Potential of household photobioreactor for algae cultivation

Climate change is a global threat that needs innovative ideas to cope with such a challenge. Agricultural practice is severely jeopardized by climate change. Indeed, extreme weather, projected increases in daily temperatures (which also increases vermin and pests), unpredictable precipitation patterns, and overall further water supply shortage will reduce agricultural productivity. The major portion of water consumption goes to agriculture and the total evapotranspiration from global agricultural land could double in the next 50 years if trends of food consumption and climate change continue. However, raising the water productivity rates is not the only solution to cover the agricultural needs. During the last 50 years, a ‘water use efficiency’ concept was applied to cover this gap and fulfill the needs but currently the concept has been enhanced to ‘maximum crop per drop’. This new concept is linked with socio-economic and environmental perspectives (UNESCO WWAP 2017). There is a need to incorporate this concept with all stakeholders’ plans and strategies to find alternate nutritious supplies that are also affordable, durable, and sustainable to avoid reaching food insecurity boundaries.

Algae are highly diverse species that are related to photosynthetic protists with relatively simple morphologies (Anderson & Lewin 2015). They are usually seen in shallow water that is rich in nutrients on which they thrive, causing eutrophication. Algae species that thrive in this uncontrolled manner in unclean water bodies are harmful for human consumption. This fact has overseen its potential as a natural treatment process for nutrient recovery. The gross chemical compositions of algae are protein, carbohydrates, and lipid. The chemical composition of microalgae varies from one species to another (Chang et al. 2017; Velazquez-Lucio et al. 2018). Microalgae are more efficient at photosynthesizing and thus they capture more CO₂ efficiently than regular land-based plants (Packer 2009; Sayre 2010). They thrive on nitrogen, phosphorus, and potassium in water, which are major run-off pollutants from farmlands.

Algae are so diverse and so complex that it is difficult to classify them all under one name. There are several algal lineages. The most common ones are cyanobacteria, green algae, red algae, diatoms and brown algae (kelps) (Anderson & Lewin 2015). Cyanobacteria are known to be able to fix nitrogen, they can be used as food (Spirulina) and soil conditioners (Stal 2015). Green algae are the most harvested microalgae as they can grow quickly, common species include Chlorella (Silva et al. 2019). Red algae dominate the seaweed species. They have been used as a source of food for thousands of years as they are rich in antioxidants, vitamins, and minerals (Gomez-Zavaglia et al. 2019). Diatoms are known to be rich in oil contents, so they are important in biofuel research (Graham et al. 2011). Finally, kelps are large brown algae seaweed found in marine water bodies and they are used in many products such as toothpaste, shampoos, salad dressings, cakes and much more (Shelar et al. 2012).

Algae have been traced back as a food source as early as 300 A.D., where written records claim that the Chinese Emperor consumed it (Wells et al. 2016). Fast forward to the current century, in 2013, the total harvest of seaweed (macroalgae) was estimated to be worth US$6.7 billion (FAO 2015). Microalgae are integrated into food additives, skincare products, and nutritional supplements. Overall, there is a growing trend towards the nutritional demand for algal products globally due to their vast health benefits. There are also numerous industries practicing commercial-scale cultivation but there are no records concerning cultivation on a household scale.

Several studies have addressed all the above-mentioned issues without integrating the concepts in a decentralized system at household level. Moreover, there is a serious challenge to bridge the gap between research efforts and a community's needs. Accordingly, finding alternate nutritious supplies through decentralized technology that are tuned towards addressing core life problems and meeting population demands becomes a must.

In this respect, this manuscript is introduced to propose microalgae cultivation at the household level for the first time in developing countries in order to cope with the environmental challenges and needs of poor communities. The first part of the article provides a wide literature review for microalgae cultivation including species, cultivation parameters, photobioreactor design, downstream processing, and economics of cultivating microalgae on household levels, while the second part proposes a business model for cultivating microalgae via a decentralized PBR and the expected challenges that will be encountered with this approach will be discussed. Last but not least, the way forward is proposed to capitalize on these outcomes.

Photosynthetic organisms such as plants, algae, cyanobacteria, and euglena are all high-potential bioresources that can be used in manufacturing food supplements, animal feed, pharmaceuticals, cosmetics, biofuels, and even wastewater treatments (Priyadarshani & Rath 2012; Huang et al. 2017). Algae, however, are a highly diverse group of organisms that include photosynthetic protists and cyanobacteria. There are over 40,000 algae species that have been identified (Suganya et al. 2016). They may be unicellular (microalgae) or multicellular (macroalgae). Macroalgae are mostly seaweed that are found near the seabed while microalgae are microscopic algae found in freshwaters and saline water (Scott et al. 2010). Macroalgae require a large area for cultivation while microalgae in photobioreactors (PBRs) take up much less space and require less labor. Both have nearly equivalent potential in food production and therapeutics. However, microalgae have several untapped applications that are still under investigation. Moreover, the cultivation of microalgae can be used to combat global warming. Microalgae can be cultivated in wastewaters and high saline water (certain species) which help tackle environmental problems. All in all, microalgae cultivation could be a doorway to added-value products and applications while tackling global warming and waste problems. Figure 1 depicts an overview of the several processes involved for algae cultivation. First, the selection between microalgae and macroalgae, followed by species selection, and the cultivation conditions that should be followed. In all cases the cultivation would take place in a photobioreactor which is either closed or open. Finally, in the downstream stage, there are several harvesting and dewatering methods which are selected based on the primary wanted bioproduct.

Microalgae are genetically a very diverse group of organisms with a wide range of physiological and biochemical characteristics; thus, they naturally produce high-value chemical compounds, such as pigments, enzymes, and minerals which serve as renewable sources for many commercial applications (Suganya et al. 2016; Gujar et al. 2019). Their production costs depend on how they are cultivated (open or closed reactors) and for what purpose (nutritional, pharmaceuticals, etc.). Their production cost ranges from a minimum of 5 €/kg for raceway reactors to 50 €/kg for tubular photobioreactors (Acién Fernández et al. 2013). Figure 2 illustrates the potential benefits from cultivating microalgae for humans, animals, aquatic life, agriculture and climate change.

Microalgae are rich in protein, greater than other vegetable sources carbohydrates, and fats (Mata et al. 2010). Some species contain similar amounts of protein found in sources such as milk, soybean, egg and meat (Becker 2007; Bleakley & Hayes 2017; Koyande et al. 2019; Amorim et al. 2020). They are sold as tablets and capsules and are also incorporated into pastas, snack foods, candy bars and beverages (Suganya et al. 2016). They are rich sources of vitamins and minerals such as potassium, iron, magnesium, calcium and iodine (Becker 2003). Additionally, WHO recommended Spirulina spp. to be included in NASA's diet as it is an ideal, compact food for space travel and is nutritious even when consumed in small amounts (Khan et al. 2005).

Algae can be a source of improved immune response and fertility, and better weight control for several animals by providing a large profile of natural vitamins, minerals and essential fatty acids (Suganya et al. 2016). Indeed, 30% of the microalgae production is sold as animal feed (Richmond 2007). Meanwhile, in the aquaculture field, microalgae are added as fish feed to stabilize and improve the culture medium (Muller-Feuga 2000) and they also act as coloring for farmed salmonids (Chuntapa et al. 2003). It also enhances the immune systems of fish while inducing essential biological activities (Pulz & Gross 2004).

Microalgae are rich sources of oils and fats containing omega-3 which is part of a healthy diet that helps lower the risk of cardiovascular diseases (Suganya et al. 2016). They are also an excellent producer of carotenoids such as β-carotene and astaxanthin (most commercially produced carotenoids) (Suganya et al. 2016). Carotenoids are pigments which can be used as food coloring, supplements, therapeutics, human and animal nutrition, and cosmetics due to their ability to act as provitamin.

Most industries are attracted to the cultivation of microalgae to produce biofuels (Khan et al. 2018). Microalgae can in fact make it possible to produce several types of biofuels: biodiesel, biomethane (or biogas), hydrogen, and bioethanol (Giordano & Wang 2017). The usage of biofuels is renewable, sustainable, biodegradable, carbon neutral for the whole life cycle and environmentally friendly (Suganya et al. 2016). However, the commercial production of microalgae for biofuel is limited as large volumes of cultures are required to produce significant amounts of biofuels at low prices compared to fossil fuels (Giordano & Wang 2017). However, biofuel is an emitter of greenhouse gases, although it is claimed to be carbon neutral as released CO₂ would be absorbed by the microalgae. The process of biofuel production (extraction, transportation to fuel production plants, then to consumers) makes it release more GHG gases overall and thus it is not recommended to be produced as a potential product of algae cultivation (Hanaki & Portugal-Pereira 2018).

Microalgae can be used as soil conditioners as the majority of the cyanobacteria are capable of nitrogen fixation (Priyadarshani & Rath 2012). They can also produce growth promoting substances which can increase maintenance and soil fertility (Song et al. 2005). Microalgae can be converted by pyrolysis to ‘biochar’ that has potential agricultural applications as it can be used as soil with some additives, and for carbon sequestration (Marris 2006).

With more than 40,000 species of microalgae, selection of the appropriate microalgae strain requires certain main features that should be taken into account during the microalgae selection process. Table 1 summarizes the main features for selection of microalgae species. Not all microalgae species can be mass cultivated; some are still cultivated on lab-scale. Chlorella, Dunaliella, and Spirulina species have dominated commercial opportunities (Suganya et al. 2016). Table A1 in Supplementary Materials provides a summary of the growth conditions, applications and market value of Chlorella, Dunaliella, and Spirulina.

Microalgae are found in lakes, lagoons or ponds and are usually associated with the negative environmental impact phenomenon – eutrophication. They are not present in all ponds naturally, but they thrive upon the existence of high levels of nutrients, specifically, nitrogen and phosphorus. Their optimum pH to grow at is 8.2–8.7 but they can tolerate pH ranges between 7 and 9 (Bitog et al. 2011). They grow at temperature ranges between 16 and 35 °C (Bitog et al. 2011).

Microalgae are cultivated in open ponds or closed systems called photobioreactors (PBRs). Around 90% of microalgae production industries continue to harvest algae in natural or artificial ponds, i.e. open PBRs (Płaczek et al. 2017). However, closed types of PBRs have been proposed in the last decade. Open PBRs are easy to construct and are relatively cheaper than closed PBRs (Moejes & Moejes 2017). There are three types of artificial open ponds: (1) unstirred open ponds; (2) circular controlled ponds; and (3) controlled raceway ponds. Controlled ponds were introduced to accelerate the microalgae growth, but the basic ones were the simple open ponds. The maintenance cost for the artificial open ponds is only limited to the paddlewheel (for raceway ponds) and adequate CO₂ concentrations and they require simple cleaning up procedures. However, the downsides of this type of cultivation are grave. First, they require a large area (Płaczek et al. 2017), depending on the production rate, with high solar incidence, so they are typically set up in deserts. Therefore, they are susceptible to contamination brought around by sandstorms or even parasites. This problem usually exists in regions such as the Middle East or Africa. The idea of controlled cultivation has allowed this problem to be overcome worldwide. Second, they are susceptible to thermal shock or photoinhibition due to the extremely high light incidence. It is also difficult to control the temperature of the pond and water loss by evaporation is high (Moejes & Moejes 2017; Muñoz & Gonzalez-Fernandez 2017). These problems can vastly decrease production rate efficiency. Third, although they require low capital costs, it is more expensive to harvest algae from raceway ponds than from tubular PBRs due to their inefficiency and low biomass density, between 0.1 and 0.5 g/L (Moejes & Moejes 2017; Muñoz & Gonzalez-Fernandez 2017).

Consequently, closed-air cultivation systems, such as tubular, column, flat, flat panel airlift, and plastic bags, are being established to have better control over the operating conditions and improved quality, and produce high value added products for pharmaceutical applications (Acién et al. 2017). In fact, microalgae PBRs are gaining increased commercial interest due to their widely acclaimed industrial benefits: (1) they are feasible in culturing and harvesting products; (2) they have low contamination; (3) they have high biomass productivity – generally between 0.5 and 8 g/L (Moejes & Moejes 2017); (4) they require a smaller area than traditional crops when grown in photobioreactors; and (5) there is prevention of water loss caused by evaporation (Xu et al. 2009; Huang et al. 2017). However, PBRs require a high capital cost (Moejes & Moejes 2017). Their operating cost is high as they require proper mixing, and the perpetual cleaning of the vessels. The downstream process for both closed and open PBRs is still expensive.

There are several factors that must be considered besides the capital costs and operating costs. When designing a PBR shape or size or volume, the light distribution, mixing, mass transfer, and abiotic conditions, along with the growth kinetics of the algae, are all factors that must be taken into account (Huang et al. 2017; Płaczek et al. 2017). Thus, this section aims to break down these factors while elaborating on their respective conditions that must meet the algae growth requirements.

Besides availability, there are three important variables of light that must be considered to ensure proper photosynthesis: spectral quality, light intensity and photoperiod (Pattanaik et al. 2018). PBRs may be employed indoors or outdoors. In the case of outdoors, natural solar radiation is the light source. It is free and plentiful, so it is usually employed for mass cultivation. Solar radiation covers a wide spectral range, but microalgae make use of the light that falls between 400 and 700 nm (Huang et al. 2017). This range is known as the photosynthetic active radiation (PAR). PAR accounts for only 50% of the sunlight spectra, radiance falling outside this range is converted to heat energy which warms up the PBR or is lethal to cells if they lie within the ultraviolet range. Moreover, to ensure all microalgae absorb light equally, the PBR surface to volume ratio must be considered (Huang et al. 2017; Moejes & Moejes 2017). This is because light intensity decreases almost exponentially with distance away from the illuminated surface of the PBR. However, microalgae do not require an enormous amount of light intensity to carry out photosynthesis (Moejes & Moejes 2017). In fact, oxygenic photosynthetic organisms can reach a maximum theoretical conversion efficiency of 8–10% solar-to-biomass energy (Posten 2009; Moejes & Moejes 2017).

Photoinhibition will take place if the light intensity surpasses the saturation level (the light saturation intensity), and the growth of the algae will be inhibited. Light radiated will be wasted first as fluorescent then as heat. At the same time, if the light intensity is below that required to maintain photosynthesis (the light maintenance point), the growth would be limited, i.e. photolimitation occurs, and the culture would collapse. The range at which light intensity is needed for photosynthesis to take place varies slightly from one species to another. Another final aspect is the photoperiod. Dark hours are important for the algae to carry out respiration to retain the energy needed to carry out photosynthesis. Accordingly, the optimum period of illumination is between 12 and 15 h (Posten 2009). This period, however, is interchangeable outdoors depending on the weather.

In the case of outdoor cultivation, it is preferred to orient the PBRs in the west/east direction as it yields 1.4 times biomass more than those oriented in the north/south direction (Huang et al. 2017). This only applies to regions in the Northern Hemisphere. In the case of indoors, it is advisable to use an LED lighting system that is designed to emit wavelength within the PAR, for 12–15 h. Also, an unusable wavelength spectrum can be eliminated by adjusting the power supply which in fact lowers the LED power consumption (Pattanaik et al. 2018). Additionally, LED lighting systems also produce the same lighting intensity as fluorescent lights but consume less than half of the electric energy used (Narukawa et al. 2006). They are small, lightweight, durable, easy to install, and do not produce that much heat compared to other light sources (Koc et al. 2013). Moreover, it was found that using red LED lighting produces the highest number of cells with the highest biomass productivity while blue LED lighting produces larger cells, i.e. larger in diameter and rounder (Koc et al. 2013). So, another approach to cultivating algae was to initially use a red LED to produce the desired cell concentration, then switch to blue light to increase cell size (Shu et al. 2012; Koc et al. 2013). Another tip to increase the light flux received by the PBR is to attach a reflecting mirror or paint the ground white material at the back of or under the PBR (if they are tubes) (Abu-Ghosh et al. 2015; Płaczek et al. 2017). This is to increase the light reflection by creating a continuous weak light background which increases the biomass (Abu-Ghosh et al. 2015).

Microalgae can grow at temperature ranges between 15 and 35 °C (Ras et al. 2013). Temperatures lower than 16 °C lower the growth rate of the cells while temperatures higher than 35 °C kill the microalgae cells (Bitog et al. 2011). The optimal temperature for culturing is between 20 and 25 °C (Bitog et al. 2011; Pattanaik et al. 2018). A PBR is subjected to a significant variation of temperature due to seasonal changes, especially if it is placed outdoors (Huang et al. 2017). Without temperature controlling systems, temperatures could vary from 10 to 30 °C from where it started (Wang et al. 2012). The disruptions in temperature make the cultivation inefficient. Thus, a temperature control system is needed to maintain a favorable temperature.

For an open PBR system, especially those constructed in desert areas where the temperature could rise more than 40 °C, the temperature could be controlled easily if it is already constructed in a greenhouse. On the other hand, there are several methods to control the temperature in the closed PBR system. First, installing sprayers on the top of the PBR that spray water when the reactor exceeds the limiting temperature enables cooling by evaporation and decreasing the temperature when needed (Wasanasathian & Peng 2007); this is a good method for cooling but costly as a huge amount of water is needed to be pumped and sprayed from above. Second, partially or completely submerging the culture into a pool of water; this is an affordable method and has demonstrated that it could increase the average light intensity in the part that is submerged in the water (Carlozzi et al. 2006). Third, use dark sheets that serve as shading the PBR that decreases the solar radiation and increases the accumulation of total chlorophyll and carotenoids which increases the biomass productivity (Ugwu & Aoyagi 2008); this is the cheapest yet most inefficient method as it reduces the light intensity. Finally, there are other heat exchange methods such as overlapping the warm tubes with cool ones or regulating the temperature of the feed (Singh & Sharma 2012).

Cellular processes inside the algae are pH sensitive. Microalgae prefer to culture in a media that has a pH between 8.2 and 8.9 but they tolerate pH ranges between 7 and 9 (Berberoglu et al. 2008). So, pH ranges outside these parameters cause a breakdown to the culture due to the disruption of cellular processes inside the microalgae. There are also species with extreme abiotic conditions (high temperature, salinity, etc.), also known as extremophiles (Wang et al. 2012).

In freshwater, when the pH is 6.5, equal proportions of the CO₂ and bicarbonate form. At pH 8.3, nearly all inorganic carbon is in bicarbonate form. At that point, the bicarbonate dissociates into carbonate until equal proportions form bringing the pH to 10.4. When almost all the inorganic carbon is in carbonate ion form, the pH is 12.

When microalgae carry out photosynthesis by consuming CO₂, the chemical equilibrium shifts towards the left, increasing the pH. Although a high pH level is not favorable, most microalgae species can take up CO₂ in the form of bicarbonate (Sayre 2010). Therefore, substances such as HCl or sodium bicarbonate may be added to control the culture pH and keep it from rising too quickly (Granum 2002).

As dissolved oxygen (DO) builds up in the media from the photosynthesis of microalgae and is not removed, given the media is at constant temperature, and due to poor mixing techniques and low mass transfer rates, the concentration of DO in the media increases. When the DO concentration exceeds 100% air the reversible reaction is shifted to the left, decreasing photosynthetic efficiency (Carvalho et al. 2008).

The CO₂ is considered the carbon source needed for microalgae to carry out photosynthesis. However, it can only be consumed in liquid form, i.e. CO₂ needs to be absorbed by the medium, to be fixed by rubisco. Therefore, the dissolved carbon dioxide (DCD) is the consequence of the balance between the consumption of CO₂ by the microalgae cells and the CO₂ mass transfer from the gas phase to the liquid phase. To increase the DCD levels in water, the surface area for mass transfer from gas to liquid phase can be increased through either direct bubbling or absorption in packed beds.

Supplying CO₂ through bubbling has been studied by many researchers. According to Henry's Law, as the concentration of CO₂ increases in the air supplied, it would result in a proportional increase in dissolved CO₂ concentration in the aqueous phase (Mook 2008). This law was demonstrated by Hill et al. (2006), where 10% CO₂ by volume in the gas phase resulted in a total of 150 mg/L of DCD while for air containing only 0.037% by volume of CO₂ in gas phase resulted in 0.7 mg/L of DCD. Hence, it is recommended to directly bubble pure CO₂ also because the bubbles increase the surface area of contact between air and water. However, a major disadvantage of direct bubbling is that it could cause shear stress over the biomass particles which could damage it and it has high power consumption (Vasumathi et al. 2012). Another method of increasing DCD in water is by using the packed bed absorption column. This is where the gas phase is fed in a countercurrent manner against the medium flow inside a column. Just like the bubbling method, it is an even more power consuming application due to the energy needed to overcome the pressure drop in a bed (Vasumathi et al. 2012). In conclusion, the best criterion for the gas supply method is one where it consumes as little power as possible, has maximum surface area per unit volume of water, and does not induce cell damage.

Although there are several ways to supply CO₂, there are two abiotic factors that increase the desorption rate (Vasumathi et al. 2012). First, blowing of wind (in the case of outdoor cultivation). Second, increasing the water medium temperature will reduce the CO₂ solubility. To increase the absorption rate and retention of CO₂ in water, one method is to use an alkaline solution where CO₂ is being absorbed as carbonate or bicarbonate (Wang et al. 2008). In fact, microalgae with carbonic anhydrase enzymes are capable of better utilizing carbonate and converting it to CO₂ to facilitate CO₂ assimilation (Wang et al. 2008).

Another aspect worth mentioning is the quality of the air supplied to the medium. Supplying pure CO₂ proves to be better than supplying air as the DCD level would differ greatly. However, supplying flue gas from thermal power plants has proven to increase biomass productivity by 30% compared with direct injection of an equivalent concentration of pure CO₂ (Douskova et al. 2009). This is not only because flue gases have high CO₂ concentration, but also because of the lower partial pressure of O₂ in the flue gas compared to the conventional air supply reducing the phenomena of photorespiration and photoinhibition (Douskova et al. 2009). Flue gas from municipal waste incinerators (Douskova et al. 2009), coal-fired power plants (McGinn et al. 2011), industrial heaters using kerosene as fuel (Chae et al. 2006), and gas boilers (Doucha et al. 2005) have all been used for cultivation. Flue gases, however, whether they are untreated or not, contain pollutants (NO_x, SO_x, C_xH_y, CO, halogen acids, particulate matters, and heavy metals) and are high in temperature which could impose a threat on the microalgae growth and their toxicity (Van Den Hende et al. 2012). Fortunately, there is progress on isolating Chlorella sp. mutants that would tolerate high levels of NO_x and SO_x in flue gases (Chiu et al. 2011) as well as high temperatures (Chou et al. 2019). Moreover, according to Douskova et al. (2009), the toxicological analysis carried out on the microalgae cultivated using untreated flue gas from municipal waste incinerators has shown that while there was high mercury levels, other critical compounds were below the limits recommended by the European Union foodstuff legislation. To conclude, there is a need for research on the influence of these pollutants on the biomass of microalgae while searching for solutions to minimize the pollutants.

Perpetual mixing inside the PBR has proved to considerably enhance the productivity of the biomass (Acién et al. 2013). Mixing basically ensures (Wang et al. 2012; Acién et al. 2013; Abu-Ghosh et al. 2016):

• 1.

  Uniform light and nutrient distribution

• 2.

  Improved gas exchange between the culture medium and the air phase

• 3.

  Prevention of microalgae sedimentation

• 4.

  Avoidance of cell attachment to the reactor wall

• 5.

  Facilitation of heat transfer and avoidance of thermal stratification.

However, if not done with caution mixing can cause cell damage if the microalgae are susceptible to shear stress (Acién et al. 2013). Accordingly, it was found that liquid velocity exceeding 100 cm/s would produce micro eddies (with diameters less than 50 micrometers) which would induce cell damage (Posten 2009). Therefore, liquid velocities must not exceed 20–50 cm/s (Posten 2009). Mixing can be carried out by mechanical agitation using an impeller, by aeration by directly pumping CO₂-enriched gas bubbles, or using a combination of both methods, all of which depends on the type of reactor.

Mixing also brings about light/dark (L/D) cycle known as the flashing light effect (FLE) which could enhance photosynthesis and improve the quality and quantity of the products (Fathurrahman et al. 2013; Abu-Ghosh et al. 2016; Martín-Girela et al. 2017). FLE is when the microalgae cells, as a result of mixing, perpetually move from saturated light (near the surface of the reactor/pond) to darker zones (middle of the reactor/bottom of the pond), hence, cells are being subjected to ‘flashing-light’. Inducing FLE in an outdoor microalga open pond can be done by adjusting the mixing velocity of the culture. For indoor PBR, FLE can be done by adjusting the circulating velocity or bubbling rate and by using shading to make it simpler (Abu-Ghosh et al. 2016). Accordingly it was found that biomass productivity of green microalgae Nannochloropsis salina increased more than 55% when the frequency of the L/D cycles was set at 60 Hz (Iluz & Abu-Ghosh 2016). Moreover, there are other studies that found FLE to be successful (Fathurrahman et al. 2013; Martín-Girela et al. 2017). However, several experiments conducted have concluded that the production of microalgae was decreased when the L/D cycle or the frequency of the flashes was not optimized (Sforza et al. 2012; Vejrazka et al. 2012, 2015). For example, according to Vejrazka et al. (2012), L/D cycles below 10 Hz decreased the biomass yield by 10% on average while setting at a frequency of 100 Hz the biomass yield increased by 35% on average. Most recently, Schulze et al. (2020) showed that the FLE inhibited growth of green microalgae T. chui and C. stigmatophora cultures. In general, there are no common conclusions obtained about the effect of FLE. This technique needs to be developed and experimented on several species to conclude whether it is suitable for all species and the optimum FLE frequency.

Different microalgae species utilize different sources of energy to proceed with their growth. Not all microalgae are photoautotroph. In fact, they are split into two major groups: autotrophs and heterotrophs. Autotrophs rely on converting solar radiation and CO₂ absorbed by chloroplasts into adenosine triphosphate (ATP) and O₂ while heterotrophs consume organic carbon produced by other organisms, as they cannot produce their own food, to satisfy their material and energy needs. A third and final group is mixotrophic which combines both chemical pathways to achieve growth. Table A2 summarizes the types of autotrophs and heterotrophs.

It is essential to have a carbon source alongside main macro-nutrient nitrogen (N), potassium (K), and phosphate (P). Other elements that need to be present include calcium (Ca), magnesium (Mg), and trace elements such as copper (Cu), manganese (Mn), selenium (Se), or zinc (Zn), which are needed for enzyme functions (Płaczek et al. 2017).

Macro-nutrients can be obtained from commercial chemical grades (Geada et al. 2017). Globally, there is an abundance in potassium and phosphate mines. The trace elements may or may not need to be added because they are required in very small amounts and could already be present in the freshwater or seawater source. Pharmaceutical-based cultivated microalgae require extremely pure chemicals which are expensive. Recently, there is an increasing use of wastewater as the culture medium while bioremediating them. This approach is more environmentally sustainable and cost-effective. With the combination of flue gas, this way of cultivation has proven to be a significant approach that can tackle environmental issues (Mata et al. 2010). The wastewater used as a culture medium can be derived from livestock, food industries, and even municipalities (Geada et al. 2017). These waste streams contain a high source of organic carbon content that represents 80% of the entire medium cost which can be used to cultivate mixotrophic and heterotrophic microalgae. Table 2 summarizes the findings of the general parameters for microalgae cultivation.

There are several studies designed for better understanding of optimization conditions and parameters with a lower number of experiments through utilizing different statistical tools (Eris et al. 2018). The most popular tool for microalgae cultivation that has been used during the recent publications is response surface methodology (RSM) based on central composite design (CCD) (Skorupskaite et al. 2015). The statistical analysis of this methodology was conducted using the analysis of variance (ANOVA). ANOVA determines the statistical difference of the independent variables (Wang et al. 2019). Table 3 summarizes four research outcomes that used the RSM and CCD for the optimization parameters of different microalgae species. First and second order multiple polynomial models are developed based on the investigation of independent variables regions (Mohamed et al. 2013; Chellamboli & Perumalsamy 2014; Skorupskaite et al. 2015; Wang et al. 2019). Statistical properties of the developed models that are presented in Table 3 give adequate information about the significance, accuracy, and reliability of each equation. Also, average reported values gathered from many references are presented in Figures 3 and 4 to visually illustrate the effect of temperature, time and sodium bicarbonate on the microalgae cultivation presented in the biomass (Chellamboli & Perumalsamy 2014; Munir et al. 2015; Wang et al. 2019).

PBRs can be categorized into two main groups: open PBRs and closed PBRs. Under each category comes a subcategory that could be based on the PBRs shape, agitation mode (if any), mechanical parts etc. Figure 5 illustrates the subcategories in open and closed PBRs. In general, open PBRs are much simpler in their design than closed PBRs as it is only required to choose the shape of the reactor based on the available area. For open PBRs the main differences lie in the flow of the culture, i.e. stirred or unstirred. For closed PBRs the subcategories have different features as some are more robust, others have better surface/volume ratio, and some are economically more feasible than others. Figure 6 demonstrates the different types of closed and open PBRs. Tables 4 and 5 summarize the design criteria, advantages, disadvantages, and applicability for household scale for open and closed PBRs.

After the process of cultivation, the biomass is separated from the bulk medium and recovered as part of the downstream process. Harvesting involves two steps:

• (1)

  Primary harvesting or bulk harvesting, where the microalgae are separated from their growth medium. The total solid mass can reach up to 2–7% using flocculation, flotation, or gravity sedimentation (Brennan & Owende 2010).

• (2)

  Secondary dewatering or thickening, where the microalgae slurry is concentrated typically by centrifugation or filtration (Lam & Lee 2012). The total solid mass can reach 15–25%. This step requires more energy.

Harvesting techniques depend on the physiology of the microalgae species, the density, the preferred end-product and the possibility of reusing the medium once again (Borowitzka & Moheimani 2012). This section discusses the suitability of each method to be used on household scale.

Flocculation is a process in which the dispersed particles are aggregated together to form large particles for settling. In the culture medium, flocculants are added to neutralize the negative charged microalgae so it could eventually slowly settle by sedimentation. There is autoflocculation, chemical flocculation, electroflocculation and, bio-flocculation.

Autoflocculation occurs naturally due to changes in environmental factors such as nitrogen levels, pH and dissolved oxygen levels (Uduman et al. 2010). The floc is mediated by gravity and absence of light. Electroflocculation is a method where electric rods connected to an electric field are immersed into the culture medium to: (1) generate coagulants by electrolytic oxidation of the sacrificial electrode followed by (2) destabilizing the microalgae suspension eventually forming flocs. This method is commonly used in the removal of microalgae from industrial wastewater (Azarian et al. 2007). This process has an efficiency of 80–95% of microalgae removal (Chen et al. 2011). The efficiency of this process depends on the electrode material, electrolysis time, current density, pH and composition of the microalgae suspension. Aluminum rods are the most suitable electrode material (Lee et al. 2013; Dassey & Theegala 2014). Electroflocculation and flotation are used in conjunction (Chen et al. 2015).

Bioflocculation is when microorganisms produce bio-flocculants which flocculate the algae. These bio-flocculants could be the organic matter produced during the algae growth which are known as extracellular polymeric substances (EPS). EPS may also be produced by the bacteria added as an organic carbon source or even fungi. For example, Wan et al. (2013) harvested Nannochloropsis oceanica using bioflocculant from the bacteria strain Solibacillus silvestris with 90% efficiency. Similarly, Zhou et al. (2013) and Xie et al. (2013) have used fungi to harvest Chlorella vulgaris.Table 6 compares the advantages and disadvantages between the aforementioned flocculation.

Chemical flocculation is the addition of chemicals to induce flocculation. These chemicals may be from organic or inorganic sources. Table 7 enlists the different types of chemical coagulants used, their optimal dosage, and optimal pH.

From this classification, it is easy to say that autoflocculation is the simplest and cheapest type of flocculation which can be applicable at household level and does not require expertise or knowledge. Simply, at the end of the growth stage, the aerator will be switched or the PBR can be covered to simulate autoflocculation. Electroflocculation is a more efficient, quicker and also affordable solution but it requires sufficient knowledge about the mechanism, which can be passed along by the local manufacturer. Finally, biochemical flocculation is costly and contaminates the microalgae making it inconvenient as feed which makes it the least favorable choice in terms of flocculation at the household level.

Gravity sedimentation relies on the characteristic of the suspended solids. Density, radius of algae cells and the induced sedimentation velocity influence the settling characteristic of suspended solids. Microalgae with high density can settle properly, unlike microalgae with low density that do not settle well and are unsuccessfully separated by settling. To accelerate sedimentation, flocculants are usually added. To improve it, lamella separators and sedimentation tanks may be used (Chen et al. 2011). Regarding the household level application, the gravity sedimentation for low density microalgae will not be suitable and would be expensive as lamella separators and sedimentation tanks require huge costs and space and are time-consuming.

Through the usage of electrolysis or pressure relief into the algal suspension, the air bubbles are generated and the microalgae adhere on the bubbles and are separated (Liang et al. 2015). The microalgae are then simply scooped off the surface. This method is effective with low-density microalgae species (Hanotu et al. 2012). This process is combined with flocculation or the addition of surfactants to increase the probability for the air bubble and suspended particle (biomass) to adhere (Gerardo et al. 2015). Flotation efficiency depends on the type of collector (surfactant or flocculants), pH and ionic strength in the medium, type of bubble formation, recycling rate, air tank pressure, hydraulic retention time and particle size (Phoochinda & White 2003). Flotation types are based on their bubble size production. Table 8 summarizes the different types of flotation methods.

In general, the flotation method is usually used for pilot scale as it requires air tanks, chemicals, and experts for operation etc. In other words, it will add more to the capital and operational costs making it inapplicable for household level practice.

The filtration method is based on passing the microalgae culture through filters operating under gravity, vacuum or pressure. The microalgae particles get stuck at the filter, allowing the water to fall through. The size range of micro-algae is typically between 2 and 30 μm. There are different filters based heavily on the microalgae size and, also on the solvent/solute properties, hydrodynamic conditions (Pahl et al. 2012). Table 9 summarizes membrane filters based on size.

The membrane performance also depends on the construction materials and surface properties, i.e. charge and hydrophobicity (Rossi et al. 2004). Accordingly, it was found that positively charged membranes perform worse than neutral ones (Rossi et al. 2004). This is because microalgae usually carry a negative charge (Pahl et al. 2012) on their surface which would increase its affinity with positive charged membranes leading to fouling of membrane and low permeate fluxes. Moreover, hydrophilic membranes perform better because they have low fouling tendency by extracellular organic matter than hydrophobic membranes (Rossi et al. 2004; Sun et al. 2013). For hydrophobic surfaces, the fouling formation may be reduced by coating the membrane surface with hydrophilic polyvinyl alcohol polymer. Consequently, the maximum flux can be increased by 36% with surface-coated membranes (Hwang et al. 2013). Pretreatment with ozone is recommended as it reduces cake layer formation (a layer formed by organic matter secreted by microalgae) by 70–93% which improves filter performance (Borowitzka & Moheimani 2012). To conclude, negatively charged/neutral hydrophilic membranes are recommended for microalgae filtration because they foul less and have high permeate flux (Sun et al. 2013). Table 10 lists the typical membrane materials.

The flow of the culture-to-be-filtered feed is a determinant factor in the efficiency of the filtration method. There is tangential flow filtration (TFF) or dead-end filtration (DEF) using either UF or MF membranes. For tangential flow or cross flow, the flow of diluted algae passes parallel to the membrane while the filtrate passes perpendicular through the membrane. It can achieve about 70–80% removal efficiency. For dead end or direct flow, the flow of diluted algae passes perpendicular to the membrane. It is not economical to use due to quick fouling of the membrane. There are vacuum/pressure filtrations which were found to recover a large amount, but they are not suitable to microalgae with dimensions similar to that of bacteria (Molina Grima et al. 2003). A major problem with filtration is that it gets clogged up eventually which reduces its efficiency.

Filtration is the easiest and cheapest dewatering method that requires no prior experience to implement at the household scale. Cheap filter membranes, like nylon, are attainable at local markets and can do the job. As for the direction of flow, it is simpler to use direct flow under the force of gravity as it requires no energy input.

Centrifugation is the quickest yet most efficient way of harvesting microalgae. For example, about 80–90% microalgae can be recovered within 2–5 min on pond effluent at 500–1000xg (xg: relative centrifugal force (RCF)) (Molina Grima et al. 2003). There are several types of centrifuge that have been examined, including disk stack centrifuges, perforated basket centrifuges, imperforated basket centrifuges, decanters and hydrocyclones (Mo et al. 2015). Selection of the appropriate centrifuge is based on its RCF range, the microalgae size it can harvest, effectiveness vs. power consumption, and capital cost. However, exposure of microalgal cells to high gravitational and shear forces can damage cell structure (Chen et al. 2011). Also, they consume a huge amount of energy and they are time-consuming for mass-cultivated microalgae (Singh & Patidar 2018). Pretreatment of the algal with flotation, for example, would reduce the energy consumption by the centrifuge.

Centrifuge is not recommended to be used on a household scale level. It requires energy and the cost is extremely high. Nevertheless, it requires expert or prior-use knowledge to set up the appropriate RCF.

Table 11 is an assessment of the aforementioned harvesting methods adapted from Chen et al. (2011), while it also rates the respective methods’ applicability rating from high to low.

In conclusion, the best harvesting method, according to the MENA region, would be filtration as it is cheap, filter membranes are available at local markets, and they do not require energy input or specialized personnel. To make the process more efficient, it can be preceded by flocculation to allow microalgae to be easily filtered.

During the drying process, 90–95% of dry solid is achieved (Borowitzka & Moheimani 2012). There are several methods of drying, as presented in Table 12.

Present studies have supported the idea of using microalgae as a novel approach to treat wastewater (WWT). This comes with the fact that microalgae can deal with different types of wastewater constituents, i.e. high nitrogen and phosphorus levels. Microalgae can also be used to treat different types of wastewater sources, i.e. municipal water, livestock effluent, aquaculture, and industrial water. Table 13 displays an overview of the research studies conducted using microalgae to treat different sources of wastewater. One of the widely cultivated species in wastewater is the Chlorella sp. (Li et al. 2019). The treated water can be used as irrigation water in agriculture, contributing towards the water scarcity problem. As for the microalgae harvested, it can be used as feed for poultry, it is safe and has shown no toxicity (Abdo et al. 2019). Cultivating microalgae for wastewater treatment, unlike the conventional activated sludge process, would require longer hydraulic retention times and a vital supply of CO₂ and light. In fact, PBRs designed for WWT take secondary treatment designs in WWT plants as a reference, i.e. PBRs for WWT are classified into two types based on the microalgae cell condition: the suspended system (mobilized microalgae) and the fixed system (immobilized microalgae) (Hoffmann 2002). In the suspended system, the microalgae freely mobilize within the wastewater culture's pollutants. Examples of possible PBR designs would be open pond, tubular PBRs, flat panel PBR, and plastic bag systems. In the fixed system there is a biofilm bioreactor, immobilized bioreactor and microalgae membrane bioreactor (Ting et al. 2017). The separation in suspended systems may be done using coagulation, membrane filtration etc., which in general increases the costs in comparison with conventional methods of WWT. For the fixed system the separation process is improved but there is difficulty in measurement of the biomass (Ting et al. 2017). Suspended systems are more applicable to treat wastewater with high organic loading rates, i.e. effluents from agriculture or livestock. Fixed systems deal well with municipal wastewater as it has a lower organic loading rate, TN and TP. The design of suspended system is determined by the illumination over the PBR while in fixed systems the operation mode and HRT are the factors that conclude its design. Unfortunately, both systems are at lab-scale or pilot scale, and require more research and understanding of the parameters to scale up the system. Table A3 summarizes some of the characteristics of different types of systems, further details about the design may be found in Ting et al. (2017).

Cultivation in wastewater increases susceptibility of infestations. There are many kinds of potential contaminants in algal ponds. There are predators such as amoebae, ciliates, other predatory algae (that feed on cyanobacteria), rotifers, flagellates, and crustaceans (Carney & Lane 2014). There are pathogens like viruses, bacteria, and fungi that can infect the culture. Finally, there are competitors, like other microalgae that can outcompete the microalgae that was intended to be grown. These kinds of contaminations, when they take place, cause complete loss of the biomass. In fact, production ponds are estimated to be non-productive 12–30% of a growth season due to contamination incidents. In addition, recovery from such accidents is slow, expensive, and labor intensive. One of the common methods to mitigate contaminations is to deploy an integrated pest management plan (Carney & Lane 2014). This plan consists of a method to identify pests/predators, developing tracking devices to monitor the population of pests and finally creating intervention responses. Intervention and control systems include salvage harvest, using chemicals such as abscisic acid, fungicides, altering growth conditions such as pH controls and ammonia exposures (McBride et al. 2014; Ganuza et al. 2016). Physical methods include sonication and filtration. Finally, there are biological methods such as introducing predators to parasites. For example, zooplankton can be used to prey on fungal spores (Kagami et al. 2004).

Climate change is the biggest challenge the globe is facing right now. It is the trigger that is pushing other disasters to take over, such as famine, water scarcity, health problems, etc. Earlier, the threat posed by climate change to the agricultural sector was discussed. People living in developing countries, specifically the inhabitants of rural areas, are the most vulnerable to such threats. Aside to malnutrition issues, their standards of living are perpetually jeopardized by an unhealthy surrounding environment. Climate change also impacts their health. The air they breathe is usually contaminated by nearby factories, especially elevated CO₂ levels in the air, which is a worldwide problem as well. Studies have shown that high CO₂ concentrations causes health problems, even if it is for a few hours (Jacobson et al. 2019). So, poorly ventilated classrooms, office environments, and bedrooms, where people spend most of their days inside, are areas where CO₂ concentrations are between 600 and 1,000 ppm and could exceed 2,000 ppm (Colton et al. 2014; Fisk 2017; Persily & de Jonge 2017). Health problems at such concentrations lead to inflammation, bone demineralization, behavioral and physiological changes, and oxidative stress (Jacobson et al. 2019). Consequently, this could impede regular daily activities making them less active and affecting their employment. Alongside climate change, most rural areas are not connected to a wastewater grid. The waste produced by the locals and their livestock is stored in septic tanks which are emptied on a weekly basis, if ever. These sceptic tanks are a hub of diseases that could affect vulnerable people such as children. Moreover, for farmers, there is a struggle to find affordable fertilizers and feed for the cattle. All in all, there is a desperate need for action, whether by the government or individuals, to mitigate such threats.

After investigating these problems, we present the idea of a household photobioreactor as a potential solution. As mentioned previously, the potential of microalgae is vast. It can help reduce CO₂ in indoor environments more than common indoor plants. It provides a means of water management as it can grow in wastewater while detoxicating it which can then be used for irrigating the plants/crops. This could also encourage the farmers to plant trees around their homes, which could protect their homes from severe winds and cold days. Microalgae is also undeniably a nutritional source of feed, however, if cultivated in wastewater it would be safe to use simply as a soil conditioner or feed for poultry (Abdo et al. 2019).

There are several easy configurations that are suitable for H-PBR, as shown clearly in Tables 4 and 5. In the following paragraphs, two H-PBR configuration scenarios will be discussed. One scenario is based on the availability of a backyard for a customer which means there is an adequate surface area and the other scenario assumes that the customer has no backyard which means perhaps this is a building, apartment, office, or rural area. For the first scenario, the raceway pond configuration would be most applicable. A raceway pond outline can be constructed by cementing bricks to one another and creating an inner outline of polyethylene (or any unreacted material) while separating the two lanes using plexiglass. Other technical inputs would be proper lighting (sunlight is sufficient) and a paddle. The volume of the pond should vary between 500 and 1,000 L to harvest a profitable quantity of microalgae. For example, according to Piccolo (2011), 1 m² of water will produce 10 g of spirulina, so to produce 1,000 g, 100 m² of pond is needed, i.e. 5 m wide and 20 m long. However, such size is not applicable at the household scale. In the second scenario, a bubble column or plastic PBR would be applicable, taking a column reactor as an example. The outline of the column reactor can be obtained from any local market, and the material can be plexiglass. To satisfy the geometric relation mentioned in Table 5 for the column PBR design parameter while also suitably sized for an indoor H-PBR, the diameter of the column can range from 0.1 to 0.25 m while the height can be from 0.3 to 0.5 m. Based on these specs, the volume can range from 9.5 to 100 L. Bubble column reactors require air spargers attached to an air pump and an LED lamp if placed indoors in case of little or no available sunlight.

For both scenarios the inoculum must be obtained from local labs initially. Later, the owner must not harvest the microalgae entirely. The media (wastewater) must be first evaluated by local laboratories to see if all nutrients are available at sufficient concentrations, and if not, the nutrients can be supplied readily by the manufacturer. The harvesting is based on Tables 11 and 12, using simple autoflocculation followed by filtration then solar drying. To conclude, the procedure itself is not complicated and does not require expertise, given it is only for household scale.

Finally, H-PBR should not be a solution solely for farmers, as urban citizens can also benefit from growing microalgae indoors. They can act as mere decorations while purifying indoor air quality and be harvested as supplements. The idea of this is to encourage people to realize the immense potential of microalgae. In fact, in 1974, microalgae was declared as ‘the best food for the future’ by the United Nations World Food Conference. Moreover, in 1993 during the United Nations World Health Organization (WHO), it was stated that ‘We at WHO consider it [Spirulina] a very suitable food.’ It is unfortunate that the idea of ingesting microalgae is repulsive. Hence, through the utilization of H-PBR, humans shall understand the potential health benefits microalgae can induce and their impact on the environment. Indeed, H-PBR can be a step forward towards sustainable living, a goal that is preached by global organizations.

The exponent value n is specific to the type and quality of the equipment, but it can be assumed as 0.85. However, Equation (4) is not applicable when the ratio between the sizes is greater than 10 (Acién et al. 2017). For such inconvenience, it is advisable to use multiple units of the same equipment. In conclusion, when investing into an H-PBR is not only about creating a profitable revenue stream but it is mainly about integrating a sustainable lifestyle in day-to-day lives. Table 15 demonstrates the business model of the H-PBR manufacturer.

The opportunities presented in this paper for microalgae cultivation prove that they are sustainable, climate-friendly, environmental and a feasible proposal at a household level. They are also deemed as an appropriate technology, i.e. it is a technology that is not technically complicated yet is sophisticated through its simplicity that tackles a detrimental local problem by finding a solution which would be environmentally safe, fulfil human needs and help the local economy to flourish. In developing countries climate change impacts are increasing dramatically, and food insecurity is a focal issue. While governments may be drafting long term solutions, the resources allocation would still be prioritized in mega projects rather than decentralized approaches. Indeed, permanent solutions need to be created to sustainably develop decentralized communities. Household scale is an investment opportunity. Its implementation would not only resolve the environmental issues but also contribute towards the general health of the public. Despite the simplicity in understanding the cultivation of microalgae, the technology, economic viability, and relevant socio-cultural factors impede the microalgae cultivation from being easily executed in practice. This section presents the palpable challenges of microalgae cultivation, especially when scaling-up for household level.

The technological aspect in the microalgae cultivation is not entirely fulfilled from the research base for household scale (Xu et al. 2019). Indeed, this scale has limited research in terms of design bases for flow and mass transfer. In addition, there are insufficient studies that can fully monitor and predict the growth of microalgae which could give insights into the amount expected to be harvested. Thus, household PBRs encounter a challenge in capturing light properly, the volume to surface ratio ought to be improved further. Further research and studies are needed to optimize the design of the PBRs while reshaping the cost-structure to make it economically feasible and simple to construct. Current industrial-scale productions dealing with major bottlenecks are associated with downstream processes. The density of microalgae cells is similar to that of water, making the harvesting process inefficient and time-consuming. For household scale, this may not be a major issue since the amount harvested is not huge and local materials can be used as harvesting tools. However, as for household scale, technical problems include media costs and cultural instability. Media costs (water, nutrients, trace elements) could be compensated in case the desired end product is a soil fertilizer or feeder for cattle and fish using domestic and agricultural wastewater as the source of nutrients, while for producing high quality species that can be used to boost individual immune systems and productivity, media with high purity is required and in this case wastewater could not be used. In both cases, the carbon dioxide level in the surrounding environment will be absorbed in the photosynthesis of microalgae cultivation.

As mentioned previously, due to the low efficiency and high energy required, about 50–60% of the microalgae cultivation process goes to the downstream process. It is both high in energy and requires a vast amount of cost, especially for the industrial scale. Developing economically efficient methods is needed for financial stability. Technologically, there is still ongoing research into developing inexpensive yet fast methods. Business-wise, designing centralized downstream processing and using solar energy for drying processes that could serve two or more industrial plants could overcome the economic challenges of using PBR. However, a cutting-edge solution would be one that could make use of the microalgae without the need to go through the entire down-stream processing, i.e. utilize the microalgae in its slurry phase. In addition, microalgae cultivation requires several resources (water, land, nutrients, energy, etc.). Some of these resources could be compensated. For example, using wastewater as cultivation media would compensate for the water and nutrient costs. Using wastewater requires perpetual maintenance and monitoring to ensure the microalgae are not outcompeted. Such activities involve costs as well.

The use of microalgae is not exclusive to the 21st century. In fact, microalgae were used as a source of therapeutic medicine thousands of years ago by the indigenous population in China, the Mayan civilization, and some communities in Africa such as Niger and Chad. However, there are still several social challenges around acceptance of the utilization of microalgae, especially as a source of vitamins in developing countries, and this is perhaps due to a lack of awareness. There is still a huge gap in the market for microalgae. Recently, research has showcased microalgae integrated with flour to create edible spaghetti to prove its increased nutritional value (Fradique et al. 2010). Moreover, the declaration made by the UN that it is the ‘the best food for the future’ and recommendations made by WHO would surely encourage people to include it into their diet. However, beyond the mindset issue with microalgae, there is a lack of learning culture and constructive criticism and contractors look for traditional and safe ways to secure their business and do not seek innovative methods. There is also a lack of skilled personnel in Egypt in rural areas. Consequently, there is a need for raising awareness about the potential of microalgae, training on the use of household scale bioreactors, and monthly follow-ups to ensure smooth operation of the cultivation process.

Theoretically, the potential of the household scale is feasible to achieve and the business model behind it is promising. The idea is to fulfill this both from a social and economic aspect. However, practically, there is still more empirical-based information needed to learn about the definite specs, costs, challenges and so on. To do this, first, screening for technical feasibility, materials, and microalgae species that are available locally should be carried out. The screening would allow for the designer to identify the challenges and limits of the household scale. Conventional microalgae species that have been cultivated locally must be known and based on that, the growth conditions and nutrients required must be identified. Then, an empirical study must be carried out to determine the optimum specs (volume-to-surface ratio) and mixing rate for an improved biomass productivity rate and gas exchange. There must also be a full investigation into the prime methodology for harvesting and drying microalgae, as there are a series of methods that are integrated. For example, chemical flocculation followed by filtration is a speedy and economical method for harvesting followed by solar drying. Based on the previous data, an economic feasibility study must be performed in order to accurately conclude whether the household scale would be applicable for a particular community, taking into account the social status and environmental conditions (urban or rural area) of the customer.

There are undeniably environmental crises taking place around the world that challenge current practices in the industry and call for change. Untapped potential through biotechnology can be a solution to several environmental issues. One competitive biotechnology approach is the PBR for microalgae cultivation that is able to mitigate the climate change impacts and fulfill food security needs. Several countries around the world have considered the implementation of large scale PBRs for microalgae cultivation but it requires a huge investment into advanced cultivation and downstream processing. Therefore, scaling-down the industrial PBR to household level could be a global emerging solution to cope with environmental and economic challenges, to tackle the rising environmental concerns and cope with economic challenges. This paper is designed to be a comprehensive source for technical aspects related to microalgae cultivation and proposes household PBR potential and a business model that would have significant economic, social and environmental implications. Economically, microalgae produce bioactive compounds which are a rich source of nutritional feed for humans and can be also be fed to fish and cattle. Socially, the process of understanding the concept of microalgae cultivation is a means of educating unprivileged individuals, providing them with environmentally related insights and handcraft skills. Environmentally, the photosynthesis process of microalgae will absorb CO₂ and provide a healthier environment. Although there are several challenges with regard to decentralized PBR technology, the potential of this tailor-made approach will enrich the circular economy, enhance environmental conditions, and create a new affordable channel for developing communities to cope with climate change implications.

All relevant data are included in the paper or its Supplementary Information.