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How does chlorine get into a private septic system? 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.

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review, content suggestions, critique are welcomed and are listed at "References."

This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

CHLORINE INTO SEPTIC TANK WASTEWATER - Chlorine Dosing Systems for Wastewater Treatment

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

• Chlorine Tablets - NO - Disinfection of Septic Effluent Discharged from Aerobic Septic Systems - use of Swimming Pool Chemical
• Calcium Hypochlorite - YES - Aerobic Septic Systems: Calcium Hypochlorite for Disinfections In Onsite Aerobic Waste Treatment Septic Systems

Is it OK to Flush Swimming Pool Backwash into a Septic Tank and Drainfield?

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.

Effects of Backwashing 5000 Gallons of Swimming Pool Water into a Septic System 3x Weekly

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.

Effects of Backwashing 320 Gallons of Swimming Pool Water into a Septic System Once a Month

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.

Effects on the Septic System of Shocking or Sanitizing a Well

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.

• If we assume that the viscosity of commercial bleach is about the same as water, a gallon of bleach contains 90840 drops that would "treat" about 11,000 gallons of water in the conditions described in that article - starting with water that is "close to drinkable" - that is, it is not sewage in a septic tank.
• One U.S. gallon = 128 oz., so one ounce of bleach is about (90840 drops / 128 ounces) or 700 drops of liquid per ounce.
• One cup (8 oz) of bleach is about (8 oz x 700 drops) or 5,600 drops of liquid in one cup
• Finally (5,600 drops / 8 drops to treat one gallon) means one cup of bleach will treat about 700 gallons of water for drinking purposes with a 30 minute exposure time.

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.
When we flush out the water system after shocking the well let's put about our 150 gallons of water right into a typical 1,500 gallon septic tank, without allowing for the additional dilution that will actually occur as we continue to run water out of our well and through the plumbing system until we no longer smell any chlorine at our sink faucets.

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

• The additional volume of water run during the flush-out of the building plumbing system
• The additional volume of wastewater as the building returns to normal use
• The deterioration of strength of volatile chlorine in water, which tends to leave standing water on its own

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.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

DRINKING WATER - EMERGENCY PURIFICATION
WELL CHLORINATION & SHOCKING
WHAT CAN GO INTO TOILETS & DRAINS?
NEVER FLUSH INTO SEPTICS
BETTER NOT TO FLUSH
HOUSEHOLD CHEMICALS
OTHER CHEMICALS
CHLORINE IN DRINKING WATER
CHLORINE IN SEPTIC WASTEWATER
  Effects of Septic Treatment
  Effects of Pool Backwash?
  Effects on Septic of Well Shock?
DISHWASHERS vs SEPTICS
DRUGS INTO the SEPTIC TANK?
GARBAGE GRINDERS vs SEPTICS
REVERSE OSMOSIS CONCENTRATE DISPOSAL
TOILET TISSUE?
TOILET TISSUE TEST
WASHING MACHINES
WATER SOFTENERS
* NO FLUSH SUMMARY LIST

• Ohio State University article on the concentration of chlorine necessary to act as an effective disinfectant, and the effects of the water's pH and temperature: See http://ohioline.osu.edu/b795/b795_7.html for details.
• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

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More Information on Building Diagnostic Inspections and Repairs

• The Septic System Information Website home page for this topic


• Additives & Chemicals: Septic System Maintenance - Are they even needed? Are they even legal?
• Biomat Formation in the Septic System Drainfield Absorption System - what leads to drain field clogging and expensive drainfield repairs
• Buyer's Guide to Septic Systems - what to do when buying a home with a septic tank
• Cesspools Home Buyer's or Owner's Advice
• Cheating on Water Tests - (Chlorine in Wells and Septics) Testing Water for Real Estate Transactions - Water Test Cheating Warnings for Home Buyers and Home Inspectors
• Clogged Drain Diagnosis - is the problem the septic system or the Building drain system?
• Drywells Home Buyer's or Owner's Advice
• Contamination in Buildings how to test buildings for bacterial contamination from Sewage and Septic backup, remediation, & references
• Leach Field Inspection Procedure Septic Leach Fields - how to inspect and diagnose septic drainfield failures - what causes septic field failure?
• Maintenance: Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• Septic Systems Inspection, Testing, & Maintenance Book - online textbook: how to inspect, maintain, repair septics
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Septic Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
• Sewage & Septic Backups and what to do about sewage spillage in buildings
• Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
• Tank Pumping Frequency Guide for Septic Tanks: When, How, What to Watch For when pumping or cleaning septic tanks
• Water Usage: Capacity: Determining Needed Septic Capacity - Typical residential water usage for people, appliances, and activities.
• What is a Septic System An Engineer's View & Septic System FAQ's

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