Calibre : Are SEQ wastewater policies missing an opportunity to reduce costs?

The draft of the South East Queensland (SEQ) Regional Plan predicts an extra 2 million people living in the region within the next 25 years and an additional 900,000 dwellings. It's expected that 60% of dwellings will be what is called 'infill housing' as opposed to 'greenfield', driven in part by the efficiency in supplementing existing infrastructure in comparison with building new networks. There are significant cost differences in servicing the two, given the nature of the infrastructure and the expected loads. Wastewater (sewerage) is one of the services that has a significant bearing on development costs, but ironically, current standards do not usually differentiate between wastewater design for infill or greenfield. Additionally, they vary from one local authority to the next.

A strong case can be made for the need to update policies to reflect these differences in infrastructure requirements. Wastewater networks to service infill development should be subject to reduced design flow rates in comparison to greenfield development. This would reduce mains sizing requirements for infill developments, which in turn would reduce capital and whole of life costs. Given the SEQ Regional Plan expects 60% of future housing to be infill, this is a significant consideration.

Certainly, significant savings are possible if these standards are harmonised across the region and are changed to reflect contemporary design principles and new technologies. Capital cost and whole-of-life savings could be achieved by challenging assumptions around sewage flow rates and by utilising proven innovative sewage conveyance approaches that minimise inflow and infiltration. Such cost reductions could then be passed on to homeowners through a reduced connection cost and utility bill.

Wastewater loads are calculated based on projected growth and described as the number of Equivalent Persons (EPs) per dwelling and an assumed flow rate per person. The EP per dwelling applied throughout SEQ varies across Local Government Area (LGA) catchments and across dwelling type. Typically, flow rate per EP varies between 180 to 210 litres per day depending upon the type of wastewater reticulation network. These variables have a direct impact on the design flow rates for wastewater systems and in turn the sizing of pipes and pumps.

Varying SEQ demand rates, baseline flows and peaking factors prescribed for different LGAs within SEQ results in substantially different design flow rates. For example, a 1,000-lot development within Moreton Bay has a design flow rate 4.84 l/s less than that of an equivalent development within Brisbane. As the size of the catchment increases so too does the difference in design rates. This increased design flow rate triggers a requirement for larger infrastructure, a larger asset base and increased operation and maintenance costs.

A comparison between Moreton Bay and Logan City indicates that a gravity wastewater of the same diameter is capable of servicing an additional yield of 16% in Moreton Bay due to the difference in EP / Dwelling standard. Given that the nature of wastewater catchments across Moreton Bay, Logan and Brisbane are broadly similar we must ask, 'Why do we not have consistent standards for the sizing of wastewater infrastructure?'

The range of wastewater load assumptions currently applied across SEQ results in an array of dwelling triggers for infrastructure sizing. Consequently, the standards and methods of determining infrastructure requirements are inconsistent. As improved information relating to actual wastewater loads from developments becomes available we have the opportunity to harmonise standards and improve investment efficiency and utilisation of infrastructure.

Now is an ideal time to recalibrate water and wastewater standards to reflect contemporary materials and thinking. This would mean lower infrastructure costs, higher development yields and consistent standards, ultimately improving the viability of future developments and facilitating the vison of growth in South East Queensland.

Mal McCann
Manager Water & Environment, QLD