Caracterización del personaje

The primary focus of this set of experiments was to identify the best performing coagulant and its dose for effective turbidity, DOC, metals and silica reduction. The performance of the three coagulants was evaluated over a range of coagulation doses applied to two CSG waters sourced directly from the field in 2013 and 2014 (water quality described in Table 3.2 chapter 3).

5.2.1.1 Turbidity removal (groundwater)

Figure 5.2 provides insight into the effect of three coagulants and coagulation doses required. Turbidity reduction increased from 0 to 86% for ACH when coagulation doses increased from 0 to 60mg/L.

Figure 5.2 – Turbidity removal from groundwater composite samples (CSG water 2013) following coagulation with different doses of ACH, Al2(SO4)3 and FeCl3 pH =8.4 at

initial turbidity 81.9NTU.

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Tu rb id ity r e m o val ( % ) Coagulant dose (mg/L)

118 As can be seen from Figure 5.2, at the doses 30 to 60mg/L ACH maximum removal of turbidity occurred and achieved 88% removal at 50mg/L of ACH (as Al+3_{). Further}

increased ACH coagulation doses from 60 to 100mg/L did not show any further turbidity reduction, but rather a slight drop in efficiency to less than 70% occurred indicating overdosing of ACH. During coagulation with ACH at 30 – 50 mg/L doses, medium size floc formed immediately and the formed floc settled within a few minutes.

Measurements of floc size were determined by visual comparison. Measurements of floc size were recorded by observation of the floc size over the 45 minutes of monitoring time, Table 5.1. The observed clarity of the supernatant after decanting and effective floc formation and settling was an indication that effective charge neutralisation and inter-particle bridging reactions occurred within seconds of ACH addition.

Table 5.1 shows size formation during 45 minutes of mixing during jar tests. Size of floc were recorded after the first 2 minutes and then after each 5 minutes interval. Rate of floc formation takes 10 – 15 minutes; the floc was remained small in volume indicating relatively low sludge production by ACH. Progression of floc formation over the jar test is important because it provides an indication of the rate of flocculation, hydrolysis and charge neutralisation reactions and sweep coagulation (Stumm and Morgan 1976) taking place by careful observation of solution appearance, floc size, and settled sludge on the bottom of breaker. Coagulation with ACH did not create a large volume of floc at any dose, and Figure 3.8 in chapter 3 provides an example of this. As can be seen from Figure 3.8, sludge volume formed by coagulation with ACH for native pH8.4 and pH6.4 was relatively low compare to ferric chloride.

Figure 5.2 shows that aluminium sulphate was less effective for turbidity removal (~28%) than ACH and required higher dosing between 60 to 80 mg/L. Attempts at coagulation with aluminium sulphate produced no visible floc after the first fifteen minutes and very minor floc at the end the experiment prior to decanting, shown in Figure 3.10 (chapter 3).

119 Table 5.1 – The measurement of the sludge height between the top of the sludge and the bottom of the breaker. (Measurements of floc size during coagulation of CSG water (2013) with 50 mg/L ACH at pH8.23). Time Floc size 0mg/L 35mg/L 40mg/L 45mg/L 50mg/L 55mg/L Minutes mm mm mm mm mm mm 2 <2 <2 <2 <2 <2 <2 5 <2 <2 <2 <2 <2 <2 10 <2 <2 <2 <2 <2 <2 15 <2 <2 2-2.5 2-2.5 2-2.5 2-2.5 20 <2 2-2.5 2-2.5 2.5-3 2.5-3 2.5-3 25 <2 2-2.5 2-2.5 2.5-3 2.5-3 2.5-3 30 <2 2-2.5 2-2.5 2.5-3 2.5-3 2.5-3 35 <2 2-2.5 2-2.5 2.5-3 2.5-3 2.5-3 40 <2 2-2.5 2-2.5 2.5-3 2.5-3 2.5-3 45 <2 2-2.5 2-2.5 2.5-3 2.5-3 2.5-3

Coagulation with ferric chloride was shown to be relatively effective for turbidity removal (95%), but required much higher doses of approximately 100 mg/L to achieve turbidity removals similar to ACH. The floc volume (Figure 3.10 chapter 3) generated by ferric chloride at higher dosage rates was significant, and it was 40% more than that

120 generated by ACH. The formation of higher floc volume was expected because of the higher coagulant dose. Overall the solubility of Fe(III) is much lower compared to aluminium sulphate, but higher than ACH (Pernitsky, 2003). For CSG waters ACH with lower solubility than ferric and higher aluminium content than aluminium sulphate was the best performing coagulant. Ferric chloride does not meet the conditions for effective floc formation and low coagulation dose for CSG waters, and was considered the second best performing coagulant behind ACH.

5.2.1.2 Effect of pH on turbidity removal

The effect of coagulation pH on turbidity removals is illustrated in Figure 5.3 for the three different coagulants at doses corresponding to their optimum effectiveness at pH 8.4 for ACH (35 mg/L Al3+_{) and Al}

2(SO4)3 (50 mg/L Al3+), and at an effective dose at

pH 8.4 for FeCl3 (60 mg/L Fe3+) as no optimum was observed.

Figure 5.3 – Turbidity removal from the supernatant after sub-samples(groundwater composite) were treated with 35mg/L of ACH (as Al+3), 50 mg/L of Al2(SO4)3 (as Al+3)

and 60 mg/L of FeCl3, ( as Fe3+) respectively at initial turbidity 81.9 NTU.

0 10 20 30 40 50 60 70 80 90 100 3 4 5 6 7 8 9 10 Tu rb id ity r e m o val ( % ) pH

121 At pH 5.8, near the pH of minimum solubility for ACH (Bertsch 1996) turbidity removal was <66% at a relatively low coagulation dose (35mg/L). At pH 7, 8 and 9, turbidity removals by ACH increased for the same dose, (35mg/L). It is evident that ACH was the most effective coagulant for turbidity removal over the pH range 5.5 to 9. These features are probably tied to the characteristics of different aluminium species present in the water at different pHs.

Ferric chloride was also effective with 60 - 78% turbidity removal for a 60mg/L dose over the pH range 6 to 9, but was less effective than ACH. Coagulation with aluminium sulphate at an optimal dose of 50mg/L (pH 8.4) achieved about 40% turbidity reduction. ACH and ferric chloride were found to be more effective for turbidity removal than aluminium sulphate over the tested pH range.

It can be seen that the best pH range for ACH and ferric chloride was already close to the native pH of the water (pH 8.4), and performance declined at lower pH for all of the coagulants. This can be explained by relatively high salinity of the waters especially seen in performance of ferric chloride and aluminium sulphate compare to pre- polymerised coagulant ACH. Overall trends were increasing turbidity reduction with increasing pHs for all CSG waters studied.