Waste | Edina
The last decade has seen enormous growth and investment in the biogas sector with pioneering work taking place to implement carbon reduction programmes and widen the scope of biogas applications. UK company Edina has been closely involved in these developments across a number of sectors, says Managing Director Anthony Fenton.
T
he production of biogas in the sewage industry is long established, but the advantages of combining this with the technology of thermal hydrolysis is a more recent development and one that has been embraced particularly in the UK. TH (Thermal Hydrolysis) is one of the methods of AAD (Advanced Anaerobic Digestion) whereby the collected sludge is steam-treated before digestion, which yields many benefits.
Following the separation of the raw sewage into sludge (solid matter) and the liquids (sewerage effluent), the sludge is sent to the TH plant. Here it is treated in batches by being pumped into vessels where steam is injected raising the temperature and subjecting the sludge to pressure for a residence time of approximately 30 minutes. It is this combined treatment that affects the cells in the organic matter, making the resulting sludge less bulky and much more rapid to decompose. The steam has a pasteurising effect on the sludge and its subsequent dewatering ability is greatly improved with a corresponding reduction in the energy required for that part of the process. The heat energy required for the
TH process is provided by the CHP (Combined Heat and Power) units installed in conjunction with the TH plant. The CHP units are fed with the resulting biogas. The CHP’s exhaust and jacket water are the sources for the required heat. The jacket water heat is used to
feed the digester and to preheat the water used to produce the steam. The exhaust is fed to a waste heat boiler where the feed water is raised to a temperature of 165 °C and 6+ bar steam. The TH process injects the steam into the sludge so there is complete heat transfer and no condensate return.
Raising the environmental
standard There has been a noticeable enthusiasm in the UK for adopting this technology by sewerage and waste water treatment companies and many factors account for this - from the nationwide standards set by Ofwat (Water Services Regulation Authority), the farming requirements for sludge disposal to land and the
ADVANTAGES In brief the advantages of applying this
method are as follows: l Pathogens present in the sludge are destroyed making the resulting digestate suitable for immediate agricultural
application. lThe biodegradable properties are
improved following the process. l Odour is reduced and storage
enhanced through a more stable product. l The malleability of the sludge post- treatment means that the load to the anaerobic digestion tanks can be doubled thereby speeding up the
operation. l This technology forms part of the “closed loop” system and is implemented by waste heat from the generator (CHP
unit). l The change in structure to the sludge also increases the amount of methane released from it. Therefore the process can power itself from the biogas which drives the engine and repeats the
treatment cycle. l Less energy demanding overall than previous methods.
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investment programmes undertaken by water companies known as AMPs (Asset Management Plans) - to the challenges they face from climate change and population pressure. Not least is the need to reduce costs and raise the environmental standard. To date Edina has been involved in thirteen TH projects by providing the CHP. Any excess power is exported to the National Grid, and generates ROCS for the operator. Any access issues and/or limited available footprint of some sites and the aims to reduce waste in construction marry well with the containerisation of the engines and associated equipment. Edina undertakes the design and container manufacture in the UK. Design ideas and requests can be worked in during this time. Usually the container with the engine already inside is delivered by haulier and lifted into position by crane and then connected. This is known as a “plug and play” unit. The installation at Howdon for Northumbrian Water was the first in the UK to use all the sludge after treatment for renewable energy. This technology processes waste from a surrounding population of 2.6 million. The resulting biosolids are Class A Fertiliser and are all used locally for agriculture, which has cut transportation and hence resulted in carbon savings. Northumbrian Water estimates that two sites using advanced anaerobic digestion to produce biogas have reduced their annual £40 million electricity bill by a fifth and that by 2015, 20% of its energy requirements will be self-generated from renewable sources. Another example of innovation in the waste sector is the colossal project at Beckton and Crossness, a joint venture by Tamesis for Thames Water.
Due for commissioning in the ISSUE 01 | GLOBAL OPPORTUNITY 2014 193
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