Discover the hidden costs of wastewater grit and how they could be impacting your bottom line.
Wastewater treatment plant (WWTP) owners and operators are generally aware of grit. They may also think that the plants for which they are responsible deal adequately with grit. But how many can say they are up-to-date with the latest insights into grit and its impacts? Taken further, how many have thought about the implications of grit when it comes to changing attitudes in the industry to whole-life costs?
All treatment plants receive grit, the inorganic solid particles present in sewage that are smaller than a few millimetres in diameter and pass the coarse screens at inlets. For plants serving combined systems, much of the grit comes from surface runoff. There is long-standing awareness of the severe impacts that grit can have in a treatment plant, particularly due to its abrasive effects and settling in unwanted locations. Consequently, wastewater plants do generally include a dedicated grit removal stage.
Keeping this awareness in mind, consider also the maintenance tasks that have to be carried out at wastewater plants. Many of these are linked to the presence of inorganic solids throughout the treatment plant. This can be seen as an indication that inefficient grit removal is currently widely accepted.
Take pumps, for example. Used extensively, they have to be maintained and will periodically need parts replacing. The greater the quantity of inorganic solids present, the greater the abrasion that occurs, adding to the maintenance burden. Not only this, any decline in performance has knock-on operational effects in terms of energy use.
A less clear impact affects secondary treatment. As the amount of suspended solids present increases, so the density of the wastewater increases. This in turn affects aeration performance, tending to push up energy use.
In both cases, the end result is that the operator is incurring costs as a result of the grit that is present. This applies throughout a treatment plant, both for many routine operations and maintenance activities, and for unscheduled maintenance tasks. The impact can be as routine as having to ensure that channels are kept clear. At the other end of the scale, sediment can build up in an anaerobic digester over time, diminishing the reactor space and impacting performance. The task of taking the digester offline and removing the sediment is an unpleasant, time-consuming and costly one.
These impacts accumulate for the plant as a whole. They place a burden on the overall annual operations and budgets – even if these budgets don’t specifically include an allocation for dealing with grit. More than this, the costs accumulate over the lifespan of the treatment plant. Ultimately they can shorten the overall lifespan of the asset, meaning there are capital expenditure requirements also. There are also potentially wider costs, not least an increased risk of incurring a fine and damaging the environment if plants are not functioning fully during maintenance or if there is an equipment failure.
More is understood now about the nature of wastewater grit. There is greater evidence about the significance of fine grit, smaller than the particle sizes used in traditional specifications for grit removal. Fats, oil and grease in wastewater attaches to particles, changing the way they settle. Also, the make-up of grit varies from location to location. Consequently, research has shown that grit removal systems designed according to traditional parameters may only remove between a third and a half of the grit present.
Operators can seek to deal with inadequate grit removal by adopting more rigorous approaches to maintenance, even adopting sophisticated condition monitoring of equipment. This still leaves the burden with the operator and does not tackle the problem at source. The alternative is ensure that wastewater treatment plants include an efficient grit removal system to help drive down costs for the ultimate owners and operators of the plants.
The understanding of inorganic solids, and the need for their efficient removal from wastewater treatment plants, should give owners, operators, consultants and contractors alike reason to reflect on whether the treatment plants for which they are responsible are efficient enough.
There are also other reasons why grit will become increasingly important. The growing focus on whole-life costs is one, such as the move in the UK to use total expenditure – ‘totex’ – as a philosophy for water company asset planning.
Other reasons include the more intense rainfall that climate change is expected to bring, leading to higher peak loads of suspended sediments. Also, expansion of wastewater plant catchments or continuing development and population growth means greater capacity has to be added, often within the same plant footprint.
Maintenance for all equipment should be regularly completed, regardless of its position on the site. A piece of equipment underground still requires as much consideration as parts of the water treatment process that are visible. If maintenance fails, grit will build up and cause long-term problems.
Allowing grit to pass through the plant doesn’t only impact a particular piece of equipment. Owners and operators need to consider the impact of accumulation for the plant as a whole. Inefficient grit removal places a burden on the overall annual operations and maintenance budget.
Poor performance, due to grit, increases the overall cost of operation of the plant. Accumulated grit reduces a plant’s operational effectiveness, making it harder for operators to meet local water treatment demands within budget.