Our homemade olive oil dressings congeal in the refrig. Not solid, but it won't pour.
That was my thoughts too. At the temperature of my drainpipes (basement usually doesn't get below about 65 degrees year round) sour cream is fairly liquid. What bits of cottage cheese that would go down the drain would be liquid as well. Not sure about the run between the wall and the septic, but I would think that one person showering would get things warmed up enough to get it moved to the septic tank.