Page 317 - Water and wastewater engineering

7-34 WATER AND WASTEWATER ENGINEERING

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The GLUMRB design guidance is a maximum of 0.240 m /min · m of weir length.
Therefore the unit is acceptable for weir loading.
Because the trial unit meets all the criteria, it may be considered in the design evaluation.

Comment. If the trial selection did not meet the criteria, then iterative trials would be evaluated

to see if any of this manufacturer’s units is acceptable for the design.

7-7 OPERATION AND MAINTENANCE

The most important operation and maintenance task in softening is the selection of the appropri-
ate chemicals and adjustment of the dose to changing raw water quality and plant flow. Monitor-
ing of the chemical feed system to detect clogging of the lines and maintenance of the mixers
ranks second in the need for close O&M oversight. Encrustation is a significant problem. Annual
removal of calcium carbonate build-up during seasonal low-demand periods is customary.
Hints from the Field. Experience suggests the following:
• Open flumes are preferred over pipelines.
• Design of pipelines and flumes should include additional capacity for encrustation.

• Sludge scraper mechanisms should be kept in operation during low-flow periods when por-
tions of the plant are off-line but are not to be drained. This prevents the sludge blanket
from settling and “freezing” the scraper so that it cannot start moving again because the
settled sludge is too dense for the torque that can be applied.
• Sludge withdrawal (called blowdown ) must occur regularly to keep the draft tube in an
upflow clarifier open so that recirculation occurs in the mixing portion of the clarifier.

7-8 STABILIZATION

A stable water is one that exhibits neither scale forming nor corrosion properties. In the water
industry, a stable water is considered to be one that will neither dissolve nor deposit calcium. The
original objective of water stabilization was to adjust the pH of the treated water to prevent cor-
rosion of the water distribution system pipes by depositing a thin film of calcium carbonate as a
protective coating. Numerous investigations have revealed that although the Langelier index (a
method of calculating stability) is a reasonable predictor of the potential for CaCO 3 to precipitate
or dissolve, it does not predict how much CaCO 3 (s) will precipitate or whether its structure will
provide resistance to corrosion (Schock, 1999).
Although its validity as a method of corrosion protection is limited, the Langelier index is still
useful in predicting the potential for CaCO 3 to precipitate or dissolve. This is particularly valu-
able in designing the processes to reduce precipitation of CaCO 3 in the rapid sand filters and pipe
network as well as those processes used to reduce the corrosivity of reverse osmosis/nanofiltration
(RO/NF) treated water.
The Langelier Saturation Index (LSI)
Langelier (1936) developed the following relationship to predict whether or not a given water
will deposit or dissolve CaCO 3 :
LSI pH pH s (7-35)