Brisbane Water has undertaken an investigation into low cost options to improve the removal of nitrogen and phosphorus for two of its wastewater treatment plants. Luggage Point Stage 2 (300,000 e.p.) is a conventional activated sludge plant designed for nitrification. Gibson Island (150,000 e.p.) is an extended aeration activated sludge plant designed for nitrogen removal to less than 10 mgTN/l. Extensive modelling and plant simulation were carried out to evaluate the potential of various modified operational modes before the most promising modes were trialed on the full scale plants. Operational trials are proceeding well and improved nitrogen removal to less than 3 mgTN/l for Gibson Island and to less than 10 mgTN/l for Luggage Point have been achieved. Improved phosphorus removal has also been achieved for periods at both plants (less than 4 mgTP/L for Luggage Point and less than 2.5 mgTP/l at Gibson Island). However, phosphorus removal has not been consistent and trials are ongoing to determine the sustainable level of phosphorus removal for these plants. The conclusion of the trials to date is that operational strategies can be implemented for these plants to effect the removal of substantial quantities of nitrogen and phosphorus for a minimum of capital cost. This paper presents the results of the various operational strategies that have been trialed and implemented for both plants. When considering the upgrading of a plant for improved nutrient removal, the principle of "Optimise What You Have First" can sometimes produce surprisingly high nutrient removal levels for a very modest capital expenditure. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.
