Diverting domestic rainwater into the sewer can increase water conservation in urban catchments

Overview of rainwater diversion

Global discussion on resource security consistently places one essence of life foremost, water. A likely contribution to urban water conservation may be found in the developments of renewable energy.

Is there a water equivalent to domestic grid-connected solar power?

This solution may take the form of diverting harvested rainwater from domestic rainwater tanks, when demand is low, and reclaiming it, when demand is high. The diversion would maintain the harvesting potential of the tank, by increasing tank drawdown and creating airspace. To be efficient and sustainable, rainwater should be diverted by gravity into an existing municipal system that collects, treats and distributes water. The most readily available system is sewerage reticulation.

To many, the idea of diverting rainwater into the sewer to contribute to urban water conservation would sound absurd. This discussion will demonstrate how it is possible and how rainwater diversion has economic value for homeowners, water treatment utilities, wastewater treatment utilities, land developers and local government.

Increasing urban water conservation with rainwater diversion

The efficiency of conventional rainwater harvesting systems and their contribution to urban water conservation is compromised during periods of low rainwater demand, such as reduced irrigation in winter or a period of vacancy due to work, holiday or a change in tenants. Often during these times, rainwater tanks periodically overflow and create waste in stormwater runoff.

Rainwater diversion aims to increase water conservation by reducing this waste. To achieve this, diversion must be controlled by a system that monitors household demand and rainfall. When demand for rainwater reduces diversion would increase. In this way, regardless of household demand, the optimal production from the tank is consistently achieved. Water production becomes the sum of rainwater supplied and rainwater diverted.

But how can we reclaim rainwater after diverting into the sewer?

Wastewater treatment plants gain income from sales of treated effluent. In many cases, their consumers use treated effluent as an alternative to mains water, such as irrigation of public open space and industrial or commercial processing. Therefore, the household can avoid directly reclaiming the diverted rainwater to reduce their mains demand, as this is done by others.

As rainwater supplies are intermittent and recovery of diverted rainwater is done by others, the household maintains a strong reliance on mains water. In fact, it is inevitable that rainwater diversion will marginally reduce rainwater supplied to the household. In this way, rainwater diversion will increase the income of water production utilities.

But how can diversion operate without overwhelming the wastewater reticulation and treatment systems?

Recovering low flows in sewers with rainwater diversion

Timing is everything. Sewer flow fluctuation is complex and dependent on many parameters. The two greatest causes of flow fluctuation are 1) An inflow/infiltration spike that occurs following rainfall; and 2) The typical diurnal pattern of water use by consumers. At worst, peak sewer flow may cause the operating efficiency of the treatment plant to drop drastically causing wastewater to bypass necessary treatment. Clearly, diverting rainwater at these times needs to be avoided.

High operating efficiency of wastewater treatment plants are achieved when flow rates match the designed capacity and sewerage constituents are at or less than design thresholds. Often when flow rates reduce, such as during the night for domestic catchments, the treatment plant will start recirculating effluent. This is necessary to maintain the health of biological treatment processes. During these times the efficiency of the plant may be very low as treated effluent production is minimal, due to volume conservation, and energy consumption is high, due to pumping for recirculation.

Diverting rainwater during periods of low sewer flows can recover flows and cleans the reticulation system. Diversion can potentially reduce the need to recirculate effluent at the treatment plant, as the inflow rate will have increased. Furthermore, as the constituents of rainwater are usually within the EPA discharge requirements of wastewater treatment plants, treatment process at this time will be highly efficient. Thus, rainwater diversion could potentially increase the volume of treated effluent and the cost of producing it.

Rainwater diversion has obvious economic value to wastewater treatment utilities. It is therefore suggested households with rainwater diversion may receive a financial incentive in discounted sanitary connection fees.

The incentives for rainwater diversion are not limited to homeowners and water utilities.

Achieving reliable stormwater management with rainwater diversion

As rainwater diversion increases the capture efficiency of rainwater tanks, the runoff from urban catchments is also reduced. In fact, the frequency of runoff events and annual runoff volumes from the connected roof area begin to behave as a 'disconnected impervious catchment'. This means, the runoff characteristic of the roof area become similar to a pre-urbanised catchment. This has been recognised as a stormwater management objective for restoration of urban creeks and streams, by the Queensland Department of Natural Resources.

But what makes the solution reliable?

As previously stated, rainwater diversion is controlled by a monitoring device that adapts to changes in rainfall and rainwater demand. In this way, the reliability of stormwater management is programmed into the system.

Reducing urban runoff will also reduce the scale and construction costs of stormwater treatment infrastructure. Thus, providing an economic advantage to land developers.

Also, as the land use allocated to this infrastructure can be reduced, this can potentially increase the number of residential allotments. Thus, providing an economic incentive to local government by increasing rates.

Potential outcomes of rainwater diversion

It has been demonstrated that rainwater diversion can potentially:

Would you support rainwater diversion? Why not send your comments to the author, Benjamin Taylor.