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The Texas Manual on Rainwater Harvesting

Texas Water Development Board

in cooperation withChris Brown ConsultingJan Gerston Consulting

Stephen Colley/Architecture

Dr. Hari J. Krishna, P.E., Contract Manager

Third Edition2005

Austin, Texas


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Acknowledgments

The authors would like to thank the following persons for their assistance with theproduction of this guide: Dr. Hari Krishna, Contract Manager, Texas Water Development

Board, and President, American Rainwater Catchment Systems Association (ARCSA);Jen and Paul Radlet, Save the Rain; Richard Heinichen, Tank Town; John Kight, KendallCounty Commissioner and Save the Rain board member; Katherine Crawford, GoldenEagle Landscapes; Carolyn Hall, Timbertanks; Dr. Howard Blatt, Feather & Fur AnimalHospital; Dan Wilcox, Advanced Micro Devices; Ron Kreykes, ARCSA board member;Dan Pomerening and Mary Dunford, Bexar County; Billy Kniffen, Menard CountyCooperative Extension; Javier Hernandez, Edwards Aquifer Authority; Lara Stuart, CBC;Wendi Kimura, CBC. We also acknowledge the authors of the previous edition of thispublication, The Texas Guide to Rainwater Harvesting, Gail Vittori and Wendy PriceTodd, AIA.

Disclaimer

The use of brand names in this publication does not indicate an endorsement by the TexasWater Development Board, or the State of Texas, or any other entity.

Views expressed in this report are of the authors and do not necessarily reflect the viewsof the Texas Water Development Board, or any other entity.


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Table of Contents

Chapter 1 Introduction..................................................................................................... 1

Chapter 2 Rainwater Harvesting System Components................................................. 5

Basic Components .......................................................................................................... 5The Catchment Surface................................................................................................... 5Gutters and Downspouts ................................................................................................. 6Leaf Screens.................................................................................................................... 7First-Flush Diverters ....................................................................................................... 8Roof Washers................................................................................................................ 10Storage Tanks................................................................................................................ 10Pressure Tanks and Pumps............................................................................................ 16Treatment and Disinfection Equipment ........................................................................ 17

Chapter 3 Water Quality and Treatment..................................................................... 21

Considerations for the Rainwater Harvesting System Owner ...................................... 21Water Quality Standards ............................................................................................... 22Factors Affecting Water Quality................................................................................... 22Water Treatment ........................................................................................................... 23

Chapter 4 Water Balance and System Sizing............................................................... 29How Much Water Can Be Captured? ........................................................................... 29Rainfall Distribution ..................................................................................................... 30Calculating Storage Capacity........................................................................................ 32The Water Balance Method Using Monthly Demand and Supply ............................... 32Estimating Demand....................................................................................................... 33

Estimating indoor water demand .............................................................................. 33Indoor water conservation......................................................................................... 35Estimating outdoor water demand ............................................................................ 36

Chapter 5 Rainwater Harvesting Guidelines ............................................................... 41RWH Best Management Practices................................................................................ 41

Water Conservation Implementation Task Force Guidelines................................... 41American Rainwater Catchment Systems Association............................................. 41

Building Codes.............................................................................................................. 41Cistern Design, Construction, and Capacity ................................................................. 42Backflow Prevention and Dual-Use Systems ............................................................... 42

Required Rainwater Harvesting Systems...................................................................... 43

Chapter 6 Cost Estimation............................................................................................. 45Comparing to Other Sources of Water.......................................................................... 51


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Chapter 7 Financial and Other Incentives ................................................................... 53Tax Exemptions ............................................................................................................ 53Municipal Incentives..................................................................................................... 54Rainwater Harvesting at State Facilities ....................................................................... 55

Performance Contracting .............................................................................................. 56

Appendix A References ................................................................................................. A1

Appendix B Rainfall Data ............................................................................................. A7

Appendix C Case Studies ............................................................................................ A11

Appendix D Tax Exemption Application Form........................................................ A25


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Chapter 1

Introduction

Rainwater harvesting is an ancienttechnique enjoying a revival in

popularity due to the inherent quality ofrainwater and interest in reducingconsumption of treated water.

Rainwater is valued for its purity andsoftness. It has a nearly neutral pH, andis free from disinfection by-products,salts, minerals, and other natural andman-made contaminants. Plants thriveunder irrigation with stored rainwater.Appliances last longer when free from

the corrosive or scale effects of hardwater. Users with potable systems preferthe superior taste and cleansingproperties of rainwater.

Archeological evidence attests to thecapture of rainwater as far back as 4,000years ago, and the concept of rainwaterharvesting in China may date back 6,000years. Ruins of cisterns built as early as2000 B.C. for storing runoff fromhillsides for agricultural and domestic

purposes are still standing in Israel(Gould and Nissen-Petersen, 1999).

Advantages and benefits of rainwaterharvesting are numerous (Krishna,2003).

The water is free; the only cost is forcollection and use.

The end use of harvested water islocated close to the source,eliminating the need for complex and

costly distribution systems.

Rainwater provides a water sourcewhen groundwater is unacceptable orunavailable, or it can augment limitedgroundwater supplies.

The zero hardness of rainwater helps prevent scale on appliances,

extending their use; rainwatereliminates the need for a water

softener and the salts added duringthe softening process.

Rainwater is sodium-free, importantfor persons on low-sodium diets.

Rainwater is superior for landscapeirrigation.

Rainwater harvesting reduces flow tostormwater drains and also reducesnon-point source pollution.

Rainwater harvesting helps utilitiesreduce the summer demand peak anddelay expansion of existing watertreatment plants.

Rainwater harvesting reducesconsumers utility bills.

Perhaps one of the most interestingaspects of rainwater harvesting islearning about the methods of capture,storage, and use of this natural resourceat the place it occurs. This naturalsynergy excludes at least a portion ofwater use from the water distributioninfrastructure: the centralized treatmentfacility, storage structures, pumps,mains, and laterals.

Rainwater harvesting also includes land-based systems with man-made landscapefeatures to channel and concentraterainwater in either storage basins orplanted areas.

When assessing the health risks ofdrinking rainwater, consider the pathtaken by the raindrop through awatershed into a reservoir, through public drinking water treatment anddistribution systems to the end user.Being the universal solvent, waterabsorbs contaminants and minerals on its


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travels to the reservoir. While inresidence in the reservoir, the water cancome in contact with all kinds of foreignmaterials: oil, animal wastes, chemicaland pharmaceutical wastes, organic

compounds, industrial outflows, andtrash. It is the job of the water treatment plant to remove harmful contaminantsand to kill pathogens. Unfortunately,when chlorine is used for disinfection, italso degrades into disinfection by- products, notably trihalomethanes,which may pose health risks. In contrast,the raindrop harvested on site will traveldown a roof via a gutter to a storagetank. Before it can be used for drinking,

it will be treated by a relatively simple process with equipment that occupiesabout 9 cubic feet of space.

Rainwater harvesting can reduce thevolume of storm water, therebylessening the impact on erosion anddecreasing the load on storm sewers.Decreasing storm water volume alsohelps keep potential storm waterpollutants, such as pesticides, fertilizers,and petroleum products, out of rivers

and groundwater.

But along with the independence ofrainwater harvesting systems comes theinherent responsibility of operation andmaintenance. For all systems, thisresponsibility includes purging the first-flush system, regularly cleaning roofwashers and tanks, maintaining pumps,and filtering water. For potable systems,responsibilities include all of the above,and the owner must replace cartridge

filters and maintain disinfectionequipment on schedule, arrange to havewater tested, and monitor tank levels.Rainwater used for drinking should betested, at a minimum, for pathogens.

Rainwater harvesting, in its essence, isthe collection, conveyance, and storage

of rainwater. The scope, method,technologies, system complexity, purpose, and end uses vary from rain barrels for garden irrigation in urbanareas, to large-scale collection of

rainwater for all domestic uses. Someexamples are summarized below:

For supplemental irrigation water, theWells Branch Municipal UtilityDistrict in North Austin capturesrainwater, along with air conditioningcondensate, from a new 10,000-square-foot recreation center into a37,000-gallon tank to serve asirrigation water for a 12-acremunicipal park with soccer fields and

offices.

The Lady Bird Johnson WildflowerResearch Center in Austin, Texas,harvests 300,000 gallons of rainwaterannually from almost 19,000 squarefeet of roof collection area forirrigation of its native plantlandscapes. A 6,000-gallon stonecistern and its arching stone aqueductform the distinctive entry to theresearch center.

The Advanced Micro Devicessemiconductor fabrication plant inAustin, Texas, does not use utility-supplied water for irrigation, saving$1.5 million per year by relying oncaptured rainwater and collectedgroundwater.

Reynolds Metals in Ingleside, Texas,uses stormwater captured incontainment basins as process water

in its metal-processing plant, greatlyoffsetting the volume of purchasedwater.

The city of Columbia, Nuevo Len,Mexico, is in the planning stages ofdeveloping rainwater as the basis forthe citys water supply for new


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growth areas, with large industrialdevelopments being plumbed forstorage and catchment.

On small volcanic or coral islands,rainwater harvesting is often the only

option for public water supply, aswatersheds are too small to create amajor river, and groundwater is eithernonexistent or contaminated with saltwater. Bermuda, the U.S. VirginIslands, and other Caribbean islandsrequire cisterns to be included with allnew construction.

In Central Texas, more than 400 full-scale rainwater harvesting systems have been installed by professionalcompanies, and more than 6,000 rain barrels have been installed through theCity of Austins incentive program in the past decade. Countless do-it-yourselfers have installed systems overthe same time period.

An estimated 100,000 residentialrainwater harvesting systems are in usein the United States and its territories(Lye, 2002). More are being installed by

the urban home gardener seekinghealthier plants, the weekend cabinowner, and the homeowner intent uponthe green building practices allseeking a sustainable, high-quality watersource. Rainwater harvesting is alsorecognized as an important water-conserving measure, and is bestimplemented in conjunction with otherefficiency measures in and outside of thehome.

Harvested rainwater may also help someTexas communities close the gap between supply and demand projectedby the Texas Water Development Board(TWDB), as the states population nearlydoubles between 2000 and 2050 (TexasWater Development Board, 2002).

In fact, rainwater harvesting isencouraged by Austin and San Antoniowater utilities as a means of conservingwater. The State of Texas also offersfinancial incentives for rainwater

harvesting systems. Senate Bill 2 of the77th Legislature exempts rainwaterharvesting equipment from sales tax, andallows local governments to exemptrainwater harvesting systems from advalorem (property) taxes.

Rainwater harvesting systems can be assimple as a rain barrel for gardenirrigation at the end of a downspout, oras complex as a domestic potable systemor a multiple end-use system at a large

corporate campus.

Rainwater harvesting is practical onlywhen the volume and frequency ofrainfall and size of the catchment surfacecan generate sufficient water for theintended purpose.

From a financial perspective, theinstallation and maintenance costs of arainwater harvesting system for potablewater cannot compete with water

supplied by a central utility, but is oftencost-competitive with installation of awell in rural settings.

With a very large catchment surface,such as that of big commercial building,the volume of rainwater, when capturedand stored, can cost-effectively serveseveral end uses, such as landscapeirrigation and toilet flushing.

Some commercial and industrial

buildings augment rainwater withcondensate from air conditioningsystems. During hot, humid months,warm, moisture-laden air passing overthe cooling coils of a residential airconditioner can produce 10 or moregallons per day of water. Industrialfacilities produce thousands of gallons


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per day of condensate. An advantage ofcondensate capture is that its maximum production occurs during the hottestmonth of the year, when irrigation needis greatest. Most systems pipe

condensate into the rainwater cistern forstorage.

The depletion of groundwater sources,the poor quality of some groundwater,high tap fees for isolated properties, theflexibility of rainwater harvestingsystems, and modern methods oftreatment provide excellent reasons toharvest rainwater for domestic use.

The scope of this manual is to serve as a primer in the basics of residential andsmall-scale commercial rainwaterharvesting systems design. It is intendedto serve as a first step in thinking aboutoptions for implementing rainwaterharvesting systems, as well asadvantages and constraints.

References

Gould J, Nissen-Petersen E. 1999.Rainwater catchment systems fordomestic rain: design constructionand implementation. London:

Intermediate TechnologyPublications. 335 p.

Krishna H. 2003. An overview ofrainwater harvesting systems andguidelines in the United States.Proceedings of the First AmericanRainwater Harvesting Conference;2003 Aug 21-23; Austin (TX).

Lye D. 2002. Health risks associatedwith consumption of untreated water

from household roof catchmentsystems. Journal of the AmericanWater Resources Association38(5):1301-1306.

Texas Water Development Board. 2002.Water for Texas 2002. Austin (TX):Texas Water Development Board.155 p.


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Chapter 2

Rainwater Harvesting System Components

Rainwater harvesting is the capture,diversion, and storage of rainwater for a

number of different purposes includinglandscape irrigation, drinking anddomestic use, aquifer recharge, andstormwater abatement.

In a residential or small-scaleapplication, rainwater harvesting can beas simple as channeling rain running offan unguttered roof to a planted landscapearea via contoured landscape. To preventerosion on sloped surfaces, a bermedconcave holding area down slope can

store water for direct use by turfgrass or plants (Waterfall, 1998). More complexsystems include gutters, pipes, storagetanks or cisterns, filtering, pump(s), andwater treatment for potable use.

This chapter focuses on residential orsmall-scale commercial systems, forboth irrigation and potable use.

The local health department and city

building code officer should beconsulted concerning safe, sanitary

operations and construction of thesesystems.

Basic Components

Regardless of the complexity of thesystem, the domestic rainwaterharvesting system (Figure 2-1)comprises six basic components:

Catchment surface: the collectionsurface from which rainfall runs off

Gutters and downspouts: channelwater from the roof to the tank

Leaf screens, first-flush diverters, androof washers: components whichremove debris and dust from thecaptured rainwater before it goes tothe tank

One or more storage tanks, also calledcisterns

Delivery system: gravity-fed or

pumped to the end use Treatment/purification: for potable

systems, filters and other methods tomake the water safe to drink

The Catchment Surface

The roof of a building or house is theobvious first choice for catchment. Foradditional capacity, an open-sided barn called a rain barn or pole barn can be built. Water tanks and other rainwater

system equipment, such as pumps andfilters, as well as vehicles, bicycles, andgardening tools, can be stored under thebarn.

Water quality from different roofcatchments is a function of the type ofroof material, climatic conditions, and

Figure 2-1. Typical rainwater harvestinginstallation


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the surrounding environment(Vasudevan, 2002).

Metal

The quantity of rainwater that can becollected from a roof is in part a functionof the roof texture: the smoother the better. A commonly used roofingmaterial for rainwater harvesting is soldunder the trade name Galvalume, a 55percent aluminum/45 percent zinc alloy-coated sheet steel. Galvalume is alsoavailable with a baked enamel coating,or it can be painted with epoxy paint.

Some caution should be exercisedregarding roof components. Roofs with

copper flashings can cause discolorationof porcelain fixtures.

Clay/concrete tile

Clay and concrete tiles are both porous.Easily available materials are suitablefor potable or nonpotable systems, butmay contribute to as much as a 10- percent loss due to texture, inefficientflow, or evaporation. To reduce waterloss, tiles can be painted or coated with asealant. There is some chance of toxinsleaching from the tile sealant or paint, but this roof surface is safer whenpainted with a special sealant or paint to prevent bacterial growth on porousmaterials.

Composite or asphalt shingle

Due to leaching of toxins, compositeshingles are not appropriate for potablesystems, but can be used to collect waterfor irrigation. Composite roofs have an

approximated 10-percent loss due toinefficient flow or evaporation (Radletand Radlet, 2004).

Others

Wood shingle, tar, and gravel. Theseroofing materials are rare, and the water

harvested is usually suitable only forirrigation due to leaching of compounds.

Slate. Slates smoothness makes it idealfor a catchment surface for potable use,assuming no toxic sealant is used;

however, cost considerations maypreclude its use.

Gutters and Downspouts

Gutters are installed to capture rainwaterrunning off the eaves of a building.Some gutter installers can providecontinuous or seamless gutters.

For potable water systems, lead cannotbe used as gutter solder, as is sometimes

the case in older metal gutters. Theslightly acidic quality of rain coulddissolve lead and thus contaminate thewater supply.

The most common materials for guttersand downspouts are half-round PVC,vinyl, pipe, seamless aluminum, andgalvanized steel.

Seamless aluminum gutters are usuallyinstalled by professionals, and, therefore,are more expensive than other options.

Regardless of material, other necessarycomponents in addition to the horizontalgutters are the drop outlet, which routeswater from the gutters downward and atleast two 45-degree elbows which allowthe downspout pipe to snug to the side ofthe house. Additional componentsinclude the hardware, brackets, andstraps to fasten the gutters anddownspout to the fascia and the wall.

Gutter Sizing and Installation

When using the roof of a house as acatchment surface, it is important toconsider that many roofs consist of oneor more roof valleys. A roof valleyoccurs where two roof planes meet. Thisis most common and easy to visualize


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when considering a house plan with anL or T configuration. A roof valleyconcentrates rainfall runoff from tworoof planes before the collected rainreaches a gutter. Depending on the size

of roof areas terminating in a roof valley,the slope of the roofs, and the intensityof rainfall, the portion of gutter locatedwhere the valley water leaves the eave ofthe roof may not be able to capture allthe water at that point, resulting inspillage or overrunning.

Besides the presence of one or more roofvalleys, other factors that may result inoverrunning of gutters include aninadequate number of downspouts,

excessively long roof distances fromridge to eave, steep roof slopes, andinadequate gutter maintenance.Variables such as these make any guttersizing rules of thumb difficult to apply.Consult your gutter supplier about yoursituation with special attention todetermine where gutter overrunningareas may occur. At these points alongan eave, apply strategies to minimize possible overrunning to improve

catchment efficiency. Preventativestrategies may include modifications tothe size and configuration of gutters andaddition of gutter boxes withdownspouts and roof diverters near theeave edge.

Gutters should be installed with slopetowards the downspout; also the outsideface of the gutter should be lower thanthe inside face to encourage drainageaway from the building wall.

Leaf Screens

To remove debris that gathers on thecatchment surface, and ensure highquality water for either potable use or towork well without clogging irrigationemitters, a series of filters are necessary.Essentially, mesh screens remove debris

both before and after the storage tank.The defense in keeping debris out of arainwater harvesting system is some typeof leaf screen along the gutter or in thedownspout.

Depending upon the amount and type oftree litter and dust accumulation, thehomeowner may have to experiment tofind the method that works best. Leafscreens must be regularly cleaned to beeffective. If not maintained, leaf screenscan become clogged and preventrainwater from flowing into a tank.Built-up debris can also harbor bacteriaand the products of leaf decay.

Leaf guards are usually -inch meshscreens in wire frames that fit along thelength of the gutter. Leaf guards/screensare usually necessary only in locationswith tree overhang. Guards with profilesconducive to allowing leaf litter to slideoff are also available.

The funnel-type downspout filter ismade of PVC or galvanized steel fittedwith a stainless steel or brass screen.This type of filter offers the advantage of

easy accessibility for cleaning. Thefunnel is cut into the downspout pipe atthe same height or slightly higher thanthe highest water level in the storagetank.

Strainer baskets are spherical cage-likestrainers that slip into the drop outlet ofthe downspout.

A cylinder of rolled screen inserted intothe drop outlet serves as another method

of filtering debris. The homeowner mayneed to experiment with various gridsizes, from insect screen to hardwarecloth.

Filter socks of nylon mesh can beinstalled on the PVC pipe at the tankinflow.


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First-Flush Diverters

A roof can be a natural collectionsurface for dust, leaves, blooms, twigs,insect bodies, animal feces, pesticides,and other airborne residues. The first-

flush diverter routes the first flow ofwater from the catchment surface awayfrom the storage tank. The flushed watercan be routed to a planted area. Whileleaf screens remove the larger debris,such as leaves, twigs, and blooms thatfall on the roof, the first-flush divertergives the system a chance to rid itself ofthe smaller contaminants, such as dust,pollen, and bird and rodent feces.

The simplest first-flush diverter is a PVC

standpipe (Figure 2-2). The standpipefills with water first during a rainfallevent; the balance of water is routed tothe tank. The standpipe is drainedcontinuously via a pinhole or by leavingthe screw closure slightly loose. In anycase, cleaning of the standpipe isaccomplished by removing the PVCcover with a wrench and removingcollected debris after each rainfall event.

There are several other types of first-flush diverters. The ball valve typeconsists of a floating ball that seals offthe top of the diverter pipe (Figure 2-3)when the pipe files with water.

Opinions vary on the volume ofrainwater to divert. The number of drydays, amount of debris, and roof surfaceare all variables to consider.

One rule of thumb for first-flushdiversion is to divert a minimum of 10gallons for every 1,000 square feet ofcollection surface. However, first-flushvolumes vary with the amount of dust onthe roof surface, which is a function ofthe number of dry days, the amount andtype of debris, tree overhang, andseason.

A preliminary study by Rain WaterHarvesting and Waste Water SystemsPty Ltd., a rainwater harvestingcomponent vendor in Australia,recommends that between 13 and 49

gallons be diverted per 1,000 square feet.The primary reason for the widevariation in estimates is that there is noexact calculation to determine how muchinitial water needs to be diverted becausethere are many variables that woulddetermine the effectiveness of washingthe contaminants off the collectionsurface, just as there are many variablesdetermining the make up of thecontaminants themselves. For example,

the slope and smoothness of thecollection surface, the intensity of therain event, the length of time betweenevents (which adds to the amount ofaccumulated contaminants), and thenature of the contaminants themselvesadd to the difficulty of determining justhow much rain should be diverted duringfirst flush. In order to effectively wash acollection surface, a rain intensity ofone-tenth of an inch of rain per hour is

needed to wash a sloped roof. A flat ornear-flat collection surface requires 0.18inches of rain per hour for an effectivewashing of the surface.

The recommended diversion of firstflush ranges from one to two gallons offirst-flush diversion for each 100 squarefeet of collection area. If using a roof fora collection area that drains into gutters,calculate the amount of rainfall area thatwill be drained into every gutter feeding

your system. Remember to calculate thehorizontal equivalent of the rooffootprint when calculating yourcatchment area. (Please refer to theFigure 4-1 in Chapter 4, Water Balanceand System Sizing.) If a gutter receivesthe quantity of runoff that requiremultiple downspouts, first-flush


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First-Flush Diverters

Standpipe

The simplest first-flush diverter is a 6- or 8-inchPVC standpipe (Figure 2-2). The diverter fillswith water first, backs up, and then allows waterto flow into the main collection piping. Thesestandpipes usually have a cleanout fitting at the bottom, and must be emptied and cleaned outafter each rainfall event. The water from thestandpipe may be routed to a planted area. A pinhole drilled at the bottom of the pipe or ahose bibb fixture left slightly open (shown)allows water to gradually leak out.

If you are using 3 diameter PVC or similar

pipe, allow 33 length of pipe per gallon; 4diameter pipe needs only 18 of length pergallon; and a little over 8 of 6 diameter pipe isneeded to catch a gallon of water.

Standpipe with ball valve

The standpipe with ball valve is a variation ofthe standpipe filter. The cutaway drawing(Figure 2-3) shows the ball valve. As thechamber fills, the ball floats up and seals on theseat, trapping first-flush water and routing thebalance of the water to the tank.

Figure 2-2. Standpipe first-flushdiverter

Figure 2-3. Standpipe with ball valve


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diversion devices will be required foreach downspout.

Roof Washers

The roof washer, placed just ahead of thestorage tank, filters small debris for potable systems and also for systemsusing drip irrigation. Roof washersconsist of a tank, usually between 30-and 50-gallon capacity, with leafstrainers and a filter (Figure 2-4). Onecommercially available roof washer hasa 30-micron filter. (A micron, also calleda micrometer, is one-millionth of ameter. A 30-micron filter has pores

about one-third the diameter of a humanhair.)

All roof washers must be cleaned.Without proper maintenance they notonly become clogged and restrict theflow of rainwater, but may themselvesbecome breeding grounds for pathogens.

The box roof washer (Figure 2-4) is acommercially available componentconsisting of a fiberglass box with one

or two 30-micron canister filters

(handling rainwater from 1,500- and3,500-square-foot catchments,respectively). The box is placed atop aladder-like stand beside the tank, from

which the system owner accesses the box for cleaning via the ladder. Inlocations with limited drop, a filter withthe canisters oriented horizontally isindicated, with the inlet and outlet of thefilter being nearly parallel.

Storage Tanks

The storage tank is the most expensivecomponent of the rainwater harvestingsystem.

The size of storage tank or cistern isdictated by several variables: therainwater supply (local precipitation),the demand, the projected length of dryspells without rain, the catchmentsurface area, aesthetics, personalpreference, and budget.

A myriad of variations on storage tanksand cisterns have been used over thecenturies and in different geographical

regions: earthenware cisterns in pre-biblical times, large pottery containers inAfrica, above-ground vinyl-linedswimming pools in Hawaii, concrete or brick cisterns in the central UnitedStates, and, common to old homesteadsin Texas, galvanized steel tanks andattractive site-built stone-and-mortarcisterns.

For purposes of practicality, this manualwill focus on the most common, easily

installed, and readily available storageoptions in Texas, some still functionalafter a century of use.

Storage tank basics

Storage tanks must be opaque, eitherupon purchase or painted later, toinhibit algae growth.Figure 2-4. Box roof washer


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For potable systems, storage tanksmust never have been used to storetoxic materials.

Tanks must be covered and ventsscreened to discourage mosquito

breeding.

Tanks used for potable systems mustbe accessible for cleaning.

Storage tank siting

Tanks should be located as close tosupply and demand points as possible toreduce the distance water is conveyed.Storage tanks should be protected fromdirect sunlight, if possible. To ease theload on the pump, tanks should be

placed as high as practicable. Of course,the tank inlet must be lower than thelowest downspout from the catchmentarea. To compensate for friction lossesin the trunk line, a difference of a coupleof feet is preferable. When convertingfrom well water, or if using a well backup, siting the tanks near the wellhouse facilitates the use of existingplumbing.

Water runoff should not enter septic

system drainfields, and any tankoverflow and drainage should be routedso that it does not affect the foundationof the tanks or any other structures(Macomber, 2001).

Texas does not have specific rulesconcerning protection of rainwatersystems from possible contaminationsources; however, to ensure a safe watersupply, underground tanks should belocated at least 50 feet away from animal

stables or above-ground application oftreated wastewater. Also, runoff fromtank overflow should not enter septicsystem drainfields. If supplementalhauled water might be needed, tank placement should also take intoconsideration accessibility by a water

truck, preferably near a driveway orroadway.

Water weighs just over 8 pounds pergallon, so even a relatively small 1,500-gallon tank will weigh 12,400 pounds. A

leaning tank may collapse; therefore,tanks should be placed on a stable, level pad. If the bed consists of a stablesubstrate, such as caliche, a load of sandor pea gravel covering the bed may besufficient preparation. In some areas,sand or pea gravel over well-compactedsoil may be sufficient for a small tank.Otherwise, a concrete pad should beconstructed. When the condition of thesoil is unknown, enlisting the services of

a structural engineer may be in order toensure the stability of the soil supportingthe full cistern weight.

Another consideration is protecting the pad from being undermined by eithernormal erosion or from the tankoverflow. The tank should be positionedsuch that runoff from other parts of theproperty or from the tank overflow willnot undermine the pad. The pad or bedshould be checked after intense rainfallevents.

Fiberglass

Fiberglass tanks (Figure 2-5) are built instandard capacities from 50 gallons to15,000 gallons and in both vertical

Figure 2-5. Two 10,000-gallon fiberglasstanks


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cylinder and low-horizontal cylinderconfigurations.

Fiberglass tanks under 1,000 gallons areexpensive for their capacity, sopolypropylene might be preferred. Tanks

for potable use should have a USDA-approved food-grade resin lining and thetank should be opaque to inhibit algaegrowth.

The durability of fiberglass tanks has been tested and proven, weathering theelements for years in Texas oil fields.They are easily repaired.

The fittings on fiberglass tanks are anintegral part of the tank, eliminating the

potential problem of leaking from anaftermarket fitting.

Polypropylene

Polypropylene tanks (Figure 2-6) arecommonly sold at farm and ranch supplyretailers for all manner of storage uses.Standard tanks must be installed aboveground. For buried installation, speciallyreinforced tanks are necessary towithstand soil expansion andcontraction. They are relativelyinexpensive and durable, lightweight,and long lasting. Polypropylene tanksare available in capacities from 50gallons to 10,000 gallons.

Polypropylene tanks do not retain paintwell, so it is necessary to find off-the-shelf tanks manufactured with opaque plastic. The fittings of these tanks areaftermarket modifications. Although

easy to plumb, the bulkhead fittingsmight be subject to leakage.

Wood

For aesthetic appeal, a wood tank(Figure 2-7) is often a highly desirablechoice for urban and suburban rainwaterharvesters.

Wood tanks, similar to wood watertowers at railroad depots, werehistorically made of redwood. Modern

wood tanks are usually of pine, cedar, orcypress wrapped with steel tensioncables, and lined with plastic. For potable use, a food-grade liner must beused.

These tanks are available in capacities

from 700 to 37,000 gallons, and are site-built by skilled technicians. They can bedismantled and reassembled at adifferent location.

Figure 2-6. Low-profile 5,000-gallonpolypropylene tanks

Figure 2-7. Installation of a 25,000-gallonTimbertank in Central Texas showing theaesthetic appeal of these wooden tanks


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Figure 2-9. Concrete tank fabricated fromstacking rings of concrete

Figure 2-8. Galvanized sheet metaltanks are usually fitted with a food-gradeplastic liner.

Metal

Galvanized sheet metal tanks (Figure 2-8) are also an attractive option for theurban or suburban garden. They areavailable in sizes from 150 to 2,500gallons, and are lightweight and easy torelocate. Tanks can be lined for potableuse. Most tanks are corrugatedgalvanized steel dipped in hot zinc forcorrosion resistance. They are lined witha food-grade liner, usually polyethyleneor PVC, or coated on the inside withepoxy paint. The paint, which alsoextends the life of the metal, must beFDA- and NSF-approved for potability.

Concrete

Concrete tanks are either poured in place

or prefabricated (Figure 2-9). They can be constructed above ground or belowground. Poured-in-place tanks can beintegrated into new construction under apatio, or a basement, and their placementis considered permanent.

A type of concrete tank familiar toresidents of the Texas Hill Country is

constructed of stacked rings with sealantaround the joints. Other types ofprefabricated concrete tanks include newseptic tanks, conduit stood on end, andconcrete blocks. These tanks are

fabricated off-site and dropped intoplace.

Concrete may be prone to cracking andleaking, especially in underground tanksin clay soil. Leaks can be easily repairedalthough the tank may need to bedrained to make the repair. Involving theexpertise of a structural engineer todetermine the size and spacing ofreinforcing steel to match the structuralloads of a poured-in-place concrete

cistern is highly recommended. A product that repairs leaks in concretetanks, Xypex, is now also availableand approved for potable use.

One possible advantage of concretetanks is a desirable taste imparted to thewater by calcium in the concrete beingdissolved by the slightly acidic


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rainwater. For potable systems, it isessential that the interior of the tank be plastered with a high-quality materialapproved for potable use.

Ferrocement

Ferrocement is a low-cost steel andmortar composite material. For purposesof this manual, GuniteTM and ShotcreteTM

type will be classified as ferrocements.Both involve application of the concreteand mortar under pressure from a gun.Gunite, the dry-gun spray method inwhich the dry mortar is mixed withwater at the nozzle, is familiar for its usein swimming pool construction.Shotcrete uses a similar application, but

the mixture is a prepared slurry. Bothmethods are cost-effective for largerstorage tanks. Tanks made of Gunite andShotcrete consist of an armature madefrom a grid of steel reinforcing rods tiedtogether with wire around which is placed a wire form with closely spacedlayers of mesh, such as expanded metallath. A concrete-sand-water mixture isapplied over the form and allowed tocure. It is important to ensure that the

ferrocement mix does not contain anytoxic constituents. Some sourcesrecommend painting above-ground tankswhite to reflect the suns rays, reduceevaporation, and keep the water cool.

Ferrocement structures (Figure 2-10)have commonly been used for waterstorage construction in developingcountries due to low cost and availabilityof materials. Small cracks and leaks can

easily be repaired with a mixture ofcement and water, which is appliedwhere wet spots appear on the tanksexterior. Because walls can be as thin as1 inch, a ferrocement tank uses lessmaterial than concrete tanks, and thuscan be less expensive. As with poured-in-place concrete construction,assistance from a structural engineer isencouraged.

In-ground polypropylene

In-ground tanks are more costly to installfor two reasons: the cost of excavationand the cost of a more heavily reinforcedtank needed if the tank is to be buriedmore than 2-feet deep in well-drainedsoils. Burying a tank in clay is notrecommended because of theexpansion/contraction cycles of claysoil. For deeper installation, the walls ofpoly tanks must be manufactured thickerand sometimes an interior bracing

structure must be added. Tanks are buried for aesthetic or space-savingreasons.

Table 2-1 provides some values to assistin planning an appropriate-sized pad andcistern to meet your water needs andyour available space. Many owners ofrainwater harvesting systems usemultiple smaller tanks in sequence tomeet their storage capacity needs. This

has the advantage of allowing the ownerto empty a tank in order to performmaintenance on one tank at a timewithout losing all water in storage.

A summary of cistern materials, theirfeatures, and some words of caution are provided in Table 2-2 to assist the prospective harvester in choosing the

Figure 2-10. Ferrocement tanks, such as thisone, are built in place using a metal armatureand a sprayed-on cement.


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Table 2-2. Cistern Types

MATERIAL FEATURES CAUTION

Plastics

Trash cans (20-50 gallon) commercially available;inexpensive

use only new cans

Fiberglass commercially available;alterable and moveable

must be sited on smooth, solid,level footing

Polyethylene/polypropylene commercially available;alterable and moveable

UV-degradable, must bepainted or tinted

Metals

Steel drums (55-gallon) commercially available;alterable and moveable

verify prior to use for toxics;prone to corrosion an rust;

Galvanized steel tanks commercially available;alterable and moveable

possibly corrosion and rust;must be lined for potable use

Concrete and Masonry

Ferrocement durable and immoveable potential to crack and fail

Stone, concrete block durable and immoveable difficult to maintain

Monolithic/Poured-in-place durable and immoveable potential to crack

Wood

Redwood, fir, cypress attractive, durable, can bedisassembled and moved

expensive

Adapted from Texas Guide to Rainwater Harvesting, Second Edition, Texas Water DevelopmentBoard, 1997.

Pressure Tanks and Pumps

The laws of physics and the topographyof most homesteads usually demand a pump and pressure tank between waterstorage and treatment, and the house orend use. Standard municipal water pressure is 40 pounds per square inch(psi) to 60 psi. Many home appliances

clothes washers, dishwashers, hot-water-

on-demand water heaters require 2030 psi for proper operation. Even somedrip irrigation system need 20 psi for proper irrigation. Water gains 1 psi of pressure for every 2.31 feet of verticalrise. So for gravity flow through a 1-inch pipe at 40 psi, the storage tanks would


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have to be more than 90 feet above thehouse.

Since this elevation separation is rarelypractical or even desirable, two ways toachieve proper household water pressure

are (1) a pump, pressure tank, pressureswitch, and check valve (familiar to wellowners), or (2) an on-demand pump.

Pumps are designed to push water ratherthan to pull it. Therefore, the systemshould be designed with the pumps atthe same level and as close to the storagetanks as possible.

Pump systems draw water from thestorage tanks, pressurize it, and store it

in a pressure tank until needed. Thetypical pump-and-pressure tankarrangement consists of a - or 1-horsepower pump, usually a shallowwell jet pump or a multistage centrifugal pump, the check valve, and pressureswitch. A one-way check valve betweenthe storage tank and the pump preventspressurized water from being returned tothe tank. The pressure switch regulatesoperation of the pressure tank. The

pressure tank, with a typical capacity of40 gallons, maintains pressurethroughout the system. When thepressure tank reaches a preset threshold,the pressure switch cuts off power to the pump. When there is demand from thehousehold, the pressure switch detectsthe drop in pressure in the tank andactivates the pump, drawing more waterinto the pressure tank.

The cistern float filter (Figure 2-11)

allows the pump to draw water from thestorage tank from between 10 and 16inches below the surface. Water at thislevel is cleaner and fresher than watercloser to the bottom of the tank. Thedevice has a 60-micron filter. Anexternal suction pump, connected via a

flexible hose, draws water through thefilter.

On-demand pump

The new on-demand pumps eliminatethe need for a pressure tank. These pumps combine a pump, motor,controller, check valve, and pressure

tank function all in one unit. They areself-priming and are built with a checkvalve incorporated into the suction port.Figure 2-12 shows a typical installationof an on-demand pump and a 5-micronfiber filter, 3-micron activated charcoalfilter, and an ultraviolet lamp. Unlikeconventional pumps, on-demand pumpsare designed to activate in response to ademand, eliminating the need, cost, andspace of a pressure tank. In addition,

some on-demand pumps are specificallydesigned to be used with rainwater.

Treatment and DisinfectionEquipment

For a nonpotable system used for hoseirrigation, if tree overhang is present,leaf screens on gutters and a roof washer

Figure 2-11. Cistern float filter


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diverting 10 gallons for every 1,000square feet of roof is sufficient. If dripirrigation is planned, however, sedimentfiltration may be necessary to preventclogging of emitters. As standards differ,

the drip irrigation manufacturer orvendor should be contacted regardingfiltering of water.

For potable water systems, treatment beyond the leaf screen and roof washeris necessary to remove sediment anddisease-causing pathogens from storedwater. Treatment generally consists offiltration and disinfection processes inseries before distribution to ensurehealth and safety.

Cartridge Filters and Ultraviolet (UV)Light

The most popular disinfection array inTexas is two in-line sediment filters the 5-micron fiber cartridge filterfollowed by the 3-micron activatedcharcoal cartridge filter followed byultraviolet light. This disinfection set-upis placed after the pressure tank or afterthe on-demand pump.

It is important to note that cartridgefilters must be replaced regularly.Otherwise, the filters can actually harbor bacteria and their food supply. The 5-micron filter mechanically removessuspended particles and dust. The 3-micron filter mechanically trapsmicroscopic particles while smallerorganic molecules are absorbed by theactivated surface. In theory, activatedcharcoal can absorb objectionable odors

and tastes, and even some protozoa andcysts (Macomber, 2001).

Filters can be arrayed in parallel forgreater water flow. In other words, two5-micron fiber filters can be stacked inone large cartridge followed by two 3-micron activated charcoal filters in

another cartridge. The ultraviolet (UV)light must be rated to accommodate theincreased flow.

NSF International (National SanitationFoundation) is an independent testingand certification organization. Filter performance can be researched using asimple search feature by model ormanufacturer on the NSF website. (SeeReferences.) It is best to purchase NSF-certified equipment.

Maintenance of the UV light involvescleaning of the quartz sleeve. Many UVlights are designed with an integralwiper unit. Manual cleaning of thesleeve is not recommended due to thepossibility of breakage.

UV lamps are rated in gallons perminute. For single 5-micron and 3-micron in-line filters, a UV light rated at12 gallons per minute is sufficient. For

Figure 2-12. Typical treatment installation ofan on-demand pump, 5-micron fiber filter, 3-micron activated charcoal filter, and anultraviolet lamp (top).


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filters in parallel installation, a UV lightrated for a higher flow is needed. In-lineflow restrictors can match flow to theUV light rating.

UV lights must be replaced after a

maximum of 10,000 hours of operation.Some lights come with alarms warningof diminished intensity.

Ozone

Chemically, ozone is O3: essentially amore reactive form of molecular oxygenmade up of three atoms of oxygen.Ozone acts as a powerful oxidizing agentto reduce color, to eliminate foul odors,and to reduce total organic carbon in

water. For disinfection purposes, anozone generator forces ozone intostorage tanks through rings or a diffuserstone. Ozone is unstable and reactsquickly to revert to O2 and dissipatesthrough the atmosphere within 15minutes.

A rainwater harvesting system owner inFort Worth uses an ozone generator tokeep the water in his 25,000 gallons ofstorage fresh by circulating ozone

through the five tanks at night. Astandard sprinkler controller switches theozone feed from tank to tank.

Membrane Filtration (ReverseOsmosis and Nanofiltration)

Membrane filtration, such as reverseosmosis and nanofiltration work byforcing water under high pressurethrough a semipermeable membrane tofilter dissolved solids and salts, both of

which are in very low concentrations inrainwater. Membrane processes,however, have been known empiricallyto produce sweeter water, perhaps byfiltering out dissolved metals fromplumbing.

A certain amount of feed water is lost inany membrane filtration process. Reject

water, referred to as brine, containinga concentrate of the contaminantsfiltered from the feed water, isdischarged. The amount of reject water,however, is directly proportional to the

purity of the feed water. Rainwater, as apurer water source to begin with, wouldgenerate less brine. Reverse osmosismembranes must be changed before theyare fouled by contaminants.

Reverse osmosis (RO) equipment forhousehold use is commercially availablefrom home improvement stores such asLowes and Home Depot.

Chlorination

For those choosing to disinfect withchlorine, automatic self-dosing systemsare available. A chlorine pump injectschlorine into the water as it enters thehouse. In this system, appropriatecontact time is critical to kill bacteria. Apractical chlorine contact time is usuallyfrom 2 minutes to 5 minutes with a freechlorine residual of 2 parts per million(ppm). The time length is based on waterpH, temperature, and amount of bacteria.

Contact time increases with pH anddecreases with temperature. K values(contact times) are shown in Table 3-3.

References

Macomber P. 2001. Guidelines onrainwater catchment systems forHawaii. Manoa (HI): College ofTropical Agriculture and HumanResources, University of Hawaii at

Manoa. 51 p.NSF International, filter performance,

www.nsf.org/certified/DWTU/

Radlet J, Radlet P. 2004. Rainwaterharvesting design and installationworkshop. Boerne (TX): Save theRain.


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Rain Water Harvesting and Waste WaterSystems Pty Ltd.,www.rainharvesting.com.au

Texas Water Development Board. 1997.Texas guide to rainwater harvesting.

Austin (TX): Texas WaterDevelopment Board. 58 p.

Vasudevan L. 2002. A study ofbiological contaminants in rainwater

collected from rooftops in Bryan andCollege Station, Texas [masterthesis]. College Station (TX): TexasA&M University. 180 p.

Waterfall P. 1998. Harvesting rainwater

for landscape use. Tucson (AZ): TheUniversity of Arizona College ofAgriculture and Life Sciences. 39 p.


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Chapter 3

Water Quality and TreatmentThe raindrop as it falls from the cloud issoft, and is among the cleanest of watersources. Use of captured rainwater offers

several advantages.

Rainwater is sodium-free, a benefit forpersons on restricted sodium diets.

Irrigation with captured rainwater promotes healthy plant growth. Also, being soft water, rainwater extends thelife of appliances as it does not formscale or mineral deposits.

The environment, the catchment surface,and the storage tanks affect the quality

of harvested rainwater. With minimaltreatment and adequate care of thesystem, however, rainfall can be used aspotable water, as well as for irrigation.

The falling raindrop acquires slightacidity as it dissolves carbon dioxide andnitrogen. Contaminants captured by therain from the catchment surface andstorage tanks are of concern for thoseintending to use rainwater as their

potable water source. The catchmentarea may have dust, dirt, fecal matterfrom birds and small animals, and plantdebris such as leaves and twigs.Rainwater intended for domestic potableuse must be treated using appropriatefiltration and disinfection equipment,discussed in Chapter 2, RainwaterHarvesting System Components.

Total dissolved solids (TDS) inrainwater, originating from particulate

matter suspended in the atmosphere,range from 2 milligrams per liter (mg/lor ppm)1 to 20 mg/l across Texas,compared with municipal water TDS

1 The acronyms ppm and mg/l are approximatelyequal here because a liter of water weighs onekilogram.

ranges of 100 ppm to more than 800ppm.

The sodium content of some municipalwater ranges from 10 parts per million(ppm) to as high as 250 ppm. Rainwaterintended solely for outdoor irrigationmay need no treatment at all except for ascreen between the catchment surfaceand downspout to keep debris out of thetank, and, if the tank is to supply a dripirrigation system, a small-pore filter atthe tank outlet to keep emitters fromclogging.

Considerations for the RainwaterHarvesting System Owner

It is worth noting that owners ofrainwater harvesting systems who supplyall domestic needs essentially becomeowners of their water supply systems,responsible for routine maintenance,including filter and lamp replacement,leak repair, monitoring of water quality,and system upgrades.

The rainwater harvesting system owneris responsible for both water supply andwater quality. Maintenance of arainwater harvesting system is anongoing periodic duty, to include:

monitoring tank levels, cleaning gutters and first-flush

devices, repairing leaks, repairing and maintaining the system,

and

adopting efficient water use practices.

In addition, owners of potable systemsmust adopt a regimen of:

changing out filters regularly,


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maintaining disinfection equipment,such as cleaning and replacingultraviolet lamps, and

regularly testing water quality.

Water Quality Standards No federal or state standards existcurrently for harvested rainwater quality,although state standards may bedeveloped in 2006.

The latest list of drinking waterrequirements can be found on the UnitedStates Environmental ProtectionAgencys website. (See References.) Thenext section discusses the potentialvectors by which contaminants get into

rainwater. For those intending to harvestrainwater for potable use, themicrobiological contaminants E. coli,Cryptosporidium, Giardia lamblia, totalcoliforms, legionella, fecal coliforms,and viruses, are probably of greatestconcern, and rainwater should be testedto ensure that none of them are found(Lye, 2002). County health departmentand city building code staff should also be consulted concerning safe, sanitary

operations and construction of rainwaterharvesting systems.

Factors Affecting Water Quality

pH (acidity/alkalinity)

As a raindrop falls and comes in contactwith the atmosphere, it dissolvesnaturally occurring carbon dioxide toform a weak acid. The resultant pH isabout 5.7, whereas a pH of 7.0 is neutral.

(A slight buffering using 1 tablespoon of baking soda to 100 gallons of water inthe tank will neutralize the acid, ifdesired. Also, a concrete storage tankwill impart a slight alkalinity to thewater.) While Northeast Texas tends toexperience an even lower pH (moreacidic) rainwater than in other parts of

the state, acid rain is not considered aserious concern in Texas.

Particulate matter

Particulate matter refers to smoke, dust,and soot suspended in the air. Fineparticulates can be emitted by industrialand residential combustion, vehicleexhaust, agricultural controlled burns,and sandstorms. As rainwater fallsthrough the atmosphere, it canincorporate these contaminants.

Particulate matter is generally not aconcern for rainwater harvesting inTexas. However, if you wish, geographicdata on particulate matter can be

accessed at the Air Quality Monitoringweb page of the Texas Commission onEnvironmental Quality (TCEQ). (SeeReferences.)

Chemical compounds

Information on chemical constituentscan also be found on the TCEQ AirQuality website. (See References.)

In agricultural areas, rainwater couldhave a higher concentration of nitrates

due to fertilizer residue in theatmosphere (Thomas and Grenne, 1993).Pesticide residues from crop dusting inagricultural areas may also be present.

Also, dust derived from calcium-richsoils in Central and West Texas can add1 mg/l to 2 mg/l of hardness to the water.Hard water has a high mineral content,usually consisting of calcium andmagnesium in the form of carbonates.

In industrial areas, rainwater samplescan have slightly higher values ofsuspended solids concentration andturbidity due to the greater amount ofparticulate matter in the air (Thomas andGrenne, 1993).


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Catchment surface

When rainwater comes in contact with acatchment surface, it can wash bacteria,molds, algae, fecal matter, other organicmatter, and/or dust into storage tanks.The longer the span of continuousnumber of dry days (days withoutrainfall), the more catchment debris iswashed off the roof by a rainfall event(Thomas and Grenne, 1993; Vasudevan,2002).

Tanks

The more filtering of rainwater prior tothe storage tanks, the less sedimentationand introduction of organic matter willoccur within the tanks. Gutter screens,

first-flush diverters, roof washers, andother types of pre-tank filters arediscussed in Chapter 2. Sedimentationreduces the capacity of tanks, and the breakdown of plant and animal mattermay affect the color and taste of water,in addition to providing nutrients formicroorganisms.

Most storage tanks are equipped withmanholes to allow access for cleaning.Sediment and sludge can be pumped outor siphoned out using hose with aninverted funnel at one end withoutdraining the tank annually.

Multiple linked tanks allow one tank tobe taken off line for cleaning by closing

the valve on the linking pipe betweentanks.

Water Treatment

The cleanliness of the roof in a rainwater

harvesting system most directly affectsthe quality of the captured water. Thecleaner the roof, the less strain is placedon the treatment equipment. It isadvisable that overhanging branches becut away both to avoid tree litter and todeny access to the roof by rodents andlizards.

For potable systems, a plain galvanizedroof or a metal roof with epoxy or latex paint is recommended. Composite or

asphalt shingles are not advisable, astoxic components can be leached out byrainwater. See Chapter 2 for moreinformation on roofing material.

To improve water quality, severaltreatment methods are discussed. It is theresponsibility of the individual installeror homeowner to weigh the advantagesand disadvantages of each method forappropriateness for the individualsituation. A synopsis of treatmenttechniques is shown in Table 3-1. Adiscussion of the equipment is includedin Chapter 2.


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Table 3-1. Treatment Techniques

METHOD LOCATION RESULT

Treatment

Screening

Leaf screens and strainers gutters and downspouts prevent leaves and otherdebris from entering tank

Settling

Sedimentation within tank settles out particulate matter

Activated charcoal before tap removes chlorine*

Filtering

Roof washer before tank eliminates suspendedmaterial

In-line/multi-cartridge after pump sieves sediment

Activated charcoal after sediment filterremoves chlorine, improvestaste

Slow sand separate tank traps particulate matter

Microbiological treatment

/Disinfection

Boiling/distilling before use kills microorganisms

Chemical treatments(Chlorine or Iodine)

within tank or at pump(liquid, tablet, or granular)

before activated charcoalfilter

kills microorganisms

Ultraviolet light after activated charcoalfilter, before tap


Ozonation after activated charcoalfilter, before tap


Nanofiltration before use; polymermembrane

(pores 10-3 to 10-6 inch)

removes molecules

Reverse osmosis before use: polymermembrane (pores 10-9 inch)

removes ions (contaminantsand microorganisms)

*Should be used if chlorine hasbeen used as a disinfectant.

Adapted from Texas Guide to Rainwater Harvesting, Second Edition, Texas Water DevelopmentBoard, 1997.


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Chlorination

Chlorination is mentioned here more forits historical value than for practicalapplication. Chlorine has been used todisinfect public drinking water since1908, and it is still used extensively byrainwater harvesters in Hawaii, the U.S.Virgin Islands, and in older rainwaterharvesting systems in Kentucky andOhio. Chlorine must be present in aconcentration of 1 ppm to achievedisinfection. Liquid chlorine, in the formof laundry bleach, usually has 6 percentavailable sodium hypochlorite. Fordisinfection purposes, 2 fluid ounces( cup) must be added per 1,000 gallonsof rainwater. Household bleach products,however, are not labeled for use in watertreatment by the Food and DrugAdministration. A purer form ofchlorine, which comes in solid form forswimming pool disinfection, is calciumhypochlorite, usually with 75 percentavailable chlorine. At that strength, 0.85ounces by weight in 1,000 gallons ofwater would result in a level of 1 ppm.

In either case, it is a good idea tocarefully dilute the chlorine source in a bucket of water, and then stir with aclean paddle to hasten mixing(Macomber, 2001). Chlorine contact

times are show in Table 3-2.The use of chlorine for disinfection presents a few drawbacks. Chlorinecombines with decaying organic matterin water to form trihalomethanes. Thisdisinfection by-product has been foundto cause cancer in laboratory rats. Also,some users may find the taste and smellof chlorine objectionable. To addressthis concern, an activated carbon filtermay be used to help remove chlorine.

Chlorine does not kill Giardia orCryptosporidium, which are cysts protected by their outer shells. Personswith weakened or compromised immunesystems are particularly susceptible tothese maladies. To filter out Giardia andCryptosporidum cysts, an absolute 1-micron filter, certified by the NSF, isneeded(Macomber, 2001).

Table 3-2. Contact Time with Chlorine

WaterpH

Water temperature

50 F orwarmer

45 F 40 F orcolder

Contact time in minutes

6.0 3 4 5

6.5 4 5 6

7.0 8 10 12

7.5 12 15 188.0 16 20 24

UV Light

UV light has been used in Europe fordisinfection of water since the early1900s, and its use has now become

common practice in U.S. utilities.Bacteria, virus, and cysts are killed byexposure to UV light. The water must go


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through sediment filtration before theultraviolet light treatment becausepathogens can be shadowed from the UVlight by suspended particles in the water.In water with very high bacterial counts,

some bacteria will be shielded by thebodies of other bacteria cells.

UV lights are benign: they disinfectwithout leaving behind any disinfection by-products. They use minimal powerfor operation. One should followmanufacturers recommendations forreplacement of bulbs.

Testing

Harvested rainwater should be tested

before drinking and periodicallythereafter. Harvested rainwater shouldbe tested both before and after treatmentto ensure treatment is working. It isadvisable to test water quarterly at aminimum, if used for drinking.

Harvested rainwater can be tested by acommercial analytical laboratory, thecounty health departments of manyTexas counties, or the Texas Departmentof Health.

Before capturing rainwater samples fortesting, contact the testing entity first to become informed of requirements forcontainer type and cleanliness, samplevolume, number of samples needed, andtime constraints for return of the sample.

For instance, for total coliform testing,water must usually be captured in asterile container issued by the testingentity and returned within a maximum of

30 to 36 hours. Testing for pH, performed by commercial analyticallaboratories must be done on site; othertests are less time-critical.

A list of county health departments thatwill test for total and fecal coliform can be found on the Texas Department ofState Health Services (TDSHS) website.

(See References.) The testing fee isusually between $15 and $25.Homeowners should contact the healthdepartment prior to sample collection toprocure a collection kit and to learn the

proper methods for a grab sample or afaucet sample.

Texas Department of State HealthServices will test for fecal coliforms fora fee of $20 per sample. (SeeReferences.) A collection kit can beordered from TDSHS at (512) 458-7598.

Commercial laboratories are listed intelephone Yellow Pages underLaboratoriesAnalytical & Testing. Fora fee, the lab will test water for pathogens. For an additional fee, labswill test for other contaminants, such asmetals and pesticides.

References

Lye D. 2002. Health risks associatedwith consumption of untreated waterfrom household roof catchmentsystems. Journal of the AmericanWater Resources Association38(5):1301-1306.

Macomber P. 2001. Guidelines onrainwater catchment systems forHawaii. Manoa (HI): College ofTropical Agriculture and HumanResources, University of Hawaii atManoa. 51 p.

Texas Commission on EnvironmentalQuality, Air Quality Monitoring,www.tceq.state.tx.us/nav/data/pm25.

html

Texas Commission on EnvironmentalQuality, chemical constituents,www.tnrcc.state.tx.us/airquality.html

Texas Department of State HealthServices, county health departments,


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www.dshs.state.tx.us/regions/default.shtm

Texas Department of State HealthServices, testing for fecal coliforms,www.dshs.state.tx.us/lab/default.shtm

Thomas PR, Grenne GR. 1993.Rainwater quality from different roofcatchments. Water ScienceTechnology (28):290-99.

United States Environmental ProtectionAgency, drinking water requirements,www.epa.gov/safewater/mcl.html

Vasudevan L. 2002. A study of biological contaminants in rainwater

collected from rooftops in Bryan andCollege Station, Texas [mastersthesis]. College Station (TX): TexasA&M University. 90 p.


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Chapter 4

Water Balance and System SizingThe basic rule for sizing any rainwaterharvesting system is that the volume ofwater that can be captured and stored

(the supply) must equal or exceed thevolume of water used (the demand).

The variables of rainfall and waterdemand determine the relationship between required catchment area andstorage capacity. In some cases, it may be necessary to increase catchmentsurface area by addition of a rain barn oroutbuilding to capture enough rainwaterto meet demand. Cistern capacity mustbe sufficient to store enough water to seethe system and its users through thelongest expected interval without rain.

The following sections describe ways todetermine the amount of rainfall, theestimated demand, and how muchstorage capacity is needed to provide anadequate water supply.

Intended End Use

The first decision in rainwater harvestingsystem design is the intended use of thewater. If rainwater is to be used only forirrigation, a rough estimate of demand,supply, and storage capacity may be

sufficient. On the other hand, ifrainwater is intended to be the solesource of water for all indoor and

outdoor domestic end uses, a moreprecise reckoning is necessary to ensureadequate supply.

How Much Water Can BeCaptured?

In theory, approximately 0.62 gallonsper square foot of collection surface perinch of rainfall can be collected. Inpractice, however, some rainwater is lostto first flush, evaporation, splash-out or

overshoot from the gutters in hard rains,and possibly leaks. Rough collectionsurfaces are less efficient at conveyingwater, as water captured in pore spacestends to be lost to evaporation.

Also impacting achievable efficiency isthe inability of the system to capture allwater during intense rainfall events. Forinstance, if the flow-through capacity ofa filter-type roof washer is exceeded,spillage may occur. Additionally, afterstorage tanks are full, rainwater can belost as overflow.

Figure 4-1. Catchment areas of three different roofs


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For planning purposes, therefore, theseinherent inefficiencies of the systemneed to be factored into the water supplycalculation. Most installers assume anefficiency of 75 percent to 90 percent.

In most Texas locations, rainfall occursseasonally, requiring a storage capacitysufficient to store water collected duringrainy times to last through the dry spells.

In West Texas, total annual rainfallmight not be sufficient to allow aresidence with a moderate-sizedcollection surface to capture sufficientwater for all domestic use. Someresidences might be constrained by thearea of the collection surfaces or the

volume of storage capacity that can beinstalled.

Collection Surface

The collection surface is the footprintof the roof (Figure 4-1). In other words,regardless of the pitch of the roof, theeffective collection surface is the areacovered by collection surface (lengthtimes width of the roof from eave to

eave and front to rear). Obviously if onlyone side of the structure is guttered, onlythe area drained by the gutters is used inthe calculation.

Rainfall Distribution

In Texas, average annual rainfalldecreases roughly 1 inch every 15 miles,

Figure 4-2. Average annual precipitationin Texas, in inches


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as you go from east to west (Figure 4-2),from 56 inches per year in Beaumont to

less than 8 inches per year in El Paso. Asone moves westward across the state, theprevalence and severity of droughts mustalso be considered.

To ensure a year-round water supply, thecatchment area and storage capacitymust be sized to meet water demandthrough the longest expected intervalwithout rain. For instance, in WestTexas, the historic longest span of

continuous dry days has exceeded threemonths. For reference purposes, acontour map of historical maximumnumber of dry days in Texas is shown inFigure 4-3 (Krishna, 2003). If therainwater harvesting system is intendedto be the sole water source for ahousehold, the designer must size thesystem to accommodate the longestanticipated time without rain, orotherwise plan for another water source,

such as a well backup or hauled water.

Also, rainfall from high-intensity, short-duration rainfall events may be lost tooverflow from storage tanks or splash-out from the gutters. Although theseintense rainfall events are considered part of the cumulative annual rainfall,

the total available volume of such anevent is rarely captured.

Another consideration is that mostrainfall occurs seasonally; annual rainfallis not evenly distributed throughout the

12 months of the year. The monthlydistribution of rainfall is an importantfactor to consider for sizing a system.Monthly rainfall data for selected Texascities is given in Appendix B.

Monthly Rainfall

Two different estimators of monthlyrainfall are commonly used: averagerainfall and median rainfall. Averageannual rainfall is calculated by taking the

sum of historical rainfall and dividing bythe number of years of recorded data.This information is available fromnumerous public sources, including the National Climate Data Center website.(See References.) Median rainfall is theamount of rainfall that occurs in themidpoint of all historic rainfall totals forany given month. In other words,historically for the month in question,half of the time the rainfall was less than

the median and half of the time rainfallwas more than the median. Medianvalues and average rainfall values forrepresentative Texas cities are providedin Appendix B.

Median rainfall provides for a moreconservative calculation of system sizingthan average rainfall. The median valuefor rainfall is usually lower than theaverage value since large rainfall eventstend to drive the average value higher. In

other words, the sum of monthlymedians is lower than the annual averagedue to the fact that the arithmeticaverage is skewed by high-intensityrainfall events. For planning purposes,median monthly rainfall can be used toestimate water availability to a

Figure 4-3. Maximum number of dry days(Krishna, 2003)


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reasonable degree of certainty (Krishna,2001).

For example, in the sample calculationsat the end of this chapter, the averageannual rainfall for Dallas is about 35.0

inches, but the sum of the monthlymedians is only 29.3 inches.

Calculating Storage Capacity

Once the median or average potential forrainfall capture is known from rainfalldata and catchment area, it will benecessary to calculate storage capacity.The decision of whether rainwater will be used for irrigation, potable anddomestic use, or both, will dictate water

demand, and therefore, capacity.

A simple method of roughly estimatingstorage capacity popular among professional installers is to size thestorage capacity to meet quarterlydemand. The system is sized to meetestimated demand for a three-month period without rain. Annual estimateddemand is divided by four to yieldnecessary storage capacity using thisapproach. This approach, however, mayresult in a more expensive system due tohigher storage costs.

If a rainwater harvesting system is to bethe sole water supply, overbuildingensures a safety margin. As with manythings in life, it helps to hope for the best but plan for the worst. Even when budget constraints may not allow theuser to install as much storage capacityas a sizing method may indicate, it is

important to provide for an area whereadditional tanks or cisterns can beinstalled at a later date when financespermit.

The Water Balance Method UsingMonthly Demand and Supply

One method of determining thefeasibility of a proposed system is themonthly water balance method. This

method of calculation is similar tomaintaining a monthly checkbook balance. Starting with an assumedvolume of water already in the tanks, thevolume captured each month is added tothe previous balance and the demand issubtracted. The initial volume of waterin the tanks would be provided byhauling or capturing water prior towithdrawing water from the system. Anexample is presented at the end of this

chapter.Data and calculations can be entered onan electronic spreadsheet to enable theuser to compare different variables ofcatchment area and storage. It issuggested that homeowners experimentwith different variables of storagecapacity and, if applicable, catchmentsurface to find individual levels ofcomfort and affordability for catchmentsize and storage capacity.

As mentioned above:

catchment area and rainfall determinesupply, and

demand dictates required storagecapacity.

A commitment to conserving water withwater-saving fixtures, appliances, practices indoors, and low-water-uselandscaping outdoors is an essential

component of any rainwater harvestingsystem design. Not only is conservationgood stewardship of natural resources, italso reduces the costs for storagecapacity and related system components.

If the amount of rainwater that can becaptured calculated from roof area andrainfall is adequate or more than


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adequate to meet estimated demand, andmeets the physical constraints of the building design, then storage capacitycan be sized to meet estimated demand.If the monthly amount of water that can

be captured, accounting for dry spells, isless than monthly estimated demand,then additional catchment area orsupplemental supplies of water (such asgroundwater from a well) will need to beconsidered.

In drier areas, no matter how large thestorage capacity, catchment area mayneed to be increased with a rain barn oradditional roof area to meet demand.

At the end of this chapter, an example ofa water balance calculation is shown forthe City of Dallas.

Estimating Demand

A water-conserving household will use between 25 and 50 gallons per person per day. (Note that total gallons percapita per day figures published formunicipalities divide all the waterdistributed by the population, yielding amuch larger amount per capita thanactual domestic consumption.)

Households served previously by a waterutility can read monthly demand fromtheir meter or water bill to find monthlydemand for purposes of building a newrainwater harvesting system. Divide themonthly total by the number of people inthe house, and the days in the month toget a daily per capita demand number.

Water conservation is covered later in

this chapter. Households solelydependent upon rainwater should adopt

efficient water use practices both indoorsand outdoors.

Estimating indoor water demand

Indoor water demand is largely

unaffected by changes in weather,although changes in householdoccupancy rates depending upon seasonsand ages of household members, morewater use during the hot summermonths, and very minor changes inconsumption of water due to increases intemperature may be worth factoring insome instances. The results of a study of1,200 single-family homes by theAmerican Water Works Association(AWWA) in 1999 found that the averagewater conserving households usedapproximately 49.6 gallons per person per day (American Water WorksAssociation, 1999).

Table 4-1 can be used to calculate indoorwater demand. Many households useless than the average of 49.6 gallons per person found in the 1999 report by theAWWA,Residential End Uses of Water.The water volumes shown in the table

assume a water-conserving household,with water-conserving fixtures and goodpractices, such as shutting off the waterwhile brushing teeth or shaving. Overalldemand in showers, baths, and faucetuses is a function of both time of use andrate of flow. Many people do not openthe flow rate as high as it could befinding low or moderate flow rates morecomfortable. In estimating demand,measuring flow rates and consumption

in the household may be worth the effortto get more accurate estimates.


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Table 4-1. Estimating Indoor Daily Domestic Demand

A.

Water

consumption

using

conserving

fixtures

B.

Assumptions

from AWWA

Residential End-

Use Study

C.

Adjustments to

assumptions

(adjust up or

down according

to actual use)

D.

Number of

persons in

household

E.

Household

monthly

demand

A x (B or C )x D

x 30

Toilets (use

only

appropriate

type)

ULFT 1.6 gal/flush 6 flushes/person/day

Dual Flush 1 gal/flushliquids

1.6 gal/flushsolids

6 flushes/person/day

Baths &

showers

Showerhead 2.2 gal/min 5 minutes/person/day

Bath 50 gal/bath NA

Faucets

(personalhygiene,cooking, andcleaning of

surfaces)

2.2gal/faucet/min

5 minutes/person/day

Appliances or uses which are measured on a per-use basis (not a per-person basis):

Clothes washer

Front-loading(horizontal-axis)

1825 gal/load 2.6 loads/week

Dishwasher 8 gal/cycle 0.7 cycles/day

Miscellaneous

other

Total

One can use Table 4-1 if the designerprefers to incorporate known or expectedbehavioral habits into the water demandestimates. The values in the first columnare to be multiplied by variablesreflecting your own household water use

patterns. The average values in thesecond column are offered forinformation, but as with all averages, aresubject to wide variation based uponactual circumstances. An example isdual flush toilets multiply three flushes


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per day liquid only (1 gpf), and add threeflushes per day for solids (1.6 gpf), (3x1)+ (3x1.6) = 7.8 gallons multiplied by 3persons = 23.4 gpd household demand x30 days = 702 gallons per month. The

authors recommend verifying anyassumptions against the records ofhistorical use from a municipal water billif available.

Indoor water conservation

Indoor domestic water conservation can be achieved by a combination offixtures, appliances, and water-conserving practices. The advantage ofwater-conserving appliances is that theyrequire no change in household routine.Some water-conserving practices needuser action, such as turning off the waterwhile brushing teeth or shaving; washingvegetables in a pan rather than under astream; washing only full loads oflaundry and dishes; and keeping apitcher of water in the refrigerator, ratherthan waiting for cold water to arrivefrom a faucet.

Water conservation appliances include:

Ultralow flush toilets (ULFTs). Since1993, only ULFTs with 1.6 gallons per flush may be sold in the UnitedStates. Older toilets should bereplaced with the more efficientmodels. Some of the ULFTs requirespecial early closing flappers tomaintain their low-flow rates, so careshould be taken in purchasing thecorrect replacement flapper forleaking toilets. If purchasing a new

toilet, those that do not use earlyclosure flappers are recommended.Dual-flush toilets (using less volumefor liquid wastes) are also a goodchoice for a water-wise household.

Faucet aerators and efficient

showerheads. These fixtures are

designed to use 2.2 gallons per minuteat 60 psi, or 2.5 gpm at 80 psi (Table4-1). Studies have shown that most people feel comfortable at less thanfull flow rates, so using the new

fixtures (which are the only ones soldin the United States since 1992)should provide you with an efficientand comfortable experience.

Hot water on demand. These wall-mounted units heat water just prior touse, eliminating the waste of waitingfor hot water from the water heaterwhile cold water is allowed to flowdown the drain. Hot water loopsystems keep hot water continuously

circulating to achieve the same goal,but can use more energy. Another on-demand unit heats water quickly onlywhen activated by a pushbutton,rather than circulating water through aloop, saving both water and energy. Arebate from San Antonio WaterSystem (SAWS) is available forinstallation of this type of on-demandcirculation system.

Horizontal-axis (front-loading) clothes

washers.Because clothes are tumbledthrough a small volume of water inthe bottom of the drum (rather thanwashed in a full tub of water), thisappliance can save up to half thewater of a traditional clothes washer.It is also as much as 42 percent moreenergy efficient. A list of front-loading, horizontal-axis clotheswashers is maintained by theConsortium for Energy Efficiency

online. (See References.) Severalmunicipal utilities in Texas, includingCity of Austin, SAWS, and BexarMet, offer rebates for the purchase ofthese energy- and water-efficientappliances.


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Estimating outdoor water demand

Outdoor water demand peaks in hot, drysummer. In fact, as much as 60 percentof municipal water demand in thesummer is attributable to irrigation.

The water demands of a large turfgrassarea almost always preclude the sole useof harvested rainwater for irrigation.

Many urban dwellers capture rainwaterfor irrigation of vegetable andornamental gardens. Because it is free ofsalts and minerals, rainwater promoteshealthy plant growth. In urban areas,rainwater harvesters may reduce theirwater bill by substituting harvestedrainwater for municipal water for gardenirrigation.

For both the health of landscape plantsand water use-efficiency, the best way towater plants is according to their needs.For most plants adapted to Texasclimate, water stress is visually evidentwell before plant death. Signs of waterstress include a gray blue tint to leaves,leaf rolling, and in the case of turfgrass,a footprint that does not spring back.

Watering infrequently and deeply has been shown to promote plant health,waiting until plants need the water helpsthe water user to be sure that they aregrowing a healthy landscape.

For planning purposes, historicalevapotranspiration can be used to project potential water de

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