analytical testing - whitish foam at urban drainage site

Question from Akin Babatola
Lab/Environmental Compliance Manager
WWTF City of Santa Cruz

I have the responsibility for assessing the possibility that a whitish foam
observed at an urban drainage site about once a year (after major storms)
is a surfactant.
Is there a combination of standard tests you might suggest?
Also, are there any common candidates among urban surfactants that might
behave this way?

Answer

Possibly, the whitish foam observed is not caused by surfactants. A natural
foam formation is often observed at lakes and minor creeks in mostly
unaffected environments. The foam formed used to be very stable and has a
batting-like appearance. The precise origin of the foam forming substances
seems to be unknown. It seems to be associated with degrading natural
organic matter, maybe degradation of resins plays an important role. The
foam is formed at the beaches of the lakes during stormy weather due to
breaking of the weaves or by turbulence of streaming water.

If the cause of foaming is man-made especially proteins, eventually from a
dairy or a food farm, may be the cause too.

To analytically tackle the problem you might start with more simple tests.

Sampling of the foam and dry it. Surfactants will be soluble and
therefore extracted with alcohol, proteins should not be extracted,
denaturation should take place.
Sampling, enrichment from the water column, adapt the standard procedure
which exists for this purpose - an air stream saturated with ethyl
acetate is bubbled through the water sample with an ethyl acetate layer
on top. The surfactants are transferred to the ethyl acetate phase.
Again proteins should be denatured at the phase boundary.

After removing the alcohol or ester you may dissolve it in water for
further testing.

physico-chemical testing
measurement of surface tension as function of concentration. At higher
concentration you will observe a minimum or constant value of surface
tension in the range below 40 mN/m. If you remove the upper layer by
suction the surface tension of the solutions of concentrations near but
below the minimum should read approx. the same value as before suction.
Otherwise the low values are caused by a highly surface active component,
mostly not a surfactant.

analytical testing
standard procedure for
anionic surfactants - determination of methylene blue active substance,
cross contamination of humic like materials causes higher values than
present.
nonionic surfactants - determination with draggendorf reagent
cationic surfactants - not necessary, because these strongly adsorb to
soil, particulates etc., it is not likely that they will have residual
concentrations causing foaming

The most common surfactant is alkyl benzene sulfonate, which might be
determined directly by HPLC in a specialized laboratory.

As a general hint - you may contact a specialized lab at your location,
which is dealing with the level of surfactants in streams or wastewater.

Invitation to email further comments!