Laboratory Analysis, Sampling, and Process Control Calculations
This domain covers proper sampling techniques, laboratory analytical methods, and the calculations used to control treatment processes. The exam tests your ability to collect representative samples, interpret lab results, and perform process control math.
Sampling
- Representative samples require proper location, timing, and technique. Use clean, appropriate containers; preserve samples as required (e.g., acidify for metals, cool for microbiology).
- Chain of custody ensures sample integrity for regulatory compliance.
- Grab samples vs. composite samples: grab is a single point in time; composite is mixed over time or space.
- Sample taps must be flushed to avoid stagnant water; for lead and copper, first-draw samples are required.
Laboratory Methods
- Turbidity: nephelometric method (EPA 180.1) measures light scatter at 90°. Calibrate with formazin standards.
- pH: electrometric method using glass electrode; calibrate with buffers at pH 4, 7, and 10.
- Chlorine residual: DPD colorimetric or amperometric titration. Distinguish free from total chlorine.
- Alkalinity: titration with acid to pH 4.5 endpoint; results as mg/L CaCO3.
- Hardness: EDTA titration or calculation from calcium and magnesium.
- Microbiology: total coliform and E. coli using membrane filtration or enzyme substrate methods.
Process Control Calculations
- Chemical feed rate: Feed rate (lb/day) = (flow, MGD) × (dose, mg/L) × 8.34.
- Solution strength: If using a liquid chemical, adjust for specific gravity and percent strength.
- Detention time: DT = volume / flow. Convert units carefully (e.g., gallons / gpm = minutes).
- Weir overflow rate: flow / weir length.
- Percent removal: (influent – effluent) / influent × 100.
Worked Example
A plant treats 2.5 MGD and needs a chlorine dose of 2.0 mg/L. How many pounds per day of chlorine are required?
- Feed rate = 2.5 MGD × 2.0 mg/L × 8.34 = 41.7 lb/day.
- If using a 12% sodium hypochlorite solution (specific gravity 1.2), the volume required = 41.7 lb/day / (0.12 × 1.2 × 8.34 lb/gal) ≈ 3.47 gal/day.
Common Traps
- Forgetting the 8.34 conversion factor (lb per gallon per mg/L).
- Miscalculating detention time by mixing units (e.g., gallons and gpm gives minutes, not hours).
- Not accounting for solution strength when calculating chemical feed volumes.
- Using the wrong sample container or preservation method, invalidating results.
- Assuming grab samples represent average conditions; composite sampling may be needed.