Technical Publication # 10Auckland Regional Council 11-1

Chapter 11Rainwater tanksdesign, construction and maintenance

11.1 Introduction

Rainwater tanks are primarily water quantity management devices. There are minor water quality benefits,depending on the amount of atmospheric deposition in a given area, but their primary function is for quantitymanagement and to supply on-site water use.

Rainwater tanks are not a stand alone solution for quality and quantity issues in a catchment, but they can beimplemented as a part of an integrated approach for reducing:

> stormwater volumes entering the receiving waters, through the use of stormwater captured and usedon-site

> flows into downstream stormwater treatment practices or if roof runoff is separated from groundrunoff

> peak stormwater flows from the sub-catchment by providing permanent or temporary storage> sanitary sewer overflows by reducing the rates and volumes of stormwater that enters directly or

indirectly into sanitary sewers> roof-generated contaminants entering water bodies> demand from mains water supply leading to more

effective use of water resources,

Peak runoff reduction is limited to any sub-catchment forwhich tanks are specifically designed. Depending on therate of slow release and their relative position in a catch-ment, all devices that store and release stormwater at aslower rate from a selected sub-catchment.have the po-tential to increase the peak flow rate further downstream.

A catchment-wide approach is essential to verify the ef-fects of flow retarding devices in a sub-catchment (ondownstream catchment areas). This is particularly im-portant for catchments with existing and potential down-stream flooding issues.

Rainwater tanks have been in use for centuries for sup-plying household and agricultural water. With the emer-gence of high quality, low cost community water supplysystems, rainwater tank systems became limited to non-urban areas. In recent years, however, there has been agreater interest in rainwater harvesting in urban areaswith wider recognition of their environmental andinfrastructural benefits.

These guidelines deal only with non-potable use of rain-water.

Plate 11-1: Rainwater tank on aresidential property

Auckland Regional Council Technical Publication # 1011-2

If potable use of rainwater is intended, several health and safety related issues are involved, including treatingand disinfecting the rainwater to appropriate water standards and avoiding cross connections. Professionaladvice is required for this.

11.2 Limitations of tanks for stormwater quantity management

As with all stormwater practices, rainwater tanks have some limitations in some situations. Their primarylimitations relate to their effectiveness for water quantity management, although they have some waterquality applications.

The current perception is that roof runoff is relatively clean. Historic approaches have tended to treat runofffrom impervious surfaces other than roofs, allowing roof runoff to bypassing treatment systems. Futureresearch will provide better information on roof runoff, but water quality limitations on tank water use cannotbe established until there is more information on the need for roof runoff treatment.

What is clear is that particulates from roof runoff are extremely small and sedimentation may not be aneffective removal process. Use of roof runoff for domestic non-potable use will reduce contaminant dis-charge into receiving waters because whatever contaminants are present, some of the runoff will enterwastewater treatment systems or be discharged on to permeable surfaces such as lawn or garden.

This chapter discusses the water quantity benefits of roof water tanks. It does not promote the blanket use ofwater tanks throughout a catchment, because the timing of storm flows through the catchment when thetanks are primarily used as detention tanks may result in potential increases downstream. A catchment-wideanalysis of the sensitivity of timing may need to be done in some catchments but general guidelines may alsoprovide assistance in the absence of a catchment plan.

This chapter allows the roof area to compensate, from a water quantity perspective, for other small impervi-ous surfaces. Roof areas cannot compensate for too many additional impervious surfaces.

The issue is further complicated by household water use. Using roof water for partial water supply (i.e.. non-potable water uses such as laundry and toilet) reduces the total volume of runoff that may be dischargedduring a storm event compared with use for total water supply (both non-potable and potable water uses),because tanks are more likely to be emptier at the start of a storm. .

Water tank limitations for water quantity management include:

> the catchment time of concentration to the point where the water tank discharges to the receivingsystem cannot exceed 20 minutes

> tanks cannot compensate forother impervious areas exceeding 120 m2

> flows from water tanks will bypass downstream treatment systems> where there are documented downstream flooding problems, only partial credit should be given to use

of water tanks, as follows:- partial use systems (approximately 300 l/day) will receive a credit for up to 150 square metres of

impervious surfaces, including roof areas- full use systems (potable and non-potable - approximately 500-600 l/day) will receive a credit for

up to 250 square metres of impervious surfaces (including roof areas)

11.3 Performance

11.3.1 Water quality credit through water use

When tank water is used for non-potable purposes, the contaminants in roof runoff are redirected to sanitarysewers and planted areas, accordingly reducing the load entering receiving environments. The percentage

Technical Publication # 10Auckland Regional Council 11-3

Parameter Typical levels in

roof runoff

Water quality

guidelines

Reference

PH 6.7 6.5 9.0 ANZECC 1992 Suspended solids (g/m3) 29 - - Cadmium (mg/m3) 0.26 2.2* USEPA 1999 Copper (mg/m3) 25 9.0* USEPA 1999 Lead (mg/m3) 17.6 2.5* USEPA 1999 Zinc (mg/m3) 315 120* USEPA 1999 Faecal coliforms (cfu/100mL) 2 200 DoH 1992 Enterococci (cfu/100mL) 15 35 MoH 1999

Table 11-1Typical roof runoff quality

* at hardness of 100 g/m3

Source Gadd et al (2001)

contaminant reduction depends on the percentage of water captured for use.

Reduced runoff volumes may also decrease stream channel erosion, conferring additional water qualitybenefits (Tremain 2001). These additional channel erosion water quality benefits have not been quantified orincluded in the water quality credit given in this chapter.

Table 11-1 compares typical roof runoff contaminant concentrations with the relevant water quality guide-lines for protection of aquatic ecosystems.

It is very difficult to quantify water quality benefits of roof runoff capture as contaminant vary according togeography and roofing materials. Any benefits will largely depend on whether water is used for on-sitepurposes or whether the tank functions primarily as a peak control device.

Airborne sediments deposited on roofs are extremely small, and will not settle out in the tank.

If the tank is used for on-site water use, the actual reduction in contaminant loading will also depend on thepercentage of time during the year that the tank is full when a rainfall event occurred. If the tank is full at thestart of a rainfall event then none of the rainfall event will be stored (and later used) and hence will conferlittle water quality benefit. The number of days the tank is full at the start of a rain event is a function of theantecedent rainfall, roof area, tank size and water use.

11.3.2 Water quality credit

For the purposes of estimating the water quality benefits from rainwater tanks, a water quality credit iscalculated based on the percentage of water used multiplied by a water quality factor. The percentage ofwater used is based on Table 11-2 and the water quality factor is taken as 0.75. The water quality factor of0.75 takes into account the number of rain days that the tank is full at the start of a rainfall event. The waterquality credit is not given for other impervious surfaces for which a quantity credit is given. Runoff from non-roof areas must be treated.

Model runs using long-term (up to 20 years) daily rainfall records indicate that the percentage of rain days atank is full varies from 15% to 35% for a typical range of roof areas and water use rates. The average tankfull days is approximately 25%, so the water quality factor of 0.75 has been taken as a value for the AucklandRegion.

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Auckland Regional Council Technical Publication # 1011-4

11.3.3 Peak flow and extendeddetention attenuation

The degree of peak flow attenu-ation that can be achieved de-pends on the roof area, other im-pervious areas, storm character-istics, tank size and outlet orificesize. Note that the tank can con-trol runoff from the roof catch-ment area only. If there is an in-crease in other impervious areas,such as footpaths and paving ar-eas, the roof catchment areaneeds to be proportionately bigenough to compensate for therunoff from those areas. Oncepaved areas exceed 30% - 60%of the roof area, the incrementalincrease in roof runoff attenua-tion storage volumes becomeslimted. This contributes to the ra-tionale for the 120 m2 limitationon non-roof impervious areas.

Rainwater tanks can be designedto perform the following func-tions (refer Figure 11-1 for com-monly used components of a rain-water tank):

> non-potable water use,with a consequent benefitof quality improvement byreducing contamina