Evaluation of on-site biological treatment options for hydrothermal liquefaction aqueous phase derived from sludge in municipal wastewater treatment plants.

Aerobic treatment, mesophilic anaerobic digestion, thermophilic anaerobic digestion, and dark fermentation were evaluated for on-site biological treatment of municipal sludge derived HTL aqueous. For all four described batch test scenarios, municipal sludge-derived HTL aqueous samples obtained under 290-360 °C and 0-30 min retention time were used. In the aerobic respirometric tests, HTL aqueous samples resulted in a five-day biochemical oxygen demand range of 40.75 g/L (350 °C-25.6 min) to 54 g/L (325 °C-0 min). The calculated aerobic biodegradability index showed that approximately 50 % of the organics in HTL aqueous were easily biodegradable. Mesophilic and thermophilic biochemical methane potential tests resulted in specific yields of 151-179 mL CH4/g chemical oxygen demand (COD) and 103-122 mL CH4/g COD, respectively. HTL aqueous obtained under 360 °C-15 min condition caused total inhibition in both mesophilic and thermophilic anaerobic digestion. Possible causes for this inhibition were pyridine, pyrrolidinone, piperidinone, pyridinol, and phenolic compounds, which were higher in abundance in the 360 °C-15 min sample. HTL aqueous was found unfit for hydrogen production in dark fermentation due to inhibitory composition. In summary, on-site biological treatment of HTL aqueous was found to be most suitable under aerobic and mesophilic anaerobic conditions.

Phosphorus recovery from municipal sludge-derived hydrochar: Insights into leaching mechanisms and hydroxyapatite synthesis.

Hydrothermal liquefaction has the potential to exploit resources from municipal sewage sludge. It converts most organics into a liquid biofuel (biocrude), concentrates P in the solid residue (hydrochar), and consequently enables its efficient recovery. This study thoroughly evaluated the effects of extraction conditions on P and metal release from hydrochar by nitric acid. Among assessed factors, acid normality (0.02-1 N), liquid-to-solid ratio (5-100 mL/g), and contact time (0-24 h) had positive effects while decreasing eluate pH (0.5-4) improved leaching efficiencies of P and metals. Notably, eluate pH played a dominant role in P leaching and pH < 1.5 was crucial for complete extraction. P and metal leaching from hydrochar have strong interactions and their leaching mechanism was identified as product layer diffusion using the shrinking core model. This suggests that the leaching efficiency is susceptible to agitation and particle size but not temperature. Using 10 mL/g of 0.6 N HNO3 for 2 h was considered the best extraction condition for efficient P leaching (nearly 100%) and minimization of cost and contaminants (heavy metals). Following extraction, adding Ca(OH)2 at a Ca:P molar ratio of 1.7-2 precipitated most P (99-100%) at pH 5-6, while a higher pH (13) synthesized hydroxyapatite. The recovered precipitates had high plant availability (61-100%) of P and satisfactory concentrations of heavy metals as fertilizers in Canada and the US. Overall, this study established reproducible procedures for P recovery from hydrochar and advanced one step closer to wastewater biorefinery.

Biochar and wood ash amended anaerobic digestion of hydrothermally pretreated lignocellulosic biomass for biorefinery applications.

This study investigated the effect of biochar and wood ash amendment on the anaerobic digestion of hydrothermally pretreated lignocellulosic biomass. Hydrothermal pretreatment was performed on switchgrass at 200, 250, and 300 °C with 0, 30, and 60 min of retention times. The pretreatment method was optimized using the response surface method for enhanced methane production. At the optimum pretreatment (200 °C/0 min retention time), a specific methane yield of 256.9 mL CH4/g volatile solids (VS), corresponding to an increase of 32.8% with respect to the untreated substrate, was obtained. Hydrothermal pretreatment was beneficial for methane production at temperatures lower than 220 °C and retention times shorter than 20 min. At more severe pretreatment conditions than 220°-20 min, sugars were degraded into other products, causing a decrease in the methane yield. The hydrothermal degradation products, i.e., acetic acid, lactic acid, furfural, and hydroxymethylfurfural concentrations, were also measured and modeled. The addition of biochar and wood ash to BMP assays were tested at 2, 9, 16 g/g VSinoculum ratios and <63, 63-125, 125-250 µm particle sizes. A decline in methane production was observed for all tested doses and particle sizes of both additives. The decline in the methane potential was proportional to the doses and particle sizes. Kinetic modeling of BMP test results also supported that using the additives was not beneficial. Based on the result of this study, it was found that the use of biochar and wood ash in a pretreated lignocellulosic biomass processing biorefinery would not be beneficial.

Enhancement of lignocellulosic biomass anaerobic digestion by optimized mild alkaline hydrogen peroxide pretreatment for biorefinery applications.

Lignocellulosic energy crops are promising feedstocks for producing renewable fuels, such as methane, that can replace diminishing fossil fuels. However, there is a major handicap in using lignocellulosic sources to produce biofuels, which is their low biodegradability. In this study, the application and the optimization of a lignocellulose pretreatment process, named alkaline hydrogen peroxide, was investigated for the enhancement of methane production from the energy crop switchgrass. Four independent process variables, solid content (3-7%), reaction temperature (50-100 °C), H2O2 concentration (1-3%), and reaction time (6-24 h), and three response variables, soluble reducing sugar, soluble chemical oxygen demand, and biochemical methane potential were used in process optimization and modeling. The optimization was performed by two different approaches as maximum methane production and cost minimization. The optimum conditions for the highest methane production were found as 6.65 wt% solid content, 50.6 °C reaction temperature, 2.94 wt% H2O2 concentration, and 16.05 h reaction time. The conditions providing the lowest cost were 6.43 wt% solid content, 50 °C reaction temperature, 1.83 wt% H2O2 concentration, and 6.78 h reaction time. For maximum methane production and cost minimization, specific methane yields of 338.52 mL CH4/g VS and 291.34 mL CH4/g VS were predicted with 62.4 % and 39.8 % enhancements compared to untreated switchgrass, respectively. Finally, it was found that the predicted methane production for the maximum methane production represents 77 % of the theoretical methane yield and 82.22 % energy recovery.

Anaerobic co-digestion of oil-extracted spent coffee grounds with various wastes: Experimental and kinetic modeling studies.

The effect of oil extraction from spent coffee grounds as a pre-treatment strategy prior to anaerobic digestion besides assessing the feasibility of defatted spent coffee grounds co-digestion with spent tea waste, glycerin, and macroalgae were examined. Mesophilic BMP tests were performed using defatted spent coffee grounds alongside four co-substrates in the ratio of 25, 50, and 75%, respectively. The highest methane yield was obtained with the mono-digestion of defatted spent coffee grounds with 336 ± 7 mL CH4/g VS and the yield increased with the increase in the mass ratio of defatted spent coffee grounds during co-digestion. Moreover, defatted spent coffee grounds showed the highest VS and TS removal at 35.5% and 32.1%, respectively and decreased thereafter. Finally, a linear regression model for the interaction effects between substrates was demonstrated and showed that distinctly mixing defatted spent coffee grounds, spent coffee grounds, and spent tea waste outperforms other triple mixed substrates.

Assessment of hydrothermal pretreatment of various lignocellulosic biomass with CO2 catalyst for enhanced methane and hydrogen production.

Hydrothermal pretreatment of five lignocellulosic substrates (i.e. wheat straw, rice straw, biomass sorghum, corn stover and Douglas fir bark) were conducted in the presence of CO2 as a catalyst. To maximize disintegration and conversion into bioenergy (methane and hydrogen), pretreatment temperatures and subsequent pressures varied with a range of 26-175 °C, and 25-102 bars, respectively. Among lignin, cellulose and hemicelluloses, hydrothermal pretreatment caused the highest reduction (23-42%) in hemicelluloses while delignification was limited to only 0-12%. These reductions in structural integrity resulted in 20-30% faster hydrolysis rates during anaerobic digestion for the pretreated substrates of straws, sorghum, and corn stover while Douglas fir bark yielded 172% faster hydrolysis/digestion due to its highly refractory nature in the control. Furans and phenolic compounds formed in the pretreated hydrolyzates were below the inhibitory levels for methane and hydrogen production which had a range of 98-340 ml CH4/g volatile solids (VS) and 5-26 ml H2/g VS, respectively. Results indicated that hydrothermal pretreatment is able to accelerate the rate of biodegradation without generating high levels of inhibitory compounds while showing no discernible effect on ultimate biodegradation.

Assessing iron and aluminum-based coagulants for odour and pathogen reductions in sludge digesters and enhanced digestate dewaterability.

Anaerobic digestion (AD) is an effective way of recovering energy and nutrients from organic waste. However, several issues including the production of corrosive, highly odorous and toxic volatile sulfur compounds (VSCs) in digester biogas, and long digestion times to achieve sufficient pathogen reductions can limit its wider adoption. In this study, Kemira™ PIX-311 (ferric chloride), PAX XL-6 (aluminum chloride hydroxide sulfate), and PAX XL-19 (polyaluminum chlorohydrate) were added to the digester feeds to evaluate the effects on digester stability, organic removal, VSCs formation in digester headspace, pathogen removal and sludge dewaterability. After preliminary dose trials, two different doses of PIX-311, PAX XL-19, and a 1:1 mixture of PIX-311 and PAX XL-19 were selected. PAX XL-6 was removed from further study as dosing significantly increased VSC levels and the PAX XL-6 dosed digester exhibited signs of instability. During the total operation period of 100days, addition of PIX-311, PAX XL-19, a combination of PIX-311, PAX XL-19 at concentrations of 4000 and 4500mg/kg total solids (TS) to digester feed did not lead to process instability. Biogas yields of all metal added digesters were similar to that of the control (no metal addition) digester. PIX-311 achieved up to a 93% reduction in biogas VSCs, 82% better fecal coliform inactivation and exhibited improved dewaterability over the control digester. The PAX XL-19 dosed digester showed modest reductions in biogas VSC concentrations, pathogen levels and improved dewaterability versus the control. Metal addition can be an effective way to control odours from VSCs, pathogens and to improve dewaterability during AD.

Effect of microwave hydrolysis on transformation of steroidal hormones during anaerobic digestion of municipal sludge cake.

Fate and removal of 16 steroidal (estrogenic, androgenic and progestogenic) hormones were studied during advanced anaerobic digestion of sludge cake using microwave (MW) pretreatment. Effect of pretreatment temperature (80, 120, 160 °C), operating temperature (mesophilic at 35 ± 2 °C, thermophilic at 55 ± 2 °C) and sludge retention time (SRT: 20, 10, 5 days) were studied employing eight lab-scale semi-continuously fed digesters. To determine the potential effect of MW hydrolysis, hormones were quantified in total (sorbed + soluble) and supernatant (soluble) phases of the digester influent and effluent streams. Seven of 16 hormones were above the method reporting limit (RL) in one or more of the samples. Hormone concentrations in total phase of un-pretreated (control) and pretreated digester feeds ranged in <157-2491 ng/L and <157-749 ng/L, respectively. The three studied factors were found to be statistically significant (95% confidence level) in removal of one or more hormones from soluble and/or total phase. MW hydrolysis of the influent resulted in both release (from sludge matrix) and attenuation of hormones in the soluble phase. Accumulation of estrone (E1) as well as progesterone (Pr) and androstenedione (Ad) in most of the digesters indicated possible microbial transformations among the hormones. Compared to controls, all pretreated digesters had lower total hormone concentrations in their influent streams. At 20 days SRT, highest total removal (E1+E2+Ad +Pr) was observed for the thermophilic control digester (56%), followed by pretreated mesophilic digesters at 120 °C and 160 °C with around 48% efficiency. In terms of conventional performance parameters, relative (to control) improvements of MW pretreated digesters at a 5-d SRT ranged in 98-163% and 57-121%, for volatile solids removal and methane production, respectively.

Fate of estrogenic hormones in wastewater and sludge treatment: A review of properties and analytical detection techniques in sludge matrix.

Estrogenic hormones (estrone (E1), 17ß-estradiol (E2), estriol (E3), 17α-ethinylestradiol (EE2)) are the major contributor to the total estrogenicity in waterways. Presence of these compounds in biosolids is also causing concern in terms of their use as soil amendment. In comparison with wastewater treatment, removal of estrogenic compounds in sewage sludge has received less attention. This paper presents a literature review regarding the source and occurrence of these pollutants in our environment. The removal pathways of estrogenic compounds in engineered systems, such as full-scale wastewater treatment plants (WWTPs), are also discussed. Review of the fate studies revealed that activated sludge system with nutrient removal shows very high (>90%) removal of estrogenic hormones in most of the cases. Although, aerobic digestion showed better attenuation of estrogenic compounds, anaerobic digestion increased the overall estrogenicity of biosolids. Finally, this paper highlights the challenges involved in analytical determination of these compounds in sewage sludge matrix.