Biomass is considered worldwide as an important source of renewable energy, including electricity, automobile fuel, and as a source of heat for industrial equipment.

Cultures commonly used for energy production are sugarcane, corn, beans, beets, and many others. There are two main factors that define when a culture is appropriate for this process: good dry matter yield per unit of land (dry ton/ha), low area requirement for cultivation, and low costs of energy production from biomass (Dermibas et al., 2009).

However, some research has condemned the use of biofuels, associating its production with possible high food prices. Algae, among the aquatic biomass feedstocks, are considered one of the most promising sources of biofuels due to their unique characteristics. They can accumulate lipids that can be converted into biofuels, present fast proliferation, have the ability to sequester CO₂ from the atmosphere for growth and do not require agricultural land or freshwater for growth or higher water consumption, and also the whole plant matter can be used in converting biofuels processes (Dismukes et al., 2008; Brennan and Owende, 2010; Jena and Das, 2011; Pate et al., 2011; Yanik et al., 2013; Zhou et al., 2014; Hognon et al., 2015).

Wastewater treatment associated with algae cultivation can offer an alternative way for sustainable renewable biofuels, since the large amount of freshwater needed for algae cultivation can be saved, becoming an environmentally friendly process (Zhou et al., 2014).

In a sewage treatment system, high-rate ponds are characterized by having high algal biomass generation which is an undesirable byproduct for the environment. Its presence in water bodies decreases water quality.

High-rate algae ponds are raceway-type ponds, in which water, algae, and nutrients are continually mixed. A paddle wheel generates a mean horizontal water velocity of approximately 0.15–0.3 m/s. This movement is necessary to avoid sedimentation and stratification. The maximum biomass production (mostly algae) is achieved through better use of lighting per volume. This is ensured by the low depth of the ponds and the constant movement of biomass through mechanical mixing (Nascimento, 2001; Chisti, 2007).

Usually the algal biomass productivity is determined by the measurement of solids found in the ponds.

The algal biomass production costs are mainly covered by the costs of treatment when using wastewater high-rate ponds resulting in lower environmental impacts in terms of water, energy, and fertilizer needs.

The biomass in high-rate algae ponds assimilates the nutrients needed for its growth and becomes responsible for the removal of nutrients from wastewater. This has the advantage of controlling pollution of water resources which contributes to the sustainable use of this technology on an industrial scale (Park et al., 2011; Passos et al., 2013).

The biomass separation process requires an increase in algal suspension concentration typically from 0.02 to 0.06% total suspended solids (TSS) to approximately 2 to 7% solids, which may be higher depending on the target process objective (Uduman et al., 2010).

The algal cells have reduced size, sometimes <30 μm and their density is similar to water with a low sedimentation rate, so to be successful in separation, it is necessary to aggregate the cells. Generally, the process comprises of two steps: the first involving destabilization of algal cells using coagulation followed by sedimentation or flotation. In the second step of the process, it is necessary to increase the biomass content, which is often done by filtration, centrifugation, or thermal processes (Molina Grima et al., 2003; Granados et al., 2012; Cai et al., 2013; Udom et al., 2013).

The algae cell has a negative surface charge, which prevents aggregation. This charge may be reduced or neutralized by the addition of flocculants or multivalent cations, such as cationic polymers that change the zeta potential, which is a measure of particle stability, reducing the repulsive forces. So the action of the attractive Van der Waals forces allows algae agglutination (Wessler et al., 2003; Granados et al., 2012). Salts used for this purpose should be non-toxic, low cost, and have high effectiveness at low concentrations (Molina Grima et al., 2003).

Another advantage of using coagulation/flocculation process is nutrient removal. The presence of nutrients in wastewater, particularly nitrogen and phosphorus, is a serious environmental problem and is receiving increasing attention. Nitrogen in the form of ammonia can be volatilized and cause air pollution. Phosphorus can permeate into the soil and cause damage to the underground water (Chen et al., 2012). When there are excessive levels of nutrients in the wastewater, they cause eutrophication of water sources, possibly damaging the ecosystem (Cai et al., 2013).

The algal biomass, after thickening, may reach 5–15% solid content, and, being perishable, it must be processed as soon as possible. Essential processes such as thickening and drying usually involve high operational costs. Thus, these steps are considered determining factors regarding the economical feasibility analysis of the overall process (Brennan and Owende, 2010; Uduman et al., 2010). The methods commonly used for thickening biomass are centrifugation and filtration followed by different drying techniques, such as natural, oven, spray, and fluidized bed drying.

There are three basic components in algae biomass: proteins, carbohydrates, and lipids. These oils can then be extracted and converted in to biofuels (Um and Kim, 2009).

The pyrolysis process appears to be an excellent alternative for energy conversion, it presents the advantage of using different sources of organic matter, not being limited by the lipid content, as with biodiesel production processes. The pyrolysis process is based on decomposition of organic compounds present in the total biomass under a controlled environment in the absence of oxygen and atmospheric pressure, resulting in different phases: liquid (bio-oil), gas, and solid (char). It is an endothermic reaction that occurs at a temperature of 300–700°C depending on the characteristics of the material to be pyrolyzed (Martini, 2009; Hognon et al., 2015).

Biomass pyrolysis is considered a renewable process, because biomass is turned in several gases when pyrolyzed. Carbon dioxide, one of the gases formed, is absorbed by the algae for its growth, making the process self-sustainable with no serious contribution to greenhouse effect. The relative yield of each phase generated in the process depends on operating parameters (temperature, heating rate, residence time, and flow rate of inert gas), properties of the biomass (the particle size as well as its moisture), and type of pyrolysis used (slow, fast, or flash pyrolysis) (Balat et al., 2009; Martini, 2009; Akhtar and Amin, 2012; Yanik et al., 2013; Hognon et al., 2015).

In order to obtain high yields of aqueous products, fast pyrolysis is normally used, which is characterized by higher heating rates (1000°C/min) and lower residence times of volatiles (10–20 s). In order to favor solid char formation, slow pyrolysis process with lower heating rates (5–80°C/min) and longer residence times (5–30 min) must be used (Van de Velden et al., 2010; Jena and Das, 2011; Yanik et al., 2013).

The bio-oil generated by biomass pyrolysis is generally cleaner than that from fossil fuels, due to its lower nitrogen and sulfur content. The biomass vaporizes, passes through a process of cracking and condensation, producing a dark brown liquid, consisting of a complex mixture of many different hydrocarbons. This process is most successful in fluidized bed reactors due to high heating rates, rapid devolatilization and easy control (Doshi et al., 2005; Martini, 2009).

In this work, the association of wastewater treatment and biofuel production through pyrolysis of algal biomass obtained in high-rate algae ponds was studied. The algal productivity, at the average depth was measured as 41.8 g.m⁻² day⁻¹ for pond A and as 47.1 g.m⁻² day⁻¹ for pond B. The algae were pyrolyzed in a tubular furnace system with external heating at different temperatures. Studies have shown that the pyrolysis process is efficient and the fractions yields are greatly influenced by temperature. Operating under mild conditions, it was possible to obtain maximum yields of 45% at 500°C for aqueous phase (bio-oil and water), 44% for char, and about 11% for gas. As we can see, through this process, it is possible to offer a promising alternative for environmental pollution control with potential economic return.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.