BOD is the amount of dissolved oxygen consumed by microorganisms as they break down organic matter in water over a specified period (typically 5 days — hence BOD₅). It's used as a measure of how much organic pollution is present in wastewater.

High BOD in influent means there's a lot of organic matter for the biological treatment process to remove. High BOD in effluent means treatment isn't working effectively and too much organic pollution is being discharged.

BOD is one of the most regulated parameters in a plant's NPDES permit. Exceeding the permit limit for BOD can result in regulatory action.

TSS measures the concentration of solid particles suspended in the water that can be captured by filtration — typically expressed in mg/L. Like BOD, TSS is a primary permit parameter for most treatment plants.

High effluent TSS means solids are escaping treatment and reaching the receiving water body. This can be caused by poor clarifier performance, bulking sludge, hydraulic overloading, or equipment problems.

MLSS is the concentration of suspended solids (both biological and inert) in the aeration basin of an activated sludge system. It's measured in mg/L and represents the total "working material" in the basin. A typical conventional activated sludge system operates at 1,500–3,000 mg/L MLSS.

MLSS is a key process control parameter. Too low, and the system lacks the microbial mass to treat the incoming load. Too high, and the secondary clarifier may struggle to settle and return enough solids.

MLVSS is the "volatile" — or burnable — fraction of MLSS, representing the active, living biomass in the aeration basin. The MLVSS/MLSS ratio gives operators insight into how much of the sludge is living biology versus inert material (grit, ash, non-biodegradable solids).

In a healthy activated sludge system, MLVSS typically represents 70–80% of MLSS. A declining ratio over time may indicate inert solids accumulating in the system.

SVI is a measure of how well activated sludge settles and compacts. It's calculated from the Settleometer test: mixed liquor settles for 30 minutes in a 1-liter cylinder, and the volume of settled sludge is compared to the MLSS concentration. The result is expressed in mL/g.

A normal, well-settling sludge has an SVI of roughly 80–150 mL/g. An SVI above 200 mL/g indicates settling problems — most often filamentous bulking.

The F:M ratio compares the amount of organic "food" (BOD or COD) entering the aeration basin to the mass of microorganisms (MLVSS) available to treat it. It's expressed in units of lbs BOD / day per lb MLVSS (or kg/kg).

F:M controls the "hunger" of the microorganism population. A high F:M means lots of food relative to biology — organisms grow rapidly and may form poor floc. A low F:M means the organisms are food-limited — they're in starvation mode, which can promote filamentous growth in some conditions.

SRT — also called sludge age or mean cell residence time (MCRT) — is the average amount of time that a microorganism spends in the activated sludge system before being wasted out. It's controlled by the rate of wasting (WAS).

SRT has enormous influence on what types of microorganisms dominate the system. Short SRT produces young, rapidly growing sludge. Long SRT produces older, more stable sludge and is required for processes like nitrification — the conversion of ammonia to nitrate, which requires slow-growing nitrifying bacteria.

HRT is the average amount of time that water (the liquid portion) spends in a treatment unit — a basin, clarifier, or digester. It's calculated by dividing the volume of the unit by the flow rate. Unlike SRT, which applies to solids, HRT describes how long the liquid is in contact with treatment.

HRT and SRT are independent. In activated sludge, the system is specifically designed to have a longer SRT than HRT by recycling solids (RAS) — you keep the biology in the system longer than the water stays.

DO is the concentration of oxygen dissolved in the water, measured in mg/L. In the aeration basin, maintaining adequate DO (typically 1.0–3.0 mg/L) is critical for the aerobic bacteria that perform biological treatment.

Too little DO starves the aerobic bacteria, reduces treatment efficiency, and can promote the growth of filamentous organisms that cause bulking. Too much DO wastes energy and provides no additional benefit (and sometimes causes pin floc by providing excess shear).

These two terms describe the two destinations for sludge leaving the secondary clarifier. RAS is the settled sludge pumped back to the aeration basin to maintain the microbial population. WAS is the portion removed from the system to control sludge age and prevent the system from becoming overloaded with solids.

The RAS rate controls the sludge blanket level in the clarifier — too low and the blanket rises, too high and MLSS in the aeration basin drops. WAS rate controls SRT — the primary lever for long-term process control.