Mystery chemical (alkane?) in wastewater

[B_DeW]

Hi all,
In a forward osmosis membrane process we recently found unexpected loss of performance due to damage or fouling (not sure yet).
We use treated wastewater (final effluent) as our feedwater for the process.
To find out what was causing the damage or fouling we sent a sample off for GC-MS analysis. Their automated library search of chemicals identified a relatively strong presence of dodecane but no other alkanes.
Has anyone heard of dodecane being present in wastewater? There is no evidence of any other alkanes. I looked at the treatment process but this site only uses silicone oil and silica as antifoam, and some basic chemicals such as sodium hypochlorite.
Treatment site shows no alkanes being discharged with final effluent.
My questions:
1. How could a chemical such as dodecane end up in wastewater with no other alkanes detected? OR:
2. What kind of chemical could look like dodecane in a GC-MS analysis?

I'm out of thoughts here... The only additional information I have is that when we boiled and condensed a sample, we got an unknown liquid dissolved in the distillate that smelled acrid and had a conductivity far higher than distilled water should have. We also did GC-MS on this sample and found only dodecane and .... heptane?
Sample fouling is not possible - multiple samples, multiple GC-MS units.
Oil leak is unlikely - would show way more than alkanes than just dodecane.
What mystery chemical are we dealing with that has a high electrical conductivity and looks like an alkane?
Thanks in advance for any thoughts!

 

[bimr]

Would you describe what the "treated wastewater (final effluent)" consists of?

 

[B_DeW]

Thanks bimr,
It's treated residential wastewater from a medium sized city (800000 pop eq.) with some industrial wastewater (but not a lot) and including storm/rain water.
Treatment consists of sedimentation, flocculation, and aeration. No chemical treatment. Sludge removal and sludge polymer addition before going through a belt press.
We have not identified any possible source of contamination inside our system.

 

[bimr]

For water reclamation of secondary treated wastewater, additional advanced treatment is necessary to remove ammonia and phosphorus. Precipitation of calcium phosphate scale on RO membranes continues to be a major challenge for wastewater recycling facilities.

Have you discussed your problem with the RO membrane manufacturer?

 

[B_DeW]

We have an activated glass media and carbon filtration pretreatment to remove 50-70% of ammonia. Scaling isn't as much of a problem for us as we use forward osmosis at low pressure <5 bar. Thanks for the suggestions though, I'll make sure we look at the phosphorus as a possible problem. We haven't dealt with that specifically yet.

We're currently still running pilot tests which is when the dodecane problem showed up.

 

[bimr]

There are several thousand chemicals present in the environment that are currently unknown and unidentified. Dodecane is what is termed an emerging contaminant like the "forever" chemical substances termed PFAS, which have been in the news lately. Dodecane occurs in the paraffin fraction of petroleum. Dodecane is released through general use of paraffins, petroleum oils, and tars. Dodecane probably passes through the residential treatment plant untreated.

A better understanding of the forward osmosis membrane processes will come from working with actual wastewater. These processes have some intrinsic challenges. For commercialization of the processes to occur, it will be necessary to predict membrane fouling as well as to develop membrane cleaning strategies.

 

[1503-44]

Jet and kerosene fuel?

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[B_DeW]

Thank you bimr and 1503-44 for the suggestions.
I'll see if there are tests available in our lab to isolate/quantify alkanes and possibly identify them. I think jet fuel is unlikely as it's a mixture and more alkanes should show up.

bimr: we've currently got a pilot plant up and running with actual wastewater, performing well. This is the main issue we're seeing so a targeted prefiltration for this chemical is what we're after. We're afraid it could be a major problem once we scale up if we don't treat it.

Some members on another chem.eng forum suggested it could just be a faulty test (although we repeated the tests) so we'll go back and ensure the analysis was done properly.

 

[bimr]

One would think that targeted filtration would be expensive. There probably are 2 options:

Some type of oily water treatment consisting of chemical treatment, maybe emulsion breaking (lower pH);

Or membrane ultrafiltration:

Link

 

[TugboatEng]

1503, fuel in the waste steam was my first guess. In my experience it's gasoline that has the alkanes, not diesel or kerosene. Do correct me if I'm wrong. My experience is that engines that run gasoline have varnish (alkyd) deposits in the crankcase while other fuels (propane and diesel) do not produce such deposits. I have assumed that gasoline contains these alkanes. Leaking storage tanks could possibly be a source.

 

[1503-44]

As you probably know they all are blends of 50 or more compounds with each of those having tens or a hundred isomers. They are differentiated primarily by specific gravity which determines the major constituants making up each one, but there is a lot of overlap. Gasoline is composed mostly of carbon chains with less than C12, but even diesel and gasoline share a few similar C numbers. Dodecane is C12, so it will undoubtedly be present in at least some quantity in gasolines, although it is more likely to be present in higher quantities in jet fuel, as the specific gravity of each are equal, then again in diesel, but probably to a slightly lesser extent.

Gasoline C4–C12
Diesel C8–C25
Jet JP-4 C10–C16
Biodiesel C4–C16

Unfortunately, that information hit the limitations of my pay grade.





A black swan to a turkey is a white swan to the butcher ... and to Boeing.