It can be done: processing organic kitchen waste from multi-storey buildings sustainably using the sewer system

Who will take the first step?

Municipal authorities want to process kitchen waste in sustainable, high-grade, ways in order to contribute to the circular economy. However, kitchen waste (vegetables and fruit) from multi-storey buildings does not reach the processing stage. Organic waste (vegetables, fruit and garden cuttings) is collected but that is not very sustainable and relatively expensive because there is no garden waste in this case and small quantities are involved. Many people living in multi-storey buildings do not separate kitchen waste from residual waste properly, for example due to a lack of space.

Collection in the waste-water system does provide openings: technically, it is possible to use leftover food grinders for the efficient recovery of raw materials from waste water. This has been demonstrated by the TKI project Organic kitchen waste via sewers. Furthermore, this system facilitates the recovery of raw materials from the sewer because it increases the concentration of raw materials. “Now it’s time for a larger pilot project over a period of several years,” says Ewald Oude Luttikhuis of the municipality of Leidschendam-Voorburg. “So that this solution can also prove itself in practice.”

“Now it’s time for a larger pilot project over a period of several years.”

Processing kitchen waste from multi-story buildings sustainably

Waste is not waste; it’s a raw material. The same applies to organic kitchen waste. At the moment, municipalities collect organic and garden waste for sustainable processing. But in districts with multi-story buildings, separate collection is often not a viable option. Other approaches are needed to achieve higher waste separation rates. “The national government still prohibits the use of food waste grinders in Dutch homes. There is a worry that they can cause blockages in sewage systems in buildings and place an additional burden on the sewage system,” says Eric van der Blom of Techniek Nederland. “But it might be a good way to process organic waste from those buildings sustainably.” “And sustainability is high on the agenda at many municipal authorities,” adds Oude Luttikhuis. So there are good reasons to look at the impact of food waste grinders on the sewage systems in multi-storey buildings.

Lab array

In order to investigate how the input of ground kitchen waste affects a building’s sewage system, KWR built an array in the laboratory to look at the technical aspects. A food waste grinder, a sink, and various configurations for sewage systems in the home were used for experiments with seventeen types of mixed food (with variations in inclines, length, diameter, bends, horizontal and vertical pipes). There appeared to be no technical problem with using food waste grinders. If the incline of the horizontal pipes is adequate (1 in 50) and assuming compliance with the existing design guidelines, a food waste grinder can be connected to the sewage system of a building without causing blockages. To confirm this, the researchers believe a larger pilot project covering about 200 homes will be needed over a longer period of time.

Test array with sink and drainage pipes and a collector at the end, together with a picture of the residue that may be left in the pipe.

Pilot project wanted

“We would prefer to use a new building project,” says Van der Blom. “Existing buildings don’t have an incline in the sewage pipes of 1 in 50; the minimum requirement is 1 in 200. Even now, engineers often find it difficult to meet this requirement and if the incline has to be steeper, that means they will have to improvise even more. It means that the pipes go lower below the floor and so they take up more space in the storey below. That is the only way to install the pipe with an adequate incline so that there is no risk of it getting blocked. It’s easier to introduce that incline in new buildings.” “You could also solve that problem to some extent by installing more sewer connections in each building,” adds Oude Luttikhuis. Of course, doing this also transfers some of the risks and costs to another party.

Benefits and consequences for different partners

Oude Luttikhuis: “As is often the case with infrastructure, the benefits, drawbacks and risks devolve to different partners. That makes decision-making complex. What we need now first is a pilot project that is large enough. Here in Leidschendam-Voorburg, I can perhaps find a few dozen homes. That is not enough. But in the Delfland area, the municipal authorities, the water authority and the water companies have adopted a shared vision about drinking water, sewage and treatment: we want to close the water and raw materials cycle and produce not waste but raw materials and energy. Additional production at the WWTP may outweigh the extra costs. Moreover, the easy disposal of kitchen waste as an additional function for the sewage system may increase public support for paying for wastewater disposal. And now it has been demonstrated that this approach is more sustainable, while the waste collection charge can be cut slightly. It will, incidentally, be easier to launch a pilot project when the Environment Act comes into effect: the discharge requirements will then be transferred from the national to the municipal level, and municipal authorities will be able to decide for themselves whether a food waste grinder is allowed or not.”

Extra convenience in expensive apartments

Van der Blom: “What we now need is a party who benefits and who is committed to following through. That will lead to solutions for the objections of the different parties. And to adjustments that make the sewage system slightly less robust. Even so the changes must never be at the expense of the reliability and safety of the building sewage system. And the first project could be in housing in the more expensive segment: developers may also see a food waste grinder as an extra convenience for buyers and so cost will be less of an issue. You won’t have to lug your rubbish around as much to dispose of it at the street level. And don’t forget that more and more apartments are in the expensive segment. In cities, you often pay more for an apartment than a standard home. So this may be the road to the introduction of a proven and popular technology.”