Organic kitchen waste via the sewer

Within the framework of the circular economy, it is important for municipalities to make the best possible use of kitchen waste (fruit and vegetable waste, or ‘FVW’) through high-value, sustainable processing. In neighbourhoods with high-rise buildings this is practically impossible, because FVW is not collected separately. But the possibility does arise when the collection is done via the wastewater. This project has demonstrated that, from a technical perspective, there is no impediment to using food waste disposers for a more efficient recovery of resources from wastewater. And for high-rises the collection of FVW using this technique is the solution with the lowest environmental impact.

Variable indoor pipe configuration of the experimental setup.


For a more efficient recovery of resources from wastewater, the resource concentrations need to be increased. This can be done by adding more organic waste to the water and by reducing the amount of water. This is the so-called ‘new water cycle’: after being processed by a food waste disposer, FVW ends up in the sewer. Together with concentrated toilet wastewater (‘black water’), it is then digested at a wastewater treatment plant (WWTP).

The objective of the OSKAR project – the name is the Dutch acronym for ‘organic kitchen waste via the sewer’ – was to study the possibility of using a food waste disposer in a home. This involved investigating several aspects, namely: technical (focused on the indoor sewer installations), governance (what is permitted under Dutch law) and sustainability (comparison with FVW collection through other routes). The user convenience aspect was not included.

The research examined whether the current indoor sewer installations are capable of transporting FVW processed by a disposer. Of course, this waste must not cause clogging or any other undesirable effects and problems, such as corrosion and/or cement rot caused by H2S, or increased methane emissions. A literature study showed that the effects of FVW on the sewer system and the WWTP have been studied, but research on the indoor sewer installations has lagged.

In this project an experimental set-up was built including a food waste disposer, a sink and several configurations of indoor sewer installations (variations in slope, length, diameter, bends, horizontal and vertical pipes), and a wide variety of types of (mixed) foods. Our laboratory experiments show that there are no technical impediments to the use of food waste disposers. Nevertheless, it would be advisable to confirm this through a practical, long-term pilot study. The most important framework conditions are the application of sufficient slope and compliance with existing design installation guidelines.


Food waste disposer experimental set-up.


The environmental impact was determined by comparing three different scenarios: the discharge of organic food waste into a kitchen and garden waste compost bin (KGW scenario); discharge into residual waste (residual waste scenario); and discharge of food waste into the sewer system, after it is processed by a food waste disposer (sewer scenario). The STOWA 2015-07 report provided the basis for the research; the model was updated with a new Life Cycle Inventory (LCI) database, and the impact assessment method was reviewed. The conclusion remained the same: the discharge of kitchen waste into the KGW bin has a lower environmental impact that does the sewer discharge, but the difference is less significant than in the STOWA report’s original model.

The use of greener forms of energy is particularly beneficial in the sewer scenario. The environmental impact of this scenario can be decreased by reducing the household’s water consumption, and by decreasing the impact of the disposer itself. This impact can be decreased by processing more FVW per disposer – for example, if several households were to discharge their FVW via a common disposer – or through a significant extension of the disposer’s service life. In the case of high-rises, the discharge of FVW via the sewer is considerably better than it is via residual waste.


An important obstacle to the discharge of FVW via a disposer is the current prohibition on their use. Food waste disposers are deemed undesirable by the Ministry of Housing, Spatial Planning and the Environment (VROM) because they ‘impose an additional load on the sewer system’. This is a reference to the possibility of increased clogging and to the fact that the processing of the effluent at the WWTP results in higher emissions.

The current legal framework does however offer the possibility of using FVW disposers. This relates to the exemption provided to municipalities for the introduction of alternative waste collection methods in specific areas. But for FVW disposers this also includes the stipulation that the wastewater must be processed separately, outside of the WWTP. This means that a pilot can be carried out.

The successful implementation of a pilot is currently the most promising means of bringing about changes in today’s policy. Being able to demonstrate that the enriched waste steam from the FVW disposer constitutes no additional load for the WWTP plays a key role in this regard.


Indoor pipe section after test with a high content of FOG.