The term ‘waste’ needs to be used with caution as nothing is wasted in nature. The by-products from one process are used as the raw material for the next. In this context waste is the collective name we call the organic material we no longer have a use for. It comprises the contents of our dustbins (after recyclables have been separated), kitchen waste, human faeces, animal slurries, animal by-products and food processing wastes. We also include here used waste cooking oil.

Anaerobic Digestion (AD)

All organic material degrades quite naturally if oxygen is present, by what is known as an aerobic process and is familiar to us as composting. If oxygen is excluded, by enclosing the process in a sealed chamber, the process is known as anaerobic. The advantage of using an enclosed chamber is that the by-products of organic breakdown (methane) can be captured easily. The methane can then be used as a fuel. If the source material is predominantly wet, a process of anaerobic digestion can be used. If the source material is predominantly dry a process of gasification can be used.

The waste streams considered here are:

• Municipal solid waste (incl. commercial waste arisings)
• Animal slurries
• Sewage sludge

All anaerobic digestion processes rely on naturally occurring bacteria and enzymes to breakdown the source material. Some bacteria work better at 35 degrees C, others better at 56 degrees C. Lower temperature bacteria take about 7 weeks to work, the higher temperature bacteria take about 2 weeks, but are fussier about the consistency of the feedstock. There are a number of different systems based on variations on AD, each with advantages and disadvantages according to different circumstances they may be required to operate within.

Anaerobic digestion produces both heat and power (CHP/Combined Heat & Power). Based on the DARE calculation for CHP plants (see 7) a 2:1 ratio of heat:power is delivered at CHP plants

Municipal Waste

Statistics show 70% of waste arisings go to landfill and 30% is recycled or composted

However this is the value for raw Municipal Solid Waste (MSW) that is made up of kitchen waste and other dry waste such as paper.

Using the Ludlow Experience¹ and translating their analysis to T&D

Animal slurries

Methane emissions from manures and slurry management make up 14% of the total methane emissions from livestock husbandry in the UK. The most promising option for the exploitation of manures and slurries to reduce greenhouse emissions is the controlled generation and recovery of methane from slurry systems through anaerobic digestion².

It is very difficult to assess accurately the available resource, as information is needed on the number of animals in the holdings, how the animals are housed (i.e. how the slurry may be collected). It has not been possible to quantify how much slurry would be available for AD. Instead a guide is taken from comparison with the Holsworthy Biogas Plant.

Holsworthy Biogas Ltd collects cattle slurry from 30 miles within a 20 km radius and food waste from food processing plants. This company has two 0.9 MW generators fuelled by the methane gas collected from the digesters. There is sufficient gas to maintain one engine working full time and the second engine part time. The AD process is very effective in killing pathogens and weeds; the liquid digestate is available to be returned to the farms as a liquid fertiliser. The electricity generator and its exhaust, provide the heat needed for the plant and the excess is piped to Holsworthy less than 2 miles away where it heats the hospital, the health centre, 2 schools, a sports hall, the council offices and 150 homes³.

Sewage Sludge

Previously, the technical expertise required to maintain anaerobic digesters coupled with high capital costs and low process efficiencies had limited the level of its industrial application as a waste treatment technology. Anaerobic digestion facilities have, however, been recognised by the United Nations Development Programme as one of the most useful decentralised sources of energy supply, as they are less capital intensive than large power plants⁴.

Sewage treatment is a significant user of energy requiring about 50kWh/y per person. An investigation in Queensland, Australia demonstrated that the dilute feedstock (compared with other bio solids stream normally used for AD) could convert the suspended organic solids by 66% to methane⁵. Sewage plants are capable of producing energy: electricity and fuel from sewage gas, and heat from wastewater and combined heat and power plants. The resulting digestate can be recycled back onto the agricultural land. The digestate can carry heavy metals according to the industrial component of the catchment area (agricultural land where the digestate is recycled is therefore monitored for heavy metals), however this is not a high risk in T & D.

Totnes Waste water and sewage treatment plant:

Based on the DARE calculation for CHP plants (see 7) where a 2:1 ratio of heat: power is delivered at CHP plants.

*This plant also produces heat some of which is currently used on site. Currently this is used to heat the site offices and as these are then too hot in the summer months, energy is used to cool the offices. This could be used to heat water for nearby houses as a CHP system.

Gasification & Pyrolysis

Gasification & Pyrolysis are advanced thermal processes for the disposal of any combustable material. Historically incineration has been used to dispose of waste, however there are concerns that this process can release highly toxic dioxins (carcinogens) into the atmosphere as well as not making best use of the heat generated. Feedstocks include: municipal solid waste, refuse derived fuel, miscanthus, clinical waste, old tyres, sewage sludge cake, animal litter, animal by-products (rendered) and various plant residues such as palm oil husks.

Gasification takes place when combustible material is heated up within a sealed chamber where most of the oxygen has been excluded. Incomplete combustion takes place so the gases given off still retain the ability to burn when they come into contact with a further supply of oxygen. The gases are piped away as a fuel.

Pyrolysis takes place where combustible material is heated up in the complete absence of oxygen. The heating breaks down the material into its basic components some of which form the basis of a fuel. Gasification and pyrolysis work best with dry materials. Wet material uses some of the energy to burn off moisture before the process can start.

It is highly unlikely that any kind of incineration plant would be acceptable in T&D, however for the benefit of completeness and to demonstrate the energy potential from current waste residues (which are expected to reduce as plastics – oil based – decline in the waste stream) potential energy generation based on this process is detailed.

There would be other feedstocks available for the above energy from waste producing technologies such as construction industry waste, abattoir waste etc. but data is not currently available.

Combined Heat & Power (CHP)

Combined heat and power is a generic term used to describe a process that produces energy in a more efficient way. CHP is not a renewable energy, it is a conversion technology able to make more efficient use of other renewable energy sources. When any combustible fuel is burned the primary energy given off is heat. The heat can either be used to drive a diesel generator or to heat water to make steam to drive a steam turbine. Approximately 1/3 of the energy in the primary fuel is converted into power whilst 2/3 remains as heat. The power can be transmitted many miles in the form of electricity along power lines. The further electricity is transmitted, the more power is lost. Energy as heat can be transmitted in the form of hot water in pipes, but however well insulated, heat is eventually lost. Unless there is a use for the heat close to the power generating station, this heat is frequently wasted via large cooling towers. As noted above in 6a, anaerobic digestion, Totnes Sewage Works produces CHP, but currently wastes a lot of the heat, as it does not have a suitable nearby use at present.

CHP seeks to overcome the inefficiencies inherent with centralised power generation by installing a smaller scale generating plant close to where both heat and power are needed. Any combustible fuel source can be used, so it is perfectly possible to have a logwood, woodchip or pellet boiler to heat water to drive a generator and the residual heat used in nearby properties. New CHP gas boilers are available for domestic properties, however current grid gas CHP is not renewable energy, simply a more efficient way of using fossil fuel. (This may change with Language coming on line at some point, natural gas could drive heat pumps; no data exists to assess this further at this stage)

The Bedzed project in South London uses waste timber from the construction industry and arboreal waste from trimming the city trees; the plant has been sized to supply all the local electricity needs so when demand is low, the surplus is sold to the grid⁷.

Assuming the CHP plant is 90% efficient, 77,387 MWh could be delivered as an alternative to the individual contribution from the various renewable energies listed (see table at 6c in Appendix C)

N.b. not included in Table 4 – already accounted for under the separate sources.

Recycled Waste Cooking oil

Totnes Rickshaw Company has made a rough assessment of 2,500 litres of waste cooking oil being available from local food commercial outlets each month for recycling into vehicle fuel. Some of this is already being processed and used in their 2 rickshaw taxis. Production is set to increase (2009) when they move into the woodshed at Sharpham as a pilot larger production system to supply fuel for more vehicles including agricultural vehicles.

Grape Seed Waste

Grape seed waste has also recently been considered in Totnes for its waste oil value. Sharpham Estate at Ashprington produces around 8,000 litres of wine each year with a high tonnage of residual grape seeds that are currently disposed of. Information about the energy inputs to crush the seeds and produce oil is not yet available. A preferred use of this high-grade oil may be for body therapy and cosmetic products. No estimation for this resource as an energy fuel is offered here.

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