Removal of Plastic Materials Contaminants etc from Compost
Traditionally, India has avoided much waste tipping or landfilling by simple composting.
Unfortunately, the rise in the use of plastics is making this more difficult as increasing amounts of plastics waste are to be found in otherwise compostable organic waste.
Packaged out of date food and catering waste from the retail and catering industries, is a growing additional source, as India’s affluence rises. The food producers and retailers are also tending to use ever more sophisticated and attractive packaging, and most of it is made from non-biodegradable plastics.
Furthermore, when these foods remain unsold and become out-of-date, they require disposal at the manufacturing or retail level. For composting and anaerobic digestion (AD) the packaging must be opened, and if the compost or digestate (from AD), is to have any value, the plastic contamination requires removal. 
Manual separation is the normal solution but it is time consuming and costly, and the smaller pieces do tend to remain to spoil user perception of the compost quality.
In particular circumstances, where large volumes of meat are involved, concerns arise with the health and safety of the pickers.
Considering the potential materials and volumes, it appears sensible to turn to technology and look for some means of mechanical removal.
The problem facing disposal through composting or AD is how to remove the plastic and foil packaging to ensure the end product (compost or digestate) is free from contamination and a viable commodity.
We describe some of the technologies below.
Plastics separation technology
The main driver for plastics separation technology is has been generated in Europe by the need to achieve improvements to the quality of composted products from mechanical and biological treatment (MBT) plants.
Several manufactures have recently announced improvements to their designs and the ability to remove more contaminants, not only for plastic films, but also for foils.
The principal techniques used are:
Screening techniques
These can remove the fine compost (eg less than will pass a 10 mm sieve) which is generally contaminant free and, providing the composting process successfully achieved sanitisation and maturation, a good product will result.
Unfortunately, screening will usually generate a large volume of “oversize” material. In fact the majority of the material will often comprise “oversize”, and contain plastic, foil and paper contaminants.
If these were to be landfilled the composting process will be uneconomical. Thankfully, however, most of the oversize is organic material that can be reintroduced to the composting process and if necessary after several passes all but the most intransigent organic material will have decomposed to below the 10 mm screen size.
However without removal of the plastic and contaminant foils, these materials will become increasingly brittle and shatter in the shredding process thus contaminating the fines. Also, above a certain proportion of contaminants in the feed stock, over time, the recycled contaminants tend to progressively build-up within the system. Ultimately, composting will fail if the contaminants are not continually removed at a rate high enough to avoid build-up.
Removal of plastic films and foils post composting has the advantage, in Health & Safety terms, of handling a sanitised material free from pathogens, provided that composting quality standards such as the UK Animal by Products Regulations (eg ABPR and PAS100 in the United Kingdom) are adhered to.
Shredding technology
Low speed, high torque “shredders” have been developed that are will reduce the size of both organic compostable materials and the contaminants.
Shredders are designed more to tear apart large particles (for example meat joints) rather than cut objects into smaller pieces.
Shredders must not reduce the particle size below optimum size if they are to maintain a suitably “open” fibrous structure capable of sufficient air exchange and general circulation to maintain aerobic conditions.
However, if the particles created by shredding remain too large, the composting process may become excessively dry, or be incapable of generating sufficient temperature rise for sanitisation.
Shredding technology has advanced significantly over the past few years, and there are now a number of reelable suppliers in the market.
Separation technology
We have already discussed simple screening. Sieving or screening has no doubt been in use for many thousands of years. It is a size separation technology which coincidentally may also achieve materials separation as discussed above.
Several other mechanical means to segregate materials with differing properties in the context of composting are currently available:-
Trommel screening; utilises a (usually inclined) rotating drum, with progressively larger holes arrayed across the drum. Improved separation is achieved by collecting the different size fractions separately. Oversize material also passes right through the drum for further sorting.
Other screen types; which comprise established technologies are:
- star screens;
- vibrating deck screens;
- and finger screens.
Density or air classification; Appears to be a promising technology,.
Equipment suppliers such as Komptech-Farwick, Doppstadt Stadtler (ballistic separator), OKAY, Hese Umwelt, and General Kinematics (a US company with a UK office supplying vibratory density separators), all have model ranges which remove contaminants from compost.
Conclusions
No system can produce a 100% separation.
The following good practise guide is suggested below:-
- Carry out initial shredding or particle size reduction before composting;
- Once composted, screen the material. The screen type generally used is the trommel type screen with a 10 mm opening size, but ballistic and density separators may also be used. (After successful composting the undersize should form a good compost, largely free of contamination, and in most cases acceptable for commercial markets);
- Most of the compost is removed in stage 2. above and this enables the separation stage which follows, to be more effective;
- Separation of plastics contamination from the organic materials in the oversize stream. This is important as this fraction can form a significant proportion of the overall mass. Once separated, the organic oversize material can be reintroduced into the process, and items such as compost and plastics sold into potential markets.
Disclaimer: While every effort is taken to ensure that information is correct when published we can accept no liability for the information provided. Listing of suppliers does not constitute a recommendation.
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