Taking Storm Runoff Out Of Sewers
Reconsidering the quality of runoff generated by current engineering practices with the help of stormwater micro-management
Wastewater collection and treatment has developed over at least the past two thousand years, but if you step back and look, it does not seem to have evolved much. Animals do not seem to pay much attention to where they discharge their waste, even if confined to rather small spaces. When in the "wild," animals let the natural environment take care of this issue.
Humans, being a more sensitive species, felt inclined to be more organized about controlling waste, at least in more developed countries and societies. While you can still find numerous groups in numerous countries that directly deposit waste into soil and water, most societies have evolved to the use of sewers for collection and treatment plants for cleaning wastewater streams.
Population growth and higher wastewater treatment standards have placed ever-increasing demand upon our waste infrastructure. Sheer volumes of wastewater demand, and the associated costs to provide treatment, are beginning to overwhelm many communities -- especially those burdened by combined sewers.
The purpose of this article is to present a case for understanding and acceptance of alternative engineering methods to supplement the existing standard tools of stormwater technology -- which in most cases requires lower levels of treatment than sewage. These alternative tools are based upon sound engineering practices that existed early in wastewater treatment, and in agriculture, but are updated with current products and knowledge.
Isolating Sewage Treatment
Sewage treatment facilities perform best when there is a relatively steady incoming flow of source material. Storms upset this steady flow with tremendous "peak" flow volumes that overwhelm treatment plant capacity and often result in direct discharge of untreated wastewater directly into clean water bodies.
Thus, came a time where storm runoff volumes were isolated from the sewage flow into separate pipe networks -- unfortunately called "storm sewers," which puts a very negative connotation upon relatively clean water. Sewage treatment plants could then be constructed, operated and expanded in direct response to demands from population growth within the network served. This approach has worked successfully in new development areas, where independent gravity-fed systems could be created.
However, older communities that were established with combined sewers remain hindered by the cost and practical difficulties associated with building a parallel system for stormwater within the confines of restricted easements, existing utilities, traffic and parking demands. The first logical solution was to build larger waste treatment to accommodate peak storm flows, but the cost of new and larger facilities is becoming prohibitive.
One alternative solution to this problem has been the use of off-line storage, such as deep tunnel facilities built by rock boring machines placed well below the typical depth of utilities, foundations, etc. Chicago has perhaps the best known example of this approach. Numerous large vertical shafts intercept the sewer system, and are placed at levels hundreds of feet below the surface. Shafts provide access for large tunnel boring machines, which then created miles of lined chambers sized to store the excess flows generated by storm events into the combined sewers. Large pumps remove the stored effluent to treatment plants for processing before treated water is then discharged into rivers and lakes.
Aside from the tremendous cost of building and operating a deep storage system, such as that in Chicago, large treatment plants must be provided to handle the total combined flow volume. The plants must also be able to treat added pollutants from surface runoff and industrial sources that would not be commonly found in dedicated sewage sources. This higher expense further justifies the need to remove storm runoff flow from sewage flow.
Separate Storm Runoff
Once it was determined that it is better to separate stormwater from sewage flows, engineering practice followed old principles -- get the runoff away from the surface, into a pipe and to the nearest stream as quickly as possible. When stream beds downstream become overwhelmed by higher flows generated from more developed hard surface in the watershed, flooding occurred and channels were deepened and widened. Industrial facilities found storm sewers a convenient means to dispose of wastes instead of burdening the treatment plants or landfills.
Storm runoff sources were merged into the new storm sewer systems and minimizing "time of concentration" became the primary design objective. Water moving across our hard surfaced sidewalks, streets and parking lots carried every bit of debris and trash it could lift or float. Trash screens and filters were discouraged in the system as they would impede flow and increase "time." As this trash material became more synthetic in nature, it floated greater distances, and in many cases was be deposited on the beaches of our lakes and oceans.
Water quality of runoff sources became a primary concern. Industrial polluters using sewer systems were identified, challenged and mandated to isolate, treat and/or dispose of waste independently of sewer systems and the environment. These sources became known as "point" sources -- usually dealing with "end of pipe" locations. Today, even non-point source pollutant sources have been identified and required to be kept from our surface and subsurface water assets. Increasing the time of concentration has actually become an ally in the process of cleaning our surface runoff water, via the use of bio-filtration and even mechanical methods to remove trash, debris and sediment.
Off-line runoff collection, filtration and dispersement practices, such as detention basins and bio-swales, became established as favored practices (otherwise know as "best management practices, or BMPs) to improve runoff water quality and reduce volume loads upon centralized storm sewer systems.
Micro-Managing By Runoff Source
Today's U.S. Environmental Protection Agency (EPA) Phase II National Pollutant Discharge and Elimination System (NPDES) guidelines place a lot of emphasis upon use of BMPs to manage stormwater for maximum benefits to our communities and the environment. In general, these BMPs should play a role in meeting the following objectives:
- Minimize environmental impact by maintaining the natural water cycle and pathways as much as possible
- Keep storm runoff as clean as possible
- Minimize use of pollution-generating materials on roofs and pavements
- Treat where necessary to remove human-made pollutants
- Keep surface runoff flows short and slow to avoid accumulation of sediment and debris
- Minimize time of exposure of rainfall directly upon bare soils
- Keep any runoff volumes that exceed natural state historical volumes within the confines of each development site
If these objectives are followed stringently, and BMPs used extensively, it would be possible for new developments to expand outwards (and upstream) from existing developed areas with no added storm runoff impact downstream. Communities blessed with aquifer sources of drinking water could realize recharge volumes that at least match extraction volumes -- provided legal water rights' issues can be resolved in a sensible manner.
Traditional engineering approaches will not meet these objectives. If our design community understands and embraces its key role as stewards of our communities and the environment, then we must take time to step back and review our design practice and approach relative to stormwater management. We must educate ourselves, and our clients, about the life-cycle impacts (costs and capabilities) of new design approaches.
The best way to start is to identify the path that each drop of rain must take over each design area of development and determine the best means to manage this resource for maximum benefit to all.
For instance, if rain is falling on an undisturbed area of pasture, prairie or woods, there likely is not any need to provide additional management practices at all. If, however, new development above an undisturbed area would generate runoff into these areas, then practices should be employed to prevent or minimize new volumes to avoid erosion potential and overloading natural infiltration capacities. Historical runoff volumes from these undisturbed areas should be maintained and allowed to continue downstream to maintain the natural balance.
Rain falling upon freshly disturbed re-graded soil surfaces will be the least clean, at least for the period of time necessary to cover the surface with hardscape or vegetation. Hydromulches, erosion blankets, sod and a myriad other forms of erosion control methods and products are widely used and accepted means to speed cover and minimize soil loss. Runoff from these surfaces should be directed into temporary and/or permanent bio-swales or detention basins where excess sediments can be captured or removed following cover establishment completion.
Runoff velocity across vegetated surfaces must be kept relatively slow -- from 2 feet per second (fps) to 6 fps (varies by soil type) to prevent soil loss into the runoff. With the addition of most forms of turf reinforcement mats (TRMs), velocities can be increased to a range from 12 fps to 18 fps. These mats can be most useful at the tops of slopes, or in swales, where water begins to concentrate and move in a mostly linear direction.
Rain falling upon roof surfaces is generally considered very clean and even a potable water source in most countries -- given suitable roof materials and safe air quality. Metal (most), slate, concrete tile, single-ply membrane and other materials are the most likely to avoid polluting runoff from roofs. Vegetated roofs, even the thinnest "extensive" cross-section, have the capability to filter and clean runoff from roof surfaces and provide water clean enough to capture and store for potable purposes, or percolate directly into ground aquifer sources.
Because roof runoff is so clean, strong consideration should be given to the capture and isolated storage of this water for reuse, at least once, before its return to nature. Such reuse can reduce demands upon community water treatment facilities for non-potable functions such as fire protection, irrigation and toilet flushing. In addition, such storage can provide secure and strategic off-line sources of drinking water supplies within every community.
New forms of subsurface storage provide means to create custom storage volumes, using simple excavation and construction materials, with placement below heavy load pavements and landscaped areas. Thus, storage devices can be smaller and placed close to each source, avoiding the grading and other design issues involved with a large centralized storage device. Simple submersible pumps can extract stored water as needed, without the need for complicated and expensive pump stations.
Rain falling upon pavement surfaces can be directly infiltrated through porous pavement surfaces into sub-soils, become surface runoff across hard materials such as concrete and asphalt, or a combination of both. Pavement surfaces exposed to traffic by vehicles will be exposed to various forms of pollutants from heavy metals to hydrocarbons, with runoff from these surfaces requiring at least minimal forms of treatment to remove pollutants.
Areas used infrequently by vehicles can be dedicated porous pavements -- usually some form of grass reinforced by plastic load-bearing structures below the surface. Reinforced grass porous pavements are specifically designed to support design loads with a porous cross-section of simple sand and stone materials, and porous living "wearing" course on top. In addition to allowing water to move rapidly (up to about 35 inches per hour) through this section, the excellent root zone medium also captures and cleans vehicle pollutants by rapidly oxidizing metals and converting hydrocarbons with active soil bacteria.
As traffic volumes increase in number and/or frequency, aisles can be paved with either reinforced gravel or hard surfaces that are sloped toward porous grass-paved parking bays. Parking areas with high daily volumes of vehicle turnover should be all hard paved, but with strategic use of grass-paved filter strips to intercept surface runoff, cleanse pollutants and then infiltrate clean water to sub-soils. In areas of clay, rock or otherwise slow to percolate sub-soils, subsurface horizontal drainage strips can be used to capture runoff volumes after cleansing, and convey them to subsurface storage devices or storm sewers.
An added, and somewhat less appreciated, benefit of porous pavements and bio-filter strip or surfaces is the ability to capture coarse debris, trash and sediment frequently associated with vehicular pavement surfaces. This feature allows for easy mower/vacuum removal, and removes these materials from further concerns related to storage, infiltration and/or exfiltration, reuse, discharge, etc. This form of debris removal should also compare favorably with mechanical forms of removal used after debris has entered storm sewer pipe systems.
Subsurface vs. Surface Storage
Surface detention storage basins are being rapidly substituted by subsurface storage devices, created with extensive manifold systems of pipe and gravel, arched chambers and gravel or new excavation efficient structural "tank" systems. Reasons for the substitution movement include 1) expense of land dedicated to surface pond; 2) safety issues that require complete perimeter fencing; 3) public health concerns related to stagnant water and mosquitoes, animals that might drown and quality of water that might percolate directly into shallow aquifers; and 4) maintenance issues of controlling weed growth and periodic removal of sediments.
Subsurface storage devices allow for multiple uses above the device, therefore maximizing efficient land use, while minimizing the land area to be disturbed. Many of the devices mentioned serve only to detain water stored for a short period of time, and either exfiltrate into sub-soils, or discharge slowly via small outlet pipes direct to sewers or surface waters. Long-term retention of water is best served by sealed pipe systems or the new modular structural storage tank systems.
Restoration of Old Communities
While all of these BMPs are relatively easy to apply to new development, they can also be used quite extensively and inexpensively with older existing developments -- especially those still left with combined sewer systems and/or with desires to draw population back "downtown."
While "new" land is hard to create in established downtowns, old buildings can be removed, buildings need new roofs, old pavements require renovation and old combined sewers can often be relined and dedicated to serve a "sewage only" function -- with or without controlled storm runoff flow to assist flushing the line.
Parks can be built above parking structures, or upon reclaimed land. Vegetated roofs can bring more "green space" and character into the city, while improving air quality and lowering temperatures at the same time. Alleys can be converted to cool porous surfaces, with possible stormwater storage below. Public and private surface parking lots can introduce porous paving and bio-filter areas to capture and clean runoff, with storm storage below. Storm inlets from private lots, alleys and streets can be reconstructed to intercept debris and then direct storm flows to subsurface storage for reuse, slow exfiltration or controlled discharge back to the combined sewer.
These techniques can be applied to any scale project, whether house by house, or block by block. If a community is faced with the enormous expense of separating sewers, or constructing a community-wide combined sewer storage/treatment system (like the Chicago Deep Tunnel), it might make economic sense to look at introducing new BMP micro-management techniques -- even with taxpayer support for implementation on private property.
Regardless of our individual ages within the design community, we are never too old to reacquaint ourselves with simple principles related to water and air moving through soils, and the extensive capacity for natural processes to assist with sublimation, conversion and disposal of byproducts of our human existence.
Can we change the image and performance of our built environment and afford the change? Yes! New technology and old principles can be combined to better manage stormwater than traditional techniques. We owe it to ourselves, and our clients, to learn new practices and implement them project by project.
This article originally appeared in the 01/01/2004 issue of Environmental Protection.