Once your harvested rainwater passes through leaf guards and first flush diverters, it enters the storage tank visually clean. However, a glass of this water held to the light will often reveal tiny, floating particles: dust, pollen, and microscopic roof grit. To protect your home's plumbing and appliances from this abrasive grit, you need physical filtration. Welcome to the world of Sediment Filters.
What is a Micron?
When shopping for filter cartridges, the most prominent number on the packaging is the "micron rating." A micron (short for micrometer) is one-millionth of a meter. To put that in perspective, a single grain of table salt is about 100 microns across. A human hair is roughly 70 microns thick. Anything smaller than 40 microns is generally invisible to the naked human eye.
Nominal vs. Absolute Ratings
Understanding the difference between these two terms is critical for designing a reliable system:
- Nominal Rating: A 5-micron nominal filter will trap most particles that are 5 microns or larger (usually around 85% to 90% of them). It is an average rating.
- Absolute Rating: A 5-micron absolute filter is guaranteed to trap 99.9% of particles that are 5 microns or larger. If you are using a filter directly before a sensitive piece of equipment (like a UV sterilizer), you should always insist on an absolute-rated filter.
The Stepping-Down Approach
If you run your tank water directly through a 1-micron filter trying to get it perfectly clean in one step, that filter will clog and drop your water pressure to zero within a week. Proper sediment filtration requires a "stepping down" approach using housing arrays.
A typical residential setup involves two or three filter housings connected in a series. The water first hits a "coarse" filter (usually 20 or 50 microns) to catch the heavy grit. It then flows into a "fine" filter (usually 5 or 1 micron). By stepping down, the coarse filter protects the much more easily-clogged fine filter, extending the lifespan of the entire array significantly.
Types of Sediment Cartridges
The material the cartridge is made from dictates how it handles dirt:
- Pleated Cellulose/Polyester: These look like accordions. They have excellent flow rates and a massive surface area for catching dirt. Plus, they can often be removed, gently hosed off, and reused several times. They are best for the "coarse" first stage.
- Spun Polypropylene (Melt-Blown): Look like dense white cylinders of cotton. As water is forced through the increasingly dense layers toward the center core, dirt is trapped deep within the matrix. They cannot be washed; once clogged, they must be discarded. They excel as the "fine" final stage (5 or 1 micron).
- String Wound: Made of tightly wound yarn. They function similarly to spun poly but are an older technology and generally offer less precise micron ratings.
Conclusion
Sediment filters are the workhorses of a rainwater system. By understanding micron ratings and utilizing a step-down approach with pleated and spun cartridges, you can achieve crystal clear water while minimizing your maintenance costs and pressure drops.