TRATAMIENTO DE LODOS DEL SISTEMA DE TRATAMIENTO

The normal range of available laboratory turbidimeters (0-2000 NTU) were found to be insufiScient to record the full turbidity proflle from a backwashing filter, particularly when using high rate fluidising water washes with high influent concentrations (up to 500 mg/1 kaolin clay). This is also a problem in industry when monitoring filter backwash water, where it is conventional to monitor light transmission rather than turbidity, making it easier to cover the wide range of turbidities during a backwash, Russell (1994). Laboratory turbidimeters have recently become available using a surface scatter technique which have a measuring range of 0-9,999 NTU and this may or may not be suitable to measure the complete backwash suspension profile over its frill range. However, the problem still remains in converting Nepelometric Turbidity

Units (NTUs) to suitable concentration units. In order to measure and record the

high backwash concentration profiles within and emerging from the filter bed, a laboratory scale instrument was developed based on a technique of determining the ratio of 90 * light scatter to forward light attenuation of a sample illuminated by a suitable light source.

Two concentration measuring devices were constructed each consisting of a flow cell, single light emitting diode and two light-to-voltage optical sensors. The light emitting diode used was a high-power gallium aluminum arsenide (GaAIAs) infrared emitting diode (type OD-880F). These diodes emit 880 nm non-coherent infrared radiant energy when forward biased. The light-to-voltage optical sensor consists of a

combined photodiode and transimpedance amplifier. The output voltage of the sensor is directly proportional to the light intensity (irradianee) of the photodiode.

Construction details and an electronic circuit diagram of the concentration measuring deviees are shown in figure 4-12 and 4-13. The light emitting diode and light-to- voltage sensors were mounted on a flow cell as shown in figure 4-12. The light-to- voltage sensors are positioned to measure forward light attenuation and 90° light seatter of the sample as it flows through the eell arrangement. Figure 4-3 shows a photograph of the flow cell arrangement.

The output range of the light-to-voltage sensors is 0-3 volts. The output voltage Ifom each sensor was fed to an amplifier with a gain set to give a eorresponding output voltage of 0-4 volts, suitable for input to an analogue to digital converter installed within a desktop computer.

F igure 4-3. Flow cell and sensor arrangement for concentration measurements.

The analogue to digital (A/D) converter and data acquisition system were eontrolled by a software program written in BASIC language. A program listing is included in appendix III. The program controls the A/D sequenee and provides data processing

and storage facilities. Before the program starts the sampling frequency and sampling duration must be set along with a data file name to store data from each of the light- voltage sensors.

For each flow cell the program computes the ratio of 90° scatter voltage to forward light attenuation voltage. The ratio is then input to an equation, depending on its magnitude, to give a concentration output in mass per volume units. It was found from the calibration procedure that using the ratio of scattered to attenuated light each device could be calibrated from clean water up to a concentration of 100 g/1 kaolin clay. Four stages were required to model the concentration over this range. Three stages were modelled using a linear equation of the form:

y = ax+ b (90)

where y is the concentration in g/1 kaolin, x is the voltage ratio, a and b are

coefficients determined from the calibration procedure. Above a concentration of 45 g/1 kaolin clay, the light transmission became very small in magnitude and unstable, eventually felling off to zero. However, the 90 ° light scatter signal gave stable readings up to 100 g/1 and it was found that the concentration could be modelled by an exponential equation of the form:

y = exp(ûr 4- bx^^ ) (91)

Details of the calibration procedure and the model curves are given in appendix IV. A detailed analysis of the possible sources of error in the concentration measurement device is given in chapter 6.0, section 6.3.

4.4.1 Flow cell connections

Connection between the sample ports and to the flow cells during each experiment was made using 6 mm internal diameter nylon tubing. For control purposes valves

were connected to sample port No 4 and the imderdrain section in order to measure the influent and effluent concentrations.

Influent and effluent concentration values were measured throughout the filtration period every 15 seconds. The concentration measurements taken at one filter were taken as representative of all four filter columns.

During the water only backwash experiments the pair of flow cells were connected to sample ports on each filter column. Using this method, backwash suspension

concentration profiles were measured at various depths using all four filter columns.

The same method was used during the air scour backwash experiments. However, during the air scour period, the outlet valve for each flow cell was closed. The valves were opened at the end of the air scour period and at the beginning of the water wash.

The connection arrangement for the simultaneous air and water experiments was not as straightforward because of the continuous presence of air bubbles in the spent backwash water. Air bubbles in the flow cell cause spurious readings, and some method of removing the air bubbles fi'om the suspension sample was therefore required. In order to achieve this, an air trap jar was introduced to the sample line ahead of the flow cells. The air trap jars consisted of an open top glass jar with a bottom exit spout. The sample fi-om the filter column was introduced into the air trap jar using nylon tubing connected directly to the sample port. The air trap jar was

initially charged with a measured volume of clean water. The bottom exit spout was connected directly to the flow cell fi-om where the sample was routed to drain. Using this arrangement, air bubbles trapped within the backwash suspension sample rise to the firee surface within the air trap jar. The suspension sample exits the air trap jar to the flow cell where a normal concentration measurement is made.