Christopher J Pratt .... This was based on a device previously used by Pratt .... authors also wish to thank Mike Jenkins for assistance in the early part of this work.
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Development of an Effective Sampling Device for Sampling Oil from Experimental Pervious Pavement Structures Développement d'un dispositif performant pour prélever des échantillons d'hydrocarbures sur une chaussée poreuse expérimentale. Tim Puehmeier, Alan P Newman, Jim Philips and Christopher J Pratt Coventry University, School of Science and the Environment, Priory Street, Coventry, CV1 5FB, UK.
RESUME Cet article présente le développement d'un dispositif destiné à l'échantillonnage correct des effluents chargés en hydrocarbures provenant de chaussées à structure réservoir expérimentales. Le principe consiste à recueillir l'échantillon dans un auget basculant qui déverse à intervalle régulier son contenu sur un plateau "tourbillonnant". Le but est de séparer les hydrocarbures du reste et de les disperser avant de collecter une fraction fixe de l'échantillon et de jeter le reste. Les données présentées démontrent que cette technique a une performance acceptable pour l'utilisation envisagée.
ABSTRACT This paper reports the development of a device to allow the representative splitting of oily effluents originating from experimental pervious pavement systems. The device works by collecting the sample in a tipping bucket system which periodically tips the sample rapidly into a swirl tray to break up the oil in the sample and disperse it prior to collecting a fixed fraction of the sample, passing the remainder to waste. Data is presented which shows that the device has acceptable performance for the required application.
KEYWORDS Hydrocarbons, free product, pervious pavement, sampling, tipping bucket
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1.
INTRODUCTION
1.1.
Background
When sampling effluents containing oil and grease from experimental pervious pavements, or in the field, workers are often faced with the problem of how to collect a representative sample of a whole rain event (to provide an event mean concentration of the oil and grease). Very often this involves splitting a large sample, whilst allowing for the fact that there are two phases in the system. There has been considerable effort put into systems which can split water samples containing suspended material of various particle sizes (Hatzell et al.,1995) In the case of samples containing oil and grease, as a required determinand, it has to be remembered that in most cases the oil will be in the form of both free product and a dissolved/colloidal phase. If the event mean concentration is required for such an analyte, which forms a separate phase to the water, then the ideal situation would be to collect the whole volume of water passing through the system during the entire rain event and either analyse the whole sample or, representatively, sub-sample the collected water prior to analysis. For experimental pervious pavement systems, which are of a laboratory scale, this is a satisfactory approach. However with larger scale experiments or with live drainage systems this is not a practical solution. Given that a 13mm rain event on a small (4 bay) experimental car park would potentially generate almost 600 litres of effluent it can be recognised that the sample would be difficult to accommodate. Even worse, with all practical systems we a position will eventually be reached where the rainfall volume exceeds the size of the container. This would result in the free phase oil, which floats to the surface of the sample, being preferentially lost as the container overflows. Even with relatively small scale situations, where a large enough collection vessel can be used to capture a reasonable rain event, this large sample would be very difficult to meaningfully subsample. Collecting a number of grab samples over an entire rain event can clearly give, at least, an indication of the event mean concentration. The use of grab samples for oil and grease determination is widely promoted in standard methods, for example the British Columbia Field Sampling Manual (Province of British Columbia, 2003) states: Grab samples are generally specified when
or when the analyte is such that the procedure of compositing would destroy the sample integrity or representativeness (VOCs, oil and grease) However workers who have tried to take grab samples over a rain event will realise that the time of onset of rain is far from predictable and the start of a rain event in the middle of the night seems to be the rule rather than the exception.
1.2.
Alternative Approaches
As pointed out by Strenstroms group (Jiun-Shiu et al., 2002), in their study of the problems of sampling for oil and grease determinations, the usual types of automatic flow weighted sampler are not suitable for such purposes. This group reported that by selecting a particular time into a rain event it was possible to obtain a grab sample, which represented the event mean concentration. This might allow the use of an automated non-discriminatory sampler, which could be triggered by a rainfall detector and timer. However this approach, whilst highly suitable for some situations, is far from universally applicable. It is likely that the optimum time will be site dependent; it will almost certainly be weather dependent; and may vary with time as the nature of the catchment changes.
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The two alternative types of sample divider, which have found favour, are similarly unsuitable for this application. The churn splitter (Wide et al., 1998)) requires an input of energy to turn the churning device and, whilst it is not impossible to redesign this system to work as a continuous or intermittent churning device, it would increase complexity considerably and make it difficult to use in situations without power supplies. The cone splitter (Hatzell et al.1995; Wide et al., 1998) works in such a way that a quiescent zone is likely to be created in the upper part of the device, thus leading to the possibility that oil would separate out in the splitter. This particularly difficult sampling problem therefore needed a different approach.
1.3.
System Requirements
The method of analysis of samples, such as those in question which would contain significant amounts of free product, would normally be either to solvent-extract with a totally halogenated solvent for an IR determination (e.g. Bond, 1999), or to extract into a non-halogenated solvent for a gravimetric method (e.g. US EPA, 1999). It has been found by experience in the authors laboratory that, in circumstances where there is significant free product, the best way to obtain good recoveries is to extract the entire sample by repeated use of a separatory funnel and then extract the sample container several times with solvent, pooling the extractions prior to analysis. The practical size limit of a sample to allow this will be limited. If we are to study oily effluents from such systems, it is important that we are able to collect a representative sample for large storm events. The work reported here set out to meet the need to sample effluent from heavily oil-loaded parking surface models ranging in size from a single bay to a four bay unit. Remembering that it might be desirable to collect a reasonably sized sample from a rain event which produces over 1000 litres of water, it was necessary to find a means of splitting the sample such that the proportion of free oil to water in the collected sample was the same as that in the rejected excess. An approach, which might be taken, would be to spread the sample out representatively as it flowed over a flat surface and then to cut a proportion of the sample, as the remainder flowed to waste. With care, this can be used when the water is flowing relatively quickly, but the system will not work in all flow conditions, nor will it be efficient if the free product is not reasonably dispersed over the surface of the flowing sample stream for example where there is only a small amount of free product, which is insufficient to form a continuous film across the surface. The principle used to overcome this problem was, essentially, to collect the sample in a container then rapidly tip the entire contents onto a metal tray (swirl tray) intended to swirl the sample rapidly, break up any free oil into small droplets and then collect a representative sample between a pair of knife edges as the temporarily homogeneous sample flows mainly to waste. This process would need repeating many times in a rain event and thus an automatic tipping bucket system was developed. Whilst tipping bucket devices have been shown to have problems when measuring water volumes (Frankhauser et al., 1998), the requirements of the tipping bucket as a sampling device are somewhat different, when the ability to split the sample representatively is more important than any quantitative issues. Particularly attractive about a tipping bucket system was its relative simplicity and the fact that the device would require no power supplies, using, as it does, the kinetic energy contained in the sample to produce the mixing required. Logically it might be expected that the more times the sampler tipped within a rain event the better the expected precision since random sampling errors at each tip would be averaged out. This would suggest that there would be an improvement in the accuracy of the event mean concentration estimate if the tip volume were small. However the principle of the device is to use the energy of the water ejected in the tip to produce the mixing required. Initial trials indicated that to get sufficient energy to
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produce an adequate swirling motion; a volume of at least 200ml was required but that the system appeared to perform better up to a tip volume of around 1 litre. The tip distance (distance from tipping bucket edge to the base of the upright wall of the swirl tray) was also an important parameter. A series of tests were carried out using a variety of tip height and tip volume combinations and using the reproducibility of the collection volume for a single tip (measured as the relative standard deviation of the tip volume for at least 5 tips) as the factor of merit. See Figures 4A and 4B. The first device developed was a double-sided tipping bucket device with a tipping element 600mm long, 150mm wide and 180mm maximum depth, leading to a maximum tip volume of 4 litres. This was based on a device previously used by Pratt et al., (1990) for volumetric flow measurements. The intention was initially to use this to collect the volume of effluent required at each side of the tipping devices motion. However it was found difficult, in practice, to adjust the tip volumes on this design and the device proved difficult to operate in the confined spaces available in the inspection chambers used as sampling points for the pavements under test. The design was modified to be a one-sided device in which the bucket was returned to its load position by a counterweight, as shown schematically in Figure 1 below and on Figure 3, which is a scale drawing. Figure 2 is a photograph of the device during its tip cycle, which clearly demonstrates the mixing energy available when the bucket tips. This one-sided device has a slight disadvantage over the two-sided system, because as the bucket discharges it is necessary to allow a small amount of effluent to run to waste during the period in which the bucket is tipped forward. Failure to ensure that this is the case results in the effluent continuing to run into the bucket in its forward tipped position. At certain flow rates this can lead to the bucket being held forward by the mass of flowing water, stopping the effective mixing process, In all such cases the time for which the bucket is held forward is dependent on the effluent flow rate. In a long-term experiment, it was considered that there would be sufficient tips to allow the potential loss in sampling precision to be averaged out. On the other hand the one-sided device does have the advantage that the tip volume can be adjusted easily by moving the position of the counterweight. A very good linear
Figure 1: Schematic of the tipping bucket sample splitter in action.
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relationship was found between tip volume and the distance of the counterweight from the end of the adjustment screw (labelled K in Figure 3).
2.
METHODS
2.1.
Validation of the Tipping Bucket Device
Once the device had been constructed a series of experiments were carried out to enable us to establish the validity of the sampler. The question asked was: does the mixing and sampling system produce a sufficiently representative sample when presented with water containing a known amount of free product? The worst-case scenario when sampling water-containing oil is a situation in which oil arrives at the sampler in individual globules at random time intervals, rather than as a continuous flowing stream. This situation was replicated in this experiment.
Figure 2: Device during tip cycle showing violent mixing available to homogenise sample.
The experiment was performed by setting up a water flow, controlled by a constant head device, into the tipping bucket device via an aluminium channel and measured volumes of oil, of known density, were injected into the flow, by means of an autopipette. The injection was made directly onto the V-shaped channel used to carry the water flow only 10cm from the effluent outlet, thus maximising sample heterogeneity. Oil injection was scheduled such that the injection cycle was slightly shorter than the tip time for the device thus ensuring that, over the 10 tips used to collect the sample, oil arrived at the sampling device during a different part of the tip cycle for each tip. The split sample was collected and was then analysed by extraction into tetrachloromethane and IR analysis, as reported previously (Bond, 1999) using a calibration curve produced by dilution of the same oil as was used to prepare the samples. Table 1 shows the results of this experiment. The efficacy in mixing and sampling the dissolved component was also investigated. A similar series of experiments to that described above was carried out using manganese chloride solution as the surrogate pollutant. This was injected as a concentrated standard solution, as for the oil injections, with the resultant split sample being analysed by atomic absorption spectrometry for manganese. A sample of the tap water used to feed the system was also taken to allow blank corrections, but this was found to be unnecessary. Again the cycle was timed to give injection events at different parts of
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the tip cycle in each tip. At the time of writing, further experiments are underway to investigate the effectiveness when sampling suspended solids.
Figure 3: 1:20 Scale diagram of sampling device.
3.
RESULTS AND DISCUSSION
Figures 4A and 4B below show the variation in the relative precision of collected volume at each tip against tip distance and tip volume. It can be seen that precision improves as tip volume increases and that there is an optimum tip distance at around 200mm. 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0
200
400
600
800
Tip Volume ml
A
B
Figure 4 Variation of relative standard deviation (RSD) of Collected Volume against: A-Tip Distance and B-Tip Volume
The results of the validation of the tipping bucket device, with respect to the collection of free phase oil, is shown in Table 1 below. It can be seen that the mean value of the determined concentration is very close to the expected mean value, but for a ten tip cycle the typical relative error is quite large, with the mean relative error being 27%. For this system, under the conditions tested, there needs to be a relatively large number of tip events to allow the sampling errors to be averaged out and thus produce an acceptable estimate of the event mean concentration.
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Measured g/l
Applied g/l
0.4
1.0
1.8
1.4
1.8
1.8
0.8
1.0
1.9
1.4
2.1
1.8
Mean Value 1.47
Expected Mean 1.4
Table 1 Measured and Applied Concentrations for Six 10 -Tip Sampling Events-Oil
It should be remembered that this system was tested under very rigorous conditions and that under normal circumstances, when free product is present in effluent from a highly contaminated car parking surface, the effluent usually arrives at the sampler with the oil dispersed as a film on the surface and with some of the homogenisation process already completed as the effluent passes through the matrix of the water storage element and then out through the drain. Table 2 gives the results, for a totally soluble analyte, which show a slight bias in the data towards predictions of higher concentration Measured mg/l
Applied mg/l
5.0
4.0
5.9
5.8
5.8
5.8
5.4
5.0
5.4
5.0
Mean Value 5.5
Expected Mean 5.12
Table 2 Measured and Applied Concentrations for Five 10 Tip Sampling Events-Manganese
It is believed that this might be an artefact of the analyte injection process but in any event the bias is not sufficiently great to invalidate the device
4.
CONCLUSION
The device described here is capable of representatively sampling oil and grease from experimental and field pervious pavement systems which are contaminated with oil to the extent that there is free product in the effluent. Although the typical relative error for a small number of tip cycles is quite high it is believed that this sample offers a worthwhile improvement over grab sampling. A considerable amount of mixing effort is provided as the sampler tips a large volume of water against a surface. When tip volume and tip distance are optimised this produces an energetic mixing effect. Despite the obvious advantages that this sampler demonstrates it must be remembered that it is not a universally applicable sample splitter. If the intention is to analyse for volatile organic compounds this device is likely to cause considerable evaporative losses.
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5.
ACKNOWLEDGEMENTS
The authors wish to thank Waste Recycling Environmental Ltd who funded most of this work via the Landfill Tax Credit Scheme; EPG who provided the required matching funding; and the Nuffield foundation who supported Jim Phillips. The authors also wish to thank Mike Jenkins for assistance in the early part of this work and Alan Cranston and Paul Whitehall for their efforts in building and modifying the device.
REFERENCES Bond, P.C. (1999). Mineral Oil Biodegradation Within Permeable Pavements: Long-term Observations. Unpublished PhD Thesis, Coventry University, United Kingdom. Fankhauser, R. (1998). Influence of systematic errors from tipping bucket rain gauges on recorded rainfall data. Wat. Sci. Tech., 37(11),121-129. Jiun-Shiu, M., Khan, S., Ying-Xia, L., Lee-Hyung, K., Haejin, H., Sim-Lin, L., Kayhanian, M. and Stenstrom, M.K. (2002). Implication of oil and grease measurement in stormwater management systems. In: Proc. 9th Int. Conf. on Urban Drainage, Portland OR. USA. (CDROM) Pratt, C.J., Mantle, J.D.G. and Schofield, P.A. (1990). Porous pavements for flow and pollutant discharge control. In: Proc. 5th Int. Conf. on Urban Storm Drainage, Osaka, Japan, 839-844. Province of British Columbia (2003), British Columbia Field Sampling Manual for Continuous Monitoring and the Collection of Air, Air-Emission, Water, Wastewater, Soil, Sediment, and Biological Samples, last accessed 2nd Nov 2003 at http://wlapwww.gov.bc.ca/wat/wamr/labsys/field_man_pdfs/fld_man_03.pdf United States EPA (1999) Method 1664, Revision A: N-Hexane Extractable Material (HEM; Oil and Grease) and Silica Gel Treated N-Hexane Extractable Material (SGT-HEM; Non-polar Material) by Extraction and Gravimetry, last accessed 2nd Nov 2003 at http://www.epa.gov/waterscience/methods/ 1664f051.html#18 Wilde, F.D., Radtke, D.B., Gibs J. and Iwatsubo, R.T. (1998). Selection of equipment for water sampling, in National field manual for the collection of water-quality data: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 9, Chap. A2, last accessed 2nd Nov 2003 at http://pubs.water.usgs.gov/twri9A1
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