Filtration Systems and Backwashing
One of the most important ways we keep our pools clean is by filtering out debris and other larger particulate matter. There are two systems commerciallyused all over the world. We’ll spend a little time talking about both. Regardless of what system you are using at your facility, the system will eventually need to be cleaned. As all things that clean do, your filters will get dirty from filtering out all the stuff you don’t want in your pool.
First, let’s take a look at the two systems used, how they work, and what the differences are, then we’ll look at the process to clean, or “backwash” each one.
Types of Filtration Systems
The fact that there are only two types of filtration makes it relatively simple to do a side by side comparison, but one important thing to remember is that no matter what, each system is proven to work and work well.
The two systems used are sand filter systems and diatomaceous earth (D.E.) filter systems. While D.E. filter systems have been around longer, they still have benefits that the newer sand filter systems cannot provide. Conversely, there are of course some areas in which the sand filter systems are an improvement over the D.E. systems. Both systems utilize a filter medium to more effectively filter water than a membrane of cloth or other material could.
Let’s first take a look at D.E. Systems, how the system works, and its particular advantages.
D.E. Filter Systems
D.E. filter systems utilize a temporary and disposable filter medium. This filter medium is colloquially called “dino-dust” as D.E. is fossilized remains of microscopic organisms called diatoms.
These tiny organisms can link together to form a medium that can filter out particles down to 3-5 microns. To put that into perspective, a single strand of your hair is probably around 100 microns thick. And a single red blood cell is about 8 microns. That’s pretty spectacular! So filtering out even the smallest debris is one of the strengths of this filtration system!
How the System Works
The D.E. system works by utilizing a series of filter membranes in a small tank, called a filter pit. These filter membranes have a impeller pump that draws the water through the membranes, and coats them with D.E.
Water is then taken from the pool’s main drain, and skimmers, and passed through the now D.E. coated filters in the pit, to filter the water. All the way down to 3-5 microns.
Advantages of D.E.
When we talk about the advantages and disadvantages of D.E., and indeed of sand filtration systems, it’s important to note that these (dis)advantages are always going to be in comparison with each other.
While many pools utilizing D.E. systems are still running those systems effectively, it is important to note that most new pools are being constructed using sand filtration systems.
However, there is still one advantage to using a D.E. system. That is its ability to filter out matter down to 3-5 microns in size. Sand filters can’t reach that level of filtration. So, professionals will notice that D.E. systems provide clearer water than sand filters; though of course filtration is just one factor that contributes to water clarity.
Disadvantages of D.E.
D.E. has become less common to install when building new pools compared to sand filters, and this is due to a number of factors.
One reason is that D.E. filters have continual chemical costs associated with them; because D.E. needs to be rinsed off the filters and disposed of every time the system needs to be cleaned (backwashed), and that means operators must have a supply of D.E. on hand to re-coat their filters.
Secondly, the backwashing procedure is much more labor and time intensive than backwashing with sand filters. But we’ll get into those processes more later.
Lastly, D.E., aside from requiring additional chemicals in your pump room, is a hazardous material, that requires handling procedures, and training in order to safely and properly use it.
Sand Filter Systems
Sand filtration systems utilize sand as a filter medium. Sand is a semi-permanent filter medium, which only needs to be changed or treated (maybe) every 5-10 years.
Utilizing sand as a medium, this system can filter down to around 20 microns. Compared to a D.E. system, 20 microns seems huge. but keep in mind, the average human hair is about 100 microns thick, so 20 microns is still plenty small.
How the System Works
A sand filtration system uses sand as the filter medium but not by coating a filter membrane, as in D.E. systems. Instead sand filter systems use a tank (or sometimes multiple tanks) that is (are) filled with sand and larger (but still small) rocks. Using an impeller pump, unfiltered water is forced down through the sand and rocks, filtering the debris out of the water, and pushing it out the other side.
While initially the sand systems cannot filter down to the 3 microns that D.E. systems do, the dirtier the system gets (until it’s time to backwash), the smaller it will filter down. Making that difference even more negligible.
Advantages of Sand Filters
While D.E. filter systems only have one big advantage, sand filter systems have many advantages over D.E. systems.
Notably, the disadvantages of D.E. filters are all but remedied by sand filters. There is no continual cost of filter medium; while sand does need to be either replaced or cleaned (it’s jagged edges can get filled in with the material it filters out), it is only necessary every 5-10 years, maybe even longer depending on your facilities usage!
Additionally, the backwashing procedures of a sand filter, require a fraction (a quarter or less!) of the time required to backwash a D.E. system, but more on that later.
Finally, there are no dangerous, or hazardous chemicals that staff have to work with.
Disadvantages of Sand Filters
So, we know that there are many advantages over D.E. systems, but that doesn’t mean that sand filters don’t have their own disadvantages; they do.
Sand filters, as mentioned before, only need to have the sand cleaned or changed once a decade, or so. And while it isn’t a frequent cost, when it does come around, it can be costly, and also time consuming, leaving your pool without circulation (which means closed) for days, potentially.
Secondly, without treatment/replacement of the sand, the filters can lose their efficiency as the originally jagged edges of sand become smoothed out, or filled in by the matter it filters from the water.
Lastly, sand filters can experience something called channeling. Channeling iis when water, trying to find the path of least resistance through the sand, creates a channel of sorts for the water, or at least a portion of it, to flow through the system without being filtered. This is most often a symptom of aging sand, and is usually remedied with either a treatment, or replacement of the sand.
Backwashing
Now that we’ve had a chance to discuss each of the two systems, let’s take a look at what each of their backwashing procedures entail, so you can have a better idea of what to expect.
It’s important to remember, however, that these instructions are general instructions, and your filter system may have more, or fewer, steps depending on the additional systems (chemical feeds, multiple pools, energy saving systems) at your particular facility.
The Purpose of Backwashing
Regardless of the type of system you use, the purpose of backwashing is the same. When we backwash a pool it is to relieve built up pressure on our filter system that is built up from filtering out materials. That additional pressure slows down the circulation of your pool, and causes additional stress on your pumps, whether pushing or pulling the water through the filters.
Below are step by step guides of what your backwashing procedures (likely) look like. But remember, some things may be different in your actual system. ALWAYS follow the established/reccommended instructions for backwashing your system.
Backwashing a D.E. Filter System
Turn off, and allow your boiler to cool.
Turn off chemical feeds.
Isolate your filter pit by closing valves bringing water from the pool and pushing water back to the pool, and immediately turn off your circulation pump.
Drain off the water in the filter pit, exposing the filter membranes.
Use a hose with a pressure nozzle to rinse off ALL D.E. and filtered debris from the filter membranes into the waste drain (30-40 minutes depending on size).
Refill isolated filter pit past the top of the filters with fresh water.
Once filled, open bypass / precoat valve to create a small, closed loop of water circulation.
Turn on circulation pump. Water will now circulate in a small closed loop, with no water going out to your pool.
Mix D.E. into buckets of water (making a slurry) while wearing proper protective equipment. (The number of pounds of D.E. needed will vary based on pool size/usage.)
Add D.E. slurry into the filter pit / closed loop. The pit will become very cloudy at this point.
Wait while the closed loop coats the filter membranes in D.E. The pit will start clearing up as the D.E. begins to stick to the filter membranes, and then to itself (5-15 minutes).
Once the filter pit/ closed loop is completely clear again, reopen the valves restoring flow both to, and from, the pool.
Close the bypass/precoat valve completely.
Turn the boiler back on, and chemical feeds.
Backwashing a D.E. System
Keep in mind this process is something that takes 45-90 minutes (excluding boiler cooling time), depending on the number of filters to be cleaned, how dirty they are, and your flow of fresh water.
Once you’ve restarted your system completely, monitor the water level in the pit to ensure the filters are not exposed to air while the pump is on. Adding additional fresh water may be necessary. You will also want to monitor your chemical feeds, to ensure you do not overfeed chemicals.
Backwashing a Sand Filter System
Turn off, and allow your boiler to cool.
Turn off chemical feeds.
Turn off main circulation.
Open backwash priority valve and waste water valve.
Adjust filter valves to reverse flow through filter.
Turn on main circulation for 1-3 minutes. During this time water will be pushed backwards through your filter, and forced out to waste with the debris collected in the sand.
Watch sight-glass/discharge water for clarity.
Once discharge water is clear, turn off circulation pump.
Return filter valves to normal operating position, close main drain and backwash priority valve.
Restart main circulation pump.
Turn on bleeder valve. Air will be forced out through the bleeder valve; leave the valve open until a solid stream of water only comes out.
Close bleeder valve.
Turn on chemical feeds and boiler.
Backwashing a Sand System
This process usually takes 5-15 minutes (excluding boiler cooling time).
Once you’ve restarted your system completely, monitor your chemical feeds to ensure you don’t overfeed chemicals.
Have any questions? Have some differences in your pool system you’d like to discuss? Want more details about the systems themselves? Let us know in the comments below!