In terms of clay, when I was shooting with Penny Banks, we did a full VERY thick clay bath day in a rented apartment. Loads of us had a go, and then hopped into the shower nearby, which seemed to handle the run off with no issue. Then to dispose of the content of the bath, we scooped out half buckets and then diluted them with another half bucket of water will it became a thin slurry, and then poured them down the toilet. Flush and repeat, and there seemed to be no issue at all. Highly recommend giving it a try! It tends to dilute better than most other substances, so unless you have particularly feeble drains and no water pressure, I don't see an issue.