Cultivation of Arthrospira (Spirulina) platensis in olive-oil mill wastewater treated with sodium hypochlorite
Highlights
► Olive-oil mill wastewater treatment with NaOCl and Ca(OCl)2 was studied. ► NaOCl had a stronger effect on the OMWW phenol and turbidity than Ca(OCl)2. ► The treated OMWW was used as a cultivation medium for A. platensis. ► N addition in the medium enhanced the growth of A. platensis, while C addition did not. ► After the treatment and cultivation the removal of certain pollutants was measured.
Introduction
Olive-oil, a valuable product from olive fruits, plays a very important role in the Mediterranean diet (Visioli and Galli, 1998). However, olive-oil mill wastewater (OMWW), which is generated by the olive-oil extraction process, is one of the most serious environmental pollutants in the Mediterranean countries, such as Spain, Italy, Greece and Turkey, which are the major olive-oil producing countries in the world (http://faostat.fao.org/). The OMWW generated at the three-phase olive-oil extraction process amounts to 1–1.6 m3 per ton of olive fruits processed (Paraskeva and Diamadopoulos, 2006). The polluting potential of this wastewater is mainly related to its high chemical oxygen demand (COD; 50–150 g O2/l) and low biodegradation due to its antibacterial activity. The high polluting organic load of this wastewater is due to its high content of sugars, tannins, polyphenols, polyalcohols, pectins and lipids. Especially the polyphenolic compounds are supposed to be responsible for the antibacterial activity of this wastewater (Dareioti et al., 2010).
The cultivation of microalgae in wastewater has been proposed since the 1960s , but recently there is a raise in the interest of this topic due to the potential of microalgae to be used as a substrate for biofuels production (Park et al., 2011). Nevertheless, microalgae biomass products could be used in many applications, including animal nutrition and in the agricultural sector (Spolaore et al., 2006). In general, the cultivation of microalgae in wastewater has a dual aim: on the one hand to produce valuable microalgal biomass and on the other hand to treat the wastewater by reducing its organic and inorganic (mainly N and P) load. OMWW derived from the three-phase extraction olive-oil process has the potential to be used as a cultivation medium for microalgae growth, since it includes all the necessary nutrients. However, the OMWW after dilution might be nitrogen deficient (Hodaifa et al., 2009, Sánchez Villasclaras et al., 1996).
The cyanobacterium (blue-green alga) Arthrospira platensis has been extendedly studied due to its potential commercial applications as a source of proteins, vitamins, essential amino acids, fatty acids etc. (Rangel-Yagui et al., 2004). In addition, A. platensis is thought as one of the most appropriate microalgae for wastewater treatment (Vonshak, 2002). It has the ability to utilize organic compounds as an energy and/or carbon source. This ability is called mixotrophy. The mixotrophic growth of A. platensis is supposed to be advantageous over the photoautotrophic, in which the needed energy derives from light energy and carbon from inorganic molecules (Andrade and Costa, 2007). The mixotrophy contributes to the removal of the organic pollutants from the wastewaters through the biodegradation and/or the utilization of the organic compounds. Moreover, cyanobacteria are capable to biodegrade phenolic compounds through the mechanism of bio-transformation (Lika and Papadakis, 2009). However, OMWW contains high amounts of phenols which inhibit the algal growth (Pinto et al., 2002). In addition, the suspended solids of the OMWW contribute to its turbidity. Wastewaters with high turbidity might affect the photosynthetic potential of the microalgae, resulting in low biomass production (Borowitzka, 1998).
Several studies exist, in which raw OMWW (Hodaifa et al., 2009, Sánchez et al., 2001, Sánchez Villasclaras et al., 1996) or anaerobic digested OMWW (Travieso Córdoba et al., 2008) was used for the cultivation of micro-algae. In all of these studies, the micro-algae used were green algae. In the best knowledge of the authors, no study exists dealing with the cultivation of cyanobacteria using OMWW as a cultivation medium.
In the laboratory scale, the cultivation media is sterilized mainly through autoclaving or filtration, while in practice, in the aquaculture hatcheries, bleach (sodium hypochlorite) is frequently used (Kawachi and Noël, 2005). Chlorination is a widely used method for the disinfection of water and for wastewater treatment (Black and Veatch Corporation, 2010). Recently, post-chlorinated domestic wastewater was used for Chlorella cultivation (Mutanda et al., 2011). In addition, hypochlorites (sodium or calcium) are used as oxidants for the OMWW purification (Niaounakis and Halvadakis, 2006). Thus, this work aims to study the effects of various concentrations of hypochlorites (sodium and calcium) on several physico-chemical characteristics of OMWW and to cultivate A. platensis in OMWW treated with hypochlorite and with the addition or not of nitrogen and carbon as nutrients.
Section snippets
Olive-oil mill wastewater
The OMWW used in the study was obtained from an olive-oil mill in Korinthos, Northern Greece. The raw OMWW (OMWWRaw), was generated by the three-phase olive-oil extraction process and was left to settle for 10 d (Markou et al., 2010). The light supernatant after the sedimentation was kept and used as substrate for the experiment. Some physico-chemical characteristics of the raw OMWW (OMWWRaw), of the suspended fraction of the OMWW (OMWWSusp) after centrifugation for 5 min at 5000 rpm and of the
Effect of hypochlorites
In Fig. 1 the effects of hypochlorite treatment on various physico-chemical characteristics of the OMWW10 are presented. Gradual increase of pH and electrical conductivity (E.C.) was observed as the mass of the applied hypochlorite increased. Both characteristics were higher using NaOCl. Turbidity decreased gradually as the hypochlorite dosage increased. The NaOCl treatment had a stronger turbidity decrease than the Ca(ClO)2 treatment and the remained turbidity was 25% and 70% of the initial
Conclusions
OMWW is suitable to be used as medium for the growth of A. platensis after NaOCl treatment, which causes a strong decrease in the OMWW phenol concentration and turbidity. After the NaOCl treatment and the cultivation of A. platensis, a considerable removal of certain organic and inorganic pollutants is obtained. However, the use of NaOCl raises some environmental issues and thus more research is needed in order to secure that the method of hypochlorites treatment of OMWW is safe. Finally, more
Acknowledgements
We thank Dr Despo Kritsotaki and George Kyriakarakos for their comments on the manuscript.
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