Storm Turbidity Spike at a Low-Alkalinity Surface Water Plant
A 6 MGD conventional surface-water plant draws from a river intake. A two-day storm raises raw turbidity from 5 to 190 NTU and color from 10 to 70 CU. Raw alkalinity is 24 mg/L as CaCO3, pH 7.1, temperature 9°C. The plant feeds alum at 28 mg/L, polymer as filter aid, and chlorine pre-filter. Jar tests on the new water show settled turbidity of 2.4 NTU at 40 mg/L alum, 0.8 NTU at 55 mg/L, and 1.3 NTU at 70 mg/L; filtered turbidity tracks the settled results. The shift operator wants to go straight to 70 mg/L 'for safety.' Filter run times have already shortened from 40 to 18 hours, and the clearwell is at 62% of its 1.0 MG capacity.
1. Which alum dose should the plant run for the next 24 hours, and what else must change for that dose to actually work? Show the arithmetic.
Read the worked answer
Dose: 55 mg/L, not 70. The 55 mg/L jar test gave the best settled turbidity (0.8 NTU); 70 mg/L was worse (1.3 NTU) and would cost more and make more sludge.
But 55 mg/L will crash the pH unless base is added:
- Alum consumes roughly 0.45 mg/L of alkalinity (as CaCO3) per mg/L of dose.
- 55 x 0.45 = 24.8 mg/L as CaCO3 demand against 24 mg/L available.
- Residual alkalinity ends up at about zero. Below pH 6.3 coagulation is poor and soluble aluminum carries onto the filters; below about 4.5-5 the floc redissolves entirely.
Actions before the dose goes on:
- Start caustic (NaOH) or soda ash feed and trim to hold post-mix pH 6.5-7.0, with finished pH in the corrosion-control range. Roughly, 1 mg/L NaOH contributes about 1.25 mg/L as CaCO3, so 12-15 mg/L NaOH adds 15-19 mg/L of alkalinity - but let the jar test set the number.
- Re-run jars with the base included; raise dose in 5 mg/L steps, not one big jump.
- At 9°C floc forms slowly - lengthen flocculation time, raise polymer slightly, hold filter rates down.
- Do not chase turbidity with coagulant: without base, the practical ceiling is 24 / 0.45, about 53 mg/L.
2. Filter runs fell from 40 hours to 18 hours. What is causing that, what are the trade-offs of the higher dose, and what will you watch each shift?
Read the worked answer
Solids loading, not the filters, changed. At about 1 mg/L TSS per NTU, 190 NTU x 6 MGD x 8.34 is roughly 9,500 lb/day of solids reaching the filters, versus about 250 lb/day at 5 NTU - nearly 40 times the load on the same media.
First question to answer: what ended the run?
- Headloss-limited: solids loading is the cause. Improve settling, not backwashing.
- Turbidity breakthrough: floc shear, wrong pH, or underdose. Compare settled turbidity to the jar test; if settled water is better than the filter effluent, the problem is filtration, not coagulation.
Trade-offs of the higher alum dose: more sludge and backwash water, more alkalinity demand, more sulfate, and a lower coagulation pH that must be corrected. A bigger dose does not rescue a bad pH.
Watch each shift: post-coagulation pH and alkalinity every 2-4 hours, settled turbidity every 4 hours (target at or below the jar result), individual filter effluent below 0.3 NTU, headloss trend per filter, filter rate at the time of breakthrough, and backwash quality - air scour, full bed fluidization, mudballs, media loss. Log everything; a sanitary survey will ask for it.
3. The clearwell is at 62% of 1.0 MG with 6 MGD demand. What is the operational risk, and what do you do about it this shift?
Read the worked answer
Available storage: 0.62 MG. At 6 MGD that is 0.62 / 6 = 0.10 day, about 2.5 hours. That is thin on a day with 190 NTU raw water and 18-hour filter runs.
Risks:
- One filter out for backwash cuts production while demand keeps pulling the clearwell down.
- A caustic feed trip or failed jar test sends non-compliant water toward the clearwell with almost no recovery time.
- Lower clearwell level means less chlorine contact time - verify CT is still met at the reduced volume and baffling.
Actions now:
- Hold production at or above demand; do not backwash during peak; stagger backwashes so only one filter is out at a time.
- Set a 50% low-level action alarm and watch the trend hourly, not once a shift.
- Confirm finished-water chlorine residual and log it each hour.
- Line up the emergency interconnection or purchased-water option and notify the superintendent before the level reaches the action point, not after.
- If level falls below the action point while settled turbidity is still above about 5 NTU, reduce to priority service per the emergency plan and make the required state notification.
- Document the event - the questions will come later.