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In Focus – Anaerobic Digestion and Sustainability

By 22nd November 2016 Blog, CSR, Sustainability No Comments

In the first in our series of In Focus articles, we take a look at Anaerobic Digestion and how it’s helping to produce cleaner energy whilst diverting waste away from landfill.

Anaerobic Digestion or AD has grown considerably in the UK during the last decade.

According to www.biogas-info.co.uk, there are now 316 operational AD plants in the UK with the capacity to process over 8.1 million tonnes of feedstock between them.


How Anaerobic Digestion works

Anaerobic Digestion sees organic feedstock consumed by micro-organisms in the absence of oxygen, producing methane (CH4) as a byproduct. In many ways, it is similar to the process that takes place in the stomach of a cow.

It involves four stages:

Hydrolysis - breaks down the complex organic matter – carbohydrates, fats and proteins – into simple sugars, fatty acids and amino acids. Carbohydrates, long chains of simple sugars, are broken down into single glucose molecules; proteins, long folded chains of amino acids, become individual amino acids; while fats, made up of head groups and fatty acid chains, have the latter part removed from the head groups and cut into smaller and smaller pieces.

Acidogenesis - sees those single sugar molecules, fatty acids and amino acids broken down further into alcohols and volatile fatty acids (like ethanol and propionic acid), with by-products of carbon dioxide, ammonia and hydrogen sulphide.

Acetogenesis - is the third stage: here, those volatile fatty acids and alcohols are converted again, this time into hydrogen, carbon dioxide, and acetic acid.

Methanogenesis – where methanogenic archaea convert the remaining hydrogen and acetic acid into methane, and more carbon dioxide.

Outputs of Anaerobic Digestion

Anaerobic Digestion produces two principle outputs: methane gas and digestate.

The methane can be used in a variety of ways.

For instance, it can be burned in an engine on site to generate electricity for supply into the national grid or heat for a related process. Or, it can be cleaned-up to remove contaminants like Hydrogen Sulphide and Ammonia and then, provided it meets the necessary specification, can be injected directly into the gas grid – where it can then be used in home heating, large-scale electricity generation or even as a transport fuel that acts as a cleaner-burning alternative to diesel in heavy goods vehicles.

The digestate is a nutrient-rich organic byproduct that’s left over at the end of the process and is typically removed and applied to fields as a natural fertiliser.

Process optimisation

Because Anaerobic Digestion relies on living organisms, getting it to work properly can be challenging.

For instance, if you vary the ‘diet’ of a digester, you can impact gas yields and digestate quality.

Fats and glycerol can be expected to produce as much as 700 Newton Metres (Nm) of gas per tonne of feedstock; fresh grass only around 60 Nm / tonne feedstock; and dairy cattle manure even less at between 10 and 20 Nm / tonne of feedstock.

Water content can also affect gas production, and even if fed with what appears to be a single, known and consistent feedstock, natural variability can also impact operational efficiencies.

Benefits of Anaerobic Digestion

Recent growth in the popularity of Anaerobic Digestion has undoubtedly helped to divert significant quantities of organic waste, particularly from the food and beverage sector, away from wasteful and potentially polluting landfill.

Biomethane used as a transport fuel in HGVs and buses not only helps to reduce Carbon Dioxide (CO2) emissions, but is also cleaner burning and can play an important role in improving air quality in towns and cities because it produces lower levels of Nitrogen Oxides and harmful particulates than diesel.

Electricity produced from burning biogas (as it’s also known) in Combined Heat and Power (CHP) engines is generally lower in lifecycle CO2 emissions than fossil fuel gas (but it depends on the feedstocks used and whether, like certain plant products, they sequester CO2 during their growth).

Notable drawbacks of Anaerobic Digestion

Like any industrial process or energy source, Anaerobic Digestion is not without its unwanted impacts. Notable drawbacks include:

Landscape impacts - the surface footprint of an Anaerobic Digestion site obviously varies dependent on its capacity, but 3-4 hectares is typical for many farm-fed installations. Waste-fed facilities are often much bigger, because they are geared-up to processing significantly larger quantities of feedstock, but they also tend to be located in industrial areas where there is a less noticeable visual impact. Farm-fed facilities can be quite imposing and create a permanent change to the local landscape.

Transport impacts – getting feedstock to the plant necessitates traffic movements, whether that’s agricultural plant and machinery (like tractors and trailers of spoilt crops), HGVs delivering food waste, and road tankers are often employed to take away digestate for spreading on local farms. These vehicle movements are year-round, and can be quite significant. A recent planning application for an Anaerobic Digestion facility in Gloucestershire forecast that annual deliveries of feedstock would be responsible for over 15,000 traffic movements a year, peaking at 159 a day in May. In the case of farm-fed Anaerobic Digestion facilities, many of these vehicle movements will typically take place on narrow rural roads.

Climate impacts – the Methane generated in Anaerobic Digestion contains Carbon which is released as CO2 when burned – whether that’s via the gas grid and used in large-scale Combined Cycle Gas Turbine (CCGT) plants to make electricity or in domestic central heating boilers. In the case of restorative feedstocks, like crops that convert atmospheric CO2 into Carbon via photosynthesis and lock it away, the lifecycle CO2 are likely to be much smaller, but for feedstocks like bakery and brewery wastes, it’s not so clear cut.  And then there are the pollutive effects of transporting all the feedstock and digestate.

Upgrading the gas - Biomethane contains a number of contaminants that make it unsuitable for injection into the gas grid, including Hydrogen Sulphide and Ammonia. These have to be removed, and CO2 needs to be removed or reduced. There are a number of techniques available to achieve these upgrades in gas quality and composition, including the use of chemicals and processes that add cost and complexity to an Anaerobic Digestion scheme.

Fugitive Methane emissions – as with any enclosed containment system that contains pipe connections and valves, there is a risk of fugitive emissions. Methane is a potent greenhouse gas, and so avoiding unplanned releases is important. A paper by the Intergovernmental Panel on Climate Change (IPCC) suggests that fugitive Methane losses in Anaerobic Digestion typically run at 5%. It also refers to research that indicates fugitive losses of unburnt Methane in CHP engines could be as high as 10%. System design, good operations and planned preventative maintenance are all factors in preventing or at least minimising fugitive losses.

Gate fees

If you’re paying to have your organic waste taken away and processed, then Anaerobic Digestion can offer substantial savings over conventional landfill routes.

According to the latest annual survey of ‘gate fees’ (the charge per tonne for processing waste) compiled by the Waste and Resources Action Programme (WRAP), “AD facility operators report the median contract AD gate fee is £15/tonne, with a gate fee of £0/tonne for organic waste supplied from municipal sources in a small number (6 out of 43) cases. […] interviews with the waste contractors affirmed
gate fees have been dropping in the last 6 to 9 months due to the constrained supply of food waste from local authorities and excess of capacity in some regions” and that downwards price pressure is expected to continue as a trend.

Building your own Anaerobic Digestion plant

If you produce a big enough quantity of suitable material, it may be worth developing your own Anaerobic Digestion plant. There could still be advantages to doing so even if you don’t quite produce enough to warrant the capital costs by opening-up unutilised capacity to other local businesses, farms and landowners.

You’ll need planning permission and a range of environmental permits (although, if you can comply with the stipulated requirements, you might be able to benefit from a Standard Rules permit) and can expect to spend between £3 and £10 million depending on the size of your scheme.

As well as saving waste costs, you’ll also benefit from generating some of your own energy and may also be able to benefit from subsidies and other government incentives aimed at encouraging the up-take of Anaerobic Digestion.

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