How to Choose the Right Components for a Commercial Rainwater Harvesting System

June 5, 2019

Rainwater harvesting is a practice of collecting, storing and treating rainwater to be used as an alternative water source and has been used since ancient times throughout the world. Today, rainwater harvesting is becoming a vital part of commercial buildings due to increased interest in potable water conservation and regulatory emphasis on stormwater management compliance.

As simple as it might sound, designing a rainwater harvesting (RWH) system that actually works can be challenging. A RWH system that performs successfully in one site may completely fail if installed in another site.  Rainwater harvesting system designs and required components vary in complexity and sophistication. The level of complexity depends on many factors including source water quality, water budget (demand and supply), and the desired use for the rainwater and applicable regulations. It is crucial to have an effective and rigorous design process that addresses all these factors and allows accurately budgeting for a rainwater harvesting system.

Some considerations when designing and budgeting a commercial rainwater harvesting system include:

Water Quality Considerations

The main objective of any rainwater harvesting system is to treat the source water to match with the water quality requirements of the desired end use by incorporating appropriate treatment components into the RWH system.

Source water quality is a site-specific parameter and varies from one site to another.  Quality of the collected rainwater is affected by the drainage area composition, type of the activities within the drainage area, surrounding land use, and local rainfall patterns.  A complete site analysis must be conducted to determine the common pollutant found in the site and associated impact on source water quality. The source water quality will determine both the type and level of treatment that is required to achieve the desired end use water quality criteria. Water quality requirements for non-potable uses are application specific and are determined based on the potential risk to human health and the environment. Unfortunately, water quality standards, plumbing and building codes provide only a general direction on minimum performance criteria for rainwater harvesting systems for non-potable applications and don’t adequately address the risk associated with rainwater reuse. It is important to characterize and assess potential risks related to the site and end-use application, and ensure the risks are correctly managed through system design, operation and maintenance.

Water Budget Analysis Importance

Another important step when designing a rainwater harvesting system is water budget analysis. Water budget analysis helps to estimate the size of the rainwater system based on the site’s water demand and supply, rainfall data and local climate factors.  Accurate estimation of water supply and demand is critical to achieve the intended water saving and maximize the overall cost-effectiveness of a project.

Rainwater Treatment Systems

A rainwater treatment system consists of the treatment equipment, pumps, a control system.

Treatment Equipment

The design of the treatment process and selection of the treatment equipment for a RWH system will depend both on the quality of source water and the intended end use.  A typical rainwater treatment system includes pre-treatment and post-treatment steps. Pre-treatment step is used to remove debris and coarse solids before water being delivered to the storage tank.  The level of pre-treatment is based on the quality of the source water. Typically, runoffs collected from the ground surface and green roofs require more stringent pre-treatment than runoff collected from hard surfaces.  Post-storage treatment is usually more sophisticated and can be comprised of biological or chemical treatment, advanced filtrations and disinfection processes. Post-storage treatment is used to remove dissolved pollutants such as metal and organics, microorganism, colour, hardness or turbidity from harvested rainwater.  Selection of the right equipment for each treatment process not only depends on of the equipment’s specifications and limitations but also requires comprehensive knowledge and understanding of the whole treatment process and strategy, as well as the system maintenance requirement.


The proper pump selection is critical to ensure optimal performance of a RWH system. They are required to deliver flow from the cistern to the RWH system components and to the point of use. The choice of pump is dependent on many factors including required flow and pressure, cost, system configuration, space constraints, ease of operation and anticipated maintenance.

Control System

A rainwater harvesting control system consists of a programmable logic controller (PLC) that receives real-time information from various instruments such as valves and sensors, provides equipment status, alert/alarms for maintenance, and responds to specific operating conditions automatically based on site specific customized programming.  Proper and precise programming is critical for the successful operation and maintenance of a RWH system and is based on a thorough understanding of the treatment process, sequence of operation, and equipment capabilities.

Codes, Regulations, and Standards

Similar to any major plumbing work, a RWH system must be installed in accordance with applicable codes and regulations and must receive the necessary permits for the work to be performed.  Lack of compliance with the required codes and regulations could result in construction delay and legal risks, which could increase overall project cost. Designers should familiarize themselves with the requirements of the legislation that apply to RWH systems, including relevant sections of the Building Codes, the Electrical Safety Codes, the Fire Codes and health and safety regulations.

Additionally, although not in force, newly published standard CSA B805-18/ICC 805-2018 regulations addresses rainwater harvesting system design and installation as an alternative source of water for both potable and non-potable applications. This standard covers all aspects of design, installation, water quality considerations, maintenance, and risk management of rainwater harvesting systems for residential and commercial applications. For RWH system designers, CSA B805-18/ICC 805-2018 will help to outline the goals and requirements of a properly designed Rainwater Harvesting System.

From Cooling Towers to Rainwater Tanks, Effluent Pumps to Rainwater Cisterns – How to Design Your Commercial Rainwater Harvesting System

Effectiveness of a RWH system depends on appropriate design of the system.  Each rainwater harvesting system is site-specific, and it is not possible to give a generalized design.  There are lots of elements to consider in addition to selection of the RWH system components.  The omission of certain key elements can lead to poorly functioning or nonoperational systems, and/or under-estimating budgets.  Net Zero Water has developed a guide to help planners, architects, civil engineers, mechanical engineers, and design-build contractors to understand the important design elements of a rainwater harvesting system and adequately budget for it.

This in-depth guide to budgeting for your commercial rainwater harvesting system provides:

  • an introduction to rainwater harvesting and the important role it plays in water resource management,
  • an overview of the components of a commercial rainwater harvesting system,
  • steps to budgeting for a rainwater harvesting system including a budgeting worksheet, and
  • an outline for implementing a commercial rainwater harvesting system.

Download the Guide to Budgeting for a Commercial Rainwater Harvest System.