Why grit removal is critical when upgrading your wastewater treatment plant

Long-term impacts on capacity and treatment works performance are so significant that grit removal is becoming a hot topic when considering an upgrade.

Faced with changing regulations and a need to upgrade treatment processes, opting for better grit removal often seems counterintuitive to utilities because it is the one process that can be bypassed without an immediate problem becoming evident. As a result, it is often an early casualty if the budget is tight, regardless of whether TOTEX (total expenditure) or whole life costs are to be considered. However, its long-term impacts on capacity and treatment works performance are so significant that it is becoming a hot topic when considering an upgrade.

Regulations are becoming more stringent

Wastewater-related regulation in North America and Europe has traditionally focused on a few key parameters, ensuring reductions of biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solids (TSS). Regulators have now tightened requirements for this basic range and added new rules to reduce eutrophication in receiving water bodies, in the form of reductions in nitrates, phosphates and ammonia.

This inevitably leads to additional discharge consent requirements and, as a result, a need to upgrade treatment works. To guarantee that these and other upgraded processes work effectively it is vital to ensure that sufficient grit is removed at the head of the treatment train.

Grit removal can no longer be neglected

Traditional methods of grit removal are typically designed to unrealistic specifications that don’t reflect real-world grit, so even at lower flows there is still a significant amount of fine grit bypassing the process. Generally, grit settles out in pipes and chambers during dry weather periods and is only transported to the treatment works during wet weather.

As a result, the works will only see its peak grit load during times of intense storms. This is when the plant needs to be at its most efficient, but experience shows that during rain events, grit that has settled in traditional chambers and basins is re-entrained in the turbulent flow and carried out of the grit removal system and into downstream processes.

One of the reasons that grit removal has been neglected is that planning and permissions are rarely required. Because the detritor is at the head of the works and not at a discharge point, it is not regulated. Unfortunately, this means there is no agreed standard or definition for advanced grit removal, or any agreed test method to reliably establish grit removal process performance.

However, the demonstrable benefits mean these are required, and in the US at least, utilities are responding.

Conclusion

When answering the question ‘why upgrade your wastewater treatment plant in the face of changing regulations’, utilities already fully understand that failing to meet their consents can result in fines and negative publicity, in extreme cases, that can have a significant effect on their bottom line and reputation.

However, funding is universally tight and there is pressure to ensure that costs are kept to a minimum. This means that looking at grit removal, and the significant benefits that improving this one process can bring, is a shrewd first step in the upgrade process.

Takeaways:

Demands on wastewater treatment plants are tougher than ever before. Upgrades are often required to meet changing and more stringent regulations.

Grit removal can no longer be neglected. Its potential to cause wear and tear can make breaches in consents far more frequent.

Failing to meet consents can result in fines and negative publicity, in extreme cases, that can have a significant effect on bottom line and reputation.

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