Maximising the energy potential of sewage

Within weeks of Russia’s invasion, the European Commission outlined proposals to reduce the EU’s dependence on Russian gas by two thirds before the end of 2022 as part of a plan to become independent from all Russian fossil fuels ‘well before 2030.’ To put this monumental shift in European energy policy into perspective, before the invasion Russian gas accounted for 40% of European gas demand, with imports around 155 bcm per year, although some countries such as Germany are much more dependent on Russian gas than others. That means that new biogas targets would replace some 20% of Russian gas – now biogas supplies just 5% of total European gas demand.

Within weeks of Russia’s invasion, the European Commission outlined proposals to reduce the EU’s dependence on Russian gas by two thirds

To wean itself of Russian gas, Europe is adopting a multi-faceted approach, including increases in energy efficiency, increasing imports from other sources such as liquified natural gas (LNG) from the United States and Qatar, and increasing domestic production of green renewable gas in the form of biomethane and hydrogen. These steps, and others, were presented by the IEA as a ten-point plan in March to significantly reduce Europe’s dependence on Russian gas within 12 months. Although the targeted under the REPowerEU plan increase to 35 bcm of biogas is ambitious, as a target it still falls below the potential 130 bcm of capacity that the IEA suggests is possible.

One of the advantages of ramping up biogas production is that the technology is proven, mature and affordable. While additional investment is required to upgrade biogas to biomethane, this too uses established technology, and biomethane can be transported and distributed using existing gas infrastructure.

However, there are political concerns over the increased use of energy crop feedstocks (both in terms of direct and indirect competition for food and feed). The war has also seen massive increases in food commodity prices and crop inputs including fertiliser, fuel and crop protection chemicals. Not only will these cost increases also increase the cost of crop feedstocks for biogas, but any measures which reduce Europe’s capacity to produce food (for example by producing biogas feedstock crops including maize, grass and sugar beet) which could further rise food prices will be fiercely resisted by politicians and the public – before any environmental considerations around land use, biodiversity and water use are considered.

Therefore, while crop-fed anaerobic digestion plants will play an important role, all other potential sources of biogas will also need to be explored and utilised, including livestock manures, food waste, green wastes and wastewater treatment.

According to the European Biogas Association (EBA), sewage-fed biogas plants are the second most numerous after farm-based plants, with the Czech Republic and Scandinavia having particularly strong representation. Overall, the EBA estimates that just under 10% of the total installed biogas electricity capacity of 10,532 MW is from sewage-fed plants.

Many of the first generation of wastewater AD facilities are now looking to upgrade, with many switching from producing electricity to biomethane to take advantage of increased incentives for biomethane production. However, small-scale improvements have been no less important in helping to boost the sector’s energy output and upgrading an existing plant is an ideal opportunity to improve its overall efficiency, to maximise both energy production and overall greenhouse gas savings.

This is where smart thinking comes in – not only in terms of adopting the latest adaptive technology and remote monitoring, although these are important – but also in terms of taking a smart approach to the overall energy efficiency of AD facilities. In the United Kingdom, the water sector has been at the heart of AD efficiency and improvements. Between 2010 and 2015, the installed capacity for the anaerobic digestion of sewage sludge in the UK rose 12% to 216 MWe, but wastewater biogas plants actually generated over 25% more power over the same period.

Not only will biogas production need to be maximised, but the efficiency of all the associated processes, from feedstock processing to digestate handling will also need to be improved. Recapturing process heat is one of the easiest ways to improve efficiency, and heat exchangers represent the best way of doing this. They are an established technology, but despite their widespread use in industries such as food manufacturing and the chemical sector, they are often under-used in AD plants.

Not only will biogas production need to be maximised, but the efficiency of all the associated processes will also need to be improved

Captured heat can be utilised in the AD process itself, for example to pre-heat feedstock or digesters to improve gas production efficiency, or anywhere else that heat is required; from water treatment, pasteurisation and concentration processes – to office and space heating, or to provide hot water for cleaning.

Using surplus heat in this way is free, without the need to buy additional fuel, and all these applications can be carried out using a suitable heat exchanger. Such an approach may also provide additional benefits compared to other technologies, such as the tank heating systems often used for pasteurisation. A well-designed system could recover and utilise 40% of the heat produced by the plant.

Heat exchangers resolve efficiency challenges

Using heat exchangers for pasteurisation is more efficient than using tanks with heating jackets as they have a much lower heat requirement; up to half of that of some systems. This is because tank systems have lower heat transfer efficiency and usually dump the hot water after use, rather than reclaiming it. Using heat exchangers means that the effective pasteurisation of digestate is possible using surplus heat rather than needing to install an additional heat source such as a biomass boiler, which could add hundreds of thousands of Euros to project costs.

However, 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 proving popular with wastewater AD operators across Europe is the DTI series from HRS, which is a double tube heat exchanger. The inner tube is corrugated to ensure improved heat transfer performance and superior resistance against tube wall fouling, resulting in reduced maintenance periods. In addition, the tube in tube design permits the processing of fluids with particles without any tube blockage, making it particularly suited to sewage AD plants.

But having recovered this 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.

Using a well-designed heat exchanger system can provide a continuous pasteurisation process that uses less energy than alternative systems, while allowing additional thermal regeneration, or recovery, levels of up to 60%. This saved heat can then be used elsewhere, such as an evaporation plant.

The HRS Digestate Pasteurisation System (DPS) provides continuous pasteurisation, with one tank being pasteurised while one is filling, and another being emptied. The DPS uses a double tube heat exchanger to heat the digestate to 75 °C above the required pasteurisation temperature. This allows for variation in the sludge consistency and its incoming temperature, making sure that the digestate is always properly pasteurised. The tanks can also be used individually, for example for routine maintenance.

Concentrated digestate is easier to manage

By improving the efficiency of their wastewater AD plants, many of the UK’s water companies are enjoying increased return-on-investment

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. A system such as the HRS Digestate Concentration System (DCS) will include measures to retain the valuable nutrients in the digestate, while the evaporated water can be condensed and returned to the front end of the AD 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.

Another benefit of the DCS is odour and ammonia control, which helps increase the nutrient content of the digestate. The high temperatures and vacuum conditions needed to concentrate digestate can cause the release of ammonia, largely responsible for the odours associated with digestate. The DCS overcomes this by acid-dosing the digestate with sulphuric acid, thereby decreasing the pH levels. This turns the ammonia into ammonium sulphate, which is not only less odorous, but is also an ideal crop nutrient.

By improving the efficiency of their wastewater AD plants, many of the UK’s water companies are enjoying increased return-on-investment helping to make their services more affordable and sustainable; particularly important as the water industry uses around 3% of all the electricity generated in the UK.