Problem: Insufficient treatment capacity and tightening ammonia–nitrogen limits.
Solution: Installation of integrated fixed-film activated sludge technology.
To manage a rapid increase in population, The Colony, Texas, needed to expand the capacity of its Stewart Creek Wastewater Treatment Plant from 12,900 m3/d (3.4 mgd) to 17,000 m3/d (4.5 mgd). However, along with the flow expansion, the Texas Commission on Environmental Quality issued the city a new permit requiring that its plant’s effluent meet strict ammonia–
nitrogen and total phosphorus limits.
The Stewart Creek Wastewater Treatment Plant included two parallel contact stabilization trains only designed to meet biochemical oxygen demand and total suspended solids limits. Even with more capacity in the system, the existing process could not meet the new 3.0-mg/L ammonia–nitrogen limit.
The Colony conducted an evaluation of expansion alternatives and decided to install an integrated fixed-film activated sludge (IFAS) technology in its existing facilities to treat increased flow and provide the required nitrification. The technology is a hybrid biological wastewater treatment process that uses both suspended and attached growth bacteria to treat wastewater. IFAS technology adds high-surface-area media directly into aeration basins in activated sludge plants to increase the amount of biomass available in the system.
In March 2005, The Colony began to modify the existing contact stabilization basins. To manage the attached biomass environment, the city installed Webitat, a BioWeb™ IFAS system manufactured by Entex Technologies Inc. (Chapel Hill, N.C.). The project’s design engineers chose the fixed-media system to accommodate a fine-bubble aeration system in the plant and avoid headloss associated with retention screens for floating media.
The system includes four vertical-side shrouds, which channel air through the module. Polyester fabric media on steel frames provide the surface for a thin biofilm to form, and dedicated bubble mixers underneath the media provide mixing and oxygen transfer. The enclosed system acts as a high-rate biological reactor.
Under normal operating circumstances, the media are maintenance-free. If the water level in the basin drops for a significant period, the biomass has to be washed off the media to prevent odors, and the media have to be covered to prevent ultraviolet degradation.
All of The Colony’s 24 modules were converted to fixed-media modules. The first train of 10 modules was installed and returned to operation within 48 hours of initial draining. Improvement in treatment performance was immediate, and the plant has been able to comply with its new ammonia discharge limits since the upgrades became fully operational in June 2007.
Using IFAS technology allowed nitrification to occur and enabled the plant to comply with its new discharge limits while expanding capacity by 33% without building additional basins.
Other benefits of installing an IFAS system include better solids settling, reduced solids production, and better overall system stability. Fixed IFAS media also are more energy-efficient than moving media because the IFAS system uses fine-bubble diffusers instead of coarse-bubble diffusers, which generally are required to keep moving media well-mixed. Compared to coarse-bubble diffusers, fine-bubble diffusers provide better oxygen transfer because smaller bubbles increase the ratio of surface area — where oxygen transfer occurs — to the amount of air pumped.
Using an IFAS system led to costs approximately 70% less than would have been required by conventional treatment. The Colony saved more than $1 million in aeration basin construction costs by choosing to modify its existing basins. Final modifications to the system were completed in September 2007, and ammonia–nitrogen concentrations continue to remain well below the new permit limit.
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