Over the years, different wastewater companies have adopted different approaches to sludge treatment, but there is now a trend towards anaerobic digestion (AD) and away from incineration. While the number of AD facilities treating sewage sludge may be growing steadily, rising just 12% from 2010-2015, these 159 plants actually generated over 25% more power over the same period*. This can be largely attributed to efficiency improvements within the wastewater sector, which is renowned for its approach to innovation, often setting the standard for operational efficiency within the entire AD industry.
The age of some of the original wastewater AD facilities are now looking to upgrade, switching from producing electricity to biomethane in order to take advantage of the Renewable Heat Incentive (RHI). At the same time, the closure of the Feed-in Tariffs (FIT) scheme to new entrants in April this year means that biomethane production, (supported under the RHI) is now more attractive for new plants. As well as such major changes to some plants, small-scale improvements have also been important in helping to boost the sector’s energy output. Upgrading an existing plant or installing a new one provides the ideal opportunity to improve overall efficiency, to maximise both energy production and greenhouse gas savings.
Heat exchangers improve efficiency
One of the easiest ways to improve efficiency is by recapturing heat. Heat exchangers represent the best way of doing this, having a much lower heat requirement than tanks with heating jackets (up to half of that of some systems). In fact, a well designed heat exchanger system could recover and reuse 40% of the heat produced by a wastewater AD plant. But not all heat exchangers are equal and one size does not fit all – the AD industry covers many different sectors processing a variety of feedstocks from food waste to farm residues, to liquid by-products.
One range which is popular with wastewater AD operators is the HRS DTI Series: a double tube heat exchanger featuring a corrugated inner tube to ensure improved heat transfer performance and superior resistance against tube wall fouling.
Another option is the HRS Unicus Series of patented scraped-surface heat exchangers. These use a reciprocal scraper mechanism to remove sludge and solids from the tube walls, ensuring maximum performance in heat-intensive operations such as evaporation and pasteurisation.
Having recovered valuable heat, what are water companies doing with it? With a typical 1.5 MW wastewater AD plant producing as much as 40,000 tonnes of liquid digestate each year – bringing significant economic and logistical challenges associated with its storage and transportation – many operators are using their surplus heat to improve their digestate management systems. After all, if it isn’t concentrated, the volume and consistency of digestate can quickly become a costly bottleneck in plant efficiency.
Concentrated digestate is easier to manage
Using surplus heat to separate water from digestate by concentration can reduce the overall quantity of digestate by as much as 80%, greatly lowering the associated storage and transport costs. HRS’ Digestate Concentration System (DCS) includes measures to retain the valuable nutrients in the digestate, while the evaporated water can be condensed and returned to the front end of the treatment process, reducing the amount of energy and water used by the plant. After concentration, the treated digestate dry solid content can be as much as 20% (often a fourfold improvement), making it much easier, and cheaper, to transport and handle.
Other uses of heat
Even if you don’t need to concentrate digestate, there are other ways in which recaptured heat can be utilised to improve efficiencies. This could be anything from preheating sewage at the start of treatment to pasteurisation so that sludge can be used as a biofertiliser. The multi-tank HRS Digestate Pasteurisation System (DPS) provides an efficient and easy to install way to provide continuous sludge pasteurisation. Heat can also be utilised in the biogas to biomethane upgrading process, and depending on the technology used, as much as 75% of the heat used for biogas upgrading can then be recovered.
By improving the efficiency of their wastewater AD plants, many of the UK’s water companies are enjoying increased ROI, helping to make their service more affordable and sustainable; particularly important as the water industry uses around three per cent of all the electricity generated in the UK. HRS’ customers and end-users already include Southern Water, Severn Trent Water, Welsh Water, Yorkshire Water, Anglian Water, Northumbrian Water, Wessex Water and Thames Water. And with Ofwat’s Water 2020 report looking to kick-start a market for treated sewage sludge, it is highly likely that the next few years will lead to even more partnerships between water companies and firms developing new treatment methodologies.
*ADBA’s Anaerobic Digestion Market Report, July 2016
