Flocculation - Tech Talk 115
Polymers or polyelectrolyte flocculants can be used to concentrate waste solids collected from recirculating systems and other aquaculture applications. An example would be the concentration of backwash wastes from microscreen filters. The flocculation process involves the optimization of the electric charge of the wastes and bringing floc particles together to form larger particles that can easily settle in a sedimentation basin.
A wide range of polymers are available. They include organic polymers that are cationic (positively charged), anionic (negatively charged) or nonionic (no charge). Because wastewater particles are generally negatively charged, cationic polyelectrolytes are more often used to neutralize or reduce the negative charge on the particles, thereby allowing aggregation and floc formation.
Since everyone's water and waste chemistry is different, a jar test must be completed to determine the optimum flocculant dosage and the duration of mixing. The standard jar test apparatus uses several containers, each with a sampling port about 10 cm below the water line, so that samples may be taken during the test. A paddle stirrer is placed in each container. These stirrers are all driven by a variable speed motor from 0 to 300 rpm. Polymer solutions are prepared according to the manufacturer’s recommendation and added in varied concentrations to the containers along with water being tested.
The flocculation process includes 1) a period of rapid or flash mixing when flocculants are added to the wastewater and stirred at high speed, 2) a period of slow mixing when the wastewater is slowly stirred to form large flocs that can be easily settled and 3) a period of rest when the floc formed is allowed to settle. The jar test procedure is a small scale representation of the larger scale treatment system and is used to optimize flocculant dosage and time periods for mixing and settling. With this information you can design either a batch or a continuous flow sedimentation system.
A control sample is included to evaluate the effect of the polymer addition. The percent reduction of the wastewater parameter of interest is calculated in relation to this untreated, unflocculated, wastewater sample. For most applications, the evaluation of the effectiveness of polymer treatment is completed by analyzing the percent reduction of turbidity, which is an indicator of suspended solids removal. In aquaculture and other applications, the percent reduction of phosphorus may also be used to evaluate effectiveness of treatment. It has been shown that the majority of the phosphorus discharged from intensive aquaculture systems (50-85%) is contained in the settleable solids. Thus, any method that removes solids will also reduce the total phosphorus.