samples were first filtered through 2.7 mm Whatman GF/D filters (Whatman,

Kent, UK) and then through 0.45 micron MSI MicronSep nitrocellulose disc filters. The

samples were acidified below pH D 2 and stored at 4ıC in the refrigerator. The pellets

after centrifugation were transferred into crucibles and dried overnight at 105ıC for TSS

determinations. VSS concentrations were determined by ignition at 600ıC for 0.5 h.

SCOD experiments were conducted by the closed reflux colorimetric method. VFAs

experiments were done by GC 610 Series ATI, UNICAM model gas chromatograph.

Total gas volume was measured by a displacement method and its compositions were

characterized by HP 4890A Model GC (American Public Health Association, 1998).

Reactor and Operation

A batch fed anaerobic bioreactor with 3 L capacity was used and mixed by mechanical

mixers continuously. The digestion was carried out in a water bath at a constant tem-

perature of 35ıC ˙ 1. The temperature was controlled by a digital-display electronic

temperature control instrument (OMRON General-purpose Controllers-E5CN/E5CN-U,

Matsuzaka City, Mie, Japan). Sludge was fed and discharged once a day. This is an

absolute requirement to avoid dissolving gas in which the water contained 10% NaCl

and 2% H2SO4. The gas via a gas washing bottle entered into a wet gas flow meter. The

experimental setup and process are shown in Figure 1.

Rationale

The system was operated 55 days and demonstrated its capacity to achieve reduced

excess sludge production. Different operating conditions were applied to find a suitable

operating strategy to maximize its efficiency with respect to the sludge production. An

Figure 1. Schematic diagram of the experimental setup.

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902 L. Zhidong

anaerobic digestion system is made, and the impact of the sludge holding tank on overall

sludge production is addressed. An approach to study the mechanisms involved in the

reduction of excess sludge production in the anaerobic digestion system is discussed and

the results are presented.

Results and Discussion

Effects of pH on System

The effect of digestion at 35 ˙ 1ıC on pH were compared in Figure 2. In general, fol-

lowing 1–10 days of digestion at 35ıC, the pH gradually increased to approximately 7.5,

regardless of the composition of the feed. Further digestion from days 10 to 55 resulted

in a decrease in pH. The pH increased from 6.3–7.2 in the sludge that was digested for

45 days at approximately 35ıC. The changes in pH shown in Figure 2 agree with the

reported pH changes in full-scale facilities. The pH increasing increased because organic

compounds became volatile fatty acids (VFA) under the action of anaerobes in the system.

VFA was gradually accumulated, which resulted in a decrease of pH. So the VFA were

gradually decomposed into CH4 that VFA accumulation alleviated, and pH increased.

COD Removal

The effluent COD of the anaerobic digestion system was excellent, as shown in Figure 3.

The influent COD were from a range of 29,868.05–34,400.45 mg/l, and effluent COD

concentrations were from a range of 9,727–24,124 mg/l. The COD removal was from

19.2–0.3%.

As shown in Figure 2, COD removal was lower, which was mainly because the

anaerobic digestion system was not stable and the anaerobic bacteria could not adopt the

digestion environment. At 1–10 days of digestion at 35ıC, the COD removal was less than

40%. Further digestion from days 10 to 55 resulted in an increase in COD removal. The

COD removal increased to 70.2%. This implies that the process is an effective treatment

effect to COD.

Figure 2. The effects of digestion at 35ıC on pH.

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Digestion of Excess Petrochemical Plant Sludge 903

Figure 3. Influent and effluent VSS in the anaerobic digestion system.

VSS Removal

The effluent VSS concentration was from a range of 9.46–4.24 mg/l. The VSS removal

was achieved at 55.6% for 25 days. As shown in Figure 4, initial VSS removal was lower,

which was mainly because the anaerobic digestion system was not stable and anaerobic

bacteria could not adopt an environment of digesting.

VFAs Production

The VFAs were continuously measured daily in the process of anaerobic digestion.

Changes in VFA with retention time and ratio of sludge dosage were shown in Fig-

ure 5.

Experimental results showed that the VFA had shown a sharp decline under the low

ratio of sludge dosage conditions. The concentration variation of VFA was gradually

Figure 4. Influent and effluent COD in the anaerobic digestion system.

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904 L. Zhidong

Figure 5. Effect of retention time and ratio of sludge dosage on VFA.

stabilizing with an advanced ratio of sludge dosage. It is explained that concentration

variation of VFA was in accord with pH variation. The VFA had shown an obvious

increase when the ratio of sludge dosage continued to increase. VFA and pH values had

reached a plateau when the system reached the tendency of stabilization.

Gas Production

The gas produced in the reactors (Figure 6) varied between 332 and 884 ml. Methane

compositions higher than 60.4% indicate proper anaerobic process. The gas production

and type of gas were important indicators in the anaerobic digestion.

Figure 6. Effect of rate of sludge dosage on gas production per day.

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Digestion of Excess Petrochemical Plant Sludge 905

Table 1

Properties of excess sludge before reducing

Parameter pH

CODcr,

mg/L

BOD5,

mg/L

TS,

%

TP,

mg/L

TN,

mg/L

VSS,

g/L

VFA,

mg/L

Moisture

content,

%

Alkalinity,

mg/L

Value 7.1 42,213 13,425 4.45 1,424 1,036 9.56 230.5 97 170

Experiment with Sludge

Excess activated sludge was taken from Fushun Petrochemical Corporation Wastewater

Treatment Plant (FPCWTP) in China. The activated sludge was stored at 4ıC in the

refrigerator. The temperature of the primary sludge was equal to the sludge in the reactor

before usage. The sludge characteristics are shown in Table 1.

Conclusions

(1) The maximum removal of COD and VSS is 70.2 and 55.6%, respectively, at

the temperature of 35ıC. This implies that the process is effective treatment to

them.

(2) The gas production added up to 884 ml when sludge retention time was 40 days,

high methane content (average 60.3%) was observed.

(3) The pH gradually increased to approximately 7.5 during 10 days. Further diges-

tion from days 10 to 55 resulted in a decrease in pH. The pH increased from

6.3–7.2, which reached a considerable measure of agreement with the reported

pH changes in full-scale facilities.

(4) Experimental results showed that the VFA had shown a sharp decline under the

low ratio of sludge dosage conditions. The concentration variation of VFA was

gradually stabilizing with an advanced ratio of sludge dosage.

(5) This research has answered some initial questions about the anaerobic digestion

process. It has also proven that the proposed technology has a good potential of

success. However, more questions have come up, thus opening several important

lines of investigation for future work.

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