The value of waste

For more than 100 years the wastewater treatment process in this country has been subject to the ageless wisdom of it ain’t broke don’t fix it and has remained largely unchanged.

Having served many generations well, it’s a process where companies are finding room for improvement – as a focus on innovation brings fresh approaches to age old techniques.

Last month, Severn Trent (ST) opened the Resource Recovery and Innovation Centre at its Spernal treatment works, a medium-sized plant south of Birmingham that processes 1.8 billion litres of water each day.

The thinking behind it is to test innovative techniques on a scale bigger than a bathtub but without any of the public health risks that using the live works could pose.

The sewage treatment process is strictly regulated to protect the health of waterways and the public. This means companies are unavoidably risk-averse, with limited opportunity to trial different treatment processes at scale, hence ST’s interest in launching such a site.

The water will be re-treated at the regulatory-approved plant to negate any potential risks with trialling new treatments.

The wastewater demonstrator testbed at Spernal sits alongside a dedicated laboratory. Both are included within the works giving them access to 500m3 of crude and 1000m3 of settled sewage each day.

Room for improvement

“It works, but it’s ripe for innovation,” says Bob Stear the chief engineer for Severn Trent who explains the company’s desire to make improvements. “The bottom line is always we want to do more for less and serve our customers better.”

The industry is facing challenges relating to energy prices, chemical availability and population growth among others, which all companies have factored into their business plans.

Severn Trent has based its innovation on the principles of creating a carbon-neutral company; extracting all value from waste; reducing service failures; exploring AI and robotics; eliminating leakage; and reducing abstraction.

In urban areas where space is a premium and populations are higher, there’s greater need for higher technologies to process large quantities of waste. Whereas in rural spots with less waste to process each day and more space available for plants, the requirements are different. Low-tech options have the benefit of needing less maintenance so technician site visits can be reduced – especially beneficial in more remote locations.

Value in waste

ST looks at urban and rural processing plants differently to recognise the practicalities of managing sites of differing sizes and varying remoteness with one common theme, as Stear explains.

“We’ve got to stop thinking about waste as a problem but think about it as a resource, a plentiful one,” he says.

Sewage contains cellulose, biopolymers, phosphorus, nitrogen, methane, organic fertiliser as well as reusable water – all valuable commodities that can be reused in the right place.

The innovation team works to find ever more efficient ways to extract these materials to make every drop of wastewater play a role in the circular economy.

The waste is rich in nitrates and phosphorus, which are essential to agriculture. There is biogas and potential recovered energy, which is used to power the plant.

The hydrogen removed from sewage waste could be used to power fuel cells for electric vehicles, which ST has plans for a fleet of. This valuable by-product is part of the company’s focus on looking for technologies to improve energy efficiency.

Treating primary sludge is cost effective and makes good fuel for anaerobic digestion, whereas the secondary stage of treatment is more expensive, and the end result is harder to use for anaerobic digestion.

The design of the sludge tanks remains largely unchanged over the past 70 to 100 years but there is room for improvement to be made on separation technology and the team is trialling a design that can be retrofitted to tanks.

Currently, the separation in the sludge tanks takes out 60 per cent of solids and 30 per cent organics. This means the costlier, more difficult secondary stage must remove the remaining 40 per cent solids and 70 per cent organics.

The method called microfloation is being trialled to remove 77 per cent solids and 40 per cent of organics – which means more biofuels as well as better performance and cheaper costs to the process by reducing the work done in the secondary sludge tanks.

Removing more solids earlier in the treatment process results in better fuel going in to the digestors and less going into the secondary process, which is the expensive part.

Currently, the secondary stage involves streams of bubbles being blown into the wastewater for the bacteria to grow on. The microfloatation system features membrane aerated biofilm reactors, which look like strands of hair with pores that pump bubbles around the tanks. The fine strands of “hair” increase the surface available for bacteria to cling on to while working their magic.

Stear explains that the process uses less energy and produces better results, so the team is keen to scale up following the successful trial.

Further improvements to be made at the secondary stage of the process include cultivated bugs, which perform the anaerobic part of the process. Imported from Brazil, the bugs ordinarily prefer warmer temperatures, so the team has worked to find ways to encourage the bacteria to not only survive but also work at cooler temperatures

From June next year the team plans to scale up from the test bed to be operational at a scale of 2,500 litres each day with the bugs working at cooler temperatures, which will make it Europe’s first anaerobic site at this scale.

Stear says they have always known that anaerobic bacteria work well at body temperature, but the challenge was to get them working at colder temperatures without prohibitively expensive heating.

The anammox plant is currently achieving an ammonia removal rate of over 80 per cent, which has contributed to a 15 per cent reduction in downstream aeration power usage.

The Brazilian biomass reacts anaerobically to convert ammonia into nitrogen. The process uses significantly less air to complete the conversion, in comparison to the conventional approach.

These will replace the activated sludge part of the treatment plant process with anaerobic membrane reactor. These bacteria favour 37 degrees Celsius but the ST team has found a way to get the bugs to work between 8 and 18 degrees Celsius.

Global learnings

ST is working with water treatment companies around the world to hone and share best practice. It is a founder of the World Water Innovation Fund, which aims to utilise expertise from different companies, countries and industries. United Utilities is another member of the consortium, which explores and shares trials, research, seed funding and new technologies.

Like all utility companies, ST is tasked to do more for less to serve its customers without raising bills. To enable it to develop and test drive its ideas for improving wastewater treatment it applies for pots of funding made available through the EU. Peter Vale, head of innovation at ST says the innovation team has worked with funding councils in the UK and EU to secure funding when it can be qualified for.

Vale explains the nature of the work means not every project will be seen to fruition, even if it is promising in early stages. “We are doing pretty revolutionary stuff and some of it will come off and some won’t.”

With every part of the processes trialled at Spernal the team learns and can build on knowledge to make incremental improvements to a cleaning process that is being forced to work more efficiently each year as the population grows. The need for sewage treatment will never cease.