Septic Tank or System Drain Backup due to Heavy Use or Area Flooding: Prevention Methods

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Septic or sewer backup prevention:

This document explains how to avoid septic system backups when heavy use of the system is anticipated and for homes connected to a municipal sewer we discuss how to prevent sewer or storm drain backups into a building during rain or heavy flooding.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Guide to Avoiding Septic Backups During Heavy System Use

I bought a home in August 2007 that uses a 2000 gallon Septic Tank.  At that time, it was inspected, possibly pumped out and was in fine working order.  We are a family of 4 and for us, we’ve had no problems.  On occasion, we have maybe 8 people visit at a given time.  We are expecting about 60-70 people over for a Birthday Party and my husband is freaking out! 

He thinks the septic tank will back up into the drains if we have that many people using the toilets, over a 6 hour period.  My question is: Is my husband correct in thinking that way, will the septic system back up??????  Could you PLEASE answer these questions  AS SOON AS POSSIBLE??????  The party is on 04/26/08.

Why Does the Septic System Back Up During a Party?

Indeed it seems to be just the luck that we are living happily along not giving the septic tank a thought until we have a bunch of guests over for Thanksgiving, Chanukah, Christmas, or a graduation party. Why is it that at events we often see the septic system backing up?

The explanation is that a septic system that backs up during a party was already in trouble, but our usage was modest enough that we just weren't noticing it. The surge of waste water entering the septic tank cannot flow into a flooded drainfield so sewage may back up into the home, usually at the lowest plumbing fixture. Sometimes it's not the wastewater surge but someone flushing something down a toilet that blocks a drain - that's a problem that can be cleared by a plumber using a plumbing snake or drain router. But often the problem is in the septic field itself.

Here are some simple tips for avoiding a septic backup during times of surges in use such as during a party

  1. Pump the septic tank before the party: When a septic system is otherwise in good working condition, the septic system stress created by having many people use the sinks, showers, and toilets over a short time is not so much the solid waste as the high wastewater volume in gallons. Very high water usage over a short time can flood a drainfield or septic mound which in turn could cause a septic backup. Pump the septic tank tank right before the event, or the day before. This will give some extra capacity to absorbing this high use resulting in a sudden surge in wastewater volume.

  2. Avoid using water unnecessarily after pump out. That means don't leave running toilets un-repaired, don't leave water running unnecessarily. See WATER QUANTITY USAGE TABLES for a table of typical daily residential water usage under normal conditions.

  3. See TOILETS, DON'T FLUSH LIST for a list of things that should not be flushed into a septic system: a list of what's ok and what's not ok to put into septic tanks and building drains.Some of these can cause a system drain blockage and thus a sewage backup into the home.

  4. Regular septic system maintenance, pumping on schedule, is the long term way to protect the system tank and drainfield from early failure and thus to protect from septic system backups. See SEPTIC CLEARANCE DISTANCES and see Ten Steps to Keeping a Septic System Working for a more thorough discussion of how to care for a septic system, or review our complete list of septic maintenance articles at SEPTIC SYSTEM CARE GUIDE
  5. See BLOCKED DRAIN REPAIR METHODS for simple methods to clear a blocked drain yourself.
  6. Provide additional or emergency toilets when an unusually large group of people are expected at a site served only by a normal residential septic system, consider renting one or more port-a-potty toilets from a local supplier. This is an especially good idea if you already know that your septic system is old, failure prone, and/or of limited capacity.


How Much Wastewater Will Be Produced by 60 Visitors During a Party?

In the email example above where our writer assumed there would be up to 70 people visiting the home over a 6 our period, if every visitor uses the bathroom and at each use flushes a toilet twice, that'll be 120 x 4gals = 480 gallons which will be less than the septic tank can hold.

So if otherwise conserve water, we have a typical home septic tank of 1000 gallons or larger, and if we start our festival with the tank nearly empty, we're sure to protect the drainfield from flooding during the event.

If the septic system is already in good working order you should be ok.

If your drains are already backing up, especially during a time of heavy use such as with guests in the home, see SEPTIC BACKUP REPAIR.

If toilets are overflowing or not flushing, see TOILET OVERFLOW EMERGENCY.

Guide to Preventing Septic or Sewer Backups Due to Flooding or Sewer Main Backups

Please see our full article on this topic, found at SEWER BACKUP PREVENTION

Question on basement sewer line backup:

I had a sewer backup into my home again yesterday at a basement toilet. Our main line has a sewer back-up flap [a wastewater check valve or backwater valve]. ... Is there such a thing as a soft plastic or other material filled that would take the shape of the toilet and prevent water coming back up as overflow. Like I did with a ball of cloths, and a brick wrapped in plastic. It did the trick in an emergency, but it's not practical. - Anonymous by request.


The advantage of installing a main sewer line backup prevention valve is that this device will avoid having to plug multiple drains in the home, and the valve, basically a big check valve, is always in place - you don't have to do a thing to get it to work.

If you are having drains backup, including at the toilet, then either your main sewer line check valve is not working, or your backup is occurring (as you suspect) because water or wastewater is draining into your in-house building drain/waste/vent system before or ahead of the main sewer drain check valve. While you could install another check valve at or near the basement toilet waste line, it makes more sense to install just one such valve to protect the entire building and to make sure that one is working properly.

Stuffing a rag or any other temporary "block" into a toilet or other drains is not the best approach to this problem. Not only can it be unsanitary and a health risk in some cases, but also, who is going to stuff drains when flood conditions occur and no one is at home?

Install a Main Sewer Line Check Valve or Make Sure Installed Valves are Working

First you may want to have your main waste line backup check valve inspected -if the sewer line is backing up from the street into your home, your main sewer sewer backup valve (waste backwater valve) or a flood guard valve is not working.

Here is a Check Valve Maintenance Guide from the City of Ann Arbor, MI.

Don't Route Roof or Surface Drainage into the Sewer Piping System

Second, you should disconnect your roof drainage from the sewer piping system entirely, routing it to a nearby storm drain, or to the ground surface (at least 12 feet away from the building and to a location that drains away from the building to avoid basement flooding).

If connecting roof runoff drains to the sewer system is actually permitted in your neighborhood, try changing the drain connection to one that is downstream from your main sewer line trap and check valve.

By the way, in some communities it is illegal to rout roof runoff into the sewer piping. Doing so significantly increases the wastewater volume load on the municipal sewer treatment system so severely that during a storm the sewage treatment plant simply overflows, dumping raw sewage into nearby rivers or waterways.

Please see our full article on this topic, found

Also see the special sewer line backup check valves discussed
at BACKUP PREVENTION, SEWER LINE. If your drains are already backing up, especially during a time of heavy use such as with guests in the home,

If toilets are overflowing, see TOILET OVERFLOW EMERGENCY.

After the emergency, see CLOGGED DRAIN DIAGNOSIS & REPAIR.

For homes connected to municipal sewer,

And for emergency toilets, see CAMPING & EMERGENCY TOILETS.

Septic or Sewer Backup Articles


Continue reading at SEWAGE BACKUP, WHAT TO DO or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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