Rainwater Collection and Reuse

In this blog, I am going to address rainwater collection and its potential to be reused for personal use. Obviously water is an essential part of day to day life; besides drinking it, we shower in it, wash our dishes, flush our toilets, clean our clothes with it, and it really adds up. Americans use an astounding amount of water. On average, a family of four will go through 400 gallons of water per day according to the EPA (Source 1). The majority of Americans do not collect their own water. Reusing rainwater is an excellent way to reduce our dependence on the water grid and also to reduce our environmental impact. I am interested in how much rainwater can be collected and how much rainwater has to be collected to become independent of the water grid. I theorize that even if houses in PA collect 100% of the rainwater possible, they will still be dependent on the water system because of their sheer usage. However, it will significantly reduce the dependence on the water system. This information is intended for environmentally conscious citizens and lawmakers because this data could encourage or discourage people from creating their own at-home water collection systems.

For the purposes of this blog, I will be focusing on the state of Pennsylvania. In order to calculate anything, I needed to gather some basic information about rain in this state and houses. In PA, the average amount of rainfall is 41.45 inches annually (Source 2).  Getting an estimate on average roof size was tricky, but reported by several roofing companies a 2,300 square foot house has a roof area of 1,500 square feet on average (Source 3). This information was difficult to track down because the information that was the most readily available was focused on reroofing cost, not the actual size of the roof. However, I am confident in that figure because according to the 2010 census, the average house size for houses in the North East is 2,336 square feet (Source 4).

To calculate the amount of rainwater that can be collected I multiplied the collection area by the amount of rain per year by 0.623. According to the University of Arizona, 0.623 represents the amount of water in gallons, 1 inch deep per 1 square foot (Source 5).

\[1500\text{ft}^{2}\times\frac{41.45\text{in}}{{1}\text{year}}\times\frac{0.623\text{gal}}{{1}\text{ in}\cdot{ft}^{2}}=38,735\text{ gallons annually}\]

To calculate the amount of water used annually in the US per year, I took the 400 gallons per day and multiplied it by 365 (days in a year).

\[\frac{400\text{gal}}{{1}\text{day}}\times\frac{365\text{day}}{{1}\text{year}}=146,000\text{ gallons per year}\]

Comparing both pieces of data you will see that even if we collect 100% of the annual rainfall we only can supply ourselves with 26.5% of our yearly needs.

\[\frac{38,735\text{gal}}{{1}\text{year}}\div\frac{146,000\text{gal}}{{1}\text{year}}=0.265\text{ or 26.5%}\]

While that may seem like a small percentage, it could cover a wide variety of our personal water usage. Personal use includes showering, brushing your teeth in the sink, taking baths, washing clothes and dishes, gardening (small scale), sprinklers, recreational activities, and even flushing the toilet. Unsurprisingly, one of the biggest wastes of water inside the home is toilets. There is a wide range in how much water toilets use per flush because as technology is advancing, they are becoming more efficient. Older toilets can use as much as 7 gallons per flush whereas high efficiency toilets only use 1.28 gallons per flush (Source 6). And according to the AWWA Research Foundation, people flush the toilet 5.05 times per day (Source 7).  Below you can see the difference in annual water use between the two types of toilets. FPD: flushes per day GPF: gallons per flush

Old Toilets

\[5.05\text{ FPD}\times4\text{ people}\times7\text{ GPF}\times365\text{ days per year}=51,469.6\text{ gallons per year}\]

High Efficiency Toilets

\[5.05\text{ FPD}\times4\text{ people}\times1.28\text{ GPF}\times365\text{ days per year}=9,437.4\text{ gallons per year}\]

(All calculations are based on the average American household with four members)

If we, as a nation, commit to switching to low flow toilets, it would drastically impact how much water we use annually. Beforehand, collecting rainwater might not have seemed like a feasible option because it wouldn’t cover a substantial amount of our demand, but if you look at increasingly efficient fixtures it becomes more attractive.

For rainwater reuse in general, there is a concern that people are not going to do it correctly and get themselves sick by consuming (or even simply coming into contact with) the water. However, the Environmental Protection Agency, otherwise known as the EPA, in conjunction with state agencies, like the Minnesota Pollution Control Agency, detail the quality of water people need to maintain in order to be safe. The system is also used for reclaimed water, but it controls for insect breeding, contamination, pathogens, and toxic spills (Source 8). As long as quality standards are being met, rainwater reuse is an excellent way to minimize our usage and therefore reduce our environmental impact.

In summation, rainwater collection and reuse is a great way to lessen our dependence on the government water supply. It is safe and allows us to capitalize on the natural process of the water cycle. In regards to its impact on our demand, my hypothesis was correct. At our average current rate of usage, the average household will still need to depend on the governmental system to supply most of their water. However, the use of home collections for rainwater reuse can substantially reduce how much homes need. We could get over 26% of our current annual water demand from it!  Additionally, the amount of rainwater that could be collected each year would be enough to cover high efficiency toilet water usage 3 times over. The newer and more efficient our fixtures are, the more rainwater collection can provide the necessary amount of water. And while that might seem like common sense, having the math to back up the assertion provides legitimacy. Ultimately, if we keep improving our fixtures, a future where we are independent from the water grid is possible.




Source 1 http://www3.epa.gov/watersense/pubs/indoor.html

Source 2 http://www.usclimatedata.com/climate/pennsylvania/united-states/3208

Source 3 http://www.houselogic.com/home-advice/roofing-gutters-siding/roofing-guide-options/

Source 4 https://www.census.gov/const/C25Ann/sftotalmedavgsqft.pdf

Source 5 http://cals.arizona.edu/cochise/waterwise/waterharvest.html

Source 6 http://www.conserveh2o.org/toilet-water-use

Source 7 http://www.waterrf.org/publicreportlibrary/rfr90781_1999_241a.pdf

Source 8 http://stormwater.pca.state.mn.us/index.php/Stormwater_re-use_and_rainwater_harvesting

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