This technical note is the last one of the section on recycling of organic waste. In this section, it is also question of treating organic waste by biological mode but in addition to the production of compost, a bio fuel known as biogas will be generated by the process.
Indeed, the anaerobic treatment of organic waste, if it is carried out properly, allows not only the transformation of the waste into compost but also into a gas composed of methan (CH4), and carbon dioxide (CO2) and nitrogen and sulfur components.
Biogas as a by-product of the methanic digestion of organic substrate is a fuel gas, which uses air as combustive agent in the same way as a fossil fuel, and which generates heat through oxidation.
The other difference is linked to the humidity level of the metha compost as it is not a dry product. It is always in a humid and even semi liquid form when it results from the digestion of pig manure or other waste in solution such as sludge from wastewater treatment plant.
The recent awareness of the population on global warming associated to the greenhouse gases GHG (methane is a GHG 21 times more harmful than carbon dioxide), the increasing costs and scarcity of fossil fuels are resulting in the growing interest for biogas technologies even if it is a technique which has been used for more than thousand years in China.
Indeed, it not only allows the sustainable treatment of waste but it is also attractive for its production of organic fertilizer and particularly bio fuel.
However, as a promising and efficient technology, biodigestion is also more complex and costly to implement than aerobic composting. It is only since a few decades that biodigestion is the subject of research and high investment technological applications, namely in the industrial sector, but as we will see, its technicity limits its viability in a certain number of contexts.
Energy recovery and agronomic uses
The conception and the implementation of the organic waste treatment system by anaerobic mode can be analyzed on three major parameters:
Efficiency of the treatment of waste
The efficiency of the system depends particularly on the adequation of the process with the nature of the flow of waste to be treated.
The digestible organic fraction of domestic waste exists and is particularly important in priority regions of ReCoMap but it can only be considered if sorting at source, voluntary deposit schemes or separate collection systems have been implemented.
The digestibility and productivity of the substrate are values which are linked. They should be considered together with the frequency and regularity of supply to the station to determine the type of process which should be privileged. For example if the flow of waste contains an important part of semi solid waste (animal manure, fats and oils, organic sediments and WWTP sludge), the addition of material rich in fibres (green waste, saw dust and rice husks) to remain in the configuration of a solid treatment and to respect a balanced C/N ratio.
It is only with such an approach that the treatment will be able to operate effectively without big variations and costly or complex requirements in terms of “raw material” and energy.
Indeed the measure of efficiency of the treatment of waste through a biodigestor is undertaken by first observing the regularity of the biological process occurring in the biodigestor (conditions of start, duration of primary phases, productivity of the methanogenesis) and this regularity, if it depends on a good initial configuration, remains strongly dependant on the capacity of the system to absorb the quantitative and qualitative variations of the incoming flow. In this regard, a biodigestor is much more sensitive than a composting station. To summarize, we can say that if we envisage to set up an anaerobic digestion unit for a given load of waste and if we aim at achieving the proper treatment of the waste, we must first have a perfect control on the upstream waste management.
Feasibility of the technology transfer
It is clear that the implementation and operation of a biodigestion unit for the organic fraction of domestic waste is a complex project which require, from the design stage to the operation and maintenance phases, that the beneficiary possess the minimum knowledge to be able to evaluate risks and to take the responsibility of its management.
This difficulty can be overcome by good training and transfer of competencies and the recruitment of specialists to cover the technology needs. However, the level of technicity of this type of installation also results in high investment costs and therefore brings the question of economic feasibility.
At this stage of reflection, we must not only consider the costs of the technology and its maintenance but also the income that are expected from the sales of the products of the biodigestor. They can be differentiated in three categories:
- Public service for solid waste management
- Energy recovery (biogas)
- Agronomic use of by product
It is the waste manager of the local, regional or national authority who will be in charge of establishing the balance and ensure that the return on investment will match with the budget available.
We can however note that if it is correctly designed, a methanic digestion unit can beneficiate from funding under the Cleaner Development Mechanism (CDM) and that this source of financing can be significant.
Introduction to Recycling of : Plastics, Paper & cardboard, Non-ferrous metal and Glass
This group is the last one of the series of technical notes proposed for the ReCoMap project within the sustainable coastal waste management theme. They deal about the recycling of non organic waste for the four major group of waste:
- Recycling of plastics
- Recycling of paper and cardboards
- Recycling of non-ferrous metals
- Recycling of glass
The choice of recycling as a major issue for waste management in the ReCoMap countries is based on the fact that a global international market for recycling of non organic waste exists but it often encourages waste managers in developing countries to collect and export the sorted waste whereas simple techniques are available for the setting up of complete recycling options at national, regional or even urban level.
In this context, the present TNs will not aim at listing the industrial or commercial exporting options for sorted recyclable waste, this global industry being very active in terms of commercial prospection and communication. We will rather opt for the presentation of a few recycling techniques which appear to be the most adapted to the ReCoMap targeted countries.
The objectives of these TNs therefore consist in describing to the waste managers the techniques which are adapted to their countries and regions in a view to recycle locally a large part of their non organic waste by supporting the implementation of financially sustainable and environmentally positive activities.