Filtration is an important step in drinking water treatment, especially where the source water is not from a fully protected catchment and has a pathogen risk. It is the process of passing water through material (e.g. sand or other granular substance) to remove particulate and other impurities from the water being treated. These impurities consist of suspended particles (fine silts and clays), biological matter (bacteria, plankton, spores, cysts or other matter) and floc. Filtration processes can generally be classified as being either slow or rapid.
Figure showing a simple slow sand filter concept.
[Reproduced from: Bruni, M. (no date). Slow Sand Filtration. Sustainable Sanitation and Water Management]
Over the years, standards for filtered water turbidity have become stricter, as scientific knowledge on risks to public health has improved. However, filter designs are generally the same, and for many utilities the filters were installed/constructed a fair while back. Based on the raw water quality and general filter maintenance practices, filter media life is impacted (can be shorter or longer).
The main traditional indicators for media health are:
water quality data (e.g. filtered water turbidity, which is critical to monitor and act upon) and
visual inspections (e.g. observation during filter backwash)
However, on top of these, knowing the condition/quality of the filter media will help you to understand many performance and operational issues. The size and shape of media grains can change over time. Some media grains can get crushed by the weight of the media above them, some grains get worn down as they bump together during the backwash process while some grains can grow in size if they get coated with organic and inorganic precipitates. In some cases there might not be enough media (depth) present for effective filtration (e.g. through media lost during filter backwash process). All these can impact filter performance.
Detailed assessment of filter media can provide valuable insight into media condition and filter health. Issues caused by incorrect layering, depth, media grade, mudballs and inclusions can all be identified. Moreover, to claim log credits for filtration as part of Health-based targets (HBTs) assessment, the filters need to consistently perform effectively. As such, the filters need to be in good working condition (monitored through periodic checks of filter media condition) and the process needs to be working efficiently (optimised).
Proper assessment of the filter and filter media condition can provide useful information to guide maintenance, optimisation and renewal projects.