This is what the clarifier section of a biologically healthy and properly operated Aquaklear 500F unit looks like. Notice how the water is clear enough to see below the surface. If there is a sludge build up or scum layer on your system, it means the following.

The bacteria, is either dead or in a weakened state due to the use of harsh chemicals of some kind. Aerobic treatment systems regardless of brand cannot maintain the appropriate amount of micro organisms if harsh chemicals are used, in any application. Laundry, house hold cleaning, prescription medications, hair and perm solutions, insecticides from pet bathing, etc. can be a few of the causes of sludge build up, or, erratic water usage.  Water usage in the home needs to be spread out to eliminate large surges in usage. Large surges over load system capacity, and hydraulically push untreated waste, and microorganisms prematurely through the system. This will cause odor and create the need for premature pumping of tank. This creates a nuisance condition and adds to operating cost.