Abstract
The recovery and recycling of nutrients (N&P) from wastewater are one of the major topics to save primary energy and resources, to raise the efficiency of wastewater treatment plants, and to foster a future circular economy. In the present study, the removal of ammonium (NH4+) and phosphate (PO43−) using natural and Ca-treated zeolite is investigated in detail. Special emphasis is put on the simultaneous removal of both species from model solutions followed by elaborate mineralogical analyses (XRD, EPMA, FEG-SEM) for zeolite characterization and in order to determine the type, structure, and crystal sizes of CaP-phases precipitating on the zeolites surface. The effectivity of the phosphate segregation and chemical composition and the crystalline structure of the CaP-phase precipitating on the surface of the zeolite depend on the physico chemical conditions in particular on pH, molar ratio of Ca and P (due to zeolite modification), and the presence of NH4+. Results of simultaneous removal experiments of N&P revealed that Ca pretreatment enhances P segregation and increases the obtainable P-loadings of Ca–zeolites. Maximum P-loadings of 25 mg g−1 Ca–zeolite in binary solutions containing both ammonium and phosphate were obtained. Simultaneous phosphate removal by surface precipitation of CaP-phases does not significantly influence ammonium ion exchange and the type of CaP-precipitates formed on the zeolite surface is assumed to be mainly brushite and apatite.
Highlights
In the actual discussions of promoting renewable energy, biogas and sewage treatment plants try to enhance their capacity for anaerobic digestion to produce green fuels
The P-loadings of Ca-modified zeolite reveal significantly higher values over the whole concentration range with a maximum of 25 mg P O43− g−1 at 2500 mg PO43− L−1. These results indicate that the P-loading capacity of the natural zeolite is affected by Ca-limitation at higher P-concentration levels, where the natural zeolitic Ca2+-ions are already expended
Natural zeolitic calcium is exchanged with sodium ions in N a3PO4-solutions leading to the precipitation of free PO43− as calcium phosphate (CaP)-phases on the zeolite
Summary
In the actual discussions of promoting renewable energy, biogas and sewage treatment plants try to enhance their capacity for anaerobic digestion to produce green fuels (e.g., biomethane). This leads to increasing amounts of liquid digestates and sludge liquor, which contain ascending concentrations of dissolved ammonium and phosphorous due to intensified co-substrate fermentation. N H4+ and PO43− are usually removed separately by biological and chemical processes All of these standard processes are expensive and do not offer the possibility to recover, concentrate, and utilize NH4+ and P O43− for example as nutrients in fertilizers. In contrast to these enormous amounts of N and P that are lost via global wastewater treatment systems, the food supply of an
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