The study was conducted at the Entsorgungszentrum Leppe, a regional waste treatment hub operated by Bergischer Abfallwirtschaftsverband in Lindlar, North Rhine Westphalia, Germany. The catchment comprises 21 municipalities, namely Bergisch Gladbach, Bergneustadt, Burscheid, Engelskirchen, Gummersbach, Hückeswagen, Kürten, Leichlingen, Lindlar, Marienheide, Morsbach, Nümbrecht, Odenthal, Overath, Radevormwald, Reichshof, Rösrath, Waldbröl, Wermelskirchen, Wiehl and Wipperfürth, with a total area of 1,356 km2 and 0.56 million inhabitants in 2019. The region is characterised by a temperate oceanic climate with a mean annual temperature of 9.5°C and annual precipitation of about 1,259 mm, with higher precipitation in winter months.

A repeated sampling campaign was conducted over one year (March 2019 to March 2020), with approximately two sampling events per month. Composite samples were collected at the waste reception prior to any sorting or shredding, representing the biowaste as delivered from kerbside collection. Before sampling the biowaste was thoroughly mixed using an industrial scale excavator. For each sampling event, approximately 60 kg of mixed biowaste was collected from around 30 locations across the reception pile using ‘grab’ sampling, with each ‘grab’ typically 1 to 3 kg. The composite sample was then spread on a white polyethylene sheet and manually sorted by visual inspection into food, fresh grass, dry grass, winter leaves, garden cuttings, Christmas tree, inseparable organics and non organic material. The inseparable organics fraction comprised highly degraded organic material that could not be assigned to a specific category by visual inspection and composite pieces where multiple fractions were physically bound and could not be separated without invasive procedures that would alter the material. Fraction shares were calculated % of total wet mass. All organic fractions excluding the non organic fraction were shredded using a garden shredder (GreenBay GB WRC 50L). A proportional subsample of each shredded fraction, typically 0.5 to 2 kg, was collected and stored at -20°C. The shredded fractions were then recombined, mixed thoroughly and shredded again. A further mixed sample, typically 3 to 6 kg, was collected and stored at -20°C for subsequent laboratory analyses. Bulk density of the mixed biowaste was determined both before and after shredding using an open top 1 m³ container without any compaction.

Frozen samples were comminuted using a rotor cutting mill (Powteq CM200, Germany) with a 1 cm2 sieve. The comminuted material was thawed at room temperature for 12 hours and then total solids (TS) and ash were measured according to Standard Methods 2540 G (APHA/AWWA/WEF, 2005). For elemental analysis, the frozen sample was dried at 85°C for 24 h (Memmert GmbH UN55, Germany) and then ground using a cutting mill (Retsch ZM 200) with a 4 mm sieve. Carbon, nitrogen, hydrogen and sulphur were determined in triplicate using an elemental analyser (Elementar Macro Cube, Elementar GmbH, Germany) according to the DIN EN ISO 16948, 2015. Oxygen was calculated by difference on a dry mass basis.

Laboratory measurements were available only on sampling dates. Daily values between sampling events were therefore assigned using a piecewise carry forward approach, where the most recent measured value was applied to subsequent days until the next sampling event (Lachin, 2016; Ortigueira and Machado, 2020). This approach was applied to biowaste fraction shares, TS, ash, elemental composition and unshredded and shredded bulk density.

Daily inflow (t wet d^− 1) was aggregated to monthly inflow (t wet month^− 1) by summation. To avoid inflating load based indicators, monthly organic inflow was calculated by subtracting the monthly non organic wet mass from the total monthly inflow. Monthly load indicators were then calculated using the monthly organic inflow, while composition and laboratory properties were retained as measured for the analysed organic fraction.

Carbon and nitrogen resource potentials were calculated as monthly loads by multiplying monthly organic inflow by total solids content and the respective elemental mass fraction on a total solids basis:

where is carbon resource (t C month^− 1), is nitrogen resource (t N month^− 1), is monthly inflow, is total solids (% wet), is the carbon mass fraction in TS for month and is the nitrogen mass fraction in total solids for month , respectively.

Theoretical oxygen demand on a total solids basis was estimated from elemental composition as a stoichiometric proxy for the relative oxygen requirement of aerobic oxidation across months (OECD, 1992). The calculation followed the standard theoretical oxygen demand approach based on complete oxidation of carbon, hydrogen and sulphur with correction for oxygen present in the substrate.

where is theoretical oxygen demand in kg O₂ per kg TS and are elemental mass fractions on a TS basis (dimensionless, sum with ash to 1) of C, H, S and O, respectively.

Oxygen demand intensity, expressed as kilograms oxygen per tonne wet waste:

where is total solids fraction of wet mass

Monthly oxygen demand load, expressed as tonnes oxygen per month:

where is monthly inflow of organic fractions.

Humus carbon potential was estimated as a scenario based stabilised carbon indicator using a humification coefficient style fraction. The monthly humus carbon potential was calculated by multiplying the monthly carbon resource potential by an assumed stabilised fraction , analogous to the humification coefficient used in soil organic carbon models (Andrén and Kätterer, 1997).

where is humus carbon potential in t C month^− 1, is carbon resource potential in t C month^− 1 and is the stabilised carbon fraction. A central estimate and a scenario band were reported using as the central value with a low and high range of 0.25 and 0.50 to reflect uncertainty in stabilisation efficiency across organic amendments and soil contexts.

Growing media volume supported and peat displacement potential were calculated as central scenario indicators. Monthly compost mass was estimated from monthly organic inflow using an assumed compost yield . A conservative value taken from the published literature range of 0.4 to 0.6 (Breitenbeck and Schellinger, 2004; Oshins et al., 2022)

Compost volume was calculated from compost bulk density = 0.6 (Sullivan and Miller, 2001).

Growing media volume supported was calculated assuming a compost volume fraction , the maximum allowed proportion of compost in the professional growing media blend (Saveyn and Eder, 2014).

Peat displacement potential was calculated assuming a peat volume fraction in conventional consumer growing media.

The peat fraction used for the central scenario was based on published German production statistics for consumer growing media, which report an average peat share of 41% for the German consumer market and consumer growing media sales of approximately 5 million cubic metres (IVG e. V., 2024)

Daily inflow included non-operating days with 0 t wet d⁻¹ and reached a maximum of 352 t wet d⁻¹ (Fig. 1a). Most operating days lay between approximately 100 and 250 t wet d⁻¹, with higher daily inflows occurring more frequently from late spring to early autumn and several days exceeding 250 to 300 t wet d⁻¹. Lower daily inflow levels occurred more often in winter, with many days below 150 t wet d⁻¹. A gap of around one week in late December corresponded to the Christmas shutdown period. The late spring to summer period showed the highest daily volatility, combining high peaks with low troughs. Monthly inflow ranged from 3,007 t wet in February 2020 to 5,648 t wet in July 2019 (Fig. 1b), while TS ranged from 32.5% wet mass in January 2020 to 43.9% wet mass in July 2019. Periods of higher monthly inflow coincided with relatively high TS in summer, while periods of lower inflow coincided with lower TS in winter. TS during autumn and winter was relatively stable at 34 ± 1.5%, whereas spring and summer showed greater variability. Previous studies also reported pronounced temporal variation in TS, although the direction differed. Hanc et al. (2011) reported the lowest TS in summer (23.2%) and the highest in spring (34.2%), while Dronia et al. (2023) reported moisture contents of 49.2 to 77.8%, corresponding to TS of 50.8 to 22.2%, with wetter material in autumn and winter. (Sailer et al., 2021a, b) reported TS varying within a narrower band, with maxima of about 34.49% for urban material and 37.01% for rural material in late winter to spring, and minima of about 27.67 to 27.72% in summer to autumn.

Across sampling dates, food waste ranged from 1.45 to 34.84% of wet mass, inseparable organics ranged from 0 to 96.20% of wet mass, and the combined garden-related fractions, defined as grass fresh, grass dry, winter leaves, garden cutting and Christmas tree, ranged from 2.30 to 94.02% of wet mass (Fig. 2). Non-organic material, including plastics, paper, glass, stones, bricks and metals, ranged from 0 to 32.03% of wet mass. Changes in composition were driven primarily by shifts between inseparable organics and garden-related fractions, while food waste contributed a smaller but persistent share. (Sailer et al., 2021a, b)Sailer et al. (2021) reported higher food shares and a narrower upper range for green waste, with food varying from about 20 to 85% and green waste from about 15 to 80% across the year, whereas (Dronia et al., 2023) reported a tighter food range of 37 to 50% and garden waste of 35 to 55%. (Sailer et al., 2021a, b) reported impurities on a TS basis and observed substantially lower levels than the non-organic shares measured here on a wet mass basis, with annual mean impurities of 2.83 ± 1.67% TS for rural material and 5.07 ± 2.71% TS for urban material. In contrast, the present study showed non-organic material from 0 to 32% of wet mass across sampling dates, indicating more frequent and more extreme contamination events even allowing for differences in reporting basis.

On a TS basis, carbon ranged from 31.79% in April 2019 to 42.11% in March 2020, oxygen from 25.41 to 33.32%, ash from 18.26% in March 2020 to 36.78% in April 2019, hydrogen from 4.35 to 5.86% and nitrogen from 1.38 to 1.83% of TS (Fig. 3). On an annual mean TS basis, the present study yielded C 37.08%, N 1.64% and ash 26.75%, closely matching Hanc et al. (2011) (C 37.05%, N 1.57%, ash 27%) but showing lower carbon and nitrogen and higher ash than Sailer et al. (2021b), who reported C 42.37 to 44.31%, N 1.99 to 2.15% and ash 15.41 to 17.78% for rural and urban material. Seasonally, the present study showed lower carbon and higher ash in spring and summer (C about 35.5 to 35.6%, ash about 29.2 to 29.6%) and higher carbon with lower ash in autumn and winter (C about 38.8 to 39.4%, ash about 21.7 to 24.5%). Hanc et al. (2011) reported a similar pattern with a summer minimum (C 31.0%) and an autumn maximum (C 42.2%), whereas Sailer et al. (2021b) showed consistently higher carbon and nitrogen with smaller seasonal amplitudes. At a monthly resolution, both datasets showed a relative carbon depression in late spring, but the present study spanned a wider range, increasing from 31.79% in April to 42.11% in March, while Sailer et al. (2021b) varied within a narrower band with urban carbon reaching about 38.23% in May.

The annual mean C:N ratio was 22.6 and ranged from 18.67 in June 2019 to 27.92 in December 2019, increasing from late summer into winter before declining. (Sailer et al., 2021b) showed a similar rise into late summer for rural material, peaking at about 26.2 in August, while the urban stream showed a winter maximum of about 23.0. The annual mean C:N aligned closely with Hanc et al. (2011) (23.6) but exceeded values reported by Sailer et al. (2021b) for rural (21.3) and urban (20.6) material. Seasonally, the present study showed C:N of 21.5 in spring, 20.6 in summer and 25.0 in both autumn and winter, indicating a wider month-resolved amplitude than reported in the reference datasets..

Unshredded bulk density ranged from 117 kg m^− 3 in June 2019 to 393 kg m^− 3 in February 2020, while shredded bulk density ranged from 290 to 537 kg m^− 3 over the same period (Fig. 4). The highest unshredded densities occurred in winter, with monthly means of 281 to 383 kg m^− 3 from December 2019 to February 2020, while the lowest densities occurred in late spring to early summer (May to June). In comparison, Sailer et al. (2021b) reported consistently higher and more variable unshredded bulk densities, with monthly means spanning 313 to 920 kg m^− 3 for rural material and 371 to 1,139 kg m^− 3 for urban material, including pronounced peaks in April to June. Shredding increased bulk density in every month, with shredded-to-unshredded ratios ranging from 1.36 to 3.65. Sundberg et al. (2011) reported bulk densities of 433 to 776 kg m^− 3 for compost substrate mixtures containing structural amendments. In the present study, unshredded bulk density was substantially lower and more variable, but shredding increased density to 226 to 541 kg m^− 3, overlapping the lower end of the substrate mixture range (Sundberg et al., 2011).

To translate month resolved variability into decision relevant metrics, three classes of derived indicators were evaluated. Oxygen demand intensity and load provided a stoichiometric proxy for the aeration burden associated with the incoming biowaste and allowed comparison between months on both per tonne and total load bases. Carbon and nitrogen resource loads quantified the monthly availability of recoverable nutrients, while humus carbon, growing media volume and peat displacement translated these resources into downstream circular economy relevant quantities defined in the Methods.

Oxygen demand intensity ranged from 366.89 kg O₂ t^− 1 wet in May 2019 to 502.91 kg O₂ t^− 1 wet in July 2019, while monthly oxygen demand load ranged from 801.06 t O₂ month^− 1 in December 2019 to 2,114.42 t O₂ month^− 1 in July 2019 (Fig. 5), and the highest oxygen demand load coincided with the month of highest inflow. Oxygen demand load varied more strongly than oxygen demand intensity, indicating that monthly process burden was driven primarily by throughput rather than by shifts in material stoichiometry. July 2019 combined the highest inflow with the highest oxygen demand load, identifying it as a month where aeration capacity and turning frequency were most likely to be limiting. Across winter months, oxygen demand intensity remained between 372.55 and 421.41 kg O₂ t^− 1 wet, while oxygen demand load decreased with lower inflow. This indicated that winter operational pressure decreased mainly through reduced throughput rather than improved material quality. The observed oxygen demand intensity range overlapped with stoichiometric oxygen demand values reported for mixed household biowaste and garden waste streams under aerobic treatment in temperate European systems (Boldrin et al., 2009). Carbon resource ranged from 269.69 t C month^− 1 in December 2019 to 683.77 t C month^− 1 in July 2019 and nitrogen resource ranged from 9.70 t N month^− 1 in December 2019 to 32.24 t N month^− 1 in July 2019 (Table 2), while humus carbon potential ranged from 107.88 t C month^− 1 in December 2019 to 273.51 t C month^− 1 in July 2019. Monthly carbon and nitrogen resource loads therefore identified months of highest absolute resource availability, which is the relevant metric for regional allocation rather than annual average composition. Growing media volume supported ranged from 5,269 m³ month^− 1 in December 2019 to 10,686 m³ month^− 1 in May 2019 and peat displaced potential ranged from 2,160 m³ month^− 1 in December 2019 to 4,381 m³ month^− 1 in May 2019. The divergence between peak resource months and peak market facing months indicated that compost availability alone did not determine peat displacement potential at monthly scale. Operational focus varied by month, with moisture monitoring and avoidance of over drying indicated in April, July and August 2019, additional carbon rich bulking to conserve nitrogen indicated in May and June 2019, increased aeration and turning indicated in July 2019, and bulking addition with reduced pile height indicated in January to March 2020; taken together, these indicators revealed distinct months dominated by process burden and by circular economy potential. The order of magnitude of the peat displaced potential aligned with reported substitution capacities of compost based growing media in the German consumer market, where annual peat substitution has been linked to compost availability rather than quality constraints (IVG e. V., 2024).

Seasonal shifts in composition were expressed as changes in material quality and processing proxies. Periods with higher shares of garden related fractions coincided with higher carbon to nitrogen ratios (Fig. 3) and lower bulk densities, while periods with lower carbon to nitrogen ratio coincided with higher bulk densities (Fig. 5). Oxygen demand intensity varied across the year (Fig. 4), but oxygen demand load followed monthly throughput and peaked in July 2019 at 2,114.42 t O₂ month^− 1 (Table 2), defining July 2019 as the highest burden month based on oxygen demand load. In Table 2, carbon and nitrogen resource loads also peaked in July 2019 at 683.77 t C month^− 1 and 32.24 t N month^− 1, while market facing indicators peaked earlier, with growing media volume supported and peat displaced potential reaching maxima in May 2019 at 10,686 m³ month^− 1 and 4,381 m³ month^− 1. This month resolved workflow separated months dominated by process burden from months dominated by downstream circular economy potential using a single consistent data chain from reception measurements to monthly indicators. The humus carbon, growing media and peat displacement indicators therefore functioned as translations of measured resource potentials into decision oriented quantities rather than as predictions of realised market uptake.

The revised Waste Framework Directive requires the separate collection of municipal biowaste across the European Union, and many regions are currently in the process of designing or scaling systems without long-term empirical data on month-resolved variability (EEA, 2022; Favoino and Giavini, 2024; WMW, 2025). The present study provides evidence from a full annual cycle showing that incoming biowaste streams are structurally variable at monthly scale, and that this variability has direct implications for capacity planning, logistics and downstream valorisation when source-separated collection is implemented.

At the regional catchment scale considered in this study (approximately 0.56 million inhabitants), the quantified peat displacement potential corresponds to about 38,400 m³ year^− 1, equivalent to roughly 8% of the estimated annual peat demand in North Rhine-Westphalia. While illustrative rather than predictive, this order-of-magnitude comparison indicates that extending separate biowaste collection to additional municipalities and increasing compost utilisation in growing media, currently estimated at around 45% of collected compost, could materially reduce peat imports at state level. Realising such potential would depend on coordinated regional system design, including collection schemes that accommodate month-resolved variability, treatment capacity sized for peak aeration and storage demand, compost quality specifications aligned with growing media standards, and contractual integration between waste management operators and substrate manufacturers.

For regions establishing source-separated biowaste systems, the results demonstrate that annual average values are insufficient for infrastructure and process design. Instead, month-resolved information on inflow, TS, composition and bulk density is required to size reception areas, aeration capacity, turning regimes and storage volumes to manage peak conditions rather than mean loads. The identification of months dominated by high process burden and months dominated by higher resource quality illustrates that operational stress and circular economy potential do not coincide uniformly across the year, and should therefore be addressed through flexible, time-adaptive system design.

Beyond compliance, the derived indicators show that source-separated biowaste represents a predictable and quantifiable resource stream with relevance beyond waste treatment. Month-resolved carbon and nitrogen resource loads, together with translations into humus carbon, growing media volume and peat displacement, frame composting as a mechanism for nutrient recycling and partial substitution of fossil peat in consumer markets. For regions implementing source-separated collection, this enables early alignment between waste management authorities, compost operators and downstream users, supporting the development of new value chains and business models linked to fertilisers, soil improvers and fossil-free growing media rather than viewing biowaste treatment solely as a regulatory obligation.