Hexavalent chromium removal from leachate

[enviroguy1]

I was recommending a hexavalent removal treatment from groundwater water runoff using the co-precipitation method to a client (use FeSO4 + acid and then raise pH to 8.5). But someone mentioned that the system proposed was great in theory, but that in practice we would have difficulty in achieving settling, which would manifest as both a scum on top of the batch and a poorly settled precipitate with little better than 1% solids. He suggested that we use Dissolved Air Flotation (DAF) to achieve 4-5% solids, and a cleaner supernatant.

I was wondering if you have any thoughts on it. Just to mention, the waste is not an industrial wastewater. It is a leachate.

 

[IFRs]

Air sparged hydrocyclone sound like a winner.

 

[kenvlach]

Scum from what? I don't see any point to Dissolved Air Flotation (DAF). Cr(III) co-precipitates really well with Fe(III) from the reduction rxn. It's an old method used in quantitative Cr analysis. Precipitates even better with Ca, Al, clay, etc. present from your landfill or other leachate source. Just add a little polymer flocculant. Get large, powder blue Cr(OH)₃ 'snowflakes' that readily settle.

For batch treatment, just use a settling tank or two. For continuous flow, use a slant plate clarifier.

If you want to skip the acidification step and speed up the rxn. kinetics (minimize residence time), use sodium hydrosulfite for the reducing agent. It also works as a 'fix-it' if residual Cr(IV) is present after initial precipitation. Just re-stir, reduce, check pH & let settle. It does require careful handling (combustible, rather falsely accused of being spontaneously combustible) and a method of powder feed.

Jar testing is a good idea before spending on equipment. Some suppliers will run trial samples & recommend a polymer flocculant for you.

Where is this project located? You may need EPA or equivalent agency permission for treatment and disposal.

 

[bimr]

Depending on how you add the chemicals, you may get a colloidal solution. If so, you will need to add a polymer to clarify the solution.

Use of a DAF will not productive, most people use conventional precipitation.

You are probably going to have to filter the effluent to meet discharge limitations as well.