The Complete STP Terminology Guide: 40+ Terms Explained — BOD, MLSS, F/M, SRT, HRT, RAS, KLD, CTE/CTO and every other acronym you meet in a sewage treatment plant — grouped by theme and explained in one or two plain lines each. The dictionary the rest of this library links back to.

An engineer's clipboard and gauges beside the aeration tank of an Indian sewage treatment plant, dissolved-oxygen probe in the frothing water, clarifier and control panel behind

Every field has its private language, and sewage treatment has more than most. Walk onto any STP site or open any design report and you are met with a wall of three-letter acronyms — BOD, MLSS, SRT, RAS, KLD, CTO — traded back and forth as if everyone was born knowing them. Most of them are simpler than they sound. Each one is just a shorthand for a single, concrete idea: how strong the sewage is, how many microbes are in a tank, how long the water stays, how much sludge comes back.

This guide is the dictionary the rest of our STP guide library points back to. The terms are grouped the way an engineer actually thinks about a plant — the water's journey, the numbers that describe it, the biology that does the work, the hardware that holds it, the way a plant is sized, and the paperwork that lets it run. Keep it open in a tab; you will meet these words again and again.

Almost none of these terms describe anything exotic. They describe one of four things: how dirty the water is, how many microbes are eating it, how long everything stays in the tank, and whether the government will let you switch it on.

1. The water's journey — process and stage terms

These describe where the water is and what is being done to it at each point in the plant. For the full walk-through, see how an STP works and the treatment process flow.

Term	What it means
Sewage	The raw used water leaving a building — from toilets, kitchens, bathrooms and washing.
Wastewater	The broad umbrella term for any spent water; sewage is the domestic kind. See sewage vs wastewater vs effluent.
Influent	The raw sewage entering the plant — the "before" water.
Effluent	The treated water leaving the plant — the "after" water that gets reused or discharged.
Preliminary treatment	The first stage: screening and grit removal that catch rags, plastics and sand to protect the machinery.
Primary treatment	Plain gravity settling — heavy solids sink, floating matter is skimmed, before any biology.
Secondary treatment	The biological heart of the plant, where microbes eat the dissolved organic waste.
Tertiary treatment	The final polish — filtration and disinfection that make the water reuse-grade.
Sludge	The concentrated solids removed at every stage — the plant's one real waste product.
Greywater / Blackwater	Greywater is lightly soiled water from baths and basins; blackwater is toilet sewage. See greywater vs blackwater.

2. The numbers — wastewater quality parameters

These are the measured values that say how polluted the water is, at the inlet and the outlet. They are covered in depth in wastewater characteristics: BOD, COD, TSS, pH.

Parameter	What it measures
BOD (Biochemical Oxygen Demand)	The oxygen microbes need to eat the organic waste — the headline "strength" of sewage, in mg/L.
COD (Chemical Oxygen Demand)	The oxygen needed to break down all matter chemically; always higher than BOD.
TSS (Total Suspended Solids)	The weight of solid particles floating in the water — what makes it cloudy.
TDS (Total Dissolved Solids)	Dissolved salts and minerals the water carries; important for reuse quality.
pH	How acidic or alkaline the water is; microbes want it near neutral (6.5–8.5).
DO (Dissolved Oxygen)	The free oxygen actually present in the water — the microbes' air supply, kept around 2 mg/L in aeration.
FOG (Fats, Oil & Grease)	The greasy fraction from kitchens that must be trapped early or it clogs everything.
Nutrients (N & P)	Nitrogen and phosphorus — good for plants, but they choke lakes with algae if discharged.
Coliform / MPN	A count of gut bacteria used as the disease-risk indicator; disinfection must knock it down.
Turbidity	A quick optical measure of cloudiness, in NTU — a fast proxy for TSS.

The one comparison worth memorising is BOD versus COD, because their ratio tells you what kind of waste you are dealing with:

	BOD	COD
Measures	Only the biodegradable waste	All oxidisable waste
Test time	~3–5 days	~3 hours
Typical value	The smaller number	Always larger
A high ratio (BOD/COD) means	Mostly digestible — ideal for a biological STP	Lots of hard, industrial matter — may need an ETP instead

3. The living machine — biological process terms

Close-up of frothing brown water churning in the aeration tank of a sewage treatment plant as fine bubbles rise to the surface

This is where the real jargon lives. Once the sewage reaches the aeration tank, engineers stop talking about water and start talking about the microbial culture doing the eating — the "activated sludge." For the master process, see the Activated Sludge Process.

Term	What it means
Activated sludge	The dense, living mass of bacteria that consumes the waste — the "workforce" of the plant.
MLSS (Mixed Liquor Suspended Solids)	The concentration of that microbial mass in the aeration tank, in mg/L — how "thick" the culture is.
MLVSS (Mixed Liquor Volatile Suspended Solids)	The living, organic fraction of the MLSS — the microbes that are actually active.
F/M ratio (Food-to-Microorganism)	The balance of incoming food (BOD) against the microbe population; the master dial of plant health.
HRT (Hydraulic Retention Time)	How long the water stays in a tank — usually hours. Too short and the microbes cannot finish eating.
SRT (Sludge Retention Time, or sludge age)	How long the microbes stay in the system — usually days. It sets how mature and stable the culture is.
RAS (Return Activated Sludge)	Settled microbes pumped back from the clarifier to the aeration tank to keep the population high.
WAS (Waste Activated Sludge)	The surplus microbes deliberately removed to stop the population overgrowing — sent to the sludge line.
SVI (Sludge Volume Index)	How well the sludge settles; a high SVI means light, bulking sludge that refuses to sink.
Nitrification	Microbes converting ammonia to nitrate — the first half of removing nitrogen.
Denitrification	Other microbes converting that nitrate to harmless nitrogen gas — the second half.
Aerobic / Anaerobic / Anoxic	With free oxygen / with none / with no oxygen but nitrate present — the three worlds different microbes live in.
Anaerobic digestion	Oxygen-free breakdown of sludge that produces biogas (methane) — the basis of UASB systems.
Bulking / Foaming	Two classic upsets — filamentous bacteria making sludge that won't settle, or a stubborn scum on the tank.

Getting the F/M ratio, HRT and SRT right is essentially the whole art of running an SBR, MBBR or MBR plant. Every technology is just a different way of holding a lot of hungry, well-fed microbes in contact with the water for the right length of time.

4. The hardware — equipment and unit terms

An Indian plant operator in a hard hat inspecting rows of pumps, blowers and pipework in the equipment room of a sewage treatment plant

The tanks, pumps and filters the process actually runs in.

Term	What it means
Bar screen	The grille at the inlet that rakes out rags, plastics and large solids.
Grit chamber	A settling channel that drops out sand and grit before they wear down pumps.
Equalisation tank	A buffer that smooths the daily surges (the 7 a.m. shower peak) into a steady flow.
Aeration tank	The main biological reactor where air is pumped in and the microbes eat.
Blower	The air pump that forces oxygen into the aeration tank — usually the biggest power draw on site.
Diffuser	The membrane at the tank floor that breaks blower air into fine bubbles for efficient transfer.
Clarifier / Settling tank	The quiet tank where fattened microbes clump and sink, leaving clear water on top.
SOR (Surface Overflow Rate)	The design flow per unit of clarifier surface area — sets whether solids get enough time to settle.
PSF / ACF	Pressure Sand Filter and Activated Carbon Filter — the tertiary units that strip fine particles, colour and odour.
Disinfection (Cl₂ / UV)	The final kill step — chlorine dosing or ultraviolet light — that destroys remaining pathogens.
Sludge drying beds	Where wet sludge is dewatered before disposal.
SCADA / PLC	The automation that monitors and controls pumps, blowers and valves without constant manual work.

5. Sizing the plant — capacity and load terms

The numbers a design starts from. Turn your building's headcount into these figures with the STP Capacity Calculator and the sewage generation calculator.

Term	What it means
PE (Population Equivalent)	The design headcount — how many people's worth of sewage the plant must handle.
Per capita sewage	The litres of sewage one person generates per day (roughly 80–90% of water used).
KLD	Kilolitres (thousand litres) per day — the standard unit of STP capacity in India.
MLD	Million litres per day — the unit for large municipal plants.
Peak factor	The multiplier applied to average flow to size for the busiest hour, not the daily average.
Organic load	The total BOD arriving per day (flow × concentration) — the real measure of the job, not just water volume.
Hydraulic load	The volume of water arriving — sized alongside organic load.
Design margin	The spare capacity built in for future occupancy and off-days.

6. The paperwork — compliance and regulatory terms

An STP is not legal until the regulator says so. These are the approvals and standards that govern it, directionally aligned to CPCB and the National Building Code.

Term	What it means
CPCB / SPCB	The Central and State Pollution Control Boards — the authorities that set and enforce discharge norms.
CTE (Consent to Establish)	The permission needed before you build the plant.
CTO (Consent to Operate)	The permission needed to run the plant, granted once it proves it meets norms.
Discharge norms / standards	The legal maximum BOD, COD, TSS, pH etc. the effluent must meet (e.g. BOD often ≤ 10–20 mg/L for reuse-grade water).
NBC	The National Building Code, which directs when and how buildings must provide on-site treatment.
ZLD (Zero Liquid Discharge)	A regime where no effluent leaves the site at all — everything is recovered. See ZLD explained.
DEWATS	Decentralised Wastewater Treatment Systems — low-energy, community-scale treatment. See DEWATS.
STP vs ETP	An STP treats domestic sewage; an ETP treats industrial effluent with chemicals a domestic plant cannot handle.

How to use this glossary

You do not need to memorise all forty terms. In practice they cluster into a single sentence that describes what any STP is doing at any moment: water of a certain strength (BOD, COD, TSS) enters at a certain rate (KLD, PE), is held for the right time (HRT, SRT) in contact with the right amount of microbes (MLSS, F/M), which are recycled (RAS) and trimmed (WAS), until the effluent meets the norms (CPCB, CTO) and can be reused. Everything else is detail hung on that frame.

From here, the natural next steps are the foundational what is an STP explainer for the big picture, and the wastewater characteristics guide to go deeper on the parameters that start every design. Bookmark this page — when a term trips you up anywhere else in the library, this is where to come back to.