The Cost Of Grit - Calculating Savings
While wastewater treatment plant owners and operators may understand that grit causes damage, reduces plant effectiveness and increases maintenance and repair costs, many still find it difficult to look beyond the initial purchase cost of a more effective grit removal system.
Upgrading grit removal equipment is often seen as a cost, but this view is short-sighted. Any purchase investment should be weighed against the lifetime operational savings that improved grit removal will realize. Thankfully these savings can easily be calculated, and can help WWTP owners and operators to justify upgrades to grit removal technology.
The problem with missing grit
Studies made by sampling the inlet channels at WWTPs have shown that on wet days, when 70% of grit arriving at the plant occurs, conventional grit removal of particles 200 microns and above may remove only 13% of the particle load - and on dry days, when the particle load is low anyway, it may only remove 43%. The rest of the grit - from 200 micron down to 70 micron and below - is allowed through. This gives an overall removal efficiency of only 22% of the total grit load arriving at the plant.1
If a conventional grit system removes around 2,000 tons of grit per year, the total actual grit load (100%) arriving is 9,090 tons a year. As a result, 7,090 tons of grit a year (78%) is finding its way into downstream processes and clogging them, causing abrasion damage and other wear.
The benefits of removing more grit
If an Advanced Grit Management® approach were taken and a system installed that removed 90% of the same total grit load it would capture 8,181 tons of grit per year; that would catch 6,081 tons of grit that would not have to be cleaned out of downstream processes.
Operating efficiency would be improved: equipment abrasion wear could be reduced by up to seven times. Clearing out digesters and aeration tanks would be need to take place less often: if currently every five to ten years, this could be extended to twenty years or more. Reduced downtime means less plant disruption.
For every 1% of grit that passes through a system, 1% extra electricity is required.
Processes would also have more capacity because there would be less build-up in tanks, less blanketing of biological processes and less clogging of aeration nozzles.
Minimizing energy consumption is a major concern for all wastewater treatment plants, especially for those with aeration systems, which can consume up to 65% of the net power demand of a typical plant using an activated sludge process. Energy studies at wastewater treatment plants indicate that every for every 1% of grit that passes through a system, 1% extra electricity is required, both to compensate for less efficient processes and to pump the extra grit around the plant.
Calculating savings
The table below uses real-world data2 to give a typical average cost for a 20 million gallon per day (MGD) WWTP over a 25-year life cycle. Using conventional grit separation technology as an example and based on typical average efficiency of grit removal for all grit with particle sizes of 106 micron and larger, savings from major cost centers can be quantified as follows:
Cost Center | Potential savings over 25 years with Advanced Grit Management technology |
Grit tank / channel cleansing | $1,703,775 |
Refurbishment of primary clarifier | $2,628,300 |
Digester cleanout | $1,192,400 |
Pump wear / maintenance | $1,115,575 |
Centrifuge wear | $405,650 |
Aeration basin cleanout | $155,775 |
Estimated total savings | $7,108,550 |
On an annual basis, averaged over 25 years, current typical annual costs using conventional grit removal technology are $350,468; typical annual savings, using Advanced Grit Management grit removal technology, could be as much as $284,342.
This suggests that by taking the Advanced Grit Management approach and removing significantly more grit, WWTP operating costs could be cut to an average of just $66,126 annually, representing a saving of more than 80% in annual running costs.
Clearly, the cost of investment in a new grit removal system may be a significant line on a WWTP budget, but the above example demonstrates that the new system could pay for itself in a relatively short time - and go on to provide costs savings year on year.
If they want to make meaningful savings on WWTP budgets, owners and operators should be thinking over the horizon about long-term operation rather than focusing too closely on up-front capital expenditure.
Read the seven questions that WWTPs should be asking about the cost of grit.
1. What Is Grit Costing You? Barter, P., European Wastewater Management Conference, Manchester, UK, 2014
2. Data provided to Hydro International for analysis purposes by wastewater treatment plant operators
