• Energy Research

Nowadays, Tunisia is seeking to implement cost-efficient and sustainable solutions in relation to the treatment of organic waste which, at up to 65%, makes up the largest proportion of total waste generated in the country. Therefore, an efficient tool for decision makers is needed to provide a clear approach about the potential of organic waste as well as the treatment concept, which can be adapted based on technical requirements and local conditions. Results revealed that there is a high variation in terms of the nature of the collected biowaste, which affects the selection of the adopted bioprocess for each geographical zone of the study area. Three main categories of biowaste are produced along the coastline of Tunisia: food waste (FW) (102.543 t/a); green waste (GW) (1.326.930 t/a); and cattle manure (CM) (1.548.350 t/a). Based on the results of similar projects and laboratory-scale research work, anaerobic and aerobic digestion were examined. Regarding aerobic digestion, the monitoring of several physicochemical parameters ascertained that the co-composting of FW and GW at different ratios (GW: FW = 100:0, 75:25, 50:50, and 25:75) allowed the production of a stable and mature compost. A highly qualified end-product was generated from each trial categorized as a finished compost of class V with reference to German Standards of compost. Regarding the anaerobic process, different feedstock mixtures (FW:CM = 0:1, 1:1, 2:1, and 3:1) were prepared to feed semi-continuous anaerobic reactors. However, a significant improvement in the process development was recorded for digesters including a higher fraction of FW, which produced 0.846 LN/kg VSin as the maximum biogas production. Therefore, biological treatments of food waste and different co-substrates seems to be a suitable technique for Tunisia in terms of waste management, environmental, and energy aspects. However, the evaluation of the efficiency of the proposed biological treatments was also verified by a draw-up of a technical and economic feasibility analysis. Although the cost–benefit estimations proved that the profits from both the compost and biogas plants would be very modest, the feasibility of such sustainable projects should not only be evaluated on an economic basis, but also by taking into account socio-environmental considerations including decreasing environmental threats, providing work opportunities, increasing incomes, stimulating public awareness as well as reducing the operating costs linked to landfilling.

Energy recovery from lignocellulosic solid marine wastes, Posidonia oceanica wastes (POW) with slow pyrolysis responds to the growing trend of alternative energies as well as waste management. Physicochemical, thermogravimetric (TG/DTG) and spectroscopic (FTIR) characterizations of POW were performed. POW were first converted by pyrolysis at different temperatures (450°C, 500°C, 550°C and 600°C) using a fixed-bed reactor. The obtained products (bio-oil, syngas and bio char) were analyzed. Since the bio-oil yield obtained from POW pyrolysis is low (2wt.%), waste frying oil (WFO) was added as a co-substrate in order to improve of biofuels production. The co-pyrolysis gave a better yield of liquid organic fraction (37wt.%) as well as syngas (CH4,H2…) with a calorific value around 20MJ/kg. The stoichiometric models of both pyrolysis and co-pyrolysis reactions were performed according to the biomass formula: CαHβOγNδSε. The thermal kinetic decomposition of solids was validated through linearized Arrhenius model.

The study regards para-nitrophenol (p-NP) removal by a mixed culture in a batch reactor under aerobic conditions performed at low ratio substrate (p-NP) to p-NP degrading microorganisms (0.09 < I(0)/(X(B,PNP))(0) < 0.80 g COD(PNP)g VSS(-1)). p-NP biodegradation was modelled with a dual-biomass kinetic including Haldane formalism. The purpose was to examine the effect of operating conditions of acclimation phases in the kinetic parameters estimated by respirometric measurements. The experiments were conducted with a series of successive additions of p-NP and a biogenic substrate (Ss) in different proportions (0 < R = Ss/I < 6.6). To place emphasis on decisive role played by frequency and amount of p-NP supply, a parallel was drawn with continuous processes, characterising acclimation cycles by different organic loading rate (207 < OLR < 1490 mg COD(PNP) l(-1) d(-1)). During acclimation, results showed progressively decreasing half saturation constant (K(s)(PNP)) values (11.4-1.21 mg CODl(-1)) whereas inhibition coefficient K(I)(PNP) increased (72.4-289 mg CODl(-1)), as the specific degradation rate increased. The inverse behaviour was observed during starvation periods. At the end of acclimation, higher values of growth yield (0.39 < Y(PNP) < 0.63 mg COD(X) mg COD(PNP)(-1)) and maximum growth rate (1.09 < mu(max)(PNP) < 2.01 d(-1)) were obtained for cycles with low R.

Cellulosic fibers from date palm are among the most promising lignocellulose feedstock for biorefinery purposes. The world production is between 1.9 and 2.4 million t/year. Initially, a pretreatment with dilute-sulphuric acid of these fibers was performed using a response surface methodology, with temperature and process time as factors. The aim is to produce bioethanol from young and old fibers from date palm, Phoenix dactylifera L. Optimal thermochemical pretreatment conditions for both fibers palms were 220 °C in hydrothermal conditions (without acid); in these conditions pretreated young fibers presented a maximum content in holocelluloses of 45.18% and old fibers 61.97%. Subsequently, during the enzymatic hydrolysis a maximum yield of total reducing sugars (TRS) was reached, 46.32 g/100 g for pretreated dry young fibers and 48.54 g/100 g for pretreated dry old fibers. After enzymatic saccharification, hydrolysates were fermented by Pachysolen tannophilus (ATCC 32691) to ethanol, reaching yields (YE/TRS) of 37.94 g ethanol/100 g of TRS for young fibers and 35.84 g ethanol/100 g of TRS for old fibers. Globally, considering the full process, in the fermentation of the hydrolysates, a yield (YE) of 10.64 g ethanol/100 g of dry young fibers and 10.88 g ethanol/100 g of dry old fibers was reached.

Geotrichum candidum is a yeast-like filamentous fungus that has attracted industrial interest. The present work investigated G. candidum biomass production in agro-industrial wastewaters (olive mill wastewater (OMW) and cheese whey (CW)) as the only substrate. Different solid media (Sabouraud dextrose agar (SDA), CW, OMW, and OMW/CW mixtures in different proportions) were tested. OMW/CW mixtures proved to be suitable for optimal mycelia growth of G. candidum with a very high hyphae density. The highest fungal and expansion rate growth of 83 +/- 1 mm and 12.4 day(-1), respectively, were obtained on a 20:80 mixture of OMW/CW, which was incubated for 7 days. This optimal mixture was used to study the biomass production and the OMW decolorization ability of G. candidum in the presence of CW in liquid medium. Liquid cultures were also conducted in OMW and CW separately. After 5 days of incubation, fungal biomass reached 9.26 g l(-1) in the OMW/CW mixture and only 2.83 g l(-1) in CW, while no biomass production was observed in OMW alone. OMW decolorization and dephenolization by G. candidum also improved in the presence of CW with a decolorization efficiency of 54.5% and a total phenolic reduction of 55.3%, compared with the control which yielded values of about 10% and 15%, respectively. These results suggested that OMW/CW--as the only substrate--could be used as a cost-effective medium to produce G. candidum biomass, without the need for water dilution or supplementation with other nutriments.

The aim of the present study was to evaluate the applicability of using protein-rich isolates from tomato seed as a sole source of nutrition for the growth of lactic acid bacteria. Unstructured mathematical and logistic models were proposed to describe growth, pH drop, lactic acid production and nutriment consumption by Lactobacillus plantarum in whole and defatted isolates in order to compare their suitability for the production of a fermented beverage. These media have considerable good quantities of nutriment that allowed the growth of L. plantarum, after which the cell numbers begin to decline. The maximum biomass was observed in defatted isolate (1.42 g L−1) followed by the whole isolate (1.24 g L−1). The lactic acid increased by about 5.5 and 6.5 times respectively in whole and defatted protein isolates. However, significant nutriment consumption occurred during the growth phase as well as stationary phase. A reduction of 61.90% and 95.88% in sugar content, as well as 21.91% and 16.93% reduction in protein content were observed respectively in whole and defatted isolates. In most cases, the proposed models adequately describe the biochemical changes taking place during fermentation and are a promising approach for the formulation of tomato seed-based functional foods.

As bulking agents (BA) affect the composting process, this work examined the impact of combinations of different organic components in order to obtain an efficient co-substrate for food waste (FW) in-vessel composting. To boost the occurrence of microorganisms inhabiting the compost, mature compost was firstly coupled with wheat straw, added to FW, and considered as a control (BC0). Then, two trials (BC10, BC20) including 10% and 20% of biochar were monitored. The results indicated that the temperature of the amended bioreactors was notably increased compared to the unamended one. Thermophilic temperatures were achieved at 14, 34, and 78 h after the experimental setup for BC20, BC10, and BC0, which lasted for 14, 17, and 12 days, respectively. When it came to an assessment of maturity and stability, the quality of the compost was evaluated against several indicators and compared with the compost quality standards of the UK, France, Canada, the USA, Poland, and Germany. BC10 illustrated a high-quality product in relation to the heavy metal concentration, a C:N ratio which reached 14.97, an AT4 which was lower than 6 (4.36 mg O2/g TS), and a nitrification index of 2.61 (<3). Consequently, the addition of 10% of biochar as a co-substrate showed an improvement of the process evolution and the characteristics of the biofertilizer produced.

Molasses are a potential feedstock for ethanol production. The successful application of anaerobic fermentation for ethanol production from molasses is critically dependent to the development and the use of high rate bioreactors. In this study the fermentation of sugar cane molasses by Saccharomyces cerevisiae for the ethanol production in a continuously stirred tank reactor (CSTR), an immobilised cell reactor (ICR) and a membrane reactor (MBR) was investigated. Ethanol production and reactor productivities were compared under different dilution rates (D). When using the CSTR, a decent ethanol productivity (Qp) of 6.8 g L−1 h−1 was obtained at a dilution rate of 0.5 h−1. The Qp was improved by 48% and the residual sugar concentration was reduced by using the ICR. Intensifying the production of ethanol was investigated in the MBR to achieve a maximum ethanol concentration and a Qp of 46.5 g L−1 and 19.2 g L−1 h−1, respectively. The achieved results in the MBR worked with high substrate concentration are promising for the scale up operation.