|
Rainwater Catchment Systems May be Better than the Big Pipe!
by Doug Pushard
 In a recent research project by Brad Crowley, home-based cisterns were compared against the $1.4 billion “Big Pipe” program to upgrade the current Portland, OR combined storm water and sewer system. At stake is a way to reduce the estimated 2.8 billion gallons of raw sewage and stormwater that is dumped annually into the Willamette river. The study discovered that harvesting rainwater onsite may be a more cost-effective and environmentally sensitive approach to the public works projects.
The purpose of the master thesis by Brad Crowley was to perform a neighborhood–based rainwater catchment analysis using the city of Portland as an example, with emphasis on analyzing the impact if all single family residences used rainwater for key indoor functions rather than municipal water. One of the key questions was: would a local neighborhood-based program actually reduce stormwater runoff to point where the costly and disruptive “Big Pipe” project could be eliminated or postponed. The answer was YES!
Besides eliminating the need for a massive project, other benefits of the smaller scale project included:
- Reducing of the amount of pollutants entering the watershed.
- Reducing the thermal impact of storm surges to wildlife and surrounding vegetation. In the summer, the runoff areas can be 10-12 degrees warmer, which directly influence warming of the urban streams.
- Reducing the need and cost to filter and transport water back to homes for use. In fact, rain water is generally clean until it picks up pollution and debris along the way to the treatment plant.
- Increasing water efficiency awareness - something a large out of sight, out of mind project will likely never accomplish.
The study compared the indoor water use of a sample neighborhood in Portland and discovered that just two fifty-five gallon rainbarrels would be enough to satisfy the toilet water use of most households. This small system could be used year round and have a definite impact on the rainwater runoff, diverting 30% of the stormwater in a year.
The 2005 study leveraged data from a previous study on research on 100 households performed by the American Water Works Association.
Other key points of the study include:
- The need for an improved system is driven by growth and it’s unintended consequences.
- Traditional stormwater systems channel water as quickly as possible to an underground pipe and then to a nearby body of water. This problem is compounded because storm runoff and sewer water are combined.
- Parking lot runoff is 16 times greater than runoff produced by a natural meadow of identical characteristics.
- Seattle receives about 40 inches of rain annually and the average residential lot sends about 9 inches as runoff versus 1 inch for a equivalent forested area.
- During large storms the added rain runoff overwhelms the system and consequently raw sewage and runoff are emptied untreated into the Willamette river.
In most of Portland, as is common in many cities, both the stormwater (i.e. rain runoff) and wastewater (water leaving the household from the toilet, shower drain, sink drains, washing machines and dish washers) enter the same pipe and make up the combined sewer system (CSS). Large pipes take both of these products to wastewater treatment plants, where the wastewater is treated in a process monitored by the Environmental Protection Agency (EPA) to ensure it meets Clean Water Act (CWA) standards. The treated water is released into the river. If the CSS could handle the volume of water at all times, this system would comply with the CWA.
Unfortunately, during large storm events, the added volume of rainfall overwhelms the wastewater treatment system. Stormwater combines with raw sewage in the combined stormwater and sewer pipe, which is then dumped into the Willamette river, in violation of the CWA.
According to the City of Portland’s website, “In a typical year, sewer overflows pour about 2.8 billion gallons of a mixture of stormwater runoff and raw sewage into the Willamette through 42 pipes." This is in violation of EPA standards and consequently the city needs to reduce these overflows and the city is spending $1.4 billion to modify the existing system. With a population of approximately 500,000 adults in Portland, approximately $2,000 will need to be paid by each person to pay for the “Big Pipe” project.
One way to minimize the water entering the CSS would to be to capture the rainwater coming off roofs and to divert it to indoor and/or outdoor water uses. By capturing the runoff onsite for later use, the storm surge would be greatly reduced and thereby potentially eliminate the system overflows.
Brad structured a spreadsheet to analyze different tank sizes (e.g. 110, 500, 1,500, and 4,500 gallons) and different inside household water uses (e.g. toilets, clothes washer, dishwasher, faucets, and shower and bath) against the monthly rainfall in the area. Surprisingly the analysis showed that two linked 55 gallon rain barrels would meet the needs of most households toilet flushing needs.
The most ideal cistern size for saving money and conserving water would be the 1,500-gallon cistern for both toilet and clothes washer use. This cistern would succeed for almost the same number of months as the 4,500 size, but it would cost less and have a better chance of fitting into an urban lot (either above or below ground). Using a 1,500-gallon cistern for toilet and clothes washing diverts 55% of the stormwater coming off a roof away from the stormwater system while reducing the strain on the municipal water supply during the dry summer months.
In fact, if the city had given each person $3,000 to install a 1,500-gallon cistern, it could have mitigated the stormwater on site and saved the city money while inspiring a local rainwater catchment market.
Brad compiled lots of very interesting and pertinent information in his thesis on rainwater catchment. A list of some of the tables and charts in his paper include:
In summary, Brad’s thesis examines a large public works project versus a local rainwater catchment program and discovers that two linked fifty-five gallon rain barrels would dramatically reduce runoff and meet the toilet flushing needs for most households. This unique solution would reduce runoff, be cheaper, and delay the need for the Big Pipe project.
Brad, thank you for this great analysis and for giving us a progressive alternative to traditional rain runoff solutions.
Epilogue:
Since completing his paper Brad has gone on to start his own rainwater harvesting company. Additionally, he has also discovered that the City of Portland wanted to do a more progressive runoff management option, but a lawsuit and the EPA forced their hand to do the Big Pipe.
Since starting his business Brad has been able to leverage the spreadsheet he created for his paper to determine how much rainwater they could use. Brad says, "We determine as a result of the roof area and the number of people in their household the cistern will become empty every month. Often it does not matter what the cistern size is because of water consumption. I used to see this as a negative point, but I have come to realize that if a cistern is empty at the end of the month, it means all of the stormwater has been reused.” He continues, “In the future I would like to work with the City of Portland to allow rainwater catchment to be accepted as way to deal with 100% of the stormwater on a site, thereby allowing homeowners to not have to install a drywell."
Full study and related topics
Neighborhood Level Analysis of Rainwater Catchment Master Thesis
Brad Crowley's Harvest the Sky website
Clean Water Standards
Water Quality article
Rainwater Quality article
|
