Rainwater Harvesting and Stormwater Management

ArchPlan has completed construction documents for two larger projects that include rainwater harvesting combined with stormwater management using a hybrid cistern solution.

The basic concept in both cases is to collect rainwater not only for flushing toilets and plant irrigation but to let the cistern also act as a tool in the stormwater management toolbox, both for quantity and quality requirements. In this function the cistern pretty much acts like an underground storage tank, a solution often chosen in projects where there isn't enough open space to install sufficient pervious areas, bioretention  or raingardens. The new Maryland stormwater management regulations encourage a mix of smaller solutions which can work in tandem. Maryland Stormwater Design Manual. In the case of our Sphinx Club Museum and Restaurant project we are using all these stormwater management tools:

• Green roof
• Bio-retention area with raingarden
• hybrid cisterne.

We specified initially a 10,000 gallon fiberglass tank but in the final version it was chnaged to a 8000 gallon concrete tank.

In the case of our Tacoma Langley Transit Center with its about 18,000 squarefeet of glass roof we combine bioretention and the hybrid cistern on a very tight site which is mostly paved for the buses. Here a steel tank is specified with a 20,000 gallon capacity.

Civil engineers and mechanical engineers need to closely cooperate regarding these systems, because they fall in the grey area (think grey-water!) between the two disciplines. The tank might be considered part of the stormwater management typically part of the civil scope while the pumps, filters and pipes are part of the water system typically under the mechnaical engineer.
Although a filtered rainwater harvesting system will not accept the very first minute of rain to avoid the high concentration of pollutants in the tank that come with the first wainitial flush after a longer dry period, it will contain the bulk of the critical initial rainfall that is so crucial to be kept out of our rivers and the bay. The filter will also keep leaves and objects out of tank while sediment settles in the tank itself. During very large storms the tank might fill up and excess water will flow out through overflow pipes. In that situation water quality is not so much an issue any more (later in a large storm most pollutant have already washed away) even if quantity would still be an issue. However, the delay in releasing water into the pubic sewer system that comes from filling the cistern first still helps to lower the peak loads in the municipal stormwater system.

The water in the cistern will be clear and plenty good enough for plant irrigation and flushing toilets. To achieve this a pump needs to be installed in the tank that pumps the water out of the cistern and into especially installed greywater pipes which are placed parallel to the freshwater system which is still required by law. This is for the case that the cistern should empty in a long draught or that the pump which is needed to pressurize the greywater would fail for example in a power outage.

The use of rainwater harvesting systems provides the following possible LEED credits:
From RMS website (Link)
 

·         Storm Water Design: Quality Control

Sustainable Sites Credit 6.1: 1 pt:

Limit disruption and pollution of natural water flows by managing stormwater runoff.
Sustainable Sites Credit 6.2: 1 pt: Limit disruption of natural water flows by eliminating stormwater runoff, increasing on-site infiltration, and eliminating contaminant.

Water Efficient Landscaping: Reduce by 50%

Water Efficiency Credit 1.1: 2 pts:

Use captured rain or recycled site water to reduce potable water consumption for irrigation by 50% over conventional means.

Water Efficient Landscaping: No Potable Use or No Irrigation

Water Efficiency Credit 1.2: 2 pts:

Use only captured rain or recycled site water to eliminate all potable water use for site irrigation.

Innovative Wastewater Technologies

Water Efficiency Credit 2: 2 pts:

Reduce the generation of wastewater and potable water demand, while increasing the local aquifer recharge.

Water Use Reduction: 20% Reduction

Water Efficiency Credit 3.1: Required:

Maximize water efficiency within buildings to reduce the burden on municipal water supply and wastewater systems. Use 20% less water than the water use baseline calculated for the building (not including irrigation).

Water Use Reduction: 30% Reduction

Water Efficiency Credit 3.2: 2-4 pts:

Maximize water efficiency within buildings to reduce the burden on municipal water supply and wastewater systems. Use 30% less water than the water use baseline calculated for the building (not including irrigation).

30% reduction: 2 pts
35% reduction: 3 pts
40% reduction: 4 pts

• Reduce energy demands

Rainwater harvesting water used for nonpotable needs consumes less energy than municipally supplied water, which undergoes high energy requiring water treatment and distribution processes.

• Promote green technologies and water conservation
• Reduce stormwater runoff and non-point source pollution

Reduce stormwater fees
Reduce detention pond areas
Increase usable green space
Utilize land more efficiently for buildings
Protect sensitive waterways

• Reuse water on site for nonpotable demands like:
• landscape irrigation
• green roof irrigation
• pool or pond filling
• vehicle washing
• cars
• fleet vehicles
• emergency vehicles: fire trucks, police cars, ambulances
• lawnmowers
• tractors
• toilet flushing
• clothes washing
• fire suppression
• cooling towers
• cleaning

• Provide a water supply in areas where groundwater is diminished and/or municipal water supply is not available.
• Provide an onsite safe water supply during natural disasters and terrorist attacks.
• Allow for irrigation during times of drought, which promotes plant health, thus reducing the heat island effect and subsequent energy demands around buildings.
• Collect water during heavy rains, which can not be absorbed into ground, and reapply at the appropriate infiltration rate, thus encouraging plant growth and reducing stormwater runoff.
• Recharge groundwater aquifers with collected rainwater.