Ireland is lagging behind the EU directive for renewable energy to be used for heating and transportation. Anaerobic digestion (AD) has the potential to fill this void. In the biogas up-gradation process, methane content is increased from the usual 50-75% to 95%. Biogas, with 95% methane content, can be used for heating purposes and as compressed natural gas for transport. It is more reliable than any other renewable source of energy like solar, wind, etc. Also, it is carbon neutral. AD provides better utilization of biomass and organic biomass produced by AD can be used directly by Irish farmers. However, higher hydraulic retention time (HRT), lower methane content in the biogas (50% to 75%) and lower biogas yield are a few of the challenges faced by the biogas industry. These problems can make the AD digestion business non-profitable and unattractive. This study aims to investigate the application of pulsed electric field pretreatment of feedstock materials, improvement in the yield of biogas, HRT reduction, and an increase in the biogas methane content.
Anaerobic digestion is a multi-step process in which bacteria break down complex organic matter into simpler compounds (like methane, carbon dioxide, ammonia, hydrogen sulphide, etc.) in the absence of oxygen. The steps in AD include hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis is the rate-limiting step and it determines the overall performance of AD. In hydrolysis, complex organic materials (starch, cellulose, fatty acids and proteins) break down into smaller compounds (glucose, sugar, and amino acids) which are used by the acidogenesis and acetogenesis bacteria. Inefficient hydrolysis reduces the amount of volatile organic matter available to bacteria in the next stage. This leads to a reduction in biogas production and an increase in the solid HRT (Nizami et al. 2009, Ariunbaatar et al. 2014). Several pretreatment methods have been applied to improve the efficiency of AD. Pretreatment methods either aim to reduce the HRT or increase biogas yield in AD. Some of these pretreatment methods are thermal, chemical (alkali or acid), UV radiation, or mechanical (Ariunbaatar et al. 2014). This study tests the applicability of a pulsed electric field (PEF) for the pretreatment of feedstock. PEF has been successfully used in the food and beverage industry to destroy the cell walls of fruits and vegetables for extraction and to introduce molecules into cells through temporary pores in biotechnology (Cserhalmi et al. 2006, Toepfl et al. 2007). In PEF, short, intense electric impulses are applied to the biomass, which opens the pore in the periplasmic membrane and peptidoglycan layer (Rittmann et al. 2008, Salerno et al. 2009, Zhang et al. 2009). The pores are held open for long enough to allow the release of cellular material outside the cell. This creates osmotic shock resulting in the lysis of the cell wall. The cell wall breakdown improves the amount of volatile organic matter available for use by bacteria and the subsequent improvement in biogas production and HRT reduction. Water is used as an electrical conduit to conduct impulses through the material (Bouzrara and Vorobiev 2003, Rittmann et al. 2008, Salerno et al. 2009)
The objective of this study is to investigate the application of pulsed electric field treatment on anaerobic digestion feedstock to increase the biogas production and retention time reduction.
Materials and Equipment used
The lead author was given a site tour of the Green Generation’s AD facility in Co Kildare. This uses a two-stage thermophilic AD process. The biogas produced from the AD plant will be supplied to the gas grid after the on-site biogas up-gradation. Feedstock available for use on site includes belly grass, milk sludge, grease trap fat, cow manure, pizza waste, horse stable bedding and pig manure. It is the first plant in the Republic of Ireland that will operate in a thermophilic condition using mixed feedstock. Easy access to the plant of the Green Generation’ AD facility and verification of the lab scale results are the main reasons for working in close collaboration with the company. The content of the dry matter, moisture, ash, and energy of all these feedstock items will be estimated by oven drying and bomb calorimetry prior to pulsed electric field treatment and AD.
The materials and equipment used in this study are two-litre flasks, a gas analyzer, thermometers, a pilot-scale PEF system (ELCRACK HVP-5, DIL), pH meter and feedstock (Carlsson et al. 2008, Salerno et al. 2009, Lindmark et al. 2014). The feedstock will be kept in a sealed glass flask to initiate AD, as shown in Figure 2. The biogas formation rate and methane content present in the biogas will be estimated by taking the readings of flow rate, temperature, pH, and biogas samples on a daily basis.
Stones and other unwanted materials will be removed. Belly grass, pizza waste and straw will be cut to uniform size to ensure homogeneity. PEF treatment will be performed using a pilot-scale PEF system (ELCRACK HVP-5). After pretreatment, feedstock will be treated for AD at both mesophilic and thermophilic temperatures: 1. Control sample, 2. Individual feedstock and 3.Mixed feedstock.
Pulsed Electric Field Treatment
The parameters like output voltage, pulse width, pulse frequency, pulse number, treatment time and energy emitted will be varied in order to determine the impact of pretreatment analysis and to obtain a better interpretation of the result. The energy released at each pulse is calculated as
E= 0.5 x C x U2 (1)
(where C and U are the capacitance and the loading charge respectively (Carlsson et al. 2008)
Figure 1. Schematic diagram of pulsed electric field treatment (Anon 2012).
The samples will be kept in the sealed two-litre flask, available in the lab. The medium will be sprayed with N2 to remove any oxygen in the flask. Because the flask will be sealed, the anaerobic condition will be retained. Digested slurry (semi-solid material that contains anaerobic bacteria) procured from the operational anaerobic digester plant will be used as inoculum. This will initiate the production of anaerobic bacteria in the flask. The anaerobic digestion in the flask will start producing biogas from the second week. The production of the gas will increase over time until there is sufficient feedstock available for the bacteria. A thermometer will be inserted into the container to enable temperature readings and tubes will be used to take biogas samples, as shown in Figure 2.
Figure 2. Anaerobic digestion batch bioreactor for biogas analysis (Kavuma 2013).
Gas Collection and Analysis
The biogas analyzer will estimate the content of biogas (CH4, CO2, CO, H2S) produced from the anaerobic digesters. The gas analyzer can measure any minute changes in the flow rate and concentration of the gas due to its high sensitivity. The biogas will be collected in a sealed bottle or plastic bag. Data generated by the gas analyzer will be further verified using the water displacement method by estimating the amount of water displaced by the biogas. The methane content will be estimated by injecting the gas in a closed gas/liquid separator containing an alkaline solution (e.g. 3% NaOH). The CO2 will be ‘trapped’ in the liquid (it will react with NaOH) and the CH4 will pass freely through the solution. The quantity of the collected gas will give the content of CH4.
As experiments are ongoing, it is possible to speculate on the results based on previous studies. The feedstock suitable for PEF treatment shall be identified. The experiment will also help in the identification of optimum voltage and pulse requirements for different types of feedstock. Previous studies have shown that electroporation can enhance the methane yield from organic waste. There is a two-to-eight per cent increase in yield with respect to energy, as has been reported in the previous studies (Bouzrara and Vorobiev 2003, Rittmann et al. 2008, Salerno et al. 2009, Zhang et al. 2009). Also, the operational large-scale anaerobic digester will be checked to validate the results of the lab scale experiment.
This study aims to examine the pulsed electric field pretreatment and suitable feedstock for a large scale thermophilic anaerobic digestion plant. An increase in methane volume or a decrease in retention time without any reduction in the biogas volume will support the effectiveness of the pulsed electric field. Since a higher content of degradable organic material is made available by the pulsed electric field pretreatment, it is used by the anaerobic bacteria to produce larger volumes of methane and biogas.
Bench scale anaerobic digestion treatment will help in the selection of the best combinations of feedstock for large scale uses in AD digestion for greater concentrations of methane gas and also higher output of biogas.
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