Treatment Processes and Unit Operations
What to know
Conventional surface-water treatment follows coagulation → flocculation → sedimentation → filtration → disinfection. Understand what each step does: coagulants (alum, ferric salts, polymers) destabilize particles; flocculation gently mixes to grow floc; sedimentation removes settleable floc; filtration polishes remaining particles; disinfection inactivates pathogens. Know jar testing as the tool to select coagulant dose and pH, and understand that pH and alkalinity strongly affect coagulation performance. Learn filter types (rapid sand, dual-media, slow sand, membrane), backwash purpose, and the concepts of filter breakthrough and air binding. For groundwater, know aeration, oxidation (chlorine, permanganate), softening (lime or ion exchange), and iron/manganese removal. Understand CT (disinfectant concentration × contact time) and how it links treatment to disinfection credit.
Traps
Questions confuse coagulation (destabilization, rapid mix) with flocculation (gentle mixing, floc growth). Watch for claims that filtration alone removes dissolved contaminants - it does not. Another trap: assuming more coagulant is always better; overfeeding can restabilize particles or drop pH too far.
Worked example / rule of thumb
If raw turbidity rises, expect to increase coagulant dose and possibly adjust pH, confirmed by jar test - not simply increase filter run time. Rule of thumb: coagulation targets the charge; filtration targets the particles; disinfection targets the pathogens. Each has a distinct job - do not blur them.
Study tip
Sketch the treatment train and label what each unit removes.