difficult wastewater pretreatment 1

[Seda6334]

Hi

I am dealing with some wastewater with high TOC/DOC (>7), high alkalinity (>300 mg caco3/l, pH is about 7,5), high inorganics, in a pilot plant. The turbidity is so diverse ...ranging from few NTU to more than 200 or even 600.
I need a pretreatment prior to the membrane, and currently, we are using coagulation. But it fails us...the parameters change a lot. I am not really sure that coagulation is the best option to have with these conditions and so many varied influencing parameters. Right now, I only want to have a stable process!
Could somebody please give an opinion?

Thanks in advance,

 

[ashtree]

My response is predicated on the assumption that the cwater being treated can actuakly be treated with a coagulant, form a floc and settle out. Based on the detail given this assumption may or may not be correct.

A full water quality analysis is always a good start before doing pilot plant testing. If you have one please post it it.

You could try putting a balance tank with a mixer ahead of the process. This would even out the changes. The size of the tank would be dictated by the rate of quality change and the hydraulic detention time. Larger within reason would be better.
To determine the size i would start by plotting out the pattern of flow, turbidity and other relevant parameters , pH, alkalinity etc, to see if there are any patterns or inkages. For example does high flow relate to lower or higher turbidity.
Assuming you went down this road you might then link your coagulant dose with flow and turbidity , and acid/caustic to pH for example. Your clarifier would recieve water that was more consistent , by using the volume of the balance tank to buffer any changes.

You say you are using membranes. Can we assume that this is MF or UF rather than RO? MF or UF will work okay with just a clarifier ahead provided that the floc has settled and the turbidity is relatively low. Many will accept less than 100ntu with some performance penalty but less than 20ntu would be better.

If you are using RO then you will have to use something in addittion to the clarifier, media filters at least, but ideally UF or MF.

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

 

[Seda6334]

Hi

Thank you for your response.

Based on the jar test visual results, yes, the flocs form and settle. Jar test procedure : (coagulation + flocculation) + settlement + sampling
jar test made it clear that even without coagulation, the turbidity removal is high. the coagulation mainly increased the COD/UV abs removal (they are about 30%).

The membrane is UF.

we have a feed tank about 150 lit prior to the process, and also screen meshes before the feed tank.
The idea is to have an inline Coag-UF process, i.e. without any other solid/liquid separation method in between like clarifier.

I have been scrutinizing papers on Inline Coag-UF processes but in none of them, anything about the rapid mixer is mentioned. which type?

what kind of water quality analysis? we are already measuring UV abs, turbidity, TOC/DOC (in the lab), residual coagulant, conductivity,...and I just wrote here the average values.

Regards,

 

[ashtree]

An inline Coag-UF process without any other solid liquids seperation process is not going to work in 600ntu or even 200ntu conditions. In the process you propose the only mechanism for removing solids will be by accumulation on the membranes and then backwashing it off. Unless you have a lot of membrane area with very low flux rates the membrane backwash will have to be very regular, maybe as much as every few minutes. This migght function okay but will be very inefficient and the amount of backwash water will be very close to or more than you are producing.
If the water was only 20ntu at the worst what you propose would be acceptable. The cleaner the water going to the membranes , the longer between backwashes and increase in recovery.
The 150 litre feed tank may be big enough or way to small because you have not mentioned the flow rate through the plant. The idea would be to look at the amount of variation in quality that you get and to have a tank large enough to buffer those variations. That is why it woulkd be important to track those variations against time and flow. That tank needs to be well mixed.

In terms of rapid mixing after coagualnt injection, you need something that will provide a couple of seconds of intense turbulence and then maybe 20-30 secs of low turbulence before hitting the UF. You need some time for microfloc to form.

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

 

[bimr]

What is the source of this water?

 

[Seda6334]

Wastewater...a terrible one...

 

[bimr]

As in what type of industry waste.