Phosphorus speciation in municipal biosolids for efficient phosphorus recovery

Abstract

Phosphorus is a required element for industrialized crop production. Our ability to produce phosphorus fertilizers has allowed humanity to sustain its population. The current sources of phosphorus are apatite sedimentary deposits geographically concentrated in four countries (Vaccari 2009). Recent reports have suggested that these resources will be exhausted by the end of this century and that the limited geographical distribution of phosphorous deposits may prove to be a source of political conflict in the future (Smil 2000, Cordell et al. 2009, Vaccari 2009). To avoid these conflicts, the potential for spent phosphorus recovery and reuse must be explored. One promising source stream for phosphorus recovery is from municipal wastewater biosolids. Current methods of phosphorus recovery from these solids only target 10% of the total phosphorus in the biosolids. To recover greater quantities of phosphorus from biosolids, we must understand the phosphorus speciation within, and be able to predict the most effective recovery method for biosolids generated from conventional activated sludge, chemical phosphorus removal or enhanced biological phosphorus removal (EBPR). In order to understand the speciation of these three types of biosolids, nine municipal wastewater treatment plants were sampled. The results showed that in biosolids post-anaerobic digestion and gravity thickening a large portion of the phosphorus is present as particulate orthophosphate and polyphosphate and requires a process that targets these species. Comparison of the speciation of conventional biosolids to the speciation of biosolids from chemical phosphorus removal and EBPR showed that energy intensive technologies such as the Stuttgart process and KREPRO can recover significant quantities of phosphorus from all types of biosolids. Commercial technologies (AIRPREX) only recovered significant quantities of phosphorus when used only with EBPR biosolids. Finally, a life cycle impact assessment was performed to assess eutrophication and green-house gas potential under various scenarios of phosphorus recovery. The results showed that scenarios with chemical or biological phosphorus removal plants typically had a lower eutrophication potential than those with conventional activated sludge. Even though treatment plants with biological and chemical phosphorus removal require more energy and chemicals to operate, the global warming potentials were lower than for conventional activated sludge scenarios.