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Introduction

A tank is a type of storage used in domestic, commercial and/or industrial settings to store water collected from surface runoff or sources such as greywater or blackwater waste streams. Once stored the water can be released in a controlled manner, and/or used to supply water demands.

There is a trend towards installing domestic rainwater tanks in urban areas to capture roof runoff and supply non-potable water demands. The benefits of using water sourced from a rainwater tank include:

• reduced reliance on potable water supply, thus deferring potable water system upgrade or expansion and increasing the security of supply from existing water sources;
• stormwater retention/detention;
• urban water quality improvement via retention and diversion of stormwater to the sewer and garden areas, thus reducing the volume of stormwater pollutants discharging to the catchment watercourses; and
• protection of urban streams, through reducing the duration of elevated flows.

Rainwater tanks are most efficient when the retained water supplies multiple water demands within a household, eg toilet flushing, garden irrigation, filling or topping-up swimming pools, clothes washing and other appropriate non-potable uses.

In many areas health departments do not expressly prohibit rainwater tanks supplying drinking water, however, guidelines typically recommend avoiding drinking rainwater where a reticulated potable supply is available.

Tank construction

Rainwater tanks are usually constructed from plastic, or galvanised steel, and are located above-ground adjacent to the sides of a dwelling or building. Where space is limited, tanks can also be installed below-ground, under-floor and in-slab - in these situations, tanks are often constructed from concrete or impermeable plastic membranes.

Conceptualisation

Conceptually, the operation of a rainwater tank is identical to the operation of any tank-based storage infrastructure. The simulation scheme developed is generic in its applicability to all forms of tank-based storage. The tank allows for the inflow of rainwater as well as the provision of trickle top-up, triggered to start and stop at a user-specified tank level.

Tank Technical Details

First Flush Separation

First flush separation is a commonly applied practice in many rain and stormwater harvesting systems. First flush devices are typically used to separate the initial runoff from a surface from subsequent flows and operate on the principle that the initial runoff commonly contains a higher proportion of accumulated surface contaminants, such as dust, sediment, litter, animal droppings, leaves and debris. 

There are many forms of first flush separation devices. The storage tank configuration utilised by Urban Developer incorporates a simple volumetric first flush model conceptually illustrated in Figure 1. The device, shown in Figure 1 operates by allowing the first flush to fill a storage chamber, drained by a small orifice. Once full, inflow bypasses the storage chamber and leaves the device via the outflow pipe. Once rainfall ceases the first flush storage chamber is drain slowly as water leaves via a small orifice located at its base. For simplicity, the discharge is assumed to be at a constant rate.

Figure 1: First flush system.

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Q_in is the inflow volume (m³);

Q_out is the outflow volume following first flush separation (m³);

q_ff is the constant outflow rate from the chamber (m³/s); and

 V_ff is the diameter of the first flush outlet (m³);

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Storage Tank Storage Tank

The Storage Tank

The Urban Developer storage tank, shown in Figure 2, breaks the available storage volume into three distinct storage zones.

Figure 2. Urban Developer storage tank definition.

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where

V_Detention       is the Detention Storage Volume (m³);

V_Retention       is the Retention Storage Volume (m³);

V_Dead            is the Dead Storage Volume (m³);

h_off-taker         is the height of the supply off-take obvert from the base of the tank (m);

h_retention         is the height of the retention storage volume (m);                  

h_detention        is the height of the detention storage volume (m);  

h_tank              is the height of the storage tank (m);

h_{TT On}            is the height of the trickle top-up on trigger (m); and

h_{TT Off}            is the height of the trickle top-up off trigger (m).

“Dead storage” or the “anaerobic zone” is located at the base of the tank and is typically provided for the accumulation of sediment and other material. Dead storage is defined by the height of the supply off-take and once filled cannot be utilised within the model to meet any form of demand.

Retention storage is the volume of the water held above the supply off-take but below the detention outflow.  Retention storage can be used to supply consumptive demands within an Urban Developer model.

A portion of the total tank volume can be specified to as detention storage by setting the detention storage depth and defining the detention outflow orifice characteristics. Detention storage can be used to mitigate peak inflow events by storing and releasing water at a rate less than that of the inflow.

Tank Routing

The Urban Developer storage tank allows for the inflow of water, Q_in, as well as providing for an optional trickle top-up volume, Q_topup, that is triggered on and off at user-specified tank heights. Supply to meet consumptive demand, Q_supply is drawn from the base of the tank just above the “dead” storage zone.

Inflows in excess of the retention storage volume are routed through the detention outflow, which is controlled according to the capacity and configuration of the outlet. During periods of very large and rapid inflows the detention storage capacity of the tank may be exceeded resulting in spillages, Q_spills, from the top of the tank. This spillage volume represents the volume of water that is unable to enter the storage tank.

Figure 3. Rainwater tank configuration.

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The routing algorithm adopted by the tank applies a first order Ordinary Differential Equation (ODE) solution scheme to solve the governing water balance present in Equation 1.

Equation 1

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where

V_t              is the volume at the end of time interval t (m³);

V_t-1           is the volume at the end of time t-1 (m³);

Q_tⁱⁿ             is the inflow volume for time interval t (m³);

Q_t^topup             is trickle top-up volume for time interval t (m³);

Q_t^supply                  is the demand volume extracted for time interval t (m³).

h_t              is the depth of water at the end of time interval t (m)

Q_t^spill(h_t)    is the overtopping volume as a function of depth (m³); and

Q_t^detention(h_t)    is the discharge rate from the detention storage for time interval t (m³/s).

Outflow from the detention volume is calculated as a function of depth, above the detention outflow orifice obvert, Outflow using the minimum discharge of the broad crested weir and orifice flow equation 2 to account for the transition that occurs as the outflow orifice is drowned. Definition of the variables presented in Equation 2 can be found in Figure 4.

Equation 2

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Figure 4. Rainwater tank detention outflow configuration.

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where                  

h_max                        is the maximum height of the detention storage (m);

h                             is the depth of water above the outlet obvert (m);

q_{detention outflow}        is the detention storage outflow rate (m³/s); and

f_{detention outflow}         is diameter of the outflow orifice (m).

Acknowledgements

This material has been adapted from:

eWater Cooperative Research Centre (2011) Urban Developer Product Specification: Storage Tank Routing v0.4. eWater Cooperative Research Centre, Canberra. 23 June 2011.