When starting up a new activated sludge plant, what action will achieve building up MLSS to the desired concentration quickly?

• A. Increase aeration rate
• B. Do not return sludge
• C. Decrease aeration rate
• D. Do not waste sludge

Answer: (D)

Building MLSS quickly comes from keeping solids in the reactor rather than removing them. MLSS is the concentration of suspended solids in the mixed liquor, determined by the balance between solids entering and solids leaving the aeration basin. If you stop wasting sludge, you’re not removing solids from the system, so the solids accumulate and MLSS rises faster. This is why not wasting sludge is the best choice for speeding up the buildup.

Increasing aeration rate can help microbial activity but doesn’t directly ensure a higher MLSS and can be counterproductive if it disrupts floc or washout. Not returning sludge would reduce the solids in the reactor, lowering MLSS rather than raising it. Decreasing aeration rate slows oxygen transfer and biomass growth, which would also slow any rise in MLSS.