Whilst water efficiency measures have many positive benefits, the reduction in water consumption can cause changes in sewage quality. Lower water consumption increases the concentration of both trade and domestic wastes. This also leads to lower sewage volumes which increases residence times in sewers potentially leading to septicity, blockages and corrosion of infrastructure. It is critical to understand the potential impacts of changing wastewater characteristics and system hydraulics if advanced water efficiency measures are to be implemented.
The purpose of this study was to better understand the practical limits the conventional sewerage system places on water efficiency investment.
Specific concerns identified included better understanding the hydraulic limitations of the sewerage system (at the Private, Reticulation, and Trunk levels) and the likelihood of impacts such as increases in blockages, sulphide corrosion, and odour events.
- Review of water quality and quantity historical data with the aim of identifying and understanding trends over time in terms of flow and wastewater quality;
- A critical literature review focused on concerns related to reduced flows
- Soliciting of input from a panel of “experts” in related fields;
- Identification of knowledge gaps; and
- Formulation of conclusions and recommendations that will relate to ways in which impacts of water efficiency measures can be better understood, addressed, and managed.
The data obtained as part of this study was recorded as part of sampling regimes that were not specifically designed to assess the impacts of water efficiency on the wastewater network. Additionally, cost impacts are also not recorded in a way conducive to establishing the financial impact of low flows in sewer networks. Therefore, although many findings point to increases in the likelihood of significant impacts on the sewerage system, it is difficult to directly assess the current situation in Melbourne accurately. However, a number of specific conclusions can be made:
- Despite variation in wastewater flows, there has been an overall decrease since 1995;
- Concentration and key constituents and flow are significantly related.
- The per capita generation of wastewater is decreasing as key wastewater constituent loading is remaining constant over the same timeframe.
- Blockages are occurring within the sewerage system; most notably in pipes of diameter < 225 mm. It is anticipated that the frequency of blockage occurrence will increase as potable water consumption decreases through the uptake of water efficiency initiatives. Private sewers are considered most at risk of blockage.
- Hydraulic limitations are not applicable catchment-wide and need to be determined through hydraulic modelling of the existing system.
- A number of emerging technologies could potentially be implemented to assist in mitigating the impacts of reductions in potable water consumption at the range of sewer levels.