Membranes and alternative processes
The advanced processes the higher-grade exam adds: membranes by pore size and what each removes, plus ion exchange, carbon, and aeration.
Membranes sort by pore size. Microfiltration and ultrafiltration have larger pores and remove particles, turbidity, and pathogens as a filtration barrier. Nanofiltration and reverse osmosis have smaller pores and remove dissolved material, hardness, salts, and small organics, with reverse osmosis the tightest. Smaller pores mean more removal, more pressure, and more reject water. Ion exchange swaps unwanted ions for harmless ones and softens water or removes nitrate or arsenic. Activated carbon adsorbs organics, taste-and-odor, and some PFAS. Aeration strips volatiles and oxidizes iron and manganese. Match the process to the specific contaminant.
You can sort membrane types by pore size and state what each removes. You can explain why smaller pores mean more pressure, more removal, and more reject water. You can match ion exchange, activated carbon, and aeration to the contaminants each targets. You can name the operating factors that govern a membrane plant: flux, fouling, recovery, and reject. You can avoid the exam's common mix-ups, like expecting carbon to remove salts.
When the standard train is not enough
Coagulation, sedimentation, filtration, and disinfection handle most surface water. But some contaminants slip through that train, and at the higher grades the exam expects you to know the processes that catch them. These are mostly about matching the process to the contaminant.
Membranes by pore size
Membranes are sorted by how small a thing they stop. Microfiltration and ultrafiltration have larger pores and remove particles, turbidity, and pathogens, working like a very fine filter and serving as a filtration barrier. Nanofiltration and reverse osmosis have far smaller pores and remove dissolved material, hardness, salts, and small organic and inorganic contaminants, with reverse osmosis the tightest. The smaller the pore, the more it removes, the more pressure it takes, and the more reject water it makes. Operators watch flux, the flow through a unit area, fouling, the buildup that chokes the membrane and gets cleaned chemically, recovery, the share of feed water that becomes product, and the concentrate or reject stream, which is a disposal problem.
The other advanced processes
Ion exchange swaps unwanted ions for harmless ones on a resin: it softens water by trading hardness for sodium, and removes nitrate or arsenic, and the resin is regenerated with a brine. Activated carbon, granular or powdered, adsorbs dissolved organics, taste-and-odor compounds, disinfection-byproduct precursors, and some synthetic organics including certain PFAS. Aeration and air stripping drive off volatile organic compounds, radon, and some taste-and-odor, and the same aeration adds oxygen that helps oxidize iron and manganese.
Matching the process to the problem
The source water and the specific contaminant pick the process. Membranes for hardness, salts, or a tight pathogen barrier. Ion exchange for a specific ion like nitrate or arsenic. Carbon for organics and taste-and-odor. Aeration for volatiles. The exam tests whether you can pair the contaminant with the right tool.
Where operators lose points
- confusing microfiltration and ultrafiltration, which take particles, with nanofiltration and reverse osmosis, which take dissolved material
- forgetting that the reject or concentrate stream is a disposal problem
- expecting carbon to remove salts when it adsorbs organics
- ignoring fouling, the thing that decides how a membrane plant actually runs
Quick reference
- Microfiltration and ultrafiltration remove particles and pathogens; nanofiltration and reverse osmosis remove dissolved salts and hardness.
- Smaller pore means more removal, more pressure, and more reject.
- Ion exchange swaps specific ions; activated carbon adsorbs organics and taste-and-odor; aeration strips volatiles.
- Match the process to the contaminant; watch flux, fouling, recovery, and the reject stream.
Where this fits
This is higher-grade ground on the water treatment exam. The loading and recovery math is in the free practice tool, and the details for your exam are on your state's page.
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