At a time of heightened concerns about waste, climate change and the need for cleaner energy, it is worth pointing out that not all the news is bad. Technologies are redressing the balance -- and one of these is Advanced Anaerobic Digestion (AAD).
AAD will not turn muck into brass, or gold, but it does offer the potential to transform the sewage treatment process from a simple clean-up to one that recovers significant quantities of energy.
In the Northumbrian Water region, in the north-east of England, there are more than 400 (437 to be exact) sewage treatment works that all produce varying amounts of sludge. This material has to be removed from every works but, inevitably, it is difficult to handle and, to say the least, rather smelly.
To make this sludge stable to further degradation and (nearly) odour free, Northumbrian Water Ltd (NWL) has long employed anaerobic digestion techniques for about 10% of its total sludge.
These technologies harness natural oxygen-free decomposition by which organic materials break down to produce biogas – roughly made up of 65% methane and 35% carbon dioxide – along with a much reduced residue of stabilized organic material. The latter can be safely deployed as fertilizer. In fact, by returning it to the soil in this way, nutrient and organic matter cycles that occur naturally are completed.
In the last five years, however, technology has advanced significantly and a technique has been perfected that can do much more.
Advanced Anaerobic Digestion significantly enhances the benefits of anaerobic digestion by separating and optimizing the key process stages used in more conventional digestion systems.
A More Sophisticated Process
There are two main pre-digestion processes used in AAD in the UK — thermal hydrolysis (the Cambi process) or enzymic hydrolysis (the Monsal process). Currently there are examples of each in operation and under construction.
Regardless of which process is used, the key to the AAD process is a phase that significantly enhances the breakdown of organic materials by, for example, breaking down cell walls. With thermal hydrolysis this is achieved by an initial high temperature of 165°C combined with high pressure (6 Bar) for less than one hour, or with enzyme hydrolysis this is achieved by phasing an increased temperature from 42°C to 55°C over several days.
The result is a far greater conversion of organic matter into biogas when the material is transferred into the anaerobic digestion phase. Following this digestion phase, there is a 50% reduction in sludge volumes, combined with the additional biogas/CHP- derived energy being produced, and ultimately a better quality bio-solids fertilizer.
One of the major benefits of this, of course, is that energy from biomass, including sewage sludge, are classed as renewable and therefore contribute to meeting Britain’s international commitments to address climate change.
Using AAD reduces the mass of material that is required to be transported off site and offers the benefit of nutrient recovery from materials that are presently wasted.
Indeed, some particularly difficult materials, such as food wastes under the Animal By-products Order (ABPO), need the conditions of AAD to render them safe.
One other benefit that is not to be sniffed at, AAD results in reduced odour.
The digested sludge cake remaining after the process will be a Class A biosolid – a safe and low odour product containing no detectable levels of pathogens, such as E. coli, and may be used as a valuable agricultural fertilizer.