The technology of “dry fermentation” o “biocel process” is an anaerobic digestion technology for solid organic wastes. It is based on a batchwise digestion at high solid concentration (30-40% w/w) and at mesophilic temperature.
In San Pietro in Casale (Bologna, Italy) a full scale Biocel plant was constructed. It processes 10.000 tons per year of source separated organic fraction of Municipal Solid Waste (MSW) in yielding energy and organic fertilizer (compost).
Figure 1: Simplified lay out of the process
Biogas typically refers to a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, green waste or food waste. Biogas can be produced by anaerobic digestion with anaerobic organisms, which digest material inside a closed system. Biogas is primarily methane (CH4) and carbon dioxide (CO2) and may have small amounts of other gases.
The fermenting process of the organic substance in the absence of air involves various anaerobic bacteria. Their composition depends on the organic source materials and specific conditions of the process.
Considerations on the basis of technology choise
This technology permits to use organic waste (MSW) as well as produced avoiding expensive, in term of money and energy, pre-treatment. In some cases, especially where the separated collection is just started, the energy produced from waste is much lower than that used to make waste useful to liquid fermentation. If you use a wet processes where dry matter of the substrate < 15% you have to transform solid wastes in a liquid. The fermentation substrate needs stirring and pumping so you must eliminate any things able to damage the equipment.
Moreover with this kind of plant you produce a high amount of process water. This by product can become a high managing cost.
In figure 2 the main advantages and disadvantages of wet technologies.
Figure 2: Advantages and disadvantages of wet technologies
In figure 3 the main advantages and disadvantages of dry technologies.
The most important limit of the dry technology is space requirement. If you have to treat a large amount of MSW you need a higher surface than wet technologies.
For the reasons described above in Bologna dry technology was chosen.
Figure 3: Advantages and disadvantages of dry technologies
Description of the digesters
The biocel system is well known as “garage system” causes of his construction form, see figure 3.
Figure 4: Panoramic view of the fermenter
The biocel fermenter are made in concrete. Inside wall are coated with spark preventing paint. The fermenter is closed with air tight doors.
For continuous biogas production at least 3 fermenters must be available. The fermenter dimensions have to be adapted to the size of equipment used for loading and unloading and the amount of input.
The modular system allows adding on of digesters without problems.
The loading is made by shovels and after the loading is finished the digester is closed. Inside perforated floors, a drainage system and percolation sprayers are installed.
Pratical operation and energy output
The fermenter is filled with a substrate obtained mixing inoculants and fresh material. As inoculant is used the digestate. The digestate is the organic materials residual from an anaerobic process. This is important for digesters, which solely work on the dry system with very little or no additional liquid. In these systems, for the optimal performance of the digester, specific bacteria have to be cultured for the substrates and have to be mixed in beforehand. The optimal ratio between inoculants and fresh material has to be found and kept up in each batch mixture. In order to keep up a loose consistence in the digestion heap structure material like straw, bark, or wood chips are mixed in.
Through the pre-treatment of mixing and storing of the substrates for one to three days a first aerobic biological conversion in the material is provoked. This is counteracting to an acidification at the beginning of the anaerobic phase. The temperature increase in this aerobic phase is catalyzing the start of the anaerobic mesophilic or thermophilic operation. The initial self-heating reduces the heat requirements during the process. The biomass is digested under airtight conditions after inoculation or flooding with digested substrate. The further inoculation with bacterial matter is taking place through recycling of a bacteria rich percolation liquid, which is sprayed over the biomass. When necessary, through a built in floor heating system in the digester and an installed heat exchanger in the tank of the percolation liquid the temperature of the process can be regulated.
Figure 5: Dynamics of methane gas production into a fermenter
The biological process is becoming stable after 2 to 5 days after the start of the anaerobic phase and reaches optimal methane concentrations of 60 to 65 %. In order to secure a continuous gas production in batch dry fermentation systems several digesters have to be run in different time intervals. The biogas production is at its maximum between day 10 and 28 and is decreasing due to general substrate degassing. The decrease of biogas production can however be compensated, if a new batch is taken into production after 4 weeks. A biogas holder installation is suggested
The different degrading reactions (hydrolysis, acid and methane formation) take place in one digester. After a retention time of around 30 to 40 days biogas production is almost zero and the digesters are unloaded.
Figure 6: Biogas production into a fermenter
Figure 7: Data collected from a fermenter working in Bologna. Single cycle.
Although odor control is less complicated than in aerobic plant, the biocel plant has to deal with odor emissions from the installation.
Odor control is essential at two specific moment: during unloading the biowaste from the track. Since the incoming biowaste is already sour, the unloading of the trucks results in major odor emission into the section of the plant where it is stored some days before fitting the fermenter, after opening the fermenter and transporting and mixing the digested waste by the shovel.
To remove malodorous composts the air inside the plant building is replaced 2 times every hours. The contaminated air flow is treated with a biofilter bed with a compost – wood chips mixture as active medium.
In bologna biogas is used to produce electricity and heat but others applications are possible. Below a short description of the biogas uses.
Generation of electricity and heat (CHP). The stationary use of biogas in combined heat and power plants for generating power and heat achieves a very high degree of efficiency. The electricity produced can be fed into the public grid or used as an independent power supply for industrial and commercial areas, or it can be used to provide power to rural settlements that are off the grid. The waste heat can be utilized in downstream systems for additional power generation, but also for use in heating, drying or the operation of refrigeration machines
Feeding biogas into the natural gas grid. Another attractive option is to feed biogas into the natural gas grid. After processing it into natural gas quality – biomethane with a methane content of up to 98 per cent – the biogas can be used in areas with a high demand for heat, and can achieve a high measure of efficiency by simultaneously generating power (CHP). In the demand-driven supply of power, the natural gas grid can play a key role as a long-term storage facility for renewable energy. By feeding in renewable hydrogen (power to gas), the natural gas grid facilitates both the storage of wind power and of biogas processed into biomethane – and hence into natural gas quality. In this way, the natural gas grid can help bridge seasonal fluctuations in electricity supply from solar and wind power plants.
Use in the mobility sector. The decoupling of production and utilization also makes it possible to use biogas as fuel for natural gas vehicles. Biogas processed into biomethane and used as fuel can make an important contribution to climate protection, enhance supply security and reduce noise emissions.
Apart from biogas the main product of the biocel process is compost. At the end of the anaerobic process the fermented waste (digestate) depending on quality and local legislation can be used in a different ways.
Figure 8: Characteristics of compost produced in bologna compared with Italian legislation
In Bologna the digestate is fermented in aerobic conditions (composting process). Normally this stage of treatment is ten days long. This stage is very important to obtain the complete sanification of the end product.
The compost chemical characteristics are strictly correlated with quality of incoming materials. The managing operators have to put big attentions at the waste used to make fermentation substrate. The compost very rich in organic matter is sold to farm and to fertilizer industry.
Mr Claudio Orsi
Partner and founder of IGW S.r.l.
- Infrastructure and construction;
- Environment and waste water treatment;
- Renewable energy
Mr Claudio Orsi, degree in Agricultural Science in Università Cattolica del Sacro Cuore in Piacenza, has been working for year on environmental protection: wastewater treatment, recycling sewage sludge in agricultural, and as CEO of AD plant from collecting waste in Bologna (Italy).
Now is the CEO of I.G.W. Srl
More info: www.igwsrl.com