I had a problem disposing of waste sudsoil. My local recycling centre would not take it and the only place that would take it was miles away.
I was eventually advised (by the local council environmental officer !!!) to pour it down the drain (YES). In the end I put it in the bin in a closed container.

In your case, I would imagine that neutralising the solution with sodium carbonate (washing soda) would convert the effluent to chemicals which the sewage system could cope with. I.E. sodium sulphate, aluminium sulphate and carbon dioxide. The Ali sulphate is used in water treatment anyway - remember Camelford ?

Cheers,

Rob