chemical oxygen demand 1

[PBFSI]

Working on a waste water treatment process. Customer has requested 50mg/l chemical oxygen demand on release downstream. Can someone explain how I can ensure this? And exactly what this means... Will be using particulate filter to 1 micron absolute and carbon (GAC)to remove hydrocarbons.

 

[moltenmetal]

How to "ensure this" will depend on what's in the water. COD is just like it sounds: the mass of oxygen required to oxidize everything which can be oxidized in the water, per unit volume. The conditions of the test are quite vigorous (i.e. a very strong oxidant- think the tests usually used an Hg-catalyzed chromic acid solution, heated for a prescribed period), so anything which can be oxidized will be. This goes for reduced metal ions (e.g. Fe+2), organics regardless of source (dissolved or otherwise), etc.

If your water was formerly in contact with petroleum hydrocarbons for an extended period (i.e. tank bottoms, hydrocarbon contaminated groundwater etc.), chances are there will be some dissolved organic crap in there which will show up on a COD test and go right through activated carbon, even though the carbon may remove the BTEX and the "mineral oil and grease". Where does it come from? Probably from biological partial decomposition of a fraction of the petroleum. Fe+2 is also commonly found, resulting from bio-driven corrosion of steel tanks, and also from the facultative organisms which dump electrons onto Fe+3 found in soils etc. Is this material of toxicological concern? Probably not. But it will still make your effluent flunk the test...

I'll re-state my warning: it may be possible to run this kind of water through carbon for some time before the soluble stuff will saturate the carbon and start to elute- but it may begin to elute after quite a short period. You may find the COD of the effluent gradually rises, even though your effluent is still free of BTEX and similar identifiable micropollutants. If it's important to you that you not flunk the discharge test, and if your water has a COD greatly higher than your discharge limit, I'd recommend a carbon column test with a fresh composite sample of the actual water prior to selecting a carbon-only treatment system, so that you know the REAL carbon adsorption capacity for the crap that makes up the COD.

How do you "remove" this material, if the carbon sorption capacity is less than economically desirable? Again- depends entirely what it is. Fe+2 is relatively easy to oxidize to the (neutral pH) low solubility Fe+3 form with peroxide, permanganate, or even air (i.e. it can happen inside an air stripper, should you go that route, so choose one designed so that it can be disassembled and cleaned periodically). The soluble organics are often amenable to aerobic biodegradation (if the BOD5 is roughly equal to the COD, you're pretty much in luck), if you have the space for a large enough reactor and the money for an operator to maintain it. Sometimes you can use Fenton's reaction to oxidize an order of magnitude or so of the COD, though you do produce an iron sludge as a by-product upon neutralization, and there's a cost for the equipment and the reagents.

Best to hire somebody in the wastewater treatment business who is familiar with all the options.