Wastewater Recycling Problem 1

[ivanhoe374]

We are currently running a trial plant on the wastewater from a food manufacturer. The existing plant is an activated sludge plant with the decant entering into a 100,000L tank before being discharged to sewer.

From this tank we are attempting to recycle the water using a combination of ultrafiltration, activated carbon and reverse osmosis.

The problem we are having is quite severe fouling of the RO membranes (10% drop over 3hrs). Our lab has identified the problem to most likely be Calcium Phosphate.

Can anyone recommend an effective method for removing the calcium phosphate, i have heard that Lanthanum chloride can be used to precipitate out calcium phosphate, which could then be filtered out in the UF system....does anyone have any experience with this? and if so what pH, dose rates etc would i be looking at?

Does anyone have any other suggestions?

One other thing we have noticed is that with an SDI test we seem to be getting a light yellow coloured stain on the SDI paper...could this be the calcium phosphate.

Cleaning with acid removes all fouling quite effectively.

 

[orenda1168]

We have a substantial amount of experience with application of lanthanum chloride for reaction with, precipitation and removal of soluble phosphates as lanthanum phosphate. Calcium phosphate, however, is highly insoluble and is in fact a mechanism for phosphate removal by addition of a soluble calcium salt such as calcium chloride, though not as efficient a removal process as application of lanthanum salts. In any case, lanthanum chloride will not react with calcium phosphate.

If the calcium phosphate is being formed downstream of the UF, then treatment at or immediately after the wastewater tank may be a viable option to prevent RO fouling. Certainly, UF will remove lanthanum phosphate, as will relatively coarse filters such as DE or sand.

Orenda


Orenda

 

[ivanhoe374]

So what are our options if the calcium phosphate is already formed before the UF(given that you mentioned that the lanthanum chloride) will not react with calcium phosphate......or if it is, should it be insoluble? (or is there any ideal pH range where it would be insolube and we could filter it out with the UF by adjusting pH?)

 

[orenda1168]

If the calcium phosphate is formed upstream of the UF, it should be completely removed there.

If it is formed between the UF and RO, then an intermediate filtration will be an alternative. You probably will need to assay the water going to each process to identify the location of calcium phosphate formation.

Again, it would appear from your description of the flow that the problem is occurring somewhere between the UF and RO. At any likely pH of the water, calcium phosphate will be essentially insoluble.

Orenda

 

[bimr]

Recycle water for what purpose?

Most food wastewater probably have fats in them. I doubt that you will get all of the fats out with an activated sludge process.

Forget the lanthanum chloride. 99% of the precipitating processes use either ferric salts or alum to precipitate phosphate. You can precipitate the salts in the activated sludge clarifier.

Don't understand why you are using RO on this water. You can recycle the water with an RO process. Are you trying to desalt? Is desalting the reason for the RO process?

If in fact you are trying to desalt, you are going to have terrific problems using RO without a serious attempt at pretreatment. You are proabably going to have to use a two stage filter process to get enough of the solids out to allow the RO process to work.

 

[ivanhoe374]

Purpose of recycled water is for boilers and cooling towers. The TDS is 1400mg/L so RO is definately required. And yes we are having terrific problems with the RO . I agree that the phosphate could be knocked out in the activated sludge plant with alum or ferric however the client has very shallow pockets!

Currently we have UF, ozone, activated carbon, antiscalent dosing as pretreatment.

Is there any reason why the calcium phosphate would form after the UF? (and not be formed before the UF) Essentially all the UF is doing is removing the solids....

There are also many different forms of the calcium phosphate

Calcium metaphosphate, diOrthophosphate, mono-orthophosphate, tri-orthophosphate, pyrophosphate...all with different solubilities i would imagine (can't find any literature on it)

 

[bimr]

Shallow pockets and you are using RO? LOL.

Alum or ferric salts are not expensive, don't know why you consider them to be expensive. It is the least expensive step in your process.

The ideal process is to use a membrane bioreactor. That is an activated sludge process with a membrane process to remove the suspended solids instead of using a clarifier. You also have to be sure to remove the ammonia in the activated sludge process as well.

You can add the alum to precipitate the phosphate salts before the membrane filter.

The food plant may be phosphates for cleaning. (Phosphoric acid is good for cleaning fats off kettles.)

Then you can filter and then disinfect, probably using a UV process.

Finally, you are ready to desalt using the RO process.

I doubt that this will be economical process. There has to be a cheaper source of water.

 

[ivanhoe374]

There is a cheaper source of water, but when that source of water is running out, government grants are plentiful for this type of project(In this country). (hence the shallow pockets comment - the capital costs will be covered by grants....it is the running costs that the client will be worried about). Having said that, if dosing ferric into their activated sludge plant is the only way to get the reuse plant up and running, then that is what they will have to do.....if there is a cheaper alternative that we can offer, then even better for them.

If we are to dose ferric prior to the UF what sought of residence times should i allow for to allow ferric/alum to react with the phosphate prior to filtering through the UF system?

 

[bimr]

You do not really need a residence time. Just mix the chemical into the waste stream thoroughly.