Balancing ammonia and alkalinity for nitrification at Walnut Creek Wastewater Treatment Plant
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The Walnut Creek Wastewater Treatment Plant in Austin, Texas, has recently experienced increasing influent ammonia concentrations. Nitrification, the biological process used to treat ammonia, consumes alkalinity, which makes it difficult to properly treat ammonia while still maintaining the pH above the required discharge level of pH 6. Operators have looked to the addition of chemicals to supplement alkalinity; one creative alkalinity source was CaCO₃ solids, which are generated by the lime-softening process at Davis Water Treatment Plant. In 2011, the utility began transferring solids to Walnut Creek and immediately noticed improvements in both the nitrification efficiency and the effluent pH. However, undissolved solids accumulated at Walnut Creek and had a detrimental effect on the biosolids treatment efficiency at Hornsby Bend Biosolids Management Plant. Ultimately the costs of the poor biosolids treatment forced the utility to examine an alternative alkalinity source. The objective of this thesis is to help Walnut Creek optimize the use of various alkalinity sources to find a long-term solution that will improve the alkalinity and ammonia balance for adequate nitrification. Analysis of the plant’s influent characteristics suggested that industrial users, especially the semiconductor industry, are major contributors of ammonia and sulfate to the wastewater. A theoretical modeling based on chemical equilibrium predicted that using the CaCO₃ solids would provide a maximum alkalinity benefit of 47 mg/L as CaCO₃. Experimental dissolution jar tests were conducted to verify the model predictions and estimate the kinetics of dissolution. Results from these tests showed no significant dissolution of CaCO₃, and that the solids remained unchanged throughout the test. These results indicate that CaCO₃ solids are not recommended to provide alkalinity at Walnut Creek. Finally, the use of Mg(OH)₂ for alkalinity was employed at Walnut Creek and allowed operators to reduce the blowers that provide aeration. To quantify this observation, bubbling column tests were conducted to measure differences in the oxygen transfer rate at various Mg(OH)₂ concentrations. However experimental results did not match the expectations, so future work is required.