12.2 Wastewater Chlorination & Dechlorination
Key Takeaways
- Wastewater chlorination is dominated by ammonia, so most of the residual is combined chlorine rather than the free chlorine typical of drinking water.
- Chlorine residual in wastewater is measured as total chlorine residual, and effective disinfection is judged by CT and coliform results rather than by free residual.
- Dechlorination is required because chlorine residual is acutely toxic to aquatic life, and permits commonly set total residual chlorine limits below detection.
- Sulfur dioxide, sodium bisulfite, and sodium metabisulfite all reduce chlorine to chloride, and each consumes alkalinity and a small amount of dissolved oxygen.
- Chlorine contact basins must be baffled to approach plug flow, because short-circuiting delivers far less contact time than the theoretical detention suggests.
12.2 Wastewater Chlorination & Dechlorination
Chlorination is still the most common wastewater disinfection method in the United States. The chemistry differs fundamentally from drinking water chlorination, and confusing the two is a reliable source of wrong answers.
Why Wastewater Chlorination Is Different
Secondary effluent contains ammonia — often 15 to 30 mg/L at a plant that does not nitrify, and still some at plants that do. Chlorine reacts with ammonia immediately:
The practical consequences:
| Drinking water | Wastewater |
|---|---|
| Free chlorine residual is the target | Combined chlorine (chloramine) is what you actually have |
| Breakpoint chlorination is achievable | Breakpoint requires an enormous dose and is rarely attempted |
| Free residual measured | Total chlorine residual measured |
| Low demand | High and variable demand |
Chloramines are weaker but more persistent disinfectants than free chlorine, which is why wastewater contact times are long — typically 15 to 30 minutes at peak flow — compared with drinking water practice.
Do not chase a free chlorine residual in wastewater. It generally does not exist in any meaningful amount, and an operator trying to produce one will grossly overdose. Compliance is judged on total residual chlorine, contact time, and bacteriological results.
Dosage and Demand
Worked example. A plant treats 3.2 MGD. Chlorine demand is 7.5 mg/L and the target residual is 1.0 mg/L.
Using 12.5 percent sodium hypochlorite at approximately 1.043 lb available chlorine per gallon:
Chlorine demand in wastewater rises with ammonia, nitrite, sulfide, organic matter, and suspended solids, all of which vary through the day and with plant performance. A sudden demand increase almost always means the secondary process has slipped.
Contact Basin Hydraulics
Detention time is only as good as the basin's hydraulic efficiency.
Worked example. A contact basin holds 92,000 gallons and the plant flows 3.2 MGD, which is 2,222 gpm.
At a 1.0 mg/L residual, CT is 41.4 mg-min/L — comfortable. But that is the theoretical figure.
[!WARNING] Short-circuiting destroys contact time. An unbaffled rectangular basin can pass a substantial fraction of flow to the outlet in a small fraction of the theoretical detention. The fix is serpentine baffling to force a long, narrow flow path approaching plug flow. Baffling factors range from about 0.1 for an unbaffled basin to 0.7 for a well-baffled serpentine basin — meaning an unbaffled basin may deliver only a tenth of its calculated contact time. Dye tracer testing is how actual detention is measured.
Basins also accumulate solids, which reduces volume and exerts chlorine demand. Periodic cleaning is a real maintenance item.
Dechlorination
Chlorine residual is acutely toxic to aquatic life at very low concentrations, so AZPDES permits routinely set total residual chlorine limits at or below the analytical detection limit. Nearly every plant that chlorinates must dechlorinate.
| Chemical | Form | Approximate ratio to chlorine |
|---|---|---|
| Sulfur dioxide (SO₂) | Gas | ~0.9 to 1.0 : 1 |
| Sodium bisulfite (NaHSO₃) | Liquid | ~1.5 : 1 |
| Sodium metabisulfite | Dry or liquid | ~1.4 : 1 |
| Sodium thiosulfate | Liquid | ~2 : 1 |
| Ascorbic acid | Dry | Higher; used for small applications |
All reduce chlorine to chloride, which is harmless. Common considerations:
- The reaction is essentially instantaneous with good mixing, so the injection point needs turbulence rather than a long basin.
- Alkalinity is consumed, and pH drops slightly.
- Dissolved oxygen is consumed — a modest amount, but at plants with a tight effluent DO limit an overdose can push DO below the limit.
- Overdosing wastes chemical and depletes oxygen; underdosing leaves a toxic residual. Control is usually by residual analyzer feedback downstream of the injection point.
Sulfur dioxide is fed with equipment nearly identical to gas chlorine and carries similar hazards. Many utilities have moved to liquid sodium bisulfite specifically to eliminate a second toxic gas from the site.
Safety
Gas chlorine and gas sulfur dioxide are both severe inhalation hazards.
| Requirement | Detail |
|---|---|
| Separate, dedicated rooms | Each gas in its own room with exterior access |
| Ventilation | Low-level exhaust — both gases are heavier than air and collect at floor level |
| Gas detection | Continuous monitoring with local and remote alarm |
| Emergency repair kits | Chlorine Institute Kit A for 150 lb cylinders, Kit B for ton containers |
| Self-contained breathing apparatus | Stored outside the chlorine room, immediately accessible |
| Cylinder securing | Chained upright; valve protection caps in place when not connected |
| Ton container orientation | Positioned so that the two valves are vertically aligned — the upper draws gas, the lower draws liquid |
| Leak detection | Ammonia vapor produces a white cloud at a chlorine leak; never spray water on a chlorine leak, since the resulting acid accelerates corrosion and enlarges the hole |
Risk Management Program and Process Safety Management requirements may apply above threshold quantities, which is another reason many utilities have converted to hypochlorite and bisulfite.
A wastewater operator attempts to establish a 1.0 mg/L free chlorine residual in secondary effluent containing 22 mg/L ammonia and finds the required dose to be extremely high. What is the correct interpretation?
A chlorine contact basin holds 60,000 gallons and the plant flows 2.0 MGD. The basin is a single open rectangular tank with no baffles. What is the practical concern?
Which statement correctly describes safe practice for locating self-contained breathing apparatus and for detecting a chlorine leak?