Application of Nostoc sphaericum and Opuntia ficus-indica Mucilage in the Coagulation–Flocculation Process of Sanitary Landfill Leachate: An Optimization Study

Leachates generated in sanitary landfills are a mixture of contaminants harmful to adjacent ecosystems. Coagulation and flocculation are common treatment methods; however, their efficiency depends on the type of coagulant–flocculant and the operating conditions. This study addressed leachate treatment using two natural biocoagulants, Nostoc sphaericum (CNS) and Opuntia ficus-indica mucilage (CMN), in combination with aluminum sulfate (CSA). Optimization was performed using response surface methodology, employing a Box–Behnken design with five factors, namely CNS, CMN, and CSA doses, as well as agitation time and agitation speed, evaluated at three levels, on turbidity reduction. Fourier transform infrared spectroscopy (FTIR) showed that the biocoagulants possess anionic surfaces with carboxyl and hydroxyl groups. The particle size of CNS exhibited a bimodal distribution with a zeta potential of −28.74 mV, while CMN showed a unimodal distribution with a zeta potential of −21.95 mV. Under optimal conditions (88.97 mg/L CNS, 105.60 mg/L CMN, 9.13 mg/L CSA, a mixing time of 25.96 min, and an agitation speed of 24.21 rpm), a turbidity reduction to 48.15 NTU was predicted. During the experimental validation of these optimal conditions, turbidity was reduced to 49.02 NTU, achieving a removal efficiency of 70%. Total organic carbon (TOC) was reduced by 65%, and metals such as arsenic were reduced by 56%. Reductions in phosphates and Biochemical Oxygen Demand (BOD5) were moderate, while the removal of Chemical Oxygen Demand (COD), surfactants, and ammoniacal nitrogen was limited. These results indicate that the combination of CNS and CMN is a viable alternative for leachate pretreatment.