What contribution do detergent alcohols make to sewage discharges and the marine environment?

Abstract

To investigate the potential sources for fatty alcohols arriving at a WWTP and entering the receiving waters, a study was conducted in North Wales in the catchment of the Treborth treatment plant. Two dimensional stable isotope analyses (13C and 2H) had been shown to be a suitable analytical tool in an earlier study and so was used here to separate the different sources of fatty alcohols. Since the fatty alcohols may arise from natural biological synthesis as well as from synthetic production, this approach was deemed the most appropriate as the stable isotopes were characteristic of the sources.
Samples were collected from four soils, four marine sediments, four detergents used in the catchment and at different parts of the WWTP. Samples were collected to establish the temporal variability of both the influent and effluent. The solid samples were refluxed in KOH in methanol, partitioned into hexane and derivatised with BSTFA to form the trimethylsilyl esters of the fatty alcohols. Liquid samples had KOH added to induce saponification and were then extracted by liquid – liquid separation into hexane. All environmental samples had an internal standard added to allow quantification of the compounds. Detergent samples were extracted by conversion of the alkyl chain to an iodide.
All samples were analysed by traditional gas chromatography – mass spectrometry to identify and quantify the compounds present. The samples were then analysed by stable isotope ratio mass spectrometry for both the 13C and 2H in each compound.
The fatty alcohol profiles varied according to the source but there was much commonality in the detergent range of C12 to C18. Soil samples had fatty alcohols up to C28 in length but the majority were shorter in chain length. Marine samples principally had compounds in the C12 to C18 chain length range. There was variability in both the concentration of fatty alcohols in the influent as well as the profile. However, the C12 dominated overall. The total concentrations decreased in the liquid phases through the treatment works with the majority of the compounds accumulating in the sludge (biosolids).
The 13C signal alone was good enough to separate terrestrial from marine sources in the environment but it does not separate faecal sources from either natural or oil-based detergents. Natural plant based detergents have 13C values between -26 and -32‰ while oil-based detergents occupy a range between -25 and -30‰. The corresponding 2H values are 250‰ for natural sourced materials and -50‰ for oil-based detergents which does enable these two sources to be separated.
Of the supplied detergents which are typical of the local catchment, samples 3 and 4 appear to exclusively derived from oil-based raw materials while detergents 1 and 2 have C12 and C14 components from natural sources combined with some oil-based longer chain fatty alcohols.
The influent to the WWTP contained fatty alcohols which originated mainly from faecal sources and natural surfactants (~75%) with a smaller amount potentially derived from oil-based surfactants (~25%). This mixture is compound specific and only realistically applies to the C12 and maybe the C14 fatty alcohols. Longer chain compounds do not appear commonly in the influent. The effluents from the WWTP contained mainly short chain compounds with a chain length less than C16. Their 2H stable isotope signature was different to the other potential sources examined and suggests bacterial synthesis during the treatment processes.
The sludge produced from the WWTP had relatively high concentrations of fatty alcohols as would be expected from their low water solubility. The stable isotopic signatures were consistent with a mixture of faecal and detergent sources although this again was variable depending on the particular compound examined. For instance, the C18 does not appear to have any detergent influence although the C13 might have but this compound is also synthesised by bacteria within the WWTP. The sludge in this area is routinely spread on agricultural land as a fertiliser and may find its way back into the sea via land runoff.
The marine sediment samples had fatty alcohols that are typical of marine production (short chain) and with stable isotope values that indicate exclusive marine production for the C14 with potentially mixed terrestrial for the C16 and C18 compounds. Therefore, the fatty alcohols in the marine sediments are not the same as those that were discharged in the liquid effluent and these fatty alcohols were not the ones that entered the works through the influent but were synthesised or recycled within the works.
Only the 13C value is available for the C12 and this might indicate some detergent contribution to this system for that compound alone at one site. It is also possible that Combined Sewer Overflows (CSOs) might contribute direct surface water runoff to that location.
On the basis of the mean WWTP discharge rates and the mean C12 concentration in the effluent, this works would contribute ~300 g of C12 per day to the receiving waters with a total of around 640 g of fatty alcohols per day (in ~10000 m3 of water) but it must be stressed that these are not the same compounds that entered through the influent.