Introduction to Surface Water Treatment

Surface water is the second most common source of drinking water on the planet after ground water. It also frequently the most contaminated. Everything that can dissolve into water and is on the surface of the earth ends up in surface water. From the image below that of all the water on earth only 3% of it is fresh water, and of that three percent only 0.3% is surface water. This is partly why clean fresh water is such a precious resource.  Many people have a surface water  source as their backup should their primary source become contaminated.  After an emergency event, the surface water sources will become contaminated and proper treatment will be more essential than it usually is.  One benefit of surface water is that is refreshes itself rapidly in the case of rivers and other fast moving water sources and because they are exposed to oxygen they favor bacteria that are much better at removing biological contamination.

Current municipal drinking water standards for the treatment of surface water are stricter than any other source of drinking water. What is required at the bare minimum is chemically assisted filtration and disinfection. Going beyond this level is possible and in many instances recommended. Without the assistance of chemicals many sand based filters are not fine enough to remove very small particles and microscopic organisms like viruses.  Many commercial water filters are chemically activated and usually contain activated charcoal.  The filters at the water treatment plant that I work at have greensand filters, activated with potassium permanganate to remove iron and manganese.  There are many different types and each one is best at removing something different from the water.  This is another reason to get your water source tested so you know what needs to be removed and therefor which filter is best.

The quality of surface water is extremely variable.  Meaning that as time progresses the treatment demands will change.  After heavy rains the suspended and dissolved materials in lakes and rivers increases dramatically.  Any treatment of surface water needs to take day to day and seasonal variations into account.  Learning what to monitor and what adjustments to make will take a lot of research and experience actually treating water from your source.

When setting up a surface water drinking source, filtration is essential. Filtering removes turbidity. Turbidity is the suspension of sediment in the water that causes a cloudy appearance.  Turbidity is caused by anything suspended in the water, organic, inorganic, biological or otherwise.  You may be wondering why removing turbidity is important, it is important because sediment can hide pathogens from disinfection, increase demand for your disinfection chemicals and increase the formation of disinfection byproducts, some of which are carcinogens.  Like I have mentioned before, the better the filtration, the easier the other steps will be.  Using a sock is better than nothing using a charcoal filter is better than a sock and using a nano filter is better than charcoal.  The goal is to remove as much as possible from the water.  Remember that disinfection is the removal or inactivation or pathogenic organisms.  Filtration is the removal part of disinfection.

Each filter has a life span called the filter run time.  Sometimes it is expressed in possible treatable volume, other times it is just expressed in time.  Filter run times are not static.  They vary greatly with the quality of the raw water.  Most commercially available filters give their run time based on filtering tap water.  This is not surprising as most people have municipally treated water, and the higher number makes their filter appear better to consumers.  Do not expect the reported filter run number to be true when filtering lower quality water.  A filter basically traps sediment on the surface of the filter media, and removes fine particles through the process of adsorption and absorption (see glossary for definitions).  As the filter is used the contamination works its way down into the spaces between the filter media.  As it becomes physically clogged it becomes more effective at filtering fine particles in a ripening process, then gets rapidly worse as it nears the end of it’s run time.

When the filter is at the end of it’s life span, two things are possible, filter breakthrough and the addition of contaminants back into the water often at higher levels then were present before.  Filter breakthrough is the water forcing it’s way through the filter media to create a short cut or a hole completely bypassing any filtration, you can see why this is undesirable.  Filter breakthrough is often easily noticeable by a rapid decrease in the quality of water treated.  The second thing that happens when the filter is done is that they can add contamination back into the water.  As the volume of water treated increases, particles begin to become dislodged from the filter and get added back into the water.  Think about it this way, a filter treats 100 gallons of water, removing the contamination and in the process reaches it’s filter run point.  If you filter an additional gallon, that gallon will be exposed to something with 100 times the contamination already present.  When you are primarily trying to filter microscopic organisms, this can be very hard to monitor outside of a laboratory.

If you have a commercial water filtration system the only thing you can do now is to replace your filter cartridge  with a new one.  What is done at large water treatment facilities is we backwash the filter.  We run water backwards through the filter, pushing all the entrapped particles out into the waste system and add any chemicals for reactivation of the filter media.  This method is cheaper in the long run but requires some basic knowledge of chemistry and chemical handling.  Some commercial cartridge filters can be washed and reused.  Always be sure to follow the manufactures instructions while washing.

I have written a few articles on disinfection so far.  In them I have already covered the basics of disinfection.  I suggest you read them because disinfection is then next step of treating surface water.  Like I said before, filtering makes the chemical disinfection step easier, it doesn’t replace the addition of a disinfectant or boiling or U.V..

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5 thoughts on “Introduction to Surface Water Treatment

  1. Pingback: Guidelines for Choosing a Water Filtration System « Omega Man Journal

  2. Pingback: Water Security Before, During & After Hurricanes | The Ωmega Man Journal

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