By: Charles “Bud” Mason, Senior Rural Development Specialist, Great Lakes RCAP
Common water quality issues in distribution systems raise both regulatory and aesthetic concerns. Iron and manganese often find their way into the distribution system and cause staining and poor water taste for users. If a system does not treat for the removal, these contaminants enter the system directly Even if systems do treat for the removal of iron and manganese, they are bound to find a way around your treatment process: a short cut. A system can continuously flush clean up its distribution system, but if its treatment process has a short circuit then flushing will be to no avail until the system eliminates shortcuts.
What is your raw water quality?
First, consider the raw levels of iron and manganese a system is dealing with. Often a system will not know for sure but will have an average idea from a previous yearly sample. The next question is where are operators drawing the samples from — at the individual wellhead, or at the raw line coming into the treatment system? If two or more wells pump into a shared line, and a system draws its raw sample at the plant, then, at best, the system is testing a composite sample. In a perfect world, a system should test individually at each well and take a composite sample prior to any treatment. Though groundwater quality does not change as dramatically as surface water quality throughout the year, all raw water should be monitored seasonally.
What is your pre-filter water quality?
After water reaches a plant, operators usually add an oxidant or air and have some sort of detention tank to allow iron and manganese to begin to precipitate and settle. As water is pumped from the detention tank, a process control check should be performed on the water prior to it entering the filters. Compare this to wellhead and composite sample results to determine the effectiveness of the treatment system. The last cleaning date of the detention tank should also be confirmed, because if the residuals are not removed, then they will impact the treatment process.
What is your post-filter quality?
Pre-filter water from the same source entering each filter will be the same quality. The post-filter water leaving each individual filter will not. For this reason, the effluent from each filter should be tested. This is seldom done. Most systems will check a composite post-filter sample, which is the blended water of all the filters usually drawn at the lab sink. If a system is experiencing shorter filter run times or complaints of water quality, they need a much closer process control to determine what filters are filtering and more importantly, which ones are not. A common problem that impacts filtration is a broken underdrain system.
What are your backwashing procedures?
Backwashing is another suspect area when looking for short circuits. A common reason operators have for backwashing a certain way in a system is, “this is how we have always done it.” Backwashing of a filter is a very important part of water treatment. When done correctly, no one notices. When done incorrectly, no one forgets.
What triggers a backwash? Commonly this is the day of the week, gallons filtered, sample results showing higher iron or manganese levels, or pressure differential in the filters. The appropriate answer for a system should be whatever method provides the best results in keeping filters running and involves proper process control sampling of pre- and post- filter water. What water is being used to backwash the filters? If a system is running high-service pumps while backwashing, then make sure that the post-filter water being used is clean enough to do an adequate job. If not, it is like washing clothes in dirty water. The next question is, how do you start and end the actual backwash? Filters are like hydrants — start the flow slow and end it slower. This allows the filter media to have a chance to rise without separation and settle down back into the proper layers. If this is not done properly, it could cause a short circuit through the filter media. Finally, how long do you backwash? A system should backwash until the backwash water quits clearing up, but always follow the manufacturer’s recommendations.
Does your system have a plan to ensure filters are filtering and that the system is producing the best water that exceeds regulatory standards and gives your customers the confidence to wash their “good towels”? If you are experiencing any water quality issues and want someone to review current processes or help write new or update standard operating procedures (SOPs), give RCAP a call.