4.9.6 Construction

The technology for rain water harvesting is well developed and the components are commercially available. Placing a cistern underground may result in a considerable amount of excavation and grading (see Figure 4.9.4). Where rain water harvest systems are used for non-potable uses, the sophistication of the design will benefit from an experienced contractor. Contractors should confirm the following has occurred:
  • A cistern should be located where the surrounding area can be graded to provide good drainage of surface water away from the cistern. Avoid placing cisterns in low areas subject to flooding. This will reduce the chance of untreated storm runoff contaminating the stored cistern water.
  • Cisterns should always be located upslope from any sewage disposal facilities.
  • Below-grade cisterns should be provided with manhole risers extending a minimum of 8 inches above surrounding grade. Underground cisterns should have tie downs per manufacturer’s specifications, or the excavated site should have a daylight drain or some other drainage mechanism to prevent floating of the cistern resulting from elevated groundwater levels (WAC 51-56-1628.4).
  • Manhole openings should have a watertight curb with edges projecting several inches above the level of the surrounding surface. The edges of the manhole cover should overlap the curb and project downward a minimum of 2 inches. Manhole covers should be provided with locks to further reduce the danger of contamination and accidents.
  • Place the manhole opening near a comer or an edge of the structure so that a ladder can be lowered into the cistern and braced securely against a wall.
  • All cistern openings should be protected from unintentional entry by humans or vermin. Manhole covers should be provided and secured to prevent tampering. Where an opening is provided that could allow the entry of personnel, the opening shall be marked, “DANGER - CONFINED SPACE” (WAC 51-56-1628.4).
  • Cisterns should be protected from sunlight to inhibit algae growth to ensure the life expectancy of the tank (WAC 51 56 1628.4).
  • The floor of the cistern should be constructed to slope slightly toward the drain to facilitate cleaning. The valve and drain line should be insulated by a sufficient depth of earth to prevent freezing during even the most severe winter weather.
  • Cisterns should be vented to allow fresh air to circulate into the storage compartment. The openings, located several feet above ground level, should be oriented to face the direction of the prevailing winds, west in most cases, to maximize ventilation. Four- or six-inch diameter plastic pipe is adequate for vents. The contractor should confirm that each vent pipe has a watertight seal through the top of the cistern.
  • Cisterns should be located as close as possible to the structure benefitting from the water reuse or landscape planned for irrigation.
  • Cisterns should be installed in accordance with the manufacturer’s installation instructions. Where the installation requires a foundation, the foundation shall be flat and be capable of supporting the cistern weight when the cistern is full.

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Figure 4.9.4 - Excavation for an underground cistern
Excavation associated with the installation of an underground rain water harvest system supporting the non-potable water demands of a residential structure. Source: David Hilgers, R Miller, Inc.