This chapter details the methods used to conduct the laboratory experiments, as there were many factors to consider when constructing and operating the filter column. The processes that are described in more detail include the physical dimensions of the apparatus, the flow and backwashing regimes, the coagulant dosing, and the processes involved in recording and viewing the particle breakthrough. It provides a comprehensive review of how the experiments were conducted.
3.1: Filter column apparatus
The laboratory experiments used a Perspex column which was 2m in height and 0.166m internal diameter. The base of the column was conical in shape and contained ballotini (glass spheres with a diameter of approximately 2cm). This not only supported the media but also dispersed the backwash flow of water and air, thus eliminating the need for a supporting layer of gravel. A wire mesh separated the media and balotini so that no sand was lost from the column during normal filtration. Inserted into the bottom half of the column were five ports for video recording purposes, six manometer ports and five sampling ports to enable testing of the water at various depths of the filter bed. At the top of the column were two outlets for backwash water. This enabled the filter to be backwashed at various bed expansions without the loss of the filter media. All of the above are illustrated in the schematic diagram of the entire apparatus. Figure 3.1.
Figure 3.1 also shows the supply tank, which was fed from the main supply tank for the department of Civil and Environmental Engineering at University College London. The tank had a capacity of 210 litres and was fitted with a ballcock to ensure supply on demand. AIR BLEED CONSTANT HEAD TANK \ ALUM TANK SUPPLY 1.6M H e a d o f w a t e r FILTER COLUMN KAOLIN TANK TANK
DRAIN
0 = VIDEO PORT X = MANOMETER TAP = SAMPLE PORT P = PUMP ■ = VALVE 1 = AIR FLOWMETER 2 = BACKWASH FLOWMETER 3 = FILTRATE FLOWMETER 4 = PARTICLE COUNTER AIR SUPPLY BACKWASH WATER SUPPLYSuspended in this tank was a thermostatically controlled water heater. This allowed the influent suspension to pass through the filter at a pre-set temperature, and was installed for the final set of experiments examining the effects of the water temperature on filter efficiency. The kaolin suspension was fed into the tank and the water was pumped up to the constant head tank which supplied the column. The tank was high enough to provide a head of 1.6m onto the media, when the bed depth was Im.
The coagulant was added just before entering the column via a baffle mechanism which was a flat spiral coil of metal within the tubing. This ensured sufficient mixing energy for flocculation. The mixing time available before reaching the media bed was 12 minutes when the water flow rate was 5 m/h. This direct filtration method was used to avoid coagulant recycling via the constant head overflow. A Hach turbidimeter was used to continuously monitor the turbidity of the suspension in the constant head tank. This enabled visual confirmation that the kaolin was always being dosed at the correct rate.
3.1.1: Data collecting equipment
Once the suspension had passed through the filter, the flow was controlled using a gate valve, and the rate was measured by both a standard rotameter and an electromagnetic flowmeter. This monitored the flow continuously and gave an electrical reading which could be downloaded into a PC software package, Winwedge. This enabled a continuous flow measurement to be made and meant that
any unintentional changes in flow could be recorded during the night or when the column was unattended.
Finally, once the flow had been measured, a small amount of flow was diverted into the particle counter. This was a Met One particle counter, model WGS26, and was capable o f reading particles in the range 2-15pm, in 6 separate size channels. These channels were 2-3pm, 3-5pm, 5-7pm, 7-10pm, 10-15pm and those particles larger than 15pm. Midway through the laboratory experiments, a new particle counter was purchased and was identical to the existing one. In comparative trials, the new particle counter provided similar particle count data to the existing instrument, providing evidence to suggest that the previous particle count data would still be valid.
3.2; Media Selection
The initial experiments were conducted using Leighton Buzzard sand of grain size 0.5 - 1.0mm, as it is commonly used in filters for potable water applications. To ensure that the grading stated on the bag was accurate, three samples of 200g were taken from various sections of the bag and sieved for 20 minutes through a sieve set of 1200pm -500pm. Table 3.1 below details the fraction weights.
SIEVE SIZE WEIGHT (g) % CUMULATIVE
% <500pm 0.9 0.46 0.46 500pm 16.0 8.15 8.61 600pm 105.6 53.77 62.38 710pm 58.4 29.73 92.11 850pm 13.4 6.82 98.93 1000pm 1.8 0.92 99.85 1200pm 0.3 0.15 100