Energy from biogas

Methane fermentation of sludge and waste 

Methane fermentation as a method of decomposition of complex organic compounds has also been used in relation to the stabilization of biological sludge in municipal sewage treatment plants and as a method of utilizing all kinds of waste containing organic compounds.

Methane fermentation of this type of substrates is similar to the fermentation of highly concentrated industrial wastewater, with the difference that solid organic compounds contained in waste should be converted into a hydrated form, available for anaerobic bacteria. Therefore, in order for the liquefaction process to run at maximum speed, it is necessary to pre-crush and mix the substrates. The liquefaction of substrates, i.e. hydrolysis, is the initial stage of decomposition, limiting the speed of the entire fermentation process, and thus has a decisive influence on the size of biogas plant reactors. Important elements of proper fermentation are the right amount of anaerobic sludge, the optimum temperature in the range of 36⁰C – 38⁰C and the reaction at the level of 7.0 – 7.8 pH. The daily dose of substrates must depend on the quantity and quality of the active anaerobic sludge. This has a direct impact on the efficiency of the entire technological system, including in particular the achieved degree of substrate biomass reduction and the amount of energy generated from biogas.

Methane fermentation of this type of substrates is similar to the fermentation of highly concentrated industrial wastewater, with the difference that solid organic compounds contained in waste should be converted into a hydrated form, available for anaerobic bacteria. Therefore, in order for the liquefaction process to run at maximum speed, it is necessary to pre-crush and mix the substrates. The liquefaction of substrates, i.e. hydrolysis, is the initial stage of decomposition, limiting the speed of the entire fermentation process, and thus has a decisive influence on the size of biogas plant reactors. Important elements of proper fermentation are the right amount of anaerobic sludge, the optimum temperature in the range of 36⁰C – 38⁰C and the reaction at the level of 7.0 – 7.8 pH. The daily dose of substrates must depend on the quantity and quality of the active anaerobic sludge. This has a direct impact on the efficiency of the entire technological system, including in particular the achieved degree of substrate biomass reduction and the amount of energy generated from biogas.

The transformation products of anaerobic bacteria are biogas and water. The mixture of sewage with biological sludge is referred to as post-fermentation.

Methane fermentation has been used as:

• method of stabilizing excessive sludge generated during the treatment of municipal wastewater in municipal sewage treatment plants,
• a method of utilizing all kinds of organic waste, especially from the agricultural and food industry
• a method of obtaining renewable energy from biogas by fermenting agricultural surpluses and as a method of utilizing waste from animal husbandry, including manure and slurry.

Justification and need to build industrial biogas plants

The construction of industrial biogas plants results from two reasons:

• as a method of utilizing organic waste, which by law must be disposed of due to the total ban on their collection and storage, e.g. in landfills,
• as a method of producing renewable energy in the form of biogas, from which electricity and heat are produced in a cogeneration system.

Due to the benefits of energy production and financial profit as well as the possibility of 90% – 95% waste decomposition, the use of this method is the most optimal solution in this regard. Another important advantage of this method is the possibility of fermenting all kinds of mixtures of various wastes, the preparation procedure of which is based on the characteristics of their composition. Of all naturally occurring organic compounds, only organic polymers, such as cellulose and lignin, are not biodegradable under the conditions of methane fermentation, although recently work has been carried out on the initial decomposition of these compounds by means of enzymes into compounds of a simpler structure that are easily decomposition in an anaerobic reactor.

Substrates used in industrial biogas plants

In the methane fermentation process, all kinds of waste containing sugars, proteins and fats can be used as substrates. They undergo gradual decomposition in anaerobic conditions. Sugars are broken down the fastest, followed by proteins and finally fats. For this reason, it should be taken into account when preparing waste mixtures. Industrial waste for biogas plants can also be mixed in the right proportion with biological sludge in a co-digestion system. In the technology of their use, their maximum fragmentation is very important, which increases the rate of decomposition and thus shortens the fermentation time.

Management conditions of the post-fermentation and its fertilizing properties

The products of anaerobic decomposition of waste, apart from biogas, are a mixture of anaerobic sludge and sewage. Depending on the chemical composition of the substrates, the concentration of fertilizing compounds in the post-fermentation will be at a different level, however, both nitrogen and phosphorus are in mineral form and assimilated by plants.

Post-fermentation can be managed by direct distribution on fields and meadows without prior processing (observing the conditions set out in the Regulation of the Minister of Environment on the recovery process R10 of 20.01.2015) or by separation of sludge, its dehydration, drying and sale as fertilizer, and the remaining sewage exported and distributed to the fields.

Waste from biogas plants must meet the following conditions:

• the rules for natural fertilizers specified in the Act of 10.07.2007 on fertilizers and fertilization are met,
• waste is applied evenly over the entire surface of the soil to a depth of 30 cm,
• the material after the fermentation process of animal origin meets the requirements of Regulation (EC) No. 1069/2009 of the European Parliament and of the Council of October 21, 2009 laying down sanitary rules for animal by-products not intended for human consumption.

The new Act on waste of 14.12.2012 (Journal of Laws item 21) in Article 2 point 6c specifies the following: “The provisions of the Act shall not apply to other, non-hazardous, natural substances from agricultural or forestry production used in that are not harmful to the environment and do not pose a threat to human life and health.

Energy from biogas

One of the main products of methane fermentation of organic compounds is biogas, i.e. a mixture of methane and carbon dioxide and, in small amounts, hydrogen sulfide, nitrogen and oxygen. Depending on the method and efficiency of the methane fermentation process, biogas with a different content of 50% to 80% methane is also obtained. The greater the degree of decomposition of substrates in the anaerobic reactor, the greater the percentage of methane in the biogas. The calorific value of biogas ranges from 17 – 27 MJ/m3, with the calorific value of pure methane 35.7 MJ/m3. Before being burned in a boiler or power generator, biogas must be dehydrated and desulfurized, i.e. hydrogen sulphide must be removed from its composition.

An example of the fermentation efficiency and the effects obtained:

1. Type of substrate – sludge from industrial sewage with a predominance of fats in the composition
2. Substrate load – 1000 kg d.m.
3. With a 70% reduction of the organic part of the substrate, 530 m3 of biogas, 1.4 MW of electricity and heat are obtained, the value of electricity is approx. PLN 560

With 85% reduction of the organic part of the substrate, 1.7 MW of electricity and heat is obtained, i.e. 21% more energy from the same mass of substrate, the value of electricity is approx. PLN 680