Storage TanksStorage tanks are often a significant cost component of roofwater harvesting systems and selecting an appropriate capacity is a focus of design. Roofwater storage tanks are typically above ground steel, polyethylene or fibreglass tanks. Below ground precast concrete or fibre-reinforced plastic (FRP) tanks are also used where above ground tanks are not suitable.

STORAGE AND TREATMENT

Storage is needed to balance irregular rainfall with typically regular demand for water. Roofwater is screened before entering storage tanks and depending on the application, it is also typically passed through a filtration and disinfection system before use.

Suitable RoofsMost roofs are suitable however there are a number of contamination issues that need to be considered including waste from human access, atmospheric pollution, chemical contamination from roofing materials and biological contamination due to birds or devices such as cooling towers.

Diverting RoofwaterBends are typically inserted in to downpipes to divert roofwater. The diversion point can be at high or low level, depending on the location of downpipes and storage tanks and if diversion pipes can be supported to a building wall.

ROOFWATER HARVESTING

Roofwater harvesting is the use of rainfall runoff from roofs for water supply. It typically involves the diversion of roof downpipes in to above ground storage tanks where the roof-water is filtered and disinfected for use in non-potable applications such as toilet flushing and landscape irrigation.

DIVERSION

Roofwater firstly needs to be diverted in to the harvesting system. This requires access to suitable roofs and the ability to divert roof downpipes in to storage.

AlternativeWater Solutions

Pre-Screening and First FlushRoofwater is screened prior to entering the harvesting system to remove debris such as leaves. Gutter mesh, rain heads and storage tank inlet screens are common. First flush devices are also used to trap built-up roof silt and leaves from the initial rainfall runoff.

TreatmentA typical treatment process involves micro strainer filtration and UV disinfection. Particular end uses such as cooling towers may require further chemical treatment. Larger storage tanks may also require a constant circulation system to prevent the build-up of poorer quality water low in oxygen.

DISTRIBUTION

Getting water to the application point requires delivery pumps and a dedicated non-potable water delivery line. A back-up potable water supply is commonly incorporated into the delivery line to ensure water is always available.

Delivery PumpsPressure pumps that activate each time an outlet is opened in the distribution system are the most common delivery pump. An alternative is a pressure vessel pump that pressurises a spherical vessel rather than directly pressurising the distribution system. This system results in fewer pump starts however also delivers fluctuating pressures.

Potable Back-UpMost roofwater harvesting systems have a back-up mains water supply that either tops-up the storage tanks or feeds directly in to the distribution system.

MONITORING AND CONTROL

The level of control required for an alternative water supply system varies depending on the end uses being serviced. Most systems are controlled by a small programmable logic controller (PLC) using a network of sensors.

StorageLevels

DistributionPressures

DeliveryPump

PLC

PLCsA PLC is a small computer used to automate system processes that typically sits in a cabinet with the treatment and delivery plant. PLCs also store data such as pump run times and flow meter readings which can be useful in making maintenance decisions and tracking water savings.

SensorsPressure sensors are used to activate the delivery pumps when a drop in pressure is detected in the distribution network. Ultrasonic level sensors or float switches are also used to monitor storage levels in storage tanks and activate the back-up mains water supply.

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