Improving Mixing, Foam Control and Rehabilitation of Anaerobic Digestion Process at the City of Winnipeg’s North End Water Pollution Control Centre

The North End Water Pollution Control Centre (NEWPCC) is one of the largest of the three wastewater treatment facilities owned and operated by the City of Winnipeg (City). The facility also provides a central sludge processing where thickened primary and waste activated sludge generated at the NEWPCC and truck hauled from the remaining two other Water Pollution Control Centres (WPCCs) are stabilized in six (6) mesophilic anaerobic digesters (MAD). The City presently utilizes a confined gas mixing system which is dated and is inefficient in keeping the digester contents completely mixed. In addition, the operation of the existing gas mixing system is severely impacted by the accumulation of grit and stringy materials which are poorly removed at the headworks. The NEWPCC experienced significant foaming in the summer/fall of 2009 resulting in the failure of the Digester No. 11 roofing system due to build-up of excess internal pressure. It is believed that over pressurization caused the roof of the digester to lift, resulting in the concrete to crack and release some sludge and foam materials on the roof as well as below the surface membrane. The remaining digesters also experienced significant operational problems, with foam becoming so thick (almost like a pancake batter) that it reached the level of the exit pipe submerged up to 3 m below the liquid level. Following a review of several alternatives, a cost-effective solution was developed for the roof failure of Digester 11 by pouring a concrete slab over the existing damaged roof as a remedial measure. To address the problems associated with poor mixing, a Pumped Jet Mix System was implemented. This paper discusses the challenges associated with the rehabilitation of Digester 11 including the feasibility of implementing sludge screening and degritting; a review of the alternative mixing systems, design, implementation and performance testing of a new mixing system and key safety upgrades and improvements implemented.