Dimitris Georgakakis

Microalgal carbohydrates: an overview of the factors influencing carbohydrates production, and of main bioconversion technologies for production of biofuels

Microalgal biomass seems to be a promising feedstock for biofuel generation. Microalgae have relative high photosynthetic efficiencies, high growth rates, and some species can thrive in brackish water or seawater and wastewater from the food- and agro-industrial sector. Today, the main interest in research is the cultivation of microalgae for lipids production to generate biodiesel. However, there are several other biological or thermochemical conversion technologies, in which microalgal biomass could be used as substrate. However, the high protein content or the low carbohydrate content of the majority of the microalgal species might be a constraint for their possible use in these technologies. Moreover, in the majority of biomass conversion technologies, carbohydrates are the main substrate for production of biofuels. Nevertheless, microalgae biomass composition could be manipulated by several cultivation techniques, such as nutrient starvation or other stressed environmental conditions, which cause the microalgae to accumulate carbohydrates. This paper attempts to give a general overview of techniques that can be used for increasing the microalgal biomass carbohydrate content. In addition, biomass conversion technologies, related to the conversion of carbohydrates into biofuels are discussed.

Cultivation of Arthrospira (Spirulina) platensis in olive-oil mill wastewater treated with sodium hypochlorite.

The subject of this paper is the cultivation of the cyanobacterium Arthrospira (Sprirulina) platensis in olive-oil mill wastewater (OMWW) treated with sodium hypochlorite (NaOCl). The main positive effect of NaOCl on the OMWW characteristics is the decrease of the phenol concentration and turbidity, rendering the OMWW suitable for A. platensis growth. Maximum biomass production (1696 mg/l) was obtained when the concentration of OMWW in the cultivation medium was 10% with the supplementation of 1g/l NaNO(3) and 5 g/l NaHCO(3). However, the addition of NaHCO(3) has no significant effect, indicating that the only limited nutrient in this wastewater is nitrogen, while carbon is provided by the organic compounds of the wastewater. The maximum of the removals of chemical oxygen demand (COD) and carbohydrates was 73.18% and 91.19%, respectively, while phenols, phosphorus and nitrates in some runs was completely removed.

Optimal use of the Hosoya system in composting poultry manure.

A study was undertaken to optimize the use of the Hosoya system in composting poultry manure in a typical layer poultry farm in Greece. The farm is located about 50 km north of Athens with a Hosoya system installed which has been in operation for more than 4 years. During the study the performance of the system was investigated and samples of the material under process were taken for moisture determination and total and volatile solids analyses. The temperature of the material, as well as the temperature and the relative humidity of the surrounding air, were also monitored. The results showed that the composting process could not be completed in the oval Hosoya installation. This could be attributed to the required intensive daily turning and pushing of the manure in the installation, in order to reach the exit at the desired rate. As a result, an early drastic temperature drop of the material occurred. A further step is then necessary for efficient completion of the composting process of this type of manure. In such a case, the Hosoya system can be considered as the necessary mechanical precomposting step required for the high moisture and muddy-textured layer poultry manure prior to its being fully composted in piles or windrows.

Forced ammonia stripping from livestock wastewater: the influence of some physico-chemical parameters of the wastewater

In highly alkaline aqueous solutions (pH >10), the main form of dissolved ammoniacal nitrogen is the unionized free ammonia. Free ammonia, being a gaseous molecule, is easily stripped out from the solution. Increasing wastewater pH is frequently used to force ammonia removal. Herein, the effect of the variation of some physico-chemical characteristics of liquid cattle wastewater on ammonia stripping was investigated. The results show that at pH 11.5, ammonia volatilization and consequently its removal through stripping, was not affected by the variation of total solids (1-10%), alkalinity (2,500-20,000 mg/L as CaCO3) and electrical conductivity (20-50 mS/cm), or by the alkali type (NaOH, KOH and Ca(OH)2) employed to increase pH. The only parameter which affected ammonia stripping rate was the variation of the concentration of bedding material (straw 0.5-5%). The results suggest that the process of forcing ammonia stripping rate at high pH is unaffected and is not limited by the variation of any of the investigated parameters, except bedding material. The parameter kOLa (1/min) of the mass transfer model of ammonia stripping regarding the control experiments (raw wastewater without increasing pH) was ca. 0.0002, while by increasing pH it increased more than 20-fold (0.004 to 0.0045).

Anaerobic mono- and co-digestion of maize silage and low solid swine wastewater in a novel two-state (liquid/solid) anaerobic digester

In this study, maize silage was anaerobically mono- and co-digested together with low solid swine wastewater in a novel two-state (liquid/solid) anaerobic digester. The special subject of the two-state digester is that operates with both, liquid and solid fractions, which are present in a segregated form inside the digester. The feeding and the withdrawal of the liquid and the solid fraction can be performed independently to allow different retention times for each fraction. In this study, as solid substrate, maize silage was used, which was nested in a plastic mesh in order to be kept inside the digester in a segregated solid state against the liquid state. As liquid substrate, low solid swine wastewater was used. In general, it was easy to remove separately and independently the two solid and liquid fractions. Two types of digesters were used: one batch type (BT) digester, in which maize silage was mono-digested as sole substrate and a combined type (CT) digester, in which maize silage was fed batch and...