25th November 2015, by Martin Wells, Institute of Water
The end product of Thames Water’s sewage treatment works is getting a brand makeover, as Martin Wells reports
One such disposal route for water companies, especially those serving large urban areas, would be to load countless tons of the stuff onto barges and send it out to the middle of the North Sea where it would be dumped. We live in more enlightened times, of course, and the sea disposal route was closed off for water companies in 1998 by European legislation. The challenge then was to find cleaner and more innovative ways to solve the sludge problem.
For Thames Water, that meant treating sludge less as a problem and more as a resource and for years the company has refined processes that go back almost a century to extract biogas from sludge, generating electricity and reducing its final volume in the process. But after processes like anaerobic digestion and thermal hydrolysis, the resulting dry cake still has to be disposed of. Which is where Alex Harrison and his team come in.
Alex, Thames Water’s biorecycling operations manager, is ultimately responsible for the safe, sustainable, ecological, cost-effective recycling of sludge. One of the best ways is to sell it to farmers to use as agricultural fertiliser but in an overcrowded market it can be difficult to get your product noticed. Hence the rebranding of the company’s sludge cake as ThamesGrow. “We’ve been distributing cake to land for more than 20 years,” said Alex, “but now we have a brand identity, supporting the product that all our sludge treatment centres produce, including the newly commissioned THP plants. The key is to regard the cake as a marketable product rather than a waste that needs to be disposed of.”
THP plants at treatment works such as Chertsey and Oxford (which produces particularly good quality cake), process not just their own sludge but also import a proportion of the 100m cubic litres of sludge exported from our smaller STWs every year.
“For the cake, we have a useable land bank of around 150,000 hectares right across the Thames Water region and beyond, and a customer base of around 5,000 farmers, who hold our product in great esteem,” said Alex. “We actually need more than 33,000 hectares annually to be able to spread the 770,000m3 of cake that TW produces each year. Due to agricultural regulations we cannot go back to the same fields year after year, hence why we need such a large landbank available to us.”
Regulations also restrict what crops the cake can be used for. Cereals and grain-type crops are most suitable (although the brewing industry generally steers clear of sludge-grown hops and barley) but using it on fields of fruit and vegetables – foodstuffs directly consumed by humans – is prohibited. Nevertheless, the farmers who do use it acknowledge that it’s a vital part of their agronomy, providing essential organic nutrients that they would otherwise have to source. “The cake that comes out of our THP plants tend to be around 30-35% dry solids, the rest being water,” said Alex, “compared with the 20-25% generated by other methods. And there’s a real benefit to Thames Water in that we’re getting more gas generated from our sludge and consequently more electricity generation. The other key benefit of the dryer THP cake is a smaller volume to haul to land, hence a smaller haulage cost.”
Driven by the need to dispose of between 700,000 and 800,000 tons of cake every year – and a limited number of farmers able to use it on their land – the sale of ThamesGrow is never going to be a profitable business. Despite sales generating £500k-£600k a year, the haulage and spreading costs are borne by Thames Water. “The income we get is a side benefit really,” said Alex. “The cake we deliver to farmers could be sat on their land for months before they utilise it so we use their land as a storage asset. Maintaining our landbank is essential and we can only achieve that by providing excellent customer service. “If we didn’t have the farming outlets we’d have to go to landfill and pay something like £100m a year, plus all the environmental downsides that go with it, as this is the least sustainable means of disposal.”
Whilst further technology advances are on the horizon that will mean even more gas and energy extracted and less cake produced, the recycling to agriculture remains a vital step in the wastewater treatment process, which will continue to keep the bio-recycling team busy.