Removal of heavy metals from water outfall

[LittleInch]

Hello,

I'm looking for any useful information on the options available for removal of heavy metals, specifically arsenic, copper and mercury, from an oily water discharge system.

current levels and requirements are approx. 0.4 mg/l down to 0.1 or less for arsenic and copper and 0.1 down to 0.02 mg/l for mecury. We're talking about 20kg of metal annually. dilution is not regarded as being acceptable, at least at this stage.

We've found removal systems for individual elements, but would prefer some sort of absorption bed or relatively small unit to avoid any flocculation systems etc to remove this, even it means dual beds and changing the material on a regular basis. Any suggestions as to suitable technologies would be gratefully received. Site is in Europe, but could source equipment from anywhere in the world.

Thanks

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Also: There's usually a good reason why everyone does it that way

 

[moltenmetal]

Hmm, removing metals at these low concentrations at the end of the pipe could be expensive, depending on how big the pipe is- you're fighting entropy here in a big way, trying to push things way up a concentration gradient. Better to find out which part of the piping system is the source of the majority of the bad stuff, and treat as close to the source as possible if you cannot eliminate the source entirely.

Selective adsorption that will take out only the heavy metals and arsenic and leave behind the light ones (which are at much higher concentrations of course) is asking for a bit of a magic bullet. You may find something, but I'm not sure what it would be. I see some notes about activated alumina, iron filings, iron oxide etc. being effective for arsenic but I don't know how well that would work for copper or mercury.

 

[bimr]

As moltenmetal has pointed out, it is difficult to make a recommendation without knowing the capacity and the water parameters (calcium, magnesium, sodium, TDS, pH, temp, SS, etc.).

One would expect that you would still have to adjust the pH and flocculate. You may be able to use inline flocculation and roughing and polishing filters.

 

[LittleInch]

Guys,

thanks for the response and we're seeing that the magic bullet might not exist. Although it would take out everything else, do you think an membrane filter would remove all this as well? The end chamber where they measure the discharge takes water from a wide variety of locations and systems so finding the source, which has slowly increased over time, is proving very difficult and could be a number of them all contributing to the total. Client thinks it better to deal with it all at the end point just prior to discharge.

I'll dig out the rest of the data from my colleague tomorrow and let you know what the other parameters are.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[bimr]

Yes a membrane systems will take the metals out. However, the problem with the membrane systems is that there is a residual waste stream associated with it. The membrane systems concentrate the waste into a smaller flow but do not solve the problem.

 

[LittleInch]

bimr, Appreciate that and before I go looking, is there a guide as to what the respective volumes are i.e. clean water / contaminated water on the basis that the vast majority of the liquid is "clean" water? - 10:1, 20:1 ?? Disposal of the more heavily contaminated is an issue, but at least it is an option worth considering.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[bimr]

The membrane recovery will depend somewhat on the TDS, but an estimate of the maximum recovery is 85% or so. That means a waste stream of approximately 15% of the incoming flow.

 

[moltenmetal]

Again, the best solution depends greatly on the flowrate, flow variability, your options for disposal or re-use of the treated water and any reject stream you produce, and what else is in the water other than these target metals. If the flow is large, membrane treatment of the whole stream is going to be prohibitively expensive. A membrane system is only going to be useful to you if rejecting that 15% stream to another treatment process is possible and the rejection of the 85% clean water from that stream is worth the energy and replacement membrane cost.

 

[LittleInch]

Ummm, sounds like that's a bit of dead end. I'll have some more data soon but I'm beginning to understand why they've asked us to look at it now. Thanks for response.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way