Bioavailability of phosphorus in purified municipal wastewaters

The phosphorus (P) load from municipalities to surface waters can be markedly reduced using current wastewater treatment technology. However, the choice of optimal treatment method and assessment of the eutrophication caused by wastewaters are confounded by the unknown bioavailability of various P forms in the effluents. We estimated the potential bioavailability of total phosphorus (TP) and of three P fractions in 20 purified wastewater samples from five municipal treatment plants in Finland. All the plants had an activated sludge process and simultaneous P precipitation. In addition, two plants had a post precipitation unit and the others nitrified wastewater. On average 36% (0–67%) of TP in effluents was utilized by the green alga Selenastrum capricornutum Printz in bioassays lasting 2–4 weeks. If P became available according to first-order kinetics, the amount of P converted into an available form during an extended time frame was usually negligible. Dissolved reactive phosphorus (DRP) appeared to be entirely bioavailable, whereas only an average of 22% (0–74%) of dissolved unreactive phosphorus (DUP) and 25% (0–54%) of particulate phosphorus (PP) was utilized by algae. Higher relative and absolute amounts of P were generally available in the effluents from the nitrifying plants than from the post precipitation plants. In addition, when the capacity of a plant was exceeded, the concentration of bioavailable P in effluents appeared to increase. Our results suggest that TP alone is not a satisfactory criterion for use in eutrophication control measures of municipal P loading.