Water Conservation and Management: A Vital Flow in Green Construction

Water conservation and management constitute a vital flow in green construction, given that buildings consume 12-15% of global potable water (UNEP, 2022). Average water consumption in a Spanish office building is 50-80 l/person per day, and in residential buildings 120-150 l/person per day (INE, 2022). The first strategy is reduction at the point of use through low-flow sanitary fixtures: dual-flush toilets at 3/6 liters (compared to 9-12 l for conventional models: 50-67% savings), faucets with aerators and flow limiters (5-6 l/min compared to 12-15 l/min: 50-60% savings), low-flow showerheads (6-9 l/min compared to 15-20 l/min: 50-55% savings), and waterless or 0.5 l/flush urinals (compared to 3-4 l: 85-100% savings).

The LEED WE (Water Efficiency) standard uses the concept of a baseline (reference consumption calculated with conventional fixtures per UPC/IPC) and requires a minimum reduction of 20% (prerequisite) for certification, with additional points up to 50% reduction (5 points). BREEAM Wat 01 awards up to 5 points for potable water consumption reduction: level 1 (12.5% reduction), level 2 (25%), level 3 (40%), level 4 (50%), level 5 (55%). The WaterSense label (EPA, USA) certifies sanitary fixtures that use 20% less water than the federal standard, and in Europe, the WELL Water (W) v2 label requires potable water filtration and measurement of turbidity (<0.5 NTU), lead (<10 ppb), and THMs (<80 ppb).

Rainwater harvesting captures precipitation on roofs and impervious surfaces for non-potable uses (irrigation, toilets, cleaning). The harvesting potential in Spain varies by climate zone: 800-1,200 l/m2 per year in the north (Galicia, Cantabria), 400-700 l/m2 per year in the central plateau, 200-400 l/m2 per year in the southeast (Almeria, Murcia), and 500-800 l/m2 per year along the Mediterranean coast (Barcelona, Valencia). A 1,000 m2 office building roof in Madrid (400 mm/year precipitation) can capture 320,000 l/year (80% capture efficiency), covering 30-50% of toilet and irrigation demand.

The harvesting system includes: a first-flush diverter (diversion of the first 2 mm of rainfall that carries roof contaminants), a storage tank (underground or in basement, with 0.1 mm mesh filter and overflow system), a pressure group, and UV treatment if used for toilets (required by RD 1620/2007 on water reuse in Spain). The cost of a rainwater harvesting system for a 5,000 m2 office building is 15,000-30,000 EUR, with a payback period of 5-10 years depending on local water prices (1.5-4.0 EUR/m3 in Spain). The Bullitt Center (Seattle, 2013, Living Building Challenge) uses rainwater as its sole source of potable water (95 m3/year captured, filtered, and treated to potable quality through slow sand filtration + UV + chlorination), demonstrating the technical viability of a water self-sufficient building.

Greywater (from sinks, showers, and washing machines, excluding toilets and kitchens) represents 50-70% of domestic wastewater flow and can be reused for toilets and irrigation after basic treatment. An office occupant generates 20-30 l/day of greywater (sinks), which when treated by membrane bioreactor (MBR) or constructed wetland reaches sufficient quality for toilet flushing (turbidity < 10 NTU, E. coli < 100 CFU/100 ml per RD 1620/2007). The potential savings amount to 30-40% of potable water consumption.

Reclaimed water (wastewater treatment plant effluent treated to reuse quality) is an alternative for landscape irrigation and industrial uses, regulated in Spain by RD 1620/2007 (quality 2.1 for garden irrigation: E. coli < 200 CFU/100 ml, SS < 20 mg/l) and EU Regulation 2020/741 (quality requirements for agricultural reuse). The cost of tertiary treatment for reuse is 0.20-0.50 EUR/m3, compared to 1.5-4.0 EUR/m3 for mains potable water. In Spain, 500 hm3/year of reclaimed water is reused (10% of treated flow), making it the 4th country worldwide in reuse after China, the USA, and Israel. Reference projects: the City of Arts and Sciences (Valencia) uses 100% reclaimed water for its ornamental lakes (8 hm3/year), and the UPC campus (Barcelona) irrigates with reclaimed water from the Prat de Llobregat wastewater treatment plant.

SuDS (Sustainable Urban Drainage Systems) -- known as LID (Low Impact Development) in the USA and WSUD (Water Sensitive Urban Design) in Australia -- manage stormwater runoff at source through infiltration, retention, detention, and natural treatment. The main techniques are: permeable pavements (open-joint pavers, porous concrete, gravel: infiltration rate of 50-200 l/m2 per hour), rain gardens (vegetated depressions that filter and retain water: retention of 50-100 l/m2), green roofs (retention of 40-80% of precipitation depending on substrate depth), and infiltration swales with vegetation.

In Spain, Royal Decree 1/2024 amending the RDPH (Public Water Domain Regulation) introduces at-source stormwater management requirements for new developments, aligning with European practice (SuDS Manual, CIRIA C753, 2015). The cost of implementing SuDS is 20-50 EUR/m2 of treated surface, with additional benefits: urban flood risk reduction (20-40% peak flow reduction), improved urban biodiversity, reduced urban heat island effect, and aquifer recharge. LEED v4.1 SS (Rainwater Management credit) requires managing the 95th or 98th percentile of daily precipitation through SuDS, and BREEAM Pol 03 awards points for sustainable drainage management that limits runoff flow to pre-development rates.

Xeriscaping uses plant species adapted to the local climate with minimal irrigation needs. In the Spanish Mediterranean, native species (rosemary, lavender, thyme, mastic, dwarf fan palm, olive) require 0-2 l/m2 per day of irrigation (or none once established), compared to 5-8 l/m2 per day for conventional lawn (Lolium perenne/Festuca arundinacea). Savings in outdoor irrigation can reach 50-100% with comprehensive xeriscaping. LEED WE (Outdoor Water Use Reduction credit) awards 2 points for a 50% irrigation reduction and 4 points for total elimination of potable irrigation.

Integrated water cycle management in buildings combines all strategies: low-flow fixtures (40-60% reduction at point of use), rainwater harvesting (15-40% of total consumption), greywater reuse (20-30% additional), SuDS for runoff management, and xeriscaping to eliminate potable irrigation. The Bullitt Center achieves a net potable water consumption of 0 l/person per day (100% self-sufficient). The Pixel building (Melbourne, 2010, Studio505) consumes 60% less water than the Australian baseline through rainwater harvesting, MBR-treated greywater, and vacuum toilets (1 l/flush). In Spain, the objective of the PERTE for water cycle digitalization (3 billion EUR, 2022-2026) is to reduce distribution network losses from 25% to 15%, complementing building-level efficiency strategies.