Experimental Investigation of Biogas Production from Wastewater Sludge

Research Article

Austin J Chem Eng. 2015;2(1): 1014.

Experimental Investigation of Biogas Production from Wastewater Sludge

Mousa H1,2*, Al-Muhtaseb A¹, Abu Qdais H³ and Abd Alaa ¹R²

¹Petroleum and Chemical Engineering Department, Sultan Qaboos University, Oman

²Department of Chemical engineering, Jordan University of Science and Technology, Jordan

³Department of Civil Engineering, Jordan University of Science and Technology, Jordan

*Corresponding author: Hasan Abdellatif Hasan, Department of Petroleum and Chemical Engineering, Sultan Qaboos University, P.O. Box 33, P.C. 123, Muscat- Oman

Received: December 28, 2014; Accepted: March 18, 2015; Published: April 27, 2015


The paper has information about experimental investigation of biogas production from wastewater sludge. The effect of incubation temperature, volatile suspended solids contents, and pressure on the amount of the biogas produced was studied in 2L plastic flasks. The experiments were repeated in 25L vessel using the above conditions showed that the CH4 content increased with time reaching a maximum value in 16 days then decreased. No H2S gas was detected indicating that biogas produced from wastewater sludge is environmental friendly source of energy. The results show the optimum conditions under which digesters should be operated.

Keywords: Biogas; Methane; Sludge; Wastewater treatment; Anaerobic digestion; Volatile suspended solids


Anaerobic Digestion (AD) of organic waste has been recognized as a cost-effective and environmental friendly process to convert organic solid waste into biogas which can be used to produce heat, electricity and fuel [1]. In this digestion process the biodegradable organic wastes are converted to simple molecules by anaerobic bacteria producing a combustible biogas containing about 50–75% methane and 25–50% carbon dioxide [2]. Several feedstock have been used to produce biogas using anaerobic digestion, such as biomass waste [3,4], algae [5,6], food waste [7,8], municipal solid waste [9-16]. Waste Activated Sludge (WAS) is a highly putrescible residue generated from wastewater treatment plants. Activated sludge contains abundant organic matters, bacterial pathogens, nutrients, and high water content. Therefore sludge handling is a big problem in itself from environmental and economic point of view since it accounts for 50% of the operating costs of wastewater treatment plants [17]. Therefore, anaerobic digestion is considered to be an interesting option for WAS treatment as it can allow sludge stabilization to reduce odors and pathogens, sludge decrement, and biogas recovery [18].

Elango et al. [19] investigated the production of biogas from domestic sewage in Chennai metropolitan city using anaerobic digestion process at 26 and 36°C for 25 days. Biogas generation was enhanced by the addition of domestic sewage to MSW. The maximum biogas production (0.36 m³/kg VS added-day), the maximum reduction of Total Solids (TS) (87.6%), VS (88.1%) and Chemical Oxygen Demand (COD) (89.3%) occurred at a feeding rate of 2.9 kg of VS/m³-day.Kalloum et al. [20] studied biogas production from the wastewater treatment plant in Adrar city, Algeria. The diluted sludge with a content of 16 g/l of Total Solids (TS) was fermented under anaerobic conditions during 33 days. The biogas produced was 280.31 Nml with a yield of 30 Nml of biogas/mg of COD removed. The biogas production increased for 26 days then started to decrease. During the first 5 days the pH dropped from 7.0 to 6.3 due to formation of Volatile Fatty Acids (VFA). After this period pH increases to 6.9 and remains unchanged.

Several attempts have been done to enhance biogas production. Feng et al. [21] enhanced the anaerobic digestion of wasted activated sludge, from municipal wastewater treatment plant in Dalian, China, by the addition of Zero Valent Iron (ZVI) as a reducing material. The production of VFAs was enhanced by 37.3% with ZVI during the hydrolysis and the acidification steps. After the digestion for 20 days, the methane productivity at ZVI of 20 g/L increased by 43.5%, and the sludge reduction ratio increased by 12.2%.Wang et al. [22] enhanced the anaerobic digestion of wasted activated sludge, from wastewater treatment plant in Brisbane, Australia, by 26% using combined Free Nitrous Acid (FNA) and heat pre-treatment. Debowski et al. [6] studied the effect of Magneto-Active Filling (MAF) on the effectiveness of methane fermentation of dairy wastewaters. It has been found that MAF incorporation into the technological system significantly improved effectiveness of biogas production, increased methane concentration and lowered content of hydrogen sulfide in gaseous metabolites of fermentative bacteria. A significant increase was also observed in the effectiveness of COD removal from dairy wastewaters.

The objective of this study is to investigate the potential production of biogas from the sludge produced at the wastewater treatment plants and to investigate the effect of various operating parameters such as pH, VSS concentration and reactor pressure and temperature on the biogas production rate.

Materials and Methods

Experiment procedure

Two sets of experiments were performed, the first set was performed in a 25L vessel, and the second set was performed in 2L plastic bottles both to mimic batch bioreactors. Details of these two sets of experiments are discussed below:

Set 1: The second part of the experiments was carried out in 2L plastic bottles filled by 1 L sludge. The purpose of this part is to study the effect of sludge concentration, temperature, and pressure on biogas production. Conditions at which maximum biogas production obtained were used for set 2 experiment carried out in a 25L reactor. The bottles were hooked with a hollow pipe sealed with rubber tube which can be closed to prevent gas escape from the bottles. When sampling is needed, a syringe is inserted through the rubber tube which is then opened allowing the gas to flow into the syringe. The withdrawn gas sample was then sent to the gas analyzer. The sludge as received contains 6% VSS. To study the effect of sludge concentration on biogas produced, the sludge was diluted with distilled water to obtain sludge of Volatile Suspended Solids (VSS) concentrations of 1%, 2%, and 4%.The effect of pressure was investigated by preparing 3 groups of bottles. In the first group the gas in the bottles was analyzed and then it was released till the pressure in the bottles reaches atmospheric pressure. In the second group, the bottles were left without any interference till the end of the incubation period (9 weeks) when agas sample was taken for analysis. In the third group, a sample from the gas was taken for analysis and the bottles were immediately sealed keeping the bottles pressurized. The reason for the third group is to determine biogas production with time under pressure and compare its results to that of group 1 conducted under atmospheric pressure condition. The effect of temperature on methane generation was studied by carrying out the same tests under constant temperature of 35°C and 25°C. Three bottles for each test were prepared to assure the reproducibility of the results. The data presented is the average of the three readings. The initial weight of the bottles and their weight after sampling were recorded. Knowing the difference in the weight and the concentration of CH4 gas enables calculating it mass (see results and discussion below, section 3.1).

Set 2: The anaerobic digestion was carried out in a 25L vessel containing a heating element and a thermostat to control the temperature of the vessel at 35°C. The vessel contains a pressure gauge, a gas sampling point at the top of the vessel and a drainage valve at the bottom for liquid sample withdrawal. The vessel is filled with 23L sludge and gas samples were taken periodically every 2 days for 44 days for analysis. The pressure was read directly from the pressure gauge. The gas samples were withdrawn by a syringe and analyzed for its CH4, CO2, and H2S contents. Every time a gas sample was withdrawn, the pressure was released until it reaches atmospheric pressure. Knowing the pressure of the vessel, the temperature, and methane gas content its amount can be calculated assuming that the gas is ideal (see results and discussion section, section 3.2).

Sludge characterization

The Volatile Suspended Solids (VSS) and the Total Suspended Solids (TSS) of the sludge were determined following the standard methods for the examination of water and wastewater [23]. The pH was measured using portable pH meter (pH 3310 IDS, WTW, Germany). The Chemical Oxygen Demand (COD), the PO4-P, and the NH4-N were determined using photometer photo lab (photoLabS12, WTW, Germany). Heavy metals including Fe, Mn, Zn, and Cu as well as other metals such as Na and K were measured using atomic absorption spectrophotometer (Varian SpectrAA-10 and SpectrAA-20, Australia). The alkalinity was determined using the potentiometric method. The sludge characteristics are presented in Table 1.