Which organism would most probably be associated with moderate sludge age and effluent water quality?

Free-swimming ciliates are commonly associated with moderate sludge age and good effluent water quality due to their role in the microbial ecosystem of activated sludge systems. These organisms are efficient grazers of bacteria and other microorganisms, which allows them to thrive in environments where there is a balanced microbial community.

When sludge age is moderate, it means that the microbial population has had enough time to stabilize and reach a point where it can efficiently process organic matter. This stable state allows for the effective removal of biomass and contaminants from the wastewater. Free-swimming ciliates indicate a healthy microbial community, as they thrive when there is sufficient food (bacteria) and stable environmental conditions. Their presence can be a sign that the treatment process is functioning well, leading to improved water quality in the effluent.

Other organisms, while present in activated sludge systems, have different implications for sludge age and water quality. Amoebas often indicate low food availability, flagellates can suggest fluctuations in nutrient levels, and rotifers typically thrive in older, well-established sludge but are not as good indicators of water quality as ciliates. Thus, the association of free-swimming ciliates with moderate sludge age and effective effluent treatment is a key reason they are considered the correct answer.