Nova-Tech Floc-Tester ET 750 - Manuel (Page 12)

12 Floc-Tester_ET740_ET750_1 12/2008
GB Analytical metods
Coagulation and flocculation of wastewaters
Chemical coagulants are added to wastewaters for the primary purpose of removing
suspended solids and phosphorus . The addition of chemicals ruduces heavy metal con-
centrations and improves disinfection efficiency . Chemical coagulation can be obtained by
adding defined amounts of lime (calcium hydroxide), alum (aluminium sulfate) or iron salts
(ferric or ferrous) to wastewaters .
The coagulation-sedimentation process typically involves:
• Injection and mixing of the coagulant that neutralize the electric charge, prevalently
negative, on suspended particles . Good and rapid mixing of the coagulant and
wastewater is important to ensure efficient use of the chemical . Typical detention
times in plant basins for turbolent mixing are 15 to 120 seconds .
• Agglomeration of the coagulated particles into large settable flocs. This
agglomeration is accomplished by stirring the water slowly so to allow the solids
formed by the addition of chemicals grow in size so that they can be removed by
gravity settling .
• The stirring is usually performed by slowly rotating paddles during 10 to 30 minutes.
• Sedimentation of the flocculated material by gravity separation. The solids are allowed
to settle by gravity from the chemically treated standing wastewater . The effluent
from the chemical settling tank is very clear and does not usually require further
filtration .
The quantity of chemical coagulant required to achieve good coagulation varies with time
and from wastewater to wastewater .
Typical coagulant doses are:
75 to 250 g/m3 for alum,
45 to 90 g/m3 for ferric chloride,
200 to 400 g/m3 for lime .