Quote Originally Posted by Roger Cole View Post
Are you so sure of that in normal hobbyist quantities? It contains silver ion but the ions won't stay ions for long in nature much less a sewage plant. Metallic silver is microbicidal but in hobbyist quantities is it really any worse than washing with antibacterial soap and washing it down the drain?
Roger Cole is right. In moderate quantities (pounds, not tons) silver tetrathionate and similar compounds which characterise used fixer don't harm sewerage treatment systems. The silver very quickly gets converted to silver sulphide in the presence of the free sulphide ion. Silver sulphide is geologically stable and inert and has one of the lowest solubility products known in chemistry. The stability and inertness of silver sulphide is the key to the remarkable archival properties of sepia toned photographs.

Before my darkroom was approved by my local council I had to calculate the silver concentration in my total household effluent. I'm pretty busy and use a few thousand sheets of film and paper per year but the result came to about 5 parts per billion. By the time this mixes with the output of the other 20 000 households that don't process photographic materials the silver concentration is below any conceivable detection limit down at the sewerage treatment plant.