It is essential that the organic acids formed in the first stage of the waste treatment process are converted to methane at the same rate at which they are formed. If not, they accumulate and ultimately lower the pH, leading to inhibition of the second stage of the digestion process and digester failure. The temperature must be maintained within certain ranges - heating increases the activity of the anaerobic bacteria reducing the required digestion time. A pH of 7.0 to 7.5 is recommended to encourage the methane-producing stage. A correctly operating digester will have sufficient buffer capacity (alkalinity) introduced from the breakdown of organic matter. Also, the gas leaving the digester is almost saturated with water vapor which condenses as the gas stream cools there causing problems – the problem is more severe when digesters are heated. To mitigate the problem, it is essential to remove as much of the moisture as possible before the gas comes into contact with the gas system devices. For this reason, water traps should be located as close to the digester as possible and all piping should be sloped a minimum of 1% toward the water trap, which should be situated at a low point in the gas line.
Several types of digesters have been developed including the floating drum, the fixed dome, the bag, the plastic tube, the plug flow, and the up-flow anaerobic sludge blanket digesters. A digester is an airtight vessel or enclosure in which bacteria decomposes biomass in water to produce biogas. Both batch and continuous digesters are commercially available. While batch systems may be suitable for some applications, they are labor intensive, and in developed countries are probably not appropriate to the steady production of gas on any significant scale.
Mixing may be used in both batch and continuous digesters to enhance contact between the bacteria and substrate. It has, however, been argued that while mixing may increase conversion rates, it is not energy efficient. In two-stage systems, only the first stage is mixed. Multistage systems provide optimum conditions for the successive digestive steps, and further development may prove them to be economical for some biomass materials.
In a process of manure and straw mixture digestion, for the first 72 hours, the yield of methane was minimal (essentially 0%) and the yield of canon dioxide is virtually quantitative (approximately 100%) – in this period, digestion occurred as aerobic fermentation to carbon dioxide. The yields of methane and carbon dioxide gases were approximately equal after 250 hours, and after 500 hours, the digestion reached the stationary phase. The methane content of the biogas was in the range of 73 to 79% for the runs, the remainder being principally carbon dioxide. During a 720-hour digestion period, approximately 80 to 85% v/v of the biogas was produced in the first 360 to 430 hours which is an indication that the digester retention time can be designed to a shorter period (360 to 430 hours) instead of the longer period (720 hours).
The final stage in the process is the disposal of the any waste. Essentially all the water entering the system will be present in the effluent. Direct recycle of the water after separation from the sludge cannot be practiced, as this will result in a buildup of toxic substances and eventual failure of the system. It is possible that some fraction of the water may be safely recycled, although data on this aspect have not been seen. Treatment of this large volume of water to remove the toxic constituents is costly and is likely to be uneconomical other than in large-scale operations.
Typically, it can be assumed that the water, which is considered to have some nutritional value, can be used for irrigation of growing crops. However, serious consideration must be given to the effect of trace metals and other toxic materials present in the water and the effects of these constituents on the crops. In addition, subject to the presence of non-toxic constituents (or the lack of toxic constituents) in the sludge, it also may be used for land application and will generally have higher nitrogen content than the original material. On an absolute (feedstock-to-sludge) basis, there may be less nitrogen retention due to losses of the nitrogen as ammonia (NH3), and there may be additional losses on land application since the nitrogen in the sludge is in a more volatile form than the nitrogen in the original feedstock. Nevertheless, the sludge resulting from the digestion of animal manure is generally considered (subject to a critical analysis of the constituents of the sludge) to have improved fertilizer value over the original feedstock. During digestion, the volatile fatty acid concentration is lower and the pH higher. Nearly all digester plants have ancillary processes to treat and manage all of the by-products. Before storage and use, the gas stream is dried and sweetened by removal of sulfur compounds. The sludge liquor mixture has to be separated by one of a variety of ways, the most common of which is filtration. Excess water is also sometimes treated in sequencing batch reactors (SBR) for discharge into irrigation systems.
Essentially all organic material can be digested except for the stable woody materials since the anaerobic microorganisms are unable to degrade lignin. The biogas which is formed has a high calorific value (heat content) and is considered as a renewable energy source. The main disadvantages of this process are (i) the possible presence of volatile siloxanes in the biogas that can cause serious damage in the energy users’ engine and boiler due to the formation of microcrystalline silica; and (ii) the increased concentration of heavy metals and various industrial organic chemicals in the residual sludge due to the significant reduction of the organic fraction during digestion, leaving the mineral and non-degradable fraction untouched.
Anaerobic digestion produces a clean and environmentally friendly fuel, although it contains carbon dioxide, water vapor, hydrogen, hydrogen sulfide, and siloxane derivatives (i.e., compounds having a molecular structure based on a chain of alternate silicon and oxygen atoms, -O-Si-O-Si-, especially – as in silicone derivatives – with organic groups attached to the silicon atoms).
As a note of caution, a mixture of biogas (predominantly methane) and air can be explosive. Methane in concentrations of between 5% and 15% v/v in air is explosive, and air should not be allowed to enter the digester or gasholder. All piping and equipment must be sealed properly to prevent gas from escaping to the outside. In addition, all electrical installations must be of the explosion-proof type, as the smallest spark could ignite escaped gases.
Biomass feedstocks generally produce a biogas rich in methane. This medium-to-high Btu (heat content) gas can, in some instances, be upgraded to a substitute natural gas (SNG). However, depending on the feedstock, non-negligible amounts of sulfur are also produced (Table B-13).
Table B-13 Comparison of different biogas feedstocks.
| Feedstock | Sulfur content* | Product sulfur* |
|---|---|---|
| Liquid and solid manure | 300-500 | 0.5 |
| Organic waste | 100-300 | 0.3 |
| Wood chips | 300-1000 | 0.3 |
| Sewage sludge | 300-500 | 0.6 |
| *mg/m3 | ||
See also: Anaerobic Digestion – Gas Production.
Biological Hydrogen Production
Biological