Effect of Low-Temperature Conditioning of Excess Dairy Sewage Sludge with the Use of Solidified Carbon Dioxide on the Efficiency of Methane Fermentation

Technological, Ecological, and Energy-Economic Aspects of Using Solidified Carbon Dioxide for Aerobic Granular Sludge Pre-Treatment Prior to Anaerobic Digestion

The technology of aerobic granular sludge (AGS) seems prospective in wastewater bio-treatment. The characteristics as well as compactness and structure of AGS have been proved to significantly affect the effectiveness of thus far deployed methods for sewage sludge processing, including anaerobic digestion (AD). Therefore, it is deemed necessary to extend knowledge on the possibilities of efficient AGS management and to seek viable technological solutions for methane fermentation of sludge of this type, including by means of using the pre-treatment step. Little is known about the pre-treatment method with solidified carbon dioxide (SCO₂), which can be recovered in processes of biogas upgrading and enrichment, leading to biomethane production. This study aimed to determine the impact of AGS pre-treatment with SCO₂ on the efficiency of its AD. An energy balance and a simplified economic analysis of the process were also carried out. It was found that an increasing dose of SCO₂ applied in the pre-treatment increased the concentrations of COD, N-NH₄⁺, and P-PO₄^3- in the supernatant in the range of the SCO₂/AGS volume ratios from 0.0 to 0.3. No statistically significant differences were noted above the latter value. The highest unit yields of biogas and methane production, reaching 476 ± 20 cm³/gVS and 341 ± 13 cm³/gVS, respectively, were obtained in the variant with the SCO₂/AGS ratio of 0.3. This experimental variant also produced the highest positive net energy gain, reaching 1047.85 ± 20 kWh/ton total solids (TS). The use of the higher than 0.3 SCO₂ doses was proved to significantly reduce the pH of AGS (below 6.5), thereby directly diminishing the percentage of methanogenic bacteria in the anaerobic bacterial community, which in turn contributed to a reduced CH₄ fraction in the biogas.

Biohythane Production in Hydrogen-Oriented Dark Fermentation of Aerobic Granular Sludge (AGS) Pretreated with Solidified Carbon Dioxide (SCO2)

Though deemed a prospective method, the bioconversion of organic waste to biohydrogen via dark fermentation (DF) has multiple drawbacks and limitations. Technological difficulties of hydrogen fermentation may, in part, be eliminated by making DF a viable method for biohythane production. Aerobic granular sludge (AGS) is a little-known organic waste spurring a growing interest in the municipal sector; its characteristics indicate the feasibility of its use as a substrate for biohydrogen production. The major goal of the present study was to determine the effect of AGS pretreatment with solidified carbon dioxide (SCO₂) on the yield of H₂ (biohythane) production during anaerobic digestion (AD). It was found that an increasing dose of SCO₂ caused an increase in concentrations of COD, N-NH₄⁺, and P-PO₄^3- in the supernatant at the SCO₂/AGS volume ratios from 0 to 0.3. The AGS pretreatment at SCO₂/AGS ratios within the range of 0.1-0.3 was shown to enable the production of biogas with over 8% H₂ (biohythane) content. The highest yield of biohythane production, reaching 481 ± 23 cm³/gVS, was obtained at the SCO₂/AGS ratio of 0.3. This variant produced 79.0 ± 6% CH₄ and 8.9 ± 2% H₂. The higher SCO₂ doses applied caused a significant decrease in the pH value of AGS, modifying the anaerobic bacterial community to the extent that diminished anaerobic digestion performance.

Characteristics of Solidified Carbon Dioxide and Perspectives for Its Sustainable Application in Sewage Sludge Management

Appropriate management is necessary to mitigate the environmental impacts of wastewater sludge. One lesser-known technology concerns the use of solidified CO₂ for dewatering, sanitization, and digestion improvement. Solidified CO₂ is a normal byproduct of natural gas treatment processes and can also be produced by dedicated biogas upgrading technologies. The way solidified CO₂ is sourced is fully in line with the principles of the circular economy and carbon dioxide mitigation. The aim of this review is to summarize the current state of knowledge on the production and application of solid CO₂ in the pretreatment and management of sewage sludge. Using solidified CO₂ for sludge conditioning causes effective lysis of microbial cells, which destroys activated sludge flocs, promotes biomass fragmentation, facilitates efficient dispersion of molecular associations, modifies cell morphology, and denatures macromolecules. Solidified CO₂ can be used as an attractive tool to sanitize and dewater sludge and as a pretreatment technology to improve methane digestion and fermentative hydrogen production. Furthermore, it can also be incorporated into a closed CO₂ cycle of biogas production-biogas upgrading-solidified CO₂ production-sludge disintegration-digestion-biogas production. This feature not only bolsters the technology's capacity to improve the performance and cost-effectiveness of digestion processes, but can also help reduce atmospheric CO₂ emissions, a crucial advantage in terms of environment protection. This new approach to solidified CO₂ generation and application largely counteracts previous limitations, which are mainly related to the low cost-effectiveness of the production process.

Ultrasonic Disintegration to Improve Anaerobic Digestion of Microalgae with Hard Cell Walls-Scenedesmus sp. and Pinnularia sp

Microalgae are considered to be very promising feedstocks for biomethane production. It has been shown that the structure of microalgal cell walls can be highly detrimental to the anaerobic digestibility of biomass. Therefore, there is a real need to seek ways to eliminate this problem. The aim of the present study was to assess the effect of ultrasonic disintegration of Scenedesmus sp. and Pinnularia sp. microalgal biomass on the performance and energy efficiency of anaerobic digestion. The pretreatment was successful in significantly increasing dissolved COD and TOC in the system. The highest CH₄ yields were noted for Scenedesmus sp. sonicated for 150 s and 200 s, which produced 309 ± 13 cm³/gVS and 313 ± 15 cm³/gVS, respectively. The 50 s group performed the best in terms of net energy efficiency at 1.909 ± 0.20 Wh/gVS. Considerably poorer performance was noted for Pinnularia sp., with biomass yields and net energy gains peaking at CH₄ 250 ± 21 cm³/gVS and 0.943 ± 0.22 Wh/gVS, respectively. Notably, the latter value was inferior to even the non-pretreated biomass (which generated 1.394 ± 0.19 Wh/gVS).

Effect of ferrate pretreatment on anaerobic digestibility of primary sludge spiked with resin acids

Resin acids are mixtures of high molecular weight carboxylic acids found in tree resins. Due to higher hydrophobicity and low solubility, they tend to adsorb on the suspended solids in pulp and paper (P&P) mill wastewater and accumulate in primary sludge through settling. Anaerobic digestion (AD) is a common practice stabilizing sludge; however, high concentration of resin acids affects the AD process. The aim of this research was mainly to determine the impact of ferrate (Fe (VI)) oxidation on selected resin acids and anaerobic digestibility of ferrate-treated primary sludge (PS) spiked with the resin acids. First, batch control oxidation of model resin acids with Fe (VI) was conducted to identify an optimum dosage, pH and contact time using a Box-Behnken design approach. Thereafter, anaerobic treatability studies of primary sludge spiked with resin acids both under control condition and optimum ferrate pretreatment were conducted. Up to 97% oxidation of resin acids occurred in pure water, while only 44%-62% oxidation of resin acids occurred in PS with an increasing Fe (VI) dosage from 0.034 to 0.137 mg Fe (VI)/mg tCOD_fed. The pretreatment did not affect the anaerobic biodegradability of resin acids; however, it lowered their negative influences on the PS digestibility. About 0.076 mg Fe (VI) dosage/mg tCOD_fed solubilized the sludge increasing the methane production by 40% compared to the untreated digester. The potential benefits of ferrate pretreatment of P&P primary sludge include resin acids oxidation and subsequent toxicity reduction, higher sludge solubilization enhancing methane production and enabling anaerobic digestion at higher COD loading.

Helianthus salicifolius as a New Biomass Source for Biogas Production

Renewable energy is becoming a widely discussed topic in the European Union (EU), due to a desire to reduce the negative effects of fossil fuels on climate change and biodiversity. About 60% of the total renewable energy produced in the EU is derived from biomass. Anaerobic digestion (AD) is an important pathway to convert biomass into biogas and then into bioenergy. Helianthus salicifolius is a perennial plant, whose biomass can serve as a co-substrate in biogas plants. Biomass composition, in addition to the biomethane and biogas potential, were investigated in raw green biomass and silage obtained from Helianthus salicifolius plants grown under different types (mineral and organic) and doses (0, 85, 170 kg N ha−1) of nitrogen fertilization. The biomethane production efficiency from Helianthus salicifolius was recorded for 25 days and found to range on average between 169.4 NL kg−1 VS for raw biomass and 193.2 NL kg−1 VS for silage. It follows from the current study that ensiling increases substrate digestibility and has a positive impact on methane concentration, but the biomethane and biogas production outputs from those substrates did not differ significantly at the end of the process.

Anaerobic Reactor Filling for Phosphorus Removal by Metal Dissolution Method

A commonly indicated drawback of anaerobic wastewater treatment is the low effectiveness of phosphorus removal. One possibility to eliminate this disadvantage is the implementation of active fillings that contain admixtures of metals, minerals, or other elements contributing to wastewater treatment intensification. The aim of the research was to present an active filling produced via microcellular extrusion technology, and to determine its properties and performance in anaerobic wastewater treatment. The influence of copper and iron admixtures on the properties of the obtained porous extrudate in terms of its functional properties was also examined. The Barus effect increased with the highest content of the blowing agent in the material from 110 ± 12 to 134 ± 14. The addition of metal powders caused an increase in the extrudate density. The modification of PVC resulted in the highest porosity, amounting to 47.0% ± 3.2%, and caused the tensile strength to decrease by about 50%. The determined values ranged from 211.8 ± 18.3 MPa to 97.1 ± 10.0 MPa. The use of the filling in anaerobic rectors promoted COD removal, intensified biogas production, and eliminated phosphorus with an efficiency of 64.4% to 90.7%, depending on the type of wastewater and applied technological parameters.

Wastewater Treatment and Biogas Production: Innovative Technologies, Research and Development Directions

The development of wastewater treatment methods and the processing of sewage sludge is associated with the search for new, efficient and technologically justified solutions, the use of which will be an alternative to the systems used thus far [...]

Raw Biogas as Feedstock for the OCM Process

The paper presents the research results obtained in the process of oxidative coupling of methane, in which unpurified biogas was used as the feedstock. Biogas obtained from two kinds of biomass materials, i.e., plant materials (potato and beet pulp, Corn-Cob-Mix—biogas 1) and animal waste (waste from fish filleting—biogas 2) was considered. The influence of temperature, the ratio of methane/oxygen and total flows of feedstock on the catalytic performance in oxidative coupling of methane process was investigated. Comparative tests were carried out using pure methane and a mixture of methane-carbon dioxide to simulate the composition of biogas 2. The process was carried out in the presence of an Mn-Na2WO4/SiO2 catalyst. Fresh and used catalysts were characterised by means of powder X-ray diffraction, X-ray photoelectron spectroscopy, and low-temperature nitrogen adsorption techniques. In oxidative coupling of methane, the type of raw material used as the source of methane has a small effect on methane conversion (the differences in methane conversion are below 3%), but a significant effect on the selectivity to C2. Depending on the type of raw material, the differences in selectivity to C2 reach as high as 9%. However, the Mn-Na2WO4/SiO2 catalyst operated steadily in the tested period of time at any feedstock composition. Moreover, it was found that CO2, which is the second main component of biogas in addition to methane, has an effect on catalytic performance. Comparative results of catalytic tests indicate that the CO2 effect varies with temperature. Below 1073 K, CO2 exerts a small poisoning effect on methane conversion, while above this temperature the negative effect of CO2 disappears. In the case of selectivity to C2+, the negative effect of CO2 was observed only at 1023 K. At higher temperatures, CO2 enhances selectivity to C2+. The effect of CO2 was established by correlating the catalytic results with the temperature programmed desorption of CO2 investigation. The poisoning effect of CO2 was connected with the formation of surface Na2CO3, whose concentration depends on temperature.

Progress in the Production of Biogas from Maize Silage after Acid-Heat Pretreatment

One of the most effective technologies involving the use of lignocellulosic biomass is the production of biofuels, including methane-rich biogas. In order to increase the amount of gas produced, it is necessary to optimize the fermentation process, for example, by substrate pretreatment. The present study aimed to analyze the coupled effects of microwave radiation and the following acids: phosphoric(V) acid (H3PO4), hydrochloric acid (HCl), and sulfuric(VI) acid (H2SO4), on the destruction of a lignocellulosic complex of maize silage biomass and its susceptibility to anaerobic degradation in the methane fermentation process. The study compared the effects of plant biomass (maize silage) disintegration using microwave and conventional heating; the criterion differentiating experimental variants was the dose of acid used, i.e., 10% H3PO4, 10% HCl, and 10% H2SO4 in doses of 0.02, 0.05, 0.10, 0.20, and 0.40 g/gTS. Microwave heating caused a higher biogas production in the case of all acids tested (HCl, H2SO4, H3PO4). The highest biogas volume, exceeding 1800 L/kgVS, was produced in the variant with HCl used at a dose of 0.4 g/gTS.

Microwave Radiation Influence on Dairy Waste Anaerobic Digestion in a Multi-Section Hybrid Anaerobic Reactor (M-SHAR)

Whey is a primary by-product of dairy plants, and one that is often difficult to manage. As whey processing units are costly and complicated, only 15–20% of whey is recycled for use in the food industry. The difficulties in managing waste whey are particularly pronounced for small, local dairy plants. One possible solution to this problem is to use advanced and efficient digesters. The aim of this study was to present an innovative multi-section hybrid anaerobic bioreactor (M-SHAR) design and to identify how microwave radiation heating (MRH) affects methane fermentation of liquid dairy waste (LDW) primarily composed of acid whey. The MRH reactor was found to perform better in terms of COD removal and biogas production compared with the convection-heated reactor. The heating method had a significant differentiating effect at higher organic load rates (OLRs). With OLRs ranging from 15 to 25 kgCOD∙m−3∙d−1, the M-SHAR with MRH ensured a 5% higher COD removal efficiency and 12–20% higher biogas yields.

Evaluation of Dairy Wastewater Treatment Systems Using Carbon Footprint Analysis

Modernisation of municipal and industrial wastewater treatment plants (WWTPs) should be carried out, taking into account its impact on global warming, e.g., through carbon footprint (CF) analysis. An important industrial sector in Poland is the dairy industry. In dairy WWTPs, the aerobic sewage sludge stabilisation applied thus far is being replaced by the anaerobic process. This change is positive due to the possibility of energy production, but it is unclear how it affects greenhouse gases (GHG) emissions. The aim of the research was to perform CF analysis for two scenarios of dairy WWTP operation. The analysis was based on the real operating data of the current system (current scenario) and project of its modernisation (alternative scenario). The current scenario consists of mechanical and dissolved air flotation (DAF) treatment, biological treatment in sequence batch reactors (SBRs), aerobic sewage sludge stabilisation and its final farmland usage. The alternative scenario assumes replacing aerobic stabilisation with anaerobic stabilisation and a combined heat and power (CHP) system. The CF calculations were based on empirical models, taking into account different emission input parameters, expressed in CO2 equivalents (CO2e). The total CF of the current scenario was 22 kg CO2e PE−1 year−1, while the alternative was 45 kg CO2e PE−1 year−1. The largest share in the current scenario belongs to emissions from WWTPs and energy use, while in the alternative, there is the addition of emissions from biogas use.

Structure and indicators of electric energy consumption in dairy wastewater treatment plant

The aim of the research was to determine the indicators of electricity consumption in every stage of the dairy sewage treatment process in relation to the sewage flow and the load of removed organics (BOD₅, COD) and nutrients (TN, TP). The research was conducted in a dairy wastewater treatment plant (WWTP) consisting of mechanical treatment, averaging tank, dissolved air flotation (DAF) and biological treatment with sequence batch reactors (SBRs). Energy consumption was measured with the help of transducers. Indicators of unit electricity consumption were determined on the basis of 95 measurement series of energy consumption, sewage flow and removed load. The mean value of total unit energy consumption relating to the flow for the entire WWTP was 2.29 kWh·m^-3, while for biological treatment 1.17 kWh·m^-3 and 0.05 kWh·m^-3 for DAF. The mean values of indicators relating to removed pollutants load for the entire WWTP were: 1.89 kWh·kg_rem BOD₅^-1, 1.30 kWh·kg_rem COD^-1, 48.61 kWh·kg_rem TN^-1 and 160.01 kWh·kg_rem TP^-1. During biological treatment, energy consumption indicators were on average: 1.65 kWh·kg_rem BOD₅^-1 and 1.19 kWh·kg_rem COD^-1, 52.90 kWh·kg_rem TN^-1 and 141.26 kWh·kg_rem TP^-1, while for DAF: 0.12 kWh·kg_rem BOD₅^-1, 0.09 kWh·kg_rem COD ^-1, 3.85 kWh·kg_rem TN^-1 and 16.17 kWh·kg_rem TP^-1. It was found that the biological treatment in SBRs was responsible for 54.1% of the total energy consumption of dairy WWTP. Aerobic sewage sludge treatment accounted for 17.0% of total consumption, mechanical treatment 17.1%, deodorization 2.6%, and other (social, lighting etc.) 6.9%, while DAF only 2.3%. The real-time electricity metering system enabled the optimisation of the electricity consumption in the WWTP, taking into account its consumption in unit processes and the removed pollutants load. The application of this system enabled to make corrections that reduced energy consumption while maintaining the required treatment efficiency.

The Effect of Static Magnetic Field on Methanogenesis in the Anaerobic Digestion of Municipal Sewage Sludge

The present study aimed to determine the effect of a 17.6 mT static magnetic field (SMF) on the efficiency of anaerobic digestion (AD) of municipal sewage sludge (MSS). The SMF had a significant impact on methane (CH4) production efficiency, the levels of fermentation rate (ηFMSS) vs. removal rate (ηVS), and the structure of the anaerobic bacteria consortium, but it did not affect cumulative biogas production. The highest CH4 yield (431 ± 22 dm3CH4/kgVS) and the highest methane content in the biogas (66.1% ± 1.9%) were found in the variant in which the SMF exposure time was 144 min/day. This variant also produced the highest ηFMSS and ηVS values, reaching 73.8% ± 2.3% and ηVS 36.9% ± 1.6%, respectively. Longer anaerobic sludge retention time in the SMF area significantly decreased AD efficiency and caused a significant reduction in the number of methanogens in the anaerobic bacteria community. The lowest values were observed for SMF exposure time of 432 min/day, which produced only 54.8 ± 1.9% CH4 in the biogas. A pronounced reduction was recorded in the Archaea (ARC915) and Methanosaeta (MX825) populations in the anaerobic sludge, i.e., to 20% ± 11% and 6% ± 2%, respectively.

Influence of the Heating Method on the Efficiency of Biomethane Production from Expired Food Products

The aim of the study was to determine the effect of heating with microwave electromagnetic radiation (EMR) on the efficiency of the methane fermentation (MF) of expired food products (EFP). The research was inspired by the positive effect of EMR on the production of biogas and methane from different organic substrates. The experiment was carried out on a laboratory scale in fully mixed, semi-continuous anaerobic reactors. The technological conditions were as follows: temperature, 35 ± 1 °C; organic load rate (OLR), 2.0 kgVS·m−3∙d−1; and hydraulic retention time (HRT), 40 days. The source of the EMR was a magnetron (electric power, 300 W). There was no statistically significant influence of the use of EMR on the achieved technological effects of MF. The efficiency of biogas production was 710 ± 35 dm3·kgVS−1 in the variant with EMR and 679 ± 26 dm3·kgVS−1 in the variant with convection heating (CH). The methane contents were 63.5 ± 2.4% (EMR) and 62.4 ± 4.0% (CH), and the cumulative methane production after 40 days was 271.2 and 288.6 dm3CH4, respectively.