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Grit - Are you seeing the Full Story?

When it comes to grit, do you know what you are missing? That's the question currently being posed to operators about the scourge of the treatment plant that's simply accepted as a necessary evil.


When it comes to grit, do you know what you are missing?  That’s the question currently being posed to operators about the scourge of the treatment plant that’s simply accepted as a necessary evil.


This column returns to the issue of innovation regularly – usually to highlight the barriers to progress -  So, it’s exciting to be able to report on a technology that I believe could make real strides in operating efficiency and energy savings for our industry.


Once grit gets past the inlet works, it starts costing money. The heavy cost in energy, materials, time and money caused by grit is widely acknowledged by owners and operators. Yet poor grit removal is often accepted as a necessary evil – and frustration with grit is accepted as a fact of operating life.   


But grit is the silent enemy of the wastewater treatment plant and should be a prime target for Totex operating cost reductions. 


Downstream of ineffective removal systems, grit:


  • builds-up in channels, pipes, primary tanks and digesters


  • wears out pumps, valves and other mechanical parts


  • blinds filters and membranes


  • clogs aeration basins and blowers


  • disrupts biological processes.


All this damage has a significant impact on operating cost and efficiency.  On average, it could cost £100,000 to drain down and clean a single primary tank every 5 – 7 years, for example.


So, why are we not doing more to remove it?   The answer is because grit does not behave as we tend to assume. Conventional design for grit removal equipment at the inlet works is focused on misleading assumptions.  As a result our engineers suspect conventional grit removal systems could often be removing as little as 20 – 30% of grit at 200 microns and larger.  


Hydro is engaging with the Water Environment Federation in the USA and the WIMES committee in the UK to review current standards.  We are also currently conducting a comprehensive sampling programme with water companies in the UK.


A new skid-mounted pilot unit is now available to be delivered and installed at WTP sites of any water company free of charge.  Operators and asset managers can explore first hand just how much grit can be removed at their site and calculate the potential savings by reducing the impact of downstream processes.


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