Possibilities and limitations of computer simulation for determination of the lifetime of iron hydroxide filters for removing phosphate from standingwaters

This paper describes the possibilities and limits of a computer simulation model describing phosphate retention by filters that contain a mixture of i ronhydroxide and sand. The goal was to calculate the running time up to the point when the original phosphate elimination of 99% decreases to 80% of a n inflow concentration of 100 mu g/l P. The empirical results gained by fil tration experiments were compared to the model figures. This shows that the filter running time was insignificant underestimated in the model when por e water velocity is < 0,7 m/h. Higher velocity (> 0.7 m/h) leads to an insi gnificant overestimation of the running time in the model. The different re asons p.e. short circuit flow and diffusion for this are discussed in this paper. At a filter pore volume of 42.7%, a ironhydroxid content of 3.75 kg, a pore water velocity of 0.23 m/h, and a pH of 7.5- 8.0, according to the model s uch a filter will be functional for about 9 years. Under comparable conditi ons, but at an pH of 5.0-5.9, the filter will work for about 18 years. Taking into account that the running time of a filter will be insignificant underestimated in the model when pore water velocity is 0,23 m/h, the runn ing time in reality will be longer than the 9, and 18 years predicted by th e model.