Waihi Beach Wastewater Treatment Plant - Some Effects of High Speed Pumping and Mixing on Activated Sludge Bacteria

The Waihi Beach treatment plant, constructed in 2001 for Western Bay of Plenty District Council (WBOPDC), consists of a single-lagoon sequential batch reactor (SBR) followed by a wetland and a UV disinfection plant. Historically the plant struggled to provide enough aeration during peak periods. A new 35 year resource consent required a review of the plant and professional advisors had mixed views about the preferred re-design of the aeration system.

This paper is an on-site study of the treatment plant aeration system and how it is affecting the activated sludge process. The purpose of this paper is to assist the WBOPDC in making a more informed decision about the aeration upgrade.

Studies of the existing system found that prolonged use of the turbo-aerators caused damage to the activated sludge floc. Damage was measured using effluent turbidity and disrupted heterotrophic bacteria. The effluent was still compliant with discharge requirements. Loss of these bacteria in the effluent is considered to be irreversible and undesirable. 

A number of simulated high speed pumping tests showed that the critical factor causing permanent disruption of heterotrophic bacteria from the sludge floc was fluid velocity or speed. After three passes at the slowest pumping speed of 29 m sec-1 the heterotrophic bacteria were easily disrupted (70 to 80%). Disruption of the sludge floc released more Gram negative bacteria than Gram positive ones and the "viable" bacteria could not be re-aggregated with the floc by gentle mixing. More than half the "non-viable" bacteria and non-bacterial particles were re-aggregated. 

It is recommended that upgrading the Triton mixer/blower units rather than turbo-aerators is a better strategy.