Advanced Anaerobic Digestion: More Gas from Sewage Sludge

by Graham Neave

Northumbria, UK [Renewable Energy World Magazine]

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.

But it does more than that too.

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.

A New Sludge Strategy

With the obvious benefits AAD offers, NWL decided to invest in a complete new build AAD and CHP plant at its existing sludge treatment centre at Bran Sands on Teesside. The facility, on a 52 acre (21 ha) site, is the company’s largest, and treats sludge from Northumbrian Water sewage treatment works south of the river Tyne and in the Tees Valley.

The existing process at Bran Sands has served NWL very well since it was brought online in 1998. It involves the use of a thermal drying plant which dries wet sludge to pellets that have been used both as an alternative fuel and as a fertilizer. The downside is that the plant uses a lot of energy. The introduction of AAD will instead use the sludge to create energy and will reduce more than 500,000 tonnes of sludge — from the treatment of domestic sewage and industrial effluent from a population equivalent of 1.9 million people — to about 60,000 tonnes.

The methane produced in the process will be collected in 11 metre diameter biogas storage bags (similar to hot air balloons) before being used. The £33 million (US$50 million) contract to design, construct, install and commission the new facility was awarded to Aker Solutions E&C Ltd from Stockton in the Tees Valley.

The new plant will generate 4.7 MWe from the four on-site CHP engines. The engine heat recovery system captures a further 2 MWth, which is used to minimize the use of natural gas for steam production for the thermal hydrolysis process.

The process will also reduce Bran Sand’s reliance upon natural gas down to less than a tenth of previous requirements — from 17 MW to 1.4 MW.

Aside from Jenbacher, key equipment suppliers include Cambi, and Eurograde (boilers).

The energy recovered from the sewage sludge goes a long way towards making the entire wastewater treatment process energy self-sufficient, producing about half the requirements of the entire treatment works site at Bran Sands. This eliminates the need for large amounts of grid electricity and therefore has the dual benefits of cutting energy use and costs. Annually the advanced digestion facility has an annual output of 37 GWh, of which 22 GWh/year will be utilized to power the rest of the Bran Sands site. Financially, this equates to greater than £5 million ($7.5 million) in operational savings, which includes a renewable obligation certificate (ROC) contribution of £1.6 million ($2.4 million).

At Bran Sands, the processes also maximizes the efficiency of the solids loading for the anaerobic digestion phase. The thermal hydrolysis pre-treatment process begins with a sludge cake, produced by squeezing sludge to reduce the water content, which therefore provided the opportunity to review NWL’s sludge transport policy. By transporting cake wherever possible this avoids the wasteful transportation of large amounts of water associated with liquid sludge tankering.

These changes have resulted in a substantial reduction in the road miles associated with moving sludge.

Changing to AAD from thermal drying at Bran Sands, along with a planned change from lime stabilization at another NWL plant at Howdon on Tyneside, will reduce CO2 emissions by 62,000 tonnes a year for the group.

AAD has provided the company with a regional sludge management solution in line with their strategic direction statement, with the added benefit of a negligible odour impact on both the site and on the agricultural land when the residue produced is recycled for use as fertilizer.

There are regulatory benefits to take into consideration as well. Recycling treated bio-solids to agriculture is considered the best practicable environmental option (BPEO) by both the UK and the EU. The process produces an enhanced treated product that improves the public perception of recycling at a time when doubts have been expressed in some quarters.

The site is covered by Pollution Prevention and Control regulations (PPC), ensuring thorough monitoring of the total environmental impacts of the entire process and, when operational, the site will be registered with regulator Ofgem as a renewable electricity generation station.

To achieve these benefits does, of course, requirement investment — some £33 million [US $50 million] in total for a construction programme whose principal contractor is Aker Solutions E&C Ltd. There are a further 30 subcontracting teams and a total workforce of over 200 people.

Construction commenced in summer 2007, although the actual concept of introducing the new technology into Northumbrian Water began in 2005.

Much of the site construction work is now complete, with equipment already in place. Some 10,000 tonnes of concrete have been poured, a full 100,000 metres of cabling laid, along with 4000 metres of pipe work. Commissioning of the plant will begin shortly with biogas production commencing this summer. The full process and business benefits are due to be realized by the autumn of this year.

Construction of the new plant has involved the use of a very tight and complex scheme, which was only made possible through the integrated team approach of Northumbrian Water Ltd, the contractor and consultants working very closely together.

The AAD Advantage

The process that the Bran Sands AAD plant facilitates is not only environmentally friendly, it is economically attractive too. The plant approaching energy self-sufficiency not only reduces costs, it also shields the company from the impact of volatile and unpredictable energy prices. It further offers demonstrable operational cost savings and improves the efficiency of sludge management throughout the region.

In addition to the ever-important cost benefits, there are also significant operational benefits. The new AAD process allows reduced maintenance compared to the existing process, which has been operating on a ‘business as usual’ basis while the plant is being constructed. It continues to allow the utilization of existing sludge assets where cost effectiveness has been demonstrated and the current sludge drying facilities will be retained at Bran Sands as a strategic contingency back-up.

The final completion of the Bran Sands AAD plant (Teesside) is not the end of the process. NWL also plans to roll-out the sludge strategy to a second AAD centre at Howdon on Tyneside, see box panel on page 64.

Looking still further ahead, and aware of the growing synergy between the water and waste industries in relation to these processes, the company is actively investigating the possibility of co-digestion — the simultaneous digestion of compatible wastes — to understand the technical, regulatory and market implications. It seems that the Bran Sands development proves the old Yorkshire adage that ‘where there’s muck, there’s brass’, more advanced processes are now proving that where there’s muck there’s gas, and that is a valuable resource.

Graham Neave is a Northumbrian Water executive director and has overall responsibility for the Customer, Technical and Operations directorates.