For routine microscopic viewing of living mixed liquor, the 10X and 40X lenses will be most useful. Begin by slowly moving back and forth across the slide until the entire contents of the slide has been viewed. This is easily accomplished when using the lower power lenses, which have a wide field of view. Most of what you will see in activated sludge appears as brown and tan clumps of particles. These particles are masses of round and rod shaped bacteria, which are referred to as floc particles. The first thing that should be assessed is the floc structure. Are the floc particles large, solid, and light brown with clear supernatant between? This is a sign of a healthy and good settling sludge. If the particles are small and very dense and the supernatant has a lot of debris in it, it is a sign of an unhealthy sludge that will leave a cloudy supernatant. This is typical of mixed liquor from systems that have excessively high solids (low F:M) or that are operating with excessive dissolved oxygen. If the floc particles are not very dense, light in color and appear weak, the sludge will most likely settle slowly and leave behind large, “straggler floc”. This is typical of systems that are in start-up or are operating under too high of an F:M.
Look at the other life forms that are present. You will no doubt see a mixture of higher life forms, such as; Amoebas, free swimming and stalked Ciliates, Flagellates, Rotifers and even sludge worms. The mix of higher life forms can tell you a great deal about how the system is operating. For instance, activated sludge that is made up of loose floc particles and a mixture of mostly amoebas, Flagellates, and some free swimming Ciliates is indicative of a young sludge (high F:M). A mixture of dark, dense floc particles and Rotifers, stalked Ciliates and sludge worms is typically found in older sludge (low F:M). Activated sludge that has strong, medium sized floc particles and a mixture of all of these organisms, (especially with large clusters of stalked Ciliates and free swimming ciliates), is typical of systems that are operating at an F:M that is well suited to capturing and metabolizing the incoming waste. Sludge with these characteristics will settle and compact well and leave a clear supernatant that has low BOD and TSS.
Every aspect of these higher life forms can give you information about the health of the system. The size, number, activity level, type and diversity of the higher life forms in activated sludge can all be used as key indicators. The F:M that a system is operated at will directly influence the types of bugs that predominate. High F:M systems that process waste at a high rate and waste it out before it is fully digested will tend to cultivate fast growing, high activity organisms (free swimming Ciliates, Flagellates, Amoebas). Low F:M systems that have long MCRTs do a thorough job of digesting the captured waste and tend to operate with slower growing, lower activity organisms (Rotifers, sludge worms, Tartigrade or water flea). Systems where the F:M is correct for the waste load will be predominated by a mixture of organisms that grow fast and a few that grow slow. The activity of the organisms in this type of system will be high when first encountering the waste and then will slow as the waste is metabolized. All of this can be seen through the lens of a microscope when used with care.