Briefing
Management of seasonal, street leaf-sweeping waste – main finding
from local authority trials
A number of local authorities took part in trials involving the treatment, testing and analysis of
seasonal street leaf sweepings collected during last autumn and early winter (2012-13). The
aim of the trials was to try and identify generic circumstances where these materials could be
collected and composted without compromising compost quality and safety. We have worked
with the Organics Recycling Group, WRAP and Defra to assess the results and outcomes from
these trials. The evidence shows that we need to re-affirm our guidance to local authorities
that seasonal street leaf sweepings are not suitable for producing any compost that could be
used on agricultural land.
Background
In May 2012 we issued guidance to local authorities (to support accurate reporting for the
Landfill Allowances and Trading Scheme) on the disposal and recovery of street sweepings
and gully emptying. The guidance advised local authorities that material from dedicated,
street leaf-sweeping collection rounds is considered unsuitable for composting, due to the
contaminants that it is likely to contain.
We had no specific evidence on the levels of
contamination and said that we were running a trial to assess this and would review our
guidance accordingly.
The outcomes from this trial, with four Welsh local authorities, signalled the need for a
precautionary approach to composting this material, due to elevated concentrations of various
potentially toxic elements (PTEs) in the composted output. We notified local authorities in
October 2012 of the trial outcomes and that, based on this trial, our guidance remained
unchanged. We recognised that the Welsh trial was limited, however, and said that we would
continue to review any further evidence provided to us. We agreed arrangements with the
Organics Recycling Group (ORG) for them to oversee further street leaf sweepings
composting trials and invite local authorities to take part.
In the period up to May 2013, 21 local authorities took part in the trials, which involved the
treatment, testing and analysis of seasonal street leaf sweepings collected during last autumn
and early winter. The aim of the trials was to try and identify generic circumstances where
these materials could be collected and composted without compromising compost quality and
safety.
There are essentially four levels of quality/contamination level that we normally use to assess
the outputs from composting or other biological treatment plants. In decreasing levels of
quality (increasing levels of contamination):
1. ‘quality compost’ produced to the Quality Protocol/PAS 100 standard; this can be safely
used as a soil conditioner, fertiliser and in growing media without any waste regulatory
controls
2. compost which, although still a waste, can be safely used, under environmental permitting
controls, on agricultural land.
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3. an output (commonly referred to as a ‘compost-like output’ or CLO) which may be
beneficially used, under environmental permitting controls, in the reclamation or recovery
of previously developed land for a non-agricultural final use.
4. an output that cannot be safely recovered to land and which has to be disposed of e.g. by
landfill.
Following these trials we have worked with ORG, WRAP and Defra to assess the results.
Summary of findings and conclusions from latest trials
The main conclusion from these trials is that contamination levels in the street leaf sweepings
are too high to produce ‘quality compost’ marketed as non-waste, or compost that can be
spread to agricultural land under waste controls. This is due to the high and variable levels of
polycyclic aromatic hydrocarbons (PAHs) found in these trials, both in the street leaf
sweepings and in their composted outputs. A number of PAHs are known to be carcinogenic.
and persistent (do not readily break down in the environment) The outcomes from these trials
indicate the need to continue with a precautionary approach to composting street leaf
sweepings. Although we want to encourage recovery of biodegradable waste through
composting, we need to ensure that the resultant compost is fit for purpose and only used
where it can confer agricultural or ecological benefit without harm to human health or the
environment.
Although street leaf sweepings cannot be used to produce quality compost, they may be
suitable to produce a CLO which is used in the reclamation or redevelopment of previously
developed land. This CLO remains a waste and therefore continues to be subject to waste
controls.
Street sweepings can be washed and mechanically separated to recover grit and sand. It
may be possible to use this sort of process to treat street leaf sweepings to produce leaf litter
which is sufficiently contaminant free to compost, but we do not have any evidence yet to
confirm this.
None of these findings affects leaf litter collected from parks and gardens, which can continue
to be used to produce compost or ‘quality compost’.
Stakeholder engagement
We are liaising with Natural Resources Wales, aiming for a co-ordinated approach across
England and Wales if possible. We are briefing the following organisations on the findings of
the trial:
 Defra
 ORG
 WRAP
 Local Government Association
 National Association of Waste Disposal Officers
 Association of Directors of Environment, Economy, Planning & Transport,
 Community Composting Network
 Environmental Services Association
 Chartered Institution of Wastes Management
 NFU
We will be informing all English disposal authorities individually.
We will be producing a comprehensive trials report including a literature search and risk
assessment, which we will make freely available. We will be updating our website to re-affirm
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our guidance to local authorities and waste operators that seasonal street leaf sweepings are
not suitable for producing any compost that could be used on agricultural land. We will also
advise on other options for recovering or disposing of this material.
Options for recovering or disposing of seasonal street leaf sweepings
In many cases, local authorities may choose not to go the expense of having seasonal,
dedicated street leaf sweeping rounds and simply deal with street sweepings as they do for
the rest of the year. Dewatering street sweepings considerably reduces the volume and
weight that is sent for disposal by incineration or landfill. The use of street sweepers with
blower attachments also reduces the quantity of leaf litter entrained in the street sweepings.
Blowing the leaves off the road and pavement into the verge, where this is possible, allows
them to mulch down naturally.
There are also a few sites that treat street sweepings by washing and mechanical separation
to recover grit and sand. We are working with an operator of one of these sites to see if the
‘organic fines’ which are also produced by the process might be suitable for recovery. Using
this sort of process it may be possible to remove the contaminated particulates from street leaf
sweepings to produce an output suitable for composting.
Another option is to send them to a suitable mechanical biological treatment (MBT) facility.
MBT plants are often configured to produce a CLO which is used in the reclamation or
redevelopment of previously developed land.
Further technical detail on our initial findings is given below.
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Technical detail on our main findings
PAHs are a large group of organic compounds that are composed of two or more fused
aromatic rings. They are primarily formed by incomplete combustion or pyrolysis of organic
matter, and generally occur in complex mixtures. Several individual PAH compounds such as
benzo[a]pyrene (BaP) are widely considered to be genotoxic carcinogens and human
exposure via air or diet therefore poses a potential risk to human health. PAH 16 values, often
quoted in the literature, refer to the sum of the concentration of the 16 principal PAH
compounds on the US EPA’s priority pollutants list.
Emissions from vehicles make a significant contribution to pollution close to the roadside. The
main sources of PAH include particulate and gaseous exhaust emissions, tyre and road
surface wear, and oil leaks from the engine and transmission systems. Lighter PAH
compounds may be emitted as gaseous components but absorb readily to other particulate
matter and plant surfaces. Heavier PAH compounds including BaP are emitted as particulates
and collect on road surfaces, roadside soils, and herbage. Some PAH compounds may be
taken up by plants from the soil but this pathway is limited, especially for the heavier PAH
compounds.
We found median PAH16 levels in street leaf sweepings compost of 23 mg kg-1 dry weight
(DW) with an overall range of <1 to 52 mg kg-1 DW, which were much higher levels than
reported by the European Commission’s Joint Research Centre (JRC) and WRAP
respectively. The JRC sampled a wide range of green waste composts from across European
Member States to establish an indicative baseline understanding of contamination levels.
Over 90% of the samples had PAH levels (based on 12 compounds only) slightly over 2 mg
kg-1 DW with a median close to 1 mg kg-1 DW. WRAP found average PAH16 levels between 2
and 6 mg kg-1 DW in batches of green waste composts from six Welsh facilities in 2009 and
2010.
In the leaf litter collected, one-third of the samples had PAH16 levels less than the detection
limit (1 mg kg-1 DW), but the remaining two-thirds had a median PAH16 level of 13.3 mg kg-1
DW. This is nearly fifty times higher than the median level in English urban herbage of 0.3 mg
kg-1 PAH22 DW found by the UK Soil and Herbage Survey (which mainly sampled parks and
open spaces).
There are no internationally agreed thresholds for PAH compounds in agricultural soil and in
compost, although guidelines have been developed for foodstuffs, water and air. The JRC
Technical group has proposed a maximum PAH16 level for biodegradable waste of 6 mg kg-1
DW to meet end-of-waste but this has not been adopted. Of the street leaf sweepings only
and combined street leaf sweepings and green waste compost in the survey, 90% had PAH16
levels that exceeded 6 mg kg-1 DW.
BaP has been widely studied for its toxicity to man and the wider environment, and the
European Food Safety Authority (EFSA) has assessed a dietary minimal risk level. We used
this minimal risk level to assess modelled human exposure through the agricultural food chain
from a typical compost application (20 t/ha/yr for ten years). We also compared modelled
levels in soil with the Soil Screening Value (SSV) for BaP, which is a generic screening value
developed by the Environment Agency to protect sensistive soil organisms including
earthworms.
Results similar to those for total PAH16 were found for BaP, where 90% of the samples of
street leaf sweepings only compost resulted in dietary exposures that exceeded minimal risk
levels (typically greater than 1 mg kg-1 DW BaP in compost). Around one-third of the samples
also resulted in soil concentrations that exceeded the SSV, an indicator of ecotoxicity to soil
organisms (typically greater than 3 mg kg-1 DW BaP in compost).
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Our secondary conclusion is that about a third of the composted outputs failed to meet quality
compost PAS 100 standards for potentially toxic elements (PTEs), but most met the PTE
standards we commonly use for compost spread to agricultural land under environmental
permitting controls. The only 3 failures were in samples from the south west region and were
for arsenic. This is possibly not surprising as natural background levels of arsenic are
generally high in the south west.
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