15 June 2012

Advances in Anaerobic Digestion

advanced anaerobic digestion
Anaerobic Digestion is a process carried out by bacteria in the absence of oxygen. The process has been developed to the level at which it produces substantially less sludge than aerobic treatment and converts organic matter to energy-rich biogas which can be used as a fuel, but further advances in the technology to reduce costs are still needed for it to be taken up on a large scale and for every farm of any size to have one.

There are only about 30 anaerobic digestion plants in Britain, compared with more than 4,000 in Germany. The Department for Energy and Climate Change (DECC) has said that the enhanced tarrifs for renewable energy from anaerobic digestion will improve its uptake, and it intends to pay 11.5p per kWh up to 500kWh, but the Renewable Energy Association REA has stated that most farmers with an average sized heard need at least double that to generate a commercial return on their investment.

Anglian Water is producing enough biogas from its new advanced digestion system to deliver 980kW of energy at the engine without any further fuel requirements and it is doing it by using advanced anaerobic digestion technology. Advanced digestion using Monsal enhanced enzymatic hydrolysis has been employed at two of their largest sites at Kings Lynn, Norfolk and Great Billing, Northants. The combined processing capacity of these plants is close to 60,000 tonnes dry solids per annum. Advanced digestion using biological hydrolysis maximises the biogas potential, which is achieved by bespoke pre-reactor design to complete the solubilisation reactions and generation of volatile fatty acids (VFAs) prior to mesophilic anaerobic digestion. High VFAs concentrations (10,000 mg/L) is produced, which is converted to biogas. The digesters convert VFAs primarily to biogas. The key innovation is the full use of the heat balance from the process in an optimal way. Low-grade heat from the combined heat and power unit is used in the first stage of the process, raising sludge temperature.

A further advance in the AD process is the use of steam autoclaves work like giant pressure cookers, heating mixed waste at high pressures to ensure the breakdown of organic material. The process ensures that more of the organic waste is captured for use in an energy generating anaerobic digestor, while the non-organic materials are cleaned and sterilised ready for recycling. As well as generating renewable electricity from the anaerobic digestor, waste heat from the process can be captured for re-use.

There is further good news that this advanced AD technology will help the industry become more established. It has been announced that waste management company Viridor has begun construction on a state-of-the-art anaerobic digestion facility near Bridgwater in Somerset. The £10m project is being developed by Viridor where Monsal are providing the technology which will convert food waste into renewable energy via their Advanced Digestion process. The plant will be Viridor’s first Food waste AD facility and will be located at its Walpole landfill site. The Somerset Waste Board agreed in September 2011 to develop the facility and the plant is expected to be operational by April 2013.

Another company which implements what it call its advanced biogas system is Oaktech Environmental. That company is an experienced developer of biological treatment solutions., and holds intellectual property rights to two advanced Anaerobic Digestion technologies designed for the treatment of a wide range of organic feedstocks including municipal solid waste, source separated wastes and commercial food. Oaktech’s proprietary technologies include the ArrowBio process for the treatment of residual waste and the Induced Bed Reactor (IBR) process for the treatment of source separated food waste, energy crops and farm slurries. Both technologies are commercially proven having undergone over 15 years development processes.

Advanced anaerobic digestion can also involve thermal pretreatment of sludge with resultant changes to sludge characteristics that have challenged pump designs, as offered by other Biogas Plant EPC Contractors. This is surely wonderful news for all those that are keen to see innovation in the biogas industry, as there are advantages in the use of autoclaving pretreatment in the manner in which it enables efficient separation of recyclates in a much healthier working environment, and provides pastuerisation for a saleable digestate at the same time. The industry must be wishing this venture every success, and watching keenly to see how well the process performs, especially the plant’s peformance in removal of enhanced calorific value from the autoclaved organic content during digestion.

Imtech Process is another delivery partner offoring Advanced Digestion in the UK and is one of an increasing number of companies which are descirbed as having a proven track record with some of the leading water companies. Plus having experience in food waste digestion and practical experience in beneficially recycling Class A biosolids for agricultural use.

With increasing pressure to find alternatives to the landfill disposal of waste, in a new development, one UK water company is using advanced anaerobic digestion in its wastewater treatment process to generate biogas – and is using this in an on-site CHP unit. So, at a time of heightened concerns about waste, climate change and the need for cleaner energy, it is worth pointing out that not some of the news is good. New technologies are redressing the balance, and one of these is advanced anaerobic digestion (AAD). AAD will not turn muck into brass, or indeed gold, but it does offer the potential to transform the sewage treatment process from a simple clean-up to one that recovers significant quantities of energy.

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