MERCURY REMOVAL SYS 2

[randyandy]

Dear All,we are currently in need of your support for providing for an executive project a mercury removal system from a water stream where:

a)mercury is the only heavy metal , with a inlet concentration of maximum 5 ppm
b)water flowrate is 5 m3 / hr
c)the desired outlet mercury concentration is 5 ppb
d)the water comes from a preliminary oil removal system with CPI and flotation unit

Available analysis of feed is :
oil 500 ppm with density 500 / 900
BOD 2500
COD 5000
SS 100
ph between 6,5 and 9
conductivity no value
mercury 5 ppm max
some amine (MDEA) coming form CO2 removal plant

we thought about a precipitation system with a polymeric agent or similar, we would like your kind support in understanding if a more complicated system would be required such as resins, with all related utilities etc. or a precipitation method would be enough. The water then passes to a biological treatment, so I would like to understand if 5 mg / lt of mercury could poison the biological cycle. We are concerned about COD content in case we use resins first, on the other hand. thank you in advance for your cooperation !!Andy

 

[unclesyd]

You might want to checkout this site out as one approach. It might not have any potential as your conductivity is nonexistent. I know of a case where a low concentration of a metal was somewhat concentrated and then removed electrolytically.

http://www.electrosynthesis.com/news/m4watts.html

 

[nbucska]

what is the source of the Hg?


<nbucska@pcperipherals.com>

 

[kenvlach]

Does “conductivity no value” mean not measured, or ND (too low) for available instrument?

I don’t know whether 5 ppm mercury would poison a biological WWT system, but it will sure wreak havoc as it moves up the food chain, becoming concentrated in swordfish, tuna and humans.

Does the analysis indicate whether the mercury is in inorganic or organic form?
From your stream info, organic form seems likely.

Absorption and IX (probably limited by fouling).
Organic form can be removed by activated carbon, but the carbon may become fouled or saturated by other constituents. Methyl mercury acetate has been removed by ion exchange (Dowex A-1 Chelate, Ionac SRXL [no longer available] and Srafion NMRR. However, IX may be fouled unless you have some good pretreatment, including carbon.

You may need to convert to inorganic form for removal. This can be done by oxidation with chlorine, followed by removal of the mercuric chloride complex on anion resin. This is (or was, my reference is rather old) done by chloralkali plants.

In cation form, Amberlite IR-120 and Dowex-50W-X8 are reportedly effective.
IX can achieve levels of 1-5 ppb.

Chemical Precipitation (of inorganic mercury).
Precipitation as HgS, using sodium sulfide or sodium hydrosulfide as source, can achieve 10-20 ppb.
Coagulation with iron or alum can achieve 0.5-5 ppb. Polymer flocculant aids the co-precipitation.

Electrochemical methods.
Your stream is too dirty and too dilute for electrowinning methods. The only electrolytic method with much promise is electrocoagulation, basically the same as “The Electrochemical Precipitation of Metal Ions” in unclesyd’s link, but with many parallel, alternating anode and cathode plates and without the cation permeable membrane to foul. Electrocoagulation should be able to convert organic mercury to inorganic form and coagulate it with Fe⁺³ ions created from the plates, co-precipitating it with the iron hydroxide. You may need a lamellar, slant plate clarifier (unless you have a lot of space for settling tanks). A polymer flocculant will help. Electrocoagulation should also reduce BOD, COD and suspended solids. Not necessarily good, as the amount of precipitated hazardous waste will increase.

The fouling potential (oil) may rule out some methods.
Possible treatment sequence:
a) chlorination to oxidize organics, convert organic mercury to inorganic,
b) electrocoagulation (which will become more effective with the presence of chloride ions, creating hypochlorite), and
c) settling (aided by polymer flocculant).

I suggest extensive electrocoagulation testing. You can try both aluminum and mild steel plates to see whether Al⁺³ or Fe⁺³ is the better coagulant.

Also, test whether the biological WWT system is affected by the Hg, and how much Hg passes through and in what form. It may be easier to treat Hg after biological WWT.

Please clarify details on the influent and also whether the mercury can be treated further upstream (nbucska’s question).

 

[Peteypete]

The best technology I know of for low metal concentrations is resin/solid adsorbents. You can use GAC before the resin bed to help with fouling and that GAC also will adsorb some Hg.

Great resins are few, but there are some new ones. Romic Chemical in East Palo Alto, CA is using a resin to treat Hg from about 5 ppb to sub 0.700 ppb. This resin can get you to your discharge goal.