By Hans Peterson, Saskatchewan Research Council, Saskatoon, Jim Kidd, Sask Water, Nipawin, and Bill Andresen, Sask Water, Outlook
Astudy by the Saskatchewan Surface Water Quality Initiative (PFRA-SRC-Sask Water) has assessed the effectiveness of commonly used surface water treatment systems in removing turbidity (i.e. particles in water), algae, dissolved organic carbon (DOC), and colour.
Turbidity is a concern because some of these particles could be microorganisms such as the Beaver Fever parasite, Giardia. Occasionally, there are some inorganic problem compounds related to groundwater, such as high total dissolved solids (TDS) which can be associated with increased levels of bromine, arsenic, sodium, iron, and manganese. The presence of algae in treated water also indicates that particle removal needs to be improved.
As for dissolved material, only organics are of great concern for surface waters. The dissolved organics come mainly from photosynthetic organisms, including higher plants and algae. Organic release can occur both during decomposition and healthy growth of these organisms. They can be a nuisance on their own because they can impart taste, odour, and colour to the water, and some compounds from blue-green algae are even toxic. In addition, when high DOC water is disinfected, large amounts of undesirable disinfection by- products result. Dissolved organic carbon is a measure of how much of all of these different organic compounds (taste, odour, toxins, colour, etc.) are present in the water.
During the spring of 1994, we contacted farmers that use dugout water as their supply source. From this survey, it became clear that most systems in use have a large, back-washable granular activated carbon (GAC) filter (0.27 m3 = 1 cubic foot capacity) that is used in combination with other treatment processes, such as softening and chlorination (14 out of 16 treatment systems were in this category). These GAC filters are not the cartridge type described in the previous article (The Ultimate Water Treatment Device). For about half of these filters, chlorination was used prior to the GAC filter.
A treatment system that effectively reduces turbidity, removes all algae, and reduces DOC and colour would also be expected to substantially remove other things associated with particles and dissolved organics. These would include parasites, taste, odour, and toxic compounds.
Did the treatment systems studied produce water which meets recommended levels? The short answer is no. Turbidity was reduced by 45%, but still did not meet the Guidelines for Canadian Drinking Water Quality. The presence of algae in the tap water is also of concern; indeed there were some systems with algae levels high enough to make the water green.
The removal of DOC and colour was also generally poor (11 and 27% respectively). There have been suggestions both in Canada and the U.S. that reducing the DOC level to less than 5 mg/L would solve many of the problems associated with these compounds. This means a much greater capacity for DOC removal is required.
Colour is an aesthetic attribute, with limits under the Guidelines for Canadian Drinking Water Quality set at 15 colour units. The treated water in this survey was considerably above this level. We can also assume that taste and odour compounds, if present, are not likely to be removed either.
Although GAC filters are designed to remove dissolved organics, they have limited capacity to do so. Our work has shown that the average GAC filter may only effectively remove DOC for one month. The filters are, however, capable of chlorine removal long after their DOC removal capacity has been exhausted.
This study shows that GAC filters improved the water, but it also points out the need for additional treatment before consumption. A few farmers employed point-of-use devices, such as reverse osmosis and distillation units.
These systems removed turbidity, algae, DOC, and colour to low levels. These devices need good quality water to operate properly for extended periods of time. One goal in farm water treatment could be to produce general purpose household water that is of suitable quality for washing and bathing, in addition to serving as a good source for point-of-use devices that provide high quality drinking water at one tap.