Pretreatment with the addition of metals to anaerobic digestion in biogas production is crucial to address improper degradation of organic compounds with low methane production. Biogas production from a combination of cassava pulp and cassava wastewater in the batch system under the variation of alkaline and heat conditions as a pretreatment was investigated with the zero-valent iron (ZVI) addition after the pretreatment. It was found that alkaline pretreatment at pH 10 with the heat at 100 °C for 30 min combined with 50 g of ZVI kg of TVS−1 showed the highest methane production up to 4.18 m3 CH4 kg TVS−1. Nevertheless, chemical oxygen demand (COD) and volatile fatty acid (VFA) removals were slightly reduced when ZVI was added to the system. Furthermore, application in the continuous system showed increased COD and VFA removals after applying alkaline and heat pretreatments. On the other hand, additional ZVI in the substrate after the pretreatments in the continuous system increased the methane production from 0.58 to 0.90 and 0.19 to 0.24 of CH4 m3 kg TVS−1 in 20 and 60 days of hydraulic retention times (HRTs), respectively. Thus, a suitable combination of alkaline and heat pretreatments with ZVI is essential for increasing methane production in batch and continuous systems.

Combining microalgae cultivation with nutrient removal is a promising technique as it enables renewable energy generation with the additional potential removal of wastewater contaminants in a single process. Performance and total yield of this process are still below the standard for industrialization. Thus, optimization is needed to reach the feasibility and actualize the concept. Cultivation conditions and reactor design play essential roles in the application and feasibility of this process. Both aspects have been developed through the years to enable the industrial application of this concept. Cultivation conditions are usually categorized into trophic conditions in which each situation has its specific function and target of removal. These conditions, however, are also applied in various reactor systems. Closed photobioreactor and open pond are two central systems for the reactor. Two of the most applied reactor models in wastewater are reviewed here to create a broad picture of the algae cultivation process by emphasizing biomass production and considering different aspects.

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Combination of suitable algae species with wastewater condition is important to achieve high productivity of algae with remarkable removal of contaminants. However, the usage of algae in treating wastewater has not yet to show sufficient removal efficiency when the biomass productivity is extremely enhanced. This review aims to scrutinize and discuss: (1) several promising species for this coupling method; (2) main wastewater characteristics related to the microalgae biomass production and their removal efficiency; (3) metal occurrences and other biotic factors; and (4) constraint of microalgae biomass production and wastewater treatment process. Microalgae such as Chlorella, Spirulina and Scenedesmus are among the most utilized microalgae because of their utilities. Chemical oxygen demand (COD) total nitrogen (TN), and total phosphorous (TP) concentrations affect biomass yield of algae cultivation. Metals occurrences, light intensity and carbon dioxide availability play an important role in process of algae cultivation with diverse optimum levels of each factor. Sufficient but not excess concentration of N and P solely for building biomass and other metabolism activities, mixotrophic condition for algae to digest organic carbon, and heavy metals defense mechanisms are expected to address constraint of biomass generation demand and wastewater treatment efficiency.

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