Effects of initial volatile fatty acid concentrations on process characteristics, microbial communities, and metabolic pathways on solid-state anaerobic digestion.

Solid-state anaerobic digestion (SSAD) is vulnerable to excess volatile fatty acids (VFA), mainly acetate and propionate. The co-effects of VFAs and microbial dynamics under VFA accumulation were investigated in SSAD of pig manure and corn straw. Adding 2 and 4 mg/g acetate or propionate caused initial increases in total VFAs, followed by decreases after day 6, resulting in 'mild' VFA accumulation, while adding 6 mg/g caused similarly increased VFAs, but with no subsequent decrease, causing 'severe' VFA accumulation and poor methanation performance. Mild propionate accumulation promoted acetate consumption, whereas acetate accumulation inhibited propionate degradation by affecting crucial redox reactions. Under severe VFA accumulation, hydrolysis and acidification mainly conducted by acid-tolerant Clostridium sp. exacerbated VFA inhibition, causing a competition between Methanosarcina and Methanosaeta, and impairments of acetoclastic and hydrogenotrophic methanogenesis and interspecies formate transfer. This study provides new insights into mechanisms of VFA accumulation in SSAD, and its effects on methanogenesis.

Effectiveness of layered inoculation in solid-state anaerobic co-digestion of pig manure and corn straw: Focus on macro-, micro-, and genetic-levels.

Layered inoculation can achieve rapid start-up and promote methanation performance of anaerobic digesters. Daily specific methane yield (SMY) rapidly increased to 2.93 mL/g VS/d during 0-13 days, and cumulative SMY reached 212 mL/g VS in the solid-state anaerobic co-digestion (SS-AcoD) of pig manure and corn straw. Data were collected at macro-, micro-, and genetic-levels of each substrate layer. The results showed that layered inoculation could improve volatile fatty acids utilization and prevent adverse effects of high total ammonium nitrogen concentrations. Layered inoculation accelerated hydrolysis, acidification, and methanogenesis of substrates, as evidenced by the efficient inoculation of Bacteroidetes, Anaerolineales, Methanosphaerula, and Methanothrix, which were primarily from inocula. The various stages of SS-AcoD were synergistically initiated during the first 13 days, and acetoclastic pathway was boosted. These results further explain why layered inoculation is an efficient method for improving methanation performance of SS-AcoD and achieving efficient utilization of organic solid waste.

Methanogenic community during the anaerobic digestion of different substrates and organic loading rates.

Three anaerobic reactors using pig manure (PM), maize straw (MS), and a mixture of the two as substrates were compared for archaeal community structure and diversity, and for methanogens response to increased organic loading rate (OLR, expressed in the mass of volatile solid (VS)). Methanogenic archaeal richness during codigestion of pig manure with maize straw (ACE: 2412) was greater than that during the others (ACE: 1225, 1467) at an OLR of 4 g L-1  day-1 , accompanied by high specific methane yield. Euryarchaeota and Crenarchaeota predominated during overall digestion of different substrates; with relative abundances of 63.5%-99.0% and 1.0%-36.3%, respectively. Methanosarcina was the predominant genus that accounted for 33.7%-79.8% of the archaeal community. The diversity in the PM digester decreased with increase in OLR, but increased in the MS digester. The diversity was stable during the codigestion with increased OLR. The relative abundances of hydrogenotrophic methanogens increased by 2.6 and 2.1 folds; the methanogenic community shifted from acetoclastic to hydrogenotrophic methanogens during digestion of MS, and of the mixture of MS and PM. Canonical correspondence analysis revealed a strong relationship between reactor parameters and methanogenic community.

[Evaluation of the Combined Removal of Heavy Metals by Saponin and Citric Acid from Municipal Sewage Sludges and Metal Stability Features].

Sewage sludge contains a high level of nutrients, such as nitrogen, phosphorus, potassium and organic matter, with a high recovery value. The presence of heavy metals in sludges has become a major limiting factor in the utilization of these sludges. Therefore, the removal of heavy metals before land application of sewage sludge has become very necessary. Two municipal sewage sludges (S1 and S2) were collected from municipal wastewater plants in Wuhan city and were studied by oscillation leaching experiments with combination of saponin and citric acid to evaluate their removal efficiencies for Cu, Pb and Zn at different volume ratios (20:1-1:20), ratios of solid to liquid (1:20-1:80), reaction times (0-2880 min), and leaching times (1-4). The heavy metal fractions in the sewage sludges were analyzed before and after leaching to study changes in the stability and mobility of heavy metals by calculating a stability factor[relative bonding strength of heavy metals (IR)] and a mobility factor (MF). The results show that the highest removal efficiency of Cu, Pb, and Zn was 43.16% (S1), 32.45% (S2) and 38.69% (S1), respectively under the removal conditions of a volume ratio (saponin:citric acid) of 5:1, solid-liquid ratio of 1:60, and leaching time of 1440 min. The removal efficiencies of Cu and Pb were significantly enhanced and Zn had a small increase with longer leaching times, which was significantly different after two to three washing times. The highest removal rate of Cu, Pb, and Zn was 78.89% (S1), 77.08% (S2) and 49.39% (S1) after four washing times, respectively. Beyond acid soluble and reducible fractions, other forms of heavy metals have very low removal rates after a single leaching. The removal rates of heavy metal fractions increased by increasing the leaching time, particularly for Pb, which in the residual fraction was significantly increased. The stability and mobility of heavy metals changes after each leaching. For example, the IR of Cu, Pb, and Zn increased to 43.63% (S1), 39.44% (S2) and 32.00% (S1), respectively, and the MF of these heavy metals decreased from 30.19% to 79.45% in the sewage sludges after four washing events.

Influence of NaOH and thermal pretreatment on dewatered activated sludge solubilisation and subsequent anaerobic digestion: Focused on high-solid state.

In this study, the influence of NaOH and thermal pretreatment of dewatered activated sludge (DAS) on the high-solid solubilisation and anaerobic digestion was separately investigated by monitoring common parameters. The results indicated that COD, proteins and carbohydrates were efficiently solubilised in both NaOH and thermal pretreated DAS samples. For NaOH pretreatment, the concentrations of volatile fatty acids (VFAs) and total ammonium nitrogen (TAN) firstly increased followed by decreasing with NaOH dose increasing. However, they decreased with the severity of thermal pretreatment. During the batch digestion experiments (at 37°C), for 80mg NaOHg(-1) total solid (TS) DAS pretreatment it resulted in a 6.99% decrease in cumulative methane yield (CMY) compared to untreated DAS. While for 80, 100, 120°C and 20mg NaOH pretreatment, CMY increased by 15%, 42%, 71% and 35%, respectively, in comparison to untreated DAS.