The analysis of amine washing solutions is possible in the same way, and leachate samples from landfi lls with high carbonate contents can also be analysed


automatically (Table 1). The foaming and high CO2 content of these types of samples do not interfere with the extraction. Approximately 12 samples can be analysed per hour.


Fully automatic dosing and emptying of reagents in a compact, well-protected design makes measurement possible even in the fi eld (Figure 3). This has the advantage that samples can be measured directly at the sampling point without stabilising reagents, which can prevent side reactions, particularly in some industrial wastewater treatment plants (e.g., leather industry, food industry, refi neries, chemical industry).


Figure 5: Measurement curves for gas extraction of H2 S from


sewage sludge: primary sludge (red curve), digested sludge (blue curve), and excess sludge (black curve).


It can be seen that the analysis of samples containing solids can be carried out within 5 - 10 minutes. In order to verify the measurements, in addition to multiple determinations on different types of sludge, additions of 10 ppm sulphide solution were also carried out. Table 2 below lists examples of these sample types. The recovery from the additions is very good. This demonstrates the high selectivity of the method for very complex sample compositions.


Table 2: Repeated measurements of sulphides in sludge samples using heating and after evaluation of the step-up tests.


The measurement technology is used in corresponding industrial wastewater treatment plants for online monitoring of pressure pipes or gravity pipes by automatically drawing in and analysing a wastewater sample every 10 minutes (Figure 1). Even the slightest signs of anaerobic processes can thus be detected at an early stage, even before gas sensors can detect H2


S contamination. By transmitting the


values to the control centre in real time, automatic control loops can be developed to optimise treatment strategies and achieve savings [4].


Measurement


Figure 3: Measurements in the fi eld without stabilizing reagents.


Sulphide determination in sludge and oils


Determining hydrogen sulphide contamination in oils and sludge is more challenging. In accordance with IP 570 (Procedure B) and ASTM D7621 (Procedure B), liquid petroleum products (crude oil, gasoline, diesel, marine fuels) are heated to 60 °C in a heating module and the gases released are analysed for H2


S. The H2


Figure 1: Example of treatment of a sewage pipe with iron(II) salt solution using a two-channel online sulphide analysis.


Usual wastewater samples are taken from the relevant areas of the wastewater network, stabilised on site with sodium hydroxide and zinc acetate, and then measured automatically in the laboratory using a 40-position autosampler. For the analysis, the autosampler automatically stirs the sample vial currently being measured in order to extract a homogeneous aliquot of the water sample (Figure 2).


S headspace module consists of a heating oven that can be loaded with 20 ml vials or other vial sizes, a valve system with a bypass gas path, and a temperature control unit for a temperature range of 30 to 250 °C. The sample is weighed into the vial (max. approx. 10 g). The sample vial is sealed with a septum cap (Figure 4).


1 2


3 (recovery by step-up test)


Primary sludge [ppm]


33.7 30.1


30.0


Digested sludge [ppm]


101.0 101.2


100.2


Excess sludge [ppm]


45.7 43.3


41.4


In refi neries, the variable measuring system with heating module has proven itself for many types of samples. Petroleum oils can be extracted at 60 °C in accordance with standard regulations, while other products such as bitumen, residues, and waste oils can also be extracted at higher temperatures, up to 180 °C.


After placing the vial in the oven and after a lead time (1 – 2 min), the gas fl ow of the H2


only the volatile H2 S to the sensor unit.


One advantage of the headspace technique is that the vials used are disposable materials that do not require extensive cleaning, which signifi cantly reduces both personnel costs due to the reduced workload and solvent consumption. Automation with up to 32 sample positions also makes it easy to tolerate longer measurement times caused by slower transport processes.


The electrochemical detection of hydrogen sulphide can be distorted by the presence of organic sulphur compounds (mercaptans). To reduce this infl uence, the detector is equipped with a diffusion barrier that reduces sensitivity to mercaptans by 75 %. Interposing an special oil trap can also wash out interfering volatile mercaptans.


Figure 4: H2 S analyser coupled with an automated heating


module for oils and solids. With the additional H2


S headspace module, all types of


Figure 2: Loading the autosampler with unfi ltered wastewater samples for automatic homogenisation during analysis.


Table 1: Sulphide measurement of leachate water samples in ppm. Nr.


1 2 3 4 5 6


Sample name


3210-01 3877-01 6541-02 6623-02 7101-03 7155-03


1


0.09 1.10 5.51 6.66 4.55 1.98


2


0.10 1.07 5.46 6.61 4.56 1.97


3


0.10 1.10 5.25 6.45 4.37 2.03


sludge with 1 - 85 % dry matter can be analysed. These are acidifi ed in a gas-tight vial and then the gas phase is transferred to the analyser. Figure 5 shows a comparison of a sulphide measurement with primary sludge (red curve), a measurement from a digestion tower sample (blue curve), and a measurement from excess sludge (black curve).


Mean value [ppm]


0.10 1.09 5.41 6.57 4.49 1.99


Standard- deviation [ppm]


0.01 0.02 0.01 0.11 0.11 0.03


Relative Standard-


deviation [%] 6.0 1.6 0.3 1.7 2.4 1.6


The measurement results in Table 3 show that mercaptan can be bound by > 90 % using a special Sulfi max light oil. At the same time, Sulfi max light oil in the oil trap binds hydrogen sulphide only to a small extent, enabling reliable measurement results in oil samples contaminated with mercaptans.


Residual H2 S determination


in solid or molten elemental sulphur


The sulphur obtained from the Claus process must be highly purifi ed for sale. Trapped H2


S gas residues pose


risks. Therefore, careful analysis is also in high demand in this area.


An example of the analysis of solid elemental sulphur in Figure 6 shows that the effective extraction of volatile H2


S is completed in a short time. Very low levels of 0.010 mg/kg - 1 mg/kg are typically detected.


S, the work must be carried out very quickly. Once the vial is sealed gas-tight, the effects of storage are minimal. The heating temperature must be set very accurately.


A sample quantity of 1 to 15 g is fi lled into the headspace vials. In the case of molten sulphur, the melt is dosed directly into the vial or into suitable small portion moulds, e.g., in the shape of bars (Figure 7). To prevent oxidation of the H2


S analyser transfers


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