• Energy Research

This paper focuses on the specific growth rate estimation problem in a Polyhydroxybutyrate bioplastic production process by industrial fermentation. The kinetics of the process are unknown and there are uncertainties in the model parameters and inputs. During the first hours of the growth phase of the process, biomass concentration can be measured online by an optical density sensor, but as cell density increases this method becomes ineffective and biomass measurement is lost. An asymptotic observer is developed to estimate the growth rate for the case without biomass measurement based on corrections made by a pH control loop. Furthermore, an exponential observer based on the biomass measurement is developed to estimate the growth rate during the first hours, which gives the initial condition to the asymptotic observer. Error bounds and robustness to uncertainties in the models and in the inputs are found. The estimation is independent of the kinetic models of the microorganism. The characteristic features of the observer are illustrated by numerical simulations and validated by experimental results.

In view of climate change mitigation, the production of biogas through anaerobic digestion has emerged as a prominent renewable fuel source, also in developing countries. The implementation of household-scale digesters in these regions is instrumental in achieving the Sustainable Development Goal 7 of ensuring clean and affordable energy. However, a rigorous quantification of the biogas production and net-greenhouse gas effect of these household digesters, taking into account fuel use, biogas leakage and fertiliser types, is lacking. This study quantified both biogas production and net-greenhouse gas emissions, utilizing Rwanda as a reference region. Through a field survey, biogas measurements, and model simulations, the study revealed that a reference case featuring a fixed dome digester of 4 m3, fed with manure of on average 2 cows produces 0.38 m3 biogas.d−1 which meets 65% of the cooking need of an average family of 4 adults and 2 children. The addition of a toilet to the household-scale digester increased the daily biogas production by 16%. Conversely, the use of a bare soil floor in the cow shed results in a 28% reduction in biogas production, emphasizing the recommendation for a concrete floor. Net-greenhouse gas emissions from a household-scale digester, including an estimated 9% leak percentage, result in a reduction of 2.4 tCO2,eq.year−1 per household compared to wood-only cooking. Besides, using the digestate as a fertiliser further enhances the reduction to a total of 2.5 tCO2,eq.year−1. The main opportunities for minimizing emissions include optimizing temperature, manure flow and reducing leakages, which needs to be considered during design, operation, and policy formulation. In summary, household-scale digesters provide sustainable waste management, renewable energy, and significant greenhouse gas emission reduction, making them a promising technology for decentralized energy solutions in developing regions.

Anaerobic nitrogen removal technologies offer advantages in terms of energy and cost savings over conventional nitrification-denitrification systems. A mathematical model was constructed to evaluate the influence of process operation on the coexistence of nitrite dependent anaerobic methane oxidizing bacteria (n-damo) and anaerobic ammonium oxidizing bacteria (anammox) in a single granule. The nitrite and methane affinity constants of n-damo bacteria were measured experimentally. The biomass yield of n-damo bacteria was derived from experimental data and a thermodynamic state analysis. Through simulations, it was found that the possible survival of n-damo besides anammox bacteria was sensitive to the nitrite/ammonium influent ratio. If ammonium was supplied in excess, n-damo bacteria were outcompeted. At low biomass concentration, n-damo bacteria lost the competition against anammox bacteria. When the biomass loading closely matched the biomass concentration needed for full nutrient removal, strong substrate competition occurred resulting in oscillating removal rates. The simulation results further reveal that smaller granules enabled higher simultaneous ammonium and methane removal efficiencies. The implementation of simultaneous anaerobic methane and ammonium removal will decrease greenhouse gas emissions, but an economic analysis showed that adding anaerobic methane removal to a partial nitritation/anammox process may increase the aeration costs with over 20%. Finally, some considerations were given regarding the practical implementation of the process.

This study analyses the steady state behaviour of biological conversion systems with general kinetics, in which two consecutive reactions are carried out by two groups of micro-organisms. The model considered is a realistic description of wastewater treatment processes. A step-wise procedure is followed to reveal the mechanisms affecting the occurrence of steady states in terms of the process input variables. It is clearly demonstrated how taking into account inhibition effects by simply including additional inhibition terms to the kinetic expressions, a common practice, influences the model's long term behaviour. The overall steady state behaviour of the model has been summarized in easy-to-interpret operating diagrams, depicting the occurrence of steady states in terms of the reactor dilution rate and the influent substrate concentration, with well-defined boundaries between distinct operating regions. This knowledge is crucial for modelers as steady state multiplicity--in the sense that more than one steady state can be reached depending on the initial conditions--may remain undetected during simulation. The obtained results may also serve for experimental design and for model validation based on experimental findings.

Looking back at over a decade of research by herself and her group, the author advocates the added value of gas phase measurements and the application of mass balances, as well as the synergetic benefits obtained when combining both. The increased application of off-gas measurements for greenhouse gas emission monitoring offers a great opportunity to look at other components in the gas phase, particularly oxygen. Mass balances should not be strictly reserved for modellers but also prove useful while conducting lab experiments and studying full-scale measurement data. Combining off-gas measurements with mass balances may serve not only to quantify greenhouse gas emission factors and aeration efficiency but also to follow dynamic concentration profiles of dissolved components without dedicated sensors and/or to calculate other unmeasured variables. Mass-balance-based data reconciliation allows for obtaining reliable and accurate data, and even more when combined with off-gas analysis.

Anaerobic pig manure digestion holds potential to contribute to a bio-based economy. This work assesses the stability of the thermophilic mono-digestion process. Thermophilic mono-digestion experiments with (i) fresh liquid pig manure and (ii) the fresh fecal fraction from source separation by a pig housing construction were conducted in semi pilot-scale continuous stirred tank reactors. The effect of separation on the digestion stability was studied by comparing thermophilic mono-digestion of fresh liquid (unseparated) and fresh source separated pig manure. Influencing factors and inhibitors were identified during the experiments. An unstable thermophilic mono-digestion process was observed for fresh liquid pig manure at a digester retention time of 60 days, due to high levels of ammonia and sulfur-containing components. Thermophilic mono-digestion of the fresh fecal fraction was more promising in terms of stability, provided enough time for digestion. In addition, the effect of low temperature (70°C) thermal pre-treatment of manure on the digestion stability was investigated. In the case of liquid pig manure, no improvement in the digestion stability was noted upon thermal pre-treatment. For the fecal fraction, the stability of the thermophilic mono-digestion process did improve. Moreover, thermal treatment and subsequent thermophilic mono-digestion of the fresh fecal fractions from two different farms with a similar pig housing construction suggested an effect of the (organic) dry matter content on the process stability.

Energy-efficient wastewater treatment plants (WWTPs) utilize systems like high-rate activated sludge (A-stage) system to redirect organics from wastewater are redirected into energy-rich sludge (A-sludge). Anaerobic membrane bioreactors (AnMBRs) offer lower footprint and higher effluent quality compared to conventional digesters. In this study, the biological treatment and the filtration performances of AnMBRs for A-sludge digestion under mesophilic and thermophilic conditions were comparatively evaluated through lab-scale experiments, mass balancing and dynamic modeling. Under thermophilic conditions, a higher COD fraction of the influent sludge was converted into methane gas than under mesophilic conditions (65% versus 57%). The energy balance indicated that the surplus energy recovery under thermophilic conditions was less than the additional energy required for heating the AnMBR, resulting in a more than three-fold higher net energy recovery under mesophilic conditions. Therefore, operating an AnMBR for sludge digestion under mesophilic conditions has a higher potential to improve the energy balance in WWTPs.