Coagulation and Flocculation Essay

1.0 Introduction In this lab, we are conducting two experiments on natural surface water. The first experiment is to conduct jar tests to estimate an optimum dosage of iron salt to remove suspended solids. The seconds experiment is to observe the rate of floc formation nad sedimentation. 2.0 Background Coagulation and Flocculation: Coagulation and flocculation are important part in water and wastewater treatment. Coagulation is the destabilization of particulate matter by physical or chemical processes. Flocculation is the formation of larger particles that will settle out of the solution. They are usually the first form of treatment of water and wastewater to remove suspended matter or color. Jar tests are used as a control test for plant operations. Aluminum or iron salts can be used to for coagulation of particles and to form flocs that can settle out. Coagulation and flocculation tests provide the optimum dosages to remove turbidity and color, along with secondary effects such as adjusting pH. Jar tests also provide information on the amount of energy needed to provide the coagulation and flocculation treatments, as well as settleability of the flocs, and clarity of the water. They can also be used to study basic processes, for instance, kinetics of reactions and removal of constituents. 3.0 Procedure Determination of Optimum Coagulant Dosage To determine the optimum coagulation dosage, a series of jar tests were conducted. First, 400 mL of clay and sodium bicarbonate amended DI water was measured and poured into a 500 mL Erlenmeyer flask using a graduated cylinder. The initial pH of that sample was then measured and recorded. The predetermined dose of coagulant was poured into a 100mL graduated cylinder and DI water was added to reach a total solution volume of 100 mL. A stir bar was added to the Erlenmeyer flask along with the coagulant dose. The flask was then placed on a magnetic stir plate was rapidly mixed for one minute. After a minute of rapid mixing, the speed was reduced to low and the solution underwent slow mixing for ten minutes. The flask was then removed from the magnetic plate and was allowed to settle. Samples were extracted from the top of the flask after five, ten, and fifteen minutes had elapsed using a 10 mL sampling syringe. Special care was taken to not disturb the sediment while sampling. The sample was transferred from the syringe to a vile, shaken, and placed in a Turbidimeter to determine the turbidity. The turbidity was recorded and the vile was emptied and rinsed between each sample. After fifteen minutes had elapsed and the last turbidity reading had been recorded, the pH of the sample was measured and recorded. The sample was then dumped into a specified waste container, the flask was rinsed with tap water and DI water, and the entire experiment was repeated using a new specified coagulant dose. 4.0 Results and Discussion After plotting turbidity against dosage from our results, we found that the optimum dosage of coagulant to be 1000 mg/L Fe2O3 as shown in Figure 1. Also, longer settling times produced lower levels of turbidity, with 15 minutes being most successful. Optimum pH for a coagulant is determined empirically from laboratory testing by keeping dosage constant and testing a pH range for optimal coagulation. Generally after adding Fe2O3, final pH decreased. Although we did not perform this in the lab, our samples had best success with a pH around 6.5 as seen in Table 1. Mixing speed is also important in coagulation and flocculation. Initially â€Å"flash mixing† is used, where high mixing speeds disperse the coagulant evenly throughout the container. Later, slower mixing speeds are used to promote particle collisions, which lead to larger floc formations. The lab is performed this way because higher speeds will help disperse the coagulant but will break up the flocs that form. By reducing the speed to slow after one minute, it allows for an even dispersion but also the formation of flocs.