| AirJet aeration and mixing systems from Landia combine a highly-efficient air ejector with chopper pumps and may also be used in conjunction with supplementary mixing systems for optimum oxygen transfer rate. |
A supposed aeration problem is often nothing of the sort; it is simply the need for an efficient and appropriate mixer. When aeration equipment was first installed in wastewater tanks many years ago, the original design may have achieved the required balance, but manufacturing requirements, new process technology and legislation have inevitably seen conditions change.
We are often called in to address wastewater problems caused by changes to production lines that have increased the biological load. There can also be issues prompted by rises in temperature, seasonal loads, in-factory waste minimization, or improvements and additions to upstream processes. Because aeration and mixing are vital to a healthy treatment process, it is best to routinely review the wastewater element as part of the whole manufacturing process so that a fully-mixed and properly-aerated tank is consistently achieved.
Just as problematic as over-aeration is under-aeration. Low dissolved oxygen (DO) in the tank and reduced biological activity will produce bulking sludge, unpleasant odors and ultimately failure to meet discharge permits. Under-aeration can also be caused by changes to conditions such as COD/ammonia loading and increases in the tank's temperature.
Working with Roquette America, a producer of starch products, syrups and polyols (sugar alcohols), we saw how the use of a diffused air system (fed by large blowers) meant constant cleaning and replacing of diffuser membranes. This included the substantial expense of hiring divers to enter the tanks and perform maintenance so as to not interrupt the 24/7 operation. Installing a self-aspirating aeration system that incorporates heavy-duty chopper pumps has resulted in a completely clog-free operation and a homogenous mixture for higher air-to-water transfer. Energy bills alone at Roquette have been reduced by 30 percent, and constant laborious maintenance has now become a simple, economic, once-a-year requirement.
As with Roquette, the overall focus should be optimum configuration of aerators and mixers for maximum treatment at minimum maintenance and power use. Hydraulic mixing of the basin will depend on several factors, not just the tank's geometry, dimensions and layout. One needs to look at the actual hydraulic mixing power provided by aerators, as well as issues such as floor-sweeping velocities greater than 0.8 ft/sec, which are required to prevent the settling out of solids that lead to anoxic or anaerobic pockets.
