Course: BFM 203 Water Treatment and Control

Lecture

(7/7)

Relevant Tests

Operators perform laboratory tests to analyze a variety of factors.

Alkalinity

Calcium, magnesium, and sodium are all alkali earth metals and form carbonates, bicarbonates, and hydroxides that increase the effective hardness of the water. Alkalinity refers to the capacity of water to neutralize acids. Alkalinity level is expressed in mg/L of calcium carbonate. Alkalinity is the result of bicarbonates, carbonates, and hydroxides of the metals calcium, magnesium, and sodium. The term "alkaline" does not necessarily mean "basic" because slightly acidic water may have the ability to neutralize stronger acids, therefore meeting the definition of alkalinity. Accomplished by titrating water to a pH of 4.5 using sulfuric acid. The amount of acid required to achieve a pH of 4.5 is used to determine the alkalinity of the water. It is important to note that alkalinity and basic conditions are not necessarily the same thing.

pH and Temperature

The pH of water indicates the intensity of its acidic or basic strength. Mathematically, pH is the logarithm of the reciprocal of the hydrogen ion activity or the negative logarithm of the hydrogen ion activity. Different pH meters and electrodes are available. Follow manufacturer's instructions for calibration and storage as each manufacturer has its own approved methods. Standardize the meter against a buffer solution before each day/shift. (4.0, 7.0, and 10.0 are standards available). Rinse the probe after use with distilled or de-ionized water only. Some pH meters have temperature compensation which adjusts the meter for changes in electrode response with temperature. When collecting and analyzing samples for pH, large changes in temperature between the sample and the surrounding air may cause a delay in response time of the meter. Be patient and wait for the probe to lock in the value before recording the result.

You will use one of three thermometer types in your laboratory work:

- Total immersion
- Partial immersion
- Dial

In addition, you may be required to use either Fahrenheit (F) or Celsius (C) scales. Use the following conversions to change from one scale to the other.

C= (F-32F) / 1.8

F = 1.8 (C) + 32F

Use the correct thermometer for each analysis and routinely check the probe temperature reading against a calibrated standard.

Turbidity

Turbidity is an expression of the physical cloudiness of water; turbidity in the effluent is an indicator of the effectiveness of the treatment process. Turbidity is caused by the presence of suspended matter such as silt, finely divided organic and non-organic matter, and microscopic plants such as algae. The Nephelometeric Turbidity Unit (NTU) is an empirical value based on the amount of light that is scattered or absorbed by particles of a polymer reference standard called formazin which produces particles that scatter light in a reproductive manner.

Chlorine Demand

Chlorine demand is the difference between the amount of chlorine applied (dose) and the amount of chlorine remaining (residual) after a specific contact time. This test determines the best chlorine dose required to achieve specific goals. Dose sample water with specific amounts of chlorine and measure the residual after the specified contact time. The sample with the chlorine residual closest to the goal demonstrates which dosage satisfies the requirements. Remember that testing for chlorine demand gives a starting point for chlorination, but the actual dose may vary with changing conditions.

Chlorine Residual

You will work with three types of chlorine residuals. Free chlorine residual is the portion of the total available residual chlorine composed of dissolved chlorine gas, hypochlorous acid, and/or hypochlorite ion remaining in water after chlorination. Combined residual is the chlorine-ammonia compounds such as monochloramine, dichloramine, and trichloramine. Total chlorine residual is the sum of the free and combined residuals. Total residual will always be equal to or higher than free residual. Combined residual should be a very small portion of total residual when ammonia is not added for chloramination.

We may use one of two methods to measure chlorine residual. The amperometric titration method is performed in a lab. It generally provides a more precise measurement than the DPD colorimetric method. It does, however, require expensive instrumentation and operators with a high level of skill. The DPD colorimetric method can be performed with a field comparator kit. It is easy to use and inexpensive. Note that the color wheel is still an approved method of chlorine analysis; it just helps us to compare visually instead of by using electronic measurement of color change.

Coliform Bacteria

Tests for Coliform bacteria are critical because improperly treated or unprotected water supplies may contain pathogens. Coliform bacteria is tested as an "indicator organism" because specific pathogens are difficult to detect in a timely and cost effective manner. The presence or absence of Coliform bacteria is a good index of the degree of bacteriologic safety of the sample water. Two methods may be used to test for Coliform bacteria.

In the membrane filter method (MF), operators filter a known volume of water through a membrane filter of optimum pore size for full Coliform bacteria retention. The filter is then placed in contact with an agar medium to provide nutrients for growth and incubated. After incubation, the filters are observed and Coliform colonies are counted and recorded as density of Coliform per 100 mL of water sample.

The presence-absence Method (P-A) is a modified version of the MPN method. Using this method, operators add 100 mL of water sample to 50 mL of sterile P-A broth in a 250 mL milk dilution bottle. The sample is incubated. After incubation, a distinct yellow color or gas formation in the bottle indicates a presumptive positive sample and must be confirmed in the same manner as the MPN method. Note that Multiple Tube Fermentation (Most Probable Number or MPN) is not normally used to analyze potable water).

Jar Tests

Jar tests are designed to show the effectiveness of chemical treatment in a water treatment facility. Six jars (see photo to the left) are typically used to emulate the coagulation/flocculation process. Varying doses of coagulant chemicals are tested to see which dose or chemical combination achieves the best results. Jar testing is an under-utilized tool in most water treatment systems. Regular jar tests can be used to fine tune treatment processes, simultaneously improving efficiency and effectiveness.

All samples and laboratory tests are useless if information is not evaluated in a timely manner and used to optimize treatment processes and verify that the water produced is safe and