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CCB Envico were commissioned by Hay Shire Council and NSW Public Works Advisory to construct an intermittent decant extended aeration based secondary treatment plant (IDEA Tank Type). The new plant – with a design capacity of 3,500 EP – replaces the old Trickling Filter Plant that served the 2,300 residents of the Hay township.


The objective of the IDEA treatment process is to produce secondary treated quality effluent via:


  • Oxidation of organic material in the influent sewage providing carbonaceous BOD5 – the quantity of biodegradable organic matter contained in water – removal, and nitrification of influent ammonia (NH4) to nitrates (NO3)

  • Denitrification of nitrates (NO3) to gaseous nitrogen (N2)

  • Normal biological uptake of phosphorus (P)


The new plant consists of:


  • A concrete inlet works with mechanical and manual effluent screens, a vortex type grit tank and grit classifier, and storm bypass facilities;

  • One IDEA tank with two surface aerators, a gas locked siphon type decanting system and waste activated sludge (WAS) pumping equipment;

  • Two waste activated sludge lagoons to stabilize WAS pumped from the IDEA tank for at least 6 months stabilization;

  • A sludge dewatering concrete hardstand area

  • Catch balance tank that captures the overflowed supernatant from the IDEA tank

  • Wastewater pump station

  • Site amenities building, including a fully equipped laboratory for wastewater testing


Treated effluent from the new plant is discharged to existing evaporation ponds adjacent to site via gravity.


Major environmental initiatives/local opportunities included:


  • The use of crushed concrete from Hay Shire Council’s waste recycling facility – the recycled concrete was used as a sub-base under the inlet works and sludge dewatering slabs

  • The Hay STP provided long and short term employment opportunities to several locals and personnel from surrounding areas in the Riverina. Every opportunity for local purchases and material manufacturing was taken by CCB Envico, injecting further money into the local economy


Challenging elements of construction included:


  • The 33° (1:1.5) slopes of the IDEAT walls made construction, particularly the concrete placement very difficult and time-consuming. Compounding this difficulty was pouring the concrete in the peak of summer when the concrete batches were at risk of going off within minutes of leaving the concrete mixer. This required a joint effort from CCB Envico, the concrete subcontractor and the concrete batch plant in the town

  • Due to the remote location of Hay – 750km from Sydney and 400km from Melbourne – and some materials of construction scheduled to arrive Just in Time, deliveries to the site were challenging at times. This was made worse mid-way through the project as the local Hay delivery agent closed-down permanently.

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