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.
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.
Restrictions
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.
Multiple tanks
In the current version of Urban Developer, you can connect more than one tank to a supply point; however this model type is not currently supported and may give inaccurate results. If you need to model a multiple-tank installation, the best workaround is to regard the entire installation as a single tank, and adjust initial levels, first-flush volumes etc, accordingly.
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.
Node Inputs
• Inflow: Tank inflow is usually from a roof node (page 116) or other impervious area (page 110).There is no direct input capability for "topping-up" the tank; topup is controlled by a parameter in the tank property editor (see below).
The top-up function takes water from the mains supply, however in this version, the mains supply usage due to trickle top-up is not tracked in the mains supply aggregation.
Node Outputs
• Supply out: The tank volume extracted from a Water Use (page 143) node per unit time.
Link type (page 66): Demand
• Spill: The volume flowing out of the tank when the detention volume is exceeded.
Link type (page 66): Runoff
• Detention outflow: The flow from the overflow pipe at the invert of the detention volume, during a flood event.
Link type (page 66): Runoff
Node Properties
| Property | Notes | Units | Default Value | Recommended Range | Constraints | |
|---|---|---|---|---|---|---|
| Min | Max | |||||
Tank dimensions | ||||||
| Cross-sectional area | Defines the cross-sectional or area of the tank base. | m2 | 2.5 | NA | NA | >= 0 |
| Height | Defines the total height of the tank. Includes detention and dead zone height and dead zone height. | m | 2 | NA | NA | >= 0 |
| Height of off-take invert | Defines the height of the offtake pipe on the tank; the offtake pipe supplies demands from the tank. The invert level of the offtake is the height of the lowest point of the pipe above the tank base. Once the tank is filled above the offtake invert level, the water depth in the tank will never fall below this level. Demand off-take occurs from the base of the tank just above the anaerobic or "dead" zone; a tank should therefore have a minimum "dead zone" depth of 0.1 m to allow for accumulation of sediment and other material. Water will never be drawn from the "dead zone". | m | 0.1 | NA | NA | >= 0 |
Initial water depth (m) | Defines the initial depth of water in the tank at the start of the model run. Default: 0.1 m Minimum: 0.0 m Maximum: 1000.0 m | m | 0.1 | NA | NA | >= 0 |
Overflow outlet and detention storage | ||||||
| Detention volume depth (m) | Defines the detention (storage) volume depth of the tank. The detention volume depth is the depth from above the overflow outlet invert to below the tank spill/top of the tank; hdet in the diagram. Detention volumes are used as buffers to decrease output flow rates in situations of high input flow. If the tank has no detention storage, set the detention volume depth equal to the overflow outlet diameter. Default: 0.1 m Constraints: ≥ 0 m, ≤ 2.0 m | m | ||||
| Overflow outlet diameter (mm) | Defines the diameter of the overflow orifice, located at the base of the detention storage. Default: 100 mm Recommended minimum: 10 mm Recommended maximum: 320 mm Constraints: ≥ 1 mm, ≤ 1000 mm | mm | ||||
| Overflow outlet discharge coefficient | Discharge coefficient used in the evaluation of the orifice flow equation. | |||||
| External top up | ||||||
| Enable external top-up | If enabled, allows for the tank to be topped-up with mains water so that the tank always maintains a user-specified water level. You do not need to connect a mains water supply node to a tank node to provide inflow for topup; when you enable external top-up, Urban Developer handles the mains connection internally. | |||||
| Top-up rate (L/s) | Defines the rate of mains water top-up in litres per second. | L/s | ||||
| Top-up triggered on (m) | Defines the depth above the tank base at which the tank will start to top up. This depth must be greater than the height of the off-take. | m | ||||
| Top-up triggered off (m) | Defines the depth above the tank base at which the tank will stop automatically topping up. This depth must be greater than the Top-up triggered-on depth. | m | ||||
| First Flush System | ||||||
| Enable first flush system | If enabled, changes the tank inflow behaviour so that a user-specified volume of water will bypass the tank before inflow to the tank occurs. | |||||
| Volume (m3) | Specifies the volume of water the first-flush device diverts. Default: 0.0 m3 Recommended maximum: 100 m3 Constraints: ≥ 0.0 m3 | m3 | ||||
Outlet flow rate (m3/s) | Specifies the outflow rate to the selected discharge outlet. | m3 /s | ||||
| Discharges to | Specifies the tank outlet that first-flush outflow is discharged through. Detention outflow specifies that the first flush volume discharges via a connection to the detention outflow pipe. Spill specifies that the first flush volume discharges via spilling from the tank. | |||||
| Other | ||||||
| Tank bypass | If enabled, all inflows flow directly to the detention outflow. This allows you to temporarily disconnect a tank without removing it from the model. A bypassed tank still has a small impact on model run-time performance, due to the direct routing of flow from the inflow connection to the detention outflow connection. In testing, simulation time for a disconnected tank is approximately 10% greater than for no tank at all (ie having link with no routing in place of the tank). | |||||
Tank model operation
Tank storage zones
The rainwater tank model used by Urban Developer is illustrated below. The storage volume comprises three distinct storage zones.
where:
VDetention
is the Detention Storage Volume (m3)
VRetention
is the Retention Storage Volume (m3)
VDead
is the Dead Storage Volume (m3)
hoff-take
is the height of the supply off-take obvert from the base of the tank (m)
hretention
is the height of the retention storage volume (m)
hdetention
is the height of the detention storage volume (m)
htank
is the height of the storage tank (m)
hTT On
is the height of the trickle top-up on trigger (m)
hTT Off
is the height of the trickle top-up off trigger (m)
Tank routing
The Urban Developer storage tank allows for the inflow of water, Qin, as well as providing for an optional trickle top-up volume, Qtopup, that is triggered on and off at user-specified tank heights. Supply to meet consumptive demand, Qsupply 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, Qspill, from the top of the tank. This spillage volume represents the volume of water that is unable to enter the storage tank.
The routing algorithm adopted by the tank applies the generic first order Ordinary Differential Equation (ODE) solution scheme, to solve the governing water balance present in Eqn 1:
thqhQQQQVVtttttttttΔ−−−++=−−)()(1detentionspillSupplytopupin1
(Eqn 1)
where:
tV
is the volume at the end of time t (m3)
1−tV
is the volume at the end of time t-1 (m3)
intQ
is the inflow volume for time interval t-(t-1) (m3)
topuptQ
is trickle top-up volume for time interval t-(t-1) (m3)
supplytQ
is the demand volume extracted for time interval t-(t-1) (m3)
th
is the depth of water at the end of time t (m)
1−th
is the depth of water at the end of time t-1 (m)
)(spilltthQ
is the overtopping volume as a function of depth at time t (m3)
Urban Developer User Guide 143
thqttΔ−)(1detention
is the discharge rate from the detention storage for time interval t-(t-1) (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 (Eqn 2) to account for the transition that occurs as the outflow orifice is drowned.
where:
maxh
is the maximum height of the detention storage (m)
h
is the depth of water above the outlet obvert (m)
outflowdetention q
is the detention storage outflow rate (m3/s)
outflowdetention φ
is diameter of the outflow orifice (m)
(Eqn 2)
where:
Cd
is the coefficient of discharge
g
is gravity (m/s2)