The Official E-Newsletter of the Institution of Engineers Sri Lanka   |  Issue 51 - February 2021

Reusing / recycling water as An engineering strategy in green buildings



The Earth's surface is surrounded by water by seventy-one percentages (71%), however, approximately only three percentages (3%) of this water are freshwater. However, unfortunately, only a tiny portion of water (about 0.3%), is even consumable by humans. In the current context, water conservation has developed into a serious issue with the rapid population increase.

Reducing the usages of the planet’s resources, especially water usage is one of the main purposes of the Green buildings around the world. With the rising of water demand, water conservation through reusing/ recycling water has become an important strategy in green building practices.

Various water recycles and reuses efficient technologies such as treating grey water and black water, HVAC Condensate recovery water, and rainwater harvesting are being implemented around the world. Given the scope of terms associated with water efficiency in green buildings, Green Building Council of Sri Lanka (GBCSL) provides several conditions that describe key elements in the design and implementation of water reuse/recycle strategies, thereby, the precious water can be saved economically in a sustainable environment and will benefit the world in terms of social, economic, and environmental so that facing of thread in water scarcity by the future generation shall be eliminated.

This article overviews reusing/ recycling water as a strategy in green building practices to save water and preserve the environment.


The concept of reusing/recycling has progressively become one of the most significant strategies as in green building practices. Due to the global challenge of a water scarcity scenario, many countries and administrations have raised their awareness of water shortage risks and proposed effective strategies on water reuse/recycle.

As a portion of natural water processes, all water is recycled and reused, for instance, the hydrologic cycle. Likewise, man-made water reusing/recycling, centres on using rainwater harvest water, treated greywater and treated black water. Recycling of water can increase the range of water supplies, increase water quality, reduction in the discharge and disposal costs of wastewater, and save the usage of energy.

In addition to the above benefits, water recycling/reuse provides incredible environmental benefits. By providing a supplementary source of water, water reuse/recycling can benefit to find ways to reduce the diversion of water from sensitive ecosystems. Some other benefits consist of decreasing discharging of wastewater and reducing pollution. Further, there are many green buildings which are reusing recycled water as an alternative source of water for a variety of not-potable uses including:

  • Landscape irrigation in buildings
  • Decorative ponds and fountains
  • Indoor uses such as toilet flushing and urinal flushing and greywater applications.
  • Air condition cooling tower make-up
  • Groundwater recharging

Reusing/ Recycling Water as a Strategy

There are four water reusing and recycling strategies listed below, to reduce the dependence of buildings on potable water supply provided by utility organization (i.e.; NWS&DB in Sri Lankan context)

  • Rainwater Harvesting
  • Greywater Treatment and Reuse
  • Blackwater Treatment and Reuse
  • Reusing of HVAC Condensate recovery water

Strategy #1 – Rainwater Harvesting:

Rainwater is a free source and can be collected in a considerable quantity from roof catchments and other pavement areas which can be used for various purposes. (Muhammad Muhitur Rahman, et al., 2019)

Harvesting rainwater from the building roof and stored in water tanks facilitates that water to be re-used in toilets, urinals, air condition’s cooling towers, laundries, hot water systems and for watering gardens and landscapes in a building. There is rapid demand increase for harvesting rainwater from buildings, (as rainwater doesn’t require much treatment before consumption) such as shopping complex, residential complex, schools and hotels which have considerable roof print area, from which significant rainwater quantity shall be harvested to meet the daily water demand of the building.

Strategy #2 – Greywater Treatment and Reuse

The idea of greywater recycling and reuse has gradually become one of the most important engineering strategies in buildings across the globe. Engineers are compulsory to rethink 21st-century water supply and management concepts. It is meaningless to expect unlimited water supply from nature since aquifer and surface water depletion from ever-growing demand.

Greywater recycling helps reduce demand for potable water from drinking water treatment plants and also decreases the load on wastewater treatment plants. Which can results in reducing of supply from water service providers (ie: NWS&DB in Sri Lanka) it leads into tens of thousands rupees saved on water bills over a year.

In the technical aspect, a considerable level of treatment is necessary, if treated greywater utilized for toilet flushing, laundry, cooling tower make up, and car washing. There are a variety of technologies that treat greywater for the above types of uses. For water to be reliably used indoors without causing complications over time, it must be treated to get rid of substantial biological growth after treatment.

Strategy #3 – Blackwater Treatment and Reuse

Blackwater can be recycled in green buildings through a small-scale on-site wastewater treatment plant. On-site treatment is generally by a process of settlement, bacterial breakdown, filtration, aeration, and chemical treatment. Typically this re-use is for applications such as watering home garden landscape and for flushing toilets. It is also possible, although very expensive, to make black water suitable for drinking as done in countries like Singapore.

Recycling and reusing the black water provides many benefits such as water use reduction, energy conservation and reduce utility bills, reduce loads on country’s sewerage system and contribute to sustainability standards such as GreenSL® and LEED.

Strategy #4 – Reusing of HVAC condensate recovery water

In a usual heat, ventilation, and air conditioning system (HVAC), humid, warm air from a building is run over a cold air handler that cools the air. When this process is done, condensate water is produced and recovered for reuse.

Condensate is considerably good water — as pure as distilled water with low in mineral content, often, no special treatment required and can be utilized for several purposes such as irrigation, cooling tower make up water, ornamental fountains and ponds and even flushing toilets.

Even though harvesting condensate from commercial HVAC systems is not new to the world, GreenSL® and LEED certification is driving renewed interest in it. In the modern world, being green is imperative from a marketing public relations point of view, but that is not the only motivation for green building owners.


Implementing reusing/recycling water as an engineering strategy in green buildings not only will save water but also will throwback with the financial, and economic benefits along with preserving the environment for our next generation.

On a concluding note, implementing reusing/recycling water as an engineering strategy in green buildings will contribute to move from the traditional triple bottom line concept (weak sustainability model) to the strong sustainability model, in which the environment is the prime domain. Finally, these water strategies will boost the social and economic values of the buildings which are subsets of the environment.


BSc.Eng.(Hons), MBA, CEng., MIE(SL) Chief Engineer
National Water Supply & Drainage Board





Facebook Twitter LinkedIn Email Print