Potential of organic matter recycling in urban areas

The potential for nutrient and OM recycling is particularly high in cities, since, following the global trend of urbanization, most people of today live in urban environments. In 2018, 55%

of the global population, i.e. 4.2 billion people, were living in cities. By 2050, this figure will be nearly 70%, (Birch and Wachter, 2011). Each year, about 1.0 to 1.9 Gt of municipal waste is produced in cities globally (GIZ, 2016), consisting of 20 – 80 % organic material (Vergara and Tchobanoglous, 2012). On the other hand, only 4% of urban N and P fluxes are recycled (figures from 2000) (Morée et al, 2013). Even though recycling numbers are usually higher in industrialized countries compared to other countries, there is a high potential to improve the existing systems. As this thesis is settled in the German context, the following section will focus on the situation in Germany.

1.2.1 Fate & treatment of organic wastes for nutrient cycling

The German Circular Economy Act (Kreislaufwirtschaftsgesetz - KrWG, 2012) defines organic waste as biodegradable waste from households, parks, gardens and landscape management (§3 (7), KrWG). In Germany, the waste is usually collected by disposal companies and treated depending on its structure. Easily degradable OM like food waste is fermented in biogas plants and used for energy production. The resulting digestate can be directly used as ferti-lizer or further composted and used as soil amendment. Green waste from gardens, parks and landscape management is processed in composting plants and the product is sold as green waste compost. According to the Circular Economy Act, all private households are obliged to collect the organic waste they produce separately from other waste streams (§11 (1) KrWG, effective since 01.01.2015) and to transfer it to the local disposal companies (§17 KrWG). Exemptions are possible if a minimum of 25 m² garden area per person (Ernst and Worlitzer, 2019) are given to establish own recycling processes. In 2017, 53.8 kg per per-son of organic food waste and 71.3 kg per perper-son of garden waste was collected separately (Table 1), equalling only 48.4 % of the theoretical potential (Destatis, 2019, Krause et al., 2014). This means, that even though §11 (1) KrWG came into effect already five years ago, the share of organic waste collected by German disposal companies is still insufficient.

Reasons for this include private composting of many households in sparsely populated areas as well as a lack of economic feasibility for disposal companies (Krause et al., 2014). But also in cities and metropolitan areas not all households have access to an organic waste bin. In Berlin the connection rate in 2016 was 80 % in inner city districts and 20 - 25 % in suburban areas (Abgeordnetenhaus Berlin, 2017). A large amount of organic waste still ends up in the

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residual waste, which then undergoes mechanical-biological treatment and disposal or ther-mal utilization in a waste-to-energy plant. The potential of this waste stream for nutrient recycling is thus not sufficiently utilized.

Table 1: Separate collection and theoretical potential of different organic waste streams in Ger-many

in 2017 for 83 Mio. Inhabitants (Statistisches Bundesamt (2017)), [1] (Destatis 2019), [2] (Krause et al., 2014). Numbers for separate collection [kg a^-1 per person (p)] and potential used [%] calcu-lated from [1] and [2]

Centralized collection systems also face several drawbacks: (i) the organic waste needs to be collected and transported for several kilometres, requiring money and energy, (ii) it is often contaminated with plastic and heavy metals, caused by wrong sorting and usage of plastic bags for disposal, and (iii) one heavily contaminated influent can cause the pollution of a whole batch. Additionally, centralized systems are not able to react quickly to influx changes with a capacity adjustment, since existing plants are built for a long-term usage. Expanding central organic waste collection to 100% would probably have more costs than benefits, since bins are often not provided when the risk of wrong sorting is high, and collection via the organic waste bin would therefore lead to contamination of the final product.

1.2.2 Possible pathways to utilization of urban waste potential

To finally sum up, there is a large potential of urban organic waste streams, which are not optimally used at the moment. At the same time citizens and municipalities of many cities and metropolitan areas around the world take steps towards a regional food production and urban agriculture. In Germany this includes primarily community gardens, balcony and roof-top farming as well as allotment gardens, with the latter already existing since the beginning of the 20th century but now experiencing a comeback. An improved cycling of urban organic wastes in a local, circular economy approach could help restore organic matter and nutrients in agricultural soils. At the same time, it could help reduce energy consumption as well as environmental impacts of production and usage of synthetic mineral fertilizers. Decentral-ized organic waste recycling could therefore serve as a good alternative or supplement for the existing centralized systems in urban areas, and help supplying urban food production

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with locally produced fertilizers of controlled quality. One technology suitable for this pur-pose could be small-scale vermicomposting.

Vermicomposting is the managed processing of organic wastes to achieve a stabilized prod-uct using different types of earthworms (E. eugeniae, E. fetida, E. andrei, P. excavatus) for acceleration and promotion of microbial decomposition (Dominguez, 2004). It was already described as an artificial process for the production of soil fertilizers at the beginning of the 1940s by Oliver (1941) and is based on the natural function of earthworms in the soil. It was not long before the first small-scale vermicomposting systems were invented by Hopp (1954) and Crowe and Bowen (1954), with the intention to provide an alternative to thermophilic composting of household wastes. This approach became more and more popular in the fol-lowing decades, as small-scale vermicomposting is a low-odour and space-saving process, which requires little effort and can therefore be carried out indoors by residents lacking an own garden (Sherman and Appelhof, 2011). Vermicomposting can produce a nutrient-rich organic fertilizer which contains more available nutrients and plant growth hormones than conventional compost (ibid.).

1.2.3 Fate & treatment of human excreta for nutrient cycling

Another urban waste water streams which is seen as key sources for “urban mining”, are human excreta (Chowdhury et al., 2014). A significant proportion of the nutrients applied in plant cultivation is finally consumed by humans through agri-food products and these nutri-ents inevitably enter the sewage system via human excreta. Although human urine, for ex-ample, accounts for less than 1 % of the waste water volume, it comprises 70 to 80 % of the N and about 45 to 60 % of the P in urban municipal waste water (Simha and Ganesapil-lai, 2017; Herrmann and Klaus, 1997). According to the characteristic composition of food, other nutrients such as K, sulphur (S) or micronutrients are contained in urine and thus ulti-mately in municipal waste water. These elements, which have previously been taken from the soil during plant production, should ultimately be replaced in order to maintain soil fer-tility. The currently most common technical processes for the recovery of nutrients from waste water mainly concern individual elements, e.g. the recovery of P from waste water by struvite ((NH4)Mg[PO4]·6H2O) precipitation or P extraction from sewage sludge ash. On the other hand, conversion and removal of N from waste water via the combination of nitrifica-tion and denitrificanitrifica-tion requires most of the energy consumed by waste water treatment plants. Such waste water treatment further leads to significant gaseous N losses due to ni-trous oxide (N2O) emissions from activated sludge processes. Most of the nutrients (P, K, S)

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remain in the sewage sludge, where cross-contamination with pharmaceuticals, heavy met-als and microplastic coming from other sources than toilets, can be observed. Due to this cross-contamination, field-fertilisation with sewage sludge has been restricted or banned by many national governments (e.g. Germany, cf. German Sewage Sludge Ordinance (Klä-rschlammverordnung AbfKlärV)). In Germany, currently 65% of the sewage sludge produced is dried, incinerated and then deposited in landfills (Roskosch and Heidecke, 2018).

1.2.4 Possible pathways to utilization of human excreta potential

First approaches for novel, recycling-oriented sanitation service include the usage of dry toi-lets. Dry toilets are increasingly replacing chemical toilets at several German festivals and are becoming increasingly popular with campers. The collected excreta are thermophilically com-posted with toilet paper, sawdust and sometimes other additives. The existing systems either recycle urine and faeces together or separate them and treat them separately. However, the legal framework for the use of these composts is not yet in place. In principle, the EU’s “end-of-life”-criterion (CE marked fertilizing products and amending Regulations (EC No 1069/2009 and EC No 1107/2009) promotes the production of bio-based recycling fertilisers but does not explicitly mention (processed) human excreta, neither urine nor fae-ces. Also in many European countries, the agricultural use of excreta is not yet covered by the legal framework. Political work and a paradigm shift are thus needed to integrate this resource into waste water or solid waste management and fertilizer legislations at European and national level.