The Potential Use of Waste Stabilization Ponds for Biofuel Production By the Microalgae Biomass

The potential use of waste stabilization ponds for biofuel production by the microalgae biomass

M. V. C. Paiva*, S. M. S Barbosa¹, E. A. Pastich*,

* Federal University of Pernambuco, Center of Technology and Geosciences, Laboratory of Environmental Sanity, Recife, PE, Brazil.

E-mail marcellavcpaiva@yahoo.com.br

¹MSc Environmental Technology, Federal University of Pernambuco, Recife, PE, Brazil.

Abstract

In the work the performance of a real scale polishing pond treating domestic wastewater in Rio Formoso city Southern coast of Pernambuco State, Brazil was evaluated. The principal objective was investigate the the potential use of waste stabilization ponds for biofuel production by the microalgae biomass Samples were taken monthly at two different times (2h and 14h) comprising a period of six months (from Janury till June of 2011) covering the rainy and dry seasons. It was observed that the phytoplankton community was represented by 40 taxa belonging to divisions Cyanophyta, Chlorophyta and Euglenophyta, being the Cyanophyta the most representatives comprising (45% from total). We found that the higher and lower biomass density was observed during January and June in both periods (diurnal and nocturne) These months represents respectively the dry and rainy periods in the region, showing that a higher sunlight penetration promote a higher algae growth on the water column. By the diagnosis of the potential use of algae growing in the polishment pond from Rio Formoso WWTP, it was possible to observe the biomass concentration, and to propose ways to optimize the biomass growth for an efficient and sustainable biodiesel production.

INTRODUCTION

Waste stabilization ponds are widely used in Brazil owing to the favorable climatic conditions and the simplicity of process. At the waste stabilization ponds, the organic matter transformation occurs due the mutualistic relation between phytoplanktonic and bacterial community, the first one responsible by the dissolved oxygen production, and the second, by the aerobic degradation of organic matter. However, the high growth of phytoplanktonic organisms, may affect the efficiency of the treatment’s system, causing disequilibrium at the receptor body due the high algae density into the treated effluent. Furthermore, is possible the occurrence of cyanobacterial blooms into the waste stabilization ponds, generating risks to the environment and the public health.

Thus, several researchers and even the population, has been opposite to the wastewater ponds implantation, due the problems related to algae growth and cyanotoxins.

Some post treatment technologies has been studied and applied to promote the biomass removal, e.g., physic-chemical process, ozonization, flotation by dissolved air, rock beds, aerated biofilters, microsieves, microfiltration and runoff grass. However most of these alternatives are costly, making the treatment economically not sustainable (FABRETI, 2006).

In recent decades it has been discussed and studied regarding the potential for microalgae use as a renewable energy source, mainly for biofuel production. The wastewater from farming and industrial activities, as well the municipal effluents, may provide a sustainable medium for microalgae growth for biofuel production. Thus is possible the integrated use of algae based treatment, with the nutrients removal and biofuel production, being entirely feasible use of wastewater ponds for this purpose (Pittman et al, 2010).

Besides to promote the nutrients removal from wastewater, (e.g. NH4+, NO3-, PO4), the microalgae cultivation for biofuels production, is useful for the CO2 mitigation, emitted by industrial processes. In addition, after the oil extraction, is still possible its use as input to ethanol and methanol production, livestock feed, and biofertilizer, because the high nitrogen and phosphorus content, or be burned for heat and electricity generation (Miao; Wu, 2005).

Chisti 2007, related that microalgae seems to be the unique fuel’s renewable source, able to supply the global demand for transport and others human activities. The oil’s productivity of several species of microalgae exceeds that ones from the best oilseeds crops, widely used for biofuel production worldwide.

The main advantages of microalgae use as source for biofuel production are related to the high growth rates, and lipids content, the wastewater use for nutrients supply, being the chemical products addition unnecessary. In other hand, the main disadvantage is the harvesting of the strains. Generally the algae are unicellular, becoming difficult its separation of the wastewater (Chen, 2006).

Several microalgae species can utilize the autotrophic or heterotrophic metabolism, thereby, they are capable of promoting the photosynthesis, or assimilate dissolved organic matter respectively. Some species are also capable of grow by the mixotrophic metabolism, meaning that the cell growth is not strictly photosynthesis’ dependent (Zhang, 1999).

Microalgae cultivation for biodiesel production may be performed through open ponds or closed photobiorectors. The low costs associate to these ponds may be mentioned as a positive aspect, nevertheless, these plants require a large area for its installation and it is not possible have a pollution control (Scott et al., 2010).

Within the ponds, the microalgae utilize the sunlight to promote its autotrophic growth. It is worth emphatize that the metabolic activity of microalgae is favored in higher temperatures environments, such as those seen in tropical countries. In these regions, the intensity of solar radiation emitted to the surface is quite high and widely distributed throughout the year (Chen, 2006).

Use of wastewater for biodiesel production has been applied to improve the biomass production instead the lipid productivity. The benefits associated to the wastewater use, is mainly related to the effluent remediation, and the cost reduction regarding to the nutrients supply, particularly the nitrogen (Pittman et al, 2010).

For a successful microalgae cultivation, besides the operational equilibrium, e.g., (oxygen, carbon dioxide, pH, temperature, and light intensity), it is necessary an organic carbon source, e.g., (glucose, proteins and fatty acids), vitamins and minerals, as well, nutrients as nitrogen and phosphorus (Zhang, 1999). .

For being nutrient stocked, generally the effluents possess wide aerobic bacterial populations, capable to produce CO2 by the respiration process, which is used for the microalgae as carbon source ( Munoz e Guieysse , 2006).

The most commons algae used at the biodiesel production, are: Chlorella, Crypthecodinium, Cylindrotheca, Dunaliella, Isochrysis, Nannochloris, Nannochloropsis, Neochloris, Nitzschia, Phaeodactylum, Porphyridium, Schizochytrium Tetraselmis, Scenedesmus, Spirulina, because they produce a oil rate about 20 to 50% related to other species, however, higher productivity may be reached.

The microalgae selection for biodiesel production has to take into account, the high biomass density to extract an elevated lipid concentration furthermore, it is necessary to acquire a wide knowledge about the physiological aspects of the species that you want to cultivate, in order to extract maximum productivity of the organisms.

Thereby, the aim of this study was investigate the potentiality for biodiesel production by the microalgae species, found at the waste stabilization/polishment pond, at the Rio Formoso city, Pernambuco State, Brazil. The analysis of the treatment system, aiming its possible use for biofuel production, was done by the microalgae identification genera and its density counting, aiming investigates the lipid cell production. Obviously all method including the biomass calculations, was grounded by specialized literature.

METHODOLOGY

This work was performed at Rio Formoso city (08°39´50´´S 35°09´32), Southern coast of Pernambuco-Brazil. The wastewater treatment plant is composed by an UASB set (upflow anaerobic sludge blanket), followed by a polishment pond and a rock filter set as post-treatment applied for suspended solids removal.

This study focused only in the polishment pond, which measures: 1.5m depth, 1.42 ha of superficial area, and 8 days HRT. The average flow rate is 40 L/s.

The monitoring period extended from January to June of 2011. Samples were taken monthly, performed during two intervals (14:00h and 2:00h). The times were selected because they respond to changes in day-night cycle that influence the behavior of algae in the aquatic environment.

The qualitative the phytoplancton analysis were accomplished by an optical miscroscopy, and an inverted microscope was used to the microalgae quantification, based on the Utermöhl techniques, described by Lund et al., (1958). The algae identification and classification were based in the following literature: Chroococcales (Komarek and Anagnostidis, 1986); Oscillatorialles (Anagnostidis and Komarek, 1988); and Euglenophyta, Cryptophyta and Chlorophyta (Bourrely, 1972). The following ecological indices were measured: (Lobo and Leighton, 1986), total density (APHA, 1998) and biovolume (Hillebrand et al., 1999).

RESULTS AND DISCUSSION

The polishment ponds, aims to remove part of the organic matter after the treatment into the UASB reactor. For a successful polishing, it is necessary a shallow environment to promote the sunlight penetration leading to a higher algae growth on the water column.

The inclusion of a post-treatment (rock filters set) at the Rio Formoso WWTP (wastewater treatment plant) was particularly aimed to the suspended solids removal (composed by algal biomass), due to complaints of the population in relation to the green colored effluent discharged into receiving body. However, it is possible to use this biomass profitably and sustainably.

The phytoplankton community was represented by 40 taxa belonging to divisions: Cyanophyta, Chlorophyta and Euglenophyta. Regarding to the contribution of the divisions into the total wealth, the Cyanophyta division showed higher contribution comprising (45%) followed by Chlorophyta (42%) and Euglenophyta division (13%). Regarding the frequency of occurrence, its worth emphasize the Oscillatoria limosa specie, which showed a rate of 100% in all months of the study, ie, was present in all samples.

The genera found in the polishment pond from Rio Formoso WWTP were: Cyanophyta division: Pseudoanabaena, Oscillatoria, Microcystis, Dolichospermun, Anabaenopsis, Coelomoron, Aphazinomenon, Raphidiopsis, Aphanocapsa, Coelospharium, Merismopedia, Choroococcus, Radiocystis, Sphaerocavum, Eucapsis, Arthrospira, in the Chlorophyta division: Scenedesmus, Sphaerocystis, Monoraphidium, Closteriopsis, Tetradesmu, Desmodesmus, Eudorina, Chlamydomonas, Coelastrum, Oocystis, Keratococcus, Radiococcus, Golenkia, Franceia and Euglenophyta division: Phacus, Euglena, Trackelomonas, Lepocinclis.

Among the identified species at Rio Formoso WWTP, the Scenedesmus genera is widely used for cultivation, mainly under experimental scale for biodiesel production The specie, Scenedesmus obliquus, for example has a better growth rate in municipal wastewater when compared to Chlorella vulgaris (Ruiz-Marin et al., 2010). Moreover, they can provide high rates (> 80%) and in many cases an almost complete removal of the ammonia, nitrate and total P from secondary treated sewage (Martinez et al., 2000; Ruiz- Marin et al., 2010; Zhang et al., 2008). This genus has the ability to utilize organic substrates under light and dark conditions, and also to perform only the photosynthesis for energy.

Cyanobacteria are also promising candidates for cultivation in wastewater, they produce biomass in satisfactory quantity and can be harvested relatively easily, due to its size and structure. Furthermore, the biomass composition can be manipulated by various environmental and operational factors to produce more cells. Although cyanobacteria species do not contain large amounts of lipids (about 20%), it has a relatively high productivity of biomass. High biomass productivity species can generate energy more efficiently than other energy technologies conversion (Balasubramanian et al., 2010).

The density was analyzed on a seasonal scale considering all the sampled points inside the polishment pond from the Rio Formoso WWTP at 14h and 2h.

The highest density occurred during January (5.4 x 107cel/ mL) and lower in June (1.2 x 107 cel/mL) regarding the samples collected at 14h. For the samples taken at 2h, the highest density occurred in February (4.7 x 107 cel./mL) and the lowest also occurred in June (1.1 x 107 cel./mL). In January was also found that the largest biomass in the diurnal period (14h) occurred in January (6.1 x 107 μm 3 /mL) and lower biomass in June (1.8 x 107 μm 3 /mL). At the nocturne period (2h) the highest density was observed in February (5.3 x 107 μm 3 /mL) and lower biomass in June (1.3 x 107 μm 3 /mL). The lower densities and algal biomass in June, can be explained by the typical storms in the region in April and May (357 mm and 564 mm respectively), promoting changes in the effluent composition and consequently to the phytoplankton community. According to Chellappa et al., (2008) the temperature variation in tropical regions is not accentuated along the year, and the phytoplankton successional changes are the result of precipitation and the increased intensity of winds. These aspects represent disturbing factors for aquatic communities, contributing to the phytoplankton restructuring process.

Through the density and biomass analysis, for samples collected at 14h and 2h, it was observed that there was no significant difference between the two times. This behavior can be explained mainly by the different metabolic pathways used by some cyanobacterial genera. Although being a phototrophic oxigenic group, there are a variety of fermentation pathways in the metabolism of cyanobacteria to meet the energy demands in the dark. To maintain some growth rate, cyanobacteria has to resort to a chemotrophic mode for energy generation. In the most species, the accumulated glycogen during photoautotrophic growth, serves as an energy source in the dark (Stal and Moezelaar, 2007). There are also species of phytoplankton that can perform the heterotrophic and mixotrophic metabolism.

In autotrophic growth, the light is used as a single energy source which is converted into chemical energy through the photosynthesis reactions. The cultivation of species of autotrophic metabolism, the artificial light sources, can be used for more than 25% of the biomass produced during the day and can be lost during night, through breathing process (Chisti, 2007). However, according to the density and biomass data observed into the polishment pond from the Rio Formoso WWTP, there is no a significant drop neither to algal density nor to the biomass measurement, due to the various forms of metabolisms performed by microalgae species present in WWTP to keep growing even in the dark.

Thus, from the diagnosis of the potential use of algae growing in the polishment pond from Rio Formoso WWTP, it was possible to observe the biomass concentration, and to propose ways to optimize the biomass growth for an efficient and sustainable biodiesel production, taking into account that algae for biofuel production are currently cultivated into the high rate ponds or into of "raceways ponds".

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