Why This Glossary Exists

Wastewater treatment terminology gets thrown around a lot — often inconsistently — in vendor quotations, compliance documents, and consultant reports. This glossary defines the terms facility managers run into most often, in plain language.

Plant Types

STP (Sewage Treatment Plant) — Treats domestic sewage from toilets, kitchens, and washing areas. See our full STP vs ETP comparison for details.

ETP (Effluent Treatment Plant) — Treats industrial process wastewater, customized to the specific pollutants an industry generates.

CETP (Common Effluent Treatment Plant) — A shared, centralized facility that treats effluent from multiple industries within an industrial estate, rather than each factory building its own standalone ETP.

ZLD (Zero Liquid Discharge) — A treatment approach where no liquid effluent leaves the site; all water is recovered and reused, with only solid/semi-solid waste for disposal. Mandated in certain water-stressed regions and industrial zones.

Treatment Stages

Primary Treatment — Physical removal of large solids and settleable matter via screening and sedimentation.

Secondary Treatment — Biological treatment using bacteria to break down organic matter (BOD/COD reduction) — includes technologies like MBBR, SBR, and extended aeration.

Tertiary Treatment — Additional polishing after secondary treatment, typically filtration (sand, UF, or membrane) and disinfection (UV or chlorination), used when treated water needs to meet reuse-grade quality.

Common Measurement Terms

BOD (Biochemical Oxygen Demand) — The amount of oxygen microorganisms consume while breaking down organic matter in a water sample over 5 days; the primary measure of biodegradable pollution load.

COD (Chemical Oxygen Demand) — The total oxygen required to chemically oxidize both biodegradable and non-biodegradable organic matter; always equal to or higher than BOD for the same sample.

TSS (Total Suspended Solids) — The mass of solid particles suspended (not dissolved) in a water sample, measured by filtration.

KLD — Kilolitres per Day, the standard capacity unit for Indian STP/ETP plants (1 KLD = 1,000 litres/day).

MLSS (Mixed Liquor Suspended Solids) — The concentration of suspended solids (mostly biomass) in the aeration tank during biological treatment, a key operational control parameter.

Common Technology Terms

MBBR (Moving Bed Biofilm Reactor) — Biological treatment using floating plastic media as attachment surface for bacteria, increasing treatment capacity within a smaller tank footprint.

MBR (Membrane Bio-Reactor) — Combines biological treatment with membrane filtration in place of a secondary clarifier, producing very high quality, near pathogen-free effluent.

SBR (Sequencing Batch Reactor) — A biological treatment process where fill, react, settle, and decant all happen in the same tank in a timed sequence, rather than in separate tanks.

OCEMS — Online Continuous Effluent Monitoring System; mandated in several states for real-time SPCB data reporting from qualifying STP/ETP plants.

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