When food waste is sent to landfill, it is digested by microorganisms or, in plain English, it rots. Carbon and hydrogen in this material is converted to methane. In an uncontrolled landfill, this is released to the atmosphere where it acts as a powerful greenhouse gas. Methane is roughly 30 times more effective at trapping heat in the atmosphere than carbon dioxide. At the same time, because waste organic matter is not being put back into the soil where the food is being grown, fertilizers derived from fossil fuels are used to enrich the land. The production of these fertilizers increases CO2 emissions.
If food waste is processed, the methane can be captured for use as a fuel and the remaining organic matter then used as a biofertilizer. This reduces greenhouse emissions in a number of ways.
When the methane is burnt as a fuel, it releases carbon dioxide and water vapour. This carbon dioxide causes a much smaller greenhouse effect than if the methane was directly released into the atmosphere. It also supplants carbon, which would have been released from fossil fuels, with carbon that has recently been captured, when the food was grown. It may, therefore, be regarded as carbon neutral.
The residual biofertilizer contains nutrients that would otherwise be obtained synthetically using fossil fuels. Fertilizer production is a major contributor to carbon emissions, mostly during their production, but also because they are typically transported over large distances. Biofertilizer also contains microorganisms that help plants to obtain more nutrients through natural processes such as nitrogen fixation and solubilising phosphorus. This can greatly reduce the requirement for synthetic fertilizers and pesticides. Yet another benefit is that because waste processing is typically distributed between local facilities, the transport related carbon emissions are substantially lower.
Current Processing Methods
Anaerobic digestion (AD) is a process in which microorganisms break down organic matter in the absence of oxygen. For many years, it has been a vital step in the processing of sewage and other waste products, and it is now increasingly being used to process food waste and other organic waste, such as cardboard.
AeroThermal is a producer of industrial autoclaves, pressure chambers used to processes materials at elevated temperature and pressure. Previously these have been used for curing carbon fibre aircraft components. It has now developed a process known as Thermo-Pressure Hydrolysis (TPH) that can be used to pre-process food waste and other bio-waste organics prior to going to mesophilic (normal temperature) AD. This significantly increases the range of organic materials that can be digested, which therefore simplifies processing, while also increasing throughput and methane generation. It also substantially reduces the amount of material requiring disposal. This last point is given as a benefit although
AD works best with organic matter that decays readily, referred to as putrescibles. Woody materials, such as cardboard and the shells of nuts, contains high levels of lignin, which does not readily decay. Removing this type of material from food waste is difficult and wasteful. TPH breaks lignin down into cellulose which can be readily broken down further by AD. Other contaminants, such as metal and glass, can be easily screened following TPH as the output is a slurry. It therefore reduces the need for sorting, while also increasing the volume of material that can be converted into useable biogas.
High levels of protein can also be an issue for AD. When proteins start to decay, they can release high levels of ammonia, killing off microorganisms and therefore slowing the rate of AD. TPH denatures the proteins, enabling them to be effectively digested without releasing ammonia gas and therefore retaining more valuable nitrogen within the digestate for use as biofertilizer. This improved efficiency means that much higher organic loading and throughput can be achieved within anaerobic digesters.
The TPH operates in cycles of 2.0-2.5 hours, each processing over 20 tonnes of bio-waste. The bio-waste, inclusive of packaging, is subject to a simple maceration process prior to transfer to the vessel.
From there, it is sterilised under vacuum, followed by high-temperature saturated steam while the drum rotates, depending on the process requirements. The cellulosic material is hydrolysed and, after the TPH process is complete, the product exits the vessel as a pumpable liquid.
This is then separated into the predominant organic fraction which goes into anaerobic digesters, producing high yields of biogas, which may be upgraded and injected into the national gas grid, or used to generate electricity in a reciprocating gas engine.
The TPH process results in more organic materials being converted to useful biogas, preventing more methane from reaching the environment, and more nitrogen being retained within the digestate for use as biofertilizer. Although the process does require an energy input, every effort is taken to minimise process energy. Two autoclaves are alternately operated, enabling thermal cycling in which at the end of the first autoclave’s cycle, it is depressurised into a second autoclave, preheating it. These measures result in a net increase in the energy gained.
Tony Kimber, AD technology technical director at AeroThermal, says: “The TPH process enables a much higher loading rate than can be achieved with a “standard” AD plant: while they can be loaded at an OLR of up to 3 kg/m3, a TPH/AD plant can be loaded at up to 7kg/m3. This results in a 50% reduction in the size of a THP/AD digester vessel volume.”
Greenhouse Gas Emissions
Quantifying the reduction in greenhouse gas emissions resulting in different methods of processing food waste is difficult, since food waste varies widely in its characteristics. However, it is possible to state approximate figures for typical food waste. For one tonne of typical food waste, processed using TPH and AD, the following reductions in tonnes of CO2 equivalent (tCO2eq):
● 79 kg of methane diverted from the atmosphere = 2.38 tCO2eq
● Net energy recovered, replacing natural gas = 0.22 tCO2eq
● Potential reduced natural gas leakage = 0.13 tCO2eq
● 12 kg N biofertilizer, reduced production and transport = 0.03 tCO2eq
● 1.7 kg P biofertilizer, reduced production and transport = 0.001 tCO2eq
● 4.7 kg of K biofertilizer, reduced production and transport = 0.002 tCO2eq.
The calculation for net energy recovery assumes that the biogas replaces natural gas, which releases 2.75 kg of CO2 per kg of gas burnt. A separate calculation was made for the significant amount of methane that natural gas extraction and long distance transportation leaks to the environment. This is estimated to be 1%-9%, therefore a value of 5% was used here. If the recovered methane replaces another fossil fuel, such as coal, this would also increase this benefit.
TPH is currently being operated at two sites. Jones Celtic Bioenergy (JCBE) is using it to produce biogas that is then upgraded and injected into the national gas grid. “JCBE is very pleased to now have the unique TPH technology demonstrated in a full-scale application where the future opportunities for the technology could truly change the face of the industry given the range of organic materials that can be processed that would otherwise be unavailable to digestion,” says Dr Andrew Walsh, technical director of JCBE.
A second plant that operates a reciprocating gas engine combined heat and power plant is currently installing Aerothermal’s TPH process.