KWR rapport - KWR 2021.121

Organic kitchen waste through the sewage system: an environmental, legal and governance aspects analysis


In this study, the impact on the environment and the legal and governance aspects of disposing organic kitchen waste through the sewerage are studied. This is an addition to the technical analysis reported in ‘Assessing the impact of food waste disposers on the indoor sewerage system’ KWR 2020.079.
In determining the environmental impact, the focus was mainly on the question: which ‘levers’ can be pulled to get more out of this concept. For this, a comparison has been made of three scenarios: disposal of organic food residues via organic waste container (organic scenario), disposal via the residual waste (residual waste scenario) or disposal in which the food residues are discharged through the sewerage via a food waste disposer (sewerage scenario). The STOWA 2015-07 report serves as the basis for this study. It is assumed that the inventory of how much material, energy, chemicals, etc., has not changed, but this model has been updated with the help of a new life cycle inventory (LCI) database and a revised impact assessment method. As a result, the new results deviate significantly from the results in the STOWA report. The conclusion that the removal of organic kitchen waste via organic (VGF) waste has a lower impact than via the sewerage system, however, is also confirmed with this new model (with accompanying assumptions), although the difference is smaller. The parameter sensitivity analysis shows four important key points. Firstly, the use of a greener form of energy in the process appears to have a particularly favourable effect on the scenario with discharge via the sewerage system. Secondly, reducing the water consumption in the sewerage scenario from 16.8 to 10L appears to reduce the environmental impact by ~5%. Thirdly, it follows from this study that in the scenario with discharge via the sewerage, the impact of the grinder is very large. Since the impact is calculated per kg of food residues, the impact of the grinder decreases if more food residues can be ground with it. It turns out to be worthwhile (~20%) if several households were to dispose of their kitchen waste via the same grinder. Finally, the quality of compost appears to be of great importance for the comparison with the organic scenario. The research by Bolzanella et al., 2003 shows that the quality of the compost can vary greatly, and this distribution means that at best the impact of the organic scenario is about 35% lower than with an average compost quality. This obviously has a major influence on the mutual comparison of the different scenarios. The advice now is to focus primarily on extending the life of the grinder. The disposal of food waste is mainly considered in high-rise buildings, where food waste is mainly disposed of via residual waste. In this model, the removal of food residues via the sewerage system scores significantly better than the removal via residual waste.

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