Ian,
With such a lot of chemicals that go down the drains nowadays, it is no wonder that the old filter and settlement beds packed up, leaving the only alternative, scrubbing the water with even more chemicals.
With regards to the citric acid pickle, if you were to put a bit of sugar in it, I am sure it could be classed as a pseudo lemonade (before it was used by the way, drinking it after use, you might start to glow in the dark). This could readily be discarded down the drains quite safely, if you use non cadmium silver solder. If you have any doubts, soak it into some cat litter in a plastic bag, and dispose of it at your local tip in the hazardous waste bin, they usually incinerate it.
I have never used anything stronger at home, so I wouldn't know how to dispose of the stronger acids.
John