Plano, S., Alford, H. and Giuffre, G, Stantec (UK) Ltd, UK(free)
Oxygen is fundamental to life and, as such, to Activated Sludge Plants (ASPs). Providing the right amount of oxygen for an ASP is a key task, during both design and operation of wastewater treatment plants, to enable compliance with regulatory effluent consents. Oxygen has a low solubility in water – therefore an ASP’s aeration system must constantly provide sufficient air to maintain a stable residual Dissolved Oxygen (DO) concentration, whilst meeting the oxygen demand of the wastewater. With increasingly stringent Total Phosphorous effluent consents enforced by the Environment Agency, there are numerous opportunities to employ Biological Nutrient Removal (BNR) within ASPs in the UK, avoiding more costly and less environmentally friendly processes such as tertiary solids removal or chemical dosing systems. The challenge for implementation of BNR plants nationwide is optimisation of the aeration system – finding the DO concentration which allows the biomass to function effectively, at the maximum aeration efficiency.
A simple Oxygen Mass Balance can be expressed as follows: dC/dt = OTR/V – OUR, where dC/dt is the variation in DO concentration in the wastewater with time, OTR/V is the oxygen transfer rate into the wastewater per unit volume and OUR is the oxygen uptake rate of the biomass. This last term is often determined by empirical equations from literature (such as Metcalfe and Eddy) or in Asset Standards developed by water companies. The calculated value will affect the size of aeration system required.
This paper presents a benchmark study of the different Actual Oxygen Requirement (AOR) formulae (for Carbonaceous, Nitrification, Nitrogen Removal and BNR ASPs) which are used within the UK against dynamic mathematical tools (BioWin and GPS – X). Differences between the empirical formulae and the dynamic mathematic tools were significant, and were greatest for BNR plants, where the mechanisms of oxygen uptake by different biomass fractions are more complex. Finally, an intensive measurement campaign was conducted to compare the mathematical tools and empirical equations used by engineers with reality.