Using Municipal Compost in Highway Schemes
Introduction
Highways schemes provide a large potential market for the products of municipal composting. In order to raise the awareness of highways scheme clients and contractors to the possible cost savings and environmental benefits in using municipal compost from the new generation of composting facilities, we recommend that the use of recycled materials (including compost) be in future made an aspect of tender value to be considered during pre-qualification and tender assessment.
To be so, under European Union rules, this fact must be declared within the tender details when tenders are published in the European Journal or equivalent.
Specification of Composts for Highway Schemes
This is easy - the specification should clearly identify municipal composts as being considered to be "equal approved" alternatives to other organic sources, such as topsoil, forest bark mulch or peat moss.
The advantages of municipal composts
Research and practical experience in the USA and Europe has shown that there are some highway landscaping applications in which the use of municipal green waste derived compost is actually more beneficial than the traditional alternatives.
In addition to the obvious ability of a compost to provide improved soil structure, and additional nutrients for planting, rigorous temperature control applied over the initial stages of composting to high quality standards ensures that these composts, when spread consistently to adequate depth, can provide excellent weed suppression.
The fibrous nature of particularly the coarser sieved composts, has also been shown to provide good erosion control. (American specifications for erosion/sediment control, can be found on the RAA web site here.)
Highways uses for compost are also some of the few applications where compost derived from mechanically separated mixed wastes are likely to be acceptable, away from the public gaze particularly when buried on embankments and in the centre of roundabouts; where the unsightly presence of residual glass particles and small plastics items would otherwise (if seen at close quarters), be unacceptable.
The various ways in which municipal compost can be beneficial in highways schemes are described in more detail below.
General soil improvement
Compost (or soil/compost mixtures) can be utilised for soil improvement, to increase the organic content of roadside soils, thereby improving soil health both physically and chemically, improving the survival rates after seeding and planting and raising the soil's water content for reduced drought susceptibility.
Improvement of the roadside environment for vegetation
The roadside environment is often hostile to the establishment of vegetation, due to lack of irrigation, run-off of road salts and other carriageway pollutants, and a frequent lack of funding for sustained management of the sort which would permit later application of fertilisers. Municipal waste derived compost is ideally suited to reduce damage from all these factors, and provides for long-term slow release of nutrients.
Soil amendment for planting
Compost/soil mixtures provide soil amendment for the establishment of planting beds, and as backfill mixes for trees and shrubs. In many countries including parts of Europe and in Hong Kong, municipal compost has been accepted by Highways Departments as the growing media of choice in roadside tree planting for such schemes as sound mitigation ("living walls") where the rapid growth of a dense woody foliage is sought for optimum sound attenuation.
Weed suppression
Coarse municipal composts provide an excellent weed control mulch for maintenance contracts. Despite the high organic content and the effectiveness of this material to retain moisture, they are effective in controlling weed growth.
This apparent paradox for coarse municipal compost is thought to be caused by the fact that the predominantly dark coloured material absorbs heat well in sunshine, causing the surface to dry out rapidly. This hot dry surface is presumed to desiccate almost all seeds soon after germination, and before they can penetrate into the undoubtedly damp and nutrient rich layers below. The municipal compost must of course, itself have attained a sufficiently high temperature for a sustained period to be free from weeds and weed seeds.
Municipal compost used as a mulch for weed suppression should be coarse in texture, weed free, and low in inert materials. As it is to be used as a surface dressing it must be aesthetically appealing (free from shreds of plastics, glass granules etc), and easy to handle and spread.
Erosion control
Municipal compost may also be used as a surface application for slopes and embankments in order to control erosion. Coarse composts including carefully chosen municipal composts, will have a beneficial effect during heavy storms due to their ability to soak-up rainfall, provide fibrous matter to bind the soil mass together, slow down the generation of run-off, and thereby substantially reduce the peak run-off flows generated which would otherwise cause erosion. A mulch on the surface also helps to absorb the energy of raindrop impact to avoid the initial suspension of solids in the run-off which starts the erosion process.
Coarse municipal composts produced by co-composting sewage sludge with green waste, containing a high percentage of wood chips have shown excellent erosion control properties in tests, when applied as a surface application on 1:2 slopes. Mixtures of coarse compost and sand have yielded similar results; attributed in this case to improved water infiltration properties.
Sustainable Urban Drainage Schemes (SUDS) and Bioswales
The availability of large quantities of municipal green waste compost will also be useful to authorities applying Environment Agency policy to sustainable surface water drainage systems for all new developments
Larger areas are being landscaped than hitherto for stormwater balancing and bioswales, to reduce flood risk and aid groundwater recharge.
Bioswales - swales utilising the natural attenuation mechanisms provided by plant growth within drainage channels enlarged to provide storm storage as well as drainage, will require additional soils suitable for this purpose, and again municipal waste compost will be a suitable means of meeting this demand.
This article was adapted from work carried out by Enviros for Remade. Contact compost@enviros.com for more information on specification and use of municipal compost in highways schemes.
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