A few years ago, I started noticing that my mound system was showing areas of very green grass on top of the mound. Why was this happening when all other area mounds seemed to be rather dry. Being that mine is the oldest system on the neighborhood, the reason was that a layer of bio sludge had built up under the distribution piping casing water to move up rather than down into the mound. After a bit of research, I found a possible way to fix the mound without digging into the mound.
The reason the bio sludge was building was likely due to the fact that this is a anaerobic system. This means that very little oxygen is flowing through the system. With little oxygen, an entirely different collection of organisms rule the tank. These anaerobic organisms eat slowly and excrete noxious, stinky hydrogen sulfide gas that can actually eat the concrete risers and tank covers.
With the simple injection of air, the tank organism population will quickly change to aerobic organisms. Aerobic organisms are much larger and has a faster respiration rate, making them vigorous feeders of raw sewage. In our system, we have placed aeration units in both the solids tank and the grey water tank. By the time this organism rich effluent exits the grey water tank, they are still very hungry and will eat the bio sludge build up out on the mound, essentially cleaning the mound and making it more efficient.