What is a Biogas Reactor

For those that ask "What is a biogas reactor" the definition we use is as follows:
 The biogas reactor within a biogas plant (anaerobic digestion plant) is the vessel or vessels in which anaerobic treatment technology produces two main products.

 These are:

(a) a digested slurry (digestate) that can be used as a fertilizer, and

 (b) biogas that can be used as an energy source.

Biogas is a mixture of methane, carbon dioxide and other trace gases which can be converted to heat, electricity or light.

Small-scale Biogas Reactors

Small-scale biogas reactors are typically designed to produce biogas at the household or community level in rural areas.

In most Small-scale biogas reactors the airtight reactors are buried in the ground and are filled with animal manure from the farm and house. Kitchen and garden wastes can also be added and toilets can directly be linked to the reactor for co-treatment of excreta.

Schematic of a small scale Biogas Reactor Large Commercial Scale

Biogas Reactors Although the process in large scale commercial biogas reactors is the same as in the small ones.

Large scale commercial biogas reactors vary from the digesters built to service a small farm (i. e. "small" in western terms) up to the digesters built to digest the sludge from a large sewage treatment works. In all cases the definition of the reactor is the same, although the majority of large scale biogas reactors are made from circular steel tanks, which are lagged for the necessary heat retention.

 A More Detailed Alternative Definition for "What is a Biogas Reactor" 

 A biogas reactor is an airtight chamber that facilitates the anaerobic degradation of blackwater, sludge, and/or biodegradable waste (e.g. animal manure, kitchen and garden wastes).

It also facilitates the collection of the biogas, a mixture of methane (CH4) and carbon dioxide (CO2) produced in the fermentation processes in the reactor. The gas forms in the slurry and collects at the top of the chamber, mixing the slurry as it rises.

 The pressure exerted by the rising gas can be used to transport the gas to the collection vessel or directly to where it is going to be used. The digestate is rich in organics and nutrients, almost odourless and pathogens are partly inactivated.

 Biogas Reactors Are Living Organisms: Operator Responses When Biogas Output Drops 

Biogas Reactors work well most of the time when the operator uses feedstocks with which the plant operator has experience, but when new feedstocks are introduced, or even when familiar feedstocks are used, the biogas quality and/ or quantity may drop. It is necessary to take action when the biological process shows signs of imbalance. In this context the operator has only a few options available at most AD plants.

The operator can reduce the feed rate, or cease feeding in which case the reactor may stabilise itself, dilute the biomass, or add fresh or degassed biomass to rectify an imbalance in reactor chemistry where the cause is known.

 Biogas Reactor feeding changes are an important tool to correct reactor slowdowns, but it is difficult to give a precise recipe for doing it successfully.

 Most biogas plants have, to a greater or lesser extent, been subjected to operational disturbances, and several plants have experienced that the process at worst may break completely.

Such collapse can have serious financial consequences for the plant, and Biogas Reactor chemistry problems are an issue that focuses the minds of all AD plant owning business men.

 Downward drifts in biogas output, and even a collapse in the biogas produced in a biogas reactor, are often linked to the types of biomass that the plants are fed with, and unfortunately it has been found that a combination of manure and other organic waste of varying strengths and composition, can become a dangerous cocktail.

 If there is a lot of protein and fat in the mixture in the reactor, it can cause high concentrations of ammonium and long chain fatty acids (LCFA) that may inhibit the process.

The best way to avoid such inhibitions is to have a thorough knowledge of the type of biomass that the plant is supplied, both in terms of the chemical composition and how the biomass breaks down in the plant.

 In addition, it is important to accurately measure the different types of waste entering a biogas reactor, as well as detailed process monitoring is very important.

 Unfortunately, it is far from always that the plants have the opportunity to feed consistently with the same types of feedstocks to provide the reactor with stable conditions.

The ability of the biogas plant operator to find consistent feedstocks are limited in number and size by the availability of materials, and the operators will therefore often be forced to mix.

 Those biogas plant which are fed from feedstock produced through the operators own business activities (e.g. on-farm manure) have the best chance of ensuring consistent feedstock availability.

 To provide a service for the disposal of organic (biomass) wastes for long-term clients makes the need to accept variations in the composition, quantity and delivery frequency of the different types of waste inevitable.

 This means that the plants may be forced to supply a certain type of waste to the biogas reactors at a time when it may cause problems with the stability of the biological process. Finally, a complete monitoring of all process parameters is a time-consuming and expensive solution, which means that monitoring at most plants is inadequate.

 As a consequence of these constraints, various drift disturbances occur at biogas plants. The question is therefore:

What can managers do to quickly restore the process when they detect a decline in gas production?

At the Department of Environment and Resources at the Technical University of Denmark they have conducted a series of trials to restore the biogas process in laboratory reactors.

The experiments supported by the Energy Research Program were primarily based on dilution of the biomass with either water, manure or degassed biomass.

 The process was inhibited by adding either ammonium or LCFA to four reactants with cattle gel.

he outcome of the different strategies as well as a description of the experiments are discussed below.

Ammonium Inhibition of Biogas Reactors

After inhibition with ammonium, it was found that the most effective method of recovery of the process was to replace half of the reactor content with degassed biomass or fresh cattle gel.

 With that strategy, it took about six days to return to the original gas production, while it took 10-11 days if 50 percent water was added instead or the daily amount of manure was reduced.

After almost six days, a significant increase in gas production was recorded in the reactor, which added 50 percent fresh cattle gel.

Thus, throughout the recovery period, this reactor produced between 42 and 74 percent more gas than the other reactors, which is associated with the additional amount of organic material that the reactor was supplied in the form of fresh slurry.

 However, one should also stick to the evolution of the oxygen level during the restoration, as it gives an indication of how stable the process is.

Here it was found that the increase in the acid level was somewhat higher by the addition of fresh manure than by the addition of degassed biomass.

Therefore,, much evidence suggests that the most effective and safe method of ammonium inhibition is a combined addition of manure and degassed biomass.

The worst approach is to do nothing and only add the daily amount of manure or dilute the biomass with water.

The supply of water provides a faster recovery than if it is not intervened, but on the other hand, this strategy leads to relatively low gas production.

 The strongest increase in oxygen levels was recorded in the reactor where no intervention was taken, indicating that the process here was more affected than in the other reactors.

 LCFA Inhibition

Inhibition of the biogas process by the administration of LCFA showed substantially the same picture as inhibition with ammonium. via www.biopress.dk

Confronting the Fear of a Biogas Digester Turning Sour

Nobody talks about what happens when a biogas digester turns sour. It's costly, can mean turning away clients, and the extra hydrogen sulphide produced can be very dangerous...

We must start confronting the fear of a Biogas Digester Turning Sour!

As more and more biogas plants are built, many organisations and plant operators will be tasked with the responsibility for reliable operation of these intricate pieces of equipment. There is a danger that unless the biogas industry airs its past errors in public, the anaerobic digestion industry will be set back by lack of understanding of the nature of the problems that can occur if biogas plants are not continually monitored and well run.

It is perfectly natural for a young industry, such as this to want to move forward and not talk about past mistakes, but on the other hand it is only by understanding the past that repeating past errors can be avoided.

For that reason, in this article we will discuss that most unpleasant outcome of poor or inexperienced biogas plant operation, which is known as a digester turning sour.

What is actually meant by this term, is when a biogas reactor is allowed to diverge sufficiently from its target pH, and intended alkalinity concentration for the methanogens (methanogenic organisms) to be killed (in effect poisoned), and other unwanted organisms to thrive. Significant quantities of dangerous hydrogen sulphide are likely to be produced in such circumstances.

To describe this as a "fearful event", is not an overstatement. The consequences in terms of lost production from a biogas plant in these circumstances are substantial. In fact it is quite possible that plant managers and/ or operators will have faced disciplinary action due to such events.

Those that sell these plants naturally like to provide the impression that these plants run themselves.  While they may appear to do so when run by experienced operators, behind the scenes there is always activity. Biogas plants should never be considered to be "black boxes" in which waste enters and methane leaves like clockwork. It simply cannot be like that!

These are complex biochemical reactors. Those that study biochemistry and microbiology will appreciate that there are at least three stages taking place, each of which has to progress successfully before biogas is produced. Phase changes are needed from solids to liquid to gas, and the right healthy micro-organisms need to be present at every stage.

The equipment can fail in a multitude of ways. Sensors can lose calibration, but still appear to be working. Pumps can, at times appear to be running but are in fact delivering no flow.

A delicate balance needs to be maintained and while it is, all goes well. However, feed materials are always changing both in their nature, and seasonally. This means that regular monitoring of digester health is always absolutely essential, and beyond that so is a proactive plant operator needed, to ensure that manual and automatic adjustments are carried out, hour by hour, day by day, year in, year out.

Wise biogas plant designers/contractors install automatic equipment and train operating and maintenance staff, to maintain that delicate balance, as a matter of routine.

But, it is vital that management and staff at every biogas facility remain vigilant, because should a digester fail and turn sour it is a lengthy procedure to bring it back to health. The worst case scenario is that the whole digester tank has to be dug out, and the whole biological commissioning process started again. The consequences of this in terms of cost, the ability to comply with contractual duties, and lost goodwill, are massive. Not to mention the dangers of the likely odour escape, the health and safety of personnel, and the risk of polluting the local environment while disposing of the contents of a "sour" reactor, which hardly need stressing.

Short of that, in most cases when it happens, it is caught in time to implement procedures to bring the reactor back to a healthy condition, while retaining the substrate in-situ. This is a slow process and may take 4 weeks or more to achieve even the recommencement of substrate feeding feed, even at a low fow rate.

Subsequently, over some weeks, the feed flow rate must be progressively increased. This must be done rapidly enough to encourage the growth of a healthy compliment of fermentation micro-organisms, but never so rapidly as to cause organic overload. Organic overload could push the reactor back into the chemical conditions which caused the problem originally, so care is needed throughout.

Once the bulk of the methanogenic organisms are lost, for example, they must be replaced. The methanogens are slow growing and have to be teased back into health, over a long period when no treatment can take place. In short, the microbiologcal system has to be allowed its own time, to recover itself, and that process cannot be rushed.

In view of the large loss of revenue and inevitable disruption which would be caused to the waste producers if their waste could not be removed by the biogas plant operator, the avoidable event of a sour digester needs to be continually borne in-mind by all those involved in the industry.

Monitoring and control systems are improving all the time in reliability and sophistication, so with time the threat is becoming easier to manage. However, the AD industry must never forget the consequences of a sour digester.

It is only when organisations in this industry continuously confront the fear of a sour digester, which should be instilled in the culture of all biogas companies, that it can be avoided. That mildly felt apprehension, needs to be ever-present throughout the company from the managing director, at all levels down to the pump fitter.

Some "fear" should especially be felt by the accountant/ maintenance budget holders who might otherwise require that essential maintenance be deferred to improve company cash-flow, just for a few months, but with dire consequences.

Paradoxically, it can be the best run biogas companies which fall hardest. It is perfectly possible for a site team to make biogas plant operation look to higher-management to be simple, and by achieving reliable plant operation for many years, to result in a loss of understanding of the duties of the plant operational staff. This is easy to creep-up on an organisation over time, and after many staff changes.

A gradual erosion of respect for the work of the site operational staff, can easily lead to corner-cutting. This can reduce monitoring and maintenance, while the site staff suffer in silence, mending and managing on reduced man-hours, and pared-back budgets. Eventually, if not corrected, this can result in a crisis, and a large plant failure.

Yes. Even the very best run companies can fall prey to this...

So, our conclusion is that the "black box" concept of a biogas plant, must always be held in-check by a willingness not to brush this age-old problem of biogas plant operation "under the carpet". You can never switch-on a biogas plant and walk away!

Instead, all in the anaerobic digestion industry must in their own ways, continuously remain open to the fear of biogas plant micro-organism failure (a "sour" digester), when operating conditions stray a long way away from healthy conditions for the unseen microbial populations, which are essential for plant operation.

Achieve that, and the problem doesn't actually recur - and the biogas industry will thrive.