Low concentration SBS dosage

[Watro]

Hi

We are designing a low flow brackish RO - 300 LPH permeate with 70% recovery, feed is well water, 2 ppm of FRC
The issue is not able to find a dosing pump. We need to dilute the SBS to meet the lowest available pump flow rate. The ideal concentration of SBS works out to be 5%.

What are consequences of diluting the 30% SBS to 5%, preparing SMBS to SBS is a challenge on even weekly basis
Not sure we can get 5% neat SBS solution. In case of dilution, is FRC neutralization will be effective as compared to dosing 30%? Since SBS also reacts with DO, so which reaction is faster. 1:3 dosage will be considered for FRC neutralization, in case of diluted SBS dosing, should we consider a higher dosage to overcome DO scavenging? Also advise the consequences of overdosing SBS.

Thanks
NM

 

[bimr]

10% solution of sodium bisulfite has a life of 7 days. Dilution further than that is not recommended as it will have a shorter life.

https://dowac.custhelp.com/app/answers/detail/a_id/2261

 

[ashtree]

Constant dosing with SBS often results in fouling by sulfur reducing bacteria, with the SBS as a major food source.
You do not mention why you need the FRC but i assume you have a biologically active well water and you are hoping to avoid bio fouling.
Doing this the way you are proposing may give you a good result at least for a while but it may also end up experiencing worse fouling.

The other question is , do you really need 2mg/l of FRC? A lower residual and more detention time may give you a satisfactory result with less SBS required. Less SBS potentially gives you less sulfur reducing bacteria potential. Have you had experience with the well water bio fouling? You might not even need the FRC at all.

You have made no mention of what pre-treatment you are using or the purpose of the treated water. There may be other options that can reduce or eliminate the SBS requirement.



Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Watro]

Thank you Bimr/Ashtree,

The purpose is for potable usage. Upstream is beyond our battery limit and end user chlorinates before TIE-IN, not sure from how long it is fed from.
FRC indicated as 1-2 mg/l, so the neutralization is for worst case

Activated carbon is a choice but the flow is too low to employ one. How is the reliability of Activated carbon cartridge filters? Any exp with this?

Thanks
NM

 

[bimr]

For the low flow that you have, you would be better served with a carbon block filter than a sodium bisulfite feed. Carbon block media is preferred over a carbon granular media filter.

It will be difficult to precisely meter the sodium bisulfite. On the other hand, the carbon block type filter is much simpler and will remove the chlorine and will require periodic replacement.

http://media.wattswater.com/S-WQ-ChloraBlock.pdf

http://waterquality.cce.cornell.edu/publications/CCEWQ-03-ActivatedCarbonWtrTrt.pdf

 

[ashtree]

I agree with bimr but the problem i see is how will you know that the carbon is exhausted. This particularly applies given that you have no control over the FRC and sudden increase may exhaust the carbon ahead of the next scheduled replacement
Maybe you should fit an ORP probe that will alarm and or shut down if chlorine is detected downstream of the carbon filter. Alternatively it might alarm but dose SBS until the situation is rectified.

A totally different approach would be to dose ammonia hydroxide to combine with the free chlorine to form chloramines. RO membranes are much more tolerant of combined chlorine than free chlorine. Given the low flow it may be difficult to dose ammonia hydroxide in small enough quantities but it would be worth investigating as another alternative.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Watro]

Thank you Gents

 

[bimr]

I agree with bimr but the problem i see is how will you know that the carbon is exhausted.

To get around this issue, end users will install two units in series. The first unit in the series is the working filter and the second unit is the polishing filter. The piping is arranged such that when a filter is replaced, the replacement is always installed as the polishing filter. The existing polishing filter will then be used as the working filter.

Replacement filters are always installed as the polishing filter in case of working filter breakthrough. This method ensures that the filters are completely exhausted (before replacement) as you can monitor breakthrough between working and polishing filters.

 

[Watro]

Thank you Bimr,

Before starting this thread I was recommending AC-CF, but our team were reluctant about the reliability and did not want to take a risk upstream of RO. Is this proven and widely accepted method to deal with FRCs? particularly before RO as compared to SBS dosing

In case of 2 stage AC-CF, are these capable of removing some suspended particles? since we need to ensure SDI at least <4 and better is 3. Feed water do not have particular TSS and turbid particles
OR Do we still need 5M CF upstream of this AC-CF? in order not to load them with traces of TSS

Further, typically how long these element lasts? Any idea of how much surface area/nos of CF element is needed to treat 2 ppm of FRCs or on what basis this is designed. I also believe these elements fit in all typical CF housings available in market

Thank you once again
NM

 

[ashtree]

The activated carbon cartridge filters will remove the FRC and is widely used on small systems. They will remove suspended particles but if this is an issue you would do better to use a dedicated cartridge filter to perform that role as they are much cheaper than clogging a carbon filter unnecessarily. Unless you are certain that the TSS levels are always very low a 5 or 1 micron cartridge filter upstream is probably a good investment anyway.

However cartridge filters are probably not the best way to improve SDI even on a small system. Cartridges are normally used as a last resort system to protect the membranes and should only be removing a tiny fraction of the incoming solids load , if this is a problem. If SDI is an issue you should be thinking about a media filter perhaps or some other system to remove the TSS loads.

You have not provided enough information on your system to determine what would be the typical life of a cartridge filter. You need to start by considering what is an acceptable life for the process based on how often the plant will be attended, what alarm systems are installed and what you know about the water quality. Most manufacturers give advice on maximum flow rates , maximum differential pressures, solids capture etc. Carbon filters are sometimes provided with estimates of how much chlorinated water can be treated with a certain carbon filter.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Carbon has a very long life when used for chlorine removal. The carbon filter will probably fail on buildup of suspended matter and high pressure loss long prior to chlorine exhaustion.

With such a small system, you will be limited by what equipment is available in the market place. Vendors do not make custom systems for such small systems, but will put a package together of the existing products.

You should keep the 5-micron cartridge filter downstream of the carbon block filters. Upstream of the carbon block filters, consider using a 15-micron cartridge filter

The frequency of filter changes depends upon your water quality and usage. For example, if there are a lot of sediment particulates in your water, then you will have to change your sediment filters more frequently than someone with little to no sediment. Having said that, manufacturers suggest sediment filter replacement every two to six months. Carbon block filters should have a service life of 6 months.

End users should have a stock of spare cartridges on hand. Over time, the end user will develop a replacement schedule based on the particular application.

The system should be 15-micron sediment prefilter, 5-micron carbon block filter, 2nd 5-micron carbon block filter, 5-micron sediment filter, and then the RO unit.

 

[Watro]

Thank you Bimr and Ash,
May I know your names and the companies you are working for? Please let me know, if that is allowed in this forum

 

[ashtree]

It is not normal to give our companies or names but some members choose to do so.
I work for a Utility in water and wastewater treatment, with responsibilities for process engineering and water quality.


Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Check the profile