The following data is taken from an AFM and a sand water treatment system working on drinking water and operated by Scottish Water.  There was no pre-coagulation or flocculation prior to either system.

The upper limit for manganese is 50 ug/l and for ferric 200 ug/l to be in accordance  with the EU drinking water directive. Manganese  is an essential trace nutrient and is required at a low concentration.  However as the concentration in water increases to a value above 0.15mg/l it may start to form black precipitates on all objects in contact with the water.

At high concentrations manganese can cause neurological  as well as kidney problems. Concentrations above  0.1mg/l (100 ug/l) have been implicated in affecting the learning ability of children ( http://ehp.niehs.nih.gov/1002321/)

Scottish Water conducted a trial comparing sand against AFM at the Fairmilehead treatment works in Edinburgh Scotland.  Two parallel systems were installed, one with Leighton Buzzard silica sand and the second with AFM.

Leighton Buzzard has been acknowledged as one of the best grades of filter sand available in Europe.The filtration system used 1000mm bed depth of 16 x 30 sand and a second parallel system used grade 1 AFM.  Both filters were operated under identical conditions.   Samples were collected by Scottish Water and analysed in Scottish Waters accredited laboratory.

The results for the parallel trial are presented in the graphs on the right. The upper graph is of manganese and the lower for Ferric. The blue line is the supply water, the green line is sand and pink line AFM. The thick green line and pink line are average trend lines. On average the AFM filter removed twice as much manganese as the sand filter.

Sand filter instability

mangaeseIt is impossible to stop a sand filter from becoming a biofilter. The bacteria glue the sand grains together which leads to transient worm-hole channelling.  This aspect has been reported on this website at the following link;  (download)  It is interesting to note that bio-instability coincided with both manganese and ferric discharge.

The concentration of manganese and iron were both low,  and both AFM and sand removed more than 50% of the influent metals.  AFM was much more stable and consistent where as the sand filter occasionally dumped solids including manganese and ferric back into the product water.

Treatment strategy

The data presented on this page was for two parallel systems.  Dissolved oxygen content was 100% saturation, however there was no pretreatment of the water by oxidising chemicals nor was there any addition of flocculation chemicals.  Manganese and ferric can be a problem in deep water reservoirs as well as ground water.  The first stage that we recommend is aeration to raise the oxidation potential  and blow-off any volatile organics.  The pretreatment and filter operating procedures are detailed below. Manganese is not the easiest metal to remove from the water, but if the procedure is followed then greater that 80% removal can be consistently achieved.  If the ORP is increased to 500mv  using chlorine dioxide hypochlorite, peroxide or ozone,  the manganese removal performance  with AFM can be increased to better than >95% removal.  The Manganese is all removed during a back-wash, so the prtocess is sutainavle and AFM never needs to be replaced.

AFM also has the advantage over manganese media or green-sands   because it does not need to be replaced, nor does it need to be regenerated.  However it is really important that AFM is very thoroughly back-washed once a day to once a week.

ferricThe removal of Manganese and Ferric from water

Chemical parameterSoluble fractionInsolublePermitted concentration in drinking waterTypical performance at 5 - 10 m/hr filtration velocity

Manganese

Mn2+

Mn4+

50 ug/l

>80 %, with no pre-oxidation

Ferric

Fe2+

Fe3+

200 ug/l

>95 %

Iron and manganese are often found in borehole / tube wells and ground water at varying concentrations depending upon the geology of the ground. The technique used by Dryden Aqua to remove the chemicals is as is as follows;

Process

  1. Oxidation reactions to convert metals from soluble ionic form to insoluble oxidized precipitate, pH correction and redox correction.  For ferric removal the ORP needs to be over 300mv, for Manganese removal is shuld be close to 500mv
  2. Decantation may be required if the concentrations are above 5 mg/l, if not proceed to AFM® filtration
  3. AFM® filtration to remove the suspended metal oxide solids, there will also be adsorption reactions and surface oxidation reactions.

Procedure

Oxidation;

100 0093This is achieved through aeration of the water. The water is aerated for a period of up to 60 minutes.  If water flow is 50 m3/hr the aeration level is 100 m3/hr of air and tank volume is 50 m3 of water. Dryden Aqua manufacture fine bubble drop in air diffusers for this application.  Aeration provides the advantage of blowing off VOC's (volatile organics)  aso the long contact time frame allows the manganese dioxide particles to grow in size,  this makes it easier to remove them from the water, and reduces any potential issues relating to AFM fouling.  However as opposed oxidation by aeration,  other oxidising chemicals such as ozone, hydrogen peroxide of hypochlorous may be applied.

pH;

The pH of the water should be  pH 7.0  to pH.7.8

Redox;

The  aeration system should increase the redox potential of the water. It is important to raise the potential to as high a value as possible. Certainly it should be over 300 mv.

1. Manganese oxidation requires a high oxidation potential, 500mv

2. Ferric oxidation is simple, the aeration system will more than suffice.


AFM® filtration

Pretreatment of the water prior to filtration by AFM® is very important. AFM® will remove the metals and metalloid by the following mechanisms;

  1. Oxidation and adsorption (similar to greensands and ferric media)
  2. Adsorption of sub-micron metal oxide particles
  3. Physical filtration of most particles down to 1 micron.

Operating parameters

AFM® filtration performance will depend upon the operating parameters. It is important to use good quality filters, we recommend filters in compliance to the German DIN standard. The follow represent the optimum filter bed operating conditions.

 RecommendedMaximumNotes

Bed depth  AFM®

1200 mm

3000 mm

Bulk bed density 1.25 to 1

Run phase water flow

<5 m/hr

10 m/hr

The slower the flow rate the better the performance

Running pressures (differential)

<0.3 bar

<0.4 bar

Do not exceed 0.4 bar differential

Back-wash water flow

>45 m/hr

60 m/hr

Back-wash for 5 minutes, or until the water runs clear. Air purge not required

Rinse phase duration

5 minutes

----

It takes a few minutes for the bed to stabilise after a back-wash

Back-wash frequency / per week

1

7

Reduce back-washing to a minimum.

pressure filters

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