Microalgae-based wastewater treatment is an alternative to physico-chemical and bacteria-based technologies. Microalgae-based wastewater treatment showed enormous potential, not only exhibiting excellent pollutant removal efficiencies but also unlimited opportunities for resource recovery. Despite its promising future, the question of selecting autotrophy or heterotrophy regimes for optimal organic pollutant removal remains. This current work juxtaposes the performance of autotrophic and heterotrophic cultures in treating organic-rich wastewater to shed light on the unsolved puzzle. This review paper details the autotrophy and heterotrophy growth regimes for microalgae, as well as highlights the source of organic-rich wastewater and its characteristics. A clear comparison between both regimes was then discussed with recent references. Heterotrophic cultures showed better parameter removal performances, especially carbon-related and N-related compounds, while the removal of P-related compounds is considerably similar. Heterotrophic regimes also resulted in higher biomass yield with higher P content as compared to autotrophy. Despite their superiority, heterotrophic regimes continuously require additional carbon sources, posing a cost-related limitation. In contrast, autotrophic culture has an added value of carbon sequestration, making it beneficial for climate mitigation and lowering operational costs. Future research should concentrate on techno-economic and cost-benefit analyses to further refine the currently discussed topic.

Tofu effluent contains a high concentration of organic materials, nutrients, suspended solids and is also low in pH. This research was aimed at applying phytotreatment using floating plant species of Pistia stratiotes to polish tofu effluent before final discharge into water bodies while also producing biogas from the resultant biomass after treatment. A range-finding test (RFT) was conducted to determine the initial concentration to be treated and resulted in 10 % tofu effluent. Phytotreatment was conducted for a period of 14 days, focusing on the removal of organic matter and nutrient contents. After 14 days of treatment, P. stratiotes were able to remove total suspended solids (TSS) by 88 %, ammonia by 42.3 %, phosphate by 50 %, chemical oxygen demand (COD) by 84 %, and biological oxygen demand (BOD) by 95 %, significantly higher as compared to control. Phytotreatment was able to stabilize pH to a neutral value, and P. stratiotes were able to transfer oxygen from air to the rhizosphere area. The maximum daily production of biogas using the plant's biomass was higher as compared to the control; however, the overall biogas accumulation was significantly lower during the 45 days of observation. Further biomass pretreatment was suggested before digestion to obtain higher biogas production since the cellulose, hemicellulose, and lignin content inside the plant biomass were subjected to being hardly degraded by the anaerobic microorganisms.

Biocoagulants have gained attention as an alternative to chemical coagulants in water and wastewater treatment due to their ability to remove pollutant parameters such as turbidity, suspended solids, color, and organic compounds. Plant-based biocoagulants are currently the most promising due to their abundant availability and reliable performance. A proper preparation of plant-based biocoagulants is required to ensure the active compounds from the plants can be extracted. Plant raw materials are prepared into biocoagulants using drying and grinding, extraction, and purification. In mechanical preparation, drying and grinding exhibit great influence on the performance of plant-based biocoagulants due to the removal of moisture and higher contact with the carrier medium during dissolution. A smaller biocoagulant's size resulted in up to 78 % higher removal of turbidity. In chemical preparation, the oil needs to be removed before extracting the active ingredients to avoid low active compound extraction yields. The defatting of Moringa oleifera seeds showed an 18 % higher protein content. Salt and alcohol extractions were mentioned as superior extraction methods to obtain carbohydrate and protein from plant-based biocoagulants compared to water extraction, with up to a 5 % increment in turbidity removal. Preparation and pretreatment protocols have a great influence on the properties and performance of plant-based biocoagulants. Further extension of research on plant-based biocoagulants should address the problems in the initial characterization of plant material, standard pretreatment and preparation protocols, and real-scale application of plant-based biocoagulants.