When it comes to grit, do UK Water Companies know what they are missing? Wastewater treatment plant operators have lived with a compromise, “one size fits all” approach to keeping their grit nuisance at bay at the inlet works. By accepting this norm, many may simply not know how much grit they are missing.
Our research shows, on average, 60% of all grit is being missed by existing conventional solutions – a major source of avoidable process inefficiency, operating, maintenance and replacement costs.
Too often, the costs are borne as part of ongoing plant operation – almost without question. Now, water companies are striving to achieve progress towards Totex objectives in AMP6 and beyond and becoming more open to innovative approaches.
Why grit matters
The effects of grit on a wastewater treatment plant are often underestimated. Grit threatens effective treatment through abrasion and accumulation; poor grit removal leads to process inefficiencies and increases electricity costs.
Too often, the costs are borne as part of ongoing plant operation – almost without question. Whether a primary tank or digester needs cleaning out, a sludge pump stripping down, or aerators need unclogging, it is just something that has to be done.
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 basins 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 for every 1% increase in grit downstream, there is a 1% increase in wasted energy.
The Grit Shake-Up
There is also a growing realisation that the standard model of grit we have taken for granted more or less since the 1940s is wrong. Conventional approaches that target 95% removal of grit particles of 200 micron or greater are based on outdated industry guidance going back more than 70 years. In addition, false assumptions about the behavior of grit mean many systems are falling well short even of this target.
False Assumptions
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’:
• Grit is a clean silicate sand particle of above 200 microns in size
• The particle has a uniform, smooth spherical shape
• The specific gravity (SG) is 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.
Are we looking at the wrong grit?
The sampling we are conducting in the UK is 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.
Change of Approach
Hydro International is introducing a radical change of approach to grit removal in wastewater treatment plants to the UK which could be fundamental to achieving progress towards water company Totex objectives in AMP6 and beyond.
Following extensive sampling, experience and observation of more than 400 installations, Advanced Grit ManagementTM offers a robust approach that closely reflects realistic grit particle behavior.
The opportunity has come about through the technology transfer of a high-performance grit removal system to the UK from the USA. The HeadCell® grit separation system is combined with the rigorous AGM approach to achieve optimum grit removal for each wastewater treatment plant.
Hydro International is seeking water company partners to conduct pilot trials at wastewater plants in the UK, as well as to set up a full operating trial site. The trials will study the potential for AGM to establish new standards based on removing more, finer grit particles at the inlet works than has conventionally been accepted as standard. Already sampling studies underway with several water companies are showing promising results.
For more information about Advanced Grit Management technologies from Hydro International, email enquiries@hydro-int.com or visit www.advancedgritmanagement.com
