rainwater harvesting systems for montana - 2

8/9/2019 Rainwater Harvesting Systems for Montana - 2

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MT 199707 AG revised 11/06

MGu

For More Online MontGuides, Visit www.montana.edu/publications

Working with the prople o Montana to design and implement innovative

approaches to conserving our most precious resource.

by Gretchen Rupp, PE, Director: Montana Water Center

updated by Ben Cichowski, EIT: Montana Water Center

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rainwater system, showing examples o each system component.

Catchment: In special circumstances, paved patios or deckscan be used to collect rainwater. Generally though, the availablecatchment area is the roo o the home, barn and outbuildings.

Any roong material is acceptable or collecting water or non-potable uses. Water to be used or drinking should not be collected

rom roos covered with asphalt composite shingles, and leadashing should not be used in these systems.

Conveyance: Gutters, downspouts and pipes convey rooruno to the storage tank called the cistern. Gutters need to besloped at least /6 inch per oot o run. Tey should be suspendedrom hangers no more than three eet apart. Aluminum orgalvanized metal are recommended because o their sturdiness,although inexpensive plastic gutters may serve beneath smallroo areas. Te downspout should be sized to provide at least one

Rr HrvsgSysms fr M

PeoPle collect and StoRe RainwateR

throughout the world. On many inhabited islands, rain is thesole source o domestic water. In portions o the U.S. wheregroundwater and surace water are scarce, expensive or o poorquality, people use rainwater. Early arm and ranch amiliesthroughout the Great Plains routinely collected rainwater to useor laundry and other purposes.

Rainwater harvesting is no longer widely practiced in Montana,but it is a simple technology that oers many potential benets.Collected rainwater can augment other water sources when theybecome scarce. Its a good standby in times o emergency, such asduring power outages or when the water table lowers and wells godry. Rainwater is sot and doesnt cause buildup o scale in waterheaters, plumbing lines or household humidiers. It does notcompromise the eectiveness o pesticides when mixed in sprayrigs, as eastern Montana groundwater otendoes. Rainwater does not stain laundry andplumbing xtures (no iron or manganese).

And it is better or landscape watering thanwaters that are naturally alkaline or rich insodium.

Simple rainwater systems or landscapeirrigation or herbicide application can bedesigned and installed by any homeowner.Systems designed to supply water or in-house use are ar more sophisticated, butthey too can be laid out by the homeowner.Tis guide describes the components o arainwater system and the process o designingthe system. Te homeowner planning asystem or outdoor use will nd most o theneeded inormation herein. Some roughguidelines or potable water systems are

included, but additional resources must beconsulted; an example is listed at the end othe guide.

System Components

Each component in the rainwater systemserves one o the ollowing unctions:rainwater catchment, conveyance, treatment(including disinection i it is to be usedor drinking water), storage or distribution.Figure is a generalized schematic o a Figure 1. Basic rainwater harvesting system


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square inch o opening per 00 square eet o roo area. Provide adownspout or each 50 eet o gutter run. I there are tall trees nearthe roo, cover the gutter with -inch hardware cloth, and placesimple wire baskets in each downspout opening to keep leaves andtwigs out o the runo. Te pipe leading to the cistern should bea minimum o 4 inches in diameter (assuming the most commonlayout gravity ow). It need not be buried beneath the rost line,but must have a slope o at least inch per oot. Sharp pipe bendsshould be avoided, and cleanouts should be incorporated wherehorizontal runs exceed 00 eet.

Treatment: Dust, leaves and bird droppings accumulate onroo suraces between rainstorms. A rst ush diverter is oten usedto eliminate most o the contamination rom the roo beore theruno reaches the cistern. A common rst ush diverter is shownin Figure 2. Water will not begin to ow into the cistern until thediverter has lled completely. A trickle drain ensures an emptychamber or subsequent rainall events. A good rule o thumb is todivert at least 0 gallons per 000 square eet o catchment area.I you are constructing your own diverter rom PVC pipe, able provides a quick reerence or determining the length o yourchamber. A roo washer should also be installed between the rstush diverter and the cistern. Te roo washer contains a screenthat will lter out large debris such as twigs and leaves. Various

devices can be constructed by the homeowner or purchasedcommercially or this purpose. A common design is shown inFigure 3. I you plan on using your collection system or potable

water, you will also want to include a 30 or 50 micron lter orpreliminary ltration. Additionally, a disinection device should belocated within the home. Tis could be either a point-o-use deviceor an in-line unit treating all o the household water.

Storage: Te cistern can be o nearly any size and material.A wood, metal or plastic barrel placed beneath a downspoutduring the warm months is all thats necessary or watering theront owerbed. Conversely, a tank that is to hold the household

water supply throughout the year may need a capacity o 0,000gallons. Any cistern to be used throughout the year in Montana

must be buried, preerably with its top beneath the rost line.Any watertight tank o non-toxic material will serve: bulk milktanks, unused lined uel tanks, or precast berglass, polyethyleneor concrete tanks, including those designed as septic tanks. As inany application, only tanks designed to be installed undergroundshould be buried. Large cisterns are oten made o concrete cast inplace. Lined or coated concrete-block vaults are not recommended,as it is very difcult to make them waterproo and structurallysound. Te cistern inlet and outlet should be designed to minimizestirring the solids settled on the tank bottom, and the tank shouldinclude a manhole, a vent, a cleanout sump and an overow pipe.I the cistern is to store potable water, a vertical ll pipe or useby water haulers is also desirable. Consult the listed reerences or

design details.Distribution: Except on steeply sloping hillsides, apressurized system is needed to distribute the water rom the

cistern. A 1 or horsepower pump, coupled with a pressuretank, usually sufces. Te outlet line rom the cistern should beburied beneath the rost line.

Rainwater is naturally acidic, and corrosive to metal tanksand household plumbing. In homes where the water lines arecopper joined with lead-containing solder, the rainwater should beneutralized beore it enters the water lines to preventing mobilizinglead and copper into drinking water. Tis can be done with acommercially-available neutralizing lter, or by adding crushedlimestone as the lter media.

First Flush

Diverter

Chamber

Trickle Drain

Removable Plug

Downspout

To Roof Washer

Figure 2. Basic rst fush diverter design

PVC Pipe Diameter

(Inches)

Chamber Length

(Inches/Gallon)

3" 32.75"

4" 18.5"

6" 8.25"

8" 4 5/8"

Table 1. Calculating the length o your rst fush diverter

chamber


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Design Approach

Te design o an individual rainwater harvesting system dependson the planned water use, the layout o the yard or armstead,and the reliability o the water supply that is needed. Dierentapproaches are taken in dierent parts o the country. ForMontana homeowners, the ollowing steps are suggested: estimate

whether rainwater could satisy the identied need; decide onthe needed reliability o the supply; size and locate the catchmentarea; size and design the cistern; select, locate and size theappurtenances.

Estimate Rainwater Availability: In theory, about six-tenths oa gallon o water is produced by one inch o rainall on one squareoot o roo area. Roughly 35 percent o the precipitation that allson a Montana roo is unavailable or use because o evaporation,leakage or diversion by the roo washer. Based on this yieldestimate, Equations and 2 (below) show the amount o waterthat can be harvested rom a given roo area or a given amounto precipitation. As the equations show, one inch o precipitationalling on one square oot o roo area yields our-tenths o a gallono usable water (the actors 0.4 and 2.5 convert among inches, eetand gallons). Te rst step is to estimate the annual water needsin gallons, and use Equation 2 to decide whether the availablecatchment area could meet the need. For making up herbicidesolution, base your estimate on your own experience. Estimatelandscape irrigation-water requirements rom Extension Serviceguidance. In-home water use ranges rom 30 gallons per personper day (low-ow plumbing xtures, laundering o ull loadsonly, no use o a garbage grinder, showers rather than baths) to00 gallons per person per day (no consideration given to waterconservation.

Establish Needed Reliability: I rainwater is to be used onlyas a high-quality supplement to plentiul groundwater, it is notcrucial to collect a rainwater volume equal to the water need everysingle year. In this case you can size your system based on youraverage annual precipitation. Each year the precipitation is lessthan average, you need to use some groundwater.

Te reason or designing a system that you know will not meetyour needs every year is to save money large cisterns are veryexpensive. Conversely, i the rainwater is to be the sole source odrinking water or your amily, you must size the system so that itcollects and stores enough water even during drought years. In thiscase, basing your design on an annual precipitation value o 50%o average is suggested.

Size and Locate Catchment Area: Te pertinent catchmentarea is calculated rom Equation 2. Catchment area should becalculated based on the ootprint created by the roo (Figure ),and not by the square ootage o the roo surace. An excessivelysteep catchment area or a dramatic rainall event may urtherdecrease capture estimates due to gutter overow. Be sure to use

roo suraces as near as possible to the planned cistern location, toshorten pipe runs. I you have the option, you may wish to avoidroos with a northern aspect: ice buildup may make it very difcultto maintain gutters there (heat tape may prevent this problem).

Size and Design the Cistern: Because precipitation and water

demand are very uneven over the course o the year (except or in-home demand), the cistern cannot be sized on the basis o annualprecipitation and demand. Instead, the total demand during thedry months must be estimated; this is the volume that must be

in storage at the beginning o the dry season (generally July ).Monthly average precipitation values are needed or this exercise;these can be obtained rom MSU Extension Bulletin 3, Climate

Atlas o Montana.

I the needed storage volume is 5000 gallons or less, alightweight berglass, metal or polyethylene tank will be possible.Teir advantage is that they can be transported to your site andlowered into place by smaller, lighter equipment, and they donot leach alkalinity into the water, as concrete does. On the otherhand, they do not have the bearing strength o concrete tanks.Tis is important i the tank is to be buried deeply, or beneath thedriveway or armyard.

Select, Locate and Size Appurtenances:A -inch or .25-inch

pipe is generally used or conducting water rom the cistern to theservice pump and pressure tank. Te cisterns overow pipe shouldbe as large as the inlet pipe rom the downspouts.

I the rainwater is to be used within the house, special care mustbe taken to prevent its contamination. Te cistern itsel, as well

Commercial Filter

(large particulate)

Hardware Cloth or

Removable Filter

To Cistern

From Downspout

OR

G=0.4XRXA Equation 1 where G = gallons o water harvested

R = precipitation, inches o water

A = 2.5xG/R Equation 2 A = roo area, square eet

Figure 3. Basic roo washer design


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Copyright 2006 MSU Extension Service

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C-1 (Structures)

Revised November 20061000-08/06JM

as each tting, must be leak proo. Manhole risers and vent pipesemerging rom a buried system should end at least eight inchesabove the ground surace, so that surace water can never owdown into the cistern. Te cistern should be located uphill and atleast 00 eet rom the nearest point o the septic system. Avoidpotential cross-connections: never plumb any part o the rainwatersystem (including the cistern overow) into the wastewater lines sothat suction could draw sewage into the system.

Costs

Te most expensive part o the rainwater system is usually thecistern. Water tanks with a capacity o 2000 gallons or less costbetween $0.30 and $.0 per gallon o storage; concrete tanksare more expensive than equivalent synthetic tanks. Te cost oconcrete construction can vary greatly, but is usually between$70 and $00 a yard plus labor. Hiring a contractor to installa large, heavy tank on a remote armstead is also expensive; calllocal contractors or estimates. A cast-in-place cistern may be anoption or some situations. For large cisterns, this approach maybe cheaper than bringing a precast tank onto the site.

Maintenance

I the rainwater is to be used or drinking water, the inside o thenished cistern should be scrubbed down with a 0 percent bleachsolution and rinsed thoroughly beore the system is put in use.Te cistern will need to be drained and emptied o accumulatedsediment every ew years. At that time, cracks should be patched

with a non-toxic sealant. Be sure to employ vigorous, positiveventilation when working inside the tank.

Other system maintenance is straightorward: keep gutters

and downspouts clean and in good repair; clean the rst ushdiverter and roo washer at least once a year; and service pumpsperiodically.

Resources

General

Harvested Rainwater, 99. Austin, exas Green Builder'sSourcebook. Website http:www.greenbuilder.comsourcebookrainwater.html

Yard and Garden Water Management. 993. MontGuide 895AG, MSU Extension Service, Bozeman, M

Te exas Manual on Rainwater Harvesting. Tird Edition2005. exas Water Development Board. Austin, exas.

Website http:www.twdb.state.tx.uspublicationsreportsRainwaterHarvestingManual_3rdedition.pd

Drinking Water Systems

Private Water Systems Handbook, MWPS-4. 979. MidwestPlan Service, Ames, IA.

Captured Rainall; Small-Scale Water Supply Systems. 98.Caliornia Dept. o Water Resources Bulletin 23. $ romCA DWR; PO Box 94283; Sacramento, CA. 9423-000.

Rainwater Cisterns: Design, Construction and Water reatment.989. E.S. Young and W.E. Sharpe. Penn State College o

Agriculture Cooperative Extension, Circular 277 UniversityPark, PA. (84) 85-785.

Household Drinking Water Protection and reatment. 99. M.Vogel. MSU Extension Bulletin EB 02. ($3.50) Bozeman,M. (40) 994-3273.

Example: Designing a System

John Riley wishes to collect precipitation to water ayoung shelterbelt that covers 500 square eet. Te countyExtension agent tells him he should apply an inch o waterto the shelterbelt each week between June 5 and Sept. 5,and that the average rainall during that time is two inches.

John calculates that he will need to collect 9,350 gallons(see example calculation below) to water the shelterbelt eachsummer. Ater pricing cistern options, he decides he only

wishes to buy 000 gallons o storage capacity, and will usewell water or the rest o his irrigation water. For a catchmentarea, he will need at least

A = 2.5 x 6000 gallons / R-annual (Equation 2)

where R-annual is the total average annual precipitation, in

inches. Te average annual precipitation on the Riley placeis 5 inches, so John will need to collect rom a roo area

with a horizontal projection o at least 000 square eet (2.5x 0005). He decides to collect rom the south side ohis barn, which has 200 square eet o roo area and is justabove where he wishes to install his cistern.

rainwater harvesting systems for montana - 2

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