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Biogas production by treating sludge in a wastewater treatment plant

E.V. Bataltsev, group EK-61

Anaerobic digestion is the bacterial fermentation of organic material in oxygen free conditions. In this context, organic means any plant or animal material that contains carbon and so can be broken down in this way.

Energy from anaerobic digestion offers great potential for using a renewable resource for electricity, heat, and combined heat and power generation. It can also reduce overall quantities of carbon dioxide in the atmosphere, reducing dangers of climate change, when it is used to replace energy from fossil fuels. Adoption of the anaerobic digestion process can lead to improvements in the management of farm and food processing residues, and there may be significant further environmental and ecological benefits.

Biogas typically refers to a gas produced by the biological breakdown of organic matter. Replacing fossil fuels (such as coal and oil) can avoid the pollution that causes climate change and acid rains. The use of sludge from wastewater treatment operations to generate energy is common throughout the U.S. and other developed countries. Anaerobic bacteria in a closed vessel (digester) produces a biogas consisting of approximately 60% methane and 40% carbon dioxide.

The complete anaerobic degradation of an organic substrate to form methane and carbon dioxide takes place in four degradation phases, each of which involves different groups of bacteria:

1. The hydrolysis phase, in which insoluble organic compounds are converted to water-soluble organic products, mainly by extra-cellular enzymes.

2. The acidification phase, in which larger soluble molecules are degraded by acidogenic bacteria (acidifiers). The products of this process are mainly hydrogen, carbon dioxide, aliphatic acids and alcohols, and small quantities of methyl amines, ammonia and hydrogen sulphide.

3. The acetogenic phase, in which acetogenic bacteria (acetic acid forming agents) degrade the organic acids and alcohols, except for the chlorine compounds, to form acetic acid, hydrogen and carbon dioxide.

4. The methanogenic phase, in which the strictly anaerobic, methane forming bacteria convert acetic acid and chlorine compounds, together with hydrogen, to form biogas (methane and carbon dioxide).

Here is the technology of biogas production. The sewage sludge is fed into the plant as a mixture of primary and secondary sludge, and it is then broken down. The sewage sludge is mixed with the sludge from the base of fermenter 1. A sludge/sludge heat exchanger is used to preheat the fresh incoming sludge while at the same time cooling the decomposed sludge pumped out of the fermenter.

The sludge flows slowly through fermenter 1 from the bottom to the top, taking about 5 - 7 days (up-flow reactor). Microbial degradation takes place as the sewage sludge rises in the reactor, and the organic components of the sewage sludge are progressively hydrolysed, depending on their composition, and further degraded. The slugging flow is modified at regular intervals of about 1 hour with mixing impulses created with the aid of the installed mixing pumps. The gas collects in the fermenter head and is passed to the mechanical gas purification mechanism by a gas collection tube.

The sewage sludge from fermenter 1 is supplied in controlled volumes into the mixing circuit of fermenter 2 where it is mixed with the sewage sludge from the base of fermenter 2. It is then heated to a constant temperature (about 55 °C) by the hot water heat exchanger and fed to fermenter 2. Fermenter 2 is identical with fermenter 1 and is operated in the same way, but at 55 °C.

The fully decomposed sludge is extracted from the upper part of fermenter 2 by the hydro-cyclone. In this cyclone, part of the biomass contained in the sludge is extracted and passed back into the mixing circuit of fermenter via a pump. This enriches the active bacterial mass in the inlet of fermenter 2, while any organic components not yet degraded are also returned and subjected to a further degradation period in the fermenter. Parallel to this process, pumps convey the fully decomposed sludge that has flowed out of fermenter 2 to the sludge/sludge heat exchanger for heat recovery, and then to the sewage plant's concentrator.

Using digesters to treat wastewater sludge reduces the volume of sludge, thereby reducing the cost of transporting and treating the sludge. The sludge after treatment and dewatering can either be landfilled or used as a soil conditioner.

L.P. Yarmak, EL Advisor,

A.M. Dyadechko, ELA


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