Performance of Multimedia filter 3

[KiwiMace]

What is a reasonable expectation/requirement for removal of particles of size 5 micron and greater from a typical multimedia filter.

We have well water which is to be filtered using multimedia filters before RO and polishing. This treated water is to be used as Boiler Feedwater.

We issued a standard masterspec with a default 98% removal, but after bidding this out, the responses were 85-90%. Was this an unreasonable specification or are our suppliers trying to cheap out.

 

[cub3bead]

Grannular media filters are usually considered effective for particles 10 um and larger.

Are you feeding a coagulant ahead of the filter? It would help with getting the small particles.

I prefer to specify a Silt Density Index (SDI) guarantee for the prefilters in the system rather than specify a TSS removal guarantee. The membrane supplier specifies the feed SDI must be <5 for their membrane warranty to apply. With the SDI guarantee from the filter supplier you link the warranties.

Where this can come into play is if you have a lot of 1 um clay particles in the source water. The particles for the most part won't be removed by the prefilters but will quickly blind off your 0.45 um filter used for the SDI test and would do the same to your RO membranes.

 

[bimr]

I agree with cub3bead's comments.

I would add that with a properly constructed well, the water should not contain micron sized particles. Micron sized particles would generally occur with a surface water supply.

Filters on well water are generally installed as a preventive measure rather than as a working filter.

 

[GarySCWSVI]

I agree with both bimr and cub3head's comments. To get a multimedia filter to remove smaller than 10 micron particulate you need to feed a coagulant, flocculant or a combination of those 2.

Gary Schreiber, CWS VI
The Purolite Co.

 

[sbush]

Your filter supplier is being honest with you. Our own testing shows that rapid rate sand filters typically remove approximately 88% of particles in the 1 - 5 micron size range and approximately 91% of particles in the 5 - 10 micron particle size range; when operating at constant and sustained rate of flow. Performance deteriorates dramatically when the filter process starts and stops frequently, as is normally the case in industrial water plants with limited storage capacity.

Adding a filter aid, like alum or polymer, might not improve filtration efficiency in this case due to the inherently low turbidity characteristics of well water. High turbidity source waters can actually be easier to coagulate and filter. There is also a risk involved with overfeeding coagulants and the impact on membrane life and performance.

We have also noticed that measuring filter efficiency by particle size reduction does not necessarily correlate to turbidity reduction and/or silt density indices (SDI) in all cases. In other words, you could have a 98% reduction of 5 micron size particles but still have unacceptable SDI levels. Therefore, Cub3bead's advice to add the RO membrane manufacturer's SDI feed water requirements to the performance specs is very wise.

As Bimr points out, well water is usually pretty low in particulate matter. However, SDI levels in well water can be high due to biological activity and/or tannins and other organic acids, and turbidity can be high due to iron and/or manganese. If you haven't already done so, a comprehensive analysis of the water source, including a particle count and SDI test, would be very useful to all the parties involved with your system.


S. Bush

http://www.water-eg.com

 

[quark]

Excellent suggestions above. I agree with sbush about overfeeding of coagulants. I faced grave problems with alum, in the past, with RO membranes clogging frequently. In addition to MMF, go for a 5 micron depth filter (pleated type are better). I would suggest to test the settling of suspended particles if at all you want to go for coagulant dosing.

 

[KiwiMace]

Thanks people, I think I need to tone down a spec. Bidder's remorse is kicking in.

Searching old threads, I gather that Fe, Mn, SDI, and SiO2 are the nasties for RO feedwater. Can I get some comment on the following levels:
* Fe (ferric) 0.92ppm (I was told that this is high)
* Fe (ferrous) 0.76ppm
* Mn 0.055ppm
* Silica as SiO2 3.5ppm
* Unfortunately, the SDI was excluded on the list of measured parameters from the water analysis.

Can anyone point me in the direction to a book or website that will put these analysis figures into perspective. I acquired Betz "Industrial Water Conditioning" but it doesn't go into this kind of detail.

 

[cub3bead]

Listed below is the link to Filmtec's Technical Manual which contains outstanding advice on pretreatment requirements in Chapter 2. You can download the whole thing (highly recommended if you have a fast connection) or individual sections. Read it thoroughly, apply the new knowledge and it will save you a lot of long hours troubleshooting your system.

http://www.dow.com/liquidseps/lit/techman.htm

It is unusual to see that much ferric in combination with ferrous iron. I suspect it is due to improper handling of the sample which allowed the ferrous to begin oxidizing to ferric.

The SiO2 reported is not that bad (Ihave seen much worse) and shouldn't be a big problem unless you have a lot of Mg and try and operate at a pH >8.0.

Your lab only gave you one result for Mn? You should have gotten a result for Mn+2 and for Mn+3.

The Fe and Mn are easily removed with greensand filters. You maybe able to modify your existing filters for this but not enough info to tell at the moment.

A second to quark's comment about alum and RO units, don't do it. If you have to use a metallic salt coagulant use FeCl3, Fe2(SO4)3 or poly aluminum chloride and stay away from cationic polymers.

Did you analyze for Ba and Sr? If not it is time to go and get another sample. If you ever intend to use any of the permeate for drinking water you need to analyze for B and Br as well.

 

[sbush]

A few more feed water constituents that should be added to your list include (but not limited to): calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, chlorine, hydrogen sulfide (if rotten egg odor is ever detected), bacteria. I would still get a comprehensive analysis of as many inorganics, organic, bacteria, and particles that your company can afford.

Water chemistry is water chemistry. The degree of reject solids as a function of conversion remains the same regardless of which RO membrane you use. There are, however, subtle differences in membrane chemical construction as well as physical construction which make one membrane more suited to one application than others. For this reason, it is highly recommended that you consult closely with the membrane manufacturer and/or the RO device manufacturer, as opposed to relying on text books.

As a rule, ferric iron should not exceed 0.05 mg/L. Reducing the pH of the feed water will help in keeping iron in solution (by retarding oxidation), but this is not a practical approach to redissolving oxidized iron.

The projected Langalier Saturation Index for the reject water will determine a lot about the scaling potential of the feed water and pretreatment requirements. These projections are typically made based upon calculation provided by the membrane manufacturers for their given products. They typically provide these and other performance calculations free for the asking. Again, I strongly recommend consulting a manufacturer for assistance rather than relying on generic programs downloaded from the Internet or from text books.

S. Bush

http://www.water-eg.com

 

[bimr]

Totally agree with sbush. You need to go back and get a complete water analysis before you do anything else.

 

[KiwiMace]

We revised the requirements down to 95% at 10 microns.

Now the issue is getting rid of the iron - making our ferrous ferric. We have compressed air at the building, which is supposed to be cheaper than adding hypo, in terms of operating cost. There is added maintenance of a porous stone used to aerate the well water and its little iron floaties. This porous stone thing sounded a little Harry Potter to me, but I'd appreciate any comment on the two systems.

 

[bimr]

The porous stone thing is not going to work for iron removal.

Iron oxidation with air is pH and time dependent. If you have the correct pH (that pH being above 7.5), you will need a tank with 15 minutes residence time to allow the iron oxidation reaction to run through completion.

The oxidation of iron with chlorine is a much faster reaction although it is recommended to have some residence time to allow the oxidation reaction to run to completion.

If you want to do the iron removal without the residence tank and time, then you should look into the use of a greensand as recommended by cub3bead or the use of a water softener. The use of the water softener should also improve the operation of the RO system.

 

[sbush]

Bimr is right about the oxidation time for iron (even longer for manganese) if you are using aeration alone. However, the normal oxidation time can be reduced to an almost instantaneous reaction within the filter vessel itself when the media contains manganese dioxide or other types of oxidation catalysts. The porous stone is only one type of air distribution system, and it isn't really necessary in a pressurized system. The key to an air saturator is introducing air to water under pressure. At 68-deg. F and 65 psi, for example, approximately 9 - 10 mg/L of air will dissolve into the water. It's the pressure than drives the process.

Pressure aeration in conjuction with catalytic oxidation media is a viable option for reducing iron and manganese. Will it reduce iron and manganese to extremely low levels? That will require pilot testing on the actual water source to answer with any certainty. For that matter, any treatment scheme needs to be field tested on the actual water supply to be certain. We routinely monitor pressure aeration systems that are continuously producing iron and manganese levels below the maximum contaminant levels for potable water, but this might not be low enough to meet the RO manufacturer's feed water requirements. So you need to define the targets for iron and manganese first.

If you decide to look at pressure aeration as a treatment option, make sure that your potential supplier is experienced with similar successful applications and that they can pilot their process for more than a few weeks. There are a lot of "water doctors" today that read something in a text book or Google and then try to reverse engineer the process, with little or no experience in what they are doing other than the assumption that someone else has done it so it must work. Just make sure you get proof and not just claims from your supplier.

Bimr is right about the ion exchange softener's ability to reduce iron and manganese to extremely low levels. And the low calcium and magnesium is an added bonus. However, if any of the iron and manganese coming into the softener is already oxidized, then fouling of the softener will occur within a year or possibly sooner. This might be an acceptable risk, given that replacing resin can be made into a relatively simple task when a softener is properly designed to facilitate hydraulic transfer of old and new resin. Again, the experience of the designer/supplier is crucial here as well.

You might also find that pH adjustment and/or chemical sequestering is an adequate solution. Here again, you need to know exactly what targets the RO membrane manufacturer wants you to meet and what the actual water chemistry is from the source. Then consult with the membrane manufacturer, the RO device supplier, the pretreatment equipment supplier, and possibly even an outside consulting firm (especially if there seems to be a lot of conflicting opinions).

I would write a performance based specification based on the RO membrane requirements (in addition to your own) and then I would demand that any supplier prove that his system will meet the targets using a pilot scale model of the proposed system (for a sufficient period of testing) before you purchase something that you might be "stuck" with later on. Then I would follow up with a requirement for a Performance Bond sufficient to cover the replacement cost of the system. (This might not be warranted if your system cost is relatively small).

S. Bush

http://www.water-eg.com