Swimming Pool Sand Filter Basics
The pool industry uses three different types of filters to remove particles of dirt and debris from water. Central Jersey Pools will discuss the ins and outs of one of them – the high-rate sand filter.
The sand filter, though not complicated, does require some knowledge to work properly. Many people don’t understand how to operate the multivalve, how much sand to put in the tank or when to backwash.
The first thing everyone should know about a sand filter is that it has valves which command certain jobs. For example, when the valve is in “filter mode,” the water is directed to the top of the filter and then flows through the sand, trapping debris as it travels to the bottom of the filter and out through the laterals.
Sand filters use one of two types of valves to accomplish this; a multivalve (which can be top or side mounted) and a side-mounted slide valve (sometimes known as a push-pull valve). A side-mounted model is strictly a two-function valve: filter and backwash modes only. The type of valve used is often dependent on regional differences, but in general, you’ll likely find more multivalves because they can also appear on DE filters.
So let’s take a closer look at the multivalve. It has several positions, known as modes. These modes often raise questions as to their proper function. We can start by breaking down the various types.
Understanding Valve Modes
First, it’s helpful to understand the positions found on a multivalve and what they mean. If you come into Central Jersey Pools we have models on display and can show you what you need.
Filter Mode: This is the mode the filter will be set to most of the time. It’s for filtering the water. In this mode, the valve directs the water to the top of the filter, where it compresses and flattens down the sand. Contaminants are removed as the water makes its way to the bottom laterals, out of the filter and back to the pool.
Backwash Mode: This is the one you turn to when cleaning the filter and its medium. You know it’s time to backwash when the pressure gauge rises 5 to 10 psi above what it normally reads when the filter is clean. In this mode, the valve directs the water to come out the bottom laterals, reversing the flow through the tank. This causes the sand to separate and lift about 7 inches above its normal position while releasing the trapped debris into the water, allowing it to exit from the filter and go out the waste line.
Rinse Mode: Water is directed by the valve to the top of the tank, just like in filter mode. The sand is reseated back in place while still sending water out of the waste line, thereby reducing the chance of particulate blow-back to the pool when the valve is moved back to the filter position after backwashing.
Recirculate: Water is directed through the valve only, not the filter. This is a position to use during certain pool cleanups and chemical treatments, when you don’t want the water contaminating the sand.
Waste Mode: This position bypasses the filter and sends all the water out the waste line – a great setting for vacuuming pools after an algae treatment, and new sand addition or filter start-ups.
Now that we are clear on which functions the valve modes perform, it is time for some do’s and dont’s that Central Jersey Pools uses and suggests that you follow to help keep filters running smoothly:
DO be aware of the filter pressure when it is clean. Keep an eye on the pressure gauge. Backwash when you see an increase of 5 psi, possibly as much as 10 psi on some filters.
DO backwash long enough to get the job done right. Wait until the water is clear coming out of the waste line or shows clear in the sight glass on the side of the valve. Backwashing can waste 50 to 300 gallons of water, so pay attention. In addition, be aware of local codes regarding pool water disposal. Many communities outlaw such procedures and could be hit with a stiff fine. Check with your local authorities on the accepted methods in your town.
DON’T move the valve handle. One thing you should never do is move the handle of the multivalve while the pump is pushing water through it. It will damage the valve and possibly other equipment as well. Fully depress the valve handle when turning it to prevent damaging the valve seat gasket.
DON’T backwash excessively. One unknown feature of this filter is that it operates most efficiently during mid-cycle. As it collects particulates in the sand bed, the debris that is caught up in the sand actually helps the filter work more effectively. Too much backwashing will reduce the efficiency.
DON’T vacuum a new pool with the filter on backwash mode. You could plug up the laterals at the bottom of the filter with dust from the new pool surface. This restricts flow to the filter, resulting in inefficient operation. All the sand would have to be moved to unclog the laterals.
DO protect the laterals while replacing the sand (using 20-30 silica). Fill the tank with a foot of water to shield the laterals and cover the upper diffuser, or stack pipe, so that said doesn’t get inside.
DO fill the tank with sand to about half-full and keep it level. Some models may vary , but the general rule is 12 inches from top of the sand bed to the bottom of the valve or top diffuser
DO start the filter in the rinse cycle for 1 to 3 minutes after starting a new filter or after replacing the sand in an existing one. Some small particles and impurities may still blow back to the pool. If this happens, always vacuum the debris to waste you may keep sending it back to the pool.
Your local pool company will have basic guidelines that will keep your sand filters in top-notch condition and your pool water sparkling clear.