“Drinking Rainwater from a Cistern – Where Water Treatment and Rainharvesting Collide”

What’s a pro to do?

By Scott O. Shaffer

Part One in Three-Part Series

Introduction:  Conflicting research has muddied the water, confused the public, created unnecessary roadblocks for water treatment professionals and possibly mis-led government about the safety of drinking rainwater from cisterns. Water treatment professionals are natural advocates for informed water treatment policy and practices informed by the best in water quality research and professional practice. This is the first in a 3-part series that aims to point the way to solutions. 

Water treatment professionals are most likely to cross paths with rainharvest system design-builders when customers are interested in drinking rainwater. It’s been our experience that licensed water treatment specialists are the “ones to call” at this intersection.

We’re not likely to be consulted by customers who use or plan to use their cistern water for landscaping, gardening or a variety of supplemental non-potable uses.  Water quality in those applications is unlikely to be much of a concern. But for prospects who are considering (or already have begun) drinking cistern rainwater, water treatment professionals have a role and an opportunity.

But what should we know about rainwater quality and how to design and build potable rainwater systems? What does the research tell us? What does experience tell us? What do we tell our customers who are becoming more and more interested in this fast-growing segment of the business?

This article addresses these questions and provides some direction for the water treatment professional interested in, or already involved with, designing and building potable rainharvesting systems.

What does ‘experience’ tell us about the quality of rainwater?

All we “know” is what we hear, and what we hear is usually some form of these three comments:

“My grandpa drank water straight from a metal cistern on his back porch all his life. He never got sick…and I’m doing the same.”

“I haven’t tested the (rain) water in my cistern but I know there’s nothing in it. I’ve been drinking it for years.”

I don’t trust rainwater for drinking, don’t know if it’s safe and have heard about bird poop coming off the roof spoiling the cistern – so I don’t drink it!”

So “experience” isn’t much help, at least, in our experience. We have been helping customers design and build rainharvesting systems for more than three years and teach classes at The Rainharvest School and have never met anyone with a RWH system who has offered to share a water analysis of their cistern water. Not a critique – just a fact.

What does the academic research tell us?

A review of several recent studies of rainwater quality by university staffs in Texas and Australia indicate that, “What we know,” depends on who we ask and how they have done their tests.

2009 research Monash University in Australia where rainwater is farther along than probably anywhere in the world,  stated its research objectives as follows: to determine the viability of microbial water quality over time; to examine specific physic-chemical water quality parameters of a subset of rainwater tanks; to determine the effect of tank maintenance and characteristics on water quality; and, to determine the effect of rainfall on microbial water quality (“Quality of stored rainwater used for drinking in metropolitan South Australia, Department of Epidemiology and Preventive Medicine, Monash University, Oct. 2009).

The study involved 300 volunteer households that were given filters to treat their rainwater. Half of the filters were real and half were not. According to Karin Leder, lead researcher in conjunction with Water Quality Research Australia, “The results showed that rates of gastroenteritis between both groups were very similar… People who drank untreated rainwater displayed no measurable increase in illness compared to those that consumed the filtered rainwater” (Water Quality Magazine, “Options for Rainwater Disinfection,” 2012)

Peter Coombs, associate professor representing Bonacci Water, Melbourne University and University of Newcastle at the American Rainwater Catchment Systems Association annual meeting in Austin in 2011 highlighted their research, including:

Use of “standard tests” can lead to false view that rainwater is unsafe

Critical review of analysis methods required for rainwater

PCR analysis revealed that majority of Coliform bacteria were Bacillus Spp. confirmation that biofilms exist in tanks - origin of bacteria is from soil, not from human, bird or animal sources thus presence of pathogens highly unlikely - results from presumptive E. coli tests unreliable -

note that majority of human pathogens are sourced from higher order mammals.

Biofilms shown to remove bacteria, chemicals and metals from the water column (ongoing research into this finding)

“Effect of Roof Material on Water Quality for Rainharvesting Systems,” published by Texas Water Development Board and conducted by University of Texas faculty in the Civil, Architectural and Environmental Engineering Department came to a very different conclusion from Coombs. EWRE faculty researchers reported, “rainwater harvested after the first flush was found to contain some contaminants at concentrations above USEPA drinking water standards (i.e., turbidity, total coliform, fecal coliform, iron, and aluminum) and that “harvested rainwater must be treated prior to potable use.”

Unfortunately the EWRE team appears to have used clear (or transparent) first flush diverters and “cisterns” in their research design– something professional RWH system installers do not do and actively counsel strongly against.  If light gets into the cistern (or first flush diverter) bacteria flourishes.

It appears that the research team used cisterns with plastic or non-metal spigots. This is not a good idea if you plan to test for bacteria and drink the water. We always recommend installing cisterns with metal spigots so the spigot can be “flamed,” not melted (see “Water Sampling for Bacteria Testing”). This should help to keep “false positives” on your coliform analysis to a minimum.

Rainwater quality research conducted at Stephen F. Austin University during the past two years titled “Demographics, Practices and Water Quality form Domestic Potable Rainwater Harvesting Systems,” concluded, “although water from roof – collected rainwater harvesting systems was generally within drinking water standards, regular testing should be encouraged to avoid potential health problems.”

What does field testing and private lab testing tell us about rainwater quality in Central Texas?

During the Summer, 2012 Water Wise Conference on rainwater quality, LCRA’s Environmental Laboratory Services reported rainwater quality test results for composite and metal roofs. These tests results were taken from water analysis requested by owners of rainharvesting systems across Central Texas during the past year. Table I below shows the lab’s findings along with maximum contaminant levels for ground and surface water. Values found to be above MCL are highlighted in red.

Table I also shows preliminary rainwater quality testing results from the past two years at our testing site in La Grange, Texas. Our pH, hardness and TDS results are very similar to ELS. Both the ELS and TWS tests suggest relatively low pH rainwater from metal roofs. Composite roofs (which represent a minority of in-place rainharvesting systems) showed TOC, turbidity clearly above MCL standards.

What do we tell our customers?

First, DO NOT TELL YOUR CUSTOMER, “You must disinfect rainwater … there’s bird poop in it!

DO RECOMMEND that any customer interested in drinking rainwater:

First: Design a system properly, recommending

1.   gutter screen or gutter foam to catch leaves and debris that can collect on the roof between rains and elevate rainwater tannin levels;

2.   a first-flush diverter to catch and capture the dirtiest water likely to come off your roof as rain begins falling;

3.   a design and materials that allow no air leaks in your PVC collection system (example: trying to tie square metal gutter downspouts to round PVC pipe);

4.   making sure there are no light leaks in your cistern by using a cistern material that doesn’t allow light to hit the cistern water;

5.    making sure the cistern is rated for potable use;

6.   using a metal (not plastic) spigot (we like lever-style) at the cistern outlet to allow proper bacteria testing (we flame all spigots when testing rainwater, city water or well water);

Second: If the customer intends to drink their rainwater:

1.   Do a complete rainwater-specific water analysis with focus on bacteria and parameters that most affect effectiveness of chlorine, ozone and UV equipment

Third: If the analysis shows bacteria present, discuss options:

1.   Chlorination

2.   Ozone

3.   Ultraviolet

Fourth: If UV is your customer’s choice:

1.   Install properly sized UV equipment

2.   Retake bacteria test to determine “present” or “absent”

3.   If test comes back “present” check UV lamp and re-take rainwater sample making sure to follow all steps for bacteria sample collection.

4.   Bacteria test should come back “absent”

Fifth: Review with customer requirement for (annual) replacement of UV light.

Sixth: If you get a “coliform present” lab report, there are several things you can recommend

1.   Do a water test designed specifically for the type of water treatment you suspect you will be recommending (chlorination, ozone, UV, etc.). On the basis of these results, you will be able to recommend the proper water treatment equipment to maximize the disinfection effectiveness.

Shaffer is a partner in Texas Water Savers Co., staff member of The Rainharvest School,

Co-host of Water Wise Conferences, and member of Texas Water Quality Association.

Copyright 2012


Rainwater Quality Research - Selected Tests & Parameters Rainwater Quality Research - Selected Tests & Parameters


WWC I Presentation : August 2012  


“Facts & Fables for Rainwater Quality” a presentation by Environmental Laboratory Services, Inc.

Ariana Dean, Tess Abbott and Susan Benevides presented, for the first time in a public forum, results of their lab

analysis of multiple samples of cistern collected rainwater from across the state. Their reported results are shown on the

following page. The rainwater samples had been submitted to the lab during the previous year and the results Dean and

Abbott presented were generally consistent with previous academic and private research.