Model maps out future sewer system scenarios

Publication selected as Editor’s Choice in Water Science and Technology

Earlier this year the editors of Water Science and Technology selected an article by the University of Bath, KWR and TU Delft as the Editor’s Choice on account of its scientific importance. Under the leadership of the British university, the collaboration partners developed a model which, on the basis of household water use, predicts the water discharge and water quality in the sewers.

The model maps out the effects of different future water use scenarios on the sewer system. According to the editor, the article is a great example that illustrates the importance of understanding all the consequences of such scenarios, including unintended and any undesirable side effects.

Ambitions to reduce water use

The research responds to the ambition of the water utilities to further reduce water use from the current level of 120 litres per person per day. In the 1990s our consumption in the Netherlands was more than 200 litres per person per day (Statistics Netherlands). Although sustainable water use and water conservation have been the focus of attention for a number of years, the general feeling is that water use needs to be reduced even more.

Much has been accomplished

Lots of progress has already been made, thanks for instance to water-saving shower heads, aerator taps, and water-saving toilets with flush interrupters or dual-flush buttons. Besides water conservation, increased attention is also being directed at alternative concepts for household water use.  Examples include the reuse of water from the shower or washing machine for toilet flushing. Research is also looking at the use of rainwater in and around the house. As a result of these measures, drinking water use in the Netherlands has dropped sharply over the last few decades. A comparable water conservation trend can be observed in the UK. And English water utilities aim to reduce water use further. Anglian Water has set itself the objective of 80 litres per person per day.

Less water through the sewers

The consequence of these water savings is that less water flows into the sewers. However, the amount of waste that has to be transported in the sewers remains the same. We still go to the toilet as often and we still use the same amount of toilet paper. The question is therefore whether the current sewer system can keep functioning. The transport of solids becomes more problematic as the amount of water decreases, so that blockages can develop more rapidly. But it also means that the concentration of substances in the wastewater increases. This might in fact be beneficial, since the effectiveness of the wastewater treatment can be enhanced. There are perhaps also possibilities of giving the sewer system a role in the treatment of the wastewater.

Modelling offers an answer

To obtain a clear picture of the effects of water conservation on the sewer system one needs modelling. Household water use is very dynamic and typically takes place in short peaks: you turn on the tap and after a short while you turn it off again. KWR developed the SIMDEUM model to simulate household water use. The model allows one to predict the total water use in a street or residential neighbourhood on the basis of simulated water use patterns.

Summary of the make-up of household water demand and discharge into the sewer under the different scenarios. A detailed description of the scenarios is available in the published article.

Extending the SIMDEUM model

The SIMDEUM model was extended to also predict, besides the household water use pattern, the wastewater composition per house, at every point in the network, and at every moment of the day. To this end, the model calculates the flow, the water level, and the composition of the wastewater. The model was calibrated and validated using data provided by Wessex Water.

Future scenarios

The model was used to test a number of future scenarios for water use in the UK. Artesia Consulting developed these scenarios on behalf of the UK Water Services Regulation Authority (OFWAT). They present a picture of UK household water use, based on different possible socio-economic developments over the next 50 years. The results were compared with the current water use. The research shows that a reduction in household water use of between 15 and 60% (depending on the scenario), would mean a drop in the morning peak discharge to the sewer ranging from 1 to 48%. It also predicted an increase in wastewater concentrations: the chemical oxygen demand increases by 55–180%, nitrogen by 19-116%, and phosphorus by 55-180%.

Measurements in Amsterdam

The project has been continued over the last year in collaboration with Waternet and TU Delft, within the framework of the TKI’s ‘New Urban Water Transport Systems’ (NUWTS) project. Specifically, the model is being applied to the Prinseneiland island in Amsterdam. This research is being supplemented with water-quality measurements in the sewer, thus further validating the model. The results of this project will be published shortly in a new article. This will mark the conclusion of the research by Olivia Bailey, a PhD student at the University of Bath. She will be defending her thesis in the near future.

Acknowledgement: This study was conducted as part of the WISE Centre for Doctoral Training, which is funded by the UK Engineering and Physical Sciences Research Council, Grant No. EP/L016214/1. Olivia Bailey is supported by a research studentship from the EPSRC WISE CDT.