Charting a Course in Coagulation Control

(March, 2003, as appeared in Opflow Magazine)

By Gary Logsdon, Dennis Ciemniecki, Alan Hess, and Michael Chipps. Gary Logsdon is a senior water research engineer with Black & Veatch in Cincinnati, Ohio; contact him at (513) 860-2021 or logsdon@bv.com. Contact Dennis Ciemniecki, vice president of water quality for Elizabethtown, N.J., Water Company at (908) 301-3153 or dciemnie@etownwater.com. Alan Hess is a senior consultant with Black & Veatch in Philadelphia. Michael Chipps is a principal research scientist with Thames Water Utilities in Redding Berks, UK.

Water treatment plant operators are responsible for carrying out a system's water production goals, some of which may conflict. For example, producing low cost water isn't always easy when minimizing turbidity and enhancing clarification are the water quality goals. Jar tests are a routine part of these processes. Anyone who has carried out jar tests knows that these are time-consuming procedures, although they are undoubtedly excellent for determining optimum coagulant dose.

In addition to jar tests, combining plant operation data with operators' practical experience on historical dosing tables or charts is an excellent way to determine the best coagulant doses for different raw water quality scenarios. Such charts and tables can be valuable plant resources for optimum operation of the coagulation process, both now and in the future.

Dosage tables or charts based on historical experience enable swift decision making on coagulant dosage. The tables can indicate when to turn doses up or down and by how much, ensuring the right dose of chemicals to consistently achieve treatment goals. Maintaining the charts can become a continuous improvement exercise, in which the tables are augmented with additional information when new situations arise. Comparing historical data to data from other monitoring techniques, such as streaming current monitors, is also a way to cross-check coagulant dosing.

Historical Water Data

Historical data, which is site specific, has been employed to determine coagulant dose for several decades. When two or more neighboring water utilities are using the same coagulant chemicals and treatment processes to treat water from a common source, one utility's treatment information may be helpful to its neighbor. Such information, however, should be considered a guide rather than an absolute answer to treatment, because the process design may differ sufficiently from plant to plant to require different dosages, even for similar source waters. Data collected from a single plant also may not provide precise guidance for determining future coagulation dosages at that plant - but may provide valuable guidance for setting up jar tests.

Data tables or charts can be based on local records over a period of years; for example, a chart of a wide range of raw river water turbidity compared to effective coagulant dosages in mg/L. Experience may show that effective coagulant dosage relates to factors other than turbidity, such as water temperature, total organic carbon (TOC) concentration, alkalinity, and coagulation pH, indicating a possible need for different dosage charts for different ranges of water quality.

Caution: Historical data may not be useful if treated water quality goals have changed since the original data were developed. Chemical dosages that produced 1 NTU or 0.5 NTU filtered water may not be appropriate for producing 0.1 NTU filtered water. Also, when chemical strength changes or a different coagulant chemical is used, a different historical database is needed.

Charting Turbidity Data

To prepare a coagulant dosage chart based on raw water turbidity, use only data:

• developed since the present filtered water quality goal went into effect.
• from days when the present filtered water quality goal was attained.
• developed using the current coagulant chemical. If the same chemical was used but supplied by different manufacturers, verify that the chemical strength is the same.
• that reflects a temperature range of about 10° F or 5° C, such as 32° to 40° F or 0° to 5° C. When temperature varies over a wider range, several charts may be needed.

Review the pH of coagulated water for successful treatment during different seasons. If water temperature seems to influence the pH value needed for successful treatment, specify the applicable pH range for each coagulation chart.

Charts or tables showing coagulant dosage plotted as the dependent variable (Y-axis) and raw water turbidity as the independent variable (X-axis) should not show a wide spread of dosage values for a given turbidity.

Consider Other Factors

If the range of dosages associated with a given raw water turbidity seems too wide for effective application of the chart data, consider using the chart as a guide for selecting the range of dosages to try in jar testing. During storm events, determine if the turbidity and dose relationship is different when turbidity is rising compared to when it is decreasing.

If raw water turbidity and coagulant dosage are not closely related (if the data show considerable scatter instead of a linear plot), one or more additional factors may be influencing the coagulation chemistry. When several variables influence coagulation, developing tables, charts, or graphs to determine coagulant dosages in the future may become too complex to be practical. The benefit of a simple, reliable means of estimating coagulant dosages is lost if dozens of charts are needed for all treatment situations that may be encountered.

Multiple variations of raw water quality sometimes can be dealt with in a data table by setting up a matrix that allows for variations in two parameters, such as turbidity and TOC. Some kinds of natural organic matter (measured as TOC) exert a coagulant demand, which could result in the need for higher coagulant dosages when TOC is in a higher range.

To develop a matrix table for coagulating water with variations in turbidity and TOC, prepare a raw water turbidity column and two or more columns for different ranges of TOC. Coagulant dosages would be filled in for each of the TOC ranges for every turbidity range in the table.

Applying Historical Dosing Data

Appropriate coagulant dosages will vary according to source water, treatment plant design, rate of flow, and other site-specific factors. Depending on local observations, the words in the table may change; for example, "increasing turbidity" and "decreasing turbidity" might be substituted with "low TOC" and "high TOC," or "low color" and "high color."

A series of similar charts covering all temperatures recorded allows the operator to adjust coagulant dosage quickly, confidently, and purposefully, without repeating jar tests. Such charts particularly benefit facilities operating on rivers, where delays in responding to rapid turbidity changes would negatively affect the coagulation process performance and potentially affect plant performance.

The charts generate a high degree of confidence because they show the dosages that delivered the best performance under identical past conditions. They also lay out the dosage plan, so the operator knows how the dosage will change and can prepare accordingly for either increasing or decreasing turbidity. Taken together, such charts increase effective decision making, responsiveness, and treatment performance.

Operators must be alert to conditions in the watershed. Source water quality changes related to watershed changes can cause the relationship between turbidity and chemical dosage to change and, thus, affect the validity of the dosage chart. At least, a decline in source water quality should spur a review of charts to determine if modification or updating is necessary.

If there is doubt about plant performance, data from what is actually happening at the plant must take precedence over the chart data. If the chart fails to provide the right results in a particular circumstance, study the conditions to determine what is different or unique about them and what can be learned for the future.

Summary

An AWWA Research Foundation study on water filtration plant operation and maintenance, Filter Maintenance and Operations Guidance Manual, showed that operators at most filtration plants do not determine coagulant dosage by one technique alone, but instead use multiple techniques. These include jar tests, pilot filters, zeta potential, streaming current monitors, and coagulant dosage charts. Good operating practices involve using multiple sources of guidance for setting coagulant dosages.

Historical dosage charts or tables are a technique for determining coagulant dosage that can be a time-saving tool at treatment plants where source water quality and coagulant dosage change from time to time. Using them in combination with other means of assessing coagulant dosage increases the operators' information base for selecting the most effective dosages.

Copyright 2003 Opflow Magazine