How University of Ghent will implement water reuse in new student homes

The client

Ghent University is a top 100 university and one of the major universities in Belgium. Our 11 faculties offer more than 200 courses and conduct in-depth research within a wide range of scientific domains. Ghent University Global Campus is also the first European university in Songdo, South Korea.

Introduction

Water Experts and University of Ghent have collaborated in several projects, where the most sustainable and economically interesting solutions for water softening, legionella prevention and sanitary warm water production were investigated. For two new student homes, University of Ghent wanted to take things further and investigate the reuse of greywater for toilet flushing.

The CHALLENGE

As a first step, Water Experts mapped the expected water flows in the buildings and simulated on a daily basis the capture and reuse of rainwater for toilet flushing. Toilet flushing contributed to about 22% of the total water use, which would mean a substantial saving.

Rainwater could provide most of the water for toilet flushing (84% on average), but city water addition was necessary. In addition, rainwater is a variable source of water, with rainfall intensity and droughts intensifying. 

Greywater, on the other hand, is a stable source of water that also comes with higher cost savings, as rainwater use only saves on the delivery of city water and not the discharge or treatment, whereas greywater reuse inherently means lower water abstraction and lower discharge.

For one of the student homes, rainwater reuse would result in a potential saving of about €3.300 per year, whereas greywater reuse would potentially save about €8.000.

From an ecological perspective, water reuse compared to rainwater reuse could be advantageous as well. When reusing water, rainwater can be infiltrated to augment our groundwater reserves. In case more reuse is wanted in the future, rainwater could still be used to compensate for the water loss in wastewater reuse treatment. 

Lastly, in case grey- and blackwater could be collected and treated together, infrastructural works would be simplified, as greywater reuse apart from blackwater discharge means that more piping is to be installed.

The Solution

Taking these things into account, several technologies for the treatment of grey- (and black)water were compared in terms of the removal of certain wastewater pollutants and cost (Table 1). 

From this first assessment, along with the practical experience with greywater reuse, membrane bioreactors, and constructed wetlands were selected as the most interesting technologies to use.

*: Suspended biomass or biofilm reactors Depending on the concentration: There is always a certain removal, but at too high concentrations of a pollutant the removal will not suffice. Depending on the type of membrane: some membrane types remove more components than others (microfiltration, ultrafiltration, reverse osmosis).

 

Table 1: Overview of the different categories of treatment technologies for greywater reuse, along with their suitability to remove different pollutants. Green: good removal. Orange: average removal + reason. Red: no removal.

 

 

Table 2: Overview of the different categories of treatment technologies for greywater reuse, along with a rating of performance and investment cost. Green: favourable. Orange: average. Red: not favourable.

*: Suspended biomass or biofilm reactors
In the next step, the option to reuse wastewater for toilet flushing was evaluated from a legislative point of view.

This is very important to assess at the beginning of such projects, as legislation can limit or affect the choice of technology and the devised concept for reuse. In Flanders, for example, when a building is closer than 250 meter from a public sewer pipe, there is an obligation to hook up and discharge wastewater in the public sewer. In such cases, it is not allowed to treat your own wastewater and discharge in waterways or infiltrate in the soil. Wastewater reuse is allowed, but one can only treat that water which is reused directly, whereas the wastewater that is not reused has to be discharged in the sewer. For the new student homes, this was the case and had to be taken into account.

A membrane bioreactor and constructed wetland was designed to treat wastewater for toilet flushing and several technology providers were contacted to estimate the cost for such systems.

A commercially available membrane bioreactor had a payback period of about 5 years, whereas a constructed wetland resulted in a payback period of about 3-4 years. This did not consider yet piping to the treatment and distribution to the toilets and the buffer tank of wastewater. A practical comparison (Table 3) showed that the provider of the membrane bioreactor only treated greywater, meaning more piping would be needed to separate grey- and blackwater. A constructed wetland usually treats combined grey- and blackwater (after passing a septic tank). Membrane bioreactors require more intensive monitoring and maintenance, resulting in higher operational costs. Also, membrane bioreactors are best operated as continuously as possible, whereas constructed wetlands can better handle downtime (e.g. in vacation periods or weekends). Constructed wetlands on the other hand, require much more space compared to membrane bioreactors but can be integrated in the scenery and be used for communication and sensibilization of students as it is very visible. Taking all advantages and disadvantages into account, University of Ghent decided to work further with constructed wetlands.

Table 3: More detailed comparison between constructed wetlands and membrane bioreactors for the treatment of grey- and/or blackwater.
Next step is to make a detailed scheme with detailed costs of all necessary elements of the system. Water Experts will also oversee the implementation to ensure the necessary quality and performance is delivered to University of Ghent.

The RESULT

The flow rates, final dimensions of the buffer tanks and the constructed wetland were calculated and put in a general diagram to incorporate in the plans of the buildings.

For your buildings, Water Experts can perform all necessary studies to determine the feasibility and to coordinate the implementation of sustainable water management in your buildings. This service ranges from water saving measures in the building to rainwater capture and reuse and even wastewater treatment and reuse.

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