Sinclair Knight Merz carried out a study into the disposal of effluent from a new sewage treatment plant serving the township of Pomona, about100 km north of Brisbane in Queensland's Sunshine Coast hinterland. The township had previously been served by septic systems. The new sewage treatment plant produces approximately 150 kL/d of effluent (for 600 EP). The options for disposal of effluent were either discharge to Cooroora Creek or irrigation onto nearby agricultural land.
The ephemeral nature of the creek and the existence of a large recreational water body downstream of the town mean that discharging to Cooroora Creek on a daily basis would not be acceptable.
Sinclair Knight Merz carried out exhaustive water balance modelling which allowed comparison between a number of cropping options including irrigation of sugar cane, silviculture, pasture and horticulture on widely varying soil types, each with different crop management techniques and design criteria. Due to the high rainfall and soil types at the proposed irrigation sites, it was found that very large storage volumes would be required to reliably dispose of the effluent by irrigation on the cropped areas available. The solution was to allow strictly controlled effluent discharge to Cooroora Creek when it was flowing above base levels.
Conceptual designs and costings were prepared by the firm for the various proposed irrigation options based on nutrient balances at the various sites, staging of development for 300 EP and 600 EP, and environmental and health regulations and requirements.