No abstract

Hydrodynamics in an oxidation ditch is a main key of an optimal treatment. Mixing and aeration are closely linked to biological performances. One parameter considered to be essential for correct design and operation is the horizontal velocity without aeration. A value of 0.3-0.35 m.s -1 for this parameter is regarded as a prerequisite (Da Silva, 1994). Hence its correct prediction is high importance. Different CFD modelling approaches have been applied and evaluated with regard to their performance in modelling the agitation in an oxidation ditch and consequently assessing the horizontal velocity. The two approaches used differ in the way they take the agitation, i.e. the impeller, into account in the model set-up : a) the "Fixed Values" approach, imposing a local axial velocity on cylinders which represent the impellers; b) the "Multiple Reference Frame" approach (MRF), using the exact geometry of the impellers. Theses approaches are compared with experimental data, i.e. with the average horizontal velocity as well as with velocity profiles. The first CFD agitation model ("Fixed Values") is highly dependant on the quality of the information provided by the supplier of the impellers. This is due to the fact that the supplier presents the value of the imposed axial velocity for a given impeller. Hence this whole modelling approach depends on the accuracy of the value provided. However, once applying the correct axial velocity imposed, this approach can gain quite some relevance as it is less computational intensive than the MRF approach . Experimental measurements have in fact revealed a too low horizontal velocity in the oxidation ditch of concern. Thanks to the CFD simulations, presented in this study, explanations of the low horizontal velocity could be put forward. This illustrates their potential for advanced troubleshooting.

Two Nitrification-Denitrification Biological Excess Phosphorus Removal (NDBEPR) systems have been operated for 8.5 months in order to compare their Biological Excess Phosphorus Removal (BEPR) performance. One of these systems, i.e. the University of Cape Town (UCT) system, exhibits mainly aerobic P uptake while the External Nitrification Biological Nutrient Removal Activated Sludge (ENBNRAS) system is characterised by high anoxic P uptake. It was observed that when operating with predominantly aerobic P uptake, the UCT system released more P than the ENBNRAS system, even though it had a lower anaerobic mass fraction. However, when the influent TKN/COD was high, i.e. > 0.1, anoxic P uptake also occurred in the UCT system and P release dropped to lower levels than in the ENBNRAS. Accordingly, P uptake of the UCT system was 5 mg P/l influent higher than that of the ENBNRAS system, when it was predominantly aerobic, but 9 mg P/l influent lower when anoxic P uptake occurred. As a result, the UCT system achieved superior P removal when aerobic P uptake was predominant (23% higher), but when high influent TKN/COD promoted anoxic P uptake the P removal of the UCT system was poorer than that of the ENBNRAS system. This study clearly showed that anoxic P uptake is not beneficial to NDBEPR systems.

The City of San Jose at its San Jose/Santa Clara Water Pollution Control Plant has embarked on a series of projects to reduce energy consumption and provide operational savings, and aeration system upgrading is one of the key projects. The plant was converted in 2000 from a coarse bubble, cross-roll in 2000 to fine bubble membrane tube diffusers, which produced significant savings. In 2009 a second project was initiated to develop an even more cost effective alternative using a greater number of diffusers in slightly different geometries. Off-gas testing was performed to confirm aeration efficiency. An economic analysis was used to determine the most cost effective alternative. Upgrading the original year-2000 retrofit with diffusers having more active surface area provided a pay back of approximately one year. The economic analysis also confirmed that the decision to upgrade the original coarse bubble system with fine bubble diffusers was sound and would provide a payback of 3 to 6 months using current costs. One counter-institutive result was the inadequacy of the original coarse bubble diffuser air piping. The in-tank headers were able to provide even air distribution for coarse bubble diffusers but were unable to do so with fine pore diffusers, which operate at much lower air flow rate. Upgrading options are continuing and additional energy conservation activities will occur. Key wordsAeration, energy saving, fine bubble diffusers, oxygen transfer efficiency, return on investment IntroductionThe San José /Santa Clara water pollution control plant (SJ/SC WPCP) is a tertiary wastewater treatment plant which treats domestic, industrial and commercial wastewaters. The Plant is looking for solutions to reduce its energy consumption in order to help the City of San José to reach its goal of reducing the per capita energy consumption by half in the next 15 years (City of San José, 2007). Reduction of energy used for aeration has been identified as a method to achieve this goal. As detailed by Readon (1995), aeration accounts for 45-75% of the process energy cost in a wastewater treatment plant.

No abstract