COD, BOD5, MLSS, SVI: Reading the Wastewater Abbreviations Before They Read You

Wastewater treatment runs on abbreviations, and a misread one can push a plant into the wrong operating move — wasting sludge that was fine, or raising aeration because a number was confused with its neighbour. The useful grouping is not alphabetical but functional: what the water carries in, what the biomass is doing, and what the operator actually logs each shift.

What the water carries: COD, BOD, BOD5, TOC, TDS

COD — chemical oxygen demand — is the amount of reducing substance in a water sample that can be oxidised, measured chemically. Under defined conditions it is expressed as the milligrams of oxygen equivalent to the oxidant consumed by the reducing substances in one litre of sample, in mg/L. The higher the COD, the more heavily the water is polluted by reducing matter. Monitoring uses two variants: CODCr, with potassium dichromate as the oxidant, and CODMn, with potassium permanganate — different oxidants, different results, so report which one you measured.

BOD is the dissolved oxygen consumed over a given period while micro-organisms break down the oxidisable, mainly organic, matter in a given volume of water. Like COD it is a composite indicator of organic pollutant content, expressed in mg/L, as a percentage or in ppm; the higher the BOD, the more organic pollutant the water carries. BOD5 is simply the standard five-day measurement, written with the 5 to separate it from the ultimate demand — when a discharge permit says BOD without qualification, it almost always means BOD5. TOC — total organic carbon — reflects how far a water body has been polluted by organic matter, counting the carbon instead of the oxygen needed to burn it. TDS — total dissolved solids — sums the dissolved constituents: ions, molecules and compounds in the water, excluding suspended solids and dissolved gases. The larger the TDS, the more dissolved impurity the water carries and the worse the pollution.

What the biology is doing: MLSS, MLVSS, SV30, SVI

MLSS — mixed liquor suspended solids — is the concentration of suspended solids formed by the wastewater and the activated sludge together in the aeration tank of an activated sludge plant. MLVSS — mixed liquor volatile suspended solids — is the same measurement with the inorganic fraction excluded, so it represents the mass of active micro-organisms in the sludge; where the inorganic share is high, MLSS flatters the plant and MLVSS tells the truth.

SV30 is the sludge settling ratio: the ratio of settled sludge volume to total mixed liquor volume after mixed liquor from the aeration tank has stood for 30 minutes in a 1000 mL cylinder. SVI — sludge volume index — is the volume occupied by one gram of activated sludge suspended solids after the same 30-minute settle, expressed in mL/g and usually written SVI30. The two readings come from the same cylinder test but answer different questions: SV30 tells you how much volume the sludge takes, SVI tells you how well a gram of it settles — the number that flags bulking before the clarifier does.

In day-to-day operation these four numbers move together in readable patterns. A rising SVI with a stable MLSS usually means filamentous growth or low-food conditions; a falling SV30 with rising MLVSS often follows a load increase that feeds the floc-formers; and MLSS that climbs while MLVSS stays flat points at inert accumulation — grit or fixed solids — that wasting will eventually clear. None of these diagnoses needs a laboratory, only the habit of reading the settling cylinder the same way every shift.

The rest of the sheet: SS, TN and TP

SS — suspended solids — covers the inorganic and organic particles that do not dissolve, together with silt, clay and micro-organisms; the suspended solids content is one of the standard measures of how polluted a water body is, and higher values mean worse pollution. TN sums inorganic and organic nitrogen in all its forms — nitrate, nitrite and ammonium nitrogen together with the organic nitrogen in proteins, amino acids and organic amines — calculated as milligrams of nitrogen per litre, and it is the routine way to express how far a water body has been enriched with nutrients. TP sums phosphorus in inorganic and organic form and rises with pollution the same way; both are the numbers behind most tightening discharge limits.

Reading the numbers together

The influent parameters — COD, BOD, SS, TN, TP, TDS and TOC — describe what has to be removed. The mixed liquor parameters — MLSS, MLVSS, SV30 and SVI — describe the state of the biomass that does the removing, and they are the figures an operator watches day to day when deciding whether to waste sludge, raise aeration or adjust the return ratio. Keep the two groups mentally separate — load parameters describe the duty, biomass parameters describe the crew — and most misreadings disappear before they reach the shift log.