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