For sewerage companies, grit is a perennial problem. It clogs up their facilities, leaving them with costly and all too frequently recurring problems which suck in valuable time and materials and push up energy costs.
One solution to this bugbear is a technology that has existed for some time but has gained little traction in the UK thanks to regulatory hoops, the old capex-dominated economic regulatory regime and the traditional risk-averse nature of the UK water sector when it comes to trying out new things.
It has, however, gained a strong foothold in the United States where the ownership structure for wastewater assets has long incentivised a whole-life approach to investment.
The technology in question is an “advanced grit management system” called HeadCell. In the wake of the introduction of a totex regime in the UK water sector, its owner, Hydro International, is hopeful that it can replicate both its stateside success in terms of installations and the benefits it has brought in cost optimisation and efficiency.
What’s the problem?
Grit is found in wastewater not only as a consequence of the sewage it carries, but also because it is washed into drains from surface run-off, something exacerbated by storm events. All this grit eventually ends up in the same place – the wastewater treatment works – and there it creates problems.
It builds up within the system, clogging the aeration machines and increasing the wear rates of other parts of the facility, such as filters. Alongside this, it decreases the efficiency of the plant, so electricity bills go up.
Currently, wastewater companies are obliged by the Water Industry Mechanical and Electrical Specifications (WIMES) to remove grit down to the size of 200 microns. However, according to research by Hydro International at least 60 per cent of grit is smaller than this. So while conventional grit removal systems remove up to 95 per cent of grit from wastewater, they count only “classified” grit of 200 microns or more. In reality, therefore, they may be removing only around 20 to 30 per cent of the true incoming grit load.
Hydro International’s UK sales and marketing manager, Keith Hayward, explains that this leads to grit accumulating in the various treatment components and adding to wear and abrasion rates, making the whole system less efficient. From a maintenance point of view, a wastewater treatment works designed to serve a population of 250,000 people needs draining and clearing every five to seven years because of accumulated grit.
“On one plant recently there has been a cost of £750,000 for cleaning out digesters,” says Hayward. In other cases, the process can cost as much as £100,000.
“That’s shutting down the digesters, draining them down, digging them out and transporting the grit away. That is occurring every seven years or so, but would not occur if HeadCell was in place,” he states. “If you assume you are draining them down just to clean them out, instead of every seven years you could reckon on every 35 years and still at a lower cost. It’s a colossal saving.”
It’s not just periodic maintenance costs that can be slashed with the installation of advanced grit management technology. So too can day-to-day costs.
Hayward says many treatment plants have their aeration specifications “over scoped” to ensure that, even when clogged with grit, they are still able to operate to the minimum required standards.
He says that for every 1 per cent increase in grit downstream in the wastewater treatment process, there is a 1 per cent increase in wasted energy. Cutting down on grit in the system cuts down on power bills.
Why hasn’t grit management been a priority in the UK?
Getting advanced grit management on the agenda in the UK has not been straightforward, despite claims of great results from technology providers such as Hydro International.
“Innovation in the UK water industry is very much like a parachute jump. You get a gang of people at whatever level and they all want to do it, but nobody wants to be the first person out the door,” says Hayward.
Furthermore, with the UK regulatory regime until recently dividing capital expenditure and operational expenditure when making its five-yearly price settlements, there has been little incentive to focus on the lifetime costs that grit creates.
Both these factors are changing. Hydro International has invested a lot of effort in wearing down UK risk-aversion and showing HeadCell to be a “proven innovation”. It has more than 200 North American deployments of the technology to point at, but to further reassure potential UK investors it has set up a mobile pilot unit for water companies to experiment with – free of charge – and conducted a range of independently verified technology tests.
Combined with these efforts, the introduction of totex metrics in the UK mean that technology providers are hopeful AMP6 will see a swathe of investment in advanced grit management. Hydro International is now in what Hutchings describes as “close discussions” with four UK water companies for the deployment of HeadCell and the realisation of associated savings.
How it works
The HeadCell system is a stack of hydraulically independent polyethylene trays contained within a concrete chamber. Sewage, which has already been through a screening process, enters the system via an inlet at the top of the chamber.
The feed establishes a vortex flow pattern within the chamber, which causes the grit to settle on the angled, layered trays.
The grit settles out by gravity along the sloped surface of each tray and then solids are swept to the centre opening, which allows them to fall to a common collection sump.
The effluent flows out of the trays, over a weir and into an effluent trough. The settled solids are pumped from the grit sump and into a washing system.
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