The coagulation of wastewater using biowaste materials as coagulants

Abstract

In recent years, there has been a growing societal, governmental, and industrial concern about chemical and biological contamination of water. Numerous domestic and industrial operations generate wastewater that contains undesirable toxic contaminants. Chemical coagulants (aluminium and iron-based salts) effectively pre-treatment water and wastewater via coagulation and flocculation. Nevertheless, accumulating these chemical coagulants in the form of sludge and metal oxides significantly affects the environment, as well as human and aquatic life. Against this background, along with biowaste generation and the desire to meet the United Nations Development Goal on Clean Water and Sanitation (UN SDGs#6,9 and 12). Addressing chemical coagulant challenges with alternative solutions, such as using biowaste as coagulants, comes in handy. Therefore, this study aimed to examine the potential of banana peel, eggshell, and seashell powder as substitutes for traditional coagulants in water and wastewater treatment. The biowaste materials (banana peel, eggshell and seashell) underwent calcination at 400°C- 800°C for 2 -3 hours. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Brunauer Emmett Teller (BET), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) were used to analyse the morphology, elemental composition of the structure, the pore surface area, the functional and molecular properties and the crystal structure, respectively. The overall analysis demonstrated the calcination of the engineered biochar was successful. Among the samples, the calcined banana peel was found to have the highest BET surface area of 4.3889 m2 /g, and the peeks on the XRD showed that calcined banana peels have the presence of calcium and potassium. The observed functional groups included O-H, C-O and C-H groups. This was followed by a feasibility study via the coagulation process using the calcined and uncalcined bio-coagulants. The results affirmed that these bio-coagulants are suited for wastewater treatment. In the preliminary studies, the removal efficiencies for turbidity and chemical oxygen demand (COD) were over 80% between the 0.8 – 8 g/L dosage range. The response surface methodology (RSM) was used for the optimisation process with an input variable coagulant dosage (1 g/L – 6 g/L), mixing speed (30 rpm – 150 rpm), mixing time (2 min – 15 min), and settling time (20 min – 120 min) and colour, turbidity and COD removal as responses. The Box-Behnken design response model had a correlation factor (R2 ) of over 0.9 at 95% confidence level. The optimum conditions were 1.6963 g/L (coagulant dose), 119.796 min (settling time), 2.25 min (mixing time), and 30 rpm (a mixing rate), achieving a removal efficiency of 92.39% COD removal, 99.77% Turbidity removal and 104.59 % colour removal at a desirability performance of 100%. The optimum conditions were then validated with various bio-coagulants for the removal of the physiochemicals (COD, Turbidity, and colour) and other emerging contaminants such as phenol, phosphate, nitrate, and ammonia. The response model's predicted results were in total agreement with the experimental results, with less than 5% deviation. In addition, the calcined seashells had the best removal efficiency, with over 80% removal for phenol, phosphate, and ammonia and under 60% removal for nitrates. The findings of this research show that bio coagulants (eggshells, seashells and banana peels) have valuable potential to be substituted for conventional coagulants in the water sector. Therefore, exploring if their economic and environmental viability for water and wastewater treatment is feasible for adaptation and implementation by policy makers and stakeholders is recommended.