Wastewater treatment systems use biological processes to treat waste, so you shouldn't put anything into them that hasn't first been ingested. Otherwise you risk having your septic tank blocked.
There are a few exceptions such as toilet paper, mild detergents and water from washing.
The following list of items are likely to upset the biology of your septic system, or make your septic tank blocked:
Food scraps – Although food is digestible, we do not recommend putting ground-up table scraps down the drain, as they will increase the solids accumulating in your septic tank. Ultimately meaning you'll need to pump out your tank more frequently. Often ground-up food scraps contain oils and fats which stop them quickly settling. Adding to Total Suspended Solids (TSS) this can be a contributing factor to making a septic tank blocked.
Chemical drain cleaner – We don't recommend using chemical drain cleaners or flushing any products containing lye into your system, as even a small amount of these substances will negatively affect the biological activity within your wastewater system. Instead, plungers or similar plumbing tools should be used to unclog your drains.
Water softener backwash brine – This can supplant the sludge layer and disrupt the scum layer in your septic tank, resulting in an early and expensive failure of your drain field. It can also negatively affect the biology in your septic system.
Medications – Many medications will harm the biology of your septic tank, and will also pass untreated through any wastewater treatment and end up dispersed into the environment.
Chemicals – It's easy for household chemicals to end up in your septic system. For this reason, we recommend using non-toxic alternatives where you can.