Phosphorus removal in reactive filter materials factors affecting the sorption capacity

Abstract: Phosphorus is an essential component in all living organisms; it is one of the components of the DNA and the key element in the energy supplying molecule adenosine triphosphate (ATP). Throughout the history, humans have been recycling phosphorus to agriculture; thereby increasing the yield, examples of this includes the burning of plants and the use of manure. Today, we rely on commercial fertilizers with high concentrations of phosphorus. The manufacturing of these products include extraction of phosphorus from phosphate rock. However, phosphate rock is a limited and non-renewable resource and the reserves are declining. The population on our planet in constantly increasing, hence the shortage of fertilizers would have consequences of catastrophic measurements, the most obvious being starvation. In order to avoid this scenario it is of uttermost importance to reuse the phosphorus that passes through our society and is subsequently released in the wastewater. The inadequate small-scale wastewater treatment facilities in Sweden of today are contributing with 20% to the release of phosphorus. By implementing the use of reactive filter materials in these systems, direct release of phosphorus to the recipient can be prevented. After a period of time these materials have to be exchanged, and they can thereafter be used as fertilizers, thereby re-cycling the phosphorus back to agriculture.The effectiveness of the reactive materials is affected by several parameters, such as pH, specific surface area, hydraulic properties of the material and the characteristics and distribution of the wastewater. In order to compare the phosphorus retention capacity in three different materials, two experiments utilizing septic-tank wastewater was conducted. The first was a pilot-scale-experiment that included Polonite and blast furnace slag (BFS) as filter materials. This experiment was conducted in two phases, using wastewater with high respectively low concentrations of organic matter. The study investigated the effect of organic matter on the phosphorus retention capacity in the materials; additionally the reduction of indicator bacteria (Enterococci) and organic matter (TOC) was studied. The second experiment was conducted at the laboratory, using wastewater with high concentrations of organic matter. In this study, the phosphate retention capacity in Polonite and Sorbulite was investigated in a re-circulatory system, where the treated wastewater was re-circulated back to the influent volume of water. The additional parameters investigated in this experiment were the removal of nitrogen and TOC as well as the retention of indicator bacteria (E. coli and Enterococci).The results showed that Polonite performed better compared to the other materials with regards to the removal of phosphate, total phosphorus and bacteria. Sorbulite had a higher removal rate of TOC and total inorganic nitrogen (TIN) compared to Polonite, which in turn removed more TOC than BFS. Furthermore, both Polonite and BFS had a higher phosphorus retention capacity when using wastewater with low concentrations of organic matter. These two materials showed a higher percentage reduction of indicator bacteria when using wastewater with high concentrations of organic matter. However, since the results showed that the bacteria count in the effluent was lower when using wastewater with low concentrations of organic matter, the higher reduction rate was therefore ascribed to a higher concentration of bacteria in wastewater with high concentrations of organic matter. Therefore, the conclusion was drawn that low concentrations of organic matter is preferable also in respect of bacteria reduction. In order to ensure a high removal of phosphorus and bacteria, as well as to prolong the lifetime of the filter material, the wastewater should be pre-treated to obtain a BOD7 value below 20 mg L-1.