Effluent Discharge Compliance Monitor

Discharge & River Flow

River flow Q_r 10.0 m³/s

Effluent flow Q_e 1.0 m³/s

Effluent Pollutant Load

Ammonia-N, C_e 5.0 mg/L

Phosphate-P, C_e 2.0 mg/L

Compliance Point (200 m downstream)

Ammonia-NMAC 0.50 mg/L

— mg/L

ratio to MAC: —

Phosphate-PMAC 0.20 mg/L

— mg/L

ratio to MAC: —

Hazard Index HI = ΣC_i/MAC_i—

—

Compliant when HI ≤ 1

How it works

Treated effluent enters the river at a fixed point. Far enough downstream the two streams reach complete cross-sectional mixing, so mass balance sets the fully-mixed concentration for each pollutant i:

C_mix,i = (Q_e · C_e,i) / (Q_e + Q_r)

Close to the outfall the plume is still concentrated near C_e,i; it relaxes toward C_mix,i over a mixing length L_mix that grows with the river/effluent flow ratio:

L_mix ≈ k · W · √(Q_r / Q_e)
C_i(x) = C_mix,i + (C_e,i − C_mix,i) · e^(−x / L_mix)

Regulators check compliance at a fixed monitoring point downstream of the outfall (here, x = 200 m). Because ammonia and phosphate both act on the same aquatic ecosystem, standards commonly apply the cumulative-ratio rule for substances with additive effect — the same logic behind the U.S. EPA's non-carcinogenic Hazard Index and Ukraine's sanitary ГДК sum-of-fractions rule:

HI = C_NH3(x) / MAC_NH3 + C_PO4(x) / MAC_PO4
Discharge is compliant only if HI ≤ 1

Q_r, Q_e — river and effluent flow rates; a bigger river or a smaller discharge dilutes faster and shortens the mixing zone.
C_e sliders — the raw pollutant concentration in the effluent before any dilution.
The colored ribbon on the riverbed and the drifting particles both show the local HI(x) — blue where it is comfortably below the standard, amber near it, red once it is exceeded.
The buoy at the monitoring point turns red the instant HI > 1 there, exactly like a real continuous compliance sensor.

Clean river flow Approaching standard Exceeds standard

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