Although to date some technologies producing bio-based phosphorus (P) fertilizers have been proposed and implemented, the efficient use of the recovered products is still limited due to legislative constraints, lack of insights in their P release with time, and in the corresponding mechanisms. The aim of this paper was to evaluate the fertilizer performance in terms of P release and use efficiency of recovered struvite, iron phosphate (FePO4) sludge, digestate, and animal manure as compared to fossil reserve-based mineral triple superphosphate (TSP). First, product physicochemical characteristics and P fractionations in the context of European fertilizer legislation were assessed. Next, a controlled greenhouse experiment was set up to evaluate plant reactions as well as changes in P availability on sandy soils with both high and low P status. P soil fractions were determined in extracts with water (Pw), ammonium lactate (PAl), CaCl2 (P-PAE), and in soil solution sampled with Rhizon soil moisture samplers (Prhizon). Based on all results, long-term field trials evaluating the P release effect of struvite and digestate as compared to animal manure and TSP on different soil types with varying P status appear to be worthwhile. These products show promise as sustainable substitutes for conventional P fertilizers and could contribute to a more efficient use of P in agriculture. A refined classification of P application standards/recommendations in terms of soil P status, texture, and fertilizer characteristics, next to the crop P demand, is recommended. Moreover, the additional use of Rhizon samplers for determination of direct available P, including dissolved organic P, is proposed for better understanding and categorization of different P fertilizers in environmental and fertilizer legislations.

Autotrophic denitrification with sulfur is an underexplored alternative to heterotrophic denitrification to remove nitrate from wastewater poor in organics. The application on ion exchange regeneration water (19.4-32.1 mS cm) is novel. Three fixed bed reactors were tested at 15 °C for >4 months, inoculated with activated sludge from sewage treatment. All were fast in start-up (<10 days) with high performance (94 ± 2% removal efficiency). pH control with NaOH rendered higher nitrate removal rates than limestone addition to the bed (211 ± 13 vs. 102 ± 13 mg N L d), related to higher pH (6.64 vs. 6.24) and sulfur surface area. Bacterial communities were strongly enriched in Sulfurimonas (63-67%) and Thiobacillus (24-26%). In an economic comparison, sulfur-based denitrification (€5.3 kg N) was 15% cheaper than methanol-based denitrification (€6.22 kg N) and both treatments were opex dominated (85.9 vs. 86.5%). Overall, the technological and economic feasibility should boost further implementation of sulfurotrophic denitrification.

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