A manganese removal system that ensures levels below 1ppb exiting the WTW with capital costs a quarter of traditional sand beds
Relatively low manganese levels in water can result in tainting of taste as well as discoloration of the water and the surfaces it comes into contact with. The Drinking Water Inspectorate (DWI) in the United Kingdom have set a maximum allowable limit of 50ppb, well below the WHO guidelines of 400ppb. While the WHO regulations are based on health concerns of the bioavailable form of manganese the DWI regulations are concerned with providing drinking water at the point of use which also looks healthy and tastes good…i.e free from discolouration and taint.
The 50ppb limit set by the DWI was intended to ensure that discolouration and taste tainting was acceptable to the consumer. However, recent studies conducted by Welsh Water and Severn Trent have shown that levels as low as 2ppb can still be problematic at the point of use.
Flushing pipework to remove the contamination on a regular basis is time consuming, disruptive to the consumer and expensive. In an attempt to develop a more viable treatment system that can produce water with a manganese concentration below 2ppb Welsh Water partnered with Amazon Filters Ltd to look at an alternative technology.
Existing Methods Of Removal
Most methods of removal rely on the oxidation of Mn(II) to Mn(IV) to precipitate out the manganese for subsequent removal through a filter bed of sand. The capital cost of these system based on sand beds can be very high and mean treatment at multiple bore holes or surface water is just not viable. A brief summary of the various available methods of removal is provided below.
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
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
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
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
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 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
Amazon Filters worked closely with Welsh Water to see if using cartridge filtration technology could result in a more viable, flexible option.
The results of the first round of testing have shown that a system based on polymeric cartridge filters instead of traditional sand beds can lead to below detectable limits of manganese with an estimated overall installation cost 25% that of a traditional sand bed system.
This has been achieved using a polymeric filter composing of meltblown fibres that have proved to be highly effective at promoting the ‘seeding’ of manganese. This ‘seeding’ on the fibres establishes areas that accelerate the removal of manganese through a catalytic process. By selecting a filter with the correct combination of fibre diameters, efficiency and water flux, the optimised system in terms of removal and lifetime can be achieved.
Test Protocol / Results
A small side stream filter was set up to process a proportion of the water from a bore hole after chlorination and pH adjustment. The initial manganese level was recorded at between 4 and 5ppb before treatment. While this is well inside the DWI regulations it was still in a concentration above that known to cause problems in the distribution network.
Flowrates were maintained during the testing between 8 and 10l/min/10” filter.
Measurement of manganese levels were recorded by submitting samples to Welsh Water’s laboratories over an initial test period of 2 weeks. Over this time a marginal decrease in levels was recorded. Following a 2 week break the test was restarted and the filtrate levels of manganese had dropped to below detectable limits.
A section through the test filter shows the discolouration disappearing as the water flows from outside to in.
Two different filter constructions were tested to establish the most optimal design with respect to retention and lifetime. While these tests were limited to just one bore hole, the results are so encouraging that additional filter systems have been ordered by Welsh Water to replicate the performance at other bore and surface water locations.
To better understand the mechanisms of removal more fully and therefore have the ability to further improve and optimise the system a research project is being initiated with Swansea University. It is hoped this will lead to further improvements in design, installation and operation of the filtration system.
The full size system for the test borehole will treat up to 8.4Ml/d guaranteeing manganese concentration below 1ppb, ensuring discolouration does not appear at the customer tap
For further information on this new filter and application please contact Amazon Filters.