@Paul73
As a school educated but hobby chemist now, I'll tell you there are three ways to get rid of your waste.
1 - Read and study the forum, specifically this message board for dealing with waste. It walks through, step by step what is required, and only used three cheap materials. Soda Ash, Iron Metal, and Sodium Hydroxide. You'll want to filter it in the end, but all in all, a simple procedure if you do the homework of making a plan, test your plan on a small amount of waste, then scale your way up to processing a larger batch of waste. I'm following this same procedure to process ~10 gallons of leftover copper nitrate from last seasons refining.
2 - Pay a company to take it off your hands. There are plenty of chemical waste companies available and they'll take your magic elixir and charge you a pretty penny for it. But it leaves you with a clear conscious and no waste to treat. (Consider it a small price to pay for the gold your just recovered!) Just make sure all your precious metals are out before handing it over. When it doubt, cement it all out (preferably on copper metal).
3 - Piss off the EPA, by which I mean either dump it somewhere, or dispose of it improperly by sealing it up and sending it to the dump. I live on a little 10 acre lake. I'm pretty sure my 10 gallons of copper nitrate would kill just about everything in that lake and poison the people who fish the lake as well. (This really isn't an option, but clearly states that illegally dumping is a bad idea.)
So, there you go, two good and one bad way to deal with your waste. Luckily for you, you're a part of this forum now, who has untold amounts of knowledge and training across a wide range to things. If you get stuck, just drop a question in here and someone will gladly give you a bit of advice on what you should think about trying next.
Elemental
Edit: In regards to your holding tank, if it's a normal septic tank with a drain field, you're going to be killing all the bacteria in there that break down your human waste by dumping your chemical waste in there. Septic tanks only work by having the bacteria eat your solids wastes, so you don't have to have them pumped as often. Frugal already hit on the heavy metals going into your drain field as well and basically making your own mini superfund site.
A tale from long ago, I grew up in a small town in Western Michigan. This little town had everyone on well water. Well, the town also had a dry cleaner, who much like yourself at first, thought dumping was no big deal. So every night, he'd dump his dry-cleaning chemicals out in the storm drain behind his store. Well, people in town started getting sick, and they tested the water and sure enough his chemicals had spread through-out the entire aquifer supporting the town. The town then had to pay (a lot of money) to have a water-tower put in along with piping to a new well station over 10 miles of out town, so they could have clean water. Please don't by like this dry-cleaner. One day you'll move on as we all do, but whatever mess you leave behind will stay.