Ephyra® and Helea®: Maximise energy recovery in anaerobic digestion

Anaerobic digestion uses naturally occurring biological processes to break down organic waste in the absence of oxygen. This process may be employed to treat a range of materials, such as food waste or crop residues, and is one of the most common processes used to treat sludges from municipal wastewater treatment plants (WwTPs).

Anaerobic digestion can play a key role in developing a sustainable and circular economy. It produces a residual nutrient-rich digestate product that may be recycled to land and recovers energy as biogas (a mixture of methane and carbon dioxide). Biogas is a sustainable fuel that can be beneficially utilised through combined heat and power (CHP) engines or grid injection as an alternative to fossil-derived natural gas.

However, while biogas is a valuable and sustainable fuel, methane is also a potent greenhouse gas (GHG) with a global warming potential 84 times that of carbon dioxide over 20 years. Therefore, it is important to maintain facilities to minimise the potential for uncontrolled emissions (such as leaks from pipework and pressure relief valve emissions) and to optimise the process to produce a stable digestate that minimises downstream emissions. 

Poor process performance increases the risk that the digestate continues to emit methane, which could negate the overall GHG benefit of the treatment process. It is estimated that wastewater treatment contributes up to 10% of total methane emissions that come from human activity, and scientists at the University of Princeton recently discovered that municipal WwTPs release nearly double the amount of methane into the atmosphere than previously measured, with anaerobic digestion potentially being a major contributor to this.

If digestate leaves the facility before being fully digested, it will continue to release methane when stored or spread on fields as a soil enhancer. This makes it vital for WwTPs to convert as much organic matter into methane as possible during treatment. Optimised anaerobic digestion is, therefore, a win/win situation for both the plant operator and the environment as it maximises revenue and reduces GHG emissions.

The easiest way to produce a more stable digestate is to increase the treatment time within the process. But delivering more capacity to achieve this – while also delivering capacity to cater for population growth – can be costly. Very long retention time processes can also be impractical and inefficient, with energy required for mixing and heating to maintain plant performance.

Haskoning provides two complementary technologies designed to optimise existing assets to increase treatment capacity and methane production, while reducing methane emissions.

Helea: A pretreatment to increase biogas production

Pre-treating the feedstock prior to anaerobic digestion increases its biodegradability and so boosts the performance of the process. Helea was originally developed by Anglian Water, which identified this as the most sustainable route to deliver advanced digestion without the need for high temperatures and pressures.

Helea comprises heat recovery, pasteurisation, and biological hydrolysis to provide a sustainable method for sludge pretreatment. The Helea process helps to break down the highly complex structure of organic matter in sewage sludge and improves sludge biodegradability by increasing its accessibility for microbial degradation.

Speeding up the degradation process reduces the required retention time, allowing anaerobic digesters to increase organic load, solids destruction, and biogas yield to boost revenue. It also ensures enhanced quality (Class A) biosolids for land application and improves digestate dewatering, minimising the volumes of material for off-site recycling.

Ephyra: Reliably efficient sludge digestion

Anaerobic digestion typically comprises a single-stage, completely mixed tank, but this is inherently inefficient due to short-circuiting and the need to restrict throughput to protect the biological processes. Ephyraanaerobic digestion is a plug flow system operating with two, three, or four reactors in series, each of which can be monitored and controlled by the Ephyra controller to ensure optimum process performance.

Connecting a series of digestion tanks operating at maximum efficiency allows the organic loading in the first reactor to be increased up to four times. The Ephyra controller also enables a portion of digestate from the final reactor to be recirculated to the first digester to bring back alkalinity (to stabilise the pH) and methane-producing bacteria for the optimum process.

Overall, the tanks have the same or lower footprint as a standard digester. This simple adaptation brings operational flexibility through the ability to occasionally take individual reactors out of service for cleaning and maintenance without the need to reduce the sludge load to the site.

Ephyra minimises short-circuiting and ensures more breakdown of the organic matter will occur in the digesters, leading to more gas production and reducing downstream methane emissions by 80–90%.

Combining technology to provide total methane capture

Deployed individually, Helea and Ephyra can both increase the efficiency of the anaerobic digestion process, with Helea boosting biogas production by 10–15% by improving the biodegradability of the sludge and Ephyra by 15–25% by increasing the rate and efficiency of degradation and minimising short-circuiting of the sludge.

However, when deployed together, these technologies will deliver the highest possible net energy yield with the lowest possible emissions. This ensures the maximum return on investment and the greatest carbon benefit for the water company, playing a key role in delivering the best value to the customer and meeting Net Zero commitments.

Helea and Ephyra can be integrated with other site-wide improvements. One example is Severn Trent’s Net Zero hub, where Ephyra is being deployed as part of the overall process flow train for the world’s first carbon-neutral WwTP.

A case study in sludge valorisation

In 2017, Tollebeek WwTP in the Netherlands became the first site to deploy Ephyra. Its anaerobic digestion now generates enough biogas to power the entire site, and the WwTP has not only become energy self-sufficient but is energy-positive, selling the excess electricity it generates to the grid.

Haskoning: Wastewater treatment experts

Every WwTP is unique. However, they are united by one goal: to increase capacity while operating more efficiently and sustainably.

Talk to one of our water tech experts for independent advice on how your facility can achieve its Net Zero goals – from maximising methane production to navigating complicated energy tariffs and regulations.