Temperature

 

 

Disinfectant

Temperature conditions also influence the effectiveness of the disinfectant.  The higher the temperature of the water, the more efficiently it can be treated.  Water near 70 to 85° F is easier to disinfect than water at 40 to 60° F.  Longer contact times are required to disinfect water at lower temperatures.  To speed up the process, operators often simply use larger amounts of chemicals.  Where water is exposed to the atmosphere, the warmer the water temperature the greater the dissipation rate of chlorine into the atmosphere.

 

 

Corrosion Control

The rate of chemical reactions usually increases as temperature rise.  Since chemical reactions are involved in corrosion, temperature generally has the effect of increasing the corrosion rate.  However, there are several exceptions to this general rule.

As the temperature of water increases, the amount of calcium carbonate that can remain dissolved in water is reduced.  This means that protective calcium carbonate film formation may be improved under this circumstance.  The deposition may be so enhanced, however, that severe scaling may result, clogging hot water lines and fittings, and coating hot water heating elements.  Heavy scaling of heating elements will increase the amount of energy used, and may result in their failure due to overheating.

Higher temperature can also alter the form of corrosion.  In cold water, pitting may be the dominant form of attack on anodic metals, resulting in short service life.  Although the rate of corrosion may increase as the temperature rises, the form of corrosion in the same water may change to a uniform or more generalized metal loss, actually resulting in a longer service life for the pipe.

Another special case of temperature effects is the influence on zinc/iron galvanic corrosion.  In hot water heaters with temperature exceeding 140° F, the normal condition of zinc being anodic to iron may reverse, resulting in the zinc becoming cathodic and the iron corroding.  Penetration of the steel tank wall can result.

 

 

Flocculation

Lower water temperature requires higher mixing turbulence, so speed should be increased.  The rate of all chemical reactions decreases with temperature.  For example, 50° F water slows down chemical reaction times to one half their speed at 70° F.

 

 

Sedimentation

A consideration in sedimentation is the effect of water temperature changes.  The settling rate (settling velocity) of a particle becomes much slower as the temperature drops.  The colder the water temperature becomes, the longer particles take to settle out.  Water is similar to syrup in this regard.  The colder syrup becomes, the longer it would take a marble to settle to the bottom of the container.  This means that longer time periods (lower flows) are required for effective settling at colder water temperatures, or that chemical dosages must be adjusted for the slower settling velocities. Temperature changes may also require that jar tests be performed to establish optimum floc settling rates.  Temperature changes are usually gradual over time so sudden changes in temperature are unlikely unless a source water change is made.

 

 

Solid-Contact Units

The sludge blanket is sensitive to changes in water temperature.  Temperature density currents tend to upset the sludge blanket.  Loss of the sludge blanket will affect the performance of the filters.

Changes in water temperature will cause changes in the density of the water.  Changes in density influence the particle settling rate.  In extremely cold water, consider using polymers, activated silica, powdered calcium carbonate, or some other weighting agent to aid sedimentation without affecting coagulation.  Simple heating by the sun on the wall of the tank or on the flocculant particles within the container will cause a certain amount of carryover of solids to occur.  Operators who are not familiar with solids-contact units tend to become upset and overreact because of the potential carryover problem.  This phenomenon is not a matter of serious concern because as the position of the sun changes, the convection currents change.

 

 

Lab Procedure

  1. Discussion

    Temperature is one of the most frequently taken tests in the water industry.  Accurate water temperature readings are important not only for historical purposes but also because of its influence on chemical reaction rates,, biological growth, dissolved gas concentrations, and water stability with respect to calcium carbonate, in addition to its acceptability by consumers for drinking.

  2. What is Tested

    Raw and Treated surface water with a common range of 5 to 25°C.
    Well water with a common range of 10 to 20°C.

  3. Apparatus

    One NBS thermometer for calibration of the other thermometers.
    One Celsius, mercury –filled, 1° subdivided thermometer

  4. Thermometer Styles

    1. Total immersion.  This type of thermometer must be totally immersed when read.  Readings with this type of thermometer will change most rapidly when removed from the liquid to be recorded.
    2. Partial immersion.  This type thermometer will have a solid line (water-level indicator) around the stem below the point where the scale starts.
    3. Dial.  This type has a dial that can be easily read while the thermometer is still immersed.  Dial thermometer readings should be checked (calibrated) against the NBS thermometer.  Some dial thermometers can be recalibrated to read at a set temperature against the NBS thermometer.

  5. Procedure

    1. Collect as large a volume of sample as is practical.  The temperature will have less chance to change in a large volume than in a small container.
    2. Immerse the thermometer to the proper depth.  Do not touch the bottom or sides of the sample container with the thermometer.
    3. Record temperature to the nearest fraction of a degree which can be estimated from the thermometer available.
    4. When measuring the temperature of well water samples, allow the water to continuously overflow a small container.  Place the thermometer in the container.  After there has been no change in the temperature reading for one minute, record the temperature.  The temperature of water samples collected from a distribution system mainly depends on the soil temperature at the depth of the water main.

  6. Precautions

    To avoid breaking or damaging a glass thermometer, store it in a shielded metal case.  Check your thermometer’s accuracy against the NBS certified thermometer by measuring the temperature of a sample with both thermometers simultaneously.  Some of the poorer quality thermometers are substantially inaccurate (off as much as 3°C).