Enhancing soluble phosphate concentration in sludge liquor by pressurised anaerobic digestion

Enhanced nutrient recovery from anaerobically digested poultry wastewater through struvite precipitation by organic acid pre-treatment and seeding in a bubble column electrolytic reactor

Limited mineralization of organic phosphorus to phosphate during the anaerobic digestion process poses a significant challenge in the development of cost-effective nutrient recovery strategies from anaerobically digested poultry wastewater (ADPW). This study investigated the influence of organic acids on phosphorus solubilization from ADPW, followed by its recycling in the form of struvite using a bubble column electrolytic reactor (BCER) without adding chemicals. The impact of seeding on the efficiency of PO43- and NH3-N recovery as well as the size distribution of recovered precipitates from the acid pre-treated ADPW was also evaluated. Pre-treatment of the ADPW with oxalic acid achieved complete solubilization of phosphorus, reaching ∼100% extraction efficiency at pH 2.5. The maximum removal efficiency of phosphate and ammonia-nitrogen from the ADPW were 88.9% and 90.1%, respectively, while the addition of 5 and 10 g/L struvite seed to the BCER increased PO43- removal efficiency by 9.6% and 11.5%, respectively. The value of the kinetic rate constant, k, increased from 0.0176 min-1 (unseeded) to 0.0198 min-1, 0.0307 min-1, and 0.0375 min-1 with the seed loading rate of 2, 5, and 10 g/L, respectively. Concurrently, the average particle size rose from 75.3 μm (unseeded) to 82.1 μm, 125.7 μm, and 148.9 μm, respectively. Results from XRD, FTIR, EDS, and dissolved chemical analysis revealed that the solid product obtained from the recovery process was a multi-nutrient fertilizer consisting of 94.7% struvite with negligible levels of heavy metals.

Integration of two-stage anaerobic digestion process with in situ biogas upgrading

High impurity concentration of biogas limits its wide commercial utilization. Therefore, the integration of two-stage anaerobic digestion process with in situ biogas upgrading technologies is reviewed, with emphasis on their principles, main influencing factors, research success, and technical challenges. The crucial factors that influence these technologies are pH, alkalinity, and hydrogenotrophic methanogenesis. Hence, pH fluctuation and low gas-liquid mass transfer of H₂ are some major technical challenges limiting the full-scale application of in situ upgrading techniques. Two-stage anaerobic digestion integration with various in situ upgrading techniques to form a hybrid system is proposed to overcome the constraints and systematically guide future research design and advance the development and commercialization of these techniques. This review intends to provide the current state of in situ biogas upgrading technologies and identify knowledge gaps that warrant further investigation to advance their development and practical implementation.

Hydraulic retention time and pressure affect anaerobic digestion process treating synthetic glucose wastewater

High-pressure anaerobic digestion (HPAD) can directly upgrade biogas (CH₄ content to 90 %) within a reactor. Understanding of how HPAD-related microbiomes are constructed by operational parameters (hydraulic retention time (HRT) and pressure) and their interactions within the biochemical process remain underexplored. In this study, an HPAD reactor was operated at five different HRT (from 40 to 13 d), with pressure around 10-13 bar. In HPAD, pressure was the driving force behind CH₄ content. Low HRTs (13-20 d) for HPAD led to volatile fatty acids accumulation, which occurred earlier than that in normal-pressure digestion. HRT mainly affected the archaeal community, whereas pressure mostly affected the bacterial community. Hydrogenotrophic methanogen Methanobacterium prevailed at low HRTs (13-20 d). When operating continuous HPAD, attention should be paid to HRT optimization, as low HRTs (e.g., 13 d) impaired the activity of CH₄-synthesizing enzyme Methyl-coenzyme M reductase.

Enhancing phosphorus recovery from efficient acidogenic fermentation of waste activated sludge with acidic cation exchange resin pretreatment: Insights from occurrence states and transformation

Achieving phosphorus (P) recovery during treatment and disposal of waste activated sludge (WAS) by anaerobic-based processes has received increasing attention. To solve the problem of low phosphorus release efficiency, anaerobic fermentation (AF) combined with acidic cation exchange resin (ACER) pretreatment was first proposed in this study. Results showed that the isoelectric point pretreatment with ACER increased the recoverable phosphorus content by 2.3 times compared to that without ACER pretreatment. Phosphorus transformation was systematically analyzed from a whole-process perspective, and the results visually revealed that the release of phosphorus during the conventional AF process (without ACER pretreatment) was limited by insufficient phosphorus release from extracellular polymeric substances (EPS) and mineral precipitation, as well as the reprecipitation of soluble phosphorus with metals. ACER enabled effective dissolution of mineral phosphorus by acidifying WAS. On the other hand, ACER adsorbed metals to promote EPS disintegration and hydrolysis, thereby enhancing the release of EPS-bound P, which also reduced the reprecipitation of soluble phosphorus during AF. Furthermore, ACER pretreatment increased volatile fatty acids production by >2-fold with enhanced sludge hydrolysis. This finding has important implications for both non-renewable phosphorus recovery and sludge resource recovery.

Salinity Influences Endophytic Bacterial Communities in Rice Roots from the Indian Sundarban Area

In "Sundarbans", the coastal regions of the West Bengal, soil salinity has always been one of the major causes of reduction in yield in these regions. The use of endophytic is a well-demanded strategy to mitigate the problems of salt stress and rice productivity. The present study attempted to analyze rice root endogenous microbial diversity and their relationship with soil salinity and physicochemical factors in the salt stressed region of Sundarbans, India using amplicon metagenomics approaches. Our investigation indicates, that the unique microbiome slightly acidic nutrient enriched non-saline zone is characterized by microbial genera that reported either having plant growth promotion (Flavobacterium, Novosphingobium, and Kocuria) or biocontrol abilities (Leptotrichia), whereas high ionic alkaline saline stressed zone dominated with either salt-tolerant microbes or less characterized endophytes (Arcobacter and Vogesella). The number of genera represented by significantly abundant OTUs was higher in the non-saline zone compared to that of the saline stressed zone probably due to higher nutrient concentrations and the absence of abiotic stress factors including salinity. Physicochemical parameters like nitrogen, phosphorus, and potassium were found significantly positively correlated with Muribaculaceae highly enriched in the non-saline zone. However, relative dissolved oxygen was found significantly negatively correlated with Rikenellaceae and Desulfovibrionaceae, enriched in the non-saline soil. This study first provides the detailed characterizations of rice root endophytic bacterial communities as well as their diversity contributed by measured environmental parameters in salinity Sundarbans areas. Since this study deals with two gradients of salinity, connecting the microbial diversity with the salinity range could be targeted for the use as "bioindicator" taxa and bio-fertilizer formulation in salt-affected regions.

Performance Evaluation of Pressurized Anaerobic Digestion (PDA) of Raw Compost Leachate

Anaerobic digestion (AD) represents an advantageous solution for the treatment and valorization of organic waste and wastewater. To be suitable for energy purposes, biogas generated in AD must be subjected to proper upgrading treatments aimed at the removal of carbon dioxide and other undesirable gases. Pressurized anaerobic digestion (PDA) has gained increasing interest in recent years, as it allows the generation of a high-quality biogas with a low CO2 content. However, high pressures can cause some negative impacts on the AD process, which could be accentuated by feedstock characteristics. Until now, few studies have focused on the application of PAD to the treatment of real waste. The present work investigated, for the first time, the performance of the pressurized anaerobic digestion of raw compost leachate. The study was conducted in a lab-scale pressurized CSTR reactor, working in semi-continuous mode. Operating pressures from the atmospheric value to 4 bar were tested at organic loading rate (OLR) values of 20 and 30 kgCOD/m3d. In response to the rise in operating pressure, for both OLR values tested, a decrease of CO2 content in biogas was observed, whereas the CH4 fraction increased to values around 75% at 4 bar. Despite this positive effect, the pressure growth caused a decline in COD removal from 88 to 62% in tests with OLR = 20 kgCOD/m3d. At OLR = 30 kgCOD/m3d, an overload condition was observed, which induced abatements of about 56%, regardless of the applied pressure. With both OLR values, biogas productions and specific methane yields decreased largely when the pressure was brought from atmospheric value to just 1 bar. The values went from 0.33 to 0.27 LCH4/gCODremoved at 20 kgCOD/m3d, and from 0.27 to 0.18 LCH4/gCODremoved at 30 kgCOD/m3d. Therefore, as the pressure increased, although there was an enhanced biogas quality, the overall amount of methane was lowered. The pressured conditions did not cause substantial modification in the characteristics of digestates.

Semi-continuous anaerobic digestion of secondary sludge with free ammonia pretreatment: Focusing on volatile solids destruction, dewaterability, pathogen removal and its implications

Our previous work has reported the pretreatment of secondary sludge with free ammonia (NH₃, FA) enhanced the methane production in batch biochemical methane potential tests. However, the batch biochemical methane potential test could only provide conservative results compared to continuous/semi-continuous anaerobic digestion. Also, the impacts of FA pretreatment on the key anaerobic digestion parameters, including volatile solids (VS) destruction, sludge dewaterability and pathogen removal, are still unknown. This study for the first time investigated these impacts using semi-continuous anaerobic digestion systems for 130 days. Pretreatment of secondary sludge for 24 h at an FA concentration of 560 mg NH₃-N/L improved VS destruction by 26.4% (from 22.0 to 27.8%), supported by a similar increase of 28.6% in methane production (from 126.7 to 162.9 ml CH4/g VS_fed). Model based analysis revealed that FA pretreatment improved the sludge degradability extent, which may be the reason for the enhanced VS destruction. Equally importantly, the dewaterability of the digested sludge with FA pretreatment was also enhanced by 9.2% (from 12.0 to 13.1% in solids content of the dewatered digested sludge), which could be partly attributed to the increased zeta potential from -16.7 to -14.5 mV. Anaerobic digestion with FA pretreatment enhanced the removals of Fecal Coliform and E. Coli by 1.3 and 1.4 log MPN/g TS (MPN: Most Probable Number; TS: Total Solids), indicating FA pretreatment was effective in enhancing pathogen removal. With inorganic solids representing 21% of the sludge used, the volume of dewatered sludge to be disposed of was reduced by 14.5% via FA pretreatment. This will substantially decrease the cost as evaluated by economic analysis. In brief, this study provides a promising strategy to enhance sludge reduction in anaerobic digestion and is of great significance in promoting the application of FA pretreatment strategy in the real world.

Comparison of the microbial communities in anaerobic digesters treating high alkalinity synthetic wastewater at atmospheric and high-pressure (11 bar)

High-pressure anaerobic digestion is an appealing concept since it can upgrade biogas directly within the reactor. However, the decline of pH caused by the dissolution of CO₂ is the main barrier that prevents a good operating high-pressure anaerobic digestion process. Therefore, in this study, a high-pressure anaerobic digestion was studied to treat high alkalinity synthetic wastewater, which could not be treated in a normal-pressure anaerobic digester. In the high-pressure reactor, the pH value was 7.5 ~ 7.8, and the CH₄ content reached 88% at 11 bar. Unlike its normal-pressure counterpart (2285 mg/L acetic acid), the high-pressure reactor ran steadily (without volatile fatty acids inhibition). Furthermore, the microbial community changed in the high-pressure reactor. Specifically, key microbial guilds (Syntrophus (11.2%), Methanosaeta concilii (50.9%), and Methanobrevibacter (26.8%)) were dominant in the high-pressure reactor at 11 bar, indicating their fundamental roles under high-pressure treating high alkalinity synthetic wastewater.

First attempt for in situ capping with lanthanum modified bentonite (LMB) on the immobilization and transformation of organic phosphorus at the sediment-water interface

LMB is a widely utilized material for the management of sediment-derived phosphorus (P) in eutrophic lakes. However, the properties of organic P at the sediment-water interface and the effect of LMB on organic P fractions in sediments are still unclear. The batch studies reported here indicate that LMB has good adsorption properties toward organic P (Glucose-6-phosphate), and the tentative adsorption mechanism relies on chemisorption. Laboratory microcosmic experiments were conducted to study the immobilization effect of LMB on the organic P in water and sediments. The results indicated that the concentration of total P, labile P and organic P in overlying water and pore water could be effectively reduced by LMB capping of sediments. After treatment, the optimal immobilization effects appeared on the 7th day and until the 60th day. However, the fractions of organic P change during the capping time. Active organic P eluted with NaHCO₃ transforms into moderately labile or non-labile P through the physical and chemical processes, as well as microbial action. Microbial community analysis showed that the addition of LMB had inhibitory effect on the phosphorus-solubilizing bacteria, which also affected the transformation between various forms of organic P. This study provided new insights of LMB in situ capping of organic P and the mechanisms of the migration and cycling of internal organic P, which is beneficial for the management of eutrophic lakes.

The transformation of phosphorus fractions in high-solid sludge by anaerobic digestion combined with the high temperature thermal hydrolysis process

Transforming inactive phosphorus (P) to active P to recover it from waste activated sludge is important. The transformation of P fractions from high-solid sludge by the anaerobic digestion (AD) and acidification phase of AD (AAD) combined with a high temperature thermal hydrolysis process (HTTHP) was investigated. The results showed that the sequence of P release effects by three processes was HTTHP + AAD > AD + HTTHP > HTTHP + AD. The PO₄^3--P release from high-solid sludge was directly affected by the temperature of HTTHP. At 140 °C, each process had more PO₄^3--P release than that at 160 °C. The total amount of PO₄^3--P release in AD + HTTHP was approximately 6 times that of HTTHP + AD. Based on the experimental results, a new process of mesophilic AD - post HTTHP was recommended, in which, enhancement of P release by sulfide ions was also proposed.

Biogas and phosphorus recovery from waste activated sludge with protocatechuic acid enhanced Fenton pretreatment, anaerobic digestion and microbial electrolysis cell

Biogas and phosphorus recovery from waste activated sludge (WAS) with sequential homogeneous protocatechuic acid (PCA) enhanced Fenton pretreatment, anaerobic digestion (AD) and microbial electrolysis cell (MEC) were investigated. The cumulation of biogas production of WAS-Fenton-AD was 330.4 mL/g VS, which was 2.05-fold of the control without pretreatment (WAS-AD) during anaerobic digestion. Biogas production of 178 mL/L/d from WAS-Fenton-AD-MEC was achieved, which was 5.23-fold of the WAS-MEC, 2.28-fold of WAS-Fenton-MEC and 1.46-fold of WAS-AD-MEC, respectively. Enhanced phosphorus recovery in form of struvite reached 1.72 g/g TS (18.03% of total P) with a purity of 74.4%. Microbial community richness and diversity analysis revealed that the pretreatment process under circumneutral condition improved the diversity of microbial community, which was consisted of Bacteroidetes (33.90%), Proteobacteria (33.14%), and Chloroflexi (10.14%), compared to a majority of Firmicutes (70.81%) in WAS-AD. This study provides a feasible strategy for the recovery of biogas combined with phosphorus from WAS.

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

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 to 58.7 ± 12.9% of total P, a factor 4.5 higher than 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.

Performance of a forward osmotic membrane bioreactor for anaerobic digestion of waste sludge with increasing solid concentration

A forward osmotic membrane bioreactor for sludge anaerobic digestion (ad-OMBR) could realize high-solid digestion via drawing moisture out by forward osmosis (FO). Methane production and microbial community evolution were monitored in an ad-OMBR as the total solids (TS) content was gradually increased. With magnesium chloride (MgCl₂) and cellulose triacetate (CTA) membrane as a draw solution and FO membrane, respectively, the ad-OMBR exhibited better performance than the conventional digester, with higher solid content, organic degradation and methane content in biogas. The conductivity of the ad-OMBR did not increase, potentially because of the formation of struvite crystals aided by the reverse-fluxed Mg²⁺ ions. Microbial diversity increased along with the increase in solid content based on the Shannon index, while the most operational taxonomic units were obtained in the 8% TS sludge Although phylum Firmicutes decreased when the TS content was raised to 11%, the relative abundance of Proteobacteria, Chloroflexi, Actinobacteria, and Bacteroidetes, which could also degrade organic matter, increased with increasing TS in ad-OMBR. FO membrane fouling in ad-OMBR was highly reversible.