Water Harvesting Design Guidelines

Today, many builders, designers, project architects, and engineers are becoming more familiar with how to reuse water after it is done serving its initial purpose. So what exactly are those guidelines for designing an effective system for water reuse? In its Water Reuse Handbook the Public Building Commission of Chicago outlines design guidelines for rainwater harvesting, greywater and blackwater systems.

Rainwater Harvesting System Design

Step 1: In interior rainwater harvesting systems the rain is harvested from a roof, filtered and purified, and then stored until it is used to flush toilets or urinals. Metal and slate are the recommended surfaces for harvesting rainwater, since water can easily flow on them. Gutters and downspouts made of half-round pvc, vinyl, seamless aluminum, or galvanized steel are then used to direct rainwater into the filtration system.

Step 2: As an initial filtering measure, large objects are removed from the water using a leaf screen or vortex filter; usually the first few gallons of water from the rainfall are automatically diverted to the sewer system since they contain large amounts of debris.

Step 3: Water is held in a storage tank until it gets treated.

Step 4: Water is sent through an in-line sediment filter or series of filters in order to remove particles greater than 25 microns.

Step 5: The water must be purified using chlorine or ultraviolet (UV) light. Chlorine is used as a liquid drip, or in tablet form. In the UV purification method, a series of in-line sediment filters are used to filter out particles greater than 1 micron before disinfection by UV light.

Step 6: A holding tank or a day tank must be used to hold the wastewater. The tank must have a connection to the municipal system as well as an overflow drain in case there is too little or too much water in the system. Along with those elements, there must be a water level censor indicating the amount of water in the storage tank. A control panel is also necessary to display information about volume of water harvested, volume of municipal water used, any potential cross contamination issue, and an indicator light for the UV or chlorination system.

Step 7: A booster pump is used to pressurize the water so that it can reach its destination.

Step 8: In the final step, the rainwater is transmitted in piping that is marked with purple or yellow paint indicating that it is “non-potable.” The toilet or urinal that it is transmitted to must also display a wall sign with yellow letter that says “This fixture is flushed with harvested rainwater. Not safe for drinking.” Water can also be colored with blue or green dye in order to discourage potable uses.

Greywater System Design

In a greywater system, water from the shower, lavatory, and laundry is filtered, purified, and stored until it is used to flush toilets or urinals. This system is recommend as a alternative to rain water harvesting, when that system fails to meet the demand for non-potable water, and when there is an adequate source of water to be harvested.

Step 1: Greywater may come from showers, lavatories, and washing machines. Water from the kitchen sink and blackwater (wastewater from toilets and urinals) are not good sources because they contain high amounts of organic and inorganic contaminants and bacteria.

Step 2: An initial filter removes hair, lint, and other solids to prevent clogging the filtration system.

Step 3: Similar to a rainwater harvesting system, the water is sent through an in-line sediment filter or series of filters in order to remove particles greater than 25 microns.

Step 4: As with rainwater harvesting, chlorine or UV light may be used to purify greywater. In a greywater system there must be dosing loop to recirculate the water from the storage tank through the purification system. In the UV purification method, an additional UV light and water agitator should be placed in the storage tank. Ozone, ultra filtration, and reverse osmosis methods can also be used.

Step 5: The water is sent to a storage tank which has sensors to alert the system if there is too much greywater present and water must be diverted to the sewer system, or if there is too little water and inputs from a municipal water source is needed

Step 6: A booster pump is used to pump water into the toilet or urinal

Step 7: In the final step, the rainwater is transmitted in piping that is marked with purple or yellow paint indicating that it is “non-potable.” The toilet or urinal that it is transmitted to must also display a wall sign with yellow letter that says ““This fixture is flushed with greywater. Not safe for drinking.” Water can also be colored with blue or green dye in order to discourage potable uses.

Blackwater System Design

Blackwater is water from flush fixtures which contains fecal matter and urine. Purified blackwater can be used for toilet flushing, irrigation, or fertilization of gardens or agriculture. There are several blackwater systems in the United States, but currently none in Illinois. There are two main methods of blackwater purification.

An aeration-based blackwater system begins with a septic tank which allows anaerobic bacteria to decompose large solids. Next, an aeration chamber uses the injection of air in order to churn the contents and allow bacteria to digest nutrients and oxygen. A sludge settling chamber then allows sludge to sink to the bottom of the tank and partially treated water is forced upward. A final chamber uses UV sterilization, chlorination or ultrafiltration to purify the water.

A wetland-based blackwater system uses natural processes to filter blackwater. The process includes an anaerobic settling tank, a biofilter to remove large solids and reduce odor, and a series of aerobic tanks with living organism such as algae, hydroponic plants, plankton, etc to remove fine particles and bacteria. The water is finally purified using UV sterilization, chlorination, or ultrafiltration.

For more specific guidelines, alternative system elements, and diagrams, visit the Public Building Commission of Chicago Water Reuse Handbook

HOWEVER, Illinois’ plumbing code needs to be updated to allow residents and businesses to take advantage of innovations in water reuse. The City of Chicago has the staff and resources to negotiate variances to the Ill. State Plumbing Code, but many other communities do not.  For three years in a row, legislation has failed that would have updated Illinois’ plumbing code to allow for rainwater harvesting systems that provide water for non-potable uses like toilet flushing. Last year, SB 38 was  passed by the Senate, then never introduced in in the House. The year before, SB 2549 was passed by the Senate, but also never introduced in the House. Hopefully year four will be more fruitful.