Backwashing is the process in which used filter media (D.E. powder) and trapped dirt or contaminants, are flushed out of your Diatomaceous Earth pool filter. Backwashing a Sand filter does not flush out the sand, just the dirt trapped within the sand bed. Backwash water contains the same chemical properties of your pool water, plus all of the "stuff" that your filter has removed from your swimming pool. It is important to know and understand local backwashing laws and regulations so you can comply and do your part in keeping your community environmentally friendly.
The Prevention of Pollution Law of 1989 states that "No person shall cause or permit the occurance of pollution, or cause or permit a risk of pollution to arise". Many localities extend this definition to include public and private swimming pools. Specifically, To prevent such pollution, many cities or water authorities do not permit the disposal of swimming pool waters containing chlorine, bromine or salt (sodium chloride), into surface water drains, land drains, streams, douits or drainage ditches.
The issue is two-fold. Some water authorities don't wan't the extra clean-up required to treat your pool backwash water. It's like doing the work twice, you filter it and flush it, and then they filter it. More money required, more taxes needed.
The second issue is that many communities have documented Fish Kills in local streams caused by pool waste water. If you have a salt water pool, the salt level of your pool water, pumped into a local stream can be enough to kill fresh water fish. If your Chlorine level is very high (over 5.0 ppm), this can also kill. DE filter powder, although natural, can "choke" the oxygen from a small stream and kill many types of fish, plants and small critters.
On the other hand, if your pool water has very low salt or chlorine levels, and especially if the water is green and nasty ~ pool water in this condition is fine. But best practice would be to pump your backwash water to an open area, at least 30 feet from any stream, and allow the Earth to filter the pool waste water before seeping into any tributary.
Several methods are available to help you comply with your local laws. A simple way to avoid wasting water and prevent discharging contaminants is to install a separation tank. A separation tank has the ability to collect used diatomaceous earth while allowing pool water to return to pool. The used media along with filtered particles will be caught in tank allowing you to discard into trash can. The downside to separation tanks are that they need to be emptied after backwashing and they tend to create a lot of back pressure, which reduces the backwashing effectiveness.
Some builders will dig a large pit, 6x6x6 feet in size, and fill it with graded sizes of rocks and pebbles. The rigid backwash pipe is directed deep into the pit. Underground sections of the pipe are drilled to create weep holes. This method is a good method, but not everlasting. It may eventually need to be redone after 15 or 20 years.
Another method designed to be economically friendly is using Purifiber. An alternative to D.E, this cellulose product is non-toxic, completely biodegradable, economically friendly, and non-abrasive. However, large volumes pumped into a small stream can still be damaging.
Another alternative to backwashing your filter is to install a cartridge filter. Cartridge filters utilize a paper element filter that does not require additional filter media, therefore backwashing is not a required procedure. Instead of backwashing, by reversing the flow through the filter tank, the cartridges are removed and cleaned with a garden hose.
The downside to cartridge filters is that the servicing person must break down the filter to access filter elements, and hosing them off is just not as fun as simply turning the filter valve to the Backwash setting. A cartridge filter also does not filter down to the same particle size as a DE filter, but is considered to be the greenest alternative in pool filtration.
So, when backwashing, or draining a pool completely, or lowering the water level for winterization, take care where you pump this water. Again, best practice is to pump to a large open area, at least 30 feet from any stream.
If draining the pool, you may need to move the hose a few times to a new location. If you can reach a city storm drain, (the curb) this would be preferable than pumping out the back into the woods, but check with your local water authority for any restrictions.