Energy or Fuel Generation from Wastewaters

Ethanol is now being used in gasoline blends and fuel for specifically designed automobile engines. Ethanol can be produced from food and agricultural wastewaters as long as there are sufficient amounts of sugar or starch. The fermentation-produced ethanol has relatively low ethanol content, which has to be enriched to 95% or higher for use as fuels for internal combustion engines. A combination of distillation and pervaporation will produce an almost 100% pure ethanol (Peng et al., 2003).

Biogas from anaerobic processes such as anaerobic sludge digesters or anaerobic reactors for reducing high-strength wastewaters has been well known and utilized to a certain degree on a small scale. But the enthusiasm for its energy generation capacity never lasts very long as people soon realize the costs associated with enriching methane gas from biogas, collection of this gas, and transportation of this gas in such small quantity. Landfills produce biogas naturally under anaerobic conditions and this gas had, for a long time, generated little interest until recently. The reasons for the attention to biogas from landfills have shared little common ground with one another. People concerned with global warming feel more depressed because methane, which is a worse greenhouse gas than CO2, and CO2, comprises the majority of biogas from landfill, whereas entrepreneurs see the same biogas as "diamond in the rough," a part of new "green revolution" that will usher in a green economy.

Biogas is not the only combustible gas produced from food and agricultural wastewaters. Research on producing hydrogen gas through fermentation of sugars in wastewaters has shown promising results (Ueno et al., 1996; Van Ginkel et al., 2005; Hussy et al., 2005). Although hydrogen production from fermentation of wastewaters reduces no significant amount of solid content, another approach, which combines hydrogen gas production and electricity generation in a single system, is gaining new attention and interests because of its waste reduction as well as energy generation (Logan, 2004; Oh and Logan, 2005). The amount of electricity generated in the microbial cell was small, but the potential of application of this technology looks bright.

Pyrolysis, particularly fast pyrolysis, offers the opportunity to obtain heating oil from solid or semisolid biomass from food and agricultural wastewaters. Pyrolysis is a thermal decomposition of organic compounds in the absence of oxygen. This process has been used for hundreds of years to produce charcoal and has been applied commercially to recover methanol, acetic acid, and turpentine. The process of fast pyrolysis produces a liquid, whose yield depends on the biomass composition and rate and duration of heating, and a char. Although the liquid (which contains up to 15-20% water) looks like oil and is called bio-oil, the elemental composition of bio-oil resembles that of the biomass rather than that of typical petroleum oils with highly oxygenated compounds providing lower heat values. Because of several organic acids in the pyrolysis liquid, the liquid is very corrosive. The char can be used as adsorbent; the components in the liquid can theoretically be separated in order to achieve their full potentials as specialty chemicals. However, the problem of chemical or physical separations is formidable in both technical and economical senses.

If the biomass is sludge, the pulverized sludge is heated to 250°C and compressed to 40 MPa. The hydrogen in the water inserts itself between chemical bonds in natural polymers such as fats, proteins, and cellulose. The oxygen of the water combines with carbon, hydrogen, and metals. The result is oil; light combustible gases such as methane, propane, and butane; water with soluble salts; carbon dioxide; and a small residue of inert insoluble material that resembles powdered rock and char. This process is called thermal depolymerization and uses hydrous pyrolysis to convert reduced complex organics to oil.

All organisms and many organic toxins are destroyed. Inorganic salts such as nitrates and phosphates remain in the water after treatment at sufficiently high levels that further treatment is required. The energy from decompressing the material is recovered, and the process of heat and pressure is usually powered from the light combustible gases. The oil is usually treated further to make a refined useful light grade of oil, such as No. 2 diesel and No. 4 heating oil.

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