Wastewater Anaerobic Digestion: challenges and opportunities

Using the right technology, in the correct way, and maintaining it properly, are key to making more of your wastewater anaerobic digestion (AD) combined heat and power (CHP) unit and combatting inconsistent feedstock.

An energy source that will never end; one that is rich and fertile for combustion? Sounds perfect. Green creds? Check! Financial benefits? Check! Job done! … Well, almost. The neatness of the solution sadly belies some underlying problems.

Firstly, the basics. AD is the process through which we harness biogas produced by the natural degradation of organic matter (which, in this case, is human waste). CHP engines combust this gas to generate heat and renewable energy. This energy can then be used to power your own facilities, neighbouring sites or sold to the National Grid.

Wastewater Anaerobic Digestion: challenges and opportunitiesAs an energy source, wastewater should be one of the best. In fact, it boasts almost ten times the energy needed to treat it. It also creates a socially valuable end-product from the two million tonnes of sludge produced every year, whilst reducing sludge solids content by up to 30 per cent.

What’s more, it also makes financial sense, as a facility can deliver noteworthy savings with a typical payback of 12-18 months. However, it is not without problems, which require careful consideration and planning.

Feedstock quality

You cannot really control what gets added to wastewater; the day, date and geography of the source all create variables. For example, feedstock at Christmas has richer food wastes and greater alcohol content than most other times of the year, and therefore generates a greater volume of methane gas, which is great for an AD plant.

Other variables include rain, which dilutes feedstock, resulting in less combustible gas. Being in Britain, there is not much we can do to stop the rain, so how do we address the problems these issues raise?

Wastewater Anaerobic Digestion: challenges and opportunitiesThe quality of the feedstock itself cannot be easily altered. But, you can counterbalance its impacts in the calibration and maintenance of your CHP engine, which is the beating heart of your AD site.

The most effective facilities see CHP engines operating around the clock, 365 days a year, 24 hours a day. However, any system operating to a continuously high intensity will need regular inspection and maintenance to avoid mechanical issues and ensure efficient operation. Poorly maintained engines can see efficiencies fall by as much as 20 per cent – which could be the difference between profit and loss.

Improving performance

We have seen countless CHPs running at less than 50 per cent performance as a result of poor control unit calibration, wrong air/methane ratio entering the combustion chamber, wrong ignition timing and poor fluid choice. Fortunately, simple maintenance can solve most of these problems.

Wastewater Anaerobic Digestion: challenges and opportunitiesYet another way to counteract feedstock quality is oil choice. Broadly speaking, the better your oil, the more reliably your engine will run and the life expectancy of the asset will be extended. My advice is to prioritise high quality oil to ensure your unit can run at the most efficient output. An additional benefit of good quality oil is that it requires less frequent changes, which in turn means less down time. The same is true of using high quality fluids and lubricants – again quality speaks volumes.

‘Seeing’ as well as controlling your engine is also a highly valuable asset. Your control system, often a touch screen unit, will provide visual detailed analysis and an easy way to change the balance between gas and air. If your system does not have this facility, it may well be worth upgrading to ensure you are making most of changes in gas quality.

By keeping your CHP equipment in top condition, you can minimise system downtime, which has the potential to cripple a plant, destroy energy production and combust your return on investment figures.