Iron, Manganese, Odor, and a Positive Coliform Result
A new 6-inch well serving a 4-bedroom home has raw water at 8 gpm: pH 6.8, iron 6.5 mg/L, manganese 0.9 mg/L, hardness 220 mg/L as CaCO3, sulfate 25 mg/L, nitrate 4.0 mg/L, arsenic 4 µg/L, and a strong hydrogen sulfide odor. The initial total coliform sample is positive, though the well was disinfected after drilling. The owner has an existing 32,000-grain softener and a 10-inch sediment filter. A salesperson proposes installing the softener first, then a 5-micron carbon block filter, and says the softener will remove the iron. The household peak demand is 8 gpm and the well produces 12 gpm. The owner wants clear water, no odor, and no staining; the local health department requires a confirmed negative coliform result before occupancy.
1. Evaluate the proposed treatment train and state the correct order of operations.
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Stop and fix the bacteria first. A positive total coliform after disinfection means either inadequate disinfection, a contaminated sample tap, or a pathway into the well. Shock chlorinate, flush, and resample with proper technique; do not install treatment to mask a positive result. The proposed train is also backwards: a softener ahead of iron filtration will be fouled by 6.5 mg/L iron and 0.9 mg/L manganese, and a 5-micron carbon block will plug quickly. Correct order: oxidation (air or chlorine) → retention/contact → filtration for oxidized iron and manganese → carbon for sulfide odor and chlorine removal → softener → sediment filter. Confirm arsenic after oxidation because it was 4 µg/L and treatment can change speciation.
2. Select and size the oxidation/filtration step for 8 gpm peak and explain the rationale.
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At 8 gpm peak, size the filter at about 5 gpm/ft², which is roughly 1.6 ft², or an 18-inch diameter tank (1.77 ft²). That gives about 4.5 gpm/ft², acceptable. But backwash at 12-15 gpm/ft² would need 21-27 gpm, more than the well’s 12 gpm. So provide a backwash storage tank and a booster, or select a filter with a lower backwash rate and a media that handles 6.5 mg/L iron. Manganese at 0.9 mg/L is harder to oxidize than iron; air alone at pH 6.8 may not work. Use chlorine or ozone with sufficient contact and raise pH to about 7.5-8.0 for manganese oxidation. Carbon after filtration controls odor.
3. What samples and monitoring schedule would you require after startup?
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After startup, sample raw and after each stage for iron, manganese, hardness, pH, sulfide, chlorine residual, and bacteria. Retest at 2 weeks and 3 months, then annually. Because the raw arsenic was 4 µg/L, include arsenic at least once after oxidation/filtration; also nitrate and the PWTA suite if the home is sold. Confirm coliform negative before occupancy. Monitor pressure drop across the filter and backwash frequency; if the softener sees any iron carryover, it will foul.