Here are our top five tips for boosting your biogas yield…
1. Take professional biological advice
There are several reasons why your gas output could be lower than expected: from sub-standard feedstock or poor feedstock management, to trace element deficiency or inhibition of the biological metabolic process. Every digester is different and you won’t be able to remedy the situation until you know what’s causing it.
A specialist biological assessment of your plant will allow you to determine the cause of inhibited gas production, and the best remedial action to take to bring the biology back to full health and quickly restore gas output. Most reputable consultants will be happy to offer a free initial biological assessment, enabling you to make an informed decision and avoid the ‘trial and error’ approach to increasing your biogas yield.
2. Select the right diet
The best performing plants are those which are designed around their feedstock – not the other way around. After all, there’s little point investing in a digester designed to operate on 100% maize if you are in the north of England, where maize is difficult to grow.
However, even the most meticulously planned plants can sometimes hit dietary problems. A poor harvest or being let down by your supplier can leave some digesters hovering around the 70% efficiency mark. In these cases, sourcing an alternative, readily-available and high yielding feedstock is crucial to bridge the gap. AD Super Energy is one such example and is ideal for boosting gas quickly; nutritionally well-balanced, it is the perfect complement to many diets. Other good options range from high protein wheat syrup to rapidly digestible whey permeate and even fibrous products such as draff and brewers’ grains.
As with any dietary modification, due consideration needs to be given to the type of material being introduced. Highly digestible materials bring a greater risk of biologically shocking your digester, so a slow-digesting, fibrous material could be perfect to give stability. Likewise, some of the greatest yielding feedstocks are high in protein or fats, which can cause complications at high inclusion rates. A proper nutritional assessment should be carried out on your existing diet before taking on new material. Seek advice from an expert AD nutritionist if you are in any doubt.
3. Optimise your feedstock
Whether you are operating an agricultural or food waste AD plant, correct management of your feedstock is key to maximising its biogas potential.
If you are processing silage, it is vital to preserve its energy (thereby maximising your biogas yield) by preventing aerobic degradation and symptomatic heating in the clamp. This can be achieved in a number of ways:
• Ensure your clamp is well-compacted (a good rule of thumb is 250kg of dry matter per m3);
• Take your silage evenly from the clamp face. Avoid leaving rough edges which will allow greater oxygen penetration and further aerobic degradation;
• Only remove what you need – don’t be tempted to take a week’s worth of feed and leave it in the feeder or in a pile on the ground, as it will degrade and yield less gas;
• Aim to reduce your clamp by a depth of 2-3m per week;
• Peel back sheeting only as far as required for feeding (i.e. expose as little of the top as possible).
In addition, silage additives can help to preserve the quality. For example, Silasil Energy XD is accredited by the German Agricultural Society (DLG) for reducing storage losses and increasing methane yield. It also protects against yeast and moulds, and reduces fermentation time in the clamp to as little as two weeks.
If you are processing food waste, the same principle of reducing aerobic degradation applies. In an industry where operators are increasingly paying for waste, a good turnaround on site will ensure you maximise the value from your feedstock. Incorporating a system whereby incoming material is logged and utilised within as short a timeframe as possible will pay dividends. Once delivered, avoid moving material as this introduces oxygen to the food, causing it to degrade and lose gas value.
4. Consider using additives
If a plant is lacking in vital trace elements, it will yield less gas and suffer process disruption. Through detailed laboratory analysis, biological experts can determine whether a plant is nutritionally deficient and prescribe the right trace element additive formula to get it back to full health.
Operators should avoid purchasing off-the-shelf supplements without first knowing the biological make-up of their digester. Laboratory testing and detailed understanding of the results are crucial, as an excess of trace elements can be just as harmful as a deficiency. In this way, a bespoke supplement tailored to your digester’s needs can be developed, optimising your plant’s efficiency in the quickest time.
For plants using high nitrogen feedstocks, such as food waste or chicken litter, an ammonium binder such as BC.ATOX Ncon can also prove invaluable. Nitrogen breaks down into ammonium and ammonia; the latter being toxic to bacteria. Left untreated, this is detrimental to the AD process and will hinder biogas production. An ammonium binder will ensure the operator gets the best out of their substrate and prevent acids from building up.
For plants using fibrous feedstocks, such as grass, maize or wholecrop rye, the use of enzymes may pay dividends. Adding the right enzymes will expose the cell membrane, speeding up digestion and improving gas yields. Again, it is important to first consult an AD specialist who can test your feedstocks and prescribe the correct enzymatic blend.
5. Don’t ignore biogas leaks
Often forgotten, the issue of biogas leakage is more widespread than most operators realise. In fact, over the last eight years, 85% of the 964 plants we have surveyed in the UK and Germany were suffering from biogas leakage.
Identifying and fixing a leak is a simple and affordable process which should form part of any responsible plant operator’s ongoing maintenance programme. Not only can it prevent a more serious and costly incident from occurring, it often results in a 12-month payback on the survey price.
Finally, once your plant is running at its optimal level, you’ll want to keep it there. Regular screening and analysis of your digester contents, backed up by ongoing support from an expert AD biologist, will enable you to make informed decisions going forward, maintain a steady gas output, and pre-empt any problems before they impact your biogas yield – and your profits.