ANAEROBIC
DIGESTION







Biomass: From the distant past to the near future

The oldest renewable energy source used by man was wood. Later, man turned to the exploitation of waterfalls, wind and solar energy. Then, a long period where non-renewable sources (mineral fuels) and the nuclear energy dominated, with all the concomitant environmental problems caused by their use. Those problems are leading to the return to Renewable Energy Sources in a reverse sequence: Solar, Hydroelectric, Aeolic and ...back to the Biomass.

Biomass is the most natural source for fuel production and for this reason it has a big potential. The means to achieve this is to convert the biomass into a compound that can be exploited in conventional machines through well-known technologies. The up to now available main responses to this query are two: biofuels and biogas. The biofuels have had a lead mainly due to the dominance of the motor vehicles but the biogas is the big chance due to a number of reasons:

  • It can be produced from any form of vegetable or animal biomass.
  • It practically consists of two components (Methane and Carbon Dioxide), regardless of its origin or production method.
  • Thanks to the development of the cogeneration methods, it can be transformed to other forms of energy by means of conventional technologies.
  • It can be stored and transported using conventional methods, tested and developed for almost 2 centuries.
  • It can be transported in long distances by virtue of the natural gas networks.

As long as a suitable amount of biomass is available, the objective is to apply the proper technology to transform the organic carbon of the biomass into biogas. The basic tool for this process is also provided by nature itself: it is called anaerobic digestion and it has been known to man for many centuries.

Numerous efforts have been made to apply anaerobic digestion at a full scale, many of which, in the case the organic fraction of the municipal solid waste and sewage sludge, are considered to be mature, successful and of general acceptance.

However, the biomass potential, whether it is included in wastes or in primary products, is huge. A very small portion of this is being used for the production of noble energy products. The vast majority of applications concern plain combustion which has low efficiency, while the local usage of thermal energy is either unproductive or practically not applicable.

The development and application of technologies that will allow the effective (reasonable biogas quantities, steady operation, clean final waste, low operation and maintenance cost) exploitation of wastes of different sources is not an easy task. It needs adjustment of the processing methods, according to the type and the condition of the waste as it comes from the main production line.

The overall cycle biomass – biogas – energy, has no adverse effects on the environment; on the contrary, it has a multiple contribution in its preservation:

  • Firstly, the biogas has been characterized as RES and the production of electric energy from it is enjoying particularly favourable terms of subsidy (the guaranteed feed-in price is about two times the buying one).
  • The production of biogas is takes place mainly through the conversion of the organic carbon into methane and carbon dioxide. The reduction of total organic carbon (TOC) in high loaded wastes such as the agroindustrial wastes can reach 95%, a level of efficiency which cannot be achieved though any other single conventional waste treatment process.
  • The anaerobic treatment of the waste is by definition a closed process and consequently, the odor emission is practically eliminated.
  • A part of the organic carbon is being assimilated in the anaerobic sludge. This results in an increase of the sludge in the reactor. By treating the excess sludge by means of modern methods, valuable organic fertilizers can be produced.
  • The characteristics of the final effluent permit its use either for flushing or as liquid fertilizer, thus contributing to savings in water resources.

All these impressive possibilities would be meaningless if the cost of transportation of the “raw material” (in this case the liquid waste) were higher than the benefit from the overall financial incentive of the investment. For this reason, the emphasis is given on the development and support of small and medium size units in the perspective of a decentralized production. This is now possible even at very remote locations, thanks to the possibility of connection to the national electricity grid (a regime similar to the one applicable to other RES like photovoltaic, wind and hydroelectric).