TABLE OF CONTENTS The presence of phosphorus in drinking water supplies does not have any known health significance, but phosphorus is considered to be the limiting factor for eutrophication of fresh surface waters so its removal from wastewaters is a concern for many. Phosphorus is present in municipal wastewater as orthophosphate, polyphosphate, and organic phosphates. The orthophosphates are immediately available for biological reactions in soil ecosystems. The necessary hydrolysis of the polyphosphates proceeds very slowly in typical soils, so these forms are not as readily available. Industrial wastewaters may contain a significant fraction of organic phosphorus, but typical municipal wastewaters do not.
Phosphorus removal in land treatment systems can occur through plant uptake, biological, chemical, and/or physical processes. The nitrogen removal described in the previous section is almost entirely dependent on biological processes, so the removal capacity can be maintained continuously or restored by proper system design and management. In contrast, phosphorus removal in the soil depends to a significant degree on chemical reactions which are not necessarily renewable. As a result, the retention capacity for phosphorus will be gradually reduced over time, but not exhausted. At a typical SR system, for example, it has been estimated that a 1-ft depth of soil may become saturated with phosphorus every 10 years. The removal of phosphorus will be almost complete during the removal period. Percolate phosphorus should not be a problem until the entire design soil profile is utilized, and the percolate then emerges or is otherwise discharged to...
