We often use the terms “young sludge” and “old sludge” to describe problem biomass. What do these terms mean when it comes to the actual bacteria and biomass? 
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Components of floc

• Living bacteria – in Activated Sludge this is often only 5 – 15% of the solids.
• Dead bacteria
• Insoluble organics & inoraganics
• Extracellular Polymers (EPS) – made by bacteria to store organics, protect cells, and for extracellular enzyme control

 
The ideal floc contains a mix of the above at ratios where the solids are bound in a dense mass that settles well in clarification. In MBR systems, floc should readily separate from effluent and not blind pores in the membrane.
 
Young Sludge
Bacteria are rapidly dividing giving a high respiration rate (OUR) and free bacteria cells in solution. Rapidly dividing cells have yet to form sufficient dense EPS for target floc density. Color is often light. With respect to the growth curve, young sludge is seen where you have higher than design F/M (excess food).
 
Old Sludge
This is a system with low rates of bacterial division, and you get higher % dead bacterial cells and inorganics in the floc. With low division rates and fewer living bacteria, you also see lower respiration rates. The EPS that holds the floc becomes a source of “food” along with slower to degrade particulates. As the EPS is consumed, you end up with smaller pin floc and “fines” increasing turbidity in the supernatant. With the growth curve chart, you are in the decline/death phase.
 
Activated Sludge systems were designed to operate in the stationery to early decline phase of growth. This is where you maintain good levels of EPS with removal of soluble BOD (organics).