The filtration process is often one of the most misunderstood and overlooked processes in a water treatment plant, despite its critical role. Filtration is the final barrier in the water treatment process, removing contaminants and making the water safe for consumption. Filters play a crucial role in protecting consumers against viral and bacterial contamination, so it’s important that the filters function as intended. Filter evaluations can be extremely beneficial in optimizing the filtration process.
But before we dive in to the importance of water filtration, let’s back up a bit.
Water Filters 101
Most consumers are familiar with the idea of water filtration in some form. Filters for kitchen faucets, refrigerators, pitchers and water bottles have grown in popularity over the past couple of decades, making “filtration” a household term. But what many consumers don’t realize is that centralized water treatment facilities use these same fundamental filtration processes to treat mass quantities of water for public use. The size and packaging is significantly different than your household Brita filter, but the results are similar.
Filtration, at the simplest level, is a physical barrier to prevent the passage of particles from one location to another. This can be achieved in a number of ways, including:
- Cloth or fibers, similar to the air filters you might have in your car or home air conditioner
- Ceramics, such as those you might see in portable camping filters
- Granular media similar to what you might see in a simple sand filter in a third-world country
Water treatment facilities often use one of these last two filtration techniques, with granular media prevalent in conventional plants.
Filter Evaluations: The Whole Story
Filters are something of a “black box” for water treatment plant operators. Operational decisions are often made based on historical data, operator convenience or passed-down knowledge. While these factors can provide valuable information, they often don’t tell the whole story and frequently result in underutilization of a plant’s filtration potential. Here’s where filter evaluations come into play. Digging deeper — both physically in the filter itself and theoretically into the science behind filtration — can provide better understanding of the filter’s true health and performance potential.
Performance indicators and quantitative tests are used during filter evaluations to assess filter health and potential. Performance indicators include turbidity reduction, filter run time and the ratio of backwash water used to filtered water produced. These indicators are often recorded by operators and can be used to evaluate filter performance over time. The quantitative evaluations consist of a series of tests that include:
- Measuring media depth
- Examining physical characteristics of the media (i.e. size, density, gradation)
- Determining the amount of solids retained within the media before and after backwash
- Analyzing backwash procedures
The quantitative evaluations provide useful information to supplement performance indicators and give a more complete portrayal of opportunities for optimization.
Compared to the general performance observed by operators daily, analysis of these factors shows specific areas where the filter is performing well and where it is struggling. For example, if a plant operator notices that filter run times are decreasing, but cannot explain why, the measurements from a filter evaluation may show that the amount of solids in the media after a backwash is similar to the amount of solids present before a backwash. This indicates the filter backwash system is not properly cleaning the media, allowing buildup in the filters and decreasing run times.
When to Consider a Filter Evaluation
In general, performance of a filter evaluation should be considered when any of the following issues occur in a water treatment plant:
- Filter run times are low
- Headloss through the filters increases significantly
- Effluent turbidity from the filters increases
- Loss of media is observed or suspected
- Upstream processes regularly perform poorly, resulting in heavy loading to the filters
- A new process is added to the plant (i.e. ozone, granular activated carbon)
- Biological operation of filters is being considered
- Additional plant capacity is desired
If the filter evaluation identifies problematic or low-performance areas, filter improvements specific to the identified issue can be recommended to optimize performance. But it’s just as important to know what to look for when things are going well. Many times, even well-operating filters can be optimized.
For some utilities, filter improvements have doubled and tripled filter run time through more effective backwash and better use of the filter’s solids storage capacity. Implementation of a regular filter inspection protocol provides operators with the tools and knowledge to maintain filter performance and enhance the lifetime effectiveness of their filters.
Time and again, filter optimization has produced proven results and allowed operators to tap into the full potential of their existing filter system. By identifying the underlying causes of filtration issues, operators can work to solve the problem at the root of the issue, rather than continually battling the symptoms.
What has been your experience with filter evaluations? Have you found them to be effective at improving the performance of your water treatment processes? I’d love to hear your thoughts on this.
Rachel Thompson is an environmental engineer in Burns & McDonnell’s Water Group. She specializes in projects for municipal water treatment facilities, specifically related to filter improvements and ozone.