In this study coagulation-flocculation method was evaluated to treat municipal wastewater using aluminum sulphate and calcium hydroxide as coagulants, and polyacrylamide as flocculant by varying pH from 5 to 7 and coagulant dosage from 100 to 300 mg L-1 at constant flocculant dosage (10 mg L-1), time (1 min for rapid mixing followed by 30 min for slow mixing), temperature (30°C ± 2°C), agitation speed (150 rpm for rapid mixing followed by 50 rpm for slow mixing) and settling time (2 h) by jar test method. The responses were percentages reduction in turbidity, chemical oxygen demand (COD) and biochemical oxygen demand (BOD). The sample was real-time wastewater from sewage treatment plant of Salalah Sanitary Drainage Services (SSDS) Company, Salalah, Sultanate of Oman. The trials were performed with single and mixed or combined coagulants. Response surface methodology (RSM) based central composite design (CCD) was used for the optimization of pH and coagulant dosage to maximize percentage reduction in turbidity, COD and BOD. The maximum of 99.7%, 98%, and 95.5% were achieved for turbidity removal using aluminum sulphate, calcium hydroxide and combined coagulants respectively. However, maximum of 60%, 58.1%, and 42.5% were achieved for COD removal using calcium hydroxide, combined and aluminum sulphate respectively. Moreover, maximum of 79.5%, 78.5%, and 54.5% were achieved for BOD removal using aluminum sulphate, calcium hydroxide and combined coagulants respectively. The obtained results indicate that treatment using combined coagulant is effective for removal of turbidity and COD, and could be used for the treatment of municipal wastewater.

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The studies on utilization of natural coagulants prepared from seeds of Citrullus lanatus (watermelon) and Cucumis melo (cantaloupe) provide insight on sewage wastewater treatment. The results were statistically analyzed by response surface methodology (RSM) based central composite design (CCD), and supported with results from Fourier Transform Infrared (FTIR) spectroscopy for functional groups present in coagulants. The batch coagulation studies were performed varying process parameters such as pH (5-7) and coagulant dosage (50-150 mg/L) at constant flocculant dosage of 10 mg/L using jar test. In jar test, coagulation was carried out with fast mixing (Mixing at 150 rpm for 1 min) followed by slow mixing (Mixing at 50 rpm for 30 min), and the final mixture was allowed to settle for 1 h at the temperature of 30±2ºC. Design-Expert® version 12 software was used to optimize pH and coagulant dosage for effective sewage wastewater treatment using natural coagulants with the objectives to maximize percentage reduction in turbidity, biological/biochemical oxygen demand (BOD), total suspended solids (TSS), and chemical oxygen demand (COD). The RSM results reveal that the optimum C. lanatus coagulant dosage of 72.3 mg/L at pH 5 achieved maximum efficacy removal of TSS and BOD by 92.8% and 92.1% respectively. The FTIR results show that the coagulants from plant seeds contain almost similar functional groups. Thus, it can be concluded that the coagulant prepared from C. lanatus would be more effective for treatment of sewage wastewater.