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It's funny how some of the smallest things can bring about the biggest irritations- a paper cut, a bit of dust in your eye or a leaky tap that just won’t stop dripping. In the wastewater treatment industry, the small things like grit bring even worse effects- yet just like that little bit of dust, we try to ignore it until the problem can no longer be avoided.
Grit can be found in most sewage, effluent and wastewater treatment plants along with anaerobic digestion process and presents one of the greater challenges to utility companies and plant operators. The make-up of grit is described by the US Environmental Protection Agency as including “sand, gravel, cinder, or other heavy solid materials that are ‘heavier’ (higher specific gravity) than the organic biodegradable solids in the wastewater.” It can also be said to include “eggshells, bone chips, seeds, coffee grounds, and large organic particles, such as food waste”.
Grit accumulates in all stages of the process and brings with it both acute and chronic effects. Short term issues include screen blinding, reduced throughput and pipe and pump blockages. The long-term effects are even more problematic and include excessive wear, reduced capacity in digesters and the resulting cleanout costs, not to mention the need for additional resources for sludge redirection. These problems have almost become tolerated in AD plants and treatment works today, with operators becoming comfortable with a duty/standby approach being adopted. However, just like ignoring that leaky tap- something eventually has to give.
The best outcomes come when we look at the root of the problem and just because grit has been tolerated in the past, doesn’t mean that it must continue to be. Solutions have been developed that can successfully remove this unwanted material and protect downstream processes and upstream equipment at the same time.
There are many options available for grit removal, as depending on the location, type of material and volume being processed. These range from simple single-stage separation solutions that can remove foreign objects larger than 6mm – for raw sewage coarse screening or sludge de-ragging, for example – to more complex multi-stage options. These may use two screens for instance to divert solids and then catch any remaining stones and grit.
Some of these solutions only occupy the same footprint as a skip and can be delivered to site using a standard skip lorry. Running off a 15kW electrical power source enables an easy hook up to site power or the use of a small 35kVA generator. Neither requirement is likely to cause any issues for utilities companies or wastewater treatment operators but would result in an improvement in grit removal.
Grit isn’t an easy problem to deal with but with the right technology and a bit of determination to get to the root of the problem, a duty/standby approach no longer has to be the norm. Having true grit, means getting rid of grit before it does the damage.
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 Wastewater Technology Fact Sheet, EPA, https://www3.epa.gov/npdes/pubs/final_sgrit_removal.pdf [Accessed 21st August 2018]