Optimization of coagulation process in drinking water treatment using Pistacia atlantica by response surface methodology (RSM)

Coagulation is one of the most important processes in water treatment. Water sources can be contaminated with pathogenic microorganisms, soluble and suspended solids, particles that change the taste, odor and color of water and make water unsafe for human consumption. The presence of such impurities and contaminants reduces water quality and must be effectively removed. Coagulation / flocculation is performed as the first step in water treatment processes to remove turbidity from suspended solids and colloidal particles. The materials used in this stage of water treatment can be inorganic coagulants, synthetic organic polymers or coagulants of natural origin. Aluminum sulfate (alum) is a common coagulant used worldwide in water treatment. Despite the high efficiency of alum in removing turbidity, the volume of sludge produced after the coagulation process is high, which raises concerns about its harmful environmental effects. The use of synthetic polymers and the presence of residual monomers is undesirable due to nerve toxicity and strong carcinogenicity. Therefore, the use of natural coagulants received more and more attention. In contrast to chemical coagulants, natural coagulants are safe, eco-friendly and generally toxic free. Natural coagulants have been found to generate not only a much smaller sludge volume of up to five times lower but also with a higher nutritional sludge value. As such, sludge treatment and handling costs are lowered making it a more sustainable option. The raw plant extracts are often available locally and hence, a low cost alternative to chemical coagulants. Since natural coagulants do not consume alkalinity unlike alum, pH adjustments can be omitted and this provides extra cost savings. Natural coagulants are also non-corrosive. In this study, Pistacia atlantica seeds was used as a natural coagulant. The seeds were obtained from coriander tree in the Zagros Mountains (Lordegan province, Rig region). These husks of the seeds were removed manually. This biomass was first subjected to coarse grinding using manually mill followed by Bosch electric mill for fine grinding. 5 g/l of seeds milled was mixed with distilled water and stirred at 500 rpm using electric stirrer for 10 min at room temperature. The filtered extract containing the active coagulant was then used in the coagulation process. Synthetic turbid water was prepared by adding kaolin a stock kaolin suspension to distilled water for all coagulation experiments. The stock kaolin suspension was prepared by dissolving 10 g of kaolin powder in 1 L of distilled water. The suspension was stirred at 20 rpm for 1 h to achieve uniform dispersion of the kaolin particles. The suspension was then permitted to stand for 24 h to allow for complete hydration of the kaolin. This suspension was used as a stock solution for the preparation of water samples of varying turbidity for the coagulations tests. 1 molar solution of HCl and NaOH were added into the synthetic turbid water to adjust the pH values. The coagulation activity of Pistacia atlantica extract was determined by Jar test 300 ml of Synthetic turbid water of different initial turbidities was filled into 600 ml beakers. Pistacia atlantica extracts with variable doses and densities were added to these beakers and mixed at 100 rpm for 4 min for rapid mixing using mechanical stirrer. The mixing speed was reduced to 40 rpm for another 25 min. The suspensions were then allowed to settle down. After 30 min of sedimentation, clarified samples from beakers were collected and their residual turbidities were measured using turbidity meter. The initial turbidity was studied 100 NTU and the solvent 0.5 M NaCl were employed. All the experiments were repeated at least twice for consistency and results averaged. On the other hand, the response surface methodology (RSM) of central composite design (CCD) at 5 levels was used to evaluate and optimize the reaction conditions of the coagulation-flocculation process in drinking water treatment. Three parameters of dose, pH and contact time, which are important and influential factors on the coagulation-flocculation process, were used as variables in the experimental design. A quadratic polynomials model with a value of R2 = 0.9654 (correlation coefficient) was presented to the data. Model accuracy tests showed a good correlation between actual and predicted values. In the treatment of drinking water obtained from experimental results, a significant reduction in turbidity, i.e. 95.13% with a dose of 90.53 mg / l, pH = 9.97 and a contact time of 44.86 minutes was obtained for Pistacia atlantica coagulant. According to the results of Pistacia atlantica seed as a coagulant in drinking water treatment showed high potential.