Analysis of the water flows in a building with vertical farming from a sustainable perspective
6.3 Results and discussion
6.3.3 Proposal for an efficient design of office buildings with water-saving technologies
This section presents a low-cost redesign of the network that might be key to improve the overall efficiency of the building, saving significant economic costs and preventing environmental impacts (avoiding additional treatment of water).
The redesigned system should use the rainwater collected directly to flush toilets, improving the utilisation of the rainwater harvesting system and avoiding filtration and chlorination. The demand of the washbowls could be fulfilled with drinking water from the network, which does not require further treatment. In this way, both rainwater harvesting and greywater reclamation would contribute to cover the demand of toilets, reducing the consumption of drinking water.
For doing so, the only change that should be implemented in the network is the connection of the 35 m3 water tank to the greywater treatment stations, which supply the toilets. This modification would only imply connecting the pipe that leaves the rainwater tank with each of the pipes going to the greywater treatment stations. These connections should be made at the point where these pipes are the closest in the building so that only the installation of two short sections of pipe would be required. All the appliances for the treatment of rainwater would remain unused, saving reagents and energy.
The evaluation of the water tank with the software Plugrisost® shows that the optimal size of the tank with the conditions of the building (considering a catchment area of 1,200 m2)would be 31 m3 of capacity. Thus, the size of the current water tank (35 m3) is adequate, approaching the optimal volume.
According to the modelling from the software, the system would be able to fulfil 45% of the current demand for flushing toilets, increasing the building's self-sufficiency significantly.
Reducing the discharge volume of the toilette cisterns in the building would be another effective measure to reduce the drinking water demand from toilets. A device for the regulation of the discharge volume and with a double-discharge option would be effective and could lessen the discharge to 3 and 6 L. Another option would be to introduce a two-litre bottle or just a ceramic, concrete or stone block with a similar volume, which could reduce the discharge by 30% for 7 L cisterns and by 20% for 9 L ones. The cost of this measure would be nearly zero, and its effect on the reduction of the water demand would be immediate.
Figure 6.8 shows a diagram representing the changes that would be implemented with the redesign of the network. The redesign of the system might reduce by 30%
the water demand for flushing toilets, and the rainwater harvesting system could provide 65% of the remaining demand with rainwater. The combination of these measures could reduce by 75% the consumption of drinking water for flushing toilets.
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Figure 6.8 Current water network system in the ICTA-ICP building and proposal for its redesign.
Future projects including water-saving technologies should estimate the water flows of the system during the design phase. This estimation will allow optimising the system to maximise the water use efficiency. In the case of rainwater harvesting, the size of the water storage tank should be optimised to prevent environmental impacts in the manufacture of the tank (specially to reduce the amount of material required) while maximising rainwater utilisation. With this optimisation in the design phase, a similar efficiency to that of the redesigned system can be achieved.
6.4 Conclusions
The ICTA-ICP building on the campus of the Universitat Autonòma de Barcelona holds ultimate water-saving technologies, including rainwater harvesting and greywater reclamation. This system represents an opportunity to assess new forms of water management at the consumption point and to obtain information about the effectiveness of these technologies.
The results show that the water use efficiency in the building is limited, due to a bad design affecting the overall utilisation of water. Over the period assessed, the building had a total water demand of 23 L/user, which is 18% lower than the reference value considered. These savings are low compared with the values reported in previous literature, and might not justify the environmental and economic costs of implementing rainwater harvesting and greywater reclamation.
The technologies applied in the building to enhance water self-sufficiency might have been effective for households, where a significantly higher percentage of the water is used in the shower and washbowls, generating more greywater that can
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potentially be reclaimed for flushing toilets. However, the generation of greywater in an office building is much lower because the water used in showers is not significant. The design of the building’s water networks should have estimated the magnitude of the different water flows to make a proper planning applying the most appropriate technology to reduce the demand for drinking water.
The study presents a proposal for a low-cost redesign of the system, which consists of reducing the discharge volume of toilets and using the rainwater collected for flushing toilets. These measures may help to reduce by 75% the external demand for water of the current system and would avoid the treatment of rainwater, saving energy and reagents. The water flows in other buildings with similar water-saving technologies should be assessed, since a redesign may imply little effort and a small economic investment but improve the efficiency of the system substantially.
Furthermore, future projects for the implementation of rainwater harvesting and greywater reclamation systems should estimate the water flows in the building during the design phase to ensure its optimisation.
Some of the flows quantified in the study rely on estimations due to the lack of flow meters in key sections of the water networks in the building. In particular, the installation of a flow meter at the outflow of the southeastern greywater treatment station would allow measuring the water demand for flushing toilets in the southeastern half of the building. Although the results of the study were coherent, the installation of these devices would be of interest to evaluate the accuracy of the results in this study. Further research is also needed to address other case studies with similar technology and to implement these water-saving systems to other types of buildings.
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