Participate Palmy

Population growth and impact on wastewater

Population growth in Palmy and how it’s been factored into Nature Calls.

Palmerston North is growing in population, and as a result the number of residential and commercial properties is also growing.

Palmy’s current population is 91,100. Our projections include a population of 112,400 by 2058 and 119,400 by 2073.

The majority of greenfield housing development is currently at Kelvin Grove, Aokautere and Whakarongo. Kākātangiata urban growth area and Ashhurst have been identified for greenfield housing development in the medium to long term.

We’re also ensuring that a range of housing is available – as well as traditional houses the District Plan enables infill housing, multi-unit housing development, minor dwellings and apartments.

Trade waste projections have been assumed to be at the same percentage growth rate as the domestic population. A further allowance of 300 cubic metres per day has been allowed for trade waste in the Longburn area.

Our best practicable option must consider the future growth rates of the city and the region

Specific to wastewater management, we projected flows and loads reaching the treatment plant from 2017 to 2073. We are continually reviewing these assumptions as the project progresses.

Factors that are considered in projecting future wastewater flows and contaminant loads include:

  • Historical flow and load data
  • Wet and dry weather conditions
  • Population growth projections
  • Potential industrial growth in Palmerston North
  • Climate change
  • Practical technologies to reduce flows and loads at source.

By understanding the flows and loads coming into the wastewater treatment plant, we can assess our options robustly; understand the potential environmental, social and cultural effects that may arise as a result of different options; and understand how the environmental effects of different options may change over the duration of the resource consent.

What is flow and load and how are they measured?

In wastewater, the term ‘flow’ is used to mean the rate of wastewater reaching the plant. It is generally measured as an instantaneous flow rate (litres per second) and longer-term flow rate (cubic metres per day). The term ‘load’ is used to mean the amount of contaminant in the wastewater before it is treated at the wastewater treatment plant.

Our investigations explore contaminants that are key to identifying appropriate treatment methods and to understanding the environmental effects of discharges. These contaminants include biological oxygen demand, nutrients (nitrogen and phosphorus) and suspended solids.

Load, like flow, is measured as a rate. In some instances this is an instantaneous rate (grams per second), but is more commonly measured or projected over a longer period (kilograms per day).

Current flows

  • Typical dry weather flow to the plant is about 250 litres per second
  • About 10% (5 litres per second) is from industrial activity
  • In wet weather the peak flow can be as much as 2,100 litres per second
  • The peak highest inflow to the plant is limited by the installed infrastructure to 2,200 litres per second.
  • Peak flows above 1,700 litres per second occur less than 0.1% of the time—less than nine hours per year

Future flows

  • In 2073 typical dry weather flow is projected to be around 340–350 litres per second, 40% higher than current dry weather flows
  • Industrial activity is expected to contribute about the same proportion of the flow as it does currently
  • The level of growth in the North East industrial zone isn’t projected to greatly influence flow to the plant. The typical range would be 10 litres per second between low and high growth scenarios
  • Peak instantaneous wet weather flows are anticipated to remain largely the same; possible increases in rainfall due to climate change are likely to be offset by improvements in the wastewater collection system which will reduce the proportion of surface and ground water in the flow
  • The report assumes there’s no impact on the size of peak instantaneous flows associated with climate change. However, peak flows may occur more often due to climate change.

Current loads

  • The current average contaminant loads reaching the plant per day are indicated below as are future projections.
  • The contribution to these loads from the city’s major industrial activities is typically around 11% for nitrogen and phosphorus and as much as 25% for biochemical oxygen demand.
  • These rates are considered typical for a city such as Palmerston North based on a per head contribution.


Current load per day

Future load per day (2073)

Biochemical oxygen demand 6,600 kilograms 10,544 kilograms
Suspended solids 7,300 kilograms 10,027 kilograms
Total Nitrogen 1,200 kilograms 1,632 kilograms
Phosphorus 196 kilograms 302 kilograms
Ammonia 740 kilograms 1,085 kilograms

Future loads

  • Future loads have been projected to increase in a similar proportion to population growth.
  • The rate of growth in the North East industrial zone is projected to have a minor influence on future loads.
  • Two growth scenarios have been developed for industrial growth, namely “low” and “high” warehousing.