Exploiting the unwanted: Sulphate reduction enables phosphate recovery from energy-rich sludge during anaerobic digestion

Abstract

Anaerobic digestion is shifting from a single-purpose technology for renewable energy recovery from organic waste streams to a process for integrated resource recovery. The valorisation of high-rate energy-and phosphorus-rich sludge creates the opportunity for their combined recovery. This phosphate is present in a precipitated form in the sludge, and its release into the liquid phase is an important issue before recovery can be achieved. The objective of this research was to exploit the "unwanted" sulphate reduction process for the release of phosphate into the liquid phase during anaerobic digestion, thus, making it available for recovery. Two different treatments were considered, i.e., a control digester and a digester to which sulphate was added, each operated in triplicate for a period of 119 days. The control digester showed stable methane production at 628 ± 103 mL CH4 L -1 d -1 , with a feedstock COD (chemical oxygen demand) conversion efficiency of 89.5 ± 14.6 %. In contrast, the digester with sulphate addition showed a 29.9 ± 15.3 % decrease in methane production, reaching an "inhibited steady state", but phosphate release into the liquid phase increased with a factor 4.5, compared to the control digester. This inhibited steady state coincided with a clear shift from a Methanosaetaceae to a Methanosarcinaceae dominated methanogenic community. Overall, the sulphate reduction process allows phosphate release during the anaerobic digestion process, yet, at the cost of a reduced methane production rate.

Molybdate delays sulphide formation in the sediment and transfer to the bulk liquid in a model shrimp pond

“…The H 2 S and total sulphide productions did show clear differences in the sediment itself, with higher concentrations of sulphide in the control treatment, indicating the effectiveness of molybdate to inhibit sulphate reduction. Residual sulphate in the bulk liquid remained present in all treatments, so despite the high availability of organic matter, sulphate reduction did not continue, as also observed in other studies on shrimp pond sediments (Torun et al, 2022) and other anaerobic ecosystems, such as anaerobic digestion (Lippens and De Vrieze, 2019). This apparent discrepancy was probably due to oxygen intrusion into the water column, halting sulphate reduction in the bulk liquid.…”

Speciation Evolution of Phosphorus and Sulfur Derived from Sewage Sludge Biochar in Soil: Ageing Effects

“…In addition, they can affect the solubility and bioavailability of each other in the environment. For example, sulfide can facilitate the release of phosphate bound to iron from SS, , whereas phosphate can increase available S through competition for sorption sites in SSB-amended soil . It is therefore expected that clarifying the speciation transformation of P and S in SSB induced by soil application will be the key to exploring the immobilization mechanisms of heavy metals and the resource recycling of the nutrients in the environment. ,, To date, fundamental knowledge on the speciation transformation mechanisms of P and S from SSB in amended soils at the molecular level is still missing , and thus critically needed.…”

Coevolution of Iron, Phosphorus, and Sulfur Speciation during Anaerobic Digestion with Hydrothermal Pretreatment of Sewage Sludge

“…During AD, sulfate can be reduced to sulfides, , and hydrogen sulfide is generated from sludges with high S content, affecting biogas production . Sulfate reduction also facilitates phosphate release to the aqueous phase during AD …”