Water Management FAQ: Grit removal

We ask Hydro International experts the questions you want answered. This month, we’re talking about successfully removing wastewater grit.

What is the case for reconsidering the importance of grit removal?

There are a number of drivers in the industry prompting operators to reconsider grit. More importantly, it is prompting them to include comprehensive solution for all types of grit removal. Due to the impact of grit at any stage of the project, especially in instances of inefficient grit removal, related concerns should be picked up by consultants.

Wastewater grit needs to be taken seriously. If left untreated, it can have detrimental effects on the plant in the long term. Grit removal systems and processes should be considered, planned for and implemented efficiently to improve efficiency, cut operating costs and lengthen the lifetime of the plant.

When do I need to start paying attention to grit?

Engineers need to think about grit and its impacts in the very early stages of any wastewater treatment plant new build, refurbishment, upgrade or expansion project. From identifying the types of grit that may be present in influent through to specifying the appropriate filters, grit removal should never be left until after the project is completed.

The costs associated with the impacts of settled grit are not generally quantified. Even so, general awareness of the impact and cost of such grit-related maintenance can prompt an operator to seek to manage this situation by implementing rigorous plant inspections or by monitoring the condition of equipment to help avoid serious breakdowns. Such measures, however, do not tackle the problem at source.

How can I specify the right technology that will remove grit efficiently?

Traditional grit removal specifications have been based on a definition of grit as spherical clean sand with a diameter of 212 micron (0.212 mm) and above and a specific gravity of greater than 2.65, and setting a 95% removal rate for grit of this kind. However, in the real world grit varies in shape, type and size, and it behaves varyingly according to these factors.

To get the specification right, engineers need to understand the different types of grit that will be encountered in the plant’s influent. The design always needs to be matched to the real-world grit that will need to be removed.

Why can maintenance be a downfall of a project?

Wastewater treatment plants need to be maintained in order to continue operating at peak performance. Grit passing through the system can both clog downstream processes, reducing the operating effectiveness of those processes, and cause abrasion damage to critical treatment technologies, increasing the need for maintenance teams to carry out repairs and install replacement parts and systems. A system with inefficient grit removal is storing up increased maintenance costs for the future.

What should I consider pre-project to ensure maximum operating efficiency?

Maintenance is often thought of as being post-project, but engineers and owners need to keep operating efficiency and product lifetimes in mind from the very start. Good project operation and easy, cost-effective maintenance begins with design and is determined by all decisions made early in the project.

Regulatory requirements will be a primary driver in setting the end goals for a project. This will include water quantity requirements for the project area, accounting for a certain selection of storm return periods. The initial design needs to bring all these aspects together in the package of measures for a site, taking account of any intermediate water quality requirements.

How is grit best controlled so that optimum performance can be achieved?

The inefficient or inadequate removal of grit underlines many equipment failures, while many maintenance tasks are related to the build-up of inorganic solids. Grit is best controlled when the system upstream of the main treatment processes provides adequate removal.

Some costs of grit are often not quantified and the biggest loss can, in fact, come in the form of poor performance. Performance goes hand-in-hand with maintenance. Efficient grit removal will improve performance, reduce maintenance requirements and, as a direct result, cut whole life costs for the treatment plant.

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