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Assessment of Chemical Mixing in Large Diameter Pipes

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Tampa Bay Water is looking to employ best management practices for effective mixing of sodium hypochlorite and other chemicals injected into large diameter pipelines.

The primary objective of this study was to investigate the degree of mixing in large diameter pipes at the Cypress Creek Facility following the original sodium hypochlorite chemical injector and determine stability of mixing and if needed, how the mixing effectiveness could be improved. The goal was to test alternative injector configurations to enhance mixing without significant capital expenditures.

The Cypress Creek Water Treatment Facility was designed to disinfect groundwater pumped from 30 wells owned and operated by Tampa Bay Water. To disinfect raw groundwater entering the facility, sodium hypochlorite is injected directly into the influent pipeline through a 3-inch injector, which protrudes 12 inches into the pipeline. Mixing occurs in 72, 84, and 42-inch diameter sections prior to ammonia injection to form chloramines. Although chlorine is not a conservative chemical, it was chosen due to the location and availability of appurtenances to perform analysis with minimal disruptions to the water treatment processes at the Cypress Creek Facility. Furthermore, the available resources at this facility offer the flexibility to consistently manage water characteristics and chlorine demand for the comparison purposes throughout the study.

Four different chemical injection scenarios were evaluated: 1) Original Injector, 2) Original Injector with dilution water, 3) 2-Inch Diffused Injector with dilution water, and 4) ½-Inch Small Diameter Injector. For this study the mixing characteristics were analyzed at influent groundwater flow rates of 10, 30, 50, and 70 mgd.

A comprehensive sampling program was conducted to determine the degree of mixing throughout the influent pipeline. Four existing sampling locations at 27, 46, 84, and 155 feet from the injection point were chosen and adjustable depth sampling probes were inserted at each location. At each flow rate, samples were simultaneously collected at each sample location at varying depths (10% intervals). Chlorine residual analyzers were used to continuously measure free chlorine concentrations, which were recorded with a datalogger. Grab samples were periodically collected and titrated to verify the residuals.

Results show significant differences in mixing patterns for each injector configuration at each flow rate and sample location. The diffused injector and small diameter injector provided the most effective mixing conditions within the shortest distance of the four injection scenarios investigated. The study concluded that mixing could be significantly improved without major capital improvements at the Cypress Creek Facility.
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Document Type: Research Article

Publication date: 2005-01-01

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