Research on a zero water consumption operation model and feasibility for office buildings Open Access

Toward sustainable development goals (SDGs) and the goal of achieving net-zero emissions by 2050, Taiwan officially unveiled its ‘Taiwan 2050 Net-Zero Emissions Pathway and Strategy Overview’ in March 2022. This comprehensive plan outlines development objectives for each phase of the planning period and the promotion strategies for different sectors. In January 2023, the Climate Act was passed, which includes a net-zero emissions target for 2050. In the field of architecture, for example, the United States, Canada, the United Kingdom, Japan, and other countries (International Energy Agency) are moving toward the goal of zero- or near-zero-energy buildings. The concept of zero- or near-zero-energy buildings aims to reduce energy consumption and carbon emissions while promoting sustainable development. In fact, water efficiency is like energy efficiency. Water efficiency is a key metric in evaluating building sustainability (Sharma et al., 2009). Across the globe, water security is already being directly and indirectly affected by climate change. The effects of climate change on water security vary significantly both within and between regions due to varying levels of vulnerability (Ayanlade et al., 2023). According to the 2019 World Water Development Report, it is projected that by 2050, over 2 billion people will reside in regions experiencing severe water shortages, and approximately 4 billion people will endure severe water shortages for at least one month annually (Niu et al., 2022). Water usage in buildings represents a significant portion of water resource consumption and carbon emissions. In recent years, water scarcity issues in Taiwan have become increasingly apparent and severe despite Taiwan having an annual rainfall of about 2.5 times the global average. In response to these environmental issues, it is essential to adopt sustainable practices in building design. Adopting sustainable practices such as rainwater harvesting and greywater recycling can reduce water usage in buildings while promoting SDGs and global sustainability (Cheng et al., 2016). Zero water consumption buildings are characterized by low water usage neutralized by rainwater harvesting systems or regenerated water in building design. They can achieve self-sufficiency through rainwater and reclaimed water circulation systems. Offsetting between these sources can facilitate water consumption approaching zero (Pimentel-Rodrigues & Silva-Afonso, 2019). These buildings primarily follow two main directions: sourcing and conservation. Sourcing involves using reclaimed water, such as rainwater and treated wastewater, to offset the original water usage (e.g., flushing toilets and irrigation), achieving the goal of recycling. On the other hand, conservation entails using water-saving devices such as toilets, urinals, and faucets, to fundamentally reduce water consumption. The present study adopted office buildings as the research subject due to their crucial water usage characteristics. The research was conducted to investigate water consumption in these buildings and assess the viability of utilizing performance-based data for the establishment of a water benchmarking system (Bint, 2012). A water-use estimation model was developed by conducting a literature review and collecting data from existing green office building cases in Taiwan.

Studies on water reuse around the world indicate that the successful implementation of new water reuse technologies is heavily dependent on the establishment of robust institutional arrangements (Marks & Zadoroznyj, 2005; Bixio et al., 2006; van Lier & Huibers, 2010; Marome & Pholcharoen, 2019; Wakhungu 2019). To achieve the goal of zero water consumption in buildings, it is necessary to focus on three main areas: equipment water usage, additional water usage, and compensatory water usage. Analyzing water usage rates in these areas is crucial. Besides equipment water conservation, rainwater harvesting, and the use of recycled water will also play key roles in determining whether a building can achieve zero water consumption.

A zero-impact building aims to achieve optimal efficiency in managing combined resources and maximizing the generation of renewable resources. The building's resource management focuses on the practicality of utilizing renewable resources such as energy and water, while striving for a closed-loop system that minimizes overall material and land use (Attia, 2016). A near-zero water system is characterized as a water and wastewater management system that operates within its designated service area, ranging from individual residential lots to expansive urban water districts, without significant withdrawals or releases of water outside this defined boundary (Englehardt et al., 2016). The implementation methods for buildings with near-zero water consumption should comprehensively address both water-saving measures and water reclamation systems (Li Cheng & Kawamura, 2023). Effective strategies include the integration of high-efficiency fixtures and appliances to minimize water use, as well as the adoption of advanced technologies for capturing and treating greywater and rainwater. Additionally, the design should incorporate systems for on-site water treatment and recycling, enabling the reuse of water for non-potable purposes. Rainwater and greywater are two common alternative water sources that can be reused in buildings because several water uses do not require high-quality water, such as toilets (Penn et al., 2013), urinals (Sahin & Manioğlu, 2019), and irrigation (Devkota et al., 2015; Unami et al., 2015; Fonseca et al., 2017). These efforts collectively contribute to reducing the overall water footprint of the building, ensuring sustainable water management practices are in place. By prioritizing both conservation and reclamation, buildings can achieve near-zero water consumption and significantly lessen their environmental impact.

In the literature review and practical applications, the estimation of building water usage is frequently associated with the scale and occupancy of the building. Typically, building water usage is estimated by converting the building floor area into an equivalent number of occupants, and then designing and calculating water facilities based on this occupancy rate (Li Cheng & Kawamura, 2023). The annual water usage per unit of building area (m³/m²·year) serves as the benchmark for water consumption. We established formulas, and calculations were performed accordingly. The information was derived from a publication by the Society of Air Conditioning, Heating, and Sanitary Engineers in Japan in 1997. The standards for personnel density in building areas primarily reference the (ASHRAE 2004) and the parameters for estimating water usage in office building spaces were standardized through the California Title 24 Alternative Calculation Method. The parameters for calculating water usage in office building spaces are illustrated. The building unit area annual water usage (water-use intensity (WUI), m³/m²·year) calculation was estimated through the average building area density (P_di, people/m²), the baseline for annual water usage per person (Q_wi, m³/person), and the facility utilization rate (F_ri). The standardized annual water usage per unit area for office buildings was determined to be 2.63 (m³/m²·year), as shown in Table 1.

The usage of reclaimed water is an effective choice worldwide for the conservation of water resources, which reduces effects on the environment in addition to reducing the expenses and energy needed for water source management (Takeuchi & Tanaka, 2020). One of the alternatives that can be practiced is to use reclaimed water. Water reuse entails repurposing cleansed wastewater for usable purposes (Bachi et al., 2023). In recent years, rainwater harvesting has become a common method for buildings to recycle water resources. This method provides a substantial alternative benefit that surpasses other sources of recycled water with the characteristic of relatively lower installation costs. Rainwater collection and utilization systems are employed in different settings, including buildings, parks, and green spaces, based on roof and ground collection methods. Typically, buildings are more suitable for roof-based collection methods, whereas larger areas such as parks and green spaces often require ground-based collection strategies. According to the annual average rainfall data from the Water Resources Agency of the Ministry of Economic Affairs in Taiwan from 1949 to 2020, the average annual rainfall is 2507 mm. Each square meter of rainwater collection area can collect approximately 6.87 L of rainwater per day. Adopting rainwater harvesting is gaining increasing attention in building practices, underscoring the growing importance of achieving zero water buildings. The primary sources of recycled water in office buildings encompass toilet flushing, plant irrigation, cleaning, air conditioning, cooling water circulation, fire protection, and other uses. Encompass five primary components: collection, conveyance, purification, storage, and power. However, the water sources collected by water recycling systems often have higher pollution levels. As such, the purification equipment associated with these systems is generally more intricate and expensive than rainwater recycling systems. Despite the increased complexity and cost of designing a water recycling system, these systems offer the advantage of a consistent water recovery volume unaffected by weather conditions. This feature makes it suitable for deployment in various locations, including residential areas, educational institutions, office buildings, and precision factories. Incorporating water recycling systems into building design is crucial to achieving near-zero water consumption objectives.

This study categorized water usage and conservation in office buildings into three groups according to the building's current design: equipment water usage, additional water usage, and compensatory water usage. Subsequently, we developed assessment formulas and calculations for water usage and conservation in office buildings and calculated the daily equipment water consumption per person for 73 office buildings, which were computed based on this classification. Additionally, the potential for achieving zero water consumption was investigated using calculation methods, as shown in Table 2.

Water consumption in office buildings primarily involves activities such as toilet flushing, handwashing, cleaning, and drinking water. Water fixtures include faucets, urinals, toilets, water dispensers, and similar equipment. In accordance with water usage habits, this study employed a flushing duration of 10 s multiplied by four times for each toilet flush. The water usage for dishwashing and cleaning was also considered based on personal habits, and handwashing was set at 1.5 min. Regarding water dispensers, considering variations in personal drinking habits and working hours, this study used half of the daily recommended water intake of 2 liters per person, set at 1 liter per person per day. In addition to regular water dispensers, a reverse osmosis (RO) water dispenser was included in the settings with a wastewater generation rate of 1:2.5. The produced wastewater can be recycled for cleaning, toilet flushing, and irrigation.

According to the nature of office buildings, water usage can be divided into the following categories: irrigation for green landscaping, water usage for landscape ponds, and cooling circulation water for central air conditioning. However, many office buildings do not include design plans for swimming pools. In addition to the replacement water for the pool itself, the overall water consumption of a swimming pool encompasses users' bathroom facility usage behaviors. Many existing studies regard this as independent water usage. Therefore, this study excluded swimming pool spaces from examining water usage in office buildings.

• W_g  is the annual irrigation water consumption per building unit area (m³/m²·year), A_g  is the planting area (m²), C_g  is the daily irrigation volume per square meter (m³/m²), S_g  is the irrigation water conservation rate, A_f is the building floor area (m²).

The essential water facilities required for daily use include toilets, urinals, water taps, and drinking fountains. The designed range for the annual water consumption per unit area, known as the WUI, was between 0.72 (m³/m²·year) and 1.79 (m³/m²·year). The median actual water consumption was 0.86 (m³/ m²·year), and the median simulated water consumption was 0.88 (m³/ m²·year). This indicates a close alignment with the designed minimum water consumption. There was a high adoption rate of water-saving equipment in green office buildings, often resulting in water consumption closely approaching the designed minimum with minimal effort. However, if the water consumption of the equipment exceeds the median despite the use of water-saving equipment, a reevaluation of the design would be necessary, as shown in Figure 5.

Without additional water usage and using only basic water-based facilities such as toilets, urinals, faucets, and water dispensers, the estimated water consumption was approximately 0.72 cubic meters per square meter per year (m³/m²·year). However, due to the limitations set by Taiwanese regulations, recycled and reclaimed water cannot replace water that comes into direct contact with human bodies. This research has established the criteria for determining whether ‘recycled and reclaimed water can be used as a substitute (compensatory water)’ and presents the corresponding data in Table 3. By using minimal equipment, water consumption of 0.24 (m³/m²·year) can be replaced with recycled and reclaimed water. With a median value of 1.23 (m³/m²·year) for available additional water, it is feasible to fully supplement water usage and have a substantial surplus of compensatory water for other purposes. However, the remaining 0.48 (m³/m²·year) cannot be replaced due to the current regulatory restrictions, as shown in Table 3.

Currently, the water conservation rates in green office-type buildings range from 26 to 76%, with a median of 53%. This finding indicates a substantial deviation from the ideal 100% conservation rate, which signifies zero water consumption. The annual water consumption benchmark for office buildings is 2.63 (m³/m²·year). The present study developed a refined water usage to calculate the water consumption of various water-use items in office-type green building cases. Recent trends demonstrate that green office-type buildings, through strategic design and water-saving equipment, exhibit a median value of 0.88 (m³/m²·year) for basic equipment water consumption. When considering all additional water consumption items, the median value for compensatory water usage is 0.8 (m³/m²·year), indicating significant water savings compared to the baseline. Further analysis suggests that, with advancements in water-saving technologies, the water consumption of basic equipment in office buildings could be reduced to 0.72 (m³/m²·year). However, due to Taiwan's regulations restricting the use of reclaimed water for human contact, the remaining water used by equipment and any additional water can be offset by increasing the reusable water compensatory rate, effectively achieving nearly zero water consumption. The potential inclusion of a recycled drinking water filtration system could enable buildings in the same area to collectively utilize a shared water purification plant in the future. Designing for equipment water usage combined with the median additional water used by current office buildings appears to be sufficient for compensation in current designs. The water consumption of office buildings can be considered as the median of the water consumption of equipment plus the additional water used by office buildings. However, there is a need to increase the rainwater collection area, adopt non-water-cooled central air conditioners or water-saving air conditioners, and implement measures such as smart watering to reduce additional water consumption or augment available water for compensatory purposes.

The study analyzed equipment water usage, additional water usage, and supplementary water usage, laying the groundwork for potentially transforming office buildings into zero water consumption structures. Generally, building water conservation has primarily focused on the water usage of building equipment. However, office-type buildings involve additional elements such as irrigation and cooling tower water. Mitigating the usage of these components is crucial for achieving the zero water consumption goal. This study redefined and integrated the water estimation method for office-type buildings, considering maximum, minimum, and design water consumption. This enables a more precise assessment of water conservation rates.

This study systematically explored methods for calculating water efficiency and zero water consumption approach in office buildings, drawing from existing literature on water conservation. The research aims to develop a refined water usage to calculate the water consumption of various water-use items in office-type green building cases. The water conservation rates in green office-type buildings range from 26 to 76%, with a median of 53%. This finding indicates a substantial deviation from the ideal 100% conservation rate, which signifies zero water consumption. The annual water consumption benchmark for office buildings is 2.63 (m³/m²·year). Despite their classification as office-type buildings, their baseline water usage can fluctuate due to additional water consumption. Using water conservation rates enables a meticulous determination of a building's water efficiency and the extent of its water-saving capabilities. The feasibility of zero water buildings was validated and determined the crucial operation for the zero water consumption goal. Consequently, design engineers can employ this methodology to compute water conservation rates for their designs, aiming for the construction of a zero water consumption building.

The authors thank the Ministry of Science and Technology of Taiwan (MOST 109-2221-E-011-034) and the Ministry of Interior ABRI (10011AV1331) for financially supporting this study.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.