Water-saving's unintended consequences

Water conservation has unintended consequences for residents and water managers, according to new research.

Victoria University PhD student Nyoman Marleni found reducing and replacing potable water lead to smellier sewers and more corrosion in sewer pipes.

"Everybody talks about the benefits of water saving measures but it's important to remember the same amount of waste goes into the sewer, so by reducing or replacing the potable water we are left with more concentrated wastewater," she said.

"This more concentrated wastewater leads to higher concentration of odour producing gases and more corrosion in sewer pipe."

In a case study beneath Melbourne's northern suburbs, Ms Marleni modelled sewer conditions under four water conservation scenarios for the residents: simple reduced water usage, grey water reuse systems, rainwater harvesting and wastewater recycling through sewer mining technology. For each scenario she also measured levels of hydrogen sulphide gas, the chemical largely responsible for pungent 'rotten egg' sewage odours and corrosion of sewer pipes.

The reduced usage scenario assumed residents were using about 22 litres a day less water per person. The impact of this scenario was calculated to reduce the average local sewer network's lifespan through corrosion by 40 years, from 147 to 107 years, and raise the hydrogen sulphide to unpleasant, yet not harmful, levels.

But when calculating what might happen if all residents installed grey water reuse systems "using bathroom and laundry wastewater for toilet flushing and garden irrigation to reduce potable water consumption by 70%" results were more alarming. Hydrogen sulphide levels would increase to be not only smelly but cause eye, nose and throat irritation, headaches and nausea. Meanwhile the average lifespan of sewer pipes under those conditions was reduced from 147 years to just 70 years.

Rainwater harvesting had less impact, resulting in slight smells and about 6 years in pipe lifespan reduction.

While other scenarios were assumed to be implemented at a household scale, the wastewater recycling through sewer mining was modelled on a sewer mining facility able to supply alternative water to 70% of households in the study area. Modelling showed the sewer pipes after sewer mining had a small increase in hydrogen sulphide gas and the average pipe lifespan was reduced by 32 years.

Ms Marleni said more research was needed so that water authorities could include precise data in planning as water saving measures became more widely used.

This study was funded by CSIRO's Water for a Healthy Country Flagship scholarship. Ms Marleni's principal supervisor is Dr Nitin Muttil from the university's Water Resources Research Group and her associate supervisor is Professor Stephen Gray from the university's Institute for Sustainability and Innovation. Dr Ashok Sharma and Professor Stewart Burn from CSIRO also are associate supervisors for this project.

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