Essential regulators of iron chemical speciation distributions in anaerobic digestion of pretreated food waste: Organic volatile fatty acids or inorganic acid radicals?

A review of iron use and recycling in municipal wastewater treatment plants and a novel applicable integrated process

Chemical methods are expected to play an increasingly important role in carbon-neutral municipal wastewater treatment plants. This paper briefly summarises the enhancement effects of using iron salts in wastewater and sludge treatment processes. The costs and environmental concerns associated with the widespread use of iron salts have also been highlighted. Fortunately, the iron recovery from iron-rich sludge provides an opportunity to solve these problems. Existing iron recovery methods, including direct acidification and thermal treatment, are summarised and show that acidification treatment of FeS digestate from the anaerobic digestion-sulfate reduction process can increase the iron and sulphur recycling efficiency. Therefore, a novel applicable integrated process based on iron use and recycling is proposed, and it reduces the iron salts dosage to 4.2 mg/L and sludge amount by 80%. Current experimental research and economic analysis of iron recycling show that this process has broad application prospects in resource recovery and sludge reduction.

Trace elements' deficiency in energy production through methanogenesis process: Focus on the characteristics of organic solid wastes

Excessive or insufficient supplementation of trace elements (TEs) limits the progression of anaerobic digestion. The main reason for this is the lack of sufficient understanding of digestion substrate characteristics, which significantly affects the demand for TEs. In this review, the relationship between TEs requirements and substrate characteristics is discussed. We mainly focus on three aspects. 1) The basis for TE optimization and existing problems: The optimization of TEs often based on the total solids (TS) or volatile solids (VS) of substrates, does not fully consider substrate characteristics. 2) TE deficiency mechanisms for different types of substrates: nitrogen-rich, sulfur-rich, TE-poor, and easily hydrolyzed substrates are the four main types of substrates. The mechanisms underlying TEs deficiency in the different substrates are investigated. 3) Regulation of TE bioavailability: characteristics of substrates affect digestion parameters, which disturb the bioavailability TE. Therefore, methods for regulating bioavailability of TEs are discussed.

Temporal Variation of Nitrogen and Sulfur Species of Food Waste and Sludge during Anaerobic Co-Digestion

Anaerobic co-digestion (AcoD) has been a widely accepted method to treat food waste (FW) and sewage sludge (SS). However, there is a knowledge gap regarding the key speciation transformation of nitrogen and sulfur in AcoD. Here, we explored the changes of nitrogen (N) and sulfur (S) compounds in liquid digestion and biogas, as well as the composition of microbial community structure and related metabolic functions. The results showed that H2S in the biogas was the main form of S in the early stage, and then, it was converted into SO42− and SO32−, while NH3 and NH4+ were the main forms of N during the AcoD. In addition, bacterial diversity was associated with N and S compounds; Syntrophomonas and Aminobacterium were positively correlated to H2S, NH3, NH4+ and SO32−, and Saccharibacteria_genera_incertae_sedis, Candidatus_Cloacamonas and Thermomonas were positively correlated to SO42− and NO2−. Additionally, the FAPROTAX prediction showed that the functional composition related to N and S metabolism was different from SS and inoculum after the AcoD. This study provides detailed information of conversion of N and S of the AcoD, which could lay a foundation for the subsequent regulation of the mechanism of nitrogen and sulfur compounds in the methanogenic process.

Nutrient augmentation enhances biogas production from sorghum mono-digestion

This study investigated the effects of the addition of micro- (Fe, Co, Ni, and Mo) and macro-(Sulfur) nutrients on mono-digestion of sorghum under mesophilic conditions. A continuous stirred-tank reactor was operated for more than 420 days under seven different experimental conditions. The experimental results showed poor performance for methane production and process stability without nutrient supplementation. Serious deficiencies in Co and S were confirmed by nutrient analysis of dry sorghum and digestate. Nutrient augmentation efficiently enhanced methane production and volatile fatty acid (VFA) removal. Methane production reached 223 mL-CH₄/g-VS, almost matching the yield predicted by biochemical methane potential (BMP) test. S was demonstrated to have a critical effect on metal availability in the digester. Consequently, to maintain stable methane fermentation, suitable supplementations of S and Co are recommended for anaerobic sorghum mono-digestion.