Systems Modeling for the Planning of Greywater Management in New Housing Developments in the City of Ensenada, Baja California, Mexico

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ABSTRACT
The implementation of a greywater reuse scheme for new housing developments in Ensenada would achieve significant savings (up to 37%) in water consumption with the corresponding monetary savings in water supply and wastewater treatment. New housing facilities would have green areas irrigated with greywater which would represent ecological and aesthetical benefits to city. A more detailed economic valuation has to be undertaken in order to determine the optimum number of houses to be included in the greywater reuse scheme. The formal evaluation of greywater systems is important to show its potential value when implemented in a large scale (city-wide, new development and re-development areas). The findings of this study will serve as additional evidence to the potential cost and water savings of implementing descentralized systems such as greywater for fast-growing areas.

INTRODUCTION
Baja California (Mexico) and California (USA) share, among other things, a drought-prone climate and rapidly increasing water demands, particularly due to increases in population. In Baja California the mean annual precipitation is approximately 145 mm per year, compared to the national average of 771 mm per year (National Water Commission, Comisión Nacional del Agua – CNA, 2004).

The city of Ensenada, located approximately 100 km south of the Tijuana-San Diego area relies entirely on groundwater as its potable water source. According to the State Population Council (Consejo Estatal de Población -CONEPO, 2005) Ensenada has currently 269,500 habitants growing at an average rate of 2.26% with a projected population of approximately 415,350 habitants for the year 2030. Therefore, estimates by the local water utility (Comisión Estatal de Servicios Públicos de Ensenada - CESPE), stipulate that water demand can only be met until year 2007. Therefore, as in California, better water management alternatives need to be quickly assessed and developed in order to guarantee the supply of potable water to the city.

According to estimates by the State of Baja California Housing Plan (2001-2007) 3,250 houses per year will have to be built in order to meet current Ensenada’s population growth demand. This will have a significant impact on the water demands in the region and, if no measures are taken, these demands will seldom be reached. The evaluation of potential new sources of supply has centered in large scale projects such as aqueducts and ocean desalination facilities, but demand management approaches have been deemed secondary to solve the water supply problem.

For years greywater reuse has been identified as an option for decreasing the overall water consumption in in-building facilities (Eriksson et al., 2002). Greywater is defined as wastewater without input from toilets, ie wastewater produced in bathtubs, showers, hand basins, laundry machines and kitchen sinks and that can account for approximately 75% of the combined residential wastewater. Although 30% of the total household water consumption can be saved by reusing greywater for toilets flushing (Karpiscak et al., 1990), greywater systems have normally considered retrofitting the new greywater pipelines into existing buildings hence, the high construction costs have made them unfeasible in most cases. Therefore, the real potential for the reuse of greywater are new residential developments (Lillibridge, 2003), in order to include dual piping during the construction of the infrastructure. Full-scale greywater reuse systems with dual-plumbed houses and commercial sites for front and backyard landscaping in California (Powell and Megerdigian, 2003) have demonstrated its viability and importance in urban water planning.

Currently reclaimed water reuse and greywater reuse are not used as alternatives to potable water in activities where water quality is not of primary concern, such as toilet flushing and garden and landscapes irrigation. Moreover, for that very reason few new housing developments have gardens and other types of recreational areas, in order to decrease overall water consumption.

The main goal of this study was to develop a dynamic systems model to work as a simulation tool for the integrated water management of wastewater, greywater and potable water in new housing developments in a drought-prone city. The objectives were to compare the potable water savings between “conventional” systems (i.e. without greywater reuse) and “non-conventional” systems (i.e. with greywater reuse). A dynamic systems model was selected as the primary analytical tool of this study since this type of models have been used for decision making in water management planning and can be developed much faster than traditional engineering models. Integrated systems models have been used to measure the performance of policy alternatives in relation to a set of objectives and performance measures established by water managers and/or stakeholders (Lopez-Calva et al., 2001). Models can represent the relationships between the elements of the wastewater service function and the relationships between the wastewater, potable water, recycled water and greywater systems.

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