Oil removal

[ivanhoe374]

Just wondering what peoples thoughts/experiences are in the best way of removing oil (both free and emulsified) at low levels..say <10mg/L

One particular application i am looking at wants to reuse some of there wastewater which will require an RO system to lower the TDS.....the emulsified oil ranges from 0-11 and this obviously needs to be removed before the RO!

 

[bimr]

It depends somewhat on the application and flow rate. What industry are you working in?

The standard method would be to break the emuslion chemically and then use air flotation, maybe followed with a filtration process to remove the residual. You may also want to investigate the use of a membrane process such as Abcor (Koch).

 

[ivanhoe374]

It is in the steal industry. Flow rate is 30m3/hr.

I was looking at using UF as pretreatment to the RO, which should remove maybe 50% of any that is remaining after an initial oil removal step.

When using air flotation is the standard method to skim the oil from the top of the aeration tank?

 

[shahyar]

This is not your answer, just some of my thoughts.
Generally speaking, for that low concentration of oil, floatation is not economical, and just filtration is enough.
But the problem is for good oil removal in filters, you need to have suspended solid (SS) at the same concentration of oil, in water. (probably you do not have).

 

[bimr]

You stated that the oil might be emulsified. The key to removing the oil is to break the emulsion in some manner. This is usually done chemically by lowering the pH. Alum is commonly used for this purpose. You also should be aware that pumping of wastewater containing oil also causes emulsification.

You have to determine the amount of oil still present. 10 mg/l is still a lot of oil and the concentration that is typically guaranteed on the effluent of an API type of gravity oil separation unit. You need to make sure the oil concentration is accurate as oil samples are typically obtained from batch samples.

Air flotation units work by dissolving air into the water and then releasing the dissolved air through a control valve as the water flow enters the flotation unit. The oil is floated off and skimmed at the top. Flotation units are also used to remove suspended solids as well. You should investigate a dissolved air flotation unit, not a dispersed air flotation unit.

Using membranes has to be carefully considered. UF membranes cost about the same as RO membranes, so UF is expensive. Membranes also require extensive pretreatment in order to remove potential membrane foulants. From my experience, use of a UF membrane without extensive pretreatment is a recipe for disaster.

Membranes have been used successfully in a limited number of niche wastewater applications. Most applications of RO on wastewater have not worked. RO is generally a water treatment technology, not a wastewater treatment technology. Before you recommend membranes, you should carefully evaluate other installations on applications similar to your own, to be sure that Membranes will work for your application.

 

[ivanhoe374]

shahyar, the suspended solids range from 10-40mg/L. I have heard that by using upflow sand filters at slow filtration rates (6m/h) 95% of free oil can be effectively removed. The filters also claim to filter down to 1 micron, so that by using inline floculation quite a clean product water should be produced.

Bimr, i agree with you in regards to the problems with membranes and wastewater. The client requires that the TDS is lowered so we will at least be looking at RO membranes. As im sure you are aware the cost of UF membranes is on the decrease. Many suppliers are now also claiming to be able to treat waters with high SS and NTU's(100+) by creating membranes with much larger surface areas and therefore lower flux rates.

 

[bimr]

The suspended solids concentration that you are describing (10-40mg/l) is too high for a direct filtration process. Generally, you start looking into clarification processes when the SS gets over 10 mg/l. High SS loadings will cause short filter runs and poor effluent quality.

Running the filters in a 2 stage working and polishing arrangement may be a possibility.

In regard to the upflow filters, my experience is that such units are generally used for wastewater applications rather than water applications. While I am not familiar with any vendors claiming a 1-micron effluent quality, such a claim would be impressive. I doubt that any conventional sand filter vendors would guarantee 1-micron effluent quality.

My understanding is that the effluent quality of an upflow filter would be of lesser quality than a standard filter because of the directions of flow of the backwash and effluent.

 

[aspearin1]

http://eng-tips.com/viewthread.cfm?qid=110540

I this thread I describe how I had success removing low level oils with bag filter media. May be helpful.


ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee