In most European countries, there has always been a clear and strong separation in biogas / biomethane plants between those fueled by agricultural biomass and those fueled by waste, although, from the chemical point of view, the composition of the digestate in the fermenter has little difference between the two. Agricultural biogas is produced, as well as a share of cultivated biomass (being phased out, in homage to the guidelines of the ILUC directive, concerning raw materials to be used for biofuels, and is concerned about the potential competition of energy crops against food crops), especially from agricultural by-products and waste .
One of the criteria that define the nature of a by-product is that of having a market value.. The digestate of anaerobic digestion plants considered agricultural has been equated to a zootechnical effluent e, as such, recognized as a soil improver.
On the other hand, FORSU is normally used in biogas / biomethane plants of waste treatment plants (for which revenues from gas production represent a minority share, with respect to the transfer rates), but that digestate is not authorized for agronomic use and must be treated, respectively in the composting unit for the solid part and in the purification plants for the liquid part.
As, from the point of view of incentives, today, there is no difference between agricultural plants and waste plants, agricultural biogas / biomethane is usually more expensive. Consequently, the vast majority of biomethane plants are fueled by waste.
hybrid agricultural plants / FORSU
The possibility of using a share of OFMSW together with agricultural biomass and of being able to spread the digestate agronomically would offer important economic advantages and would certainly contribute to a greater use of agricultural waste., making plants that are now at the limit of profitability economically sustainable.
In reality, the agronomic use of this digestate, based on the R10 regulation on sewage sludge, it is not explicitly forbidden and plants of this type have existed in Italy for some time.
There are certainly concerns about the possible negative impacts of these plants. One of these is the possible presence of plastics. However, a percentage of plastics, until 0,5% it is also allowed in compost.
From the point of view of the most recent European regulations:
- according to EU regulation 1691, the digestate is not considered a chemically dangerous material (according to REACH regulation)
- According to the EU directive 2019/1009, the digestate can be used as a fertilizer, after having undergone treatment at least 55 ° C for 24 hours and after a retention time of 20 days.
From the point of view of economic sustainability, the attached spreadsheet shows that the greater investment required to treat OFMSW together with agricultural biomass is more easily repaid with the increase in the share of FORSU. However, it must be considered that the possibility of direct use of the digestate of a plant that uses OFMSW is not unlimited: with the growth of digestate volumes, the spreading cost increases with increasing distance from the plant.
However, the possibility of creating hybrid OFMSW / agricultural plants could play an important role especially in areas where the low population density does not make plants powered by FORSU alone sustainable. Moreover, the construction of FORSU plants by private individuals is hindered by the difficulty of obtaining sufficient and stable supply contracts, necessary to obtain project funding, while, by local administrations, it is often difficult to aggregate sufficient waste streams to reach economically sustainable plant sizes.
In situations like this, hybrid plants could help overcome these obstacles and broaden the base of feasible biomethane plants. Concerns about digestate quality could be mitigated by the adoption of specific policies, times, for example to enhance the digestate, with post treatments, such as osmotic concentration or pelleting.