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How does chlorine get into a private septic system? Is it a problem? When are chlorine or other disinfectants needed for aerobic or other septic systems? Will septic tank chlorine damage the septic tank or leach fields? This document explains how to extend the life of the septic system by being careful about what goes into it.
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In locations where wastewater effluent must be sanitized by chemical treatment before it is discharged to the environment, chlorine products and various dosing methods may be used in some system designs. For example some sand bed filtration systems produce effluent that is treated with chlorine injection before the final discharge of effluent to the environment. .
These are alternative septic system designs which are intended to be chemically treated including using disinfectants such as calcium hypochlorite or chlorine.
The previous section which discussed water chlorinators for drinking water does not normally pertain to chlorine-dosing septic systems.
For aerobic septic systems which must use a disinfectant stage, see our article on what disinfectants to use (not pool chlorine tablets) at AEROBIC Septic Systems Final Treatment Stage
More Reading on Disinfection Tablets for Aerobic or Aeration Septic Systems
More about the effects of chlorine (or bleach) on wastewater and the environment is at WASTEWATER BIOCOMPATIBILITY.
Below is our informal analysis in reply to this interesting question from a reader. The reader proposed backwashing a swimming pool into a large septic tank and an unknown drainfield size, using two alternative load levels which we describe as two options.
Note: The author (D Friedman) is not a septic design engineer. The following comments are opinion based on research and experience and should be amended by a septic engineer and adjusted for the specifics of your particular case.
Option 1: backwash at 355 gpm and produce 5000 gals of waste 2-3 times per week. Chlorine levels are expected to be at 2ppm
5000 gallons of wastewater at 2 ppm chlorine dumped into a 10,000 gallon tank at 355 gpm would mean running that volume of water into the tank in about 15 minutes. Your septic tank will reach about 1 ppm of chlorine at that rate before it is further diluted by other wastewater.
A concentration of 1 ppm chlorine in a septic tank is unlikely to kill all of the septic tank and drainfield bacteria off severely immediately but over time may reduce the total bacterial activity.
We need 10 ppm (10 mg/L) concentration of chlorine in water with an exposure time of 30 minutes to kill virtually all Giardia and bacteria in water; that time needed would be increased for cloudy turbid water (in a septic tank), for higher pH water, and for cold water. -- OSU
Septic Tank Flooding by the Swimming Pool Backwash
There is a risk that putting such a large volume of water from a swim pool into a septic tank or onto the drainfield would ruin the septic system.
High entry rate in gpm of water into the septic tank is also likely to be a problem. Pumping wastewater into the septic tank at 355 gpm is also likely to be a much faster rate of water insertion than normally occurs from a residential building served by a septic tank. This surge and flow rate may agitate the septic tank contents, increasing the level of suspended solids significantly.
A typical residential septic tank and drainfield are designed to receive about one tenth of the pool backwash volume twice a day - or about 1000 to 1200 gallons a day. See Water Quantity Requirements and see the Table of Septic Tank Size Requirements
5000 gallons of water into a tank during one day is about the same as the wastewater usage from perhaps 50 people in a building. That is 2 ½ times the expected usage level for a 5000 gallon septic tank.
High volumes of water from a swimming pool into a septic tank will also flood the tank and prevent proper processing of the waste therein. Flooding a septic tank means that the users are pushing a high level of suspended solids, abnormally high in addition, out into the drainfield - there is not enough settling time -shortens the drainfield life or ruins it promptly.
Drainfield Flooding Caused by Swimming Pool Backwash
High volumes of water from a pool backwash onto a drainfield will flood the field and prevent proper processing of the septic tank effluent that is meant to be discharged there.
You would have to tell me that your septic engineer designed the drainfield to handle surges in wastewater volume at a level equal to the sum of the usage load from the building it serves plus the swimming pool. I'd be surprised if someone were willing to foot the bill for such an over design - some sites won't have sufficient space for a working drainfield of the size needed to handle what amounts to a more than 50 person load.
Effects of Frequency of Swim Pool Backwash
Spreading out the load by only backwashing every other day or two is unlikely to be sufficient to allow the tank or field to recover.
Summary about High Volume and High Flow Rate Water Sent into a Septic Tank
You should not discharge swimming pool backwash into a septic tank. Doing so will flood the tank and push solid waste into the drainfield and there may be some temporary reduction in the bacterial activity, reducing the treatment level in the septic tank and thus increasing the BOD and suspended solids.
You should not discharge swimming pool backwash anywhere within 100 feet of a drainfield. Doing so is likely to saturate the soil, interfere with drainfield treatment of septic effluent, and possibly harm soil bacteria.
Option 2: gravity for a total load of 320 gals per month. Chlorine levels are expected to be at 2ppm.
Septic Tank Impact from 320 Extra Wastewater Gallons in a 5000 g Septic Tank
A once a month additional load of 320 gallons into a septic tank, especially a large 5000g unit, is the equivalent of about 3 more people of load once a month - which should not be a significant percentage increase in load compared to the design level of a 5000 g tank.
Septic Tank Impact of Chlorine in Wastewater
The chlorine load of swimming pool water at 2 ppm may have a small impact on bacteria in the tank but at an infrequent insertion is unlikely to have any lasting impact so long as the system is also in normal use as a septic tank receiving black water from a building.
Drainfield impact of 320 Extra Wastewater Gallons in a 5000 gallon Septic Tank
As above, a once a month additional load of 320 gallons into a septic tank, especially a large 5000g unit, is the equivalent of about 3 more people of load once a month - about the same as having a few visitors for a weekend. I would not expect the water volume to have a significant impact on the system.
In general it is better for a septic system to reduce rather than increase the water load into the tank and drainfield. If the septic tank were a typical residential 1000g or 1200g tank, the impact of these wastewater loads could be significant.
At WELL CHLORINATION & SHOCKING we describe the procedure for sanitizing a drinking water well. This procedure is used following certain well, pump, or plumbing repairs and it is often used in a first-stage attempt to diagnose (and on occasion "cure") bacterial contamination of well water. For a 100' deep well column of water (that is 100' of water is in a 6" steel casing - we don't care the actual well depth, just the amount of water in the well) we suggest using 3 cups Clorox™ to shock the well.
Actually when you are running water into a building as part of flushing out and sanitizing a well, the volume of water being treated is greater as you are drawing water out of the well, more water is running into the well from the earth, and that more dilute solution is entering the building water tank and piping.
Commercial "chlorine" bleaches usually contain 3 to 6% sodium hypochlorite (NaClO). Some bleaches also contain detergents and thickening agents, and other oxygen bleaches using hydrogen peroxide do not contain any chlorine and are based on peroxy-compounds. Bleach in sufficient concentration and at sufficient exposure time will kill bacteria (by attacking certain proteins in the bacteria) and some other pathogens. The temperature of the water-bleach solution also impacts the necessary exposure time (colder means you need longer exposure for the disinfectant to be effective).
How Many Drops of Water (or bleach) are in One Gallon?
At DRINKING WATER - EMERGENCY PURIFICATION we discuss procedures for purifying drinking water, and the concentration of disinfectant needed. There we suggest that 8 drops of bleach will purify one gallon of water.
How Much Water is In the Well Being Shocked?
A 100 foot deep column of water in a standard 6" well casing will equal about (1.5 gpf x 100) 150 gallons of water.
So the health department rule of thumb that suggests we use 3 cups of bleach to shock a 100 foot deep well means we are using a concentration 12 times stronger than we would just to produce drinking water. (700 /150) x 3 cups.
This extra strength chlorine solution must be intended to accommodate disinfection of contaminated surfaces in the well casing, the pump, the piping, water tank, and fixtures, and to handle the dilution that occurs when we run water through the building plumbing system in the well shock procedure.
Conclusion of Effects of Well Shocking on a Septic System
To look at the effect of shocking the well on a septic tank, let's guess that we dilute the disinfecting solution not at all.
Our 150 gallons is joining a volume ten times larger (1,500 gallons) in the septic tank, so it is diluted by a factor of ten - we're back to a level close to the sanitation of normal drinking water.
Our dilute chlorine solution in the septic tank is further weakened by
Finally, considering that the concentration of sewage bacteria in the septic tank is enormously more than the original concentration of bleach in drinking water could have purified, it is not at all likely that following normal well shocking procedures would damage the septic system due to its chlorine concentration.
Details about the effects of chlorine (in bleach for example) on soils and on ocean bodies or freshwater bodies are at WASTEWATER BIOCOMPATIBILITY.
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