Dear Blue Lobster:
I specialise in waste treatment facility discharges, inspecting large-volume fluid discharge systems to ensure that everything is running smoothly in terms of mechanical structures, flow paths, and artificial and environmental blockages in flow systems.
Crayfish have begun to make their way up into the facilities. They occasionally need to be purged when the crayfish buildup becomes too dense. If crays were there just to harvest nutrients from the waste, why is it that is no other wildlife to be seen other than these crays?? I am stumped.
Dear Gentle Sir:
Your problem is a puzzling one. You don't mention your geography, but crayfish in general are opportunistic scavengers and so would be attracted to any palatable biomass which became available in their habitat. The factor that does not make sense is the human feces, which crays do not regard as food.
Despite the fact that crayfish feed on rotting organic material, they are not coprophages. The bulk of human feces is bacterial matter and that, combined with the fact that the human gut is efficient at absorbing nutrients from food before it is passed, leaves only a few possibilities to explain your cray problem.
One is that the waste you are processing is not simply solid human waste. Are there perhaps cat or dog feces in the mix? Their high protein content would attract crays. Has there been a large epidemic of diarrhea recently? Undigested materials passed in leaky stools would afford the cray nutrients by the gallon.
Perhaps the best measure is to chemically examine the sludge and, once certain properties that might attract crays are established, track down the source of the attraction. In the meantime, a shovel or spade and a good tight wetsuit are in order. Good luck!