Our drains back up yesterday back flooding into the bathroom. The flood was minimal internally but after a day today of shifting and flushing the drains in the garden the patio has been flooded with sewage waste (absolutely stunk so certainly a back up of a lot of toilet waste). Most of its been cleaned up now but what do we need to do to make it safe for our dc to play out there again? I assume that we need to somehow chemically clean the flags??