When looking to achieve required levels of water quality, there is increasing and relentless pressure to reach the high standards set by World Health Organisation (WHO) and Drinking Water Inspectorate (DWI).
In regards to manganese levels in water there is however a significant difference in the requirements set by these two organisations. WHO guidelines have set a maximum allowable limit of 400 parts per billion (ppb), while DWI have set a substantially lower maximum of 50ppb. Whilst both conform to health requirements the DWI’s far lower limit is also ensuring that drinking water at the point of use looks healthy and tastes good.
For the consumer, taste and appearance are of major importance and therefore the management of manganese levels are a primary consideration for water companies. Further to the DWI standards, recent studies conducted by Welsh and Severn Trent have identified that levels need to be even lower at 2ppb in order to deliver a high quality of water, both from a health perspective as well as taste and appearance.
Removal processes such as traditional sand beds that require regular back flushing with up to 20m3/hr/m2 of bed are wasteful of water as well as having very high capital costs. The higher concentration of manganese getting into the distribution network can also result in the necessity to flush pipework on a regular basis.
At a top level, most methods of removing manganese rely on the oxidation of Mn(II) to Mn(IV) to precipitate out the manganese for subsequent removal through a filter bed of sand. This can be extremely expensive and if water is accessed from multiple locations it is just not viable to treat all individual sites.
Existing methods of manganese removal include:
|
Removal Technology |
Principle |
Advantages |
Disadvantages |
Summary |
|
Oxidation -Precipitation |
Oxidation of MnII to MnIV which can be removed by dead end filtration such as sand beds |
Chemicals used to oxidise (e.g. Chlorine / permanganate) are freely available |
Chlorine may need to be removed. Permanganate can discolour the water at low concentration |
Additional use of chemicals should be avoided if at all possible. Periodic backwash required |
|
Sequestration |
Prevention of Mn precipitating out and discolouring water |
No discolouration of water visible |
Manganese still present in water with associated health impact |
Not used as a viable control method |
|
Adsorption |
Ferrihydrite, goethite, ion oxide coated sand (IOCS) used to promote oxidation process |
Readily available technology |
Effectiveness highly dependent on the pH of water. Beds need to be regenerated with Chlorine or permanganate |
Only work for specific water conditions and still required addition of chemicals |
|
Ion Exchange |
Ion Exchange |
Good for polishing up water with very low concentrations |
Can not be used for higher concentrations and can block quickly if oxidation to Mn(IV) occurs |
Not suitable for municipal water suppliers |
|
Membrane Technologies |
Reverse Osmosis (RO) and Nano-Filtration |
OK for low concentrations |
Can block rapidly, especially when oxidation causes precipitation |
Not suitable for municipal water supplies |
|
Biological Removal |
Biological mediated oxidation using aerobic bacteria such as Gallionella |
Smaller volumes of sludge produced and no chemicals added |
Complicated to run and maintain balance of microflora biosystem |
Not common practice |
In order to improve the ppb levels achieved by the existing methods and deliver a much more cost-effective solution, Amazon Filters and Welsh Water focused on the use of cartridge filtration technology.
In order to achieve a maximum level of 1ppb and reduce overall installation costs by 75%, a polymeric filter composing of meltblown fibres was developed. This solution can lead to below detectable limits of manganese that is far superior to traditional sand bed systems.
The polymeric filter solution achieves the following:
With such significant improvements to the limits of manganese and the over installation and maintenance costs of the filtration process, the Amazon Filters solution presents significant financial benefits to any water company whilst exceeding the prescribed health regulations specified by DWI.