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 as much iron and manganese needs to be removed 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.
ATi has developed the Q45/6H 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 Q45/6H 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 Q45H 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.