Hyper-Concentrated Culture System

A hyper-concentrated culture system is a promising method for the treatment of IWW. In such systems, algal biomass density increases via flocculation with chitosan, as a natural flocculent, as the rate of nutrient removal from wastewater by this culture system is directly proportional to the concentration of microalgal biomass. Studies on S. Obliquus and Oscillatoria sp. suggested that a cell concentration of 1.9 g dry weight can be obtained by utilizing sewage sludge (Hashimoto and Furukawa, 1989). Further, the highest N and P removal was archived by hyper-concentrated cultures.

The hyper-concentrated microalgal culture of 5. obliquus completely removed 1 mM of NH₄⁺ within two hours from wastewaters with a tertiary treatment. The biomass also increased during the NH₄⁺ uptake. The major advantages of hyper-concentrated cultures are (1) they allow a consistent performance, (2) there is no change in the microalgal population composition either in laboratory experiments or scale-up, despite the presence of other microalgae species, (3) they reduce residence time, and (4) they reduce the area of pond needed for WWT (Lavoie and De la Noue, 1985). However, these have only been used on a small scale until now. The economic and engineering practicability of the hyper-concentrated culture system on a large-scale still requires further study (Kiran et al„ 2017).

Dialysis Cultures

Microalgal biomass grown on IWW is generally contaminated with pathogenic microorganisms, which limits the production of value-added products. Therefore, the production of high-quality microalgal biomass is a major concern. Dialysis culturing of microalgae is good practice in WWT, particularly to obtain concentrated biomass (Dor, 1975). In such a cultivation technique, osmotic exchange takes place between the microalgal culture, and the voluminous surrounding wastewater considerably intensifies microalgal growth and enables the production of concentrated biomass. This biomass can extract nutrients from wastewater via a semipermeable dialysis-wall; however, it remains separated from the bacteria, protozoan and suspended solids of the wastewater. During biological treatment, the heterotrophic bacteria present in the IWW can influence the growth of microalgae and it will reduce the rate of nutrient removal by the microalgae, which can be controlled in dialysis cultivation systems. The low porosity of the dialysis membranes undoubtedly prevent the passage of bacteria and protozoa.

Moreover, with microalgae growth in dialysis cultivation, the duration of the lag phase and the transition time for the exponential phase to the stationary phase remain approximately the same (Lebedeba et al., 2002). Physiological parameters, such as the rate of photosynthesis and concentrations of pigments in phototrophs, do not change in the course of the cultivation. The growth rate in dialysis cultivation is significantly higher than in a batch culture. Hence, significant biomass of physiologically active cells is accumulated in the relatively small volume of the dialysis bag, and this can be used for obtaining biomass as well as exo-metabolites (Savanina et al., 2008). A high biomass can be maintained for long periods with a high nutrient utilization rate (Marsot et al., 1991; Ney et al., 1981).

Photobioreactor Cultivation

Microalgal biomasses produced from IWW are considered as the most promising feedstock for biofuels. However, the chemical and biological characteristics of IWW significantly affect biochemical compositions of microalgae, which is the major disadvantage for producing microalgae biomass from IWW. Therefore, it is not economically attractive for biomass production using IWW (Barros et al., 2015). Photobioreactors (PBR) are preferred due to a minimal risk of contamination during microalgal biomass production than in an the open pond (OP) cultivation system, and this helps to achieve maximum biomass productivity. The overall cost of materials for microalgae cultivation in an OP system is higher than bubble column PBR and tubular PBR (Dasan et al., 2019) (Table 9.2).

The PBRs play an important role in IWW treatment using photosynthetic microalgae due to their positive advantages, including high photosynthetic efficiency, high biomass productivities, less contamination, no water evaporation and accurately controlled environment (Huang et al., 2017; Raeesossadati et al., 2014; Wang et al., 2012). However, obtaining PBRs that maximize sunlight capture and conversion is one of the key challenges in profitable microalgae biomass production. Moreover, the bioremediation of IWW by microalgae also depends on the geometry of the PBR, mass flow, CO, input concentration, the intensity of light and growth temperature. Different types of closed PBR systems include the stirred tank PBR, horizontal tubular PBR, airlift PBR, bubble column PBR, vertical tubular PBR and flat-panel PBR (Huang et al., 2017). However, the majority of PBRs are still too expensive for low-cost microalgal biomass production (Gupta et al., 2015;

TABLE 9.2

Working Principle of Different Photobioreactors

Hallenbeck et al., 2016). In biofuel production using microalgae, high-energy input is required, especially for cultivation, biomass harvest and drying (Li et al., 2008; Show et al., 2015). Therefore, an advanced PBR along with low-cost downstream processing technologies are required to accomplish sufficient microalgae biofuel production from wastewater (Medipally et al., 2015).

The PBR design cost is one of the main criteria which influences the overall cost of large-scale microalgal biomass production. The reduction in PBR design costs can decrease microalgal biomass production costs. The next major cost factors are the production medium, light conditions, media agitation, microalgae photosynthetic efficiency and C0₂ supply (Gupta et al., 2015). Among these, C0₂ is an expensive consumable in biomass production through PBRs. Industrial flue gases can reduce the cost of C0₂ (Aden et al., 2012). Using IWW that contains mineral nutrients is highly recommended to reduce the production medium cost. Norsker et al. (2011) estimated that the cost for commercial-scale microalgal biomass production using an open ponds system, horizontal tubular PBR system and flat-panel PBR system was 5.47,4.59 and 6.59$/kg of dry biomass, respectively. When using either tubular or flat-panel PBRs, the biomass production cost can be reduced to 0.77 and 0.75$/kg of dry biomass, respectively; while for open raceways, the unit biomass production cost cannot be reduced below 1.42$/kg of dry biomass. Thus, the bottleneck of low-cost microalgae biomass production is to develop more dynamic PBRs. Also, large amenities capable of producing

>150 tonnes hectare-' year¹ should be operated with modest operating costs, utilizing flue gases like CO, and IWW as a production (Aden et al., 2012).