Rainwater Harvesting and Stormwater Collection
By Peter S. Cartwright, PE
Collection and consumption of rainwater has been a common practice of humankind for centuries. To minimize contamination, the normal practice is to collect rainwater from a roof by means of gutters (troughs), directing the water through downspouts and into rain barrels or cisterns. Although very clean compared to stormwater (water collected from roads, ditches, lakes, rivers, etc.), rainwater is contaminated from particles and gases in the air as well as debris from the roof (bird and animal droppings, leaves, dirt, etc.). Stormwater runoff from roads, parking lots, etc., can be collected, treated and reused; however, treatment is much more involved because of the typical heavy loading from oils, greases and other ground surface contamination.
Before the development of treatment processes, rainwater was the primary source of potable water for those people for which other clean water sources (rivers, lakes, etc.) were not available.
As cities developed, infrastructures became more centralized and municipalities took over the responsibility of collecting rainwater (stormwater), generally discharging it to the nearest lake or river. Although household rainwater catchment systems (generally homemade) utilizing gutters, downspouts and rain barrels (remember the song?) had been commonly employed to collect water for landscape and gardens, even those largely faded from the urban North American scene several decades ago. Still quite common in Europe for non-potable applications and widely used in the developing world for all applications, rainwater harvesting is a major, relatively untapped water source available for recovery and reuse.
Our air has become increasingly polluted with years of irresponsible behavior by all segments of society and is much dirtier now than when rainwater was collected and consumed thousands of years ago. Likewise, drinking-water quality standards are constantly becoming more stringent. In spite of this, treating rainwater requires generally less technology than for any other source of wastewater. The reason is because rainwater is the purest of virtually all sources of water, is naturally soft and is free of DBPs. When used for most non-potable applications (garden watering, landscape irrigation, etc.), it requires little or no treatment.
The attributes of harvested rainwater include:
- Water is free; the only utility costs are for collection and use.
- The collected water is used close to the source; little cost for distribution.
- Rainwater harvesting reduces flows to stormwater drains and reduces non-point pollution.
- Rainwater harvesting reduces expansion pressure on municipal water treatment plants.
- Collection tank overflows can recharge aquifers.
- Augments or replaces limited quantities of groundwater
- Provides a good quality alternative if groundwater quality is unacceptable
- Provides a source if tap charges are too high for water supply connection
- Reduces erosion in urban environments
- Source of water that is naturally soft (no need for water softeners)
- Provides water that is pH neutral/slightly acidic.
- Water that is sodium-free, important for those on low-sodium diets
- Good quality water for landscape irrigation
- Water for non-potable indoor uses
- Safe water for human consumption, after appropriate treatment
- Helps utilities in reducing peak water demand in the summer
- Provides water for cooling and air-conditioning plants
- Provides water for fire protection
- Saves money for the consumer utility bills
Rainwater harvesting also provides several additional benefits. As it is soft, it does not produce scale build-up in hot water heaters, plumbing, faucets and shower heads. Rainwater requires less soap and detergent than most public water supplies because of its natural softness. In particular, this source of water can be valuable for hotels, helping them reduce the use of municipal water supplies and the quantity of detergents used for daily laundry.
Rainwater collection is applicable not only for houses, but for all other buildings and structures—virtually anything with a roof. One inch of rainfall provides 620 gallons of water per 1,000 ft2 of roof area. In the metric system, one centimeter of rainfall on one square meter of collection surface will deposit about 10 liters of water. Depending on the location, legal restrictions may limit the use of such reclaimed water for potable purposes. Some laws require disinfection of all water brought into the building, even if only for toilet flushing.
Rainfall patterns and quality
The frequency of rain events and rainfall quantities vary considerably from one geographical area to another; however, historical patterns are usually available for use in rainwater harvesting system design. While rainwater is invariably cleaner than stormwater, it is not free of all contamination.
All air contains particles, liquids and gases formed by natural processes, such as erosion of land surfaces resulting in dust, salt-spray from ocean wave action, biological decay, forest fires, chemical reactions of atmospheric gases and volcanic activity, as well as those from human origins: industry, agriculture, transportation (including aviation) and construction. The composition of these airborne contaminants varies widely depending on their source; they may contain gases, salts (predominantly sulfates), minerals (such as silica), organic materials and, in most cases, water.
Bacterial action in the collected water may produce gases (such as hydrogen sulfide) that are both unpleasant and dangerous. This speaks to the value of rigorous microbial inactivation. Regarding potable uses, activated carbon adsorption is usually effective in removing these gases.
Lest one should become discouraged by the apparent level and diversity of contaminants in collected rainwater, it is important to underscore the fact that both the kinds and concentrations of contaminants in collected rainwater are much, much less than those found in most ground and surface water supplies. This is a primary incentive to consider rainwater as a resource to augment (or replace) other water sources in residential, commercial, industrial and agricultural applications.
A rainwater harvesting system is comprised of the following components:
- Collection surface (usually a roof)
- Conveying system (gutters, downspouts and piping)
- ‘First Flush’ diverter and filters for roof trash removal
- Collection tank/cistern
- Distribution system
If required, the collected water is directly treated with technologies dictated by intended use and/or regulatory requirements and sent to use. An optional approach is to include a ‘day tank’ for internal storage, distribution and municipal water makeup.
This is usually a sloping or flat roof. As the roof material can have a significant impact on the quality of the collected rainwater, a discussion of materials is in order.
— Aluminum is the best all-around metal roofing material, as it is very inert, releasing only trace amounts into the collected water.
— Galvanized sheet metals are available in a number of alloy formulations, with or without painted surfaces.
— Sheet steel, because of its poor weather resistance, is no longer in wide use today.
— Lead and copper flashing are not recommended due to the potential for these metals to be leached into the water.
- Ceramic materials
— Clay tiles are very inert; however, unless sealed, they are porous and tend to absorb a percentage (as much as 10 percent) of the rainwater.
— Concrete tiles have similar absorptive characteristics as clay, but tend to neutralize the somewhat acidic pH of most rainwater.
— Slate tiles are inert, smooth and dense. As a result, they are probably the most ideal roofing material.
- Composite materials
Sheet roofing/shingles are generally asphalt based, often containing fillers and any number of potentially contaminating materials. This, plus the fact that they are rather porous, make them generally undesirable as collection surfaces.
The design of a rainwater harvesting system is influenced by the following considerations:
- Building type (residence, office building, etc.)
- Intended uses of water (toilet flushing, culinary, etc.)
- Applicable regulations
- Local climate
- Economic situation
The basic rainwater harvesting system components have been described earlier; however, the selection and assembly of these and possible additional components, the degree of automation and monitoring, etc., are all part of the total system design. Figures 1 and 2 are illustrations of a large residential installation and an office installation.
The rainwater that falls onto such ground-level surfaces as parking lots, driveways, streets, sidewalks, etc., can be defined as stormwater. It can also include floodwaters from river or lake overflow, or other such events. As with rainwater harvesting, a tank or cistern is used to collect the stormwater for treatment and subsequent use. An example of this is illustrated in Figure 3.
This article has described the inexpensive and readily available water sources of rainwater and stormwater, and has detailed the design and distribution systems for each. An upcoming article will discuss the appropriate treatment technology choices to condition this water for various applications.
Peter S. Cartwright, PE, Master Water Specialist, is President of Cartwright Consulting Company. He has been in the water treatment industry since 1974, has authored over 300 articles, presented over 300 lectures in conferences around the world and has been awarded three patents. Cartwright has chaired several WQA committees and task forces and has received the organization’s Award of Merit. A member of WC&P Technical Review Committee since 1996, his expertise includes high-technology separation processes such as RO, UF, MF, UF electrodialysis, deionization, carbon adsorption, ozonation and distillation. Cartwright is also Technical Consultant to the Canadian Water Quality Association. He can be reached at: (952) 854-4911; fax (952) 854-6964; email@example.com or www.cartwright-consulting.com