This study investigated the anaerobic digestion of an algal–bacterial biofilm grown in artificial wastewater in an Algal Turf Scrubber (ATS). The ATS system was located in a greenhouse (50°54′19ʺN, 6°24′55ʺE, Germany) and was exposed to seasonal conditions during the experiment period. The methane (CH₄) potential of untreated algal–bacterial biofilm (UAB) and thermally pretreated biofilm (PAB) using different microbial inocula was determined by anaerobic batch fermentation. Methane productivity of UAB differed significantly between microbial inocula of digested wastepaper, a mixture of manure and maize silage, anaerobic sewage sludge, and percolated green waste. UAB using sewage sludge as inoculum showed the highest methane productivity. The share of methane in biogas was dependent on inoculum. Using PAB, a strong positive impact on methane productivity was identified for the digested wastepaper (116.4%) and a mixture of manure and maize silage (107.4%) inocula. By contrast, the methane yield was significantly reduced for the digested anaerobic sewage sludge (50.6%) and percolated green waste (43.5%) inocula. To further evaluate the potential of algal–bacterial biofilm for biogas production in wastewater treatment and biogas plants in a circular bioeconomy, scale-up calculations were conducted. It was found that a 0.116 km² ATS would be required in an average municipal wastewater treatment plant which can be viewed as problematic in terms of space consumption. However, a substantial amount of energy surplus (4.7–12.5 MWh a⁻¹) can be gained through the addition of algal–bacterial biomass to the anaerobic digester of a municipal wastewater treatment plant. Wastewater treatment and subsequent energy production through algae show dominancy over conventional technologies. Graphical abstract

Innovative breeds of sugar cane yield up to 2.5 times as much organic matter as conventional breeds, resulting in a great potential for biogas production. The use of biogas production as a complementary solution to conventional and second-generation ethanol production in Brazil may increase the energy produced per hectare in the sugarcane sector. Herein, it was demonstrated that through ensiling, energy cane can be conserved for six months; the stored cane can then be fed into a continuous biogas process. This approach is necessary to achieve year-round biogas production at an industrial scale. Batch tests revealed specific biogas potentials between 400 and 600 L_N/kg_VS for both the ensiled and non-ensiled energy cane, and the specific biogas potential of a continuous biogas process fed with ensiled energy cane was in the same range. Peak biogas losses through ensiling of up to 27% after six months were observed. Finally, compared with second-generation ethanol production using energy cane, the results indicated that biogas production from energy cane may lead to higher energy yields per hectare, with an average energy yield of up to 162 MWh/ha. Finally, the Farm²CBG concept is introduced, showing an approach for decentralized biogas production.

With the increasing use of renewable energy resources for the power grid, the need for long-term storage technologies, such as power-to-gas systems, is growing. Biomethanation provides the opportunity to store energy in the form of the natural gas-equivalent biomethane. This study investigates a novel plug flow reactor that employs a helical static mixer for the biological methanation of hydrogen and carbon dioxide. In tests, the reactor achieved an average methane production rate of 2.5 $\frac{L_{{CH}_4}}{L_R\ast \mathrm{d}}$ (methane production [L_CH4] per liter of reactor volume [L_R] per day [d]) with a maximum methane content of 94%. It demonstrated good flexibilization properties, as repeated 12 h downtimes did not negatively impact the process. The genera Methanothermobacter and Methanobacterium were predominant during the initial phase, along with volatile organic acid-producing, hydrogenotrophic, and proteolytic bacteria. The average ratio of volatile organic acid to total inorganic carbon increased to 0.52 ± 0.04, while the pH remained stable at an average of pH 8.1 ± 0.25 from day 32 to 98, spanning stable and flexible operation modes. This study contributes to the development of efficient flexible biological methanation systems for sustainable energy storage and management.

The increasing share of renewable electricity in the grid drives the need for sufficient storage capacity. Especially for seasonal storage, power-to-gas can be a promising approach. Biologically produced methane from hydrogen produced from surplus electricity can be used to substitute natural gas in the existing infrastructure. Current reactor types are not or are poorly optimized for flexible methanation. Therefore, this work proposes a new reactor type with a plug flow reactor (PFR) design. Simulations in COMSOL Multiphysics ^® showed promising properties for operation in laminar flow. An experiment was conducted to support the simulation results and to determine the gas fraction of the novel reactor, which was measured to be 29%. Based on these simulations and experimental results, the reactor was constructed as a 14 m long, 50 mm diameter tube with a meandering orientation. Data processing was established, and a step experiment was performed. In addition, a k_La of 1 h⁻¹ was determined. The results revealed that the experimental outcomes of the type of flow and gas fractions are in line with the theoretical simulation. The new design shows promising properties for flexible methanation and will be tested.

Flowchart of a model paper recycling mill water circuit, highlighting recycling, recovery, and discharge pathways. Icons indicate each unit (from http://www.flaticon.com). This study investigates nanofiltration (NF) and reverse osmosis (RO) treatment of the liquid fraction from digestate of short-fiber residues from paper recycling mills, focusing on water reuse and ammonium (NH4-N) recovery. NF removed up to 86% chemical oxygen demand (COD) and fully retained phosphorus (Ptot), but separated only 40–55% of total nitrogen (Ntot) and NH4-N. RO performed better, removing 96% COD, 86% NH4-N and Ntot, while fully retaining Ptot. RO permeate met the freshwater quality for paper recycling, with small NH4-N remaining. To improve NH4-N recovery, natural zeolite was tested with RO in three scenarios: treating the liquid fraction of digestate, RO retentate, and RO permeate. The highest NH4-N adsorption (79%, 7.8 mg/g) occurred with RO retentate, and the highest desorption (53%, 2.9 mg/g) with RO permeate. Adsorption performance varied with NH4-N concentration and pretreatment, with higher NH4-N levels leading to greater adsorption. COD was also reduced by up to 70% during adsorption. RO treatment of digestate in a model paper mill (0.7 Mt/year production capacity) could recover about 38,000 m3 of water annually, saving 71,000 €. Combined savings from water reuse and avoided digestate disposal could alone reach 0.2 million € annually.

The paper sector is characterised by high freshwater consumption and a strong need for improved resource efficiency. In this context, industrial digestates derived from short-fibre residues in paper recycling mills represent a promising substrate for water recovery within a circular economy framework. This study investigated the dewatering of short-fibre digestates as a pre-treatment for downstream membrane processes, aiming to maximise the liquid fraction (LF) recovery while minimising dry matter (DM) content. Seven scenarios were studied: sedimentation (S0); pre-sedimentation with chemical addition using iron(III) chloride (FeCl₃) + polydiallyldimethylammonium chloride (polyDADMAC) (S1), FeCl₃ + starch (S2), Nanofloc^® (S3), and polyDADMAC (S4); and direct dewatering without pre-sedimentation using polyDADMAC with cloth filtration (S5) and centrifugation (S6). With reference to the sedimentation supernatant, S4 achieved the highest DM separation efficiency of 76%, followed by S1 (64%), whereas S2 and S3 were below 40%. However, LF recovery relative to the initial digestate was limited in scenarios S1–S4 to 17% (170 g/kg_digestate), with DM concentrations of 2.0–4.8 g/kg_LF. In contrast, direct dewatering increased LF recovery substantially, with centrifugation (S6) achieving up to 690 g_LF/kg_digestate and cloth filtration (S5) 420 g/kg_digestate, while maintaining a low DM (1.7 g/kg_LF). Chemical oxygen demand (COD) and phosphorus (Ptot) were largely separated from the liquid fractions in all the scenarios. Nitrogen (Ntot) and ammonium (NH₄-N) in the LF remained more variable, ranging from 22 to 153 and 5 to 22 mg/kg_digestate, respectively. These results indicate that centrifugation with polyDADMAC is the most effective approach, suggesting that mechanical force with a chemical additive can be used for the efficient dewatering of short-fibre digestates.