From toilet flush to national grid in 20 days (Updated)
11 Oct 2010
Didcot, UK – UK households are cooking, and heating their homes with gas produced from human waste for the first time – using biomethane gas from a new unit at Didcot sewage works in Oxfordshire
The project at Didcot - a joint venture between Thames Water, British Gas and Scotia Gas Networks (SGN) - took six months to complete and cost £2.5 million. The new plant is expected to produce enough renewable gas to supply up to 200 homes.
Sewage arrives at the works from some of Thames Water’s 14 million customers to be treated and recycled back to the environment.
Sewage sludge is then treated further in warmed-up vats in a process called anaerobic digestion, where bacteria break down biodegradable material, yielding biogas. Impurities are removed from the biomethane before it is fed into the gas grid. (See fact file below for technical details).
The whole process - from flushing a toilet to gas being piped to people’s homes - takes around 20 days, the project team estimates.
Apart from the £2.5m capital investment, there are no additional costs are involved in producing gas via the new plant, compared to a conventional process, said an SGN spokesman.
“The fact that the digesters were already in situ meant that no additional work was required. The capital investment figure includes all feasibility, design and construction of plant and control systems,” he explained.
According to the spokesman, Didcot is effectively a demonstration project, providing a relatively low volume of gas to the grid and investigating to potential of contribution of biomethane-to-grid projects to renewable energy targets.
“[An] optimum project size [would] depend on the method of operation,” he explained. “For example, existing sewage treatment plants may have existing anaerobic digesters that can be used, whereas new plants would require them to be constructed and costs would therefore need to be assessed.”
As Martin Baggs, chief executive of Thames Water, stated: “We already produce £15m a year of electricity by burning biogas from the 2.8 billion litres a day of sewage produced by our 14 million customers. Feeding this renewable gas directly into the gas grid is the logical next step in our ’energy-from-waste’ business.”
According to Centrica, the average person produces 30kg – dried-out sewage sludge – of sewage per year that could be used for producing gas.
That, it said, means that the UK’s 62.5m people could, in theory, generate enough renewable gas to meet the annual demand of 200,000 homes, up to 1% of the UK’s population.
According to National Grid, biomethane from all sources could account for at least 15% of the domestic gas market by 2020.
Process equipment installed at the new facility and technical challenges involved:
1) Anaerobic Digesters: These were already in place at Didcot. These sealed tanks break down waste using microorganisms, extracting raw biogas and leaving a sludge that can be dried and used as fertiliser. Prior to implementation of the project, some of the biogas was used to heat the ADs but the rest was flared off and wasted. Challenges involved with the ADs are around keeping output up, especially in winter when the air is cold.
2) Gas clean-up plant: Often referred to as the “gas scrubber.” This removes impurities from the raw biogas, resulting in biomethane. Challenges include setting up the plant to deal with variable gas volumes.
3) Gas enrichment plant: The methane leaving the scrubber is not rich enough to satisfy the same exacting standards as fossil gas. Therefore, to ensure customers receiving biomethane are not disadvantaged, the gas is enriched with propane. Challenges include getting reliable enrichment at low flow rates.
4) Biomethane to Grid skid: This piece of equipment includes the meter to measure the energy flowing into the network, the analyser that ensures the biomethane produced meets Gas Safety regulations, odorising plant to add the characteristic smell that is added to all gas entering the network and pressure control equipment. Challenges have included designing a specification for this equipment to meet the characteristics of biomethane.
5) Telemetry: Flows into the network and outputs from the analyser are monitored in our System Control Room using telemetry so that any changes can be monitored in real time and information fed back to the biomethane producer and for metering purposes.
