Hervey Bay and the Great Sandy Straits are hosts to migrating whales in winter and the area also includes a marine park, fisheries habitat and the heritage listed Fraser Island, so when high population growth required an expansion of the Pulgul Wastewater Treatment Plant in the late 1980s, Wide Bay Water aimed to find an environmentally sustainable solution which was economically viable and supported by the community.
To avoid tertiary treatment with nutrient removal and a long marine outfall, Council chose to implement effluent reuse with conditional support from the Department of Environment. In 1990 a 385 Ha mixed agriculture farm was purchased (sugar cane, cattle and small tea tree plantation) and a storage dam and irrigation infrastructure were installed. The project proved to be successful in a number of ways: a capital saving of $6.4 million, lower operating costs, a 90% reduction in nitrogen discharged to the bay (60 tonnes/annum) and a 70% reduction in phosphorus (15 tonnes /annum).
The predominant rural activity in the region was dry land sugar cane farming. There are no major rivers or groundwater supplies in the area and therefore minimum irrigation was practiced. The farming community was very sceptical about using treated effluent on cane farms. However, following effluent irrigation on the Pulgul farm, the very low production rate of 55 tonnes/Ha improved to 80 tonnes/Ha and in 1996 it won first place in the productivity ratings in the Maryborough Sugar Factory supply area. This generated much interest and demand from the sugar cane farmers as a guaranteed reliable supply of water also provided certainty against drought and crop failure.
To cope with population growth which was greater than 5.5 % per annum, the program was expanded in stages to include effluent from the Eli Creek Sewage Treatment Plant (plus irrigation on a golf course and turf farm). The dominant application continued to be on sugar cane farms but the irrigation water demand was both seasonal (November to April) and highly dependent on rainfall pattern. As a result, in wet years the demand from the cane farmers was only 35% and in dry years there wasn’t enough effluent available.
The average recycled water usage was 75% of supply but sewage flows continued to increase and all the farms which wanted recycled water had been signed up. Stuck in a rocky hard place, in 2003, various options were evaluated including sewer mining for watering the Esplanade at Hervey Bay, irrigation of parklands, dual reticulation, industrial use and growing other crops.
Implementing all these options would still not use enough effluent and would cost ten times more than the recommended alternative: irrigating eucalypt tree plantations. Tree growing trials had been undertaken on seven eucalypt tree species. They not only thrived but could use 5 ML/Ha per annum compared to 3 ML/Ha per annum for sugar cane. It was calculated that to achieve the target of 90% effluent reuse by 2007, as required by the EPA, 200 Ha of WBW owned land would need to be developed for irrigation and an additional 40 to 50 Ha each subsequent year. The target was to plant one million trees by 2015 (with 600,000 planted actually by 2013).
The strategy has provided the necessary wet weather flexibility. In dry years, cane farmers (turf farms, golf course and the airport precinct) receive their full allocation and trees receive 75% of their allocation. Unlike sugar cane which could miss a harvest without adequate water in the growing season, trees are not significantly impacted by this reduction. In wet years sugar cane farmers use only 10% of the total volume of recycled water and trees 85%.
Wide Bay Water continues to receive the same amount of income from external users plus income due to sale of wood products at trimming and harvest time, plus carbon sequestration credits. However, the main benefit is that as required by the environmental regulator, 90% of dry weather treated sewage flows are recycled.
The Class B effluent is filtered and the tree plantations are watered by 1,450 km of drip feed lines. The irrigation system is automatically monitored from a central control room with remote in-field valve controls, flow measurement, graphing and SMS alarming. A weather station monitors temperature, humidity, solar radiation, wind speed, rainfall and evaporation rate. Additional sensors monitor effluent pH, dam levels and soil moisture.
WBWC recently commissioned a 25,000 EP MBR sewage treatment plant producing Class A water which can be used for a range of recyling applications. In the longer term, this effluent may be pumped to the Cassava Dam catchment for possible groundwater recharge with the option of indirect potable use via advanced treatment at the Burgowan ozone/BAC filtration plant.