Biological carbon dioxide utilisation in food waste anaerobic digesters

Carbon dioxide (CO2) enrichment of anaerobic digesters (AD) was previously identified as a potential on-site carbon revalorisation strategy. This study addresses the lack of studies investigating this concept in up-scaled units and the need to understand the mechanisms of exogenous CO2 utilisation. Two pilot-scale ADs treating food waste were monitored for 225 days, with the test unit being periodically injected with CO2 using a bubble column. The test AD maintained a CH4 production rate of 0.56 ± 0.13 m3 CH4·(kg VSfed d)−1 and a CH4 concentration in biogas of 68% even when dissolved CO2 levels were increased by a 3 fold over the control unit. An additional uptake of 0.55 kg of exogenous CO2 was achieved in the test AD during the trial period. A 2.5 fold increase in hydrogen (H2) concentration was observed and attributed to CO2 dissolution and to an alteration of the acidogenesis and acetogenesis pathways. A hypothesis for conversion of exogenous CO2 has been proposed, which requires validation by microbial community analysis

Recovery of ammonia from wastewater through chemical precipitation

Chemical precipitation is a consolidated technique applied in wastewater treatment to remove and recover phosphorous and ammonium that remain in the effluent after the anaerobic digestion treatment. The precipitate is magnesium ammonium phosphate hexahydrate (MgNH4PO4·6H2O), also known as struvite, and it is sold as a slow-release fertiliser. However, the value of struvite is quite low and has a limited market. Furthermore, it precipitates with heavy metals and other impurities that need to be removed to make the fertiliser commercially viable. This study looked at the thermal decomposition of struvite to recover added value products and recycle the magnesium for further precipitation. A kinetic study was carried out to understand the mechanism of decomposition and the formation of the different solid phases, which is fundamental for the design and optimisation of the technology. The thermogravimetric study confirmed that thermal decomposition is possible, but ammonia could not be completely released below 250 °C. The thermal analysis also led to the determination of the energy required for the decomposition, found to be 1.87 kJ g−1, which also includes the evaporation of water and ammonia. The kinetic study through the isoconversional method showed the presence of two major reactions, and the model-fitting approach identified the diffusion model as the best fit for the first reaction. The activation energy of the first reaction found with this method was 0.24 kJ g−1, comparable with the data obtained from the isoconversional method. The two-stage decomposition reactions were proposed, and the final calcination product was confirmed as magnesium pyrophosphate, which could be used in agriculture or dissolved in diluted mineral acids solution to separate the phosphate from the magnesium

Assessment of dynamic membrane filtration for biological treatment of old landfill leachate

This study investigated the behaviour of dynamic membrane (DM) filtration for the treatment of stabilised landfill leachate in a bench-scale pre-anoxic and aerobic submerged dynamic membrane bioreactor (DMBR). Four meshes with different openings (10, 52, 85 and 200 μm) were tested to support the development of DM. Differences were observed among the meshes in supporting the development of the cake layer constituting the DM. The treatment of landfill leachate had an impact on sludge characteristics resulting in deteriorated filtration performance of the DM. Effluent turbidity was often higher than 100 NTU for larger mesh pore size (85 and 200 μm). Low effluent turbidity was achieved with meshes with 10 and 52 μm (13 ± 2 and 26 ± 4 NTU, respectively) although at membrane fluxes lower than 10 L m− 2 h−1. The bioreactor exhibited a moderate organics removal of 50–60% and an ammonia oxidation between 80 and 90%. Incomplete nitrification was observed due to increased concentrations of free ammonia and free nitrous acid, with nitrite effluent concentrations up to 1062 mgNO2--N L−1. Due to the large presence of refractory organic matter in landfill leachate, denitrification was limited resulting in a total nitrogen removal of approximately 20%

Analysis of fouling development under dynamic membrane filtration operation

This research is a contribution towards evaluating the appropriate fouling mechanism responsible for the flux decline under dynamic membrane (DM) filtration and its formation mechanism by using gravity-driven filtration in a specifically designed experimental setup. Series of extended short term filtration experiments were performed at varying operating conditions of mixed liquor suspended solids (MLSS) concentrations, trans-membrane pressures (TMP) and mesh pore sizes. Blocking models were applied to identify the fouling mechanisms occurring in DM development. The results demonstrated that cake filtration model can adequately describe fouling mechanisms during DM filtration. According to the analysis of variance, DM development, as described by flux (J) trends during filtration, was significantly affected only by MLSS concentration while effluent turbidity was significantly affected by MLSS concentration and TMP. On the contrary, J and effluent turbidity trends during filtration were not significantly influenced by mesh pore size, at least in the range used in this study (10–200 μm)

Full-scale trials to achieve low total phosphorus in effluents from sewage treatment works

The tightening of phosphorus (P) standards has forced the UK water industry to identify options to improve P removal in sewage treatment works. This article presents results of 12-month trials testing three technologies at full-scale: a membrane bioreactor (MBR), a continuous backwash up-flow sand filter (CBUSF) and a high-rate compressible media filter (HRCMF). The aim of the trial was to determine the lowest total phosphorus (TP) concentrations that these systems can consistently achieve under stable operating conditions. MBR is a mature technology combining biological treatment with membrane filtration for solids removal. CBUSF uses granular media deep filtration as removal process while HRCMF uses a synthetic fibre compressible media to retain solids and both technologies are typically applied as tertiary treatments in combination with chemical P removal. The lowest TP effluent concentration was recorded for CBUSF (0.22 mgP/L) while the highest TP removal was recorded for MBR (95 ± 3 %). HRCMF delivered the poorest performance (32 ± 26 % TP removal, 1.7 mgP/L average TP effluent), attributed to limited chemical TP binding and solids removal under the conditions tested. Factors affecting P removal performance during the trial were the identification of the optimal coagulant dose to bind soluble P, the formation of solids that can be removed by the separation unit, insufficient reaction time for the coagulation process and the carryover of coagulant into the final effluent. The trials gave valuable insights to address the implementation of new technologies or upgrade existing assets' performance

Performance and stability of sewage sludge digestion under CO2 enrichment: a pilot study

Carbon dioxide (CO2) injection in anaerobic digestion has recently been proposed as an interesting possibility to boost methane (CH4) recovery from sludge and organic waste by converting a greenhouse gas into a renewable resource. This research assessed the effects of exogenous CO2 injection on performance and process stability of single-phase continuous anaerobic digesters. Two pilot scale reactors treating sewage sludge were operated for 130 days. One reactor was periodically injected with CO2 while the other acted as control. Two injection frequencies and injection devices were tested. The results indicated that CO2 enrichment allowed an increase in CH4 production of ca. 12%, with a CH4 production rate of 371 ± 100 L/(kgVSfed⋅d) and a CH4 concentration of ca. 60% when dissolved CO2 levels inside the test reactor were increased up to 1.9-fold. Results also indicated an improvement in process resilience to temporary overloads and no impacts on stability parameters

Coagulation–flocculation process with metal salts, synthetic polymers and biopolymers for the removal of trace metals (Cu, Pb, Ni, Zn) from municipal wastewater

To ensure compliance with regulatory standards, it is important to examine the potential of treatment technologies to enhance trace metal removal from wastewater. This study investigated the effectiveness of coagulation–flocculation at removing trace metals from humus effluent with ferric chloride (FeCl3), the synthetic polymer polyethyleneimine (PEI) and the biopolymers chitosan and floculan. Effluent samples were collected from a trickling filter treatment works operating in the UK and contained 21 ± 4 μg/L Cu, 0.8 ± 0.1 μg/L Pb, 4 ± 1 μg/L Ni and 43 ± 9 μg/L Zn. The influence of coagulant dosage and the velocity and time of the slow mixing stage were studied via a series of jar tests. Chitosan and PEI had a moderate effect on the removal of trace metals (≤ 35%). FeCl3 removed 48% Cu, 56% Pb and 41% Zn at the optimised dose of 0.10 mg/L. At the optimised dose of 0.25 mg/L, floculan removed 77% Cu, 68% Pb and 42% Zn. The dominant mechanism for particle removal by FeCl3 was enmeshment in the precipitates (i.e. sweep flocculation), whereas, for floculan, inter-particle bridging was the dominant removal mechanism. Overall, FeCl3 and floculan were found to be most effective at removing trace metals from wastewater

Two-stage anaerobic digestion of the organic fraction of municipal solid waste – Effects of process conditions during batch tests

Two-stage anaerobic digestion (AD) batch tests were performed using the organic fraction of municipal solid waste as substrate. Effects of different combination of initial pH (5.5, 7, and 9) and food to microorganism (F/M) ratio (from 0.5 to 6 gVS/gVS) were investigated for hydrogen and methane productions during the first and the second stage of AD, respectively. Results showed that both initial pH and F/M ratio had an impact on hydrogen yield, hydrogen production rate and duration of lag phase. The highest hydrogen yield of 29.8 mLH2/gVS was obtained at initial pH of 5.5 and F/M ratio of 6. However, the highest hydrogen production rate (65 mLH2/gVS/d) was recorded at pH of 9 and F/M ratio of 6. Increasing the initial pH from 5.5 to 9, led to shorter lag phases for all F/M ratios. Methane production from second phase was not significantly influenced by the F/M ratios tested in the first digestion phase. When compared to single-phase AD, two-stage AD tests resulted in enhanced methane production rates from 37.3 to 68.5 mLCH4/gVS/d, reducing by half both the lag phase and the time required to reach maximum methane production

Experimental Protocol to Test Explicit Motor Learning–Cerebellar Theta Burst Stimulation

Implicit and explicit motor learning processes work interactively in everyday life to promote the creation of highly automatized motor behaviors. The cerebellum is crucial for motor sequence learning and adaptation, as it contributes to the error correction and to sensorimotor integration of on-going actions. A non-invasive cerebellar stimulation has been demonstrated to modulate implicit motor learning and adaptation. The present study aimed to explore the potential role of cerebellar theta burst stimulation (TBS) in modulating explicit motor learning and adaptation, in healthy subjects. Cerebellar TBS will be applied immediately before the learning phase of a computerized task based on a modified Serial Reaction Time Task (SRTT) paradigm. Here, we present a study protocol aimed at evaluating the behavioral effects of continuous (cTBS), intermittent TBS (iTBS), or sham Theta Burst Stimulation (TBS) on four different conditions: learning, adaptation, delayed recall and re-adaptation of SRTT. We are confident to find modulation of SRTT performance induced by cerebellar TBS, in particular, processing acceleration and reduction of error in all the conditions induced by cerebellar iTBS, as already known for implicit processes. On the other hand, we expect that cerebellar cTBS could induce opposite effects. Results from this protocol are supposed to advance the knowledge about the role of non-invasive cerebellar modulation in neurorehabilitation, providing clinicians with useful data for further exploiting this technique in different clinical conditions

Organic waste biorefineries: looking towards implementation

The concept of biorefinery expands the possibilities to extract value from organic matter in form of either bespoke crops or organic waste. The viability of biorefinery schemes depends on the recovery of higher-value chemicals with potential for a wide distribution and an untapped marketability. The feasibility of biorefining organic waste is enhanced by the fact that the biorefinery will typically receive a waste management fee for accepting organic waste. The development and implementation of waste biorefinery concepts can open up a wide array of possibilities to shift waste management towards higher sustainability. However, barriers encompassing environmental, technical, economic, logistic, social and legislative aspects need to be overcome. For instance, waste biorefineries are likely to be complex systems due to the variability, heterogeneity and low purity of waste materials as opposed to dedicated biomasses. This article discusses the drivers that can make the biorefinery concept applicable to waste management and the possibilities for its development to full scale. Technological, strategic and market constraints affect the successful implementations of these systems. Fluctuations in waste characteristics, the level of contamination in the organic waste fraction, the proximity of the organic waste resource, the markets for the biorefinery products, the potential for integration with other industrial processes and disposal of final residues are all critical aspects requiring detailed analysis. Furthermore, interventions from policy makers are necessary to foster sustainable bio-based solutions for waste management