The demands on wastewater treatment are greater than ever before, putting upgrades and refurbishment firmly in the spotlight, but with a firm requirement to manage costs.
Wastewater treatment works upgrades are an essential part of ensuring the modern plant continues to meet its discharge consents in the face of tighter standards, population growth and increasing water use. Achieving these aims while facing reduced budgets and the demand to maximise manage ROI is a major challenge.
One obvious answer is often overlooked: grit removal. Sitting at the head of the treatment train and furthest from the regulated point of discharge, grit removal is often seen as a minor part of the treatment process. This can be detrimental to a project as it’s an essential element that can significantly help with the challenge of cutting operation and maintenance costs.
Grit removal improvements remain a less-favoured option. While the costs of removing grit and maintaining a detritor are well known, the OPEX (operational expenditure) cost of not removing grit effectively is less clear. Detritors do not cause compliance issues, and the cost of removing grit is often transferred to other sites within the plant. This particularly happens where sludge is digested, meaning that funding is deflected or deferred beyond the utility’s usual short financing cycle.
One of the key issues is that grit removal is normally restricted to particles larger in size than 200 μm (microns) (see Wastewater Engineering: Treatment and Reuse by Metcalf & Eddy). However, typically around 40% of inorganic influent material in the UK is smaller than this, and similar figures are found elsewhere, including the USA.
Without an effective and efficient grit removal and capture system, various costs rise significantly. Not only is power usage substantially increased, the maintenance commitment to all downstream processes is also negatively impacted. This is partly due to the abrasive nature of grit. Grit increases the amount of wear and tear that moving parts, such as pump impellers, are subjected to.
If grit is not captured in the detritor, it will settle out in other parts of the treatment train.The consequences of grit settling out in unwanted areas include blockages in pipes and channels, and reduced capacity in the downstream treatment tanks.
Lack of capacity impacts on the flows that can be treated (or in a digester, on the hydraulic retention time) and, as a result, upon the effectiveness of the treatment and the quality of the final discharge. It also means that expensive and time-consuming manual removal is required. This leads to increased downtime for vital processes and the need to consider health and safety implications.
Improving the amount of grit collected at the head of the works is an attractive option that can ensure downstream processes function correctly and remove or postpone the need for upgrading (and consequently, the costs of the improvements). The correct application of a high-performance grit system reduces lifetime energy costs and can help reduce the whole life costs of capital equipment. This is done by extending the time between maintenance periods, reducing blower usage in aerated systems, and cutting the amount of downtime required for grit removal.
Cutting the cost of an upgrade depends on ensuring that processes such as grit removal are working to optimum capacity.
By improving the amount of grit collected at the head of the works, general wear and tear can be avoided—eliminating high maintenance costs.
Without an effective and efficient grit removal and capture system, various costs rise significantly.