3.0       Technology that we proposed

 

3.1       Anaerobic Digestion

 

Anaerobic digestion is a biological process, in the absence of oxygen, that produces a gas principally composed of methane (CH4) and carbon dioxide (CO2) otherwise known as biogas. These gases are produced from organic wastes such as food processing waste. Biogas is used as a form of renewable energy that can be used to generate electricity and heat to power on-site equipment. A further by-product of the process is biofertiliser, which is rich in nutrients such as nitrogen, phosphorus and other elements required for healthy plant growth and fertile soil.

Anaerobic digestion can be performed as a batch process or a continuous process. In a batch system biomass is added to the reactor at the start of the process. The reactor is then sealed for the duration of the process. In continuous digestion processes, organic matter is constantly added (continuous complete mixed) or added in stages to the reactor (continuous plug flow). Here, the end products are constantly or periodically removed, resulting in constant production of biogas.

 

The two conventional operational temperature levels for anaerobic digesters determine the species of methanogens in the digesters. Mesophilic digestion takes place optimally around 30 to 38 °C, or at ambient temperatures between 20 and 45 °C, where mesophiles are the primary microorganism present.

Thermophilic digestion takes place optimally around 49 to 57 °C, or at elevated temperatures up to 70 °C, where thermophiles are the primary microorganisms present. Mesophilic systems are considered to be more stable than thermophilic digestion systems. In contrast, while thermophilic digestion systems are considered less stable, their energy input is higher, with more biogas being removed from the organic matter in an equal amount of time.

 

In a typical scenario, two different operational parameters are associated with the solids content of the feedstock to the digesters:

• High solids (dry,stackable substrate)

• High solids (wet,pumpable substrate)

 

High solids (dry) digesters are designed to process materials with a solids content between 25 and 40%. Unlike wet digesters that process pumpable slurries, high solids (dry-stackable substrate) digesters are designed to process solid substrates without the addition of water.

 

Digestion systems can be configured with different levels of complexity.

In a single-stage digestion system (one-stage), all of the biological reactions occur within a single, sealed reactor or holding tank. Using a single stage reduces construction costs, but results in less control of the reactions occurring within the system.

In a two-stage digestion system (multistage), different digestion vessels are optimised to bring maximum control over the bacterial communities living within the digesters.

 

The system chosen will largely depend on the feedstock to be processed. In this project, we are dealing with high solid materials (dry,stackable), such as food waste mixture. Therefore, the process tend to be processed at a mesophilies temperature using the single-stage continous sittred tank reactor (CSTR) system.

 

3.2       Reasons of choosing Anaerobic Digestions

 

• Anaerobic digestion provides biogas, a clean, local renewable energy source.

      Food waste is a continual source of input material that does not use any natural resources. As the amount of waste is produced      

      continually, there is a constant stream of inputs into the digester creating a stable source of electricity generation. Each dairy cow can

      produce 2kWh to 3kWh of electricity per day, which can be used to power the farm and/or be sent to the grid.

• Greenhouse Gas Reduction. Methane gas produced contribute to Greenhouse gases emissions. Anaerobic digestion could reduce methane release by 66%. These reductions in Greenhouse Gas emissions can earn carbon credits which can be sold to generate an additional source of revenue.

• Odor Reduction. Placing food waste streams in digesters, rather than leaving them exposed to the environment, can reduce odors at landfills and municipal waste plants.

• Valuable By-Products. The digestate (compost and fertilizer) can be used or sold, creating an additional revenue stream.

• Simple process yet save costing process. Compared to other technology (fermentation, pyrolysis and gasification), the technology chosen does not require water or any segregation before digesting it in the reactor which make the process much simpler and less work energy.

 

 

Why not other technologies?

 

In fermentation, the process production is very slow and the product contains impurity, needing further treatment. Fermentation has limited ability to vary output rate and has very long start-up times which require continuous skilled technical support.

In the pyrolysis technology, the process is very dependent on the moisture content of the feedstock, which should be around 10%. At higher moisture contents, high levels of water are produced and at lower levels there is a risk that the process only produces dust instead of oil. High-moisture waste streams, such as sludge and meat processing wastes, require drying before subjecting to pyrolysis.

 

In the gasification technology, is quite complex and sensitive process. Fuel is bulky and frequent refueling is often required for continuous running of the system. Handling residues such as ash, tarry condensates is a time consuming and  dirty work. Getting the producer gas is not difficult, but obtaining in the proper state is the challenging task.

 

 

3.3       Sustainability

3.3.1    Economy

Anaerobic digestion of food waste eliminate the need to transport waste to an established waste disposal facility. Utilizing the waste onsite saves upfront costs of transportation and disposal and also generates financial gains with the byproducts produced. The process creates a viable energy source that can be used for heating and electricity for homes or the facility itself. This biomass energy can be a net zero cost and offset electrical costs,provide new forms of energy to areas that might not have access to traditional electrical energy sources; or generate profit if the energy produced can be sold. Additionally, the AD systems often generate commodities such as fertilizer, which can be used at the site or sold for additional income. Creating natural, organic fertilizer in itself saves costs for these facilities, which use the fertilizers on their crops. In developing countries the biogas produced can also be used to run a cook stove, which saves a family from having to purchase alternative fuels for cooking.

 

 

3.3.2    Social

The electricity produced from biogas becomes a tremendous social benefit to communities. In rural areas, anaerobic digestion give residents energy independence and make them self-sufficient. Providing an independent energy source gives residents more reliable access to electricity, improves quality of life, and promotes economic growth. Anaerobic digestion change the way communities heat their homes, providing a constant heat source so people do not have to look for wood or other fuels. Another social benefit is the creation of jobs. Learning how to operate an anaerobic digestion system and perform routine and heating source operations and maintenance checks also improves laborer skills and can help establish the local expertise needed for additional biomass electricity systems.

 

3.3.3    Environment

Anaerobic digestion systems reduces water and air pollution, and improves human health and the environment. Biogas produced by anaerobic digesters can replace the use of fuel gas and oil. Harnessing methane lowers greenhouse gas emissions, improving air quality. The removal of manure from these facilities also improves water quality by reducing runoff and controlling pests, decreasing eutrophication and the potential for water pollution in local waterways. Additionally, utilizing manure decreases the time it sits at the farm contributing to odor and pest issues. Another benefit is from the use of the final digestate product (both liquid and solid), which is used as fertilizer at many of the facilities. This organic fertilizer reduces waste byproducts and the need for chemically-enhanced fertilizer, as well as helps improve crop production. Several countries that use AD systems report a reduction in the use of firewood for heating and cooking. The replacement of firewood with biogas-powered electricity and heating helps prevent deforestation, especially in remote populations that count on forestry as a source of exports.

 

3.4 Catalyst

 

Bio-organic catalyst (BOC) offers substantial improvement in anaerobic digestion, providing higher yields of bio-methane, improved sludge quality, reduction of total solids, and significant reduction of noxious sludge odors. BOC can increase bio-methane yields over 100%, with up to 30% reduction in total solids discharged. Sludge quality is enhanced due to more complete digestion and sludge odors will be greatly reduced, including odors in dewatering operations.BOC accelerate the anaerobic digestion processes into more optimized conversion efficiency. Case studies show a much higher yield per pound, or kilo, of organic waste biogas, while clearing the internal accumulations that build up over time. BOC requires little capital equipment expenditure, as installations involve simple injection pumps, along with a reservoir of BOCs. Results become evident relatively quickly, as a faster release of high bio-methane value components of the waste material shifts the internal biomass within the anaerobic digester into the methanogenesis phase, increasing the bio-methane yields and total consumption of volatile fatty acids.

 

In our project, the use of glycerol as an amendment to anaerobic digestion of hog manure has proven to increase both biogas and methane production because glycerol has a high amount of readily biodegradable soluble BOD, which can be easily consumed by anaerobic bacteria.