Treatment Processes

Public water systems that use surface water sources must treat to remove or inactivate microorganisms capable of causing waterborne diseases. Filtration is used to physically remove the target organisms from the water supply. Specialized filters that have proven removal capabilities must be used. Disinfection, usually with chlorine, is used to inactivate (kill or prevent from reproducing) organisms that are too small to be removed through filtration.

For more detailed descriptions of these processes and how they are used at public water systems, refer to the following fact sheets:

• Surface Water Filtration – Pathogen Removal (PDF)
• Chlorine Disinfection – Background (PDF)

For small noncommunity public water systems, the removal of pathogenic organisms in surface water sources is generally achieved using replaceable bag or cartridge filters or backwashing membrane filters.

• Surface Water Filtration – Bag and Cartridge Filters (PDF)
• Surface Water Filtration – Membrane Filters (PDF)

It is often necessary to provide pre-filtration ahead of the final filter to prevent it from clogging and to save on the cost of filter replacement. Pre-filtration may be accomplished using bag or cartridge filters with a larger pore size than the final filter or with backwashing sand filters.

• Surface Water Filtration – Pre-Treatment (PDF)
• Surface Water Filtration – Pressure Media Filters (PDF)

For small noncommunity public water systems, disinfection is achieved using chlorine. Most commonly, liquid chlorine (bleach) is injected in small amounts into the water supply using a chemical feed pump.

• Chlorine Disinfection – Equipment (PDF)

While UV technology has improved and gained popularity in recent years, there are both technological and regulatory barriers that prevent it from being used as the primary disinfection processes at public water systems in Minnesota. UV light must pass through water unimpeded to be effective, and many surface water sources in Minnesota contain dissolved organic content that gives it a brown color. Even small amounts of color may be enough to prevent UV from achieving the desired level of disinfection. For more information on the use of UV to treat surface water sources, contact MDH.

Chlorine reacts with dissolved organic matter in water to form Disinfection Byproducts (DBPs). Some DBPs are known to increase the risk of chronic health issues such as cancer and liver damage. DBPs are not regulated at transient public water supplies such as small campgrounds and resorts where most of the consumers are on site for short periods of time. Owners and employees of transient supplies that use chlorine disinfection and drink the water regularly may consider treating the water to remove DBPs at the point of use (POU), such as at a kitchen faucet. POU filters should be certified under NSF/ANSI Standards 53 or 58 for the removal of trihalomethanes (THMs).

• Disinfection Byproducts at Transient Systems (PDF)
• List of POU Filters Certified by NSF for THM Removal

The following schematics show typical layouts for small surface water treatment plants in Minnesota.

• Surface Water Plant Schematic (PDF)
• GWUDI Plant Schematic (PDF)