ALL MIXED UP — OPERATING EXPERIENCE WITH EXTERNAL DRAFT TUBE MIXERS

The Hammond Sanitary District (HSD) operates a 48 MGD treatment plant, generating 30,000 wet tons of biosolids annually. Solids are stabilized in six (6) primary anaerobic digesters that were retrofitted in 1995 with three (3) external draft tube mixers each. The mixers help process efficiency, but have been difficult to operate and maintain. The most disappointing aspect of the project was the poor quality of the as delivered machined pump components, namely undersized shafts that caused almost immediate bearing failures. Each shaft, without exception, required reconditioning. Also, the originally designed system generated unforeseen, extremely high, local fluid velocities inside the draft tubes, inducing velocity-accelerated corrosion and catastrophic failures. The remedy was to install hardened steel plates around sections of the draft tube under attack, and to slow the pump speed down. Digester solids concentration profiles are used to confirm that modified flowrates and digester turnover times are providing adequate mixing.