The Danish national resource strategy aims at a 50 % recovery of 7 selected fractions of household wastes in 2022. The newly amended EU directive on waste also has a goal of 55 % recovery of municipal waste in 2025, 60 % in 2030 and 65 % in 2035 and a demand of separate sorting of biowaste before the end of 2023 (Affaldsrammedirektivet, 2018).
This has entailed an increased focus utilization of organic resources such as source-separated organic household waste (hereafter called biowaste) in the Danish Municipalities. The high demand of phosphorus in food production stresses the importance of phosphorus recovery from biowaste. Phosphorus is an essential plant nutrient that is limited which necessitates finding alternative sources, e.g. waste resources such as biowaste.
Another emerging issue is the occurrence of microplastics in our environment and the need to investigate the sources of this pollution. The new Danish legislation on utilization of waste products in agriculture Affald til jord bekendtgørelsen (BEK nr.1001 af 27/06/2018) emphasizes a rising agricultural demand for high quality organic waste, including anaerobically digested biowaste biopulp. Affald til jord bekendtgørelsen includes a maximum limit for the content of physical impurities (>2mm) in the biowaste digestate. Microplastics is included as a quality parameter in this study. The occurrence of microplastics in biowaste biopulp and the resulting biowaste digestate after anaerobic digestion is the parameter focused on. This project investigates the occurrence of microplastic particles between 100 μm and 5 mm.
The project only deals with the number of microplastic particles in biowaste biopulp and digestate, to have a first indication of the scale of the microplastics occurrence. The project does not deal with catchment area, choice of bags, pretreatment methods or effects of microplastic occurrence in biowaste biopulp or digestate.
The biowaste biopulp investigated in this study originates from two suppliers, Gemidan Ecogi A/S and HCS A/S. The pretreatment facilities receive source-separated biowaste from households in different catchment areas and the facilities uses different pretreatment methods. This project does not account for the differences in catchment areas nor pretreatment methods.
Sampling of biopulp from the plants at Gemidan Ecogi and HCS was done twice 3 weeks apart. Sampling from Gemidan Ecogi was done both before and after anaerobic digestion at a pilot biogas plant located at Avedøre wastewater treatment plant. 6 samples (3 samples each time x 2 sampling campaigns) were analysed in triplicates, making a total of 18 samples.
For determination of the microplastic particle occurrence in the samples a combination of microscopy and verification with Fourier-transform infrared spectroscopy (FT-IR) was used. Several databases (commercially available databases and databases created by Roskilde University) with reference spectrums from known materials (e.g. biobags, other waste bags, carrier bags, food packaging) were used to identify the particles’ spectrums.
Microplastic particles were found in all analysed samples. The contents of microplastic particles were in the range of 50 to 200 particles per g total solids (TS). When comparing the occurrence of microplastic particles before and after anaerobic digestion it was found that the microplastics content per g TS increased after anaerobic digestion. This was partly due to the degradation of organic matter during anaerobic digestion.
When comparing the content of microplastic particles to the total phosphorous (TP) content the number of particles was unchanged after anaerobic digestion. Even a comparison of the types of microplastic particles did not indicate any major changes after the anaerobic digestion process. It can however, not be concluded that no degradation of bioplastics during anaerobic digestion, since degradation of some particles and fragmentation of others can have levelled out the total number.
The study unsurprisingly shows that an estimation of the occurrence of microplastic particles in organic resources is fairer when it is calculated based on the TP content. It is also more environmentally relevant to state the number of microplastic particles based on TP compared to TS or total organic matter (TOM) since fertilisers are applied on agricultural fields based on the TP content.
The calculation of the total number of microplastic particles per TS, TOM or TP do not reveal the types of particles and it is very likely that form, size and type of polymer influences the lifetime and environmental impact of the microplastic particles.
The most commonly found microplastic particles were flakes/foils followed by fibers. Most of the flakes/foils consists of polyethylene (PE). Most of the fibers were polyester (75 %) or nylon (18 %) fibers. The share of microplastic particles that origins from the biobags used throughout Copenhagen Municipality accounted in this study for less than 5 % of the total number of microplastic particles found.
RUC has in two prior studies, one together with EnviDan, investigated the content of microplastic particles in the sludge fraction from wastewater treatment at two different plants. The methods and analyses were the same as in the ones used in this study.
A comparison of the occurrence of microplastic particles in biowaste biopulp and wastewater sludge shows that there is 3-8 times higher number of particles in biowaste biopulp compared to sludge based on the TP content. When based on TS or TOM it is the other way around. This is due to the much lower TP content in biowaste compared to sludge (and higher TS). It is necessary to make further comparisons to be able to assess the quality of each of the types of organic fertilizer. This report does thus not evaluate whether the best choice of organic fertilizer is digested sludge or digested biopulp.
It is not possible from the number of microplastic particles to conclude whether application of the fertilizer product will cause problems in the fields and/or in the recipient waters as no effect studies were included in this study. Previously published studies on the effects of microplastic particles on different species of earthworms, however, indicate that there are no adverse effects on population relevant endpoints, such as survival, growth and reproduction, at environmentally realistic microplastic concentrations. The potential risk of microplastic in the terrestrial environment should, however, be further investigated with a focus on other important soil-associated organisms than earthworms.
|Translated title of the contribution||Microplastic prevalence in biopulp from source separated organic household waste |
|Place of Publication||København|
|Number of pages||34|
|Publication status||Published - Jun 2019|