Close monitoring of what is going on inside an anaerobic digestion (AD) tank is essential to identify and resolve problems, prevent expensive equipment breakdowns and maximise biogas outputs and profit. Richard Braybrooke, director at specialist AD operations company Eco Verde Energy (EVE) and David Lisle, technical sales executive at Omex Environmental, reveal their tips, tricks and advice on how to get AD plant biology right.
To start, Richard shares a simple rule of thumb for optimising the biological processes inside an AD plant.
“For efficient digestion, the feedstock which goes into the tank needs to be retained there as long as possible, to maximise the biogas yield and reduce the volume of digestate pumped into the lagoon,” he says.
“However, with the number and complexity of chemical and biological processes going on, there is scope for things to go wrong.”
He stresses that operators need to monitor the biological state of the mix in the digester, to understand whether it is healthy, or if an early intervention is required.
A visual check is a simple but important step to monitor the health of AD plant biology.
“Look into the digester at least once a day. Check for floating unmixed lumps of feed or layers, observe the level of movement, see if there are visible bubbles of methane rising to the surface and ask yourself whether it’s looking as you would expect,” advises Richard.
On at least a weekly basis, Richard suggests testing the FOS/TAC ratio and the proportion of dry matter in the tank. FOS/TAC is the ratio of volatile organic acids to alkaline buffer capacity and is an effective measure of stability in the digestion process. There should be very little variation in FOS/TAC measurements from week to week, and each plant and digester will have its own optimum range.
“The FOS/TAC test is very sensitive, so it’s best if one person takes responsibility for carrying out the test, to ensure a consistency and avoid false fluctuations,” says Richard.
“A change in gas quality is another easily spotted indication that something isn’t right,” he adds.
“Any unexpected change should be investigated. Finding a problem early can result in it being resolved with minimum disruption, whereas leaving an underlying issue unchecked can lead to expensive failures and downtime.”
Richard explains that healthy biological processes start with good quality feedstock.
“When crops are grown specifically for use in an AD plant, it is critical they are grown to a high-quality standard to ensure a maximum output of biogas.
“At Ellough AD plant, one of the sites operated by EVE, we work closely with our growers to ensure the crops are grown to have the optimum level of dry matter when harvested.
“For this to work in practice, it’s essential to develop good channels of communication with growers, and for any quality requirements to be agreed in their contract. If the contract only includes tonnage of crop required, you can’t necessarily expect a quality crop as there’s little incentive for the grower.”
Richard adds that the farmers supplying Ellough also receive digestate from the AD plant, which enhances their soil quality by adding organic matter and nutrients, contributing towards the development of a circular economy in which no resources are wasted.
On site, the way crops are stored in the clamp is important to maintain feedstock quality. “Good clamping to compress the feedstock is vital, so it remains well preserved. This means it will be more digestible and retain the maximum gas potential when it enters the digester,” says Richard.
“When cutting into the clamped feedstock, it is essential to achieve a clean cut, to prevent air getting in. When feedstock is exposed to oxygen in the clamp it triggers an aerobic reaction, and the material begins to decompose, so energy is lost before it even enters the digester.”
Richard highlights that a priority for EVE is to ensure operators and plant managers at all EVE-run sites have received the right training to enable them to understand the biological processes and the impacts of different feedstock types used on-site.
“Prioritising healthy biology inside the digestion tank keeps a plant running smoothly, and minimises the occurrence of the less desirable parts of the job, such as the 3am emergency callouts,” says Richard.
“We find that when our teams learn more about the process we start to see improvements in performance and they become increasingly engaged. Omex Environmental provide great support with training and assistance, and are a valuable asset to our armoury.”
As a technical adviser, David is familiar with the scenarios which can lead to a disruption to the biology of an AD plant. For example, when equipment breaks down or is shut down for a planned repair, plants are starved by vastly reducing feedstock input while the required maintenance work is occurring.
“Once the equipment is back on, the temptation is to bring the feedstock input back up as quickly as possible, to bring back gas production to normal levels. However, this can destabilise the biology and effectively gives the plant indigestion,” says David.
“A steady rate of increasing inputs can avoid this. Plants with a stable biology and adequate trace elements in the digester can be pushed a bit harder in this scenario, while a plant with low trace elements cannot, as a build-up of acids will occur.”
Another common pitfall is to attempt to run an AD plant at a higher temperature than it was designed for, without fully considering the implications.
“There is currently a trend towards running AD plants at higher temperatures, but I question whether it is worth it,” says David.
“At a higher temperature it is possible to get gas from feedstock quicker, but the biology in the tank is less stable and things can go wrong quickly.
“I’d recommend considering whether monitoring systems are set up to spot problems quickly enough and checking limitations of all technology. Some equipment may be unsuitable for use at temperatures above 45°C, and not adhering to this can lead to breakdowns.”
David highlights the importance of taking a holistic view when considering potential changes to an AD plant system.
“Look at the impact of a change on the daily operational cost, not just in terms of output gained or savings made,” he says.
“A free feedstock can become an expensive feedstock to an AD plant if it is poor quality and leads to lower outputs and expensive breakdowns.”
Use of food waste as a feedstock is beneficial to the environment because it provides a means to produce energy from waste and keep food waste out of landfill. However, it is challenging to operate these plants, as there is little control of the types of waste coming in.
“With inconsistent inputs it can be more difficult to maintain a stable biology inside the digester,” says David.
“But, operators can find out what types of waste are coming in and counter the potential problems. For example, if the feedstock coming in is volatile, add it very slowly. Or, if it is high in sulphur, dose it with iron to counteract the effect.
“No matter the feedstock type, the principles behind optimising AD plant biology remain consistent. Know what you’re putting in, observe how the process is performing and carefully consider the impact of any changes to management on the whole process of operating the plant,” he concludes.
As seen in Bioenergy Insight.