As for ventilation you could just baffle any fan inlet/outlet using flat black spray paint and some plywood. Making something like ||| so that the air follows a U and light can't go by

As for chemicals probably spent fixer and selenium are not good for septic tanks.
Fixer: put some steel wool in your spent fixer can, shake it a couple of times, let it stand for a day or 2 and then you can dump the rest
Spent Selenium, diluted with water can be a good fertilizer, don;t over do it though.