NAPL Extraction and O/W Sep Iron Fouling

[Cheffy]

Hello All!

I am working on a multi-phase extraction of a subsurface fuel oil plume under fairly permeable conditions. Vacuum extracted products passes through a 2 tank air stripping system before being moved via a diaphragm pump to an oil water separator. Athough we are currently experiencing some setbacks, my primary issue is the following. While this setup has proven highly effective for removal and recovery of oil, we quickly ran into problems with iron fouling of the oil water separator and sensor columns. The groundwater contains high iron levels (10 - 20 ppm), resulting in the oxidation and precipitation of iron in the oil/water separator. Normally a settling tank would be effective, but the problem is that the oil seems to form a complex with the iron, producing a thick orange scum that floats at the oil/water interface. Tests have confirmed that the bulk of the mass is oil (70%+), about 2.5% solids (only 20-25% of which is iron), and that it is primarily of an inorganic nature (bacteria plate counts = cfu ~ 10^6). Significant scum production occurs only when free product is being extracted, regardless of iron concentrations.

We are currently engaged in a trial use of an organic acid to chelate the iron, and while lab trials seemed effective, only mild to moderate success has been noted in the oil/water sep and due to other complications are difficult to gauge. Lab trials using sodium metabisulphate gave poor results unless very high concentrations were used.

I cannot find any detailed examples of this situation after extensive research, and I'm becoming quite frustrated. Cannot use acid for maintaining solubility due to problems with corrosion in current system.

Any suggestions?

Thanks,

Chef

 

[bimr]

You have not said what the flow rate is, nor the type of O/W separator, so one must guess at a practical solution.

You can try the classical oil emulsion breaking process where you take the pH down to around 5. Not sure if this pH will be low enough to completely dissolve the iron.

Don't see what the problem actually is with the O/W separator. Is it a problem disposing of the iron with the oil waste?

Have you tried a different type of O/W separator?

What about a periodic acid wash of the O/W separator?

 

[Cheffy]

Water flow rate is somewhat variable, but about 4 litres per minute with current well use (previously much higher) - NAPL removal is spotty currently, low retrieval at the moment. The scum seems to form in the water phase as oil droplets coalesce and surface in the o/w sep. The main problem is that the orange scum produced requires regular manual removal from the o/w sep, and much of this scum seems to bypass the baffles that separate the coalescing section from the section of separated water and passes into transfer tanks, clogging level sensors. Additionally, the o/w sep uses a coalescing media that has become clogged with this orange scum, thereby reducing its efficiency. OVerall oil separation is hindered significantly, placing excessive loading on downstream (expensive) activated carbon filtration. We are still in the pilot phase BTW.

The iron begins to oxidise rapidly once extracted from wells, after which it may travel 50+ feet for about 20 min before entering the air/liquid sep and 0.5 to 1 hour before entering the o/w sep. Eh values usually remain low (-100 to 120 mV), but DO is generally elevated (4 to 6 ppm). Ideally injection of organic acid should take place at the well head, but this is very impractical for our situation, and instead is injecte prior to the air/liquid separator at the vacuum manifold.

We have not tried a different o/w sep, and have on occassion used pressure washing and a vac truck to remove buildup in the sep and lines. My lab trials have shown that a low acid dose is not effective at keeping iron in solution (need pH below 3.5).

I am considering a combination of citric acid (to chelate) and HCl to as a low acid dose - does anyone have experience here using chelating agents for large volume iron control? Trying to avoid synthetics such as EDTA etc.

 

[bimr]

Your flow rate are too small to think about any type of continuous automated treatment process. Also I would not bother with an O/W separator, especially a coalescing plate unit.

I would look at a batch treatment process. Use multiple conical tanks where you can collect wastewater in one tank, while treating wastewater in a different tank.

Use a conical tank where you have the ability to skim off the top as well drain from the bottom. The reason for that is you have materials that will settle as well as float.

You can do the treatment in the batch conical tank. Mix the chemicals with a mixer, then allow the tank contents to settle before decanting.

 

[Cheffy]

Much appreciated info. I will consider this, and discuss with other technical advisors here (too many cooks in the kitchen on this project, and none can agree on how to cook the meal). We are a bit limited in space though, so it may not be feasible. Working out of construction containers on public/private land with homeowners to consider.

Thanks again!

Chef