With a growing impetus to divert food waste from landfill anaerobic digestion is both opportunity for the waste industry, and a technical challenge. Since acquiring Monsal and it’s advanced anaerobic digestion technology back in 2014, GE has developed a number of such facilities. WMW talks to GE’s Michael Theodoulou to find out how.
WMW: What is Advanced Anaerobic Digestion and how does it increase biogas production from food wastes?
Michael Theodoulou: From a food waste standpoint it’s the whole integration of the digestion process – the pasteurisation process and the mesophilic digester, as well as the way in which we integrate between the organic separation and the digestion plant.
Typically at an integrated food waste digestion plant there are several processes and technologies. You need to effectively separate the organics from the containments such as packaging, plastics, metals and grit. That’s the first step in the process to produce a slurry which can be sent to the anaerobic digestion part of the plant where that slurry is converted into biogas and a resultant digestate.
There's a holding buffer tank between the scheduled separation process which may operate 12 hours a day five days per week and the consistent feed to the digester 24/7.
WMW: We hear a lot about the benefits of using digestate as a fertiliser, does this require any additional treatment?
Michael Theodoulou: Within the digestion part of the plant you’ll have several process steps depending on what the nature and quality of the digestate needs to be. For instance it’s very common to put a pasteurisation process onto the system so that you can kill pathogens prior to it being used as a fertiliser. In some parts of the world that may not be a requirement from a regulatory point of view.
In our approach to pasteurisation we apply a hold to the slurry prior to digestion of 1 hour at 70 degrees (time and temperatuare are adjusted based on local regulations), in order to cover the full pathogen kill required by regulations.
At the back end of the digestion process to produce a quality fertiliser product you can incorporate a dewatering process to produce a cake. The water removed in the dewatering process can be recycled back into the digestion plant for dilution of the incoming food waste or it can be polished for discharge into the environment
WMW: What are the key factors to consider when designing and planning a food waste AD plant?
Michael Theodoulou: We typically use the same design approach, as long as it is a comparatively solid food waste, but we look at feedstock characteristics and the variation of the types of feedstock coming into the plant. Depending on that we would potentially look at are different ways we would design the separation process as well as different ways of introducing different blends of feedstock such that you get consistent performance out of the digestion plant.
The viable size for an AD plant depends on a number of factors including the quantity and quality of readily available feedstock, the level of feed-in-tariffs, or incentives and the potential gate fee for incoming waste. Where government incentives are favourable anaerobic digestion is commercially viable for food waste down to around 30,000 tonnes per year, where those incentives are less favourable the plant would need to be a larger, maybe around 60,000 tonnes per year.
WMW: How do you optimise the digestion process to achieve the best yield of biogas?
Michael Theodoulou: First and foremost the best yield from the digestion process comes from the effectiveness of the separation process. You need to maximise the amount of organics in the waste stream and have a slurry that is homogenous and consistent in terms of particle size and strength. The digestion process doesn't like a lot of variability when you're feeding it so you need to produce a consistent, homogenous, even strength slurry to maintain biogas production.
The other important part is to minimise the amount of non digestible inerts which go in. You need to minimise the amount of plastics and grit that get through because those materials take up volume in the digester..
You'll definitely have a more effective plant if you have separate food waste collections, you have less complications with the processing when contamination levels are less than 30%. When you have organics comingled with the residual waste, the amount of contaminants is over 60%, so the amount of separation you need to do is much greater.
WMW: Over the past few years we’ve seen an increasing number of gas-to-grid operations coming online. From a developer’s perspective how do these projects compare to CHP?
Michael Theodoulou: We have started to see a shift towards upgrading the gas into biomethane for gas to grid injection. It's definitely something that we're starting to see with the projects that we have in the UK and also in North America where it's starting to become a preferential solution for biogas utilisation.
From an investment standpoint we've seen that the cost of implementing biogas upgrading compared to CHP (Combined Heat & Power) fairly even, but you do need to condition the biogas more for injection compared to CHP
Where I am in Canada it’s getting a lot of traction because of the elimination of greenhouse gas emissions by offsetting the non-renewable natural gas. You definitely have a situation in many parts of the world where there's a carbon offset market that allows for high revenues in gas to grid projects.
WMW: Processing waste can be tough on equipment. What maintenance steps should operators take to avoid downtime?
Michael Theodoulou: In terms of maintenance the biggest consumer is the front-end separation. You're dealing with organic materials which can sometimes be laden with hard heavy materials such as metals or rocks. You do get wear and you do need some maintenance to maintain the effectiveness and the uptime of the separation technology. On the digestion side you have to maintain pumps and mixers and mechanical rotating equipment, but it's relatively low compared to the separation side.
On the biogas side, if you're using a CHP engine then routine maintenance and servicing is beneficial to consistently keep the output of those engines at the maximum. For example, that could be changing the oil and spark plugs every 2000 - 3000 hours.
For biogas upgrading and injection plants you have to maintain the method of cleaning the gas and that can sometimes consume media and chemicals.
WMW: We hear a lot about new AD developments in Europe, but there is also a lot going on in North America. Is the time right for AD to take off there?
Michael Theodoulou: I see North America starting to implement certain strategies which have the ability to drive the developer market in terms of building a financially viable solution to handling organic waste.
We're also starting to see a trend of municipalities which have control over residential organic waste looking towards anaerobic digestion as a solution. That’s because it not only reduces the impact on the environment in the form of extending landfill lives and reducing the emissions that would occur there, but also to produce renewable energy whether that be electricity or biomethane.
Michael Theodoulou is senior product manager in anaerobic technologies at GE Water & Process Technologies.
GE recently signed a definitive agreement to sell its Water & Process Technologies business to SUEZ, a global services and solutions company with operations primarily in water and waste management. The deal remains subject to customary closing conditions such as regulatory approvals and appropriate consultation.
