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 advisedlocal authorities thatmaterial 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.
  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 landfor 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 downin 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. Wewill be updating our website to re-affirm 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 weightthat issent 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.

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. PAH16 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-1dry 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-1PAH22DW 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 grouphas 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).

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.

1