G'day Stephen,

The dose rate for this concenrate is:

• Dose 60 ml per 20L waste-holding tank capacity and add 2-3L of water

Therefore, with a bit of maths we know the dosge rate for a 520L tank is 1.56L.
Thetford recommend re-dosing every 5 days when in use.
There may be a better product available commercially for larger tanks like yours, I'd probably recommend a larger quantity of BioMagic then we sell, but they'd be great to call to get some more advice.
They've always been incredibly helpful this blackwater inquiries.

If you're not going to be using to boat for a fair while, I'd probably recommend dumping/emptying the tank before you leave.

G'day Gary,

It's not essential to add the chemicals, Thetford does recommend adding 60mls every 5 days when the toilet is in use which helps to break down the waste, reduce odours and gas build-up of what's in the holding tank of your toilet.

If you were not to use chemicals the waste wouldn't be broken down and you may have some more smells. Using the chemicals helps the smell, cleans, and makes emptying the toilet easier. You shouldn't have any issues not using chemicals if it is emptied regularly. 

G'day Graham,

No, you cannot dump your portable loo with chemicals into a septic tank.
There are some products available that claim to be septic safe, but this only allows you to dump into your own personal septic tank at home where you know what the method the septic system uses, and can confirm with a septic professional that its safe.

If you are not responsible for the maintenance of the septic system, or haven't had permission to dump into it, you shouldnt be - regardless of whether or not you believe the toilet chemicals to be safe.

Google is a great resource to find a dump point close to you, & there are literally everywhere. Having said that though, if you know you'll be away somewhere without one & you can't wait to get to one, I wouldn't be using chemicals at all.

G'day Boz,

No, it's not, the cap is quite small like a standard soft drink bottle.
The Aqua Kem Blue Concentrate instructions advise you to use the measuring cup from your waste holding tank.
Alternatively, you could pick up a small measuring jug or cup from a cheap shop without too much trouble.