Los sistemas de bloqueo de trenes entre estaciones

This section deals with the analytical methods used to assess the physiochemical and rheological properties of the sludge.

5.2.1 Total and volatile solids

TS and VS concentrations were measured in duplicate for sludge and synthetic sludge samples. The procedure follows that described in Standard Methods (2540 B and E) (Eaton et al., 2005). Nickel tins were cleaned, dried and put into the furnace for 30 minutes to burn off any residual volatile matter. The tins were then cooled and weighed (W1). 20 ml of sludge was measured into the tins, which were then placed in an oven at 105 °C for at least two hours, until all liquid had evaporated. The tins were placed in desiccators and once cooled, were weighed again (W2) before being transferred to the furnace at 500 °C for one hour. After cooling in the desiccators, the tins were weighed a final time (W3). Concentrations of TS and VS can be calculated using the following equations:

_{Equation 5-2}

Equation 5-3

where TS is total solids concentration, VS is volatile solids concentration, W1, W2 and W3 are weights as indicated in method above, and V is sample volume.

5.2.2 pH

pH was measured using a Mettler Toledo Model 320 electrode and meter. The meter compensates for changes in temperature and was calibrated using buffer solutions of pH 4, 7 and 9. The electrode probe was rinsed with ultra-pure water (UPW) before and after each analysis to minimise cross- contamination of samples.

110 5.2.3 Alkalinity and Ripley’s ratio

The method for measuring alkalinity and Ripley’s ratio (RR) is based on the titration method described in Standard Methods (2320 B) (Eaton et al., 2005). 20 ml of the sample was placed in a beaker. The sample was titrated with 0.1 N sulphuric acid to end points of pH 5.75, 4.5 and 4.3. The end point of pH 4.5 is recommended for samples with high alkalinities such as wastewater (Eaton et al., 2005) and is the end point used for the calculation of total alkalinity (TA). The other two end points are used in the calculation of RR. TA, which is a measure of the ability of a solution to buffer acids, was calculated as mg CaCO3/l according to the following equation:

Equation 5-4

where TA is total alkalinity, A is volume of standard acid used to reach end point of pH 4.5, N is normality of standard acid used, and V is volume of sample used.

Alkalinity and VFAs are measured to gauge the stability of a digester. It is claimed that an end point of pH 4.3 takes into account the VFA buffering as well as the bicarbonate buffering (Jenkins et al., 1983). As such, Jenkins et al. (1983) suggested the measurement of partial alkalinity (PA) with an end point of pH 5.75, which allows for 80 % titration of the bicarbonate alkalinity, but only 20 % titration of the VFAs. Titrating to pH 4.3 gives the buffering of bicarbonate and VFAs and is termed TA. As such titrating from pH 5.75 to pH 4.3 gives an indication of the VFA buffering and this is called intermediate alkalinity (IA). RR is the ratio of IA:PA and acts as a measure of digester stability. The advantage of RR over direct measurement of alkalinity and VFAs is that it requires only one analytical procedure which reduces the introduction of error and is less time-consuming. Ripley et al. (1986) stated that RR values below 0.3 indicate a stable digester whilst values of 0.8 or higher indicate that a digester is under stress. The RR was calculated using the following equation:

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Equation 5-5

where RR is Ripley’s Ratio, TA is volume of standard acid used to reach end point of pH 4.3 and PA is volume of standard acid used to reach end point of pH 5.75.

5.2.4 Ammonia

The method for measuring ammonia was based on the ion selective electrode method described in Standard Methods (4500-NH3 D) (Eaton et al., 2005). 100 ml of the sample was placed in a beaker

and a Thermo Scientific Orion 9512 ammonium ion selective electrode was immersed in the sample. 1 ml of 10 N NaOH solution was added to the sample to raise the pH above pH 11 and the reading was allowed to settle before it was recorded. A calibration curve was created using standard dilutions of stock ammonium chloride solution. Ammonia concentration of the sample was then calculated from the sample readings and the calibration curve.

5.2.5 Volatile fatty acids

Lactate, acetate, propionate, butyrate and succinate were measured using a DIONEX LC30 chromatography oven equipped with an ED40 electrochemical detector, a GP50 gradient pump and an AS40 automated sampler. The column used for VFA analysis was an IonPac AS11-HC 4 x 250 mm. Samples were centrifuged at 6000 rpm for 15 minutes and then filtered using Whatman 540 filter paper. Samples were first diluted to 5 % with UPW. Standards containing a known concentration of each of the volatile fatty acids were used to calibrate the high-performance liquid chromatograph (HPLC) and the samples were then run, with blanks run every ten samples and standards every twenty samples. As it was not possible to run this analysis each week, samples were collected from each of the digesters on a weekly basis and were frozen until they were processed.

5.2.6 Viscosity

Viscosity measurements were carried out using a Couette viscometer (Fann Model 35) which operates by shearing a known volume of fluid in a test cell. The torque required to turn the bob at a

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known speed within the cup of fluid is a function of the viscosity of the fluid. The sample cup was filled with 350 ml of the test fluid and the cup was then raised so that the rotor sleeve was immersed in the sample to the marked line. The fluid was thus able to fill the rotor sleeve and surround the bob. Each fluid was mixed at known bob speeds of 100, 200, 300 and 600 rpm and, once the reading had stabilised, the needle deflection was recorded. Using the conversion factors given in the viscometer manual, the needle deflection was translated into a shear stress reading so that a viscosity curve could be produced for each sample.