Anke Bockreis

Waste Separation Press (WSP): A mechanical pretreatment option for organic waste from source separation

An efficient biological treatment of source separated organic waste from household kitchens and gardens (biowaste) requires an adequate upfront mechanical preparation which possibly includes a hand sorting for the separation of contaminants. In this work untreated biowaste from households and gardens and the screen overflow >60mm of the same waste were mechanically treated by a Waste Separation Press (WSP). The WSP separates the waste into a wet fraction for biological treatment and a fraction of dry contaminants for incineration. The results show that it is possible to replace a hand sorting of contaminants, the milling and a screening of organic waste before the biological treatment by using the WSP. A special focus was put on the contaminants separation. The separation of plastic film from the untreated biowaste was 67% and the separation rate of glass was about 92%. About 90% of the organics were transferred to the fraction for further biological treatment. When treating the screen overflow >60mm with the WSP 86% of the plastic film and 88% of the glass were transferred to the contaminants fraction. 32% of the organic was transferred to the contaminants fraction and thereby lost for a further biological treatment. Additionally it was calculated that national standards for glass contaminants in compost can be met when using the WSP to mechanically treat the total biowaste. The loss of biogas by transferring biodegradable organics to the contaminants fraction was about 11% when preparing the untreated biowaste with the WSP.

The greenhouse gas and energy balance of different treatment concepts for bio-waste:

The greenhouse gas (GHG) and energy performance of bio-waste treatment plants been investigated for three characteristic bio-waste treatment concepts: composting; biological drying for the production of biomass fuel fractions; and anaerobic digestion. Compared with other studies about the environmental impacts of bio-waste management, this study focused on the direct comparison of the latest process concepts and state-of-the-art emission control measures. To enable a comparison, the mass balance and products were modelled for all process concepts assuming the same bio-waste amounts and properties. In addition, the value of compost as a soil improver was included in the evaluation, using straw as a reference system. This aspect has rarely been accounted for in other studies. The study is based on data from operational facilities combined with literature data. The results show that all three concepts contribute to a reduction of GHG emissions and show a positive balance for cumulated energy demand. However, in contrast to other studies, the advantage of anaerobic digestion compared with composting is smaller as a result of accounting for the soil improving properties of compost. Still, anaerobic digestion is the environmentally superior solution. The results are intended to inform decision makers about the relevant aspects of bio-waste treatment regarding the environmental impacts of different bio-waste management strategies.

Pretreatment Of MSW For Co-digestion In Waste Water Treatment Plants

Co-digestion of organic fractions in existing digesters at waste water treatment plants is already feasible. This study shows the results of waste sorting and large scale tests to investigate the possibility of using parts of residual waste instead of common co-substrates. Most existing digesters have not originally been designed for such substrates. In particular, inert particles can settle in the digester and cause problems in the co-digestion. In a first step residual waste smaller than 40 mm was screened into eight size fractions. Each size fraction was then sorted into relevant fractions. The sorting shows that residual waste smaller than 40 mm contains about 42.5% (w/w) of organics, the residual waste smaller than 25 mm even 51% (w/w). But the sorting also shows 29% (w/w) plastics, glass, inerts and other components in the residual waste smaller than 25 mm which are not suitable for the digestion. In order to separate these physical contaminants a combination of different mechanical separation steps was tested. First a flip-flop screen was used to split the residual waste smaller than 40 mm at 5 mm and 10 mm. The fine fraction was then directed to an air jig and an organic fraction with up to 79% (w/w) volatile solids and an inert fraction with about 6% (w/w) volatile solids was achieved. The organic fraction poses a potential as co-substrate while the inert fraction could potentially be landfilled. The 5-40 mm and 10-40 mm fractions were put on a hard particle separator. The hard particle separator produced a light, intermediate and heavy fraction without increasing the organic content noticeably. However, the light fraction could meet minimum requirements for solid recovered fuel.