catch water
rain and snow melt, water grid-intertie

The following calculations do not include landscaping, washing cars, etc. This only covers defined uses within the building to illustrate the fresh water use difference between a conventional building and a sustainable building. These ‘sustainable’ water using methods can be implemented while still providing all of the expected modern expectations relative to the use of the building.

Generally speaking, a sustainable building with the methods outlined uses about half as much fresh, potable, drinking water as a conventional building.

Conventional home use calculation

Item Gallons per person, per day

Toilet (1.6 gal./flush) x (avg. of 6 flushes per day) 9.6 gallons

Toilet leakage (.17 gal./hr.) x (24 hrs. in a day) 4.08 gallons

Shower (2.5 gal./min.) x (4.8 min. avg. shower) 12 gallons

Bath (50 gal./bath) x (.14 bath per day) 7 gallons

Sinks (statistical average) 9 gallons

Dish Washer (13 gal./load) x (.17 load/day) 2.21 gallons

Clothes Washing Machine (50 gal./load) x (.3/load/day) 15 gallons

Total 58.89 (59)

One person uses 58.89 (59) gallons of water per day, on average. Two people would use 117.78 (118) gallons per day, on average.

Sustainable home use calculation

Item Gallons per person, per day

Toilet (greywater used to flush, not fresh water) 0 gallons

Toilet leakage (no fresh water used) 0 gallons

Shower (1.8 gal./min.) x (4.8 min. avg. shower) 8.64 gallons

Bath (50 gal./bath) x (.14 bath per day) 7 gallons

Sinks (statistical average) 6 gallons

Dish Washer (13 gal./load) x (.17 load/day) 2.21 gallons

Clothes Washing Machine (20 gal./load) x (.3/load/day) 6 gallons

Total 29.85 (30)

One person uses 29.85 (30) gallons of water per day, on average. Two people would use 59.7 (60) gallons per day, on average.

 

 

Catching the water

Water is caught from a roof with a potable surface. From the roof, the water is channeled through silt catches into cisterns. Cisterns are sized to the local climate and are best buried and completely protected from the sun. The water from the cistern is gravity-fed into a Water Organizing Module with a pump and filter. The pump pushes the water into a pressure tank to supply code required water pressure. The filters clean the water for consumption and cleaning.

rain water harvesting supply calculation

Rainwater harvesting calculators are important for several reasons. They help to better balance the supply and demand so that a system can be sized correctly and so the user has adequate storage to get through long periods without rainfall. This calculation determines stored water volume over the course of three years and whether supplemental water is needed.

For every square foot of roof, about .6234 gallons of water can be caught from 1 inch of rain.

total rain precipitation per month: 6” (example)

total rain precipitation per year: 72” (example)

1 square foot with 1 inch of rainfall catches .6234 gallons of water

size of roof: 1,000 sf (example)

1,000sf x .6234gal. = 623.4 gallons of water caught per inch 1 inch of rainfall.

623.4gal. x 72 inches of rainfall = 44,884.8 gallons of rain water.

In this example, we now have 44,884.8 gallons of water caught and available to use in one year. This is a calculation based on forecasted number which are based on probabilities and statistics using data from decades and decades of weather logs and research. As long as the model remains consistent, these calculations should be accurate enough.

To determine how much water can be used on average per day, divide the total available gallons of water by 365 days.

44,884.8 / 365 = 122.9 gallons can be used per day.

The following comparison of water use with the calculated water available in the example is very telling. Both buildings provide sufficient design and amenities for modern day living. The sustainable home could be a more conducive environment for working. Regardless, both buildings are at least adequate. The contrast brings up some powerful issues considering the state of the world we live in today.

Using the fresh, potable, drinking water use calculation outlined, two people in a sustainable building use, on average, about 30 gallons of fresh, potable, drinking water per day. For this example, these two people would have, on average, about 92.9 gallons of water left over. This is good, and this example is in a very wet climate.

4 people use about 60 gallons per day, 32.9 gallons leftover

Using the fresh, potable, drinking water use calculation outlined, two people in a conventional building use, on average, about 118 gallons of fresh, potable, drinking water per day. For this example, these two people would have, on average, about 4.9 gallons of water left over. This is not good and is at a high risk for running out of water, and this example is in a very wet climate.

4 people use about 236 gallons per day, 113.1 gallons in deficit.