This review presents the extractions, characterisations, applications and economic analyses of natural coagulant in separating pollutants and microalgae from water medium, known as microalgae harvesting. The promising future of microalgae as a next-generation energy source is reviewed and the significant drawbacks of conventional microalgae harvesting using alum are evaluated. The performances of natural coagulant in microalgae harvesting are studied and proven to exceed the alum. In addition, the details of each processing stage in the extraction of natural coagulant (plant, microbial and animal) are comprehensively discussed with justifications. This information could contribute to future exploration of novel natural coagulants by providing description of optimised extraction steps for a number of natural coagulants. Besides, the characterisations of natural coagulants have garnered a great deal of attention, and the strategies to enhance the flocculating activity based on their characteristics are discussed. Several important characterisations have been tabulated in this review such as physical aspects, including surface morphology and surface charges; chemical aspects, including molecular weight, functional group and elemental properties; and thermal stability parameters including thermogravimetry analysis and differential scanning calorimetry. Furthermore, various applications of natural coagulant in the industries other than microalgae harvesting are revealed. The cost analysis of natural coagulant application in mass harvesting of microalgae is allowed to evaluate its feasibility towards commercialisation in the industrial. Last, the potentially new natural coagulants, which are yet to be exploited and applied, are listed as the additional information for future study.

Increasing awareness on the detrimental effects of inorganic chemical coagulants and sustainable treatment have paved the way to develop environmentally-benign natural coagulants for water and wastewater treatment. In this research, brown, green, and red lentil (Lens culinaris) extracts have been evaluated as a natural coagulant for the turbidity reduction. The physicochemical characteristics properties of lentil extract were evaluated through field emission scanning electron microscopy (FESEM), Fourier-transform infrared spectroscopy (FTIR), zeta potential analysis, and energy-dispersive X-ray (EDX) analysis. Red lentil extract was found to be an anionic polymer with hydroxyl and carboxyl functional groups. The effects of three major operating parameters, pH, the concentration of coagulant, and settling time, were studied and optimized through response surface methodology (RSM) using the Box–Behnken Design (BBD). The red lentil extract as coagulant was outperformed alum in acidic wastewater treatment, which achieved up to 98.87% turbidity removal with an optimum concentration of 26.3mg/L in 3min. Besides, the red lentil extract as coagulant reduced 29.42% of material cost, generated 15.6% lower amount of sludge and produced flocs with better settling characteristic (SVI: 7.39) as compared to alum. Based on the experimental observation and characterization study, absorption and bridging mechanisms were proposed for red lentil extract as the coagulant in turbidity reduction.

The growing global concern with environmental issues has raised the interest in the research into natural biopolymers as a coagulant aid in order to reduce the use of inorganic coagulants. This paper investigated the feasibility of sesbania seed gum (SSG) as a plant-based coagulant aid and ferric chloride as a coagulant in drinking water treatment. Acid extraction method marked the highest and most promising extraction yield at 20.8%, as compared to other extraction methods. Further, the SSG extracted carried a weak negative charge of −3.02 mV, which is classified as a near neutral coagulant aid. Hydroxyl and carboxyl functional groups, which aid in coagulation–flocculation, were found in the SSG. These physiochemical analyses results evinced good characteristics of SSG as a coagulant aid. On the other hand, response surface methodology (RSM) with three-factor Box–Behnken design (BBD) was employed to evaluate and optimize the reaction condition of the coagulation–flocculation process in drinking water treatment. A quadratic polynomial model was fitted to the data with a high value of R² (0.9901). Model validation experiments revealed the good correspondence between actual and predicted values. In drinking water treatment, a promising 98.3% turbidity reduction was achieved with 10.2 mg/L of FeCl₃ and 4.52 mg/L of SSG. Therefore, SSG exhibited potential as a coagulant aid in drinking water treatment.