Objective:

To present the results and impacts of a prototype for harvesting rainwater in a single-family house in Bogota (Colombia), designed to save both potable water consumption as well as the embodied energy required to produce and distribute it from the city water system.

Context:

The city of Bogota has a bimodal rainfall regime with multiannual monthly minima above 30 mm, and an average above 60 mm. This high rainfall intensity and frequency constitutes a potential hydric and energetic resource, which is available to the region during most of the year, and could be extracted through new systems for rainwater collection and use (referred to as SCALLs, for its acronym in Spanish). In this article, we will focus on the initial design of a rainwater harvester prototype with a capacity for one family. This prototype has been designed to operate under the high rainfall potential of the city, in order to store and deliver water efficiently to a household connected to the main local water provider (the Aqueduct Company of Bogota, EAB in Spanish). The water supplied by the prototype is enough to substitute the consumption of tap water for the usage of toilets, washing machine, watering of gardens, and cleaning of tanks and general surfaces. In comparison to the water provider, the rainwater harvester prototype is competitive with the quantity of water it delivers, and with the reduced consumption of energy required to supply it.

Method:

To start the analysis of potential energy efficiency, we considered rainwater as a composition of two distributed resources: the water itself, and its embedded energy. Thus, rainwater has the potential to both replace the tap water consumption and reduce the energy service provided by the centralized water supply of the EAB. To evaluate the efficiency of this substitution, we generated a new 5-step method that allowed us to define the baseline efficiency as a performance projection of the centralized tap water service, and then compare it to the performance improvement of the SCALL prototype. We found that the main performance indicator is the comparative energy intensity, which enables the calculation of multiple benefits of energy efficiency.

Results:

The SCALL prototype, in the initial stages of its implementation, achieved savings of 25% in drinking water used in the home, 26% of embedded energy and a reduction of greenhouse gases by 27%. This was obtained with only 22.4% of the nominal design capacity of the prototype, in a study period of 56 months. In this period, it was possible to recover the investment from the economic savings resulting from the substitution of drinking water for rainwater. The results suggest that the EAB could develop an efficient SCALL implementation program by its users, which would allow it to obtain close to an income and monetary savings that would represent close to 2% of the total income that each residential SCALL would produce. The degree of success of which would depend on the massification of the program. This without accounting for the benefits that would be obtained by deferring investments for infrastructure expansion, security of supply and reliability provided by the implementation of efficient SCALLs.

Conclusions:

The results of the energy efficiency evaluation show the technological, economic, environmental and social viability of the SCALL prototype of a residential user in Bogotá. At the same time, we propose a new methodology for evaluating the energy efficiency potential from a distributed resource such as rainwater, allowing scrutiny of its multiple benefits compared to a centralized service supplier. For the Capital District of Bogotá, it would be beneficial (socially and environmentally) to develop a policy to promote SICCALL to efficiently manage the demand for water and energy. This policy could be made possible by the development of collaborative network economies, around distributed water and energy resources, in order to meet the objectives of sustainable development and citizen participation, of the so-called smart cities.