Tuesday, February 24, 2015

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.

Thursday, February 19, 2015

US Rendering Plant Gets State-of-the-Art GasMix Biogas Mixing Technology

(Image: Courtesy Landia)

Landia’s GasMix state of the art biogas digester mixing technology has just been successfully installed at North State Rendering in Oroville, California for the best possible biogas yield. To achieve that, very thorough mixing of the bio-reactor is essential.

Only by achieving heterogenous mixing can the micro-organisms that produce biogas methane get at all the food they can extract from the substrate, and the GasMix system (which we have described previously here) is particularly ingenious in the way it achieves that.

The rendering industry is seldom publicized.  What it does is not exactly glamorous and little usually happens in the industry to promote more general interest.  However, at North State Rendering they now have something which is making them a star of the U.S. biogas industry.

(Image: Courtesy Landia)

Biogas Energy is the company that has been contracted to build an anaerobic digestion facility for North State Energy, where Landia's gas mixing equipment is helping to pay peak dividends.

They are now achieving uninterrupted biogas production, generating electricity 24/7, fuelling the rendering company's trucks, and creating heat to run their boilers. Plus, they have estimated that they will be able to reduce by 75% their diesel costs, by introducing a gas-cleaning skid to create a natural gas grid-ready quality biomethane, that is then compressed to become their own source of CNG fuel.

Rendering waste is a high energy producing substrate, so their bio-reactor produces a lot of energy. This means so that the economic benefit of this new plant is seriously cutting their business costs.

Not only that, at times when they do not need all the power to plant produces, they can further process and sell the spare energy, and the plant also provides a form of wastewater treatment ready for its discharge and use.

This has to be a notable first for the industry in North America!

(Image: Courtesy Landia)

Brian Gannon of Biogas Energy said recently that:

"For a rendering plant, biogas is a natural fit”.

“North State Rendering were looking for ways to cut costs, secure new waste supply contracts, and improve wastewater treatment, so creating their own on-site biogas facility was a wise move.  Fuel and energy are a significant operational cost for the business, so investing in technology that eliminates electricity bills, slashes diesel costs and reduces natural gas imports all makes sense.  Modifying the anaerobic digestion process to integrate with a rendering plant took some fine tuning, including a very positive modification to the digester’s mixing system, but now, we see how we got it right”.

Renderers commonly find that food waste from kitchens, grease trap waste, plus restaurant and food processing waste, can be expensive to render, and yet it is ideal for anaerobic digestion.  The biogas plant allows them to divert such wastes from their existing and new waste disposal contracts, freeing up rendering capacity for rendering more suitable materials.

By configuring their waste reception facility flexibly they now divert their existing incoming materials to the most efficient type of process, either to the rendering plant or to the biogas plant.

The digester processes materials such as, food and yard waste and high-liquid content grease trap materials.  Wastewater produced by the rendering process is also sent to the biogas plant, as is also any dead livestock tissue, after suitable preparation. This is particularly useful during hot weather, when rendering may become difficult due to the rapid degradation of this material.

Brian Gannon also said:

“We had been using submersible propeller mixers inside our main digester, but with our re-design of the tank, we switched to a new system that meets all of our needs.  One of the main issues with submersible digester mixing systems is that the equipment is inside the tank, which from a maintenance point of view is a nightmare.  The downtime caused by having to open the digester to lift the mixers out for repairs and maintenance caused serious process interruptions and safety issues."

“We now have a Landia digester mixing system, which is mounted externally, so maintenance is much easier.  Even during commissioning when the AD biology was at a delicate stage, we were able to carry out some tweaks without any interruption whatsoever to the biogas production process.  With submersible mixers we would have had to start over again, which would have been very expensive and used up a ton of manpower”.

Ease of maintenance is not the only benefit from the Landia GasMix system, Brian also explains that the Landia (patent-pending) system is able to agitate and mix into the entirety of the digester tank (even one as big as this - at 64 feet high!), whereas competing systems frequently fail to prevent a hard-pan (or "crust") from forming on the surface of the tank’s contents.

Brian also said that:

“The anaerobic digestion facility is designed to process a very wide range of feedstock”, he said, “so its pumps and mixers have to cope. The Landia chopper pumps, which form part of the GasMix system, are absolute troopers. They just keep on working.  We wouldn’t be achieving what we are now without them”.

The GasMix system installed comprises, two 30-HP chopper pumps and a self-aspirating system. That, coupled to a clever sequencing control system ensures that the action of the meathnogenic micro-organisms is much more effective in reducing the volatile organic solids content to produce more methane, and does it in a much reduced time period.

Landia’s GasMix which is uniquely designed specifically for AD and biogas reactors, is simple to operate, and has a low energy requirement, because it only needs to run for up to 30% of the installed capacity of the equipment, in normal use.

Brian Gannon concluded:

“Renderers have a big head-start over other companies trying to develop new waste-to-energy facilities.  Unlike newcomers, renderers already have the necessary permits in place to process waste material.  They also have the trucks to collect waste, and the energy consumption that biogas facilities can help fuel."

“As energy and fuel prices climb and wastewater discharge fees escalate, waste processors can turn waste into an asset.  For Biogas Energy, our experience at North State Rendering and the introduction of Landia’s GasMix digester mixing system means that we can help our clients generate renewable energy with a system that maximizes production while facilitating operations and maintenance”.

For more information contact:
Landia: www.landiainc.com
T: +1 (919) 466 0603
Biogas Energy: www.biogas-energy.com
T: +1 (815) 301 3432

Anaerobic Digestion Community Website