What is the effect of oil on sand bed filtration?


The filtration step forms part of effluent treatment and is designed to remove Magnesium hydroxide Mg(OH)2 colloids from a high pH effluent stream. The worry is that small amount of free oil entering the effluents upstream (eg due to leak from equipment) will affect filtration. Is anyone aware of industrial practice with similar issues? Is there threshold oil concentration?




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Jeff Droll
I am in charge of the waste water treatment unit in a refinery so you could say that I am no stranger to oil making its way into my sand filters.

If the oil is presenting itself as a sheen on the surface it will be taken away with each backwash provided you don't drain the filter low enough to let it enter the sand.

Dissolved or emulsified O&G in ppm quantities (<20 ppm) will eventually collect within the sand grains and slowly reduce filter run-times. We use a very strong potassium hydroxide/solvent based detergent injected into the backwash about once a month to remove any entrained O&G.

High levels of dissolved/emulsified O&G can be a problem you need to tend to in a preceding process as it will progressively entrain and clog your media with particulates.

Another thing that could cause you a problem is that if your stream is in the high 10 pH range you could saponify certain types of oils.

Sameer Dohare
Identification of threshold quantity of Oil in the feed of sand filter is depends on the various parameters like quality and quantity of sand, pH, temperature, backwash frequency, etc.
Considering the feed conditions, in my knowledge, the effect of oil in the feed does not have major change on the filtration of suspended solids, however, the bed life reduces with requirement of quick backwash frequency, as the adsorbed oil degrades the soil quality.
Jose Martinez
If you have an open tank you could test an oli skimmer.
Andras  Paksy
Thanks very much for the response and comments. These seem to reinforce our view that oil should be avoided. At the moment we are not considering to implement a new process step for active oil removal, rather prevention of oil entering the effluent stream. The question was aimed to assess whether we are being overly cautious and also to gain an understanding of the wider industry practice.
The question of a "threshold" concentration is an interesting one. The number of 10ppm (or 20ppm) has been mentioned - although I understand that these at most are guidelines and will depend on the specifics of the effluents chemistry and the treatment system in question. We are likely to recommend further laboratory studies to establish the actual influence of oil on our filters as the concerns over its effect seems to be substantial enough to warrant this. A related but difficult issue is the fate of the oil in the treatment train – this will of course have a direct relevance to the question of how it will affect the sand bed filters. The effluent in our case is high pH, which I think could potentially cause additional problems (frothing?).
Andras Paksy
Satendra Pal Singh
Oil contents are not allowed beyond 10 ppm in sand filter. if oil contents will present beyond 10 ppm. we can not remove suspended solid from sand filter in back washing.
Johanna Ludwig
Oil is very bad for sand filters. What is the oil concentration you expect? If it is high (20-500 ppm), I recommend using a process including flotation and filtration. If it is low (< 20 ppm), a filtration step might be enough. However, it's depending on the droplet size also. Ceramic membranes are much more efficient than sand/cartridge filters and don't need regular replacement.
Btw, what's the application of this process?
Sean Roop
You need to remove the oil upstream or you will simply foul your beds with free O/G. There is a myriad of ways to do it depending on your stream and your CAPEX for buying new equipment. Bottom line to the question is that you WILL foul out the filter (s). Hope this helps. I wish I had better news for you. Best, Sean.
Ademar Cesar Ferreira
You shall remove oil before filtration, specially if you are using sand filter that works based in superficial filtration, otherwise you have to backwash your filter more frequently. free oil as you refer to is easy to be removed. In case you can not do it you can try to use a double media filter (anthracite and sand) .
jeremy dudley
Not sure about a threshold oil concentration, but treating oily water at refineries has a similar issue. I know of a flowsheet that uses oil plate interceptor, pH adjustment, induced air flotation, activated sludge, and then sand filtration. The gradual acuumulation of oil on the sand filter results in frequent backwashes and a treated water that is much closer to the consent values than would be wanted from a filtered effluent.
Lincy P.
Dear Andras,

We can control oil by placing oil and grease trap which is operated automatically or manually. This will be available based on the flow volume and needs to be replaced once in a while.

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