Bench-scale jar tests

Bench-scale jar tests in is a standard method to investigate wastewater flocculation and sedimentation properties. Jar tests aiming to determine the best conditions for coagulation and flocculation include selection of the type of coagulants and flocculants, dosage, and the most efficient pH level at which to operate. The name “jar tests” refers to the procedure of using several 1-2 litre jars to conduct the experiments in. According to the Nalco Water Handbook ⁴, “observational results in a jar test remain the best method for coagulant selection”. Bratby ³¹ also claims jar testing to be an ideal tool for coagulation optimisation, with many studies in the past 85 years supporting the validity and applicability of the method. Further supporting the choice of method, Edzwald ³² stated that:

“The chemistry of coagulation for sedimentation and DAF plants is similar, therefore coagulant dosages and pH conditions are identical to forming flocs“.

Methods

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4.2.2 Jar test experimental design

The jar test experiments were designed and executed in the following order:

1. Determination of best coagulant and pH.

2. Determination of the best polymer in combination with the coagulant from step one.

3. Dosages optimisation

In addition to the three primary goals above, the influence of mixing time was investigated, and one experiment regarding H2S removal was conducted.

The performance of five different coagulants (Table 7) was examined in step one. In the first step, a coagulant dosage equal to the one used in the full-scale process (10 mg/l calculated as metal) was used. Removal of COD, oil, phosphorus, and turbidity was investigated as a function of pH. The pH pre-coagulant addition was varied in steps of one, from pH 5 to 10. One sample was taken for each coagulant and pH combination.

The results were then plotted as the removal rate, remaining contaminant or both, parameters vs pH. The best coagulant was then chosen for the next step, based on several criteria.

In step two, the coagulant chosen in step one was combined with different polyelectrolyte flocculants. The pH, pre-coagulant addition, was varied from 7 to 9. The type of coagulant was the independent variable while the measured contaminants were the dependent variables. The coagulant dose was kept constant, at 10 mg/L of Fe³⁺. In the third step, the optimal chemical dosages were determined. The wastewater was treated with coagulant dosages (mg/l calculated as metal), in several different experiments for different feed water samples. Some experiments were done with the coagulant only, others with both coagulant and flocculants. In these experiments, only turbidity was analysed.

4.2.3 Jar test experimental procedure

Six one litre transparent cylindrical glass jars were used, and the experiments were run side-by-side. The jar testing apparatus (Flocculator 2000) was supplied by Kemira Oyj.

Each jar could be separately adjusted for rapid agitation time (400 rpm), slow agitation time (40 rpm), and settling time (no mixing). A program of 30-sec rapid mixing followed by 4 min 30 sec min slow mixing and then 5 min settling was used. The

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agitator was of the paddle type, and the axis was kept in place by a frame also working as a baffle (Figure 7).

Water samples were taken from a side stream of the flocculation tank inlet pipeline, after the API oil-water decanting treatment. Ten-litre plastic buckets were used. Before transferring the sample to the one-litre jars, the initial pH was adjusted with NaOH and H2SO4, the temperature was measured, and water was thoroughly mixed. Thereafter the six jars were filled with one litre each. All experiments were carried out at approximately room temperature (25°C).

Before starting an experiment, the jars were mixed for 5 seconds using the rapid setting (400 rpm). At t=0 s the experiment and rapid mixing were commenced, and the coagulant was added. If a flocculant was used in addition to the coagulant, it was added at t=20 s. At t=30 s mixing was slowed down to the slow setting (40 rpm). At t=4 min 30 sec mixing was discontinued. Experiments were concluded at t=9 min 30 sec, and the samples were taken from the jars for analysis from the middle part of the jar, halfway to the bottom from the surface. Floc formation was observed during the whole

Figure 7: Jar test in progress. Coagulant doses from the left:

5, 10, 20 mg/L Fe from ferric sulphate.

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experiment. The final pH was measured, and samples for analysis Floc strength was also visually observed by mixing the fluid rapidly for a couple of seconds after sampling.

Coagulants were added directly from the concentrated stock solution using a micropipette. Flocculation polymer solutions had to be prepared on site. The polymer solutions were prepared from dry powder and tap water (as in the full-scale process).

The dry polymer powder was weighed and slowly dosed into the water vortex created in the magnet stirred beaker. Stirring was sustained for 30 min, thereafter the homogeneity of the solution was inspected. After discontinuing the mixing, the solution was let to settle for 30 minutes or more. The polymer solutions were added to the jars with microliter pipettes. The solutions were stored at room temperature, covered with aluminium foil to prevent evaporation, for a maximal time of a week. New polymer solutions were made weekly, all on the same day.