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Another method to describe the various micro-algal culture strategies is as either suspended or immobilized systems (Christenson & Sims 2011). HRAPs and PBRs include suspended cultures. Matrix-immobilized or biofilm systems contain immobilized cultures (e.g., ATS). Due to their high cost, matrix-immobilized systems are unsuitable for low-tech applications. The bioreactor's economic cost is the most critical performance aspect, with examples of micro-algae wastewater treatment systems including (but not limited to) PBR, HRAP, matrix-immobilized micro-algae, and linked micro-algal biofilm systems (Mohsenpour et al. 2021). Each of these systems offers various pros and cons as shown in Table 1.

Table 1

Pros and cons of suspended- and immobilized-cell culture methods in wastewater treatment procedures

Cultivation system	Pros	Cons
Suspended-cell cultivation systems	Thoroughly researched and improved. Larger volumes of wastewater can be treated. Large-scale operations are possible.	Microalgal harvesting is essential prior to the release of treated wastewater.
Immobilized-cell cultivation systems	Simpler to harvest microalgae prior to releasing treated wastewater. Aging cultures are more resistant to photoinhibition. The immobilization matrix gives cells greater tolerance to adverse conditions, including salinity, metal toxicity, and pH.	Expensive expenditures related with the polymeric matrix (in the case of cell entrapment). Requires a large surface area (in the case of microalgal adhesion and biofilm formation). Light restrictions may arise. Only applicable for small and pilot-scale activities.

Cultivation system	Pros	Cons
Suspended-cell cultivation systems	Thoroughly researched and improved. Larger volumes of wastewater can be treated. Large-scale operations are possible.	Microalgal harvesting is essential prior to the release of treated wastewater.
Immobilized-cell cultivation systems	Simpler to harvest microalgae prior to releasing treated wastewater. Aging cultures are more resistant to photoinhibition. The immobilization matrix gives cells greater tolerance to adverse conditions, including salinity, metal toxicity, and pH.	Expensive expenditures related with the polymeric matrix (in the case of cell entrapment). Requires a large surface area (in the case of microalgal adhesion and biofilm formation). Light restrictions may arise. Only applicable for small and pilot-scale activities.

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