Drinking Water Treatment for Very Small Systems
Core concepts
A very small system still performs the same unit processes as a large one: source protection, coagulation/flocculation and sedimentation (if surface water), filtration, and disinfection. Know what each barrier removes: coagulation targets turbidity and natural organic matter; filtration removes particles and some pathogens; disinfection inactivates bacteria and viruses. Understand the difference between a primary disinfectant residual and a secondary residual maintained in distribution.
Disinfection math and CT
Candidates must be comfortable with chlorine dose, demand, and residual. The relationship is dose = demand + residual. Free chlorine residual is measured by DPD colorimeter or test kit; total chlorine includes combined chloramines. The CT concept (residual concentration × contact time) governs pathogen inactivation, and contact time depends on baffling and flow. A common trap is confusing dose with residual or forgetting that chlorine demand consumes part of the dose before any residual remains.
Filtration and turbidity
Know that turbidity is measured in NTU and is both a treatment-performance indicator and a regulatory limit. Filter backwash, filter-to-waste, and slow sand filters appear in small-system questions. A frequent trap: assuming clear water is safe water-pathogens can be present at low turbidity.
Worked rule of thumb
If a well pumps 50 gpm and the operator wants a 0.5 mg/L free chlorine residual with a measured demand of 1.0 mg/L, the required dose is 0.5 + 1.0 = 1.5 mg/L. Always add demand to target residual to find dose.