Alkaline fermentation of waste activated sludge with calcium hydroxide to improve short-chain fatty acids production and extraction efficiency via layered double hydroxides

Enhanced hydrolysis of waste activated sludge and recovery of volatile fatty acids: Performance and mechanistic analysis of synergistic treatment with sodium citrate and calcium oxide

Anaerobic fermentation has emerged as a promising method of transforming waste activated sludge into high-value products (e.g., volatile fatty acids (VFAs)). This work developed sodium citrate (SC)-calcium oxide (CaO) pretreatment to accelerate the production of VFAs by enhancing sludge solubilization and disintegration of extracellular polymeric substances. The results showed that co-pretreatment with 0.25 g/g TSS of SC and 0.05 g/g TSS of CaO effectively boosted VFAs accumulation (5823.3 mg COD/L), which was 12.2 times higher than the Control group. SC-CaO pretreatment enhanced hydrolysis and acidogenesis by providing ample organic substrates, thereby promoting the growth of hydrolytic and acidogenic bacteria. Additionally, the fermentation broth resulting from co-pretreatment exhibited lower phosphorus concentration and higher biodegradability. Economic analysis confirmed that the combined pretreatment is cost-effective. This work provides a viable strategy for enhancing high-value product recovery from sludge.

Metagenomic approach reveals the mechanism of calcium oxide improving kitchen waste dry anaerobic digestion

In light of the characteristics of excessive acidification and low biogas yield during kitchen waste (KW) dry digestion, the impact of the calcium oxide (CaO) on KW mesophilic dry digestion was investigated, and the enhanced mechanism was revealed through metagenomic approach. The results showed that CaO increased the biogas production, when the CaO dosage was 0.07 g/g (based on total solid), the biogas production reached 656.84 mL/g suspended solids (VS), approximately 8.38 times of that in the control. CaO promoted the leaching and hydrolysis of key organic matter in KW. CaO effectively promoted the conversion of volatile fatty acid (VFA) and mitigated over-acidification. Macrogenome analysis revealed that CaO increased the microbial diversity in KW dry digestion and upregulated the abundance of genes related to amino acid and carbohydrates metabolism. This study provides an effective strategy with potential economic benefits to improve the bioconversion efficiency of organic matter in KW.

Application and improvement methods of sludge alkaline fermentation liquid as a carbon source for biological nutrient removal: A review

Alkaline fermentation can reduce the amount of waste activated sludge and prepare sludge alkaline fermentation liquid (SAFL) rich in short-chain fatty acids (SCFAs), which can be used as a high-quality carbon source for the biological nutrient removal (BNR) process. This review compiles the production method of SAFL and the progress of its application as a BNR carbon source. Compared with traditional carbon sources, SAFL has the advantages of higher efficiency and economy, and different operating conditions can influence the yield and structure of SCFAs in SAFL. SAFL can significantly improve the nutrient removal efficiency of the BNR process. Taking SAFL as the internal carbon source of BNR can simultaneously solve the problem of carbon source shortage and sludge treatment difficulties in wastewater treatment plants, and further reduce the operating cost. However, the alkaline fermentation process results in many refractory organics, ammonia and phosphate in SAFL, which reduces the availability of SAFL as a carbon source. Purifying SCFAs by removing nitrogen and phosphorus, directly extracting SCFAs, or increasing the amount of SCFAs in SAFL by co-fermentation or combining with other pretreatment methods, etc., are effective measures to improve the availability of SAFL.

Layered double hydroxides for phosphorus recovery from lipid-rich waste anaerobic fermentation liquor

This study aimed to extract orthophosphate (ortho-P) from lipid-rich waste AF liquor (AFL) by Mg/Al layered double hydroxides (Mg/Al LDHs) adsorption, evaluate the influence of carbonate and investigate adsorption mechanisms. The carbonate influence experiment using synthetic P-rich wastewater indicated that low carbonate level was favorable for P extraction by LDHs. And then, real AFL rich in volatile fatty acids (VFAs), carbonate and ortho-P was applied as adsorbate to explore the Mg/Al LDHs adsorption performance. Experimental results indicated that 4 g/L Mg/Al LDHs could extract 88.3% of ortho-P from the AFL with low carbonate level (4829.83 mg CaCO₃/L), and the adsorption quantity was 62.99 mg P/g LDHs, however, negligible VFAs were extracted. Kinetics and mechanisms analysis indicated that adsorption of P onto Mg/Al LDHs was a rapid physiochemical process, including ion exchange and surface adsorption. Finally, the nutrients release test confirmed the slow-release property of intercalated P.

Simultaneous recovery of short-chain fatty acids and phosphorus during lipid-rich anaerobic fermentation with sodium hydroxide conditioning

Anaerobic fermentation (AF) could achieve simultaneous recovery of short-chain fatty acids (SCFAs) and phosphorus (P) when waste activated sludge (WAS) and meat processing waste (MPW) act as co-substrate. However, long-chain fatty acids, the degradation intermediates of lipids, always inhibit anaerobic microbial activity. Therefore, sodium hydroxide (NaOH) conditioning was applied to improve the lipid-rich AF performance in this study. The results demonstrated that 96% WAS (v/v) with NaOH addition that remaining at pH 7.5 could achieve the maximum SCFAs yield (1180.05 mg/g VS_fed) at 12 d, and ortho-P content in the AF liquor (AFL) was much more than that of without NaOH addition. Anaerovibrio and Aminobacterium, one kind of lipolytic and proteolytic bacteria, respectively, became the major genus in the lipid-rich AF system. 86% of P in the AFL from 96% WAS + pH 7.5 reactor was recovered through vivianite crystallization method, with 91% of SCFAs remaining in the post-AFL. Meanwhile, analysis results verified vivianite formation in the P precipitate products. Overall, this study provided a new idea to achieve SCFAs and P simultaneous recovery from WAS and MPW through AF with NaOH conditioning and vivianite crystallization.

Dissolution of Portlandite in Pure Water: Part 2 Atomistic Kinetic Monte Carlo (KMC) Approach

Portlandite, as a most soluble cement hydration reaction product, affects mechanical and durability properties of cementitious materials. In the present work, an atomistic kinetic Monte Carlo (KMC) upscaling approach is implemented in MATLAB code in order to investigate the dissolution time and morphology changes of a hexagonal platelet portlandite crystal. First, the atomistic rate constants of individual Ca dissolution events are computed by a transition state theory equation based on inputs of the computed activation energies (ΔG*) obtained through the metadynamics computational method (Part 1 of paper). Four different facets (100 or 1¯00, 010 or 01¯0, 1¯10 or 11¯0, and 001 or 001¯) are considered, resulting in a total of 16 different atomistic event scenarios. Results of the upscaled KMC simulations demonstrate that dissolution process initially takes place from edges, sides, and facets of 010 or 01¯0 of the crystal morphology. The steady-state dissolution rate for the most reactive facets (010 or 01¯0) was computed to be 1.0443 mol/(s cm²); however, 0.0032 mol/(s cm²) for 1¯10 or 11¯0, 2.672 × 10⁻⁷ mol/(s cm²) for 001 or 001¯, and 0.31 × 10⁻¹⁶ mol/(s cm²) for 100 or 1¯00 were represented in a decreasing order for less reactive facets. Obtained upscaled dissolution rates between each facet resulted in a huge (16 orders of magnitude) difference, reflecting the importance of crystallographic orientation of the exposed facets.

Biological recovery of phosphorus from waste activated sludge via alkaline fermentation and struvite biomineralization by Brevibacterium antiquum

Struvite biomineralization is a promising method for phosphorus recovery from wastewater treatment plant streams, and the growth of responsible microorganisms in mixed cultures is one of the most critical points for applying this process in pilot and full-scale. This study aimed to investigate the growth and bio-struvite production of Brevibacterium antiquum in mixed sludge culture. Alkaline fermentation was applied at different pH conditions to enhance the phosphorus content of sludge for an efficient recovery, and pH 8 was determined as the most feasible considering the phosphorus release and sludge characteristics. Growth optimization studies showed that NaCl's presence decreases the growth rate of Brevibacterium antiquum and bio-struvite production. At the same time, pH in the range of 6.8-8.2 did not alter the growth significantly. In addition, studies showed the ability of Brevibacterium antiquum in unsterilized fermented sludge centrate to grow and recover the phosphorus as struvite. Thus, our results indicated the potential of struvite biomineralization in full-scale wastewater treatment plants.

Enhancing phosphorus recovery from sewage sludge using anaerobic-based processes: Current status and perspectives

Anaerobic-based processes are green and sustainable technologies for phosphorus (P) recovery from sewage sludges economically and are promising in practical application. However, the P release efficiency is always not satisfied. In this paper, the P release mechanisms (regarding to different P species) from sewage sludge using anaerobic-based processes are systematically summarized. The obstacles of P release and the updated achievements of enhancing P release from sewage sludges are analyzed and discussed. It can be concluded that different P species can release from sewage sludge via different anaerobic-based processes. Extracellular polymeric substances and excessive metal ions are the two main limiting factors to P release. Acid fermentation and anaerobic fermentation with sulfate reduction could be two promising ways, with P release efficiencies of up to 64% and 63%. Based on the summarization and discussion, perspectives on practical application of P recovery from sewage sludge using anaerobic-based processes are proposed.

Regulation of anaerobic fermentation for producing short-chain fatty acids from primary sludge in WWTPs by different alkalis

Carbon source production from primary sludge in wastewater treatment plants (WWTPs) via anaerobic fermentation process has been paid more attention. However, slow hydrolysis rate and low yield of short-chain fatty acids (SCFAs) limited its application. This study aimed at improving the anaerobic fermentation efficiency of primary sludge by alkali regulation (NaOH, Na₂CO₃ and Ca(OH)₂), and revealing the mechanism. Results showed that three kinds of alkalis allowed enhancing hydrolysis and acidification, and reducing methane production in the anaerobic fermentation process of primary sludge. The Na₂CO₃ regulation contributed to highest yield and productivity of SCFAs, reaching 1626 mg COD/L and 0.189 g COD/g VSS at 4th day, respectively. Microbial community structure analysis indicated that the relative abundance of fermentative microbial community was improved in the alkali regulation system, where methanogenic archaea was effectively inhibited. The continuous flow experiment further verified that the Na₂CO₃ regulation could steadily increase yield of SCFAs in the anaerobic fermentation process of primary sludge, and the yield was also the highest among three kinds of alkali regulation.

Calcium hypochlorite enhances the digestibility of and the phosphorus recovery from waste activated sludge

Waste activated sludge (WAS) can be treated using anaerobic digestion (AD) for biogas recovery and volume reduction. However, the poor digestibility and hydrolysis of WAS limit AD applications. The current study investigated the feasibility of applying calcium hypochlorite as a WAS pretreatment strategy to improve AD treatment efficiency using laboratory reactors. The results showed that pretreatment with 5 - 20% Ca(ClO)₂ (total suspended solids basis) significantly enhanced WAS anaerobic digestibility, and led to significantly enhanced methane production rate and biomethane yield comparing to the AD of raw WAS (P < 0.05). Low Ca(ClO)₂ pretreatment (5 - 10%) significantly enhanced digestion efficiency, which can be attributed to the development of fermentative and syntrophic bacteria. However, high Ca(ClO)₂ doses (>20%) reduced microbial activities, leading to slow release of dissolved organic compounds and prolonged methane production lag phase. In addition, high Ca(ClO)₂ removed 82.7% of the initial phosphate by calcium-phosphate binding, reducing the phosphorus in liquid digestate.

Preparation of composite sludge carbon-based materials by LDHs conditioning and carbonization and its application in the simultaneous removal of dissolved organic matter and phosphate in sewage

In this work, a novel carbon-based hydrotalcite-like compounds materials (LDO-SBCs) were prepared by coupling layered double hydroxides (LDHs) conditioning and pyrolytic carbonization, and characterized by X-ray diffraction (XRD), Thermogravimetric Analyzer (TGA), X-ray photoelectron spectroscopy (XPS) and Brunner-Emmet-Teller (BET) measurements. The synthesized LDO-SBCs composites were used in wastewater treatment for simultaneous removal of phosphate and dissolved organic matter (DOM). The adsorption of DOM and phosphate were well conformed to pseudo-second-order mode. Adsorption equilibrium was better fitted by Langmuir model for phosphate, while Freundlich model for DOM. Compared with the raw sludge carbon, the removal efficiency of DOM and phosphate by LDO-SBCs were increased by 8% and 13%, respectively. Based on the fluorescence spectrum and parallel factor analysis (PARAFAC), LDO-SBCs performed well in promoting the removal of protein substances (TPN and APN). Pore filling, hydrogen bonding, electrostatic adsorption and surface complexation might be dominant in the adsorption of DOM, while, surface complexation and ion exchange between the LDO layers were mainly responsible for the adsorption of phosphate. The difference of adsorption capacity of LDO-SBCs was related to the superior channel structure of composite materials and the composition of interlayer anions of LDO.

Removal and Extraction of Carboxylic Acids and Non-ionic Compounds with Simple Hydroxides and Layered Double Hydroxides

Carboxylic acids are an important natural component as a final product or intermediates for syntheses. They are produced in plants, animals and also as products from biotechnological processes. This review presents the use of single hydroxide particles and layered double hydroxides as alternative adsorbents to remove carboxylic acids from liquid media. The proposal to use hydroxide particles is based on its affinity to adsorb or intercalate carboxylic acids. Besides, the change in properties of the adsorbate-sorbate product evinces that this intermediate can be used as a vehicle to transport and release carboxylic acids. Additional examples will also be presented to prove that layered hydroxides are capable of removing non-ionic compounds from wine, milk and tomato. The use of layered compounds to remove active ingredients could reduce the number of separations steps, costs and reduce or eliminate solvents, thus encouraging the design of industrial processes of separation using hydroxides particles.

Enhancing degradation and biogas production during anaerobic digestion of food waste using alkali pretreatment

Alkali pretreatment of anaerobic digestion (AD) was investigated as a strategy to degrade complex organic matter such as fats. AD of food waste (FW) with alkali pretreatment was conducted using batch assays and long-term experiments for 70 days in two reactors. The aim of this study was to compare the impact of alkali pretreatment on solubilization and biogas production and to evaluate the performance in reactors with that of the untreated FW. The alkali pretreatment enhanced the solubilization of organic matter. The best biogas yield (829 mL/g VS) and methane content (65.48%) were obtained by the pretreatment with 1% CaO with the highest P_{i, n} (66.06%) of biodegradable soluble materials. The long-term reactors with pretreatment performed more steadily with higher biogas production under organic loading rates (OLR) over 5 g VS/(L⋅d). The bacterial community structure was different under various conditions. Methanosaeta and Methanospirillum were the dominant archaea in this study, while Methanosaeta increased in R1 at OLR of 6 g VS/(L⋅d). The study concluded that alkali pretreatment with 1% CaO appeared as a potential strategy for AD of FW.

Short-chain fatty acids recovery from sewage sludge via acidogenic fermentation as a carbon source for denitrification: A review

Wastewater treatment plants face the problem of a shortage of carbon source for denitrification. Acidogenic fermentation is an effective method for recovering short-chain fatty acids (SCFAs) as a carbon source from sewage sludge. Herein, the most recent advances in SCFAs production from primary sludge and waste activated sludge are systematically summarised and discussed. New technologies and problems pertaining to the improvement in SCFAs availability in fermentation liquids, including removal of ammoniacal nitrogen and phosphate and extraction of SCFAs from fermentation liquids, are analysed and evaluated. Furthermore, studies on the use of recovered SCFAs as a carbon source for denitrification are reviewed. Based on the above summarisation and discussion, some conclusions as well as perspectives on future studies and practical applications are presented. In particular, the recovery of carbon source/bioenergy from sewage sludge must be optimised considering nutrient removal/recovery simultaneously.

In-situ ammonia stripping with alkaline fermentation of waste activated sludge to improve short-chain fatty acids production and carbon source availability

Alkaline fermentation of waste activated sludge (WAS) to produce short-chain fatty acids (SCFAs) has been proved to be promising to develop internal carbon source for denitrifying processes in municipal wastewater treatment plants. However, a large amount of ammoniacal nitrogen also releases during fermentation, resulting in inhibition of acidogenic process and reduction of carbon source availability. Alkaline fermentation of WAS combined with in-situ ammonia stripping was proposed to improve SCFAs production and carbon source availability simultaneously. The results showed that a maximal SCFAs production of 308.7 ± 4.8 mg chemical oxygen demand/g volatile suspended solid was achieved under the pH = 10 + In-situ ammonia stripping, which was 21.7% and 141.5% higher than that of the pH = 10 and the control, respectively. Most of the ammoniacal nitrogen was stripped out, and the WAS-derived SCFAs availability as carbon source increased by 103.2%. These findings indicate in-situ ammonia stripping could make alkaline fermentation of WAS more practical.

Carbon source and phosphorus recovery from iron-enhanced primary sludge via anaerobic fermentation and sulfate reduction: Performance and future application

Anaerobic fermentation and sulfate reduction (AF-SR) was firstly used for recovery of carbon sources and phosphorus from Fe-enhanced primary sludge (Fe-sludge). With FeCl₃ dosage of 30 mg Fe/L, 63.0% of the chemical oxygen demand (COD) and 97.3% of the phosphorus were concentrated from sewage into Fe-sludge. Batch anaerobic fermentation tests of Fe-sludge with and without sulfate addition (AF-SR and control) were performed. The results showed that volatile fatty acid concentrations of the control and AF-SR were 211.0 and 270.2 mg COD/g volatile suspended solids, respectively. Furthermore, 33.2% (control) and 56.2% (AF-SR) of the total phosphorus in Fe-sludge was released. The recovery performances of carbon source and phosphorus were calculated based on struvite precipitation. The available carbon source of the AF-SR system was 44.5% higher than that of the control. A novel integrated wastewater and sludge treatment process based on chemically enhanced primary sedimentation and AF-SR is proposed for future application.