Have your compost collections got bags of potential?

Single-use plastic bags are widely used for collecting compost, but local authorities need to be aware of the knock-on effects that the choice of bag can have. Tony Breton explains

In recent years, there has been a dramatic increase in the use of biological treatment waste processing across the UK. From an almost standing start ten years ago, there are now more than 300 collection schemes for compostable materials operating across the UK.

There are a number of different methods of collection including wheelie bins, reusable bags, compostable bags and non-compostable bags. This article looks into the use of single-use bags in collection schemes and how the choice of bag can affect the processing and the quality of the end-product.

Dest separation of materials is a key stage in most recycling processes, in particular composting, and is therefore a prerequisite of the UK's quality specification for composted materials BSI PAS 100.

Separating biodegradable wastes for composting at source not only provides the composter with a cleaner material to process but, given the weight of biodegradable wastes, it also helps producers secure land-based markets for composts and local authorities to reach their recycling targets and landfill allowance targets for biodegradable wastes.

While source separation is a vital part of reducing contamination, it can also form part of the problem if the chosen collection receptacle is a plastic bag. However, not all bags are the same and there are three types of bag currently available on the market.

The first type is made from non-degradable plastics - traditional plastic bags made from non-renewable mineral oils. These bags are typically strong and cheap, however, if they are used in composting collections, they will need to be split, emptied and removed from the biowaste prior to it the entering composting processes. Such input preparation can significantly increase overall costs.

The second type is made from degradable plastics, sometimes referred to as oxo-degradable plastics. These bags are based on polyethylene but contain a metal additive to promote early stage disintegration. Degradation occurs in the form of repeated fragmentation following exposure to ultraviolet light or dry heat, and can be built into the bag typically taking between 18 months and four years.

The third type of bag is made from compostable plastics. These are bioplastics which biodegrade through the action of naturally occurring micro-organisms. These plastics can be safely composted together with the organic fraction of the municipal solid waste to produce high-quality compost.

Compostable plastics do not hamper the composting process because they are biodegradable, they disintegrate quickly, are safe and do not contaminate the final compost. Compliance with the European Norm EN 13432 indicates that all these requirements are satisfied.

The majority of bioplastics are derived at least in part from renewable resources - starch and sugar - both of which come from annually renewable crops and are completely biodegradable.

Setting the standard
Following the development of the EU Packaging Regulations, an EU standard for compostable and biodegradable packaging, EN 13432 was introduced and adopted by national standards bodies in many of the EU member states. The standard ensures that a product is biodegradable, compostable and is safe.

In order to gain certification to BS EN 13432 (the British version of the European standard, published by the British Standards Institution) the final packaging product must be fully tested and approved. Bags, which are certified to BS EN 13432, are also an acceptable feedstock to commercial composting systems which comply with BSI PAS100 for composted products.

When looking to purchase biodegradable or compostable bags, it is essential that reference to EN 13432 is made in any tender. Purchasing authorities should also ask to see evidence that the products to be purchased are certified. It is worth noting that no degradable plastic has ever met the criteria for EN 13432 and so should not be considered compostable.

In addition, recent court cases in Australia and in Italy decided that the use of the word degradable could be misleading to the public and that there was no scientific evidence that they bestowed any environmental benefit.

Advantages of compostable bags
Further to the reassurances that bags certified to EN 13432 will compost satisfactorily within a typical composting process, compostable bags have a number of other advantages. Due to their inherent properties, once in a composting environment, the bioplastic will absorb water.

In turn, this makes the fragments of bioplastic heavier and much less susceptible to becoming wind-blown litter, which is often cited as a major problem by composting facilities that have tried to compost degradable plastics.

As the emphasis moves from the chase for green waste to collecting food wastes, compostable bags have been shown to exhibit significant benefits. Bags for food waste collection are much thinner than those used for garden waste collection and are often used in conjunction with solid-sided or vented kitchen caddies.

The thickness of the bags and their intrinsic capability of transpiring (i.e. to loose water vapour) means that they encourage the drying of the organic waste, especially when used with vented caddies, thus aiding odour reduction and helping to maintain public participation in separate food collection schemes.

This type of bag also makes them suitable for short cycle treatment such as in-vessel composting and anaerobic digestion. They also help keep both collection vehicles and outdoor caddies clean.

The greatest benefit from using compostable bags in these collections comes from the increased efficiency of collection. Since the bags can simply be lifted out of the outdoor caddy or bucket, collection operatives can visit many more properties before returning to the collection vehicle. The best example of this in the UK is in Preston, where they have no problems completing their busiest round - 1753 properties, about 60% participation - in a day which they estimate to be twice as efficient as the best separate food collection scheme which does not use bags.

Spot the difference
As always, education has a key role to play in the success of any bagged biowaste collection. Collection operatives and public alike must know how to identify an acceptable compostable bag from a standard polythene or degradable bag.
All products certified to EN 13432 should prominently display the compostable logo, given that the industry is still in its infancy, the additional use of distinctive colourings could also prove beneficial. Certification can be applied for from the Composting Association and it maintains a list of certified compostable plastics and packaging.

Compostable bags have enormous potential, especially in separate food waste collection schemes and will become more widespread as the demand for sustainable solutions increases. Maintaining and improving both efficiency and quality is key to the entire biological resource industry. Using bags certified to EN 13432 will help reach both these goals.

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