Discharging an oxygen-demanding waste (measured as BOD5/COD) sets off a chain of consequences downstream of the outfall, described classically by the Streeter–Phelps DO sag curve:
Dissolved-oxygen sag — microbial oxidation of the organic load consumes DO faster than reaeration replaces it, producing a DO "sag" that reaches a minimum some distance downstream (the critical point).
Fish kills and loss of sensitive species — DO below roughly 4–5 mg/L stresses most game fish; below about 2 mg/L causes acute mortality, and the community shifts toward tolerant, often undesirable species.
Septic/anaerobic conditions — if the load is severe enough to drive DO to zero, the water body turns anaerobic, producing odours (H2S), colour change, and sludge banks.
Loss of beneficial uses — recreational, fisheries and aesthetic uses are lost while the sag persists, and downstream water-supply intakes may require additional treatment.
