Treatment Processes
The treatment train
Most surface water plants follow coagulation → flocculation → sedimentation → filtration → disinfection. Groundwater plants often need only disinfection, with aeration, oxidation, or softening added for iron, manganese, hardness, or sulfide. Learn what each step removes and how the steps interact. Coagulation destabilizes particles so they can stick together; the jar test determines the right dose and pH. Flocculation is gentle mixing that grows floc. Sedimentation removes settleable floc; filtration polishes out remaining particles and, critically, pathogens such as Cryptosporidium. Membrane and granular activated carbon (GAC) processes address specific contaminants like TOC, taste-and-odor compounds, and synthetic organics.
Concepts to master
- Jar testing to optimize coagulant dose, pH, and mixing energy.
- Filter media: sand, anthracite, dual-media, and the role of backwashing.
- Filter breakthrough and filter-to-waste practices; turbidity as a filter performance indicator.
- Softening (lime or ion exchange) for hardness; aeration/oxidation for iron and manganese.
- CT concept (disinfectant concentration × contact time) as the bridge between treatment and disinfection.
Traps
Questions confuse coagulation (chemical destabilization) with flocculation (physical mixing). Another common trap is assuming filtration alone provides disinfection; it does not. Watch for questions implying a filter should be run to complete exhaustion rather than backwashed on headloss or turbidity triggers.
Worked example
A jar test shows the lowest settled turbidity at 15 mg/L alum and pH 6.5. If raw turbidity rises sharply, the operator should re-run the jar test rather than blindly increasing dose, because the optimum shifts with alkalinity and particle character.