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Reagents
The choice and dosing of the chemical coagulant to be adopted for the removal of suspended solids from wastewater derive
from the results of laboratory evaluations by the so-called Jar test. Multiple stirrers with reproducible stirring speeds allow to
adopt standard conditions for the test, which are the basic requirement for reproducible results.
The standard conditions are related to:
• dimensions and shape of the vessel and stirring paddle
• time and speed of stirring during turbulent mixing
• evaluation criteria for the obtained results
The most diffused adopted conditions are:
• 1000 ml glass beakers (jars), tall form, Ø 105 mm
• stirring paddles 25 mm high, 75 mm wide, 1 mm
thickness
• turbulent stirring after chemical addition: 120 rpm, 120
seconds
• first evaluation of results after 5 minutes of sedimentation
The results can be evaluated based on different criteria:
A) Floc dimension evaluation with numerical degree (0=no flocs, 2=very small, 4=small, 6=medium, 8=large, 10=very
large).
B) Time from the addition of chemicals to the first appearance of flocs.
C) Evaluation of residual turbidity of supernatant, after a determined sedimentation time, by turbidimeter.
D) Measurement of the electro-kinetic potential of suspended particles on a sample taken immediately after the addition
and mixing of chemicals. Coagulation should be easier when the zeta potential of particles is very low.
E) Evaluation of filterability of clarified water by standardized membrane filters under pressure. The reduction of water flow
with time is related to the degree of clogging of filters due to residual unsettled suspended matter.
Other measurements are related to the pH of water after chemical treatment (lime raises the value while iron and aluminium
salts lower it) and the temperature of water at which the experiments are performed.
References: American Society for Testing Materials Norm ASTM D-2035-G4 T; Passino R. and M.Beccari (1970)
"Standardization of jar test in coagulation-flocculation processes for the removal of inorganic turbidities (in Italian). Acqua e
Aria, Milano, 1-10, October 1970"; Degremont (1978) "Etude de la coagulation et de la floculation des eaux. Methode n.703,
948-950. Memento technique de l'eau. 1200 pages, Paris"; U.S. Association of Environmental Engineering Professors
(1972) "Environmental Engineering Unit Operations and Unit Processes Laboratory Manual. J.T. O'Connor Ed. 350 ppg".
Leaching test
Leaching tests are used to simulate the behaviour of waste in a dump that is submitted to the leaching activity of rainwater.
There are two different methods that use leaching solutions of different types:
F) Acetic acid, proposed for dumps receiving both organic and inorganic wastes;
G) Carbon dioxide-saturated water, for dumps receiving only inorganic wastes.
Waste sample to be examined: if a liquid phase is present, it must be removed by filtration or centrifugation and preserved
in a refrigerator at 2-4 °C. The granulometry of the solid phase must be lower than 9.5 mm (standard sieve).
Waste extraction: 20-25 g of the solid phase are weighed with a precision of ± 0.1 g.
Method a) A volume of distilled water corresponding to 16 times the weight of the sample is added.
The pH of the mixture is maintained at 5±0.2 using 0.5 N acetic acid. The stirring is performed for 24 hours, controlling the
pH every 15 minutes during the first hour and then every hour. The pH is corrected to 5±2 using again 0.5 N acetic acid. If
after 24 hours of stirring the pH is higher than 5.2, a new correction is made and the stirring is prolonged for another 4
hours.
Method b) The leaching solution is given by a volume corresponding to 20 times the sample weight of distilled water
saturated by carbon dioxide for not less than 15 minutes. The stirring is performed for 6 hours without any pH correction.
Stirring speed: must be able to maintain continuously mixed the solid phase with the leaching solution.
Analysis of extract: the leaching solution and the liquid phase if present in the original sample are filtered by a membrane
filter of 0.45 µm porosity and submitted to analysis for the single metals by the methods commonly used for waters.
• volume of wastewater
• time and speed of stirring during flocculation
• 600 ml wastewater samples
• stirring
height
of
the sample
slow speed flocculation: 30 rpm, 25 minutes
9
paddles,
the middle
height
of
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