Comparison of phosphorus recovery from incineration and gasification sewage sludge ash

Abstract

Incineration of sewage sludge is a common practice in many western countries. Gasification is an attractive option because of its high energy efficiency and flexibility in the usage of the produced gas. However, they both unavoidably produce sewage sludge ashes, a material which is rich in phosphorus, but that it is commonly landfilled or used in construction materials. With current uncertainty in phosphate rock supply, phosphorus recovery from sewage sludge ashes has become interesting. In the present work, ashes from incineration and gasification of the same sewage sludge were compared in terms of phosphorus extractability using electrodialytic methods. The results show that comparable recovery rates of phosphorus were achieved with a single electrodialytic step for incineration ashes and a sequential combination of two electrodialytic steps for gasification ashes, which was due to a higher influence of iron and/or aluminium in phosphorus solubility for the latter. A product with lower level of metallic impurities and comparable to wet process phosphoric acid was eventually obtained from gasification ashes. Thus, gasification becomes an interesting alternative to incineration also in terms of phosphorus separation.

Sewage Sludge Gasification in a Fluidized Bed: Experimental Investigation and Modeling

“…Table S5 shows the mass fraction of the elements detected in the ashes arising from gasification of the SS-A and SS-B samples. In agreement with other SS gasification ash samples investigated in the literature, ,, the major elements in both materials are Ca, Al, Fe, and P. However, their concentrations are, on average, lower than the values typically reported in the literature, in the ranges of 23–61 g/kg for Al, 84–148 g/kg for Ca, 88–123 g/kg for Fe, and 51–149 g/kg for P, dry ash basis …”

“…The mineralogical composition of the bottom ashes investigated in the present study was compared to previous research, , highlighting a high variability in the chemical composition between different types of SS gasification ash, which in other countries mainly consists of quartz and calcium-containing phosphates (e.g., Ca 9 Fe­(PO 4 ) 7 , Ca 9 Al­(PO 4 ) 7 and/or Ca 7 Mg 2 (PO 4 ) 6 ), in addition to single calcium and iron phosphates. The detected absence of calcium-containing phosphates, which include both alkaline-soluble P such as in Al/Fe­(III)–P bonds, and alkaline-insoluble Ca–P bonds, is beneficial in terms of the recovery of phosphorus from gasification ashes.…”

“…Special attention has been paid to the characterization of bottom ashes from SS gasification, which is of particular interest for their safe management or further valorization in the context of circular economy. In fact, depending on the specific chemical-physical properties, ashes from SS thermal treatment could be conveniently employed either in the development of materials for building application , or as adsorbent materials, i.e., for hot H 2 S removal, or further treated for P recovery, − and so on, rather than disposed of in landfills. However, while the properties of ashes arising from SS incineration have been extensively studied so far, to the best of the authors’ knowledge, only a few studies are available in the literature relative to the in-depth characterization and further valorization of SS gasification ashes.…”

“…To implement the circular economy targets, particular attention is paid to the sludge-to-energy (StE) technologies, which allow not only the waste volume to be reduced, but also direct energy or fuels (as energetic vectors) to be generated, , natural resources (e.g., land) to be saved, and valuable byproducts (e.g., SS ash, biochar, digestate) to be obtained for use as raw materials or marketable products, including adsorbent materials, , unconventional source of phosphorus, − construction and building materials. − Among the currently available StE technologies, the ones involving the thermochemical treatment of sludge, such as combustion (for the direct production of energy), pyrolysis (together with oxy-pyrolysis and torrefaction), and gasification (the last two for the production of a fluid with energetic value), appear particularly promising because of the very good performances in terms of short reaction times (from seconds to minutes) and high conversion efficiency (less than 20% of unconverted organic constituents at the end of the process) also in comparison to the biochemical conversion routes such as anaerobic digestion (reaction time from 7 days to 5 weeks; about 40–70% of unconverted organic constituents) …”

Partial replacement of rock phosphate by sewage sludge ash for the production of superphosphate fertilizers

“…It is therefore crucial to find the link between wastewater treatment plants and agricultural production systems and to close regional P cycles. The two main pathways for recovering P from sewage sludge are the precipitation of a phosphate‐rich recyclate ( e.g ., struvite) ( Bhuiyan et al., ; Le Corre et al., ) and the thermal utilization (mono‐incineration) of sewage sludge, resulting in P rich ashes (sewage sludge ash: SSA) ( Parés Viader et al., ). Both of these pathways show drawbacks: the precipitation of struvite results in a high quality product, but usually fails to recover all P contained in the sewage sludge ( Möller et al., ).…”