In compiling and analysing the results of the original flow variables, it became clear that there were several factors that influenced the breakthrough characteristics of the filter, other than just the magnitude and timing of the flow change itself. The external variables investigated were;
• The influent water temperature,
• The subsequent shear stress variations,
• Vibrations affecting both the laboratory apparatus and one of the full-scale treatment plant locations.
All of these factors are detailed individually in this chapter.
5.1: Influent water temperature
The variation in water temperature between summer and winter conditions in the laboratory ranged between 14°C and a maximum of 26°C. This appeared to have quite a significant impact on the amount of breakthrough, so repeat experiments were performed where the only variable was the water temperature, to analyse this difference in particle shedding.
Figure 5.11 below is a very good illustration of the trend that was found with all of the variables tested. The graph shows two 8h runs using 50 mg/L kaolin. 100% temporary flow changes were applied after 4h, and the two water temperatures were 16°C and 22°C. It can clearly be seen that prior to the flow change, both runs had a
fairly similar ripening period (although the colder 2-5 pm size range took the longest
to ripen), and subsequent good filter performance.
Comparison of particle breakthrough at different water tem peratures. 100% flow increase +4h (50mg/l kaolin)
2500 8927,11,073 _ 2000 Z 1000 08:44 09:56 11:08 12:20 13:32 14:44 15:56 Time 4 3.5 3 c 1 2.5 $ o 2 u. 1.5 --- 2-5um 22'c 1 >5um 22'C 0.5 2-5um 16'C 0 >5um 16’C Flow I/m In
Fiuure 5.11: Particle breakthrough observed at different influent water temperatures, using a Im mono-media bed and a starting flow rate o f 5m/h (1.81/min = 5 m /hi
However, during the flow change itself, the run at 22“C suffered significantly higher
levels o f breakthrough, both in the 2-5pm range, and even more so in the >5pm range,
which suggested that entire floes were being detached. A secondary peak is clearly
visible, which drops o ff the instant that the flow is reduced. However immediately
afterwards, the filter begins to fail at an increasing rate. By the end o f the filter cycle,
the breakthrough in the 2-5pm range is approximately 2000 counts /ml.
In contrast the run performed at 16°C shows very little breakthrough during the flow change, and although the particle counts remain above base counts for the entire
duration o f the flow change, once the flow is reduced, the filter quickly recovers and
the shedding remains low for the remainder of the filter cycle, with no indication of any end-of-run breakthrough.
This was quite the reverse of what was expected to happen, given the higher shear stress and reported poor performance of alum at low temperatures. It would be expected that much higher particle counts would be observed during the 16°C run. The superior performance of the filter at cold temperatures was observed in all experiments.
5.1.1: Jar Tests at varving temperatures
In order to investigate the reasons behind the variations in filter performance at different temperatures, jar tests were performed to visualise the floe development and subsequent turbidity readings.
As previously mentioned in Chapter 3.32, The suspension that was tested was 800ml of London tap water, dosed with 50mg/l kaolin and 0.19ml/l Alum (M/10). This had been established previously as the optimum dosing level. The temperatures tested were 25, 16 and 11°C. The jar test rig was set to perform 15 seconds of rapid mixing followed by 1 Omins of slow stirring. 15 minutes settling time was set before the final turbidity reading was taken. Turbidity samples were taken from the 600ml mark on the jars. The initial turbidity of the kaolin suspension was on average 58NTU.
25®C 16°C 11°C
INITIAL VISUAL OBSERVATIONS
Well formed floes after 2mins. Very
visible.
Pinprick flocculation after Smins. Larger floes
after 6mins.
Very slow flocculation. Only pinprick floes after
6mins. lOMINUTE TURBIDITY 9.2 13.2 18.9 lOMINUTE VISUAL OBSERVATIONS
Large floes settle straight away No settlement of floes Pinprick floes, no settlement. 15 MINUTE TURBIDITY 1.6 3.3 3.7 FINAL WATER TEMPERATURE 25.5 18 15
Table 5.1 : Jar tests using alum at different water temperatures
These jar tests show that larger floes form in warmer water, and have a faster settling time. The laboratory experiments have shown that more detachment occurs in warmer water, despite the lower viscosity and consequent reduced shear force with increased water temperature.
The floes do not appear to be more dense in colder temperatures, as there is no settling during the last 15 minute settling phase (although this could possibly be due to their small size). Despite this, the results suggest that small floes created using alum at cold temperatures are not weak, as they can withstand higher shear stresses caused by the lower viscosity of the water.
5.1.2: PAX and alum comparisons
Having established that the performance of alum floes is greatly influenced by temperature, and thus possibly not suited to plants which experience a wide range of water temperatures, the results were compared to the performance of PAX. Firstly,
the jar tests at different temperatures were performed, using the same protocol as the alum jar tests. To provide a better comparison, 50mg/l kaolin was used, despite only lOmg/1 kaolin being used in the filter experiments. Table 5.2 below details the floe formation and turbidity readings during the jar tests.
2S*C
16*C
11”C
INITIAL VISUAL OBSERVATIONS
Rapid flocculation, large floes after 2
mins
Similar floe development between temperatures, much slower than 25°C,
only small floes after 3 mins lOMINUTE
TURBIDITY
5.4 12.8 12.9
lOMINUTE OBSERVATIONS
Large floes show rapid settling
identical looking f sett
oc size, very slow ling 15 MINUTE TURBIDITY 2.2 7.5 7.6 FINAL TEMPERATURE 26°C 18°C 15°C
Table 5.2: Jar tests using PAX at different water temperatures
The results of these jar tests have shown that at lower temperatures, the floe development is very similar and appears not to be dependent on the actual water temperature. However, the highest temperature tested showed far superior development, with rapid flocculation commencing within 2 minutes. The floe development at this temperature was almost identical to the corresponding alum floes. Once the mixing period had been completed (i.e. after 10 minutes), the residual turbidity was lower when using PAX, and this was the case for all water temperatures. However, it would appear that the PAX floes could be less dense than alum floes as the turbidities after the settling period were all higher, and the visual observations during this period confirmed that the floes do not settle as well.
Having established the speed and degree o f floe development at different
temperatures, laboratory filter runs were repeated to compare the performance o f the
two coagulants.
25% and 50% temporary flow changes were applied after 4 hours using lOmg/1
kaolin. The water temperatures during these experiments were 14 and 24°C. The 2
graphs are displayed as block diagrams o f the total particle counts in the 2-5 pm range
for the whole runs, so as to give an indication o f overall filter performance.
Figure 5.12 shows the results o f a 25% flow increase. What is immediately apparent is
the good performance o f alum at both temperatures. This concurs with previous
experiments which suggest that a 25% flow increase is close to the lower threshold for
causing additional breakthrough for alum.