Organic Contaminants in Sewage Sludges: A Survey of UK Samples and a Consideration of their Significance
- The European Union has produced a Working Document on Sludge
(Third Draft, April 2000), described as 'an EU-initiative to improve
the present situation for sludge management'. It proposes that there
would be 'provisions on concentration limit values for heavy metals and
organic compounds' and that '…if the concentration of one or
more organic compounds in sludge is higher than the concentration
limits propose, the use of sludge (on agricultural land) should not
take place'. The document proposes 'limit values for concentrations
of organic compounds in sludge for use on land', for certain classes of
compounds. These are: 'AOX', the so-called 'sum of halogenated organic
compounds'; linear alkylbenzene sulphonates (LAS);
di(2-ethylhexyl)phthalate (DEHP); 'NPE' (nonylphenol and nonylphenol
ethoxylates with 1 or 2 ethoxy groups); polynuclear aromatic
hydrocarbons (PAHs); polychlorinated biphenyls (PCBs); and
polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs). Annex 6 of the
working Document adds the proviso that 'Sludge shall be assumed to
conform to the concentration limit values…. if, for each
concentration limit value considered individually the 90th percentile
of the samples, within a 12 month period are at or below the threshold
value and if the 10th percentile of the samples exceed only one
threshold and by less than 10%.'
- The question of whether limits should be set for organic
contaminants in sewage sludge is complex and controversial for several
reasons, which are briefly explained in the report. Amongst them is
that there is still a general lack of accepted, validated analytical
methods for the analysis of most classes of organic chemicals in sewage
sludge and a lack of background or survey data on their occurrence.
This study and report were therefore commissioned by the DETR:
- to conduct a survey for a range of organic contaminants in UK sewage sludges;
- to compile relevant data for these compound classes and other compounds which may be relevant to the Working Draft;
- to comment on the levels found, their relationship to the proposed standard and their significance;
- to provide a considered assessment of the need for and workability of standards for organic contaminants in sewage sludges.
- Several classes of compounds were included in the study; those
for which standards have been proposed (PAHs; PCBs; PCDD/Fs; phthalate;
nonylphenol; linear alkylbenzene sulphonates) and a range of other
classes, namely organochlorine pesticides, chlorinated paraffins,
polychlorinated naphthalenes, polybrominated flame retardants and
nitro-musks. The key findings were as follows:
- Polynuclear aromatic hydrocarbons.
- The EU proposals are that the 'sum of the PAHs' acenaphthene,
phenanthrene, fluorene, fluoranthene, pyrene, benzo(b+j+k)
fluoranthene, benzo(a)pyrene, benzo(ghi)perylene and
indeno(1,2,3-cd)pyrene should not exceed 6 mg/kg dry matter, although
it is not clear what criteria have been used to select these compounds
from the array of PAHs present in environmental samples.
- In the survey samples the 'EU total PAH' concentrations ranged between 18-50 mg/kg dry weight. Consequently all of the sludge samples were well above the proposed EU limit of 6 mg/kg. This observation applies even to those WWTPs for which there was 0% trade effluent and purely rural, domestic wastewater.
- The range of values found in this study is broadly consistent
with that reported in other national surveys of sewage sludge in recent
- Hence, typical sewage sludges from every country and every
survey are likely to be in exceedance of the proposed EU standard. It
therefore seems likely that implementation of this standard would
result in the cessation of a very substantial proportion of all sewage
sludge applications to agricultural land in the EU.
- Polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs)
- The EU Working Document on Sludge (third draft, April 2000) proposes that the TEQ attributable to the PCDD/Fs should not exceed 100 ng/kg dry matter if the sludge is to be used on agricultural land.
- About 25% of the survey samples exceeded this value, ranging
between 20-225 ng/kg. This was consistent with other surveys and
datasets for PCDD/Fs in contemporary sewage sludges.
- Some of the samples of sewage sludges from every country and
every survey will be in exceedance of the proposed EU standard.
Implementation of this standard would therefore have to result in the
re-routing of many sewage sludges to other outlets, or the need to
identify and reduce the PCDD/F sources, or to consider additional
- Linear alkylbenzene sulphonates, phthalates and nonylphenol
- Standards have been proposed for all three of these substances,
at 2600 mg/kg, 100 mg/kg and 50 mg/kg for LAS, DEHP and NP,
- For LAS and nonylphenol (as for PAHs), it seems likely that
the majority of sewage sludges in the UK and elsewhere will be in
exceedance of the proposed EU standard. Implementation of this standard
would therefore result in the re-routing of most sewage sludges to
other outlets. The situation is likely to be similar, though less
severe, for phthalates.
- Polychlorinated biphenyls
- The EU Working Document contains a proposal that PCBs,
specifically the sum of the individual congeners 28, 52, 101, 118, 138,
153 and 180, should not exceed 0.8 mg/kg dry matter if the sludge is to
be used on agricultural land.
- All the survey samples were consistently below the proposed EU
limit. Furthermore, the range in values between WWTPs was small,
indicative that there are very few fresh or ongoing primary sources of
PCBs to the environment.
- The low levels found are consistent with other surveys and datasets for PCBs in contemporary sewage sludges.
- Organochlorine pesticides
- No standards have been proposed for these compounds and low
levels were measured for the wide range of substances analysed in the
- Chlorinated paraffins (PCAs) and nitro-musks
- To our knowledge, we present the first data on these compounds in sewage sludges.
- Total concentrations of the short-chained and medium-chained
PCAs ranged between 7-200 mg/kg and 37-9700 mg/kg, respectively. PCAs
are widely used industrial chemicals; these appear to be very high
concentrations and indicative of chemicals with numerous and ongoing
diffuse sources. There appeared to be a stronger link to
urban/industrial regions/sources than noted for the other classes of
compounds, although even for these compounds the link to sources is not
- Given that PCAs are in active production and widespread use, it
is not surprising that they are present at much higher concentrations
than other related families of 'persistent organic pollutants', such as
PCBs and PCNs, whose use has been extensively phased out. Nonetheless,
some of the sludge samples contain very high levels of these
substances, which are known to be persistent and bioaccumulatory.
- Nitro-musks are widely used in soaps, detergents and perfumes.
They are therefore likely to have obvious routes into the wastewater
stream. There were big variations in the levels of the different
compounds screened. Some were present in all the sludges in the 0.1-100
mg/kg range, others in the 10-1000 ug/kg range and others never
detected (< 5-50 ug/kg).
- In the light of data from this study, further consideration of
the fate/behaviour of chlorinated paraffins (and perhaps also
nitro-musks) in sludge-amended soils and their potential for foodchain
transfer would be prudent, together with a consideration of their
sources to the wastewater stream.
- Summary remarks
- The standard levels proposed in the Draft Document for PAHs,
PCDD/Fs, phthalate, nonylphenol and LAS all have very serious
implications for the addition of sewage sludge to agricultural land, if
they were to ever be adopted. Virtually all sewage sludges in Europe
are likely to exceed the proposed limits for PAHs and LAS, for example,
even those originating from rural/domestic WWTPs.
- There is little agreement as to whether controls on organic
contaminants in sewage sludge are needed and - if so - which organic
contaminants to regulate. Germany and Austria have selected PCDD/Fs,
PCBs and AOX, for example, whilst Denmark has chosen LAS, PAHs,
nonylphenol and phthalate. The inconsistencies apparent between the
regulations on organic contaminants in different European countries
arise because there are inconsistencies in the technical/scientific
approach used to derive them. In some cases there appears to be little
or no scientific basis to their derivation; they are set rather
arbitrarily and are inherently pre-cautionary. The arbitrary selection
of compound classes and regulatory limits is extremely unfortunate,
because inappropriate and unnecessary regulation is counter-productive
to encouraging re-cycling and introduces significant costs through the
requirement to monitor for any compounds specified in the Draft
- This report makes clear that fundamental questions remain about:
It is argued that a more logical and scientifically defensible
approach is to consider each class of organic compounds 'on its
merits', in the pathways analysis and risk assessment framework. This
would lead to either the acceptance that current levels in sewage
sludges are environmentally benign, or would prompt the need for more
scientifically based information on the compounds in question. The
latter could perhaps ultimately result in a decision that levels of a
given compound class are undesirably high and the need to monitor
sludge quality, identify the specific sources and bring about a source
Copies of this report may be available as an Acrobat pdf download under the 'Completed Research' heading of the Research Page on the DWI website.
- the scientific justification for standards for any of these classes of compounds;
- whether it would be possible to reliably monitor for these
compound classes in sewage sludge anyway, because of the absence of
validated analytical procedures;
- crucially, whether any environmental benefits, in terms
of ensuring foodchain and soil quality, would actually ensue from
imposing them. Indeed, it can clearly be argued that the re-routing of
sewage sludge, which currently results in beneficial effects on soil
fertility, to other disposal routes will have a clear negative effect
on the environment overall.