Methods
- Chlorine (gas or sodium hypochlorite) with contact time, often followed by dechlorination.
- UV light damages pathogen DNA; no chemical residual.
- Peracetic acid and ozone at some plants.
How secondary performance affects disinfection
- Effluent TSS shields organisms from UV and chlorine. At White River, effluent TSS of 28 mg/L on September 14 after the storm coincided with an E. coli result of 260 cfu/100 mL, above the daily maximum.
- Ammonia combines with chlorine to form chloramines, which disinfect more slowly than free chlorine.
- Nitrite exerts chlorine demand.
- UV transmittance drops with color, iron and dissolved organics.
Chlorine dose
Dose (mg/L) = demand + residual
Feed (lb/d) = Q (MGD) x dose (mg/L) x 8.34
Example: 8.0 MGD, demand 4.0 mg/L, residual 1.0 mg/L: 8.0 x 5.0 x 8.34 = 334 lb/d chlorine. As 12.5% hypochlorite (about 1.04 lb available chlorine per gallon), roughly 320 gpd.
Effluent
Final effluent must meet the permit for BOD, TSS, ammonia, bacteria, pH, DO and any nutrient limits. White River's summer limits include CBOD5 10/15 mg/L, TSS 12/18 mg/L, ammonia-N 1.5 mg/L monthly and 3.0 daily maximum, TP 1.0 mg/L, E. coli 125 geometric mean and 235 daily maximum, and DO at least 6.0 mg/L.