A Process Analysis for the Expansion of a Wastewater Facility in Bullhead City, Arizona Using Advanced On-line Monitoring and Computer Modeling

A survey at the Section 10 Wastewater Facility in Bullhead City, Arizona began on June 7, 2002 for process analysis in regard to the plans for plant expansion from the existing oxidation ditch of 2.0 MGD to 4.0 MGD. This survey included the use of the MAT (Mobile Analytical Trailer) 2000 online monitoring trailer to provide information regarding the diurnal variations and the overall efficiency of the biomass. The data from this field analysis along with laboratory results were used to construct a model of the oxidation ditch process using Hydromantis GPS-X modeling software to help determine the ultimate capacity of the existing plant without risking any effluent compliance.The primary monitor within the MAT2000 (Mobile Analytical Trailer) is the respirometer along with a colorimetric ammonia monitor and a dissolved oxygen probe placed directly in the basin. These results and routine laboratory analysis data can then be used in the construction and calibration of the plant model using GPS-X to simulate various process scenarios and kinetic coefficient predictions in the process calculations for the facility upgrade.The initial process analysis revealed autotrophic inhibition as the ammonia concentration began a steady climb while the respiration rates went down. After the biomass seemed to recover the efficiency of the surface aerators was also a question and periods of dissolved oxygen limitations were evident with some ammonia present in the ditch effluent and laboratory analysis showing the presence of nitrite-nitrogen. When the data was used in the model construction there was a significant difference in the predicted nitrate-nitrogen when the dissolved oxygen set point was changed from 0.5 to 1.0 mg/L, which raised questions to the size of the anoxic zone as this dissolved oxygen range leaves little room for error. The model also ran different scenarios comparing fine-bubble diffusion for aeration to the surface aeration under mixed liquor concentrations of 2,000 mg/L and 5,000 mg/L of MLSS and how the power consumption changed. This pointed back to the initial calculation of the oxygen transfer efficiency and the importance of the maintenance of the solids inventory. The other important consideration to be carefully analyzed is the variance in some of the kinetic coefficients that will be used in the design calculations as the model calibration with the respirometric data showed different values than those used in other references.