I use a heavy metal test strips to check the THH solution and I found there is less than 100 ppb of heavy metal cations (Zn(II)) in a concentrated solution of THH I made from the final product. The tests strips check for many heavy metals, not only zinc. Metals like tin, cobalt, copper, lead, mercury etc.
If there is less than 100 ppb in the entire solution, then the THH precipitate is certainly safe.
What I do is to filter the reaction mixture (containing THH and metal acetates) first through a paper filter, then I precipitate THH freebase with 5% ammonia (slow addition, strong stirring), filter, wash, redissolve crude THH in warm vinegar, filter through a bed of diatomaceous earth and finally re-precipitate with 5% ammonia, filter, wash, dry.
Note that acetic acid is too weak to dissolve most metals unless they are oxidized.
Finally, I neutralize the spent reaction mixture with just enough NaOH. This should precipitate the zinc as zinc hydroxide without redissolving it as zincate.
If you are still concerned about pouring traces of heavy metals down the drain, they can be removed from waste water by the addition of sodium sulfide soln. It causes precipitation of the said metals as sulfides, which are practically insoluble in water. I use this method to process aqueous copper and mercury waste water in my hobby lab.
Even if some metals slip through to the product, this is not a big deal. Our bodies detoxify from heavy metals (otherwise people living in cities would be aready dead

), though slowly, not to mention these are already normally present in food (e.g. fish).