It depends. If your raw water quality changes enough everyday to cause a process upset then maybe you do need to.
If your raw water is relatively constant and your process is performing well and nothing is changing then maybe you don't. You would simply use the dose settings you used the day before. But you probably should still grab a sample of the water from the flash mixer and put it on the jar tester just to double check that everything is okay. Doing it this ways saves the time of measuring out chemicals etc and helps to test under actual plant conditions and dosing.
Even if conditions are stable and things are going well you should still do jar tests regularly to test whether the current dose rates pH etc is still optimum. The only way you can do this is to set up a number of jars and trial different rates and compare them with the current dose rates.
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
Agree with ashtree, if your raw water quality changes to cause a process upset then it is mandatory. If this is a surface water, the water quality may be constantly changing. You may be interested in doing a coagulant study to determine the optimum coagulant.
If the SDI is always high, then the pretreatment scheme is inadequate.
A preferable filter scheme is to use a 2 stage filter. The first stage is a higher flow rate filter. The second stage filter is a slower flow rate polishing filter.
It may be impossible to achieve a low SDI with just a single stage filter.
The 2 stage filter system that bimr is referring to would incorporate a filtration system in the first stage that would remove larger particles and hopefully the largest amount of the solids by mass. The second stage would have a greater degree of filtration in terms of particle size but would be much less able to handle solids. You would have to conduct a particle size analysis to know exactly what technology would be required and the sizing of each filtration unit. But for example stage 1 might be a multimedia filter that would remove 95% of particles down to a size of 10 micron.
The stage 2 unit might be a cartridge filter that will remove 99% of particles down to a size of 1 micron.
The multimedia filter should be removing a very large percentage of the total suspended solids remaining in the feed water after clarification, because this can be easily back washed at a regular interval when blocked. The cartridge filter unit should be removing very little in terms of percentage of the total suspended solids and is there primarily as a safeguard against breakthrough and a final polishing step.The cartridge filter would have to be replaced though as these cannot normally be back washed.
You might want to consider an ultra-filtration process in lieu of a 2 stage filtration process.
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
Okay.
The example i used before was simply an example and it does not necessarily mean that its correct for your circumstance. You might use a two stage process utilising 2 multimedia filters with the first stage relatively coarse media working on a high filtration rate capturing the larger particles and a second stage with very fine media but a quite slow filtration rate to capture finer particles as I think bimr is suggesting. A cartridge filter would only be used as a second stage if the amount of particles left in the water was quite low and you had a large amount of filtration area.
Cartridge filters on RO plants are typically only there to protect the membranes against catastrophes and as a last resort. Typically onl a few small amount of solids or particle removal is achieved.
If the process you have is going to be successful the coagulation/sedimentation/ clarification process should be removing most of the solids and particles. The multimedia filters should then remove most or the remaining particles down to maybe 10 micron(but perhaps lower) and turbidity should be quite low probably about 0.1 ntu. The cartridge filters should now have very little to do if everything upstream is working okay and you should get at least a couple of weeks before a change is required.
But this does not appear to be happening. But you have said that you are constantly changing cartridges and that SDI is always high.So either the process is unsuitable for the conditions or something is not working correctly, or both. SDI is a measure of the particles trapped on a 0.45 micron filter paper and the smallest micron filtration you have is nominally 5 micron.(Not that these filters should be seeing much loading if everything is designed correctly) That does not necessarily mean that it won't work but with surface water it can be very difficult to maintain a very high level of very fine particle removal with the process train you have.
So a couple of questions:
1) Has this process ever worked okay?
2) Has there ever been some form of particle size analysis done to make sure that the treatment process is appropriate for RO pre-treatment given the water source?
3) Is this a seasonal problem related run off or river flows?
4) Is there any bio-fouling issues with the cartridges and RO?
5) Has the hydraulic and solid loading on the filtration units ever been checked?
6) Do you routinely measure turbidity at various stages in the plant?
7) Which cartridge requires changing the most frequently 5 micron or 20 micron?
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
1) Has this process ever worked okay? yes
2) Has there ever been some form of particle size analysis done to make sure that the treatment process is appropriate for RO pre-treatment given the water source? this process deigned by specialist water treatment company
3) Is this a seasonal problem related run off or river flows? our water source is deep well
4) Is there any bio-fouling issues with the cartridges and RO? please see attached 5 micron cartridge filter photo
5) Has the hydraulic and solid loading on the filtration units ever been checked? No
6) Do you routinely measure turbidity at various stages in the plant? No,But we measure SDI
7) Which cartridge requires changing the most frequently 5 micron or 20 micron? 20 micron
To be clear about your process based on what you have said so far these are the steps:
1) Raw water from a deep well.
2) Coagulant dosing and i assume some sort of mixing.
3) Sand filters
4) 20 micron cartridge
5) 5 micron cartridge
6) RO.
Is this correct?
The next question is how long has this plant been in operation? You said that the plant has previously worked okay. How long ago was that and for how long did it work okay?
If the plant had previously worked okay for a long time and now it doesn't implies that something has changed.
How easy that will be to determine will depend on how good your records and testing has been over the whole period of operation.
If you have no records and no test data then it will be difficult, if you have a lot of data then it may be easy. Here are a few things to consider:
Has the raw water quality changed?
Have you changed coagulant or dosage, or injection point?
Has filter operation changed? Is there a longer or shorter period between back washing? Has the backwash rate changed? Have you changed the media? Have you inspected the filter media to make sure it has not been lost, its not mud balled and its clean?
Have you had a change in operator in this time?
With regards to the photos of the cartridge filters, are the two on top that are very brown more recently replaced than those below that have significant white patches on them?
When they dry out is the residue on them powdery and easily removed or does it stay stuck on the filter?
What do the 20 micron cartridges look like?
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
Assumed that you had a surface water application as inline filtration is more appropriate for surface water. Please post a water analysis.
This looks to be more of a problem with iron. Without an elevated pH and/or chlorine injection, it may be difficult to precipitate the iron in the short time that the water is passing through a filter.
The typical problem with wells is dissolved iron which will turn to insoluble iron with the addition of air. It is generally easier to remove the iron in the dissolved state than try to precipitate the iron.
Do you also have a problem with hydrogen sulfide as you posted about it some time ago. Hydrogen sulfide is difficult to remove and it takes an extended time to precipitate hydrogen sulfide.
A water softener may be used to remove the dissolved iron. Sources of air must be eliminated for this process.
It would seem that the iron is quite low although it does not say whether it is total iron or soluble iron. Based on the fact that the levels are roughly the same before and after the sand filters we would have to assume it is in the soluble form. This should be low enough to be giving very little problem and i would expect to see the cartridge filters more orange than brown if iron was the main culprit.
Although the conductivity is about the same many parameters are significantly different between the two analysis generally getting worse in 2013. Unfortunately not all the same parameters have been tested.
If the turbidity truly was 50NTU in 2009 i would question the process train selected. At that turbidity it would be normal to choose a clarifier rather than rely on contact filtration. I note the 2013 analysis shows turbidity at 10 ntu. This will require the media filters to be performing well to achieve a low SDI. A filter inspection may tell you a lot.
How good are your records on things like dose rates from the past?
I would suggest you work through some of the questions i posed in my earlier posts. This might show a clearer trend if you have adequate data.
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
The first observation is that the well was not properly installed, or sampled correctly, or the analysis is incorrect. It is very unusual to have turbidity and COD in a deep well. The presence of these parameters indicates the well may be contaminated with surface water.
In a deep well the iron should be dissolved and the water sample should be perfectly clear when drawn. On standing in contact with the atmosphere, the water should slowly cloud and finally deposit a yellowish to reddish brown precipitate of ferric hydroxide.
The recommended treatment would also depend somewhat on the capacity of the system which you have not stated. I am assuming the water system is small.
I recommended two stage media filtration as posted above. In addition, I would also recommend a water softener after the filtration as the filtration will not remove the hydrogen sulfide in the short retention time provided by a filter. The use of a water softener tends to significantly improve the operation of the RO system (in terms of less frequency of cleaning).
If the turbidity is due to the iron, then go with just the water softener.
You should have a laboratory determine what is causing the COD and quantify the hydrogen sulfide.
As mentioned above, some details are missing from the water anaylsis. Before you spend money to improve the system, have a laboratory do a bench test of the proposed improvements.
