project

Design guidelines for sewer systems for domestic wastewater

In the urban context there are two key water discharge streams: the dry-weather stream and the stormwater stream. The volume flows of domestic wastewater are essentially a function of drinking water consumption, with a certain time delay. A great deal of knowledge has been gained recently about drinking water consumption on the basis of the end-use (e.g., showers, toilet flushing, dishwashers and washing machines). This knowledge makes it possible to model the nature and volume of the domestic wastewater stream on a minute time-scale.

Technology

This project researches the possibilities of hydraulic optimisation in the design of the sewer system for domestic wastewater. In current practice, the dimensioning of the pipes is entirely determined by the inspection possibilities. The idea is that the hydraulic approach could result in a distribution network that is shorter and consists of pipes of smaller diameter. By taking management into account during the design phase, and through the improved hydraulic design, one can also expect that maintenance needs (and thus operational costs) will be reduced. The idea is that the new design principles can be applied in new neighbourhoods but also in the rehabilitation of the existing systems.

We also examine the impact of the addition of organic matter – ground kitchen waste, for instance – with a view to improving the treatment options for the wastewater stream. The research also studies whether a ‘thicker’ stream can be collected with the new concept.

Challenge

The objective of the project is to establish design rules for a specific gravity-drainage collection network for domestic wastewater, based on a hydraulic optimisation analogous to the hydraulic optimisation of drinking water distribution networks. These are the so-called ‘last and first mile’ of the drinking water and wastewater network. In addition, a scenario is be developed which analyses the hydraulic consequences of enriching domestic wastewater with kitchen waste.

This project aims at achieving the following results:

  • Insight into the background of current design guidelines for dry-weather stream sewers.
  • Hydraulic modelling of a sewer system exclusively for domestic wastewater, based on the dynamics of drinking water consumption.
  • Experimental verification of the models, including ‘thicker’ streams with ground kitchen waste.
  • Review of design rules for sewers for domestic wastewater.

The project began on 1 January 2014. The first model calculations have been carried out and define the preconditions for the experimental verification. The inventory of current practice is being done on the basis of various interviews and a literature review.

Solution

Two test set-ups were tested to assess the drainage capabilities of a smaller pipe, and to test the transport of objects through a smaller pipe compared to a larger one.

The first test indicated that the capacity of a round 110 mm pipe is sufficient to drain the wastewater of over 200 houses, while the second test indicated that a smaller pipe is more effective at transporting objects than a larger one. Both outcomes justify giving new consideration to the development of a discharge pipe for domestic wastewater which would be much cheaper. The follow-up step would be to test these findings in a practical pilot project.