Economical Wastewater Treatment Design for Highly Variable loads

There are many municipal wastewater treatment plants (WWTP’s) in New Zealand that are treating large industrial wastewater loads. Treating these large industrial loads can be challenging due to the characteristics and variability of the wastewater. Often these WWTP’s are located in rural towns where financial constraints have a large influence on the plant design. The Te Kuiti WWTP is one of these plants and underwent a significant upgrade in 2012/13.

The Te Kuiti WWTP is owned and operated by the Waitomo District Council (WDC). The plant receives domestic wastewater from the town of Te Kuiti and also receives wastewater from two meatworks that, when in full production, contribute up to 80% of the total BOD load entering the WWTP. Prior to the recent upgrade, the WWTP regularly breached its resource consent particularly with respect to ammonia. Problems included: the reactor was too shallow; aerators were unreliable and inefficient; the anoxic zone was too small; inadequate instrumentation; the hydraulic loading rate on the clarifier was too high (large inflow and infiltration during rainfall events); low ultraviolet transmissivity (UVT) in the final effluent during high load periods, poor disinfection (no specific disinfection).

With low cost solutions in mind and attention to the highly variable load, an innovative activated sludge (AS) reactor design was installed that utilised existing infrastructure wherever possible. To minimise the cost, the reactor was constructed with a circular earth bund and a central anaerobic zone utilising a curtain structure. An oxidation ditch type AS process was created within the reactor annulus. That eliminated the need for expensive internal pipe-&-pump recycles and also provided flexibility to increase/decrease the reactor aerobic/anoxic fractions as the wastewater loads increased/decreased. The reactor was also designed to allow the reactor depth/volume to be adjusted according to the load. A mix of submerged aspirator and floating aerators were used in the aerobic zone and both directional and non-directional mixers were used in the anoxic and anaerobic zones. Further upgrades installed included: a pre-clarifier chemical dosing system for use when required; tertiary filtration and UV disinfection; storm flow by-pass; sludge stabilisation; sludge dredge and dewatering system.