It was a hoax. The only high you'd get off inhaling sewage gas/methane is oxygen deprivation. Granted some people like to choke themselves out but helium inhalation is much funnier, and nitrous oxide is much more fun. Anyone whose done much of either knows you get a popskull headache. Most likely the original images from Africa were kids making methane for fuel. They do that, normally on a cesspool scale rather than a 2 litre bottle scale, to make gas for heating or firing a small cook stove.