Branch: : Mechanical Engineering
Optimization of Methane Production from Solid Organic Waste
Anaerobic digestion of volatile organic solids (VS) leads to the production of biogas. One constituent of biogas is methane, a clean burning, renewable source of energy. Methane could very well be used to replace typical fossil fuel as our primary energy source. VS are widely available through the collection of manure and other organic wastes. A significant problem with anaerobic digestion is the long hydraulic retention time (HRT).Typical low-rate digesters require 30-50 day HRT. If biogas production from the digestion of organic solids becomes more efficient, the usage of biogas as an alternative to standard sources of energy will become commonplace. There are many ways to increase the production of biogas, and in turn, decrease the long HRT. These methods include the use of additives, pretreatment of substrate, and mixing. In this paper, I discuss several methods used to increase biogas production and decrease the hydraulic retention time.
Methane (CH4), productivity, manure, livestock, methane productivity, volatile solids (VS), volumetric methane production
Anaerobic digestion of animal manure results in the production of a biogas composed mainly of
methane (CH4) and carbon dioxide (CO2). Uncontrolled decomposition of animal manure is undesirable as gases released are believed to have global warming effects. Methane is generated from the digestion of organic compounds in feces, primarily carbohydrates, proteins, and lipids. Methane productivity can be measured in terms of volatile solids (VS) destroyed, VS loaded, volume, or animal production (Moll ret al
., 2004). In this paper, methane productivity will be discussed chiefly with respect to volume. Fig. 1 represents the biogas production process by anaerobic respiration. Organic solids are broken down to create methane and carbon dioxide. Taking carbon dioxide, hydrogen from VS is added yielding methane.
Fig. 1 (Nagamani and Ramasamy, 1999) If anaerobic digestion is managed properly, biogas
can be captured and used in place of fossil fuels, providing a CO2 neutral energy source. In the U.S. and Canada alone, there are over 121 million cattle (USDA). The methane potential from these cattle is 435.6million m3per day (Nagamani and Ramasamy, 1999). This equals more than 25% of the average
daily consumption of Natural gas in the United States from cattle alone (Energy Information
Administration). 1.7 cubic meters of biogas has the energy equivalent of one liter of gasoline. This means
that the waste from one cow could generate the equivalent of 200 liters of gasoline in one year (Biogas
Production). The long HRT period remains a significant obstacle. The need to improve the overall
efficiency of anaerobic digestion exists. The following table gives the biogas potential from the waste of several different species.
Table 1 (Nagamani and Ramasamy, 1999) The factors that affect methane productivity include:
•Species, breed, diet, and maturation stage of the animal
•Amount and type of bedding material
•Type of bacteria selected
•Use of mixing techniques
In this paper, I will discuss methods that can be used to optimize the production of methane gas using anaerobic digestion of organic waste and decrease the HRT.
Volatile Solids (VS)
A main constituent of manure that can drastically affect the methane productivity during digestion is the volatile organic solids (VS) content. Manures with higher VS ratios will have greater methane productivity. Volumetric methane production can be increased by solid-liquid separation by producing a higher VS
concentration. A problem with separation is the liquid portion is high in contaminants and must be treated
before releasing to a waterway.
The theoretical methane potential can be calculated from Bushwell’s formula. Methane productivity in
terms of VS loaded as residence time approaches infinity is referred to as the ultimate methane yield. The ultimate methane yield will always be lower than the theoretical methane yield. If only the solid portion is being used for digestion, it is essential that the separation unit is efficient in transferring VS to the solid fraction because a part of the VS will be present in the liquid (Moller et al., 2004). Typical VS values vary
significantly depending on the source and any pre-treatment. Swine waste contains higher VS concentrations than that of cattle. Sow (530 ± 6 l kg-1VS) waste is generally highest, pigs (516 ± 11 l kg-1VS) are considerably higher than that of cattle manure (468 ± 6 l kg-1VS) (Moller et al., 2004). As shown above, the methane production does vary slightly depending on the origin of waste, but methane productivity per unit of VS degraded is relatively constant for all waste types. For most livestock the number is around 500 l per kilogram VS. (Hill, 1984).
Organic and inorganic chemicals can be added to the slurry to improve gas production. Additives
can stimulate microbial activity under different operating conditions. Some organic substances are available naturally, but are not much consequence in terms of their use in the habitat. When utilized in biogas production, some additives can greatly improve performance (Yadvika et al. 2004) Inorganic Additives .There are many inorganic additives that increase gas production. The addition of iron salts has
been found to amplify the production of biogas. Salts were added at varying concentrations; FeSO4 was added at 50 mM, FeCl3 at 70μM (Clark and Hillman, 1995 as cited by Yadvika et al., 2004). Production facilities with higher concentrations of heavy metals tended to have a greater methane yield than those that did not (Wong and Cheung, 1995). The addition of calcium and magnesium salts as energy supplements improved methane production and avoided foaming of the slurry (Mathiesen, 1989 as cited by Yadvika et al., 2004).
Nickel ions have shown to increase gas production amounts by as much as 54%. Nickel was
found to stimulate production up to 5 ppm. The optimum range appeared to be at 2.5 ppm in a water
hyacinth-bovine waste substrate. The increased in biogas is due to the activity of nickel dependen metallo-enzymes involved in anaerobic digestion (Geeta et al., 1990). Other chemicals have significantly enhanced biogas production. Eosin blue dye at a concentration of 0.1 μM found a biogas increase of 25-35% when added to manure slurry (Dhawale, 1996). Gaddy discovered a method for improving the performance of anaerobic digestion of solid substrate; at least 1-chelating agent (between 1-100 μM) and at least one nutrient (between 1-5000 μM) was added to a solid substrate to make solid nutrients soluble and enhance bacterial growth. Methane production can be increased or smaller digesters can be used to achieve the same methane production. Faster start up, greater stability and more rapid recovery from upsets were possible using this new method” (Yadvika et al., 2004). Organic Additives Additives can help to maintain conditions that are favorable for more rapid gas production in an anaerobic digester. These conditions include: pH, inhibition/promotion of acetogenesis and methanogenesis, etc. Powdered leaves of some plants have been found to stimulate biogas production between18-40% (Chowdhry et al., 1994 as cited in Yadvika et al. 2004). Increased biogas production due to selected additives appears to be due to adsorption of the substrate on the surface of the additives. Thiscan create an increased localize substrate concentration and a more favorable environment for the growth of microbes (Chandra and Gupta 1997). Plant residues treated with alkali at 1% NaOH for 7 days combined with manure at a 1:1 w/w ratio showed a two fold increase in biogas and methane production (Dar and Tandon, 1987). This
method will be discussed in more detail in the “Pretreatment” section of this paper. Tomato plant waste added to rabbit wastes inquantities greater than 40% improved methane production (Trujillo et al., 1993). Plant waste like corn stalks, rice straw, cotton stalks, wheat straw, and water hyacinth when mixed with ca
ttle manure can increase gas production from 10-80%. Biodegradation of mango processing wastes
produced significantly more methane after the addition of bean seeds, black gram, guar, and guargum at a rate of 1500 ppm. The addition of organic and inorganic substances to waste slurry increases the
efficiency of anaerobic digestion and the production of methane gas. Supplementing manure with leaves and organic residue can increase gas production up to80%. Most of these options are easy to complete and economically effective.
When manure is manipulated prior to entering the digester, this process is called pretreatment. Many pretreatment methods have been tested, and some have shown a significant improvement in terms of biogas and methane production. There are several methods of pretreatment that are quite effective.
Pretreatment of manure by maceration has shown considerable promise to increase the production of biogas. Hartmann, et al., (2000) showed a 25% increase in biogas production when implementing
maceration for pretreatment of particulate matter in manure. Maceration appears to increase the surface area of the fibers by separating them and allowing for individual treatment or recirculation. The effects due to shearing, rather than cutting, cause the increased methane productivity. Maceration is a low cost pretreatment option, which provides greater incentive to implement this process for increased biogas production (Hartmann et al., 2000). Ultrasonic pretreatment of waste activated sludge disrupts cell
membranes, resulting in the release of organic substance outside the cell. This allows the substances to be more easily hydrolyzed and improve anaerobic respiration. Methane generation increased with time exposed to ultrasonic pretreatment time. A thirty minute pretreatment resulted in a 64% increase in
methane production as compared to control. Thirty minutes is considered the optimum exposure time, as increased exposure time did not result in significant increases in gas production (Wang et al., 1999).
The following figures represent the effect of ultrasonic pretreatment of substrate. Fig. 2 show the
destruction of organic matter versus ultrasonic treatment time, and Fig. 3 shows the production of
methane versus solubilization ratio. Ultrasonic pretreatment of substrate will increase the solubilization, and in turn increase methane production. The relationships can be seen in the figures below..