Biowaste has an important role in the EU’s recycling economy plan. Its separate collection is set in many EU countries and helps in resource conservation, climate protection and soil conservation. The increasing amount of contraries is an issue.

Lately the increasing amount of contraries in compost has been endangering the high quality of the soils made with it, and the acceptance built up over so many years of hard work. The contraries come from people throwing the wrong things in the organic bin, and most of these are plastic. With our experience and our passion for composting, we are working with other technology companies on machines and process solutions to make a usable compost product out of contamined biowaste. Here we want to give you a quick look.


Careful shredding

A lot of the contraries in biowaste can be removed during processing, as long as they aren’t shredded too small. Excessive shredding of plastic materials is best avoided by using low speed shredders. Komptech offers single-shaft Terminator and double-shaft Crambo shredders for the purpose. Both can be configured for selective shredding by means of settings and equipment options for the shredding unit. Preshredding can be dispensed with in order to avoid making contraries too fine at the start of the composting process.


Screening before composting is becoming more important

Screening is the most effective process step for getting rid of contraries, and is done at nearly all sites, typically after the rotting phase. Compost has to meet strict limit values for contraries, whereas there are no such rules for biowaste. However, a move is afoot to define quality parameters for organic material after processing and before composting. For example, pre-composting screening at 80 mm and removing or sorting the overs is an option for heavily contamined feedstocks. For this purpose coarse screening drum and star screens can be used, and in some cases disc or spiral shaft separators. These technologies are available from Komptech in a wide range of capabilities, in most cases with versions for both stationary and mobile use.


New technologies on the way

Fine screening is dominated by drum screens, followed by star screens, often in combination with wind sifting. It is evident that screening machines will need to handle smaller grain sizes in future. Screening technologies combining vigorous material movement (star screens, flip-flow screens) and screen meshes of eight mm and lower are in development. Currently, automatic sorting machines that use near infrared or colour and image recognition to separate contraries from biogenic waste are not in general use. But an increasing number of plants are deploying this kind of technology for compost, compost overs or the shredded feedstock. It should be noted that there are limits to the technical possibilities, and more importantly, to the financing of such solutions. Therefore, improvement of the source material is an important step. Information campaigns as well as monitoring will help reduce the amount of contraries in organic waste. Ultimately, it’s down to all of us whether plastic gets into the green bin or not.