Anaerobic Digestion: Recovering Mesophilic Digester From Failure Due to Toxicity

The operation of Anaerobic Digesters (AD) for stabilisation and reduction of organic material is an established process in many Wastewater Treatment Plants (WWTP). One of the challenges faced by large WWTP serving both industrial and domestic sources is the vulnerability to toxicity. The Mangere WWTP is the largest WWTP in Auckland, producing 300 tonnes per day of biosolids and utilising gas from the digestion process to meet up to 60% of the power demand. In October and November 2012, the plant experienced poor digestion. The first sign of failure was a drop in the gas production from 5,500 to 3,500 m3 /day and in the pH, from 7.2 to 6.7. The alkalinity decreased dramatically from 2,700 to 50 mg/L CaCO3 while the volatile acids increased from 10 to 3,800 mg/L. Several steps were taken to recover the digesters from failure. This included reducing the load to the digesters to a minimum of 0.6 kgVS/m3 , modifying the solids stream configuration, and dosing sodium bicarbonate to increase alkalinity. By proactive management of the incident, the digesters were brought back from failure within two weeks. This paper investigates the steps during the recovery phase and some of the investigations undertaken.