Sediment, Turbidity & Color

If your water isn't completely clear

• Sediment consists of physical, macroscopic particles (sand, grit, flakes from pipe corrosion) that will eventually sink to the bottom of a glass.
• Turbidity is cloudiness caused by microscopic, suspended solids (clay, silt, possibly microbes) that may take hours to days to begin settling. Sometimes dissolved gases will appear to be turbidity but will clear upon degassing in a few moments.
• Color can be "apparent" (caused by suspended rust or clay) or "true" (caused by completely dissolved minerals or organic acids that dye the water).

Once the cause of the cloudiness is identified, several techniques useful on the scale of a household can be applied.

Physical particle mechanisms

Dissolved color mechanisms

1. Mechanisms for sediment & turbidity (physical barriers)

If the contaminant is a physical particle — even a microscopic one — it can be caught physically.

• Centrifugal separation: Used for heavy sand and grit. Water is forced into a cylindrical housing at an angle, creating a miniature vortex. Centrifugal force pushes the dense particles outward against the walls. They lose momentum and fall to a collection bowl at the bottom.
• Surface filtration (sieving): Used for uniform sediment. Water passes through a thin physical barrier (like a pleated polyester cartridge). Any particle larger than the pore size is physically blocked on the surface.
• Depth filtration (tortuous path): Used for finer sediment and turbidity that would quickly clog a surface screen. Water is forced through a thick wall of material (like a spun fiber cartridge) or a deep bed of granular media (like sand). The water must navigate a complex, twisting maze. As particles travel, they lose momentum and get wedged into microscopic crevices throughout the entire depth of the filter.

2. Mechanisms for "true color" (chemical manipulation)

If your water looks like something like tea, and the color never settles to the bottom of the glass, you are dealing with "true color." The contaminants are completely dissolved in the water, meaning physical filters will let them pass right through. You have to change their chemistry.

Oxidation & catalytic precipitation (for iron and manganese)

Dissolved iron and manganese are the most common causes of red, orange, or black water. In the ground, lacking oxygen, they are dissolved and clear (ferrous iron). When exposed to oxygen, they "rust" and turn into colored solids (ferric iron).

An oxidation filter aggressively introduces an oxidizer (like a pocket of compressed air, ozone, or chlorine) into the water. This chemically forces the dissolved, invisible iron molecules to precipitate (turn into solid rust flakes). Once the color-causing minerals are solid, the filter uses standard depth entrapment to catch the flakes in a media bed, returning clear water to the house.

Ion exchange (for tannins)

If your water has a yellow/brown tint but no metallic taste, it is likely caused by tannins — dissolved organic acids from decaying vegetation (roots, leaves, peat) in the aquifer.

Tannin molecules carry a negative electrical charge. Tannin filtration systems use a tank full of specialized plastic resin beads that are pre-loaded with chloride ions. When the well water flows over the beads, the stronger negative charge of the tannin molecules knocks the chloride off the bead. The tannin takes the chloride's place on the bead, and the harmless chloride washes into your house. The color is literally swapped out of the water.

Adsorption (for organics)

Activated carbon removes dissolved organic color compounds through physical adsorption. The non-polar organic molecules that cause the tint are drawn to the non-polar carbon surface by Van der Waals forces and get permanently stuck in the microscopic pores.

What works

In practice, intermittent cloudiness in well water can be difficult to treat. In my area (the Sierra Nevada foothills) wells often become cloudy following rains and the pump may kick up a small handful of sediment of all particle sizes — sand, silt and clay — requiring a stepwise system to remove the particles. When different particle sizes are present it is always necessary to use a primary coarse sieve mechanism of some type followed by a secondary finer sieve. Otherwise, if only the smaller-pored filter is used, it will quickly clog with large particles.

Typically sand-size particles can be easily removed by inline filters using straining (screen and disc) or centrifugal force (spin filters). All the above filters can be configured to backwash automatically to avoid clogging and allow little effect on water flow/pressure if operated properly.

Silt-sized particles can be effectively removed by a depth-type backwashing filter or large-gradient cartridge filter. Backwashing with media designed to trap particles is also very good at removing clay particles and other fine particles less than 3–5 μm, but can be overwhelmed if there is a big slug of sediment. The sub-micron sized particles are the hardest to remove because fine filters cause the most pressure drop, particularly when not well maintained, so the finest filter (1 μm) would be the filter of last resort.