Utility Water Treatment

 

Borehole and surface water wells often have water quality problems that needs to be taken care of before water is used in households or industrial processes. Most typical raw water problems are due to hardness, fluoride, chloride, arsenic, iron, manganese, and hydrogen sulfide. In addition, in surface water there can be also organics (e.g. humus) and bacteria.

Below you can find information of different treatment methods for typical harmful substances.

Fluoride, Chloride and Arsenic

Fluoride, chloride, and arsenic in water may cause health problems when guideline limits are exceeded. These substances are usually removed with membrane technology where water is forced through a semipermeable membrane with pressure. Membrane technologies are typically classified to nano and ultrafiltration and reverse osmosis. Main difference between these methods is in the permeability of used membrane. In nanofiltration and reverse osmosis processes, water is always divided into permeate (filtrated water) and concentrate (residue water stream that includes the filtrated substances). The process yield is typically between 70–80% in borehole and surface water treatment.

Nanofiltration is particularly good method for removing arsenic from water and makes the water also softer (removes calcium and magnesium salts).

Reverse osmosis is required for removal of fluoride and chloride. With reverse osmosis is possible to remove all dissolved salts from water with up to 97–99.5% rejection but typically, in drinking water treatment, fluoride and chloride are removed as much as is needed to meet local guidelines (this is done by taking only part of the raw water to reverse osmosis treatment).

Membrane treatment system consists typically of prefiltration (defined based on raw water quality), high pressure pump, membranes and required piping, valves, sensors, and control system.

Nanofiltration and reverse osmosis system output Approximate unit size (L x D x H) [m]
8m3/h 3.5 x 1.5 x 2
20m3/h 3.5 x 1.5 x 2
50m3/h 5.5 x 2 x 2

Typical reverse osmosis equipment layout.

Iron and Manganese

Iron and manganese in water may cause smell, colour and bad taste, and precipitate easily to water pipes, tableware, and storage vessels. Iron and manganese can be removed with multimedia filtration (water is led through a filtration media). To certain extent, multimedia filtration can also be used to remove humus and other organics.

Multimedia filtration systems consists of filtration tank(s) / vessel(s), piping, required valves and sensors. Additional pumping is usually not needed as system works with existing supply network pressure. Applied media is typically ion-exchange or catalytic type (polymer resin or manganese oxide).

When ion-exchange media is used, filters needs to be regenerated periodically when capacity is used by injecting brine to filtration media (at same time media is backflushed). When catalytic media is used, only backflush now and then is required. Due to required regeneration and/or backflushing sequences, more than one filtration tanks / vessels are needed when there is a need for constant flow of filtrated water. Previously mentioned regeneration and backflush sequences are taken care with automatic valves / control system.

Ion-exchange filters when raw water quality: iron <~0,75 mg/l and manganese <~0.3mg/l

System output Approximate unit size (L x D x H) [m]
8m3/h 1.5 x 1 x 2.3 + brine tank D1
20m3/h 2 x 1 x 2.3 + brine tank D1.2
50m3/h 3.5 x 1.5 x 2.3 + brine tank D1.2

Catalytic media filter – when raw water quality: iron >~0,75 mg/l and manganese >~0.3mg/l

System output Approximate unit size (L x D x H) [m]
8m3/h 2 x 1 x 2.3
20m3/h 3.5 x 1.5 x 2.3
50m3/h 4.5 x 2 x 2.3

StrongFlow’s Ion-exchange filter with three parallel (composite) tanks and example picture of steel filtration tanks (photo courtesy of LORIVAN NV).

Hydrogen Sulfide

Hydrogen sulfide causes bad smell (like rotten eggs) and may lead to corrosion of metallic pipes. Hydrogen sulfide is usually removed by aeration. Aeration is usually done in a separate tank with air distributors placed on bottom. Air pumped to water oxidizes the hydrogen sulfide to sulfate.

Bacteria

Bacteria in raw water can be disinfected with ultraviolet radiation (UV). In UV-system the water is lead through a chamber where UV-radiation inactivates microorganisms. Simple UV-system consists of UV-device (chamber + UV-lamp inside) and ballast/control system.

Example of UV-device.