Nova-Tech Floc-Tester ET 750 - Manuel (Page 13)

13 Floc-Tester_ET740_ET750_1 12/2008
GB Analytical metods
Laboratory evaluation by Jar test
The choice and dosing of the chemical coagulant to be adopted for the removal of
suspended solids from wastewaters derive from the results of laboratory evaluations by
the Floc test . Multiple stirrers with reproducible stirring speeds allow to adopt standard
conditions for the test, that are the basic requirement for reproducible results .
The standard conditions relate to:
- volume of wastewater in each stirring position
- dimensions and shape of containing vessel and stirring paddle
- time and speed of stirring during turbulent mixing phase
- time and speed of stirring during flocculation phase
- evaluation criteria for the obtained results .
The most diffused adopted conditions are:
- 1000 ml glass beakers (jars), tall form, 105 mm of diameter
- 600 ml wastewater samples
- stirring paddles 25 mm high, 75 mm wide, with a thickness of 1 mm
- stirring height of paddles roughly at middle height of wastewater sample
- quick turbolent stirring after chemical addition: 120 rpm for 120 seconds
- slow speed stirring during flocculation: 30 rpm for 25 minutes
- first evaluation of results after 5 minutes of sedimentation after the end of
stirring .
The results can be evaluated on the basis of different criteria:
a) evaluation of floc dimensions with a numerical degree of merit (0 = no flocs, 2 = very
small hardly visible, 4 = small flocs, 6 = medium size flocs, 8 = good size flocs, 10 = very
large flocs) .
b) time from the addition of chemicals to first appearing of flocs
c) evaluation of residual turbidity of supernatant, after a determined sedimentation time,
by turbidimeter
d) measurement of electrocynetic potential of suspended particles on a sample taken
immediately after the addition and mixing of chemicals . Coagulations should be easier
when zeta potential of particles is very low .
e) evaluation of filterability of clarified water by standardized membrane filters under pres-
sure . The reduction of water flow with time is related to the degree of clogging of filters
due to residual unsettled suspended matter .