Using off-gas testing to map mixing gradients and audit blower capacity.

A water resource recovery facility sited in a region at a high elevation has experienced the effects of over-designing its blowers. In this case study, we used off-gas analysis and site-specific power tariffs to quantify actual process loading and air requirements, and we quantitatively evaluated various options for blower replacement or upgrade. Off-gas analysis mapped the oxygen uptake rate at the surface of the tank, suggesting that the tanks were not evenly loaded across their sections. The local cost of energy directly affects the return on the investment calculation and limits the available solutions. The payback of partial or complete blower replacement may not be justified even in the event of excessive aeration, and the sequencing of aeration system improvements including diffuser replacement, process controls, and blower modifications should be evaluated contemporaneously. PRACTITIONER POINTS: Off-gas analysis can be used to evaluate process loading imbalances by mapping the oxygen uptake rate. Alpha factors from off-gas testing are used in process models to evaluate air requirements and blower air demand. Comparative evaluation of blowers must be done considering the net present value of the status quo, upgrades, or replacement.