| TECH-TIP - Tips for Optimizing Coagulation
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TECH-TIP - Tips for Optimizing Coagulation

Coagulation is a critical component of the surface water treatment process and plays an important role in the multi-barrier approach to enhancing particulate removal at surface water treatment plants.  During the coagulation process, a coagulant chemical is added to the water to help particles agglomerate to form larger particles that are more easily removed by downstream sedimentation and filtration processes at a typical conventional surface water treatment plant.  Coagulation is also used at direct filtration plants, although these plants have different particle formation objectives to account for the absence of a sedimentation process at these facilities.  The effectiveness of coagulation will impact the performance of downstream processes.  An optimized coagulation process can help to improve the quality of the filter influent water, which, in turn, can improve filter performance, run time, and filter effluent turbidity.

Typical coagulant chemicals, such as alum (aluminum sulfate) or ferric chloride, have a positive charge, which serves to neutralize the negative surface charge present on most particulate matter.  The positively charged metal ions formed by coagulant chemicals can also consume alkalinity and depress pH.  Since pH can impact several aspects of water treatment, such as organics removal and corrosivity, pH control is an important aspect of the coagulation process.  

Coagulants may be fed alone, or in combination with another chemical, such as a polymer or pH adjustment chemical.  The optimal coagulant chemical and dose varies from plant to plant – and even from hour to hour, depending on source water quality.  Because of this, it is important to establish procedures for control of the coagulation process, such as a Coagulation Control Strategy, for the water treatment facility.  Such a procedure helps to define the drivers for determining coagulant dose, critical process control testing parameters associated with coagulation, and standard operating procedures for modifying coagulant dose.  The Partnership for Safe Water’s treatment plant self-assessment guidance, Self-Assessment for Water Treatment Plant Optimization, contains a sample Coagulant Control Strategy that may be reviewed and customized to meet the needs of many surface water treatment facilities.  

In addition to the information included in Self-Assessment for Water Treatment Plant Optimization, this article contains tips for operators to consider when optimizing the coagulation process.

  • Water Quality Monitoring – Chemical dosing, as well as the effectiveness of the coagulation process, is dependent on several water quality factors, such as turbidity, pH, alkalinity, and total organic carbon.  Proactive staff monitor water quality upstream of the water treatment plant, in anticipation of changes in water quality that may impact coagulant dose.  The sooner a significant change in water quality is observed, the sooner steps can be taken at the treatment plant to prepare for and respond to the change.  Monitoring critical parameters in the water treatment plant, both upstream and downstream of coagulation, also allows for rapid recognition of and response to changes in water quality that may impact the coagulation process.  Streaming current detectors and zeta potential analyzers can also provide useful information about the charge state of the water and individual particles, to help fine-tune coagulant dosing.  Operators should not discount the benefits of visual observation of floc formation, in addition to water quality parameters.  By illuminating the flocculation basis and viewing the floc particles, operators can observe the particle formation to help evaluate the status of the coagulation process.  Commercial tools also exist to more quantitatively evaluate the characteristics of floc particles.  

  • Chemical Addition – For coagulant chemicals to be effective, they need to be added to the water.  A common cause of coagulation process failure is clogged chemical feed lines.  Staff may consider using flow meters or no-flow alarms on the coagulant feed lines so that a no-flow condition can be quickly recognized – or monitor a water quality parameter that is rapidly indicative of coagulation effectiveness.  It is also helpful to regularly verify the calibration of the coagulant feed pumps.  Upon being added to the water, coagulant chemicals commonly require high energy mixing for proper dispersal (rapid mix).  If proper mixing is not achieved, consider assessing whether modifications are needed to mixing procedures or to the location of the chemical addition point.  If multiple chemicals are added, but coagulation does not appear to be optimized, consider a special study to evaluate whether the order and location of chemical addition makes a difference in the effectiveness of coagulation.   

  • Jar Testing – Jar testing is a bench scale simulation of the coagulation process.  Its small scale nature allows operators to test a variety of chemical dosing and mixing parameters to evaluate their impact on the effectiveness of coagulation.  After a jar test has been completed, operators will test the water in the jars for turbidity, pH, alkalinity, organics, and other parameters that may be key to coagulation process control.  Jar testing is a helpful tool for evaluating potential changes in coagulant or coagulation conditions.  It is also helpful for determining the coagulation conditions that are optimal for treating a highly variable raw water source.  For jar testing to be most effective, operators or lab staff should be familiar and comfortable with testing procedures.  To assist with this, standard operating procedures for jar testing should be established, and it is recommended that staff running jar tests practice the procedure (or apply the procedure) on a regular basis.    

  • SOPs and Training – Regardless of how the coagulation process is controlled, it is important that the plant have standard operating procedures (SOPs) established to support the coagulation process.  While it is important that operators are empowered to make appropriate treatment decisions regarding coagulation, procedures should also clarify when to request additional assistance and who to call in this situation.  To avoid complacency and maintain operator skill, it is important that operators are adequately trained in the plant SOPs and that procedures are periodically reviewed, updated, and practiced.  By doing this, plant staff will be able to respond appropriate to protect water quality under critical conditions.  
Additional information about coagulation process optimization, including self-assessment questions, and operational recommendations, may be found in the Partnership for Safe Water’s treatment plant self-assessment guidance, Self-Assessment for Water Treatment Plant Optimization,  AWWA Manual M37, Operational Control of Coagulation and Filtration Processes, is another helpful utility resource for this critical water treatment process.