Phosphorus removal in passive treatment technologies for tertiary wastewater treatment

The treatment of phosphorus at small sewage works requires alternative approaches to the traditional chemical precipitation and biological removal pathways, as such approaches do not align well to the requirements at such a scale in relation to the use of chemicals, increased energy demands and/or increased sludge production. At small sewage works, constructed wetlands are often used as a tertiary treatment for solid removal and some associated biological degradation. The current work aims to assess the potential to upgrade such systems for phosphorus removal by replacing the traditional media with a reactive alternative. This was accomplished through a series of laboratory and pilot trials to establish the most appropriate media and understand the underlying mechanisms. Determination of key properties, such as retention capacity, mechanical strength and regeneration potential, identified steel slag and phosfate™ as media that were suitable for in depth investigation. Both were shown to be effective at phosphorus removal if sufficient contact time was provided such that 1 mg L- 1 effluent concentrations was achieved when an empty bed contact time of 48 hours was used. A detailed investigation of the media revealed that steel slag worked through a two-step process where initially calcium was dissolved into the water from the surface of the media and then precipitated with the phosphorus to form calcium phosphate. The presence of alkalinity in steel slag bed inhibited the precipitation of phosphorus through calcium, as carbonates were precipitated instead. There was also a risk associated with the leachability of aluminum and flushing of retained phosphorus during the treatment of wastewater with very low phosphorus concentrations from the bed. In the case of Phosfate™ , the binding agent resulted in very high effluent pH and the formation of colloidal phosphates that needed to be filtered out to enable low effluent phosphorus concentrations to be achieved. The results indicated that both media have the potential to remove phosphorus from tertiary effluents, but the issues of leachability of aluminum from steel slag bed and the release of retained phosphorus should be investigated further before full scale trials. The colloidal phosphorus observed escaping effluent from phosfate™ bed and that the elevated effluent pH should be solved before full scale trials.