Technically Speaking: Grit removal
Grit is a perennial problem in wastewater treatment works, but advances in removal techniques mean that even smaller particles can now be extracted
by Keith Hutchings, Group Product Manager, Hydro International
Grit entering the process stream of a wastewater treatment works has always posed a threat by raising operating costs and impacting on energy efficiency. But the heavy burden in energy, materials, time and money is widely tolerated in UK water companies because it is accepted as unavoidable.
Whether a primary tank or digester needs cleaning out, a sludge pump stripping down, or aerators need unclogging, it is accepted as something that has to be done.
Until now, most wastewater treatment operators in the UK have lived with a compromise, a “one size fits all” approach to keeping their grit nuisance at bay at the inlet works. By accepting this norm, many simply do not know how much grit they are missing.
Now, our attitudes to grit are changing and the realisation of the true cost of grit – especially finer grit- has started to undermine this stance. Utilities are considering the total cost of capital and operating expenditure (Totex) in their investment decisions, so asset managers, operations staff and maintenance engineers no longer have to put up with grit.
Why grit matters
Grit threatens effective treatment through abrasion and accumulation; poor grit removal leads to process inefficiencies and increases electricity costs.
Grit accumulates in channels, pipes, primary tanks and digesters; wears out pumps, valves and other mechanical equipment; blinds filters and membranes; clogs aeration basins and diffusers; disrupts biological processes; reduces the effectiveness of energy recovery; and reduces the quality of final effluent with a risk of exceeding pollutant limits.
As a consequence, maintenance budgets are burdened with excessive plant and personnel costs to periodically remove grit build-up in tanks and basins. In the meantime, clogged aeration basin diffusers lead to wasted energy use and greater carbon footprint.
Draining down and cleaning out a primary tank can cost as much as £100,000 and may need to be repeated every five to seven years. It is calculated that for every 1% increase in grit downstream, there is a 1% increase in wasted energy.
So what is happening to challenge the traditional norms for grit removal? First is the current AMP6 emphasis on Totex asset investment and management. Every aspect of plant operation, including the effects of grit, is under scrutiny.
Secondly, wastewater technology is looking towards greater automation and higher efficiency. That means plants with fewer personnel, a higher volume throughput in a smaller space and more biological processes to achieve better water quality and to recover energy.
Thirdly, and crucially, there is a growing realisation that the standard model of grit we have taken for granted more or less since the 1940s is wrong. There has been a large elephant in the room – or more accurately some very small ones slipping quietly through grit removal systems and into our works.
Revising the standards
Our current industry standards (WIMES standard no. 2.02 Grit Removal and Treatment Equipment 2008) make assumptions about the nature and form of grit. Similar to the current USA standards (Water Environment Federation Method of Practice 8 and Metcalf and Eddy) they make some key assumptions for ‘convenience’. These define grit as a clean silicate sand particle of above 200 microns in size, which has a uniform, smooth spherical shape, and a specific gravity (SG) of 2.65.
Based on these assumptions, the standards have established an all-purpose particle settling-out rate depending on flow rate, which defines conventional treatment and equipment design parameters. Conventional grit removal equipment is designed to remove up to 95% of this size and shape of grit. That is - if working at peak design efficiency.
Grit size matters
The trouble is, the sampling we are conducting at a number of plants in the UK is consistently showing that this cosy picture of grit is wrong. Overall, we are finding that around 60% of grit is smaller than 200 microns, and in some places an even greater proportion than that. The indication is that most grit removal systems may actually be removing as little as 20% to 30% of the incoming total grit load.
From our experience working with more than 200 installations in North America we know that, in fact, the majority of particles can be in the 106 micron to 75 micron range, or even smaller. Grit shapes are seldom spherical, and shape significantly affects the particle settling rate. Actual particles are frequently rough in texture, which allows even smaller particles and organics to bind to them.
The particles frequently carry organics, such as soaps, fats, oils and grease, which expand the surface area or clump together, and thus significantly affect the settling rate.
Conventional equipment is designed around notional rapid settling rates seldom actually seen in raw wastewater, so does not trap slower settling particles, i.e. the smaller and larger, non-round, low density particles that form the majority of the grit intake.
False assumptions about grit are exacerbated by standard industry inlet channel sampling techniques that sample from the bottom of the channel, rather than from across the whole channel depth.
Advanced Grit Management
In the USA there is growing acceptance and support for grit removal techniques that are able to remove grit particles 75 microns and larger. They focus on methods to remove the 106 micron to 75 grit particles, depending on the needs of the plant, while at the same time also removing larger particles above 106 micron.
Behind the successful deployment of these techniques is an approach which Hydro calls Advanced Grit Management (AGM). The basis of AGM is sampling inlet waters to gain an accurate profile of the incoming grit, and then recommending and tailoring a combination of technologies to meet those conditions for an individual plant.
The premium collection technology uses a stacked tray design called HeadCell, which uses a large surface area and short settling distances to capture fine particles. HeadCell achieves up to 95% removal of all grit 75 microns or larger.
To guarantee these levels of performance, HeadCell is accompanied by effective washing and dewatering technology, so that all organics are effectively removed while preventing smaller particles being resuspended. GritCup washer and classifier and SpiraSnail dewatering units have been developed for optimum integration with HeadCell.
The effects of high-performance grit collection on the typical wastewater treatment plant operation vary from plant to plant, but are usually considerable and can be calculated for every plant. Hydro International is currently undertaking sampling at WWTPs across the UK and has a pilot unit available to install on site free of charge; it is also discussing opportunities with UK water companies to partner a full equipment trial for a HeadCell system. With innovation high on the agenda, it looks like grit could turn out to be an unlikely game changer for the UK water industry.
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