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The Cost Of Grit - Q&A with Keith Hutchings

Q. Is grit a significant factor in the cost of running a Wastewater Treatment Plant (WWTP)? If so, how?

A. The abrasive nature of grit has long been recognised as a major factor causing wear and therefore inefficiencies in pumps, valves and mixers. A pump supplier recited to me the anecdote that wear on a pump impellor could be seen after 30 minutes in contact with grit. The maintenance cost of this is often overlooked as a ‘normal’ routine cost. This is only one factor that can dramatically add to the cost of running a WWTP.

Q. How well do you think WWTP owners and operators currently understand the impact of grit on their systems?

A. The impact of grit is widely known and, unfortunately, accepted. They are aware of the reduced capacity of their treatment vessels and tanks, they are aware of the cost of cleaning out aeration chambers and digesters. However, grit removal at a headworks is one of the only processes that is able to be bypassed without any immediate effect on the performance of the WWTP. The effect will only become apparent when treatment objectives are not met and energy costs escalate. As one water authority said to me: ‘we don’t have a problem with grit because we don’t have a budget to deal with it’.

Accumulated wastewater grit - Milton, FloridaQ. What could owners and operators do differently that would reduce their costs of operation?

A. The current standards are changing. Traditionally particles down to 200 microns were targeted, but it is becoming more accepted now that this is not enough. There are technologies available that can target grit down to 75 microns, which can end up being 40% of the total grit load.

Q. What conventional wisdom should be challenged about grit removal? What myths exploded?

A. That all wastewater grit is spherical, silica sand. Wastewater grit can be anything and any shape. Our research, and that of many independent research papers, demonstrates that wastewater grit consists of more than just inorganic particles. It picks up fats, grease and organic particles as it ‘snowballs’ down the pipe. This not only makes the particle larger, but also decreases its settleability.

Q. In terms of grit removal, what do contractors and owners need to be most aware of when designing a new WWTP?

A. As with all treatment chains, each individual process must be more efficient as the last. It is no good to design an advanced grit management system to target 75 micron removal only to feed that into a de-watering unit that is only capable of retaining 200 micron particles. You have just negated any benefit of keeping fine grit from entering your works.

Q. Are there any misconceptions about grit removal that contribute to increased costs?

A. The most common misconception is that ‘we just clean out the tanks every few years’ is just as acceptable employing a good grit removal system. With the increased focus on energy consumption - and indeed, energy production - this is just not good enough. Our own research shows that an advanced grit removal system will pay for itself in three years at specific locations by keeping grit from the treatment processes.

Q. If you could change one thing about how treatment plant owners and operators think about cost, what would it be?

A. Don’t just consider how much it costs to install an innovative system. You must take into account the man hours spent on maintenance, cleaning out tanks, the downtime of the processes and impact on performance of the works, the cost of temporary equipment while the processes are down, the increased cleaning of membranes that get prematurely clogged and blinded. The blight of efficient and economical wastewater treatment is short-sightedness and a five year horizon that is dictated by the AMP cycle.

Q. What should owners and operators take into account when estimating the cost of wastewater plant operations?

A. You should not only assess the cost of doing something, you must also truly assess the cost if you do not do it. Not only tomorrow or next year, but over the lifetime of the asset. We recently launched the Cost Of Grit online tool to help owners and operators to do just that.

Q. What do you think is least well understood about the cost of grit in a WWTP?

A. I think the physical impact on individual processes is reasonably known and, as I have said, accepted to a certain degree. What is not properly understood is the overall impact on operating costs: cost of maintenance, cost of cleaning, and cost of spare parts. These may be on different budgets and so the true cost may not be recognised.

Q. Do you know of any examples of plants that are approaching this problem differently? Overcoming this challenge? Any best practice?

A. While there are over 400 advanced grit management installations in the United States, the advanced git management mantra is only just gaining traction elsewhere. In the United Kingdom we have presented our findings and have received significant interest. We have conducted grit sampling and pilot trials at different locations around the UK to demonstrate the amount of grit that is being missed by old, outdated and inefficient technologies. Following this, we believe we are close to receiving our first order for an advanced grit management system in the UK.

 

Keith Hutchings is Group Wastewater Product Manager at Hydro International.

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