Picture: Mandeo River in Betanzos (Galicia) Retrieved from: http://www.herculestours.com/rutas-por-galicia/desde-coruna/rutas-por-galicia-coruna-betanzos/
Chapter 10
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Chapter 10 General conclusions
This chapter presents the general conclusions of the dissertation, providing adequate answers to the research questions defined in Chapter 2.
Research Question 1: What are the main factors affecting the environmental impacts of water supply networks in cities and which improvements should be implemented?
The environmental analysis of the water supply networks was done differentiating two parts of the life cycle: the construction and the operation.
Construction of water supply networks: pipe and appurtenances
One of the relevant factors affecting the environmental impacts of the construction of the network is the pipe diameter of the constructive solution. For constructive solutions with smaller pipe diameters, the installation of the network is the largest contributor. Thus, optimising the dimensions of the trench and the processes of the installation will be key for the reduction of the environmental impacts. In contrast, for constructive solutions with larger diameters, the pipe (its production) accounts for the highest percentage of the environmental impacts. In this case, using less impacting materials will allow the maximum reduction of the impacts.
Therefore, the pipe material is another relevant factor affecting the environmental impacts, and the selection of less impacting pipes can prevent a significant part of these impacts. The constructive solutions were assessed for the pipes made of the most common materials. The results revealed that plastic pipes (HDPE, PVC and LDPE) are more sustainable than pipes made of ductile iron and glass fibre reinforced polyester, which have between 3 and 11 times more environmental impacts than the prior ones, depending on the impact category (Table 10.1).
However, there is an existing gap regarding the variation of the lifespan of the pipes depending on its pipe material. Future findings in this regard would be of interest, but the lifespan should be significantly different to affect these conclusions because the impacts of glass fibre reinforced polyester and ductile iron are several times that of plastic pipes.
Table 10.1 Comparison of the environmental impacts of constructive solutions with different pipe materials and diameters.
HDPE=high density polyethylene, DI=ductile iron, 90/200 mm=pipe diameter of the constructive solution
At the city scale, the selection of the less impacting materials for the construction of the network has proven to be a simple but effective measure to reduce the
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environmental burdens. This analysis can be applied to new urban areas or for the redesign of old networks. The construction of the water supply network of Betanzos (Spain) with plastic pipes (the least impacting material) instead of ductile iron and cement pipes would have generated between 6 and 16% less environmental impacts.
Construction of water supply networks: municipal water tanks
The environmental impacts of municipal water tanks, more specifically cylindrical water tanks, are affected by three variables: position (buried, partially buried, superficial), storage capacity (cubic meters) and dimensions (radius and height).
For the position, superficially placed tanks are preferable from an environmental perspective because its construction requires less reinforcing steel and less soil needs to be excavated and transported, reducing the environmental impacts between 15 and 35%. In respect of the dimensions, the less impacting water tanks are the tallest ones, which also have the smallest diameter, because these dimensions imply a more optimised geometry and a limited use of steel.
Regarding the water storage capacities, tanks with volumes ranging between 1,000 and 2,500 m3 are environmentally preferable, as these cases hold a better relation of steel and concrete per volume of water stored. Volumes smaller than 500 m3 hold between 5 and 30% higher impacts and volumes larger than 5,000 m3 are between 5 and 20% more impacting.
Applying the less impactful options for each of these variables in the construction of water storage tanks is key for the reduction of its environmental impacts.
However, this is not always possible due to specific conditions of the installation points. For instance, superficial tanks cannot be installed in some densely populated urban areas. The application to real case studies shows that selecting the less impacting option for the construction of the tanks would have prevented between 10 and 40% of the environmental impacts in the specific case studies assessed.
Operation of the water supply network
Regarding the operation, the consumption of electricity for the transportation of water is the most relevant factor to consider. This consumption varies from one municipality to another according to with some key variables that affect it.
A relevant variable affecting the relative consumption of electricity and hence the environmental impacts is the size of the municipality. Small-sized municipalities (<10,000 inhabitants) tend to have larger electricity consumptions than medium-sized ones (10,000-50,000 inhabitants). In the sample analysed, the prior ones show nearly 14 times larger electricity consumption (1.15E-2 as opposed to 8.3E-4 kWh/m3 registered water·km of network).
Another significant variable is the density of the population (inhabitants/km2).
Water needs to cover relatively longer distances in less densely populated municipalities (<90 inhabitants/km2) and thus present a higher electricity consumption in comparison with more densely populated municipalities (>90 inhabitants/km2). In this case, municipalities with lower population density show approximately 7 times higher electricity consumption.
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The location of the municipality (cost or inland) and the climate (Mediterranean or Oceanic) might be linked to the electricity consumption of the network, as well as the seasonality. However, the relation found was not robust enough to reach solid conclusions, and further studies with a larger sample should be implemented.
The correlation between some of the variables considered in the study was analysed, and the results are represented in Figure 10.1. As seen, the size of the city is clearly related to the length of the network and the water consumption, but not with the electricity consumption, which was not related with any of the variable (only holds a relation in terms of kWh per m3 of water and km of network, as discussed above).
Figure 10.1 Diagram of the relations found between some of the variables analysed for the operation of the network.
Another important factor to consider is the loss of water, which should be assessed in a more thorough bottom-up study, as it is believed that external factors stated in IWA (2003), such as un-metered connections, dilute the measurement of the water losses.
Although specific measures to reduce the environmental impacts of networks were not considered in the dissertation, any reduction of the electricity consumption and the water losses would be helpful. A proper urban and regional planning to optimise the water transportation might be effective to reduce the electricity consumption. Another relevant measure would be increasing the share of renewable energies in the electricity mix of the country. Finally, the reduction of the water losses along the network would also reduce these environmental impacts.
These measures should preferably be implemented in municipalities from the clusters holding the higher electricity consumption. Small-sized municipalities with a low density of population are prone to have higher electricity consumption.
Research Question 2: How effective are water-saving technologies used at the building level in urban areas?
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Based on previous literature, there is strong evidence showing that rainwater harvesting and greywater reclamation have a significant potential to save water in urban areas (Farreny et al. 2011b; Angrill et al. 2012, 2017b, Vargas-Parra et al.
2013, 2014). Nevertheless, an appropriate application and planning of these technologies in buildings is required for their effectiveness. Otherwise, their efficacy can be limited, and they might even increase the overall environmental impacts and costs of the system.
The assessment of the ICTA-ICP building has shown being more efficient in the use of water than a reference office building from the same area, consuming 18% less water per user. However, these savings might be low when considering all the measures undertaken in the building for the efficiency in the consumption of water. The research conducted revealed that although the water-saving technologies are working properly, their effectiveness is limited due to a lack of planning during the design phase.
The rainwater harvesting system of the building holds a 35 m3 tank, whose rainwater might partially cover the large demand for toilets. However, this rainwater is filtered and chlorinated for its used in washbowls, generating further impacts with this additional treatment. Moreover, the rainwater collected in this tank is underused due to the low demand generated in washbowls.
The most significant water demand of the building is the flushing of toilets (90%
of the drinking water consumed). In contrast with the rest of the water-consuming devices, toilets hold a conventional mechanism with a discharge volume of 7 and 9 L/discharge and no double-discharge option. The greywater reclamation, which collects greywater from washbowls and uses it for flushing toilets, only covers 6%
of this demand because the volume of water used in washbowls is substantially smaller than the volume required for flushing toilets.
The water use efficiency of the building might improve if certain low-cost measures were applied. Firstly, the discharge volume of toilets in the building should be reduced. If a device for the regulation of the discharge was installed, or simply a 2 L volume that sinks was introduced into the toilet tank the discharge could be reduced by 30% for 7 L cisterns and by 20% for 9 L cisterns. Another effective measure would be using the rainwater collected directly for flushing toilets, increasing the utilisation of this resource and eliminating the need for treatment (filtration and chlorination). Washbowls would use regular water from the water supply network, which has the adequate quality without additional treatment. The implementation of these measures might reduce by 75% the external water demand of the building, improving the water efficiency and the self-sufficiency of the building significantly.
Figure 10.2 shows the diagram of the current system and the potential improvements that could be implemented.
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Figure 10.2 Diagram with the quantification of the water flows in the ICTA-ICP and the preliminary data for the potential redesign.
Research Question 3: Are integrated rooftop greenhouses an efficient and sustainable alternative for food production in cities?
This research has proven that the i-RTG is feasible and can boost the production of food in urban areas. From an urban perspective, this new system represents a new use for water in cities, providing agricultural services to buildings.
The system can grow 19.6 kg of tomato/m2·year, being in total 1,650 kg of tomato for the 85 m2 of productive area analysed. Thus, the i-RTG is an alternative to conventional production that can provide fresh and nutritious food and improve food security and self-sufficiency in cities.
The symbiosis with the building allows reducing the use of resources significantly.
Between 80 and 90% of the demand for water during the crops was covered with rainwater from the rainwater harvesting system of the building. This symbiosis also allows cultivating during both summer and winter with significant savings in the use of energy, as demonstrated in previous studies (Nadal et al. 2017). Winter crops have shown being less efficient than summer ones in the use of resources due to the lower production (increasing the use per kg of product). For instance, the winter crop conducted consumed 104 L/kg of tomato whereas summer crops consumed 47 and 64 L/kg.
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The i-RTG can contribute to reduce the environmental impacts of food production.
The environmental performance of the system is better than that of conventional production for all the impact categories analysed (except marine eutrophication) thanks to the lower needs for packaging and transportation. For instance, a summer crop in the i-RTG generates 0.58 kg of CO2 eq per kg of tomato whereas a conventional greenhouse generates 1.7 kg of CO2 eq, which proves that it can contribute to climate change mitigation.
The most important contributors to the environmental impacts of the i-RTG are the use of fertilisers, accounting for more than 25% in four of the six impact categories, and the infrastructure of the i-RTG (structure and rainwater harvesting system), accounting for between 30 and 60% of the impact. In this sense, the application of this technology to an industrial scale should consider the optimisation of the infrastructure and the adjustment of the fertilisers used to improve the environmental performance of such systems. For instance, applying available software for the optimisation of the size of the rainwater storage tank might prevent significant environmental impacts. Moreover, a proper follow-up of the concentration of nutrients in the nutrient solution and the leachates can allow adjusting the nutrients supplied to improve the efficiency of the crops.
Thus, the analysis of the nutrient flows of the crops (Chapter 7) is a central issue for improving the efficiency of the i-RTG and hydroponic crops in general. In average, 51% of the nutrients supplied to the crops end up in the leachates, whereas a significant part is uptake by plants and remains in the biomass (26%) and the fruits (production) (11%). However, these percentages are very different depending on the specific nutrient.
The perlite used for cultivation of hydroponic crops retains substantial amounts of nutrients, in particular for nitrogen (in average, 6% of the incoming nutrient), phosphorus (7%) and calcium (5%). This factor must be considered when evaluating the nutrient balance and for the management of the substrate, which could be a potential by-product at the end of life (for instance, for amending soil-based crops). Other substrates such as rockwool or coir are pending to be assessed, but they might have similar retention rates.
Although the balances for the different nutrients were not completed, a significant improvement was observed in relation to recovery values seen in the previous literature, representing the first comprehensive attempt to close the nutrient balances in open hydroponic crops. The measures that should be undertaken in future projects according to the research conducted are summarized in Table 10.2.
Future studies should provide further insights in this regard and try to get closer to a full balance.
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Table 10.2 Summary of the measures proposed for future i-RTG projects.
Subsystem Measures
Nutrient solution and leachates
A periodic follow-up should be conducted to adjust the nutrient solution to the requirements of the crop.
Recirculating or reusing the leachates utilising the nutrients and avoiding the discharge to nature.
Biomass and production
Maximise the solar radiation reaching the crop to improve productivity and reduce biomass.
To compost bio-waste generated in the crop.
To fix the biogenic carbon in biomass using it as a by-product for manufacturing goods.
Infrastructure Optimising the dimensions of the infrastructure, for instance, the size of the rainwater tank.
Nutrient retention in perlite
Reusing the substrate as a byproduct, for example for the amendment of soil-based crops.
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