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Cassava pulp is a major by-product produced in a cassava starch factory, containing 50–60% of starch (dry basis). Therefore, in this study we are considering its potential as a raw material substrate for the production of methane. To ensure sufficient amounts of nutrients for the anaerobic digestion process, the potential of co-digestion of cassava pulp (CP) with pig manure (PM) was further examined. The effect of the co-substrate mixture ratio was carried out in a semi-continuously fed stirred tank reactor (CSTR) operated under mesophilic condition (37 °C) and at a constant OLR of 3.5 kg VS m−3 d−1 and a HRT of 15 days. The results showed that co-digestion resulted in higher methane production and reduction of volatile solids (VS) but lower buffering capacity. Compared to the digestion of PM alone, the specific methane yield increased 41% higher when co-digested with CP in concentrations up to 60% of the incoming VS. This was probably due to an increase in available easily degradable carbohydrates as the CP ratio in feedstock increased. The highest methane yield and VS removal of 306 mL g−1 VSadded and 61%, respectively, were achieved with good process stability (VFA:Alkalinity ratio < 0.1) when CP accounted for 60% of the feedstock VS. A further increase of CP of the feedstock led to a decrease in methane yield and solid reductions. This appeared to be caused by an extremely high C:N ratio of the feedstock resulting in a deficiency of ammonium nitrogen for microbial growth and buffering capacity.

Abstract The aim of the present study was to assess the influence of substrate concentration on the fermentative hydrogen production from sweet sorghum extract, in a continuous stirred tank bioreactor. The reactor was operated at a Hydraulic Retention Time (HRT) of 12 h and carbohydrate concentrations ranging from 9.89 to 20.99 g/L, in glucose equivalents. The maximum hydrogen production rate and yield were obtained at the concentration of 17.50 g carbohydrates/L and were 2.93 ± 0.09 L H 2 /L reactor/d and 0.74 ± 0.02 mol H 2 /mol glucose consumed, corresponding to 8.81 ± 0.02 L H 2 /kg sweet sorghum, respectively. The main metabolic product at all steady states was butyric acid, while ethanol production was high at high substrate concentrations. The experiments showed that hydrogen productivity depends significantly on the initial carbohydrate concentration, which also influences the distribution of the metabolic products.

Improper management of pig manure has resulted in environmental problems such as surface water eutrophication, ground water pollution, and greenhouse gas emissions. This study develops and compares 14 alternative manure management scenarios aiming at energy and nutrient extraction. The scenarios based on combinations of thermal pretreatment, anaerobic digestion, anaerobic co-digestion, liquid/solid separation, drying, incineration, and thermal gasification were compared with respect to their energy, nutrient and greenhouse gas balances. Both sole pig manure and pig manure mixed with other types of waste materials were considered. Data for the analyses were obtained from existing waste treatment facilities, experimental plants, laboratory measurements and literature. The assessment reveals that incineration combined with liquid/solid separation and drying of the solids is a promising management option yielding a high potential energy utilization rate and greenhouse gas savings. If maximum electricity production is desired, anaerobic digestion is advantageous as the biogas can be converted to electricity at high efficiency in a gas engine while allowing production of heat for operation of the digestion process. In conclusion, this study shows that the choice of technology has a strong influence on energy, nutrient and greenhouse gas balances. Thus, to get the most reliable results, it is important to consider the most representative (and up-to-date) technology combined with data representing the area or region in question.

Six hydrothermal liquefaction experiments on Nannochloropsis salina and Spirulina platensis at subcritical and supercritical water conditions (220–375 °C, 20–255 bar) were carried out to explore the feasibility of extracting lipids from wet algae, preserving nutrients in lipid-extracted algae solid residue, and recycling process water for algae cultivation. GC–MS, elemental analyzer, FT-IR, calorimeter and nutrient analysis were used to analyze bio-crude, lipid-extracted algae and water samples produced in the hydrothermal liquefaction process. The highest bio-crude yield of 46% was obtained on N. salina at 350 °C and 175 bar. For S. platensis algae sample, the optimal hydrothermal liquefaction condition appears to be at 310 °C and 115 bar, while the optimal condition for N. salina is at 350 °C and 175 bar. Preliminary data also indicate that a lipid-extracted algae solid residue sample obtained in the hydrothermal liquefaction process contains a high level of proteins.

Abstract Hydrothermal liquefaction (HTL) is a promising technology for converting organic-rich waste biomass such as swine manure (SM) and sewage sludge (SS) into energy-dense bio-crude. Until now, one of the major challenges associated with HTL is the pumpability of high dry-matter containing fibrous feedstocks for continuous processing. In this context, this batch scale study presents a suitable approach for enhancing the pumpability of the fibrous material, specifically SM, by co-processing with SS. Obtained results showed that SM was not pumpable itself due to its fibrous nature, but became pumpable by the addition of SS at overall 25 % dry matter content. It was highlighted that the sample mixture containing ~80 % of the SM was smoothly pumped with 20 % SS. Subsequently, HTL experiments were carried out on samples mixed under the ratios SM:SS (100:0, 0:100, 50:50, 80:20, and 20:80). The highest bio-crude yield (42.38 %) via maximum synergistic effect was obtained from the sample SM/SS (50:50) at ratio 1:1 with the best HHV of 36 MJ/kg. Almost 60–70 % mass of all bio-crudes contained volatiles at 350 °C. ICP-AES measurements revealed that the majority of the inorganic elements were concentrated into the solid phase, while 40–50 % of the potassium and sodium were transferred to the aqueous phase. In conclusion, using SS as a co-substrate with SM not only enhances the pumpability of SM, but its co-liquefaction has demonstrated beneficial synergistic effects on improving the energy recovery of the bio-crude.

Abstract Biogas production is one of the number of tools that may be used to alleviate the problems of global warming, energy security and waste management. Biogas plants can be difficult to sustain from a financial perspective. The facilities must be financially optimized through use of substrates with high biogas potential, low water content and low retention requirement. This research carried out in laboratory scale batch digesters assessed the biogas potential of energy crops (maize and grass silage) and solid manure fractions from manure separation units. The ultimate methane productivity in terms of volatile solids (VS) was determined as 330, 161, 230, 236, 361 L/kg VS from raw pig slurry, filter pressed manure fiber (FPMF), chemically precipitated manure fiber (CPMF), maize silage and grass silage respectively. Methane productivity based on mass (L/kg substrate) was significantly higher in FPMF (55 L/kg substrate), maize silage (68 L/kg substrate) and grass silage (45–124 L/kg substrate (depending on dry solids of feedstock)) as in comparison to raw pig slurry (10 L/kg substrate). The use of these materials as co-substrates with raw pig slurry will increase significantly the biomethane yield per unit feedstock in the biogas plant.

Danish district heating systems have good water quality, but continue to suffer from biofouling and biocorrosion. Localisation analyses of bacteria using microautoradiography were performed for one system in order to obtain detailed information for solving these problems. A mass balance showed that 77% of the bacteria were located at surfaces, with 23% in the bulk water, and 9% of the total carbon originated from biomass, while 91% was dissolved in the bulk water. The presence of active bacteria was determined with microautoradiography which showed that biofilms contained 99% and 1% were in the bulk water. A high bacterial functional diversity was observed, with active mesophilic and thermophilic bacteria under aerobic and anaerobic conditions and with potentially corrosive biofilm bacteria present. The study reveals that by applying the activity based approach, the ratio of living and dead bacteria in the biofilm and bulk water in this type of system could be accurately determined. Also, the results emphasise that to minimise biofilm growth and biocorrosion, monitoring should be established focusing on the surfaces, since bulk water parameters do not reflect bacterial activity.

The kinetic parameters of anaerobic propionate degradation by biomass from 7 continuously stirred tank reactors differing in temperature, hydraulic retention time and substrate composition were investigated. In substrate-depletion experiments (batch) the maximum propionate degradation rate, Amax, and the half saturation constant, Km, were initially estimated by applying the integrated Michaelis-Menten equation. Amax was in the range from 22.8 to 29.1 micromol gVS(-1) h(-1) while Km was in the range from 0.46-0.95 mM. In general, Amax gave a good reflection of the reactor performances. Secondly, the accuracy of the applied method was evaluated by use of radiotracer methodology. Amax was found to be 14-15% lower in the substrate-depletion experiment than in the radioisotope experiment due to endogenous propionate production. By including the endogenous propionate production, a 42-49% lower Km was estimated. The results demonstrate that the rate of endogenous substrate (propionate) production should be taken into account when estimating kinetic parameters in biomass from manure-based anaerobic reactors.