New Zealand's First Full Scale Biosolids Solar Drying Facility
Annual Conference
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The Pines II Wastewater Treatment Plant (WWTP) in Rolleston, Canterbury was designed for an ultimate connected population of 60,000 and construction is being implemented progressively in two stages. The first stage of the ultimate capacity was constructed for a connected population of 30,000 and was completed at the beginning of 2013. The current Pines II WWTP consists of an inlet works (fine screens and grit removal), secondary treatment in two bioreactors comprising anoxic and aerobic zones for advanced nitrogen removal, secondary clarifiers and UV disinfection. Treated wastewater is irrigated to dedicated land adjacent to the WWTP. The solids stream consists of gravity thickening, aerobic digestion, sludge dewatering and solar drying in two halls (steel glass house structures). The current flow and load to the WWTP is in the order of 20 000 population equivalent.
The waste activated sludge (WAS) is removed from the bioreactors to maintain a target solids retention time (SRT) of 15 to 20 days in the bioreactors. The WAS is pumped into the Gravity Thickener where it settles and is thickened to 1 to 1.5 % dry solids. The thickened WAS (TWAS) is pumped into a compartmentalised Aerobic Digester with alternating aeration and mixing zones for a period of 8 to 10 days. The aerobically digested sludge is dewatered in centrifuges to 18 to 20% dry solids and transferred to the solar drying halls via screw conveyors. The solar drying halls are fed automatically by conveyors and the automated turning and transport system (SludgeManagerTM) spreads the cake across and along the hall. The SludgeManagerTM, and the ventilation and extraction fans are automatically controlled by the solar dryer controller. The amount of time the sludge is retained within the drying halls is automatically controlled according to the weather conditions.
The solar dryer was designed to achieve a minimum dryness of 70 % dry solids in winter. The solar dryer was commissioned in May 2013 and the dry solids concentration has increased gradually during winter and the current dry solids concentration at the end of the hall is well above 80 %.
This paper presents the selection process, design considerations, one year operational experience, operator input and performance of the first full scale solar drying facility in New Zealand.