Chlorine Monitor, high manganese application
Many raw waters contain dissolved iron and manganese, often present in the form of ferrous bicarbonate and manganese bicarbonate. Iron and manganese most commonly occur in well waters and impounded surface waters, but water containing these compounds can often be problematic during the treatment process. Iron compounds can cause a yellow-brown stain, and manganese-bearing waters can cause a black stain, as well as causing the growth of certain filamentous organisms, which can lead to the clogging of pipes in distribution systems.
For these reasons it is desirable to remove as much iron and manganese from water as possible during the treatment process. The removal of these contaminants involves the use of chlorine. In a typical water treatment plant, iron is removed in a first stage process. Coagulation is followed by filtration which removes the precipitated iron salts. Manganese is removed in a second stage. Typically, chlorine is used to oxidise soluble manganous salts to insoluble manganic salts. However, during the treatment process it is essential that chlorine levels are carefully monitored as chlorine can be highly toxic to fish and invertebrates, and can cause significant damage to river habitats.
The effects of manganese contamination on the chlorine monitoring process depend on the level of manganese in water. A typical case is that a sensor will be installed and calibrated with a sensitivity of approximately 150-180%. Manganese deposition will mean that recalibration is required daily for three to seven days. After this initial period the sensitivity will be reduced to approximately 80-100%.
The chlorine sensor maintains this sensitivity for a length of time depending on the level of manganese in water. Typical membrane lifetimes are between six and nine months. At this point the maintenance requirement is a 15 minute process of membrane exchange followed number of associated issues, including excessive cost, maintenance requirements, high rates of component failure, environmental and health and safety issues associated with the storage, handling and disposal of buffering reagents. The high costs and other factors, including environmental impact and health and safety issues, have caused the water industry to look for alternatives to liquid buffers; many water companies are now looking for a way to measure free chlorine residuals without the need for chemical buffering traditionally associated with such measurements.
pH Corrected Hypochlorous Acid (HOCI) Monitors
An alternative method of measuring chlorine in water is through the use of pH-corrected hypochlorous by a stabilisation period of around 20 to 40 minutes. The design of the flow cell ensures that there is a directed flow of sample across the membrane. This flow acts as a self cleaning device, and the soft scouring of the sample flow prevents build up of solids.
Chlorine is the most widespread disinfectant used for the treatment of drinking and cooling water systems. Monitoring the level of chlorine in drinking water entering a distribution system is normally considered to be of high priority. The free chlorine level needs to be constantly monitored and controlled during and after the second stage of iron and manganese treatment in order to avoid certain problems for chlorine monitors. The challenge is that the sample water is laden with un-dissolved manganese salts, which can coat and stain the chlorine monitor resulting in increased maintenance being required. In addition, manganese salts coat the measuring electrodes of typical monitors and cause zero drift and loss of sensitivity. High levels of solids can also block sample inlets.
Open Cell Amperometric Monitors
The water industry has historically relied on open cell amperometric monitors or a variation on open cell amperometric technology with buffer delivery systems for examining the levels of chlorine in the disinfection process. It is estimated that the cost to the water industry of operating amperometric monitors with buffer could exceed 20 million per year. Many amperometric monitor designs are based on engineering technology which dates back 40 years or more.
HOCl dissociates into hypochlorite (OCl) in a pH dependant manner. As a result, most monitors require the use of chemical buffering in most applications. The typical pH level of water measured in water treatment works can range from 7 to 9.2. Chemical buffering reduces the pH to between 5 and 6 and ensures that all the residual chlorine is present as HOCl. If an HOCl monitor is combined with a pH monitor it is proposed by some manufacturers that the output of the HOCl monitor can be compensated for with reference to the HOCl v pH dissociation curve.
However, there are also issues associated with the use of pH-corrected hypochlorous acid (HOCI) monitors for samples where the pH of water varies. Seen as a solution to the difficulty of measuring chlorine where there is a high variability in pH values. limitations in the use of HOCI pH-corrected monitors have been detected, with temperature, ionic strength, errors in pH measurements, response times and errors in the HOCI measurement, all diminishing the reliability of the results.
The combined effect of these associating factors and errors in the actual HOCl measurement has severely limited the use of pH-corrected HOCl monitors in the water treatment process. Reproducibility of greater than 20-25% is often difficult to achieve in any situation other than one in which pH is already stable.
Analytical Technology Inc (ATi) has developed the Q45H chlorine monitoring system which achieves accurate and reliable free chlorine measurement even in the presence of high levels of un-dissolved iron and manganese salts.
Featuring a membraned polarographic sensor, modern chlorine monitors like the ATi Q45H can overcome typical challenges. A catalytic electrode and a counter electrode anode are immersed in electrolyte behind a microporous membrane. Chlorine species diffuse through the membrane, electrochemical reduction occurs at the cathode and a current proportional to the chlorine concentration is generated. This current is measured and then converted to a chlorine concentration.
The presence of the membrane eliminates electrode contamination and zero drift is eliminated.
Typical membrane lifetimes are six to nine months and maintenance is minimal, requiring a 15-minute process of membrane exchange followed by a stabilisation period of around 20 to 40 minutes. The design of the flow cell of these chlorine monitors ensures that there is a directed flow of sample across the membrane. This flow acts as a self-cleaning device. A soft scouring prevents build up of solids.
The A10 sensor from ATi has a response time around 10 times higher than that of conventional sensors at high pH values (pH >8.3). This means that the monitor can be used reagent free over all the pH values found in the E drinking water industry. In variable pH applications the Q45H62 utilises an optional pH compensation feature to eliminate free chlorine error caused by process pH drift. If the correct sensor is used there is an extended response to chlorine at high pH levels, allowing pH correction to be employed over a much wider range.
When treating water high in iron and manganese content, the use of chlorine can be extremely effective. However, contamination from these compounds can cause problems with the chlorine sensor. If this is combined with variable pH water, chlorine monitoring can be extremely difficult.
The pH-corrected polarographic membrane monitors from ATi are now proving as stable and accurate as traditionally buffered monitors, thus offering the water industry a new way forward. A number of industry evaluations have also shown that these monitors are fast becoming the preferred instrument of choice. Additionally, the improved A10 sensor now allows reliable free chlorine measurement even in the presence of high levels of un-dissolved manganese salts.
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