ANNOUNCEMENTS
The phosphorus (P) paradox – the scarcity of P resources in the lithosphere (soil) but their excess in the hydrosphere (aquatic ecosystems) – is well recognized in the environment. Central to solving this fundamental P paradox is recovering P from different waste streams, and this research assesses the feasibility of recovering P from one such waste stream- decentralized sewage systems by focusing on the development of technology suitable for P recovery, which seeks to balance the efficiency of recovering P with the energy required to recover it. More specifically, the influence of tapered velocity gradient, which is known to be energy efficient, on the recovery process was tested to make the recovery attractive for decentralized systems. Further, a sludge blanket clarifier (SBC) which operates on the principle of tapered velocity gradient was designed. Input parameters like flow rate, volume of clarifier, velocity gradient used in designing the SBC were explained. The study also sought to find, based on life cycle analysis (LCA), the most sustainable option of supplying P to agriculture among such options as recovering P from centralized and decentralized sewage treatment systems or using the conventional source, namely chemical fertilizers, especially diammonium phosphate (DAP). The goal of this study was to make the developing nations less vulnerable to P scarcity by building a local and alternative source of P for agriculture while also augmenting decentralized sanitation systems. The study also provides valuable insights into the importance of developing vulnerability-based characterization factors of nonrenewable resources. Septic tank liquor was chosen as the source of P in this study because septic tanks are widely used in developing countries and no prior study was found that investigated septic tank liquor for recovering P.
Factors that influence P recovery, namely pH, the molar ratio of magnesium (Mg) to P, velocity gradient (G), and mixing duration (td) were tested and optimized for recovering P in the form of struvite. Interactions between the above factors and their possible influence on the efficiency of P recovery were also examined. Under optimal conditions, more than 90% of P could be recovered. Among the factors that influenced P recovery, pH and the Mg:P ratio affected recovery efficiency whereas the velocity gradient and mixing duration affected the size of struvite crystals. Tapering the velocity gradient not only increased the crystal size significantly, from 280 μm to 371.9 μm, but also reduced energy requirement by 62%. Assessment of the interaction between the influencing parameters showed that pH accounted for about 80% of the increase in P recovery, followed by G (5%), Mg:P ratio (2%), interaction between pH and the Mg:P ratio (1%), and interaction of all the three factors (4%). When all these parameters were optimized, the effluent‘s chemical oxygen demand (COD) decreased by 47%; its biological oxygen demand (BOD), by 20% and its content of total suspended solids (TSS), by 27%. Thus, recovering P from septic tank liquor makes the final effluent less polluting.
Life cycle assessment was used for assessing the sustainability of the three options of sourcing P in the context of a developing country. The options were (1) recovering P from septic tank liquor, a decentralized treatment system; (2) recovering P from a centralized wastewater treatment plant; and (3) producing DAP in a factory with two ways of disposing phosphogypsum waste, namely selling it to the cement industry or dumping it in landfills. For the decentralised system SBC designed previously was used was recovery of P. The P-recovery technology for centralized plants considered in the present study is the ‗AirPrex process‘, used in Germany, in which P is recovered from anaerobically digested sludge. Impacts of each option on the following impact categories were measured using the ReCiPe methodology, in addition to the total impact: global warming, formation of particulate matter, terrestrial acidification, eutrophication of freshwater, and consumption of non-renewable resources (fossil fuels and minerals). The cumulative energy demand of each option was also estimated. The three options differed in terms of their impact on environmental externalities. The energy demand of DAP production was the lowest, and its impact on global warming was 38% of that of the decentralized system and 37% of that of the centralized system. Production of DAP was associated with maximum consumption of the mineral resource, namely rock phosphate. The decentralized system had the least impact on eutrophication of freshwater and most on the formation of particulate matter. Overall, the centralized system had the highest impact score – 2.3 times that of DAP production – followed by decentralized process, its impact being 1.4 times that of DAP production. If the phosphogypsum waste produced in manufacturing DAP is disposed of in landfills, DAP production had the highest overall impact. The major impacts of the P recovery options are not from the process itself but from the upstream supply chain of inputs required for the process or, in other words, the resources (energy and chemicals) used by such technologies. The impacts of P recovery options are thus transboundary (a problem of shift of region) whereas the impact of DAP production is more localized. The decentralized sewage treatment systems were not only better than the centralized systems in recovering P but also had less impact (40% lower than that of the centralized systems). Sensitivity analysis confirmed that both materials and energy inputs had a major influence on the impact of P recovery options: reducing the chemical and energy demand significantly reduced the impacts of the two recovery options such that their impacts were even lower than those of DAP production. Considering the degree of uncertainty in the inventory data analysed through Monte Carlo simulations, the mean impact score of the three options can be different from the original impact obtained without considering uncertainty. Thus, it becomes imperative to include uncertainty distribution in the inventory data. Although the environmental impact of centralized sewage treatment systems was greater than that of decentralized sewage treatment systems, because of economies of scale the operating cost of the former was lower than that of the latter. Overall, this study is the first initiative to develop a technology to recover P from decentralized sewage treatment systems, a technology with low energy requirements, and to compare its impact to that of two alternative technologies on the basis of life cycle analysis. The results offer useful inputs for making policies related to controlling water pollution as well as those aimed at making agriculture sustainable.
