Net Zero wastewater treatment plants are no longer a pipedream

Author: Paul Lavender, Water Utilities Director

Water treatment companies have long hoped for a sludge treatment plant capable of generating enough biogas to power the entire facility and export the surplus. Has Anglian Water unlocked a key part of the puzzle in delivering a sustainable Net Zero wastewater treatment?

Extracting biogas for renewable energy from sewage sludge is costly, accounting for around half of all wastewater treatment costs. Sludge - rich in hydrocarbons, nutrients, and trace elements - is treated by anaerobic digestion (AD) to stabilise it, break it down, recover biogas, and create a nutrient-rich digestate soil conditioner which can be used in agriculture.

The process of anaerobic digestion for sludge typically takes around 15-20 days, although longer times are desirable for greater biodegradability and lower emissions. This impacts the volume of sludge that can be treated in the digesters. Consequently, the industry has been exploring alternative pre-treatments that could accelerate the digestion process and increase the yield of biogas. Different methods, such as chemical, mechanical, biological, and thermal processes have all been considered to address the problem.

Thermal hydrolysis was crowned the champion process and has been the favoured pre-treatment process for the past two decades. The process involves pre-heating sludge to 100⁰C before sending it through a series of pressurised reactors, which are then heated to around 160⁰C. The sterilised sludge is then fed into a tank operating at normal pressure. The steam from the cooling sludge is captured and reused to heat the incoming sludge for the next batch. Finally, the cooled sludge is discharged to the anaerobic digesters to work their magic.

Thermal hydrolysis is proven to have several benefits, such as improved dewatering, biodegradability, pathogen removal, and energy recovery. However, heat-treating sludge at high temperatures brings its own set of challenges. The process requires pressure vessels and extra fuel requirements to maintain the high temperature. Along with the heavy use of heat exchangers, this results in high capital and operating costs. Moreover, if the fuel used comes from fossil fuels, it’s the emission reduction equivalent of robbing Peter to pay Paul.

Race to Net Zero

In order to meet its net-zero targets, the industry needs a more sustainable sludge pre-treatment process.

The scientists and engineers at Anglian Water believe they’ve not only achieved that ambitious goal, but have proven it as well. A decade ago, they revisited research on biological pre-treatment and perfected a three-stage process that involves pasteurisation followed by biological hydrolysis, which eliminates the need for costly pressure vessels or chemicals.

The three-stage heat, pasteurisation, and biological hydrolysis (HpH) process involves pumping sludge into a heating tank and raising its temperature using a heat recovery system. The warmed sludge is then further heated in a pasteurisation tank using a steam injector. The steam is derived from a waste heat boiler which recovers heat from the combined heat and power engine exhaust gas, and no additional support fuel is needed. After pasteurisation, the sludge is transferred for hydrolysis and then sent to the digesters.

This process, which is largely biological in nature, boosts anaerobic digestion and leads to increased levels of biogas production. As compared to traditional anaerobic digestion, this technique can extract 25 percent more biogas and increases the plant’s overall treatment capacity. Furthermore, it consistently achieves 1 MW of electrical power generation per tonne of dry solids processed (equivalent to 3 MW of energy in the biogas). This performance is comparable to thermal hydrolysis but is more eco-friendly due to its requirement of around 70 percent less steam and energy.

The efficiency levels achieved by HpH - renamed Helea® and now owned by Royal HaskoningDHV – can substantially contribute towards the industry’s goal of achieving net zero. However, the biggest advantage of this technology is its cost-effectiveness during construction and operation.Experience shows that it requires 25-30 percent less capital expenditure than thermal hydrolysis, and the operational costs are £30-£60 less per tonne of dry solids treated. For a 20,000 TDS plant, that translates to an opex saving of around £1M annually. The saving is not just due to the improved net energy yields, but also due to reduced costs associated with the treatment of liquors from the dewatered digestate product, and less maintenance and downtime. For instance, there is no need to take down pressure vessels to inspect them, and there is no requirement for support fuel.

The biological hydrolysis Achilles heel

The long-perceived Achilles heel of biological hydrolysis is that it doesn’t kill pathogens as effectively as thermal hydrolysis. Thanks to Helea’s patented batch pasteurisation system, this is no longer the case. The system meets all HACCP requirements and results in a 6-log kill ratio, meaning it’s 99.9999% effective. Out of 1 million microbes, less than 10 microbes are left standing. Not surprisingly, the biosolid output is already being safely used in agriculture as a Class A product.

Some will argue that biological hydrolysis is difficult to control and operate. However, the team at Anglian Water begs to differ. The company was so convinced by the technology that it installed Helea at four of its regional Sludge Treatment Centres located in Basildon, Cliff Quay (Ipswich), Colchester, and Pyewipe (Grimsby). Each centre uses a patented hydrolysis tank design which has a variable level and maintains consistent biological hydrolysis, regardless of the throughput. The four plants have operated for nearly a decade and continue to demonstrate a consistent performance with 100% plant availability and safety record.

As water companies continue to face the challenging targets of the industrial emissions directive, they must consider all avenues to reduce emissions and move towards more efficient and sustainable hydrolysis. This cannot happen overnight, and there are opportunities in the market for alternative pre-treatment processes that can increase plant throughput and biogas yield. However, it is more important than ever to take the time to get a fresh look at sustainable biological hydrolysis. If all water companies in the UK adopted this technology, they would collectively generate approximately 1,000GWh of renewable energy from wastewater - a significant step forward towards the road to achieving net zero.

This article was originally published in Water Magazine on 3 January 2024.