Anaerobic digestion
The anaerobic digestion is a very important operation that can be operated on different organic solid or liquid substrates, useful to transform them in biogas, where the presence of methane is very important (more than 60 %). The residual from the digestion forms a sludge denominated digestate, that in many cases can be utilized in agriculture, in account of its high content in nitrogen, as a fertilizer or at least as a soil improver.
For the anaerobic digestion, it is possible to use different original materials, destinated to disposal after other operations or specifically produced; the most important of them are:
Animal waste and residuals from farming activities;
Sludges from wastewater treatment
Concentrated liquors from organic industrial productions (sugar factories, distilleries, amid productions, etc.)
Organic biodegradable part of municipal solid wastes
Energy crops (maize, mischantus,….) expressely produced for anaerobic digestion and biogas production.
Some of these materials can be mixed as a feed for digestion in an operation of co-digestion, considering that part of the feed is useful to introduce fermenting bacteria and consequently improving velocity of the operation (animal wastes, for example) and the other part leads to an high yield in biogas (energy crops).
The anaerobic digestion can be operated in very simple, traditional closed tanks, or in a more modern, monitored apparatus, and with batch or continuous scheme; in any case the fundamental aspect that are required to obtain a good grade of transformation of the original organic substrate in biogas are a convenient residence time (15 – 40 days), a medium or high temperature (for thermophilic operations, with the maximum transformation kinetics, until to 50 – 55 °C), a controlled neutral environment, a good suspension of solids (biomass and segregated organic substance) in the reaction liquor.
The produced biogas can be directly sent to engine to produce electric energy to be immitted in the distribution grid, or to boilers to produce thermal energy; options of co-generation are very positive from the environmental balance point of view, but in account of the localization of the digestion plants normally in remoted areas, or in general in agricultural zones, it is normally quite difficult to utilize the produced heat, and the option of electric only configuration is preferred.
Fig. 6 – Scheme of an anaerobic digestion
The produced biogas can be subjected to a further separation process, leading to production of bio-methane, with the destination of immission in natural gas systems or utilization in automotive engines, with positive aspects as concerns elimination of local atmospheric impact and substitution of fossil fuels; this technology is now adopted in some areas of North Europe, but its economic sustainability is strictly connected to the costs and prices for the original raw materials, and the awards that are introduced for this form of fuel in distribution.
Also the production of bio-hydrogen is an important branch of the anaerobic digestion, taking into account that with a controlled chemical and biological operation in the digester it is possible to stop the transformation of organic substrate to a gas quite riche of hydrogen instead to arrive to the final biogas production, but this technology for the moment is only at the stage of pilot and demonstrative plant.
The final destination of the digestate, that from the mass balance point of view is a very important by-product of digestion, can lead as it was anticipated to positive agricultural applications, but it can create some local acceptation problems in consideration of nitrogen load that can be introduced in underground waters, smell aspects during the field application, residual formation and atmospheric immission of methane; from this point of view, the right destination of digestate, its management nd its introduction must be carefully controlled.
Finally, it is important that the anaerobic digestion that is operating in MSW landfills is a well consolidated, traditional source of biogas, that must be used both for energetic and for environmental impact limitation aspects: it is in any case important to consider that, in the required and operating modification of systems adopted to treat MSW, the solution of landfill is progressively substituted from other more convenient and more controlled solutions (separation and recycle, composting, WtE), and consequently the landfill activity is progressively of lower diffusion. Also the aspect of a quite limited energetic efficiency of this system, both for times of transformation and for efficiency in biogas collection, is an aspect that limits the interest of this technology from a strict energetic point of view.