Rainwater Harvesting & Cisterns: Life Cycle Costs

A common Rainwater harvesting design configuration for residential use, with its various components shown. PEP Design Kempen.

Overview

Rainwater harvesting is the process of intercepting, conveying and storing rainfall for future use. The term “rainwater harvesting” (RWH) is used herein to refer to capturing rainfall onto rooftops (i.e. “roofwater”) and stormwater runoff from the ground surface (i.e. “stormwater”), and then reusing for irrigation or other greywater-approved uses. The rain that falls upon a catchment surface, such as a roof, is collected and conveyed into a storage facility. Storage tanks, a common method for storage, range in size from rain barrels for residential land uses (typically 190 to 400 litres in size), to large cisterns for industrial, commercial and institutional land uses. A typical pre-fabricated cistern can range from 750 to 50,000 litres in size. STEP has prepared a life cycle cost estimate for a 50,000 L below-ground concrete cistern design configuration, based on a 2,000 m² roof drainage area and runoff control target of 25 mm depth, for comparison which can be viewed below. To generate your own life cycle cost estimates customized to the development context, design criteria, and constraints applicable to your site, access the updated LID Life Cycle Costing Tool (LCCT) here.

Design Assumptions

Rainwater harvesting (RWH) is ideal for high density, zero-lot-line developments, large garden and landscape areas, and sites that cannot infiltrate water (e.g. contaminated soils, WHPAs, IPZs, etc.). Components include: catchment area (e.g. rooftop), pretreatment filter to remove coarse debris, cistern storage, and a connecting pipe network. Additional components include pumps to lift water to higher elevations, additional water filtration and treatment.

Design and operation and maintenance program assumptions used to generate the cost estimate are based on tool default values and the following STEP recommendations:

• Total daily water use of 2,000 L per day (tool default).
• User specified storage tank capacity of 50,000 L.
• Operation and maintenance (O&M) cost estimates assume annual inspections and cistern pump tests, removal of trash and debris from rooftop and pretreatment filter four times a year, and cleaning of cistern and replacement of pump and pressure tank every five years. Verification inspections are included every 5 years to confirm adequate maintenance.

Notes

• Two (2) German-style in-line filters and precast concrete containment structures are included to provide pretreatment of roof runoff.
• The tool calculates costs for new (greenfield) development contexts and includes costs for contractor overhead and profit, material, delivery, labour, equipment (rental, operating and operator costs), hauling and disposal.
  • Land value and equipment mobilization and demobilization costs are not included, assuming BMP construction is part of overall development site construction.
  • Design and Engineering cost estimates are not calculated by the tool and must be supplied by the user.
  • The tool adds 10% contingency and additional overhead as default.
• All cost estimates are in Canadian dollars and represent the net present value (NPV) as the tool takes into account average annual interest and discount rates over the 25 and 50 year operating life cycle periods.
• Unit costs are based on 2018 RSMeans standard union pricing.
• Additional costs associated with retrofit or redevelopment contexts is assumed to be 16% of the cost estimate for new (greenfield) construction contexts.
  • Retrofit construction cost estimates are included in the 'Costs Summary' section for comparison.
    

Construction Costs

Construction Costs Per Unit Drainage Area (CAD$/m²) - No Infiltration Design, 25 mm Treatment

Note: Please click on each image to enlarge to view associated construction cost estimates.

Above you can find a breakdown of construction costs by expense type for the 50,000 L below-ground concrete cistern design configuration.

The Material & Installation expenses represent the largest portion of total construction costs (86%).

Life Cycle Costs

Below are capital and life cycle cost estimates for the below-ground concrete cistern design configuration over 25- and 50-year time periods. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values. Operation and maintenance costs are predicted to represent 19% of total life cycle costs over the 25-year evaluation period, and increase to 27% of total life cycle costs over the 50-year period, due to periodic cleaning of cistern tank and replacing of components (pump and pressure tank).

25-Year life cycle cost break down

Rainwater Harvesting: No Infiltration

Note: Click on each image to enlarge to view associated life cycle cost estimate.

50-Year life cycle cost break down

Rainwater Harvesting: No Infiltration

Note: Click on each image to enlarge to view associated life cycle cost estimate.

Cost Summary Tables

Total life cycle cost estimate over the 50 year evaluation period for the below-ground concrete cistern design configuration is ($124,016.80), making it one of the cheapest LID BMP options.

No Infiltration

A residential rain barrel collecting roofwater from the house installed with a diverter to ensure water does not overflow the system when full and allows water to instead bypass the barrel and enter directly into the storm sewer through a storm lateral or enter an on site dry swale or infiltration trench, if installed.

The practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

Rainwater harvesting.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

Tank used to store rainwater (typically roof runoff) for later use.

Tank used to store rainwater (typically roof runoff) for later use.

Sustainable Technologies Evaluation Program

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

The technique of removing pollutants from runoff as it infiltrates through the soil.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.