Wastewater Treatment
Treatment train
A typical plant moves through headworks and screening, grit removal, primary clarification, biological secondary treatment (activated sludge or trickling filter), secondary clarification, disinfection, and discharge. Solids go to thickening, digestion, and dewatering.
Biological control
Activated sludge operators manage sludge age (MCRT), food-to-microorganism ratio, MLSS, dissolved oxygen (commonly 1 to 3 mg/L), and return and waste sludge rates. Nitrification needs a longer sludge age, adequate oxygen, and alkalinity; it consumes alkalinity and lowers pH, which can stall the process. Cold wastewater slows nitrification noticeably.
Key measurements
Permit limits are usually written for BOD and total suspended solids. BOD is the oxygen consumed by microorganisms over five days; COD is a faster, chemical measure and reads higher. They are not interchangeable. Sludge volume index: SVI = (30-minute settled sludge volume in mL/L x 1000) / MLSS in mg/L.
Traps
Equating BOD with COD. Reading a high SVI as good settling when it actually signals bulking. Forgetting that a clarifier blanket that is too high carries solids over the weirs, or that a low F/M with long sludge age can still fail if oxygen is short.
Worked example
A 30-minute settle gives 250 mL/L and the MLSS is 2500 mg/L. SVI = (250 x 1000) / 2500 = 100 mL/g. Roughly 80 to 150 mL/g indicates good settling; values above about 150 suggest filamentous bulking and call for a look at F/M, DO, and septicity.