The reason there is nothing that deals with home darkrooms is because it really is a non-issue. The qty of silver you dump down the drain is going to be measured in fractions of parts per billion when it arrives at the sewage treatment plant. They can't do anything with the majority of prescription drugs, including the chemotherapy ones that pass thru a persons system and are discharged into the toilet either, and that is a huge problem for waste water treatment plants.
It's not like you are printing 2000 4x6 prints per day and processing a couple of hundred films per day, which can to be the thru-put of a single mini-lab setup at the local grocery store.
Edit, there even might be enough iron and other chemicals in the sewer pipes to react with most residential home darkroom silver discharges long before it gets to the treatment plant.