Wednesday, October 03, 2012

The Water Treatment Process: Part 2

Last time, we left off at what a water treatment system may involve. Before we get into that though, let's do a short review of what the treatment process aims at accomplishing (i.e. what it's designed to remove or modify). A very short and broad list of some of those items is listed below with some examples:
  • Pathogens: bacteria, viruses, cysts, microorganisms.
  • Heavy Metals: lead, mercury, iron.
  • Taste & Odour Compounds: organic byproducts and decomposition.
  • Other Chemicals: pesticides, TCE.
  • pH: acidity or basicity.
  • Pharmaceuticals: a newer problem that may not be fully regulated yet.
Recall that the better the source water is, the lower the level of treatment required. But let's stick to the most "difficult" case that requires the most treatment, which usually occurs when the source is surface water. Below is a diagram of a typical treatment train for surface water consisting of four major unit processes. When you're working with higher quality sources like groundwater, it may be possible to skip some of the earlier steps.

Walking through the train from left to right: water is first pumped from the source (e.g. lake); a broad screen removes large debris; pretreatment is applied to remove larger solids and some dissolved compounds; the cleaner water is filtered to remove fine solids; a disinfection process is used to kill pathogens; and finally, chlorine is added to provide secondary disinfection before being distributed to consumers.
The pump and screen part is pretty simple, except it's interesting to note that the screen helps clean the water and keep debris from breaking expensive things in a plant.

Pretreatment is a bit more involving. If I remember right, there is more than one method. However, among the more common is conventional pretreatment where a coagulant is mixed into water that "destabilizes" dissolved and undissolved solids, which effectively makes everything "sticky". This mixture is then blended gently to encourage solids to bump into each other and become bigger. And finally, the water is let to effectively sit still (move very slowly) so that those large solids can settle to the bottom to be swept away.
Sample Water Treatment Train

Without pretreatment, the next step, filtration, would be difficult and inefficient to accomplish. Why, you ask? Because the filters would have to remove a lot more stuff, which means more clogged filters and more frequent cleaning. There are a variety of filters ranging from passive simple/engineered sand beds (e.g. slow sand, rapid sand) to pressurized membranes cartridges (e.g. ultrafiltration, reverse-osmosis).

And like filtration, disinfection would be difficult to accomplish without the prior process – actually, in this case, it may even be unsafe. Pathogens are microscopic and can hide within solids. If the disinfection agent can't make contact with the organic pathogen, it can't kill it. This allows them to grow back and contaminate the water again. Chlorine is a very common disinfection agent, but there are also ultraviolet light, and ozonation.

The last step is secondary disinfection using chlorine, chloramine, or some other compound to keep the water clean from pathogens on its way to the consumer.

What I just outlined is a completely dumbed down explanation of the water treatment process. I could probably write a few dozen pages on the entire subject and just start to go into detail. It is also a continuously developing and evolving field with a large amount of research being carried out.

The important takeaway is that you now have an idea of what typically happens to your water before it gets to your tap. You know about the need for pumps, constant input of chemicals and output of waste, and energy requirements. This is a very resource intensive piece of infrastructure that enhances our quality of life. Unfortunately, its complexity may be taken for granted.

Hopefully, you now have an idea of why water conservation is important in a "green" future.

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