Wa­ter En­ergy Nexus

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Santa Fe con­tin­ues to de­crease its gal­lons per capita (that’s per per­son) per day wa­ter us­age. As a com­mu­nity we are now un­der 100 gpcd. We can all be proud of our ef­forts to con­serve wa­ter. How­ever, sev­eral Euro­pean coun­tries are in the 6070 gpcd range, so ob­vi­ously there is room for im­prove­ment. But how low­can we go?

Just for fun, let’s play with some num­bers and a sce­nario to see what is pos­si­ble. Let’s use a two-per­son house­hold­with an av­er­age monthly per-per­son wa­ter use of 1,000 gal­lons and an an­nual rain­fall of 12 inches. The house has a 1,700-square­foot roof, a sys­tem that brings rain­wa­ter into the house for toi­let-flush­ing and clothes-wash­ing, and a grey­wa­ter sys­tem that is plumbed to ir­ri­gate the yard.

This 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 used for toi­let-flush­ing and clothes-wash­ing, or about 7,200 gal­lons a year. As­sum­ing there is an ef­fi­cient rain­wa­ter col­lec­tion sys­tem, the house har­vests 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 us­age fig­ure, since these ap­pli­ances would no longer be us­ing drink­ing­wa­ter.

About 30 per­cent 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 per­cent of the an­nual wa­ter use, or about 9,600 gal­lons a year. In our sce­nario the grey­wa­ter would yield about 7,200 gal­lons a year. With the left­over rain­wa­ter and this grey­wa­ter, it would be more than enough to han­dle typ­i­cal ir­ri­ga­tion needs.

The only items left in the imag­i­nary home are drink­ing/cook­ing wa­ter, shower/bath/hand-wash­ing wa­ter, and leaks. This is about 30 per­cent in a typ­i­cal home based on na­tional av­er­ages. This would be about 7,200 gal­lons a year. As­sum­ing— and this is a re­ally big as­sump­tion — that the gal­lons per capita per day was re­duced by the same per­cent­age above, we are now well be­low 50 gpcd and lead­ing those Euro­pean coun­tries.

It is pos­si­ble? Tools such as the­Wa­ter 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, are do­ing just that. “You may very real­is­ti­cally ex­pect today to de­sign and build a home that uses lit­tle to no wa­ter be­yond cap­tur­ing and stor­ing rain­wa­ter, even in a semi-arid high-mesa cli­mate like Santa Fe,” Bob said. “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 much lower.

Doug Pushard, founder of the web­site www.Har­vestH2o.com, has de­signed and in­stalled res­i­den­tial rain­wa­ter sys­tems for over a decade. He is a mem­ber of the Santa FeWater Con­ser­va­tion Com­mit­tee, a life­time mem­ber of the Amer­i­can Rain­wa­ter Catch­ment Sys­tems As­so­ci­a­tion, and an EPAWaterSense Part­ner. He can be reached at doug@Har­vestH2o.com.

DOUG PUSHARD

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