I have some suggestions for your problem, from my extensive house painting experience. I recycle kerosene by letting the sludge settle, and I use the same technique with clay and plaster water in my classroom.
Ask at the Home Depot or a local drywall/plaster contractor for donations of 5 gallon pails with lids. Pour all your dirty water into them, then seal them with their lid. They can be stacked outside, unless it freezes. Does it freeze where you are, Bunki?
If you have water dirtied only with glue, tempera, clay, acrylics, plaster or watercolor, just let the pail set undisturbed for a week, letting all the particulate settle. Pour out the relatively clean water off the top, right on the ground would be ok. Then combine all the sediment into one bucket. When a bucket is fulled with consolidated sediment sludge, sacrifice that bucket and put it in the rubbish dumpster.
Soapy water will not become safe enough to pour out on the ground, even if you leave it to settle. Use separate buckets for soapy water. Mark these buckets to be removed to a drain for emptying. Keep tight control of soapy water, limiting it's production.
You'll need some strong backs to help with this. One 5 gallon pail of water weights something like 40 pounds. Ask your willing and able custodians to help you by supervising selected middle school kids with the task of pouring and removing the buckets at the end of each week with their dolly, and I'll bet they'll just take care of it.
My art room has very poorly designed sinks that get us wet just thinking about turning them on, but I wouldn't trade with YOU right now.
Good luck. Mark
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