Water Services That Last
Life-Cycle Costs Approach
Philanthropedia medal
INGO Accountability Charter logo

Effects of first flush on rainwater quality

Updated - Monday 19 June 2006

Our reading of the literature has found that First Flush (FF) systems contribute signifcantly to reducing the contaminents that are washed off the roof harvesting area. We are looking for research on FF systems and any evidence to demonstrate effectiveness.

Research on effects of first flush on rainwater quality

Little detailed specific research has been done on effects of first flush of rainwater. The common expectation is that it will improve the chemical, physical and microbiological quality of the collected/stored water.

Debris after first flush (roof water harvesting)

Research by Warwick University (UK) in Uganda and Sri Lanka showed substantial reductions in turbidity of the rainwater; they give recommendations on how much to flush (see below).

Research in Australia indicates that the effects are limited on bacteriological and heavy metal reductions while a lot of water may be lost. The hypothesis is that geographical locations (city, traffic, dust, rural, trees etc.) may be important factors for the contamination on the roof, and the need/effect of first flush. Researchers conclude that limited evidence is available and more research is needed.

Research Warwick University (DTU)

Measuring the effects of roofing material on turbidity of first harvested rainwater and amount of first flush recommended (research in Uganda and Sri Lanka). Conclusion: rule of thumb: “For each mm of first flush the contaminate load will halve”. To design a first-flush diversion system we therefore suggest you:

  1. Measure mean runoff turbidity on wet days following at least 3 dry days,
  2. Select a target maximum turbidity at which we will allow water to enter the main tank - 20 NTU is usually sufficient,
  3. Employ the table on recommended first-flush amounts to decide how many millimeters to divert (table 1, p. 7 in the paper of Martinson and Thomas - see below)
  4. Divert that amount of water whenever rainfall follows at least 3 dry days.

Read more: Martinson, D. and Thomas, T. (2005?). Quantifying the first-flush phenomenon. Coventry, United Kingdom, DTU/University of Warwick. Paper presented at the 12th IRCSA conference, New Delhi, India.

Overall general research findings

Rainwater users can reduce their risks of disease from contaminated rainwater consumption by regular maintenance and using a well-designed system. A range of enteric pathogens has been found in roof-collected rainwater including Salmonella, Campylobacter, Giardia and Cryptosporidium. The likely sources of these pathogens were faecal material deposited by birds, frogs, rodents and possums, and dead animals and insects, either in the gutters or in the water tank itself.

Read more: Rainwater Research. Information on research by Dr Peter Coombes of the University of Newcastle in Australia and Stan Abbott, Director of the Roof Water Research Centre at Massey University in Wellington, New Zealand. Web site is set up by the Australian based company called Rain Harvesting.

Research Australia

Chemical water quality and health risk assessment of urban rainwater tanks

  • Dr Heather Chapman, CRC Water Quality and Treatment, Australia
  • Mr Ted Gardner, Department of Natural Resources and Mines, Australia
  • Mr Rob Huston, Griffith University, Australia
  • Dr Andrew Chan, Griffith University, Australia
  • Dr Glen Shaw, University of Queensland, Australia

There has been interest in the use of rainwater tanks in urban areas in the last few years as catchments yields become less predictable and urban populations continue to increase.

The Cooperative Research Centre for Water Quality and Treatment (CRCWQT) has been conducting research developed to assess the quality of water from rainwater tanks and to determine appropriate end uses. This presentation will provide results of the study to date. This will include data from (i) a reconnaissance of water quality in urban rainwater tanks in various locations around Australia, (ii) ongoing sampling and testing at facilities in Melbourne and in Brisbane (including the investigation of filters, UV devices and hot water systems as a means of water disinfection).

Research in Brisbane has also been undertaken on first flush devices that have become almost an article of faith in helping protect rainwater quality. However, detailed event studies undertaken in South East Queensland (SEQ) have shown that discarding the first millimetre of runoff usually does little to substantially reduce the event mean concentration of bacteria and heavy metals entering a rainwater tank during a storm event.

Moreover separating heavy metals into particulate and soluble forms is important in deciding on appropriate treatment technologies and allowable end uses (particulates tend to settle as tank sludge). First flush devices can also substantially reduce the catch of rainwater tanks.

The SEQ first flush study highlighted the complexity of roof runoff chemistry and a PhD project is underway in SEQ which investigates the effect of geographic location (i.e. cities, suburbs, air quality) on bulk contaminant deposition on roofs, the interaction of roofing material and rainfall characteristics on contaminant concentration in the runoff, and the transformation of contaminants that occur in a rainwater tank due to biogeochemical processes including biofilms.

The results from the research undertaken by the CRCWQT will hopefully enable a better assessment of the water quality of rainwater tanks for various end uses and also a better assessment of the suitability of certain water quality improvement devices. Conference Organiser - ICMS Pty Ltd

More from Australia

The available research data do not permit any definitive conclusions to be drawn about the effect of roof or tank materials on health-related water quality parameters. Similarly there is little information relating management practices to water quality, although there is experimental support for the capacity of first flush diversion to improve the quality of collected water. Surveys of householders indicate that cleaning of sludge from tanks is generally rare, although cleaning of gutters is more frequent.

Read more: Executive summary of Public health aspects of rainwater tanks in urban Australia by the Cooperative Research Centre for Water Quality and Treatment.

Why are first flush devices needed?

There are many studies that indicate the need for first flush devices. First flush devices remove most of the dirt, debris, and contaminants that have collected on the roof prior to the rain. In addition, these devices also minimize the effects of acid rain and other airborne pollutants that are scoured from the surrounding sky as the rain begins to fall.

As regards independent sources of information we are happy to share the following links that may offer additional insight and information on the need for first flush devices. Some general information on rainwater harvesting system design is also included.

Fitting an appropriately sized First Flush Water Diverter is critical to achieve good quality water. Water Diverters improve water quality, reduce tank maintenance and protect pumps by preventing the first flush of water, which may contain contaminants from the roof, from entering the tank.

Read more: First Flush Water Diverters, web page set up by the Australian based company called Rain Harvesting.

First flush diverters

Contaminants from a roof are usually concentrated in the first run off from the roof. After this runoff has passed and washed the roof the water is considerably safer. The amount to be removed varies and a number of studies have had differing results. Yaziz et al. (1989) found that 0.5mm was sufficient to reduce the faecal coliforms count to zero on two roofs in a Malaysian campus.

Coombes et. al (2000) have found that even after 2mm was flushed, there were still significant faecal coliforms in the runoff from a building located close to a bus depot in Australia. Field trials by the DTU in Uganda have shown unacceptable turbidity after 2mm have removed although FC counts were in the WHO "low risk" category. Despite this uncertainty, first flush systems are a popular method of improving the quality of roof runoff prior to storage, particularly in Asian countries.

Four methods of separating first flush

There are basically four methods of separating the first flush:

1. Manual

2. Fixed volume

3. Fixed mass

4. Flow rate

Manual

The manual method is the simplest and widely recommended (Lee & Visscher, 1992), (Gould & Nissen-Petersen, 1999), it does, however rely on the user both being home and prepared to go out into the rain to operate the device much reducing its usefulness.

Fixed volume

The fixed volume method, which relies on the water simply filling a chamber of a set size (usually a length of downpipe) until it overflows is the "automatic" method usually applied in low cost systems. The method can be used either with or without a floating ball seal which helps in reducing mixing between early dirty water and later clean water, however Michaeledes (1987) has found that this mixing is transient. They are also found with either automatic draining over a period of time or require manual draining.

Manual draining systems have little to recommend them as if left to drain will not only fail to work for the next storm, but can cause additional pollutants to be washed in to the tank from the first flush device itself.

Fixed mass

The fixed mass system has also been promoted, mainly in Africa but has met with little success. The devices, usually relying on a mass of water to tip a bucket or seesaw tend to be unreliable and users inevitably disable the system.

Flow rate

A newer first flush concept is to use the changes in flow rate over the course of a storm. Stormwater management designers have been using a flow rate model of first-flush for some time to reduce the large land areas required for "volumetric" facilities (Adams, 1998), however, recently an Australian company has developed a system whereby flow rate is used for roof runoff.

The SafeRain system (Church, 2001) balances the rate of water intake into a suspended hollow ball against its leakage, raising its weight and stretching its suspension until it descends into a recess, blocking the opening and allowing water into the tank. The system has the advantage of being self-cleaning and removes the need for any storage of the first flush water (and its subsequent drainage).

See SafeRain Water Diverters for Water and Cistern Tank Systems. URL: http://www.saferain.com.au/

Source: Rainwater Harvesting Technology - Filters and separators, web page of the Domestic Roofwater Harvesting Programme of DTU/University of Warwick,

The full references mentioned in the paragraph (on first flush diverters) can be found at : http://www.eng.warwick.ac.uk/DTU/rwh/components6.html

Information on some common first flush designs

Other resources

  • Small Community Water Supplies: Technology, people and partnership Edited by Jo Smet and Christine van Wijk (2002). Includes a chapter on RWH with information on Collection surface ; Filtration systems and settling tanks ; Storage provisions ; Sizing a rainwater harvesting system ; Health and water quality ; Integrated and holistic nature of rainwater harvesting ; Conclusions and recommendations. It also has a bibliography and an overview of web sites and discussion groups.
    Order code: TP40-E. More info at http://www.irc.nl/page/1917
    If you would like to order one of these (or other IRC) publications free of charge, just send an email to publications@irc.nl and mention the title and the order code. Also mention that you order via the WELL document service.
  • FAQ - What is a rain water catchment system? The experience of CREPA, http://www.irc.nl/page/10369 (Ask IRC resources page)
    “a device for first rain diversion: thanks to this device, the first rains drain out the filth accumulated on the collection area (roofs) and are not collected in the tank. The type of diversion device promoted by CREPA is of T-shaped PVC fitted with a stopper at the lower end, made of the same material. When the stopper is open, water from the gutters flows down on to the ground. But if the stopper is closed, water rises and enters the tank through the pipe connected to the upper end of the T-shaped piece of PVC”
  • Q&A - Rainwater harvesting systems, http://www.irc.nl/page/14666 (Ask IRC resources page). Provides an overview of documents, organisations and web sites on RWH.

IRC, May 2006
--------------------------
This response has been provided by the WELL Resource Centre Network with funding from the UK Department for International Development (DFID).

Would you like more information about WELL? Look at the WELL web site or send an e-mail to the WELL programme at IRC
--------------------------