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Phosphonates are widely used in various industries. It is desirable to remove them before discharging phosphonate-containing wastewater. This study describes a large number of batch experiments with adsorbents that are likely suitable for the removal of phosphonates. For this, adsorption isotherms for four different granular ferric hydroxide (GFH) adsorbents were determined at different pH values in order to identify the best performing material. Additionally, the influence of temperature was studied for this GFH. A maximum loading for nitrilotrimethylphosphonic acid (NTMP) was found to be ∼12 mg P/g with an initial concentration of 1 mg/L NTMP-P and a contact time of 7 days at room temperature. Then, the adsorption of six different phosphonates was investigated as a function of pH. It was shown that GFH could be used to remove all investigated phosphonates from water and, with an increasing pH, the adsorption capacity decreased for all six phosphonates. Finally, five adsorption–desorption cycles were carried out to check the suitability of the material for multiple re-use. Even after five cycles, the adsorption process still performed well.

Systematic investigations of the acidic dissolution of P, Al, Fe and Ca from Al-containing tertiary sludge were carried out in this work. The results were then compared with the dissolution behaviour of Al-containing anaerobically digested sludge to evaluate the P–recovery potential in the form of struvite from tertiary sludge versus anaerobically digested sludge. Additional investigations of synthetically produced Al–sludge served as a comparison for the dissolution behaviour of P and Al without the influence of other contaminants (metals, biomass). In addition, the acid consumption was analyzed as a function of the target pH during the dissolution. The dissolution efficiency of PO4-P in tertiary and anaerobically digested sludge after acid treatment at pH 2 was ∼90%. The dissolution efficiency of Al and Ca in tertiary sludge was also ∼90% at pH 2, while the release efficiency of Al and Ca in anaerobically digested sludge was lower, ∼70% at pH 2. In tertiary sludge, about 75% of Fe was found dissolved at pH 2, whereas in anaerobically digested sludge this value was higher, ∼90%. Based on the experimental data, it can be concluded that significant dissolution of phosphorus from Al–containing tertiary sludge can take place at pH < 3. The highest sulphuric acid consumption for P–dissolution was observed in the case of tertiary sludge at pH 2.