There are two very important things to remember when you are going to pour something down your drain.

The first is that your septic system does allow fluids to drain out through a leach field which will in turn, enter the environment and make its way into the local ecosystem.

The second is that your septic system requires bacteria to break down solids and prevent damage to your system.

Any substance that could be toxic to the water supply - or kill off the necessary bacteria in your septic tank, should never be emptied into your system in large quantities.

For instance, rinsing off a paintbrush that was used for varnish is fine. Pouring the leftover varnish down the drain? You'd better not.

17 Chemicals that you should limit or avoid entirely:

1. Cooking Oils

Safe to eat does not equal safe to pour. Olive oil, canola oil, or any other vegetable oil, grease runoff from cooked meats, even salad dressings can all congeal and cause clogs throughout not only the pipes in your home, but throughout your entire septic system.

2. Motor Oil

For the exact same reasons as above, plus one, you should never dispose of motor oil down your drains, or even in your yard. Motor oil is harmful to the environment and dangerous for the water supply.

3. Paints

Many consider washing hands or brushes out in the sink to be acceptable, though we would never recommend that to our customers. Paints and paint additives often contain additives that are toxic. These toxins can leach into your water supply once poured down your drains. In addition to the toxins, the paint have a tendency to congeal and cause clogs. You should never put any paints or varnishes down the drain.

4. Furniture Polish

Furniture polish can be extremely toxic. Poisoning from furniture polish can cause a vast array of damage to multiple systems of the body. Read the labels of all cleaning products before disposing for proper disposal instructions, or call the manufacturer.

5. Lye/Sodium Hydroxide (NaOh)/Caustic Soda

Your septic system depends on bacteria to break down the organic matter in your septic system. Lye can kill these bacteria, causing your septic to sludge up much quicker than it would otherwise. Lye can be found in most cleaners, and in reasonable amounts, it will not cause a huge problem. As a base, it affects pH level of anything it is added to and can increase the pH of your water supply above what is safe for consumption.

6. Drain Cleaners

Drain Cleaners such as Drano are generally lye based and can kill of the necessary bacteria in your septic tank.

7. Oven Cleaners

These are also lye based products and should be disposed of per manufacturer recommendations.

8. Pesticides

It's important to remember that anything that goes down your drain will eventually end up in the water table. Pesticides can be toxic in large quantities, and certain pesticides contain bacteriocides, or antimicrobials that can kill the necessary bacteria in your septic tank.

9. Photo Chemicals

Kodak and other popular photography companies do not recommend pouring darkroom chemicals into a septic. These products can be high in metals that can affect your septic system, as well as have a toxic effect on the water table.

10. Paint Thinners

Paint thinner is an extremely volatile chemical solution. If it mixes with acids, it can have a very bad reaction. (pouring paint thinner down your drain an hour before your spouse pours out vinegar could be a huge problem.) It's highly toxic and should never be mixed with anything other than oil based paints or substances specifically recommended by the manufacturer.

11. Varnishes

Varnish is not only toxic and harmful to the water supply, but it is designed to harden, which can cause serious problems for your septic system.

12. Antibiotics

In normal quantities, such as flushing the occasional prescription down the toilet, antibiotics will not have a major effect on your septic system's necessary bacteria. However, if attached to a residence with a large number of individuals taking such medications, such as a nursing home, etc, antibiotics can cause a lot of damage to your septic system.

13. Cleaners High in Phosphates

Detergents and other cleaners with phosphates are harmful to the environment. It depletes the vital oxygen that fish and other aquatic organisms spent on and can cause significant algae problems in nearby lakes and streams. If you live in a lake community where septic systems are prominent, please educate your neighbors and encourage the use of phosphate free detergents. Phosphorus is harmful to the environment, as it can deplete oxygen which is vital to fish and other aquatic organisms. The use of phosphate-free detergents, also helps prevent algae problems in nearby lakes and streams.

14. Cleaners High in Surfactants

It is recommended to use laundry detergents that are low in surfactants. Visually, you can notice high levels of surfactants based on how foamy or sudsy the water becomes. Many surfactants have poor anaerobic degradability in the septic tank and may inhibit hydrolysis (the chemical breakdown of a compound due to reaction with water.) This impacts the ability of your septic to break down solids. Surfactants have also been shown to negatively affect aquatic life in high levels.

15. Illegal Drug Manufacturing Chemicals

True story: A household involved in the illegal manufacturing of drugs was pouring the chemical byproduct down the sink, which in turn poisoned the well and poisoning the people who were manufacturing the drugs.

16. High Quantities of Household Acids

The most common household acid is vinegar. Normal levels of vinegar are perfectly safe to put into your septic, but if you had a case of apple cider vinegar that just didn't taste right anymore (vinegar doesn't go BAD as it's self preserving as an acid) or large quantities of citric acid perhaps used in candle-making or some other hobby, the drain is not the place to dispose of these as you could have a negative effect on the pH of your water table. Please note though, that vinegar-based cleaners are much safer for your septic than phosphate or surfactant based cleaners.

17. High Quantities of Household Bases

Ammonia and bleach are the most common household bases. As a rule, ammonia is one of the safer base chemicals to pour down your septic. Bleach is also relatively safe. However, if your cleaning business closes down and you decide to pour out all of your bottles of cleaner at once, you can affect the pH level of your water table in a negative way. It's important to be mindful about disposing of high quantities of anything besides water and check manufacturer labels for suggested alternatives for disposal.