By Tate Burckhardt
Fundamental concepts for treating water often utilize ion exchange and/or oxidation and filtration. Both can be successful, but like all treatment techniques, there are limitations depending water quality and choice of treatment method. For example, bacteria can limit traditional treatment due to biofilm coating (sliming) of the equipment and media used. This article will cover some basic treatment, with a new twist on filtering problem water, which provides reduced maintenance, water savings and chemical savings.
First things first
What is problem water? It is water that has one contaminant at higher than normal levels or has combinations of contaminants. Unfortunately, this water is also identified when traditional treatment equipment fails to provide quality water.
When treating water, there are five basic steps to follow: sanitation, oxidation, filtration, conditioning and high-purity refinement (typically for the drinking water). The water quality may allow some steps to be skipped; however, problem water needs attention at each step. Traditional approaches often include chlorine as the sanitizer and oxidizer, followed with some form of backwashing filter (typically with carbon) to remove the chlorine. The finishing steps generally use a softener followed by RO for drinking water and food preparation. There are many other approaches, but this article will focus on this type of setup. There are specifics within the filtration that can be tweaked to make this process more efficient.
Every oxidizer has pros and cons. Chlorine is the most widely used oxidizer because of its primary and secondary sanitation properties. This secondary sanitation property is why most municipalities use it to keep water sanitized in all the lines and during storage. One of the downsides is the extended contact time, which is often twenty minutes. Ozone is one of the fastest oxidizers, almost instantaneous, and doesn’t leave any residual. Greensand filters that utilize potassium permanganate are a strong oxidizer, but are a poor sanitizer. Hydrogen peroxide is another strong oxidizer that works in water with higher pH; however, NSF requires post chlorination after its use. Aeration is a good oxidizer of iron, but often promotes bacteria growth. Therefore aeration requires post sanitation. Let’s go back to the most traditional method of oxidation and sanitation using chlorine; but we are going to soup it up by combining it with aeration. Aeration is a free oxidizer and makes chlorine more efficient.
Since we chose chlorination, we want to pick a contact tank that ensures there is enough contact time for it to do its job. One major factor when dealing with contact time is the channeling of water that reduces the efficiency. Water likes to follow the path of least resistance; by interrupting its, flow we increase the efficiency of the tank’s contact time. Often baffles are used to improve this efficiency or, multiple small tanks versus one large tank also works well. Bladder tanks and long runs of straight pipe account for little-to-no contact time. Atmospheric tanks promote flocculation of oxidized particulates, off gas hydrogen sulfide and methane. They require an additional pump to provide pressurized water to the house. Many of these tanks have large flat bottoms, which makes blowing down the tank more difficult. Since problem water is what we are trying to treat, we will use a baffled atmospheric tank to give us the most flexibility and treatment options. This will complete steps one and two (sanitation and oxidation).
Normally, a backwashing filter is put in-line after the water has been oxidized. It mechanically cleans the water by trapping the oxidized particles within the media filter bed. They do a great job, but send a tremendous amount of treated water down the drain during the backwash and down-flow rinse cycle. In some cases, the backwash and rinse volume will be almost as much water as the household uses daily. Another item of concern is the large amount of oxidized particles that settle to the bottom of the contact tank. How could we make this more efficient and keep the tank clean? These inefficiencies are solved by removing the backwash filter from its normal position and make it filter the water inside the tank. Putting it before the contact tank wouldn’t do any good, because everything has not been oxidized yet and the filter wouldn’t work properly. But by pumping water from one side of the contact tank, running it through the filter, and then pushing it back into the other side, we can filter the tank water. (Figure 1) Pumping the water 24-7 with a small/low-pressure pump through the filter will filter the water several times before the large internal pump pressurizes the water to the remaining treatment equipment.
Figure 1. Contact tank filter
The clean tank has several benefits. The pressure pump now operates iin clean water, which increases the life of the pump and it also eliminates the need to blow-down the settled particulates inside the contact tank. We still have to address the backwashing on the tank filter. Normal backwash valves only allow one incoming water source, which doesn’t solve the problem of sending all the treated water used to backwash down the drain. By using a standard in-out head on the filter tank and adding a three-way valve to each side of the in-out head we can then reverse the flow of water inside the filter tank and use an untreated water source to backwash the tank filter. (Figure 2) This will clean the particulates from the media inside the tank filter and save the customer the chlorine costs from dumping treated water to drain. There is no longer any need to down rinse the tank filter, since the low flow, low pressure pump inside the contact tank will finish this when the tank filter goes back into its normal service position. The small amount of untreated water inside the tank filter will then mix with the already chlorinated water inside the contact tank and filter out before the next morning’s water use. There will be a small drop in chlorine residual, but not enough to affect the overall performance of the system.
Figure 2. Three-way valve system
The water inside the contact tank is filtered several times, but not 100-percent filtered because it is constantly blending back into the contact tank. (The tank filter is not inline like traditional applications; therefore, inline filtration is required.) Because the water is virtually clean, two 4.5”x20” cartridge filters will efficiently polish the water to the softener or home. The first cartridge filter removes the remaining particulates from the water, followed by a pleated carbon impregnated filter to remove chlorine residual. One would think these cartridge filters would become plugged and fail within a short period of time, but they have been found to last over six months without replacement. (Note: When installing a drinking water RO, the effluent water for the RO can be plumbed back into the atmospheric contact tank. The large amount of water inside the contact tank will dilute the water and make the RO 100-percent efficient. The amount of reject water blended back into the tank is nominal and already soft; therefore it shouldn’t have any effect on a softener.)
Field test results
Application 1: 7 gpm, 5 ppm Fe, 85 gpg hardness, 7.1 pH and positive for bacteria. The water from the tank tested with trace iron and after 20-micron cartridge filtration, was at a non-detect level. The first 20-micron pleated filter was replaced after seven months of use, with nominal pressure drop across the filters (see Figure 3). The carbon impregnated final filter showed slight signs of iron staining (see Figure 4). The physical view inside the tank was crystal clear with small amounts of iron particulates on the bottom of the tank. These particulates appeared to be iron that has flaked off old plumbing.
Figure 4. Pleated carbon filter
Application 2: 10 gpm, 2 ppm Fe, 2 ppm tannins, 30 gpg hardness, 7.2 pH and methane. This application required use of treated water for backwash since there was methane in the water. The water inside the tank appears clear; however, the final results are pending.
Application 3: 25 gpm, 6 ppm Fe, 35 gpg hardness, 7.4 pH and positive for bacteria. This application flowed more water than 4.5”x20” filters could handle; therefore, post filtration with traditional backwashing carbon units were used. However, the circulation filter on the contact tank was used to reduce the iron load on these carbon filters. The water inside the contact tank showed trace levels of iron with a small amount of large iron particulates on the bottom of the contact tank. The five-foot deep water was crystal clear all the way to the bottom of the contact tank.
The results of these trials far surpass the original estimations. We are currently awaiting the full results in water that exceed 20 ppm iron and another with 50 hydrogen sulfide. The short-term results are mirroring the proven results.
About the author
Tate Burckhardt is the Vice President of Better Water Industries Inc. He can be reached at (507) 247-5929 or email@example.com
About the company
Better Water Industries Inc. has tackled some of the nation’s worst well water. This pursuit is driven by the customers and the need to handle ever-challenging water conditions.