How Low Can We Go?
by Doug Pushard
Santa Fe, New Mexico continues to decrease their Gallons Per Capita (i.e. Person) Per Day (GPCD). As a community it is now well under 100 GPCD. It is one of the leading cities in the country in this measure. The community can be proud of their efforts to conserve water. However, several European countries are in the 60-70 GPCD range. So obviously there is room for improvement. But how low can we go?
Just for fun let’s play with some numbers and a scenario to see what is possible. Let's use a 2 person household, using an average per person of about 1,000 gallons of water a month. The house has a 1,700 square foot roof, a rainwater system installed that brings water back into the house for toilet flushing and clothes washing, and a greywater system that is plumbed to irrigate the yard. The household is in Santa Fe where the average rainfall is about 15”, but let's use 12 inches a year for our scenario.
This imaginary household would consume about 24,000 (1,000 gallons x 2 people x 12 months) gallons a year. By industry standards slightly over 30% percent of this would be for toilet flushing and clothes washing or about 7,200 gallons a year. Assuming they have an efficient rainwater collection system, the house could harvest over 12,000 gallons a year. So all the toilet flushing and clothes washing could be handled by the rainwater system and water savings could be subtracted from our GPCD, since these appliances would no longer be using drinking water. About 30% of water consumed inside the house produces greywater and could be used for outdoor irrigation.
Typically in arid areas, outdoor irrigation account for about 40% of the annual water use or about 9,600 gallons a year. In this scenario the greywater would yield about 7,200 gallons a year. With the left over rainwater and this greywater, it would be more than enough to handle typical irrigation needs. We have covered the outside water use, the indoor toilet flushing and clothes washing so the only items left in the imaginary home are drinking / cooking water, showering / bath / hand washing water and leaks. This is about 30% (i.e. 30% toilet flushing/clothes washing, 40% outdoors and the remaining other indoor water uses) in a typical home based on national averages. This would be about 7,200 gallons a year, versus the current load of 24,000 gallons a year. Multiply this savings times by even 50,000 households and that’s over 300,000,00 gallons a year saved! Assuming, and this is a really big assumption, that the GPCD was reduced by the same percentage above we are now well below 50 GPCD and leading those European countries, not following.
This means no new big capital projects to add more treatment plants, no new bond issues to secure new water rights, and most importantly we are living in nearly a fully sustainable way. With new technologies that are sure to enter the market over the next decade, these numbers will become even easier to reach.
Even better, instead of just using the rainwater to flush the toilet and for clothes washing, treat it to drinking water quality for all indoor use! Then treat some of the greywater for reuse in the toilets and we will be getting close to a NET ZERO water use home. Now that is truly sustainable!
It is possible? Yes, we have NET ZERO energy homes being built today and there are NET ZERO water homes on the drawing boards. Tools such as the Water Efficiency Rating Score (WERS) help builders, architects and homeowners model different water conserving devices. Builders such as Bob Kreger of Kreger Design Build, Santa Fe, New Mexico are doing just that. Per Bob, "Architects are seizing recent significant advances in building energy conservation design strategies (substantial fossil-fuel demand reduction) which allow us to seriously consider "near Zero Energy" solutions. These strategies are rapidly democratizing the transition to renewable energy at all scales. Architects are also using water conservation design strategies to substantially reduce demand for precious potable water (presently widely misused for a multitude of non-potable purposes such as landscape irrigation). These advances allow us to realistically consider "near Zero Water" solutions in not just the single family and multifamily sectors but in all sectors of our built environment (commercial, industrial, etc.) and once demand reduction strategies for precious potable water are in place, then a similar democratization opportunity eases our transition to "renewable water" (capturing and storing rainwater regardless of where we are). This "low-hanging fruit" enables multiple pathways to "near Zero Water".
In fact literally billions of dollars annually are being spent on this commonsense strategy. Performance-driven water conservation software such as WERS informs us how to offset demand of precious potable water with rain capture and storage. “
It is truly possible. We can go really low!