The use of anaerobic digestion for wastewater treatment continues to be increasingly valued due to the need for resource preservation and recovery. Different high-rate anaerobic reactors with biomass retention capacity exist for the treatment of industrial and municipal wastewaters. The anaerobic moving-bed biofilm reactor (AnMBBR) is a newer anaerobic reactor that operates with biofilm growing on mobile inert media. It is simpler in design and operation compared to other high-rate reactors and it can withstand high concentrations of suspended solids. The number of studies on AnMBBRs for wastewater treatment has been increasing; however, until now no systematic evaluation of the scientific literature on this topic exists. This review aims to identify the types of wastewaters treatable using AnMBBRs, the process configurations for best treatment performance, and advantages/disadvantages of AnMBBRs.

AnMBBR is suitable for wastewater treatment at high organic loads, as it allows for high volumetric loading rates and short retention times, resulting in a compact system. It can tolerate large variations of organic and hydraulic loads and even starvation periods. This flexibility makes AnMBBR a suitable option for the treatment of industrial wastewaters experiencing seasonal variability in production levels or changes in product lines. Overall, AnMBBR technology is a versatile and effective option for the treatment of various wastewaters, offering high removal efficiencies, stability, and flexibility in operation, even at temperatures lower than the typical mesophilic range used in anaerobic treatment. Its potential for application is expected to continue growing along the need for resource recovery from wastewaters.

The use of anaerobic moving-bed biofilm reactors was investigated for biogas production from the liquid fraction of sewage sludge hydrolysate. Anaerobic digestion requires traditionally long retention times due to the slow growth of methanogens. This work investigated biogas production with 2–3 days retention time under mesophilic (37°C) and thermophilic (50°C) conditions. Two continuous reactors were operated to treat the liquid fraction of hydrolysate sewage sludge for 196 days. The mesophilic reactor showed a more stable performance compared to the thermophilic one. The mesophilic reactor kept the soluble COD removal above 80 % at a volumetric loading rate (VLR) above 7 g COD L⁻¹ d⁻¹, and specific surface loading rate (SARL) above 20 g SCOD m⁻² d⁻¹. In contrast, the thermophilic reactor displayed a soluble COD removal above 80 % at an average VLR of 4.2 g COD L-1 d-1, and average SARL of 14 g SCOD m⁻² d⁻¹. The biogas from both systems contained 73 % methane. The microbial analysis of the biofilm showed that 36 % of the viable cells were anaerobic microorganisms.

This project has targeted utilisation of infrastructure for organic waste treatment in Sweden, in particular sewage sludge, to achieve increased production of high-value materials and energy carriers, reduced use of primary resources, and improved economic performance. We have investigated the sewage sludge management system as a socio-technical system facing a change, with integral connections to the energy and waste systems.

In conclusion, there is no silver bullet for the future of sewage sludge management. Indeed, it would have to be a full clip of silver bullets, as we found that a mishmash of different barriers –technical, economic, legal, and related to public perception – creates uncertainty that hinders progress regarding both sustainable long-term strategies and technological advancement. The Swedish sewage sludge management is largely fragmented, highlighting the need to shift directionto a more holistic approach. This can help actors address common issues rather than focussing solely on activity-specific problems. Introducing new legislation could be a key step, as the current specific legislation on sewage sludge has a seemingly insignificant role for today’s sludge management, compared to other legislation and the voluntary certification.

We have formulated six overall research highlights, to outline both published results and meta-conclusions based on combined insights. Each highlight is described separately in this report.

Pulp and paper production is one of the largest global industries producing annually 400 million metric tons of pulp and paper products and 6 million tons of pulp and paper biosludge (PPBS). From a resource efficiency and sustainability perspective, there is a need for improving PPBS management. This study assessed fermentation of PPBS as pretreatment to improve PPBS feasibility as feed for black soldier fly larvae. The impact of temperature, pH, and inoculum on the concentration of soluble chemical oxygen demand (sCOD) and volatile fatty acids (VFA) was assessed. An initial pH of 10 and the addition of inoculum from an anaerobic digester substantially increased the concentration of sCOD. The obtained concentration of VFA was low compared to the VFA concentration needed to improve the growth of Black Soldier Fly Larvae (BSFL). The PPBS is recalcitrant to fermentation because of the high content of lignocellulose. Fermentation as done in this study does not convert PPBS to a feasible feed for black soldier fly larvae; thus, further research on improved fermentation is needed. However, fermentation at alkaline pH and addition of inoculum do increase the final pH of PPBS which improves its feasibility as feed for BSFL. Future studies should explore pH > 10 and temperatures > 55 °C to increase sCOD and improving generation of VFA by removal of inhibiting substances, testing other types of inoculum (rumen microorganisms) and co-fermentation.

A statistically designed range of tests was used in order to map the impact of time, temperature and pH on the acidogenic fermentation of sewage sludge with the addition of waste fly ash. The main factors investigated were temperature (35, 55 and 65 °C), pH (7, 8 and 8.5) and retention time (1, 2 and 4 days). The initial pH was adjusted by adding ash. Up to about a third of the volatile solids could be solubilized in less than two days retention time. Higher temperatures (55 and 65 °C) and adjusted pH (7 and 8) favored hydrolysis whereas fermentation producing organic acids was faster at lower temperatures (35 °C). Sludge hydrolysis occurred fast at 55 and 65 °C, reaching a solubilized total organic carbon (TOC) concentration of 3.84 g TOC L^-1 after one day. Thermophilic conditions (55 and 65 °C) resulted in a lower volatile fatty acids (VFA) concentration compared to mesophilic conditions (35 °C). At 35 °C, the highest VFA concentration was measured after 4 days and initial pH 7 (10.0 ± 0.2 g COD L^-1). This study showed the potential of using a waste stream to increase and hasten the hydrolysis of particulate organics, resulting in higher TOC solubilized in 2 days, while promoting a higher VFA production measured as g COD L^-1.