Aeration is the most energy-intensive operation in wastewater treatment, amounting to 45-75% of plant energy costs. Fine-pore diffusers are today almost ubiquitous in municipal wastewater aeration, due to their advantageous aeration efficiency (mass of oxygen transferred per unit energy required). Nevertheless, older municipal treatment facilities and many industrial treatment plants are still equipped with coarse-bubble or surface aerators. Fine-pore diffusers are subject to two major disadvantages: a) fouling, if not cleaned periodically; b) decrease in oxygen transfer efficiency caused by dissolved surfactants. Coarse-bubble and surface aerators are typically not subject to the traditional problems affecting fine-pore diffusers. Nonetheless, they achieve oxygen transfer at the expense of increased energy intensity. The increased biomass concentration associated with high mean cell retention time (MCRT) operations has a beneficial effect on aeration. Nutrient-removing selectors are able to further increase aeration efficiency, as they sorb and utilize the readily available substrate which otherwise would accumulate at bubble surfaces and dramatically decrease aeration efficiency. We summarise here our 30-year long experience in aeration research, and results obtained with clean- and process-water tests are used to show the beneficial effects of high MCRT operations, the beneficial effect of selectors, and the decline of aeration efficiency due to dissolved surfactants.
Aeration is the most energy intensive unit operation in municipal wastewater treatment, and fine‐pore diffusers have been widely used to minimize power consumption. Unfortunately, fine‐pore diffusers suffer from fouling and scaling problems, which cause a rapid decline in aeration performance and significant increase in power consumption. Diffusers must be cleaned periodically to reduce energy costs. The cleaning frequency of diffusers is site‐specific and its effectiveness can be evaluated with oxygen transfer efficiency (OTE) testing. Off‐gas testing is the best technique for measuring OTE in real‐time. Fine‐pore diffusers have low α factors that are further reduced at high loading rate. A time‐series of off‐gas measurements were conducted to demonstrate the value of real‐time OTE data for developing energy‐conserving operating strategies. The observations confirm the inverse correlation between OTE and airflow rate as well as the economic benefits of diffuser cleaning. In addition, mathematic models were applied to simulate the transient oxygen uptake rate (OUR) and show the impact of varying load on OTE and aeration cost, especially when faced with time‐of‐day power rates. Regular diffuser cleaning can reduce average power costs by 18% and various equalization alternatives can reduce power costs by 6 to 16%.
Aeration is an essential process in the majority of wastewater treatment processes, and accounts for the largest fraction of plant energy costs. Aeration systems can achieve wastewater oxygenation by shearing the surface (surface aerators) or releasing bubbles at the bottom of the tank (coarse- or fine-bubble aerators). Surfactants accumulate on gas-liquid interfaces and reduce mass transfer rates. This reduction in general is larger for fine-bubble aerators. This study was conducted to evaluate mass transfer effects on the characterization and specification of aeration systems in clean and process water conditions. Tests at different interfacial turbulence regimes were analysed, showing higher gas transfer depression for lower turbulence regimes. Higher turbulence regimes can offset contamination effects, at the expense of operating efficiency. This phenomenon is characteristic of surface aerators and coarse bubble diffusers and is here discussed. The results explain the variability of alpha factors measured at small scale, due to uncontrolled energy density. Results are also reported in dimensionless empirical correlations that describe mass transfer as a function of physiochemical and geometrical characteristics of the aeration process.
Due to the fluctuations of different environmental parameters, the energy footprint of water reclamation and purification processes has a dynamic behavior during diurnal periods. Among the influencing factors, hydraulic load and constituent load have the highest diurnal impact on the energy footprint of the biological activated sludge process during water reclamation, and energy intensive membrane processes (e.g. reverse osmosis) during water purification. Besides the diurnal energy footprint variations, the energy-associated carbon emission also is facing a variation among the portfolio of energy sources employed by power supply agencies in different hours of a diurnal period. During the night, the carbon intensity of the unit power generation is lower due to a higher contribution of clean-source energies; while during the daytime, fossil fuelsource power has a bigger contribution in fulfilling the power augmented demand.
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