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Taking the Mystery Out of the Stage 1-DPB Rule
By Larry Rader, NDWC Environmental Consultant
On January 1, 2004, the Stage 1 Disinfection Byproduct Rule (DBPR) will finally filter down to the small drinking water systems. It will apply to systems that use a surface supply for raw water and serve a population of less than 10,000 people and all groundwater systems under the direct influence of surface water.
A total organic carbon (TOC) concentration greater than 2.0 micrograms per liter (mg/L) in the system’s raw water is the trigger that sets matters in motion. A few basic treatment changes can help you through this rule and on to the next. In most cases, complying with this rule can be as simple as moving the point of chlorination.
Although NOM-DOM-POC-TOC may sound like selection number three in a Thai restaurant, they are actually different forms of organics found in surface water sources. And when organics combine with chlorine,
a reaction takes place that forms disinfection byproducts.
Natural organic material (NOM) is just what the name implies and is found in all surface water sources. NOM is divided into two forms: particulate organic carbon (POC) and dissolved organic matter (DOM). For the purpose of differentiating between the two, DOM will pass through a 0.45-micron filter and POC will not. The Stage 1 DBPR lumps both forms together by measuring TOC.
If your plant uses conventional coagulation, flocculation, and sedimentation, and if you are bringing good quality water in the range of 1.0 to 2.0 NTU to the top of the filter, the particulate organics should be removed. Unfortunately, dissolved organics make up approximately 90 percent of the TOC in most surface water sources. Although DOMs can be removed through conventional coagulation, flocculation, and sedimentation, changing their form using a pre-oxidant prior to coagulation can improve results.
In the past, chlorine was the pre-oxidant of choice. Although chlorine oxidizes organics and some inorganics and is a good disinfectant and cheap to use, chlorine and organics react to form disinfection byproducts. While you can rely on coagulation, flocculation, and sedimentation alone for TOC removal, the addition of a preoxidant prior to coagulation, in my opinion, gives you a tremendous advantage.
Pre-oxidation Provides Advantages
In my area of the country, the two most common pre-oxidants are potassium permanganate and chlorine dioxide. Both have operational concerns, such as pink water (potassium permanganate) and carrying a residual into the finished water (chlorine dioxide), but these are operational issues and easily controlled with proper plant procedures.
Potassium permanganate requires a longer contact time and must be applied at least 15 minutes prior to the addition of a coagulant and preferably at the intake. Although chlorine dioxide can be applied at almost any point in the pre-treatment process, care must be taken to keep any residual from entering the distribution system. However, both oxidize organic material without contributing to the formation of DBPs. Once the dissolved organics are oxidized, an operator can use coagulation, flocculation, and sedimentation to easily remove them.
After the organics have been oxidized and removed in pre-treatment, chlorine can then be applied. Many systems are moving chlorine to the top of the filter when contact time calculations allow it. Again, remove the organics, then add chlorine.
Enhanced Coagulation Reduces TOCs
Enhanced coagulation is a stand-alone treatment for TOC reduction and becomes required if the percent of removal is not achieved through other means. The treatment technique requires dropping the pH
in the coagulation process to a specific target number, depending on the alkalinity of
the raw water. Organics are removed more completely at a lowered pH.
The U.S. Environmental Protection Agency (EPA) requires bench testing with either alum or ferric to ascertain their removal potential before trying other coagulants. The bench tests identify how much of either coagulant is required to drop the pH into the target zone and which provides the optimum percentage of removal. Ferric, either sulfate or chloride, usually does best because of the free acid it contains and the amount of metal (iron) in the product. Alum contains less metal and less free acid, thus requiring more chemical. Although either alum or ferric may be overfed to achieve the proper pH and better removal, there
Over-feeding any coagulant tends to increase carryover to the filter that can cause increased aluminum in the finished water (alum) or increased iron (ferric). Also, operators know that chemical floc does not settle well and tends to go through the filter and into the clear well where it forms pin floc, causing an increase in turbidity. And, of course, overfeeding any coagulant creates more sludge.
I have had some personal experience using sulfuric acid to drop the pH prior to coagulation. In my experience, alum doesn’t coagulate well in the high dosages needed for many low alkalinity waters. First, consider that each part per million (ppm) of alum reduces natural alkalinity by slightly less than 0.5 ppm. In one source that I experimented with, the raw alkalinity was 39 ppm, and I had to apply 80 ppm of alum to reach the target pH of 5.5; there was no alkalinity remaining to support coagulation, and floc never formed. Ferric on the other hand does work well in combination with acid or by itself.
The source water that required 80 ppm alum only required 55 ppm ferric sulfate to reach the same target pH. Floc formed and settled very well, leaving a lot of sludge. Using acid is much cheaper than coagulants and could allow lower coagulant dosages, which, in turn, would produce less sludge. Keep in mind, the metal in either alum or ferric assists in the removal of TOCs and reducing the feed rate can reduce the removal rate.
Using sulfuric acid also gives the option of using one of the new poly aluminum chlorides (PACL). Although PACL does not provide as good percent removal as ferric, it does floc well at the lower pH and creates much less sludge. If sludge removal is a concern, and you can get by with a slightly lower removal rate, sulfuric acid and PACL could also be an option.
All of these options require bench testing, but for the operator interested in staying ahead of the game, the time is well spent. For more information about the Stage 1 Disinfection Byproduct Rule, visit EPA’s Web site at www.epa.gov.
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