AMBIENT

HYDROGEN

WASTEWATER

SULPHIDE

LEVELS

TREATMENT

PLANT

LAWRENCE

AT A

C. C. K O E

Department of Civil Engineering, National University of Singapore, Kent Ridge, Singapore 0511 (Received 14 June, 1984) Abstract. This paper describes a novel technique of measuring ambient hydrogen sulphide (HgS) concentrations simultaneously at several locations around a wastewater treatment plant. A commercially purchased H2S monitor is modified to operate in a 'static mode' to enable degree of darkening on" pieces of lead acetate tapes to be correlated against the exposure duration and the ambient H2S concentration of sewage air. The technique can yield mapped contours of time - average H2S concentrations as low as 0.2 ppm. The methodology is exemplified for a wastewater treatment plant in Ipswich, Queensland. Isopleths of H2S concentration obtained at the wastewater site for two different meteorological conditions reveal that high levels of H2S are detected around the plant's inlet structure and primary clarifiers.

1. Introduction

Emissions of gaseous hydrogen sulphide (H2S) from a variety of treatment unit processes at a wastewater treatment plant are unavoidable. Although H2S upon release from a specific source becomes mixed with ambient air such that the resulting H2S concentration a few metres from the source is below a few parts per million (ppm), the H2S level is still high enough to cause serious corrosion and odour problems (Matthews and Boon, 1978). Mapped contours of the spatial distribution of gaseous H2S levels in the neighbourhood of a wastewater plant can therefore provide useful information on H 2 S dispersion under varying meteorological conditions and help identify major H2S emission sources (Reinsch et al., 1977). In quantifying the dispersion of gaseous H2S from an area-source such as a wastewater plant, it is desirable to obtain time-averaged values for H2S concentration at many points simultaneously. However, the simultaneous measurement of low ambient H2S levels at several locations is beset with problems of providing sufficient numbers of monitoring equipment. It is obviously uneconomic to purchase many H2S monitors in order to place one at every point of interest in the neighbourhood of a source. A procedure to overcome this problem is now described. The technique assumes that a single H2S monitor that operates on the lead acetate/sulphide principle is available. Short segments of lead-acetate tape can be freely exposed to ambient air at several locations of interest for a selected time period and each tape segment will undergo some darkening due to the formation of lead sulphide. The extent of darkening will depend primarily on the period of tape exposure and the average H2S concentration during this period. If the effects of these two factors can be quantified beforehand by experiment and related to the degree of discolouration, the resulting relationship may be used to provide time-averaged H2S concentrations at each of the tape locations. EnvironmentalMonitoring and Assessment 5 (1985) 101-108. 9 1985 by D. Reidel Publishing Company.

0167-6369/85.15.

102

AMBIENT HYDROGEN SULPHIDE LEVELS

2. Methodology 2.1,

HYDROGEN

SULPHIDE

MONITOR

A commercially available hydrogen sulphide (H2 S) monitor (Model 7010, manufactured by Universal Environmental Instruments (U.K.) Ltd.) was used in this study. The instrument utilises the principle of colour densitometry to measure the concentration of H2S in ambient air. Operation of the instrument requires a stream of sample air containing H2S to be directed onto the top half of specially impregnated lead-acetate paper tape to produce a coloured stain (lead sulphide). The optical density of this stain is proportional to the H2S concentration. A photoelectric cell monitors the degree of darkening on the tape against a reference 'uncontaminated' bottom half of the tape and causes a signal to be fed to a readout meter. The same signal is also transmitted to a set of output terminals where a chart recorder may be attached. The readout meter is calibrated and scaled linearly from 0-20 ppm. The instrument is designed to operate in a continuous mode whereby a reel of lead-acetate tape is transported continuously across the sample air inlet at a slow speed of 10 cm hr i. Under such operation, the H2S concentration of the sampled air is given directly by the reading registered on the output meter of the monitor. 2.2. H2S

CONCENTRATION

-- E X P O S U R E

DURATION

RELATIONSHIP

It is possible to operate the commercial H2S monitor in a 'static' mode whereby the reel of lead acetate is replaced by two specially prepared pieces &lead-acetate tape segments each sized and positioned in such a manner on the optical module to represent the top and bottom half of a tape section when scanned by the photo-electric cell. The upper tape segment can earlier be exposed for a known time period in a sewage air sample containing a known H2S concentration to result in a certain degree of darkening. The reflective difference between the stained top segment and the blank reference lower segment would cause a signal to register a reading o f R ppm on the output meter. Since the monitor is now operated in a non-continuous mode, the output reading R ppm does not indicate the actual H2S concentration of the air sample into which the tape segment was exposed. This numerical readout however, provides a convenient scale on which various degrees of darkening on pieces of lead-acetate tape can be graded. Bagfuls of sampled sewage air were collected from a wastewater plant and for each bag, a portion of the sewage air was bled through the H2S monitor to determine its HzS concentration. Specially prepared lead-acetate tape segments were introduced into each remaining portion of sewage air and stored for varying time periods. After appropriate durations of exposure, each stained lead-acetate segment was then quantified in the 'static' mode on the H2S monitor to yield readout values of R (ppm). The dimensionless ratio of readout value R (ppm) to the actual H2S concentration, [ H2S ] (ppm) in the sewage air sample for each tape segment is plotted against exposure duration T (minutes) in Figure 1. The central trend of these results indicate a power function relationship between time, T, and the dimensionless ratio R/[H2S ] of the form: R/[HeS] = 0.37T ~

(1)

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Equation (1) indicates that the meter reading (when the H 2 S monitor is operated in the 'static' mode) is proportional to the concentration of H2S ancl to (roughly) the square root of exposure duration. If T is about 5.6 min, then the meter reading correctly reflects the HaS concentration (i.e. R = [ H z S ] ) , but when T = 180 min (3 hr), Equation (1) becomes: [H2SI = R/6.3.

(2)

This indicates that, when H 2 S concentrations are hovering around 1 ppm where the accuracy of the H a S monitor (in the continuous mode) is somewhat limited, the major achievement of transferring to the static mode and adopting an exposure duration of 3 hours is equivalent to a meter scale magnification factor of 6.3. Since the full-scale meter reading is 20 ppm, this means that H 2 S concentrations greater than about 3.2 ppm would cause tapes exposed for 3 hr to yield off-scale readings, but this problem is rarely encountered in practice (except at locations very close to strong sources of H2S).

2.3.

FIELD

MEASUREMENTS

It is now possible to measure low ambient U 2 S concentrations commonly found near wastewater treatment plants by exposing pieces of lead-acetate tapes for a fixed

104

AMBI ENT H Y D R O G E N S U L P H I D E LEVELS

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LAWRENCE C. C. KOE

105

exposure duration T (min) at various points around the plant site, obtaining R readout values for each exposed tape on the H2S monitor and using Equation (2) to derive the gaseous H2S concentration at each selected monitoring station. This procedure is now applied to a modest-sized wastewater treatment plant in Ipswich, Queensland. Figure 2 shows a layout of the treatment plant which processes an average wastewater flow of 0.23 m 3 s - 1. The main treatment units consist of primary clarifiers, trickling filters, secondary tanks and sludge drying beds. The site slopes downward generally to the south-east, with about 6 metres of fall between the inlet structure and the secondary sedimentation tanks. H2S emissions can be expected from various points of turbulence around the treatment plant and at the inlet structure where raw sewage is processed. A total of 55 monitoring stations, each consisting of a wooden post erected such that a tape segment can be suspended at a height of 1.2 m above ground level, were selected around the 10 ha plant site. These points were considered sufficient for H2S isopleths to be obtained for the plant. Preliminary tests at the wastewater plant indicate that the ambient H2S concentration around the site rarely exceeded 3 ppm. As such, an exposure duration of 3 hr, which results in a magnification factor of 6.5 times (see Equation (2)), will not result in off-scale readings on the H2S monitor output meter. The nominal 3-hr period selected was from 6 to 9 p.m. This evening period was chosen partly to avoid suspected bleaching affects of sunlight on darkened tapes, and partly because this period commonly exhibited atmospheric temperature inversions, very light winds and suppressed rates of pollutant dispersion. It took about 25 min (from 6.00 to 6.25 p.m.) to set out all 55 tapes in an ordered sequence and a similar period (starting at 9.00 p.m.) to retrieve them in the same order. This procedure ensures that each tape segment is exposed for a duration of 3 hr. Extra precautionary tests were carried out to ensure that the tapes exposed late (e.g. 6.25 to 9.25 p.m.) yielded results negligibly different from their 6 to 9 p.m. counterparts. The complete technique described above was applied to the wastewater plant at the Tivoli site on two separate survey periods. Conditions during the two survey periods TABLE I Site conditions for H2S surveys at Tiboli WWTP Survey

I

II

Mean wind speed (m s 1) Ambient air temperature (°C) Sewage temperature (°C) Sky condition Mean wind direction General description of wind behaviour

0.5 20 22 Clear and fine NW Very light, but with occasional wind gusts up to 1.5 m s 1 before 7 pm, diminishing thereafter. Calm after 8 pm.

0.1 27 26 Clear and fine NE Fairly light, but with occasional wind gusts up to 3ms-1 before 7pm, diminishing thereafter. Calm after 8 pm.

106

AMBIENT

HYDROGEN

SULPHIDE

LEVELS

are summarised in Table I. Local wind speeds and directions were observed with a hand-held anemometer every 30 min during both sessions, and these reveal that, although both sets of conditions were relatively calm, their dominant air movement were significantly different in both magnitude and direction. 3. Results Mapped contours of average H2S concentrations at the Tivoli wastewater treatment plant between 6 and 9 p.m. on each of the two survey periods are shown in Figure 3

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LAWRENCE

107

C. C. K O E

and 4. The contours are based on point values derived by applying Equation (2) to meter readings from the HzS monitor when operated in the static mode on lead-acetate tapes exposed at the indicated locations. Since an exposure duration of 3 hr was selected, each derived H2S concentration value is time-averaged over a 3-hour period. Both these figures exhibit the following features: (a) A central area, around the primary clarifiers and inlet structure, where H2S levels are above 1.0 p p m during both survey periods. (b) A more extensive region encompassing almost all of the treatment units where HzS levels are above 100 ppb. ib

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108

AMBIENT HYDROGEN SULPHIDE LEVELS

(c) A tendency for the dispersion of H2S gases to be dominated by the prevailing wind direction that occurred during each survey session. It is worth noting that during the first survey period (see Figure 3), significant levels of H2S are detected in a gully on the eastern part of the plant, as indicated by the pronounced protrusion of the 100 ppb contour in that direction. This indicates to some extent the tendency of H2S gases to migrate slowly towards low-lying areas around the wastewater plant especially during calm conditions. Such migration however is not evident in Figure 4 perhaps because it was suppressed by the then strong drift towards the westerly direction. 4. Conclusion

The study has illustrated the possibility of measuring low level ambient H2S concentrations at several locations around a wastewater site, By operating a commercial H 2 S monitor in a static mode, the darkening on pieces of lead-acetate tape exposed to an ambient H2S level can be quantified and correlated against the time of tape exposure. Such correlation enables the quantification of pieces of exposed tape segments left for a fixed period at several locations around a treatment plant to yield time-averaged ambient H2S levels. For the Tivoli wastewater treatment plant surveyed on two occasions in this study, ambient H2S levels in ppb levels are detected at several locations and H 2 S isopleths are plotted to indicated the dispersion of H2S gases around the wastewater plant. High levels of H2S ( > 1 ppm) are usually located around the plant's inlet structure and primary clarifiers. References Matthews, P. J. and Boon, A. G.: 1978, 'Odour Nuisance in Sewerage and Treatment System: Problems and Control', Water Pollution Control, 77 (2), 248-258. Reinsch, D. A., Parr-Smith, G. A., and Cornell, M. A.: 1977, 'Odour Assessment and Control in Sewage Treatment Works', Proc. of the 7th Fed. Convention, Australian Water and Wastewater Association, Canberra, Australia, September.