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 hydrogen (biohydrogen) is hydrogen produced biologically. The main reactions involve fermentation of sugar derivatives, such as glucose:
A related reaction produces formic acid (leading to the potential for the production of formate derivatives (esters of formic acid) instead of carbon dioxide:
See also: Biohydrogen.
Biomass
Biomass refers to biological material derived from living or recently living organisms, such as plants or plant-derived materials. As an energy source, biomass can either be used directly via combustion to produce heat, or indirectly after converting it to various forms of biofuel. Conversion of biomass to biofuel can be achieved by different methods, which are broadly classified into: (i) thermal, (ii) chemical, and (iii) biochemical methods. This biomass conversion can result in fuel in gas, liquid, or solid form.
Biomass is a renewable energy source unlike other resources such as crude oil, natural gas, tar sand, coal, and oil shale. Agricultural products specifically grown for biofuel production include crops such as corn, soybeans, rapeseed, wheat, sugar beet, sugar cane, palm oil, Jatropha, as well as wood (Table B-14).
Biomass is generally produced in a sustainable manner from water and carbon dioxide by photosynthesis. There are three main categories of biomass—primary, secondary, and tertiary.
Table B-14 Major categories of biomass feedstocks.
| Algae | Prokaryotic algae, Eukaryotic algae, Kelps |
|---|---|
| Aquatic plants | Algae, Water weed, Water hyacinth, Reed and rushes |
| Biorenewable wastes | Agricultural wastes, Crop residues, Mill wood wastes, Urban wood wastes, Urban organic wastes |
| Energy crops | Short rotation woody crops, Herbaceous woody crops, Grasses, Starch crops, Sugar crops, Forage crops, Oilseed crops, Switchgrass, Miscanthus |
| Food crops | Grains, Oil crops |
| Forest products |
Wood;
|
