Coagulation Upset After Heavy Rainfall
A surface water treatment plant draws from a river that rose 1.5 feet after a 2-inch rain event. The raw water turbidity jumped from 4 NTU to 85 NTU, and the pH fell from 7.2 to 6.6. The plant uses alum coagulation with a target settled turbidity of 2 NTU. The operator on duty notices that the jar test performed at the current alum dose of 25 mg/L shows pin floc that does not settle well, and the settled turbidity in the jar is 5 NTU. The plant flow is 2.0 MGD, and the rapid mix is a mechanical mixer with a 30-second detention time. The filter run has been shortened from 48 hours to 24 hours due to increased solids loading. The operator must decide how to adjust the coagulant dose and other parameters to restore performance. The plant has a maximum alum feed rate of 60 mg/L and can adjust pH with lime or caustic soda.
1. What are the most likely causes of the poor coagulation performance, and what immediate adjustments should the operator consider?
Read the worked answer
The poor performance is likely due to a combination of increased particle loading, pH shift, and insufficient coagulant dose. The raw water pH dropped to 6.6, which is near the lower end of the optimal range for alum coagulation (typically 6.5–7.5 for turbidity removal). At pH 6.6, alum speciation may favor soluble species over the positively charged polymers needed for charge neutralization, leading to pin floc. Additionally, the higher turbidity means more particles to destabilize, so the original dose of 25 mg/L may be insufficient. The jar test showing pin floc and 5 NTU settled turbidity indicates underdosing or poor floc formation. Immediate adjustments: 1) Increase alum dose incrementally (e.g., to 35–40 mg/L) and perform a new jar test to find the optimum. 2) Adjust pH to around 7.0–7.2 using lime or caustic soda to improve alum effectiveness. 3) Consider using a coagulant aid (polymer) to strengthen floc if the dose increase alone does not achieve target settled turbidity. 4) Verify rapid mix energy and detention time; with higher turbidity, ensure adequate mixing intensity (G value) and time. 5) Monitor filter performance and be prepared to adjust backwash frequency or use filter aids. The operator should also check for any changes in alkalinity; alum consumes alkalinity (1 mg/L alum consumes about 0.45 mg/L alkalinity as CaCO3), so if alkalinity is low, pH may drop further, requiring more lime.
2. If the operator increases the alum dose to 40 mg/L, how much additional alkalinity will be consumed, and what potential operational issues could arise?
Read the worked answer
Alum (aluminum sulfate, Al2(SO4)3·14H2O) reacts with natural alkalinity (bicarbonate) according to: Al2(SO4)3 + 3Ca(HCO3)2 → 2Al(OH)3(s) + 3CaSO4 + 6CO2. Each mg/L of alum (as Al2(SO4)3·14H2O) consumes approximately 0.45 mg/L of alkalinity as CaCO3. The increase from 25 to 40 mg/L is 15 mg/L of alum, so additional alkalinity consumption = 15 × 0.45 = 6.75 mg/L as CaCO3. If the raw water alkalinity is, for example, 40 mg/L as CaCO3, the new alkalinity would be about 33.25 mg/L, which is still adequate, but pH may drop further due to CO2 production. Potential issues: 1) If alkalinity drops below about 20 mg/L, pH may become unstable and fall below 6.5, impairing coagulation and causing corrosion concerns. 2) Increased alum dose produces more sludge, which may increase sludge handling costs and shorten filter runs. 3) Higher sulfate concentrations may increase corrosivity of the water. 4) The operator should monitor pH and alkalinity after the dose change and add lime or caustic soda as needed to maintain pH in the optimal range (7.0–7.5) and alkalinity above 20 mg/L. 5) Also consider that the increased dose may require adjustment of the rapid mix and flocculation to avoid over-mixing or under-mixing.