By Hans Peterson, Saskatchewan Research Council, Saskatoon
Water has some unique properties which will allow different types of compounds to form suspensions in it.
We often refer to compounds as either being particles or dissolved. In reality the difference between what we consider particulate and dissolved is just a matter of size, with the boundary between particulate and dissolved being occupied by colloids. As particles become smaller the surface area to volume ratio increases and the settling time through the water becomes slower. A grain of gravel will settle in one second through a 1 m water column. The same trip takes a bacterium eight days and a very small colloidal particle 200 years.
"True particles" are bigger than one micrometer (1 millionth of a metre). These particles are also referred to as suspended solids and can be of mineral origin, living organisms, or material derived from organisms. Examples of mineral material are sand, silt, and clay.
Living organisms can be different types of microorganisms, including algae, cyanobacteria (blue-green algae), parasites (Giardia and Cryptosporidium), bacteria (e.g., E. coli) or viruses. Material originating from organisms would generally be decomposing animal and plant matter.
The colloids and dissolved substances can be of similar origin, but of smaller size. The colloids are 0.1 %/m or less and truly dissolved substances are around 0.001 %/m. Compounds that are of particular interest in drinking water treatment are the humic and fulvic acids; these compounds can be both colloidal and in true solution. Particles and colloids can impart colour and turbidity to the water, with dissolved substances mainly imparting colour.
Due to their small size and long settling times, some particles,
all colloids, and dissolved substances cannot settle naturally.
Mechanical sand filtration can remove some larger particles and
biological filtration (using microorganisms in sand or activated
carbon filters) can process (use as energy source) some of this
material. Membrane processes, such as nanofiltration [NF] and
reverse osmosis [RO], can effectively deal with most of both the
colloidal and dissolved compounds. However, membranes can easily
be damaged by too much material, as is typically present in prairie
surface waters. This is where coagulation comes in.
Coagulation bunches small particles together to form larger particles so that they can be removed by settling. Coagulation can change the settling time for the very small particles from several years (indeed several hundred years) to hours.
In natural water, most particles and colloids have a negative electric charge. Therefore, rather than clump together to form larger particles, particles and colloids repel each other. To get these particles and colloids to form larger clumps, this negative charge must be neutralized. This can be done by adding positive ions, such as aluminum and ferric ions, which react with the negative particles and colloids. This destabilizes the colloids and forms microflocs, which are bunches of destabilized particles. The microflocs then grow and soon becomes visible to the naked eye, and as the floc grows, its tendency to sink gets greater.
Coagulation is very effective in removing impurities in dugout water and should be considered as an essential first step in drinking water treatment. Water processed in this way will be much more suitable for further in-house treatment.
Under the approved Canada-Saskatchewan Agriculture Green Plan Surface Water Quality Initiative project, the Saskatchewan Research Council [SRC], the Prairie Farm Rehabilitation Administration [PFRA], and Sask Water, in collaboration with Saskatchewan Agriculture Diversification and Development [ADD] Boards have been exploring new ways of adapting the coagulation process to treat dugout waters. This work has resulted in the design of sedimentation chambers (around 200,000 L plastic lined ponds) where dugout water can be pumped in, treated, and protected from contamination.
To contact the author, write to the Saskatchewan Research Council, 15 Innovation Blvd., Saskatoon, SK S7N 2X8
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