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The gully pot as a biochemical reactor
8)
Pergamon 0273-1223(95)00340-1
Wat. Sci. Tech. Vol. 31. No.7. pp. 229-236.1995. Copyngbt IC! 1995 IAWQ Pnnted in Great Britain. All ngbts reserved. 0273-1223195 $9'50 + 00()(}
THE GULLY POT AS A BIOCHEMICAL REACTOR Gregory M. Morrison*, D. Michael Revitt** and J. Bryan Ellis** * Department of Sanitary Engineering. Chalmers University of Teclmology. S-412 96 G6teborg. Sweden ** Urban Pollution Research Centre. Middlesex University. Bounds Green Road. London Nil 2NQ. UK
ABSTRACT Roadside gully pots (catch basins) have been identified as potential sources that can make significant contributions to storm water pollutant loadings. Between storm events the gully pot sediment and liquor undergo cbanges in composition as a result of biochemical reactions. Sediment maturation and acidic dissolution processes release pollutants from the contaminated chamber sediments and interstitial pore waters into the relatively clean gully pot liquor. Dissolved pollutant profiles for gully pot outflows therefore show substantial early contributions from gully pot liquor and interstitial waters reflecting microbial and geochemical degradation processes which act upon the trapped chamber sedlffients both during and between stoon events. The majority of dissolved organic carbon is washed out in the gully pot outflow in the early low flow stages, suggesting that the main contributing source is the supernatant gully pot liquor. Small additional releases coincide with. and indicate additional releases of. soluble organics from the interstitial waters as the basal sediments are disturbed. Conductivity cbanges show that dissolved inorganics also exhibit efficient removal during the low flow stages of storms. with the absence of delayed peaks indicating a neglIgible contribution from the settled gully pot sediments. During storm events, low runoff rates produce marked decreases in pH levels from the initial gully pot lIquor value of pH 6.0-7.1 to a value approacbing typical rainfall levels (average rainfall pH 4.1). This lowenng of the pH indicates that the dissolved buffering agents initially present on the road surface and in the gully pot liquor have become depleted and exhaustion of dissolved Ca clearly Illustrates this effect. Initial decreases in dissolved oxygen concentrations and redox potential are indicative of exposure of the reduced basal sediments as overlying supernatant liquor is washed out. A subsequent secondary decrease in redox potential. coinciding with increasing flows. is due to the addlbonal release of reduced intersbtial waters as the gully pot basal sediments are disturbed. Finally. dissolved oxygen levels return to nonnal as the oxygenared surface waters become predominant in the outflow waters.
=
KEYWORDS
Gully pot; biochemical processes; solids; organic carbon; pH; redox potential; conductivity; dissolved oxygen.
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O. M. MORRISON et al.
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INTRODUCTION It is now widely recognised that the accumulation of sediments in urban sewer systems can lead to surcharging, flooding, premature overflow and receiving water pollution. The deposited sediment, when mixed with sanitary sewage in an oxygen-deficient combined sewer environment, provides an ideal incubation site for anaerobic biodegradation of any included organic materials. Subsequent scouring of the sewer invert during storm flow conditions can then yield high DOC as well as BOD, COD and NH 4-N loadings to the receiving water (Ellis, 1986). The majority of sediment enters the sewerage system from washoff of surface solids via roadside gully pots (or catch basins). However, there is considerable evidence to show that such inlet chambers are extremely poor sedimentation basins and retain little of the frnest. heavily contaminated particulate fractions (Sartor and Boyd, 1972; Morrison et al., 1988). Ideally. the larger and heavier solids should settle out in the gully chamber but a number of workers have demonstrated that overall trap efficiency is a direct function of inflow flow rates (Lager et al., 1977; Pratt et al .. 1987; Grottker, 1990). Inflow rates in excess of 3-41 s·1 have been shown to lead to significant disturbance. mixing and mobilisation of the inlet chamber contents (Morrison et aI., 1988) although it would also appear that particle heterogeneity, sediment age and depth are also factors influencing gully pot outflow loadings (Pratt and Adams. 1984; CIRlA. 1987; Clegg et al.• 1993; Butler and Karunaratne, 1994). A number of studies have categorised the composition and size distributions of gully pot sediments (Sartor and Boyd, 1972; Ellis and Harrop, 1984; CIRIA. 1987; Butler et al.• 1993). The total organic contents typically comprise 10-20% of the total solids weight with non-particulate material (litter, vegetation etc.) making up some 10% of the total sediment volume. Whilst the median sizes of basal gully pot sediment varies between I to 10 mm. the organic materials are typically comprised of particles less than 0.5 mm with up to 30% being associated with size fractions less than 0.06 mm. Even when the organic content is low «7%). the fact that the large majority of the total sediment COD is associated with the settleable fraction (Crabtree and Forster, 1989) might imply that the organic matter is bound to the smaller mineral size fractions. Previous papers have reported on gully pot mechanisms controlling metal speciation within the chamber and associated toxic outflows to the sewer system (Morrison et al., 1988, 1989); this paper will focus on the physical and biochemical processes occurring within gully pots following surface inflows which lead to sediment disturbance and pollutant mobilisation. EXPERIMENTAL The gully pot catchment chosen for this study is located at Chalmers University of Technology, Goteborg. Sweden and drains a 390 m2 surface. The catchment is almost completely impervious with drainage occurring only from parking areas, a road and a roadside kerb. The gully pot has a standing volume of 41.5 I and is situated adjacent to the University basement. The gully pot was cleaned before commencement of the study. As the basement is at a lower level it was convenient to dig a trench between the gully pot and the basement wall allowing electric cables and sampling tubes to connect between the two. The study was carried out over a two-month summer period between July and September. Sow measurements A gully pot flow measuring device was used to monitor the depth of road runoff passing through a 30· V• notch weir. A gully pot sampler to collect road runoff. was positioned in the gully pot chamber so that all the flow was channelled into the flume. How height was recorded and calibrated against flow volume passing through the V-notch.
The gully pot as a biochemical reactor
231
CQotinuous monjtorine system Conductivity. pH. dissolved oxygen concentration and redox potential were continuously monitored in the gully pot liquor. Gully pot liquor was pumped out of the gully pot, through an opening in the basement wall, via a strengthened 12 mm diameter polyethylene pipe and returned to the gully pot in the same manner after passing through the on-line measurement system. A water flow rate of 2.61 min-I allowed a water depth of 30 mm in the continuous system, and resulted in the gully pot liquor being recycled every 0.27 h under dry conditions. The continuous monitoring system consisted of a sealed PVC pipe of 50 mm diameter and 400 mm length. with an inlet of 13 mm diameter and an outlet of 25 mm diameter. Measurement electrodes were inserted, through holes in the pipe, to ensure sufficient immersion in the sampled gully pot liquor.
Road runoff entering the gully pot was sampled by a tipping bucket sampler located on the flow measuring device. The sampled water flowed under gravity through a polyethylene tube into the University basement where a step motor sampler produced a 1 I sample every three minutes. Gully pot liquor and outflow were both sampled manually through a 12 mm polyethylene pipe by gravity feed. Concentrations of dissolved and particulate organic carbon. dissolved calcium and suspended solids were analysed in the collected samples of road runoff. gully pot liquor and gully pot outflow. RESULTS AND DISCUSSION The contents of gully pots are subjected to a combination of biological and chemical processes during dry weather storage and. in addition, physical processes become important during periods of heavy rainfall. As a result a rapid change of gully pot contents may occur as a shock pulse of acidic, well-oxygenated rainfall enters the system via the road surface. Physical processes In this study, the impacts on the contents of a gully pot as a consequence of six storm events possessing different flow intensities and volumes have been assessed. The characteristics of each storm event including storm duration and length of the antecedent dry period are described in Table I. Two storm events (Storms 2 and 5) occurred after relatively long antecedent dry spells of 10 days. Storm 5 was the most intense storm with, at one stage, 2.7 mm of rain falling within a five minute period. Storm 2 was a long heavy storm which continued for 5.8 h with a maximum road runoff flow rate of 2.03 I s-I. This storm began with a low flow for the first hour of 0.05 I s-I, but had two intense rainfall periods during the third hour. Storm I was also an intense summer storm of two hours' duration with most of the flow volume occurring during the first half• hour. Storms 3, 4 and 6 were low intensity storms each with a short antecedent dry period.
The depression storage value for the catchment has been calculated from a plot of rainfall depth against total runoff volume for the two-month study period. The resulting linear relationship (correlation coefficient. 0.998) gave a value of 0.146 mm which represents the water storage threshold within the catchment resulting from surface ponding, water-particulate adhesion coefficients, evaporation/wind losses and infiltration capacity which must be exceeded before runoff enters the gully pot. This is a low value compared with the 0.13-\.5 mm range which has been observed in other European catchments (Ellis et aI., 1986; Falk and Niemczynowicz, 1978) but is not surprising given the small area and well-sealed nature of the contributing catchment. The gully pot outflow loadings for dissolved calcium. dissolved and particulate organic carbon and suspended solids for five of the storm events (Storms 2-6) are shown in Table 2 and can be compared to the hydrological characteristics in Table \. The measured loadings of organic material and solids show a positive relationship to the antecent dry period length. This suggests a continuous accumulation of organic material and suspended solids during the dry period if it can be assumed that an eqUilibrium between input
232
G. M. MORRISON et al.
and output is established for these parameters. Using a wet/dry vacuum cleaner to collect surface sediments from two sub-areas of the catchment it was estimated that a maximum total load of 17.2 kg of solids and 250g of particulate organic material was available for wash-off. Only a small proportion of these solids eventually appears in the gully pot outflow with 2-3% of the available surface solids being observed for Storms 2 and 5 and for the less intense storms this is reduced to less than 0.3%. A similar discrimination occurs for the particulate organic carbon with between 10 to 18% of the total available being monitored in the outflows during the higher intensity storms but only 0.6-3.0% appearing for Storms 3. 4 and 6. These results are consistent with those reported by Ashley and Crabtree (1992) in which solids washed into the gully pots were found to contain organic fractions of up to 40% and to possess a size composition in which 75% of particles were less than 250 Ilm in diameter. It is. therefore. clearly only the very fine solids which have been accumulated on the road surface, which have the ability to remain suspended in the storm water and to be transmitted directly into the gully pot system under normal flow conditions. Table I. Hydrological characteristics for the six monitored storm events Storm 1 2 3 4 5 6
Antecedent dry period, d
Storm duration, h
8
2.1 5.8 8.7 4.3 1.0 2.5
10 3 1 10 1
Rainfall, mm
18.5 2.9 4.6 9.7 6.5
Road runoff,l 1690 6930 1670 2160 3950 3990
Table 2. Gully pot outflow loadings of dissolved calcium. dissolved and particulate organic carbon and suspended solids Storm 2 3 4 5 6
Dissolved Ca, g
Dissolved organic C, g
Particulate organic C, g
Suspended solids, g
12.4 3.9 2.1 2.7 1.6
28.3 15.1 10.3 21.4 6.5
45.3 2.4 1.5 26.2 7.6
467 23 28 314 48
A major contnbutlon to the suspended sediment in the gully pot outflow during Storms 2 and 5 is derived from the gully pot basal sediment. During intense and large volume storm events a net loss of gully pot sediment occurs, while during less intense events the gully pot may gain sediment. For suspended solids the antecedent dry period relates only to road surface sediments so that when the gully pot contribution increases, the correlation (suspended solids with antecedent dry period) becomes weaker. The importance of the gully pot in contributing sediment is underlined in Storm 2 for which mass balance calculations show that the gully pot sediments produced 66.5% of the suspended solids in the outflow whereas for Storm 4 the contribution was only 11.3%. The results point to two processes which are important in controlling the loadings of organic material and suspended solids in gully pot outflow: I. Road surface sediment mobilisation which depends on both accumulation of fines and hydrodynamic mobilisation during the storm event. 2. Gully pot basal sediment mobilisation which only occurs during larger storm events (i.e. Storms 2 and 5. but not 3. 4 and 6). A threshold inflow volume/rolte therefore exists beyond which disturbance of the gully pot chamber reactor contents occurs.
The gully pot as a biochemical reactor
233
Plow,lIs
,
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....
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Dissolved oxygen. mg/l
pH
Redox. mV
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....
8
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TIme. minutes Figure 1. The effect of tncoming flow on redox potential. dissolved oxygen and pH of a gully pot liquor at the beginnmg of a 810nn event Biolo~ical
and chemical processes
The measured hydrogen ion concentration of the rainfall reaching the road surface during the study period was in the pH range 3.8·4.9. When this acidic rainfall reaches the road surface it is initially weakly buffered by road dust derived organic and inorganic salts. On entry to the gully pot the road runoff undergoes further buffering by the gully pot liquor which initially remains at the pH established between storms (Figure I). Subsequently the incoming road runoff reduces the pH to between 4.0 and 5.0. Mter the storm event both pH and conductivity show definite increases in the gully pot liquor (Figure 2) and is followed by an increase in dissolved calcium. These increases may be attributed to dissolution of cement in the concrete gully pot structure by the acidic gully pot liquor remaining after the storm event thL~
G. M. MORRISON tt al.
234
resulting in the release of calcium and carbonate species; this in tum leads to the observed changes in pH and conductivity. The relatively slow release of calcium ions compared with the initial rapid decrease in hydrogen ions on completion of the rainfall event is clearly illustrated in Figure 2. A closer examination of the data indicates that during the first 50 hours of the dry period the concentration of hydrogen ions decreases by 20 mg I-I and this is accompanied by an associated increase in calcium ions of 2 mg I-I. Dissolved oxygen levels decreased to 60-80% saturation after storm events and remained relatively constant over the subsequent dry period. Redox potential also remained relatively constant indicating that most of the in-gully-pot biological activity is restricted to the mobilisation of sediment, rather than within the gully pot liquor itself. pH
Conductivity, mS/m Dissolved Ca, mg/l 12~------------------------------------------'
•
10
•
a
pH
a 4
Conductivity 2
o~~~~~~~~~~~~~~~~~~~~
o
50
100
150
200
250
300
350
Time, h Figure 2. pH, conductivity and calcium variations in gully pot liquor during a dry period.
Gully pot interstitial sediment water mobilisation during storm events is indicated by changes in conductivity, dissolved oxygen, redox potential, ATP and dissolved organic carbon levels measured in the gully pot liquor. Interstitial waters were disturbed at the beginning of all recorded storm events and were indicated by a sharp drop in redox potential and dissolved oxygen concentration, with rapid recovery and return to normal conditions on further mixing with oxygenated road runoff (Figure I). Clegg et al. (1993) have observed variations in the redox potential of undisturbed gully pot sediments of between -50 and -450 m V indicating their capacity for contributing to the reducing conditions within the gully pot. For all the storms studied in this research programme, the minimum redox potential and dissolved oxygen concentrations recorded in the gully pot liquor were 46 mV and 6 mg I-I, respectively, with values between storms stabilising at around 250 mV and 9 mg I-I. The mechanism for redox potential and dissolved oxygen sag entails entrainment and exposure of the reduced basal sediments as overlying supernatant liquor is washed out. Further evidence for early sediment mobilisation is that dissolved organic carbon increases, by up to 41 mg I-I in Storm 2, due largely to the formation of sedimentary maturation products (Figure 3) while the road runoff contribution adds to the early flushing effect. CONCLUSIONS The results obtained in this experimental study indicate that flushing/cleaning of a gully pot is ideally required at 4-7 d intervals to provide maximum control of pollutant outflows to the sewer system. In the absence of such regular maintenance, gully pots can only be expected to act as effective grit chambers with
The gully pot as a biochemical reactor
235
rlimited contributions to the removal of persistent toxic pollutants which are washed from highway surfaces. Indeed. gully pots appear likely to accentuate storm water pollution problems through biochemical transfonnation and r~lease of pollutants from the sediment to the dissolved phase.
Flow,
Us
DOC, mg/l
2.5~-------------------------------------,50
DOC outflow
~---, -
.\
2
I-~"<\ ......;.. ~DOC I I I I
1.5 I I
, \ \ "
40
gullypot 30
I
20
1 I I
0.5
I
10
Time, minutes Figure 3. Variations of flow and dissolved organic material in Storm 2.
REFERENCES Ashley. R.M .• Crabtree. R.W. (1992). Sediment origins. deposition and build-up in combined sewer systems. Wat. Sci. Technol .• 25 (8).1-12. Butler. D .• Clark. P.• Payne. J. (1983). Management of sediment m drainage catchments. In: Proc. 6th /111. Conj. Urban Storm Drainage. MarsaJek. J•• Torno. H.C. (eds). Seapomt Publishing. Victoria. Canada. 736-741. Butler. D.• Karunaratne. S.P.H.G. (1995). The suspended solids trap efficiency of the roarlside gully pot Water Res.• 29. 719-729. CIRIA (1987). Sediment movement in combmed sewerage and stormwater drainage systems. Project Report No. J. Construction Industry Research and Information Association. London. Clegg. S .• Forster. c.F.. Crabtree. R.W. (1993). An examinauon into the ageing of gully pot sedtments. Environ. Technol .• 14. 453-461. Crabtree. R.W .• Forster. C.F. (1989). Preparation protocols for the analysIs of sewer sediment samples. Report ER 354E. Water Research Centre. Swindon. Ellis. J.B. (1986). POllutional aspects of urban runoff. In: Urban Runoff Pollution. Torno. H.C .• MarsaIek. J•• Desbordes. M. (eds). Springer-Verlag. Berlin. 1-38. Ellis. J.B .• Harrop. O. (1984). Variations in solids loading to roadSide gully pots. Sci. Tot. Environ.. 33. 203-211. Ellis. J.B .• Harrop. D.O .• Revitl, D.M. (1986). Hydrological controls of pollutant removal from highway surfaces. Water Res .• 20. 589-595. Falk. J.• Niemczynowicz. J. (1978). Characteristics of the above·ground runoff in sewered catchments. In: Urban Storm Drainage. Helliwell. P.R. (ed). Pen tech Press. Plymouth. 159-171. ReICher. I.. Prall, C.J .. Elliott. G.E.P. (1978). An assessment of the importance of roadside gully pots in determining the quality of stormwater runoff. In: Urban Storm Drainage. Helliwell. P.R. (ed). Pentech Press, Plymouth. 586-602. Groltker. M. (1990). Pollutant removal by gully pots in different catchment areas. Sci. Tot. Environ .• 93. 515-522. Lager. J.A .• Smith. W.O .• Tchoblanoglous. O. (1977). Suspended solids discharge from highway gully pots m a residenl1al catchment. Sc,. Tot. Environ .• 59. 355-364.
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G. M. MORRISON et al.
Morrison, G.M.P .• Revill, O.M., Ellis. J.B., Svensson, G., Balmer, P. (1988). Transport mechanisms and processes for metal species in a gully pot system. Wattr Res.,ll, 1417·1427. Morrison. G.M.P., Revitt, O.M., Ellis. J .B. (1989). Sources and stonn loading variations of metal species in a gully pot catchment. Sci. TOI. Environ., 80. 267·278. Pratt, CJ., Adams, J.R.W. (1984). SedlDlentsupply and transtrussion in roadsiide gully pots, Sci. To/. Environ., JJ, 213·224. Pratt, CJ., EllIott, G.E.P., Fulcher. G.A. (1987). Suspended solids discharge from highway gully polS in a re5ldenual catchment. SCI. TOI. Environ .• 59, 355·364. Sartor. JD .. Boyd, G.B. (1972). Water pollution aspects of street surface contaminants. US EPA Report No. R2·72-08I, US Government Printing Office. Washington DC. Verbanck, M. (ed). (1993). Origin, occurrence and behaviour of sediments in sewer systems. Wat. Sci. Technol., 25 (8), 2J4 pp.
Pergamon 0273-1223(95)00340-1
Wat. Sci. Tech. Vol. 31. No.7. pp. 229-236.1995. Copyngbt IC! 1995 IAWQ Pnnted in Great Britain. All ngbts reserved. 0273-1223195 $9'50 + 00()(}
THE GULLY POT AS A BIOCHEMICAL REACTOR Gregory M. Morrison*, D. Michael Revitt** and J. Bryan Ellis** * Department of Sanitary Engineering. Chalmers University of Teclmology. S-412 96 G6teborg. Sweden ** Urban Pollution Research Centre. Middlesex University. Bounds Green Road. London Nil 2NQ. UK
ABSTRACT Roadside gully pots (catch basins) have been identified as potential sources that can make significant contributions to storm water pollutant loadings. Between storm events the gully pot sediment and liquor undergo cbanges in composition as a result of biochemical reactions. Sediment maturation and acidic dissolution processes release pollutants from the contaminated chamber sediments and interstitial pore waters into the relatively clean gully pot liquor. Dissolved pollutant profiles for gully pot outflows therefore show substantial early contributions from gully pot liquor and interstitial waters reflecting microbial and geochemical degradation processes which act upon the trapped chamber sedlffients both during and between stoon events. The majority of dissolved organic carbon is washed out in the gully pot outflow in the early low flow stages, suggesting that the main contributing source is the supernatant gully pot liquor. Small additional releases coincide with. and indicate additional releases of. soluble organics from the interstitial waters as the basal sediments are disturbed. Conductivity cbanges show that dissolved inorganics also exhibit efficient removal during the low flow stages of storms. with the absence of delayed peaks indicating a neglIgible contribution from the settled gully pot sediments. During storm events, low runoff rates produce marked decreases in pH levels from the initial gully pot lIquor value of pH 6.0-7.1 to a value approacbing typical rainfall levels (average rainfall pH 4.1). This lowenng of the pH indicates that the dissolved buffering agents initially present on the road surface and in the gully pot liquor have become depleted and exhaustion of dissolved Ca clearly Illustrates this effect. Initial decreases in dissolved oxygen concentrations and redox potential are indicative of exposure of the reduced basal sediments as overlying supernatant liquor is washed out. A subsequent secondary decrease in redox potential. coinciding with increasing flows. is due to the addlbonal release of reduced intersbtial waters as the gully pot basal sediments are disturbed. Finally. dissolved oxygen levels return to nonnal as the oxygenared surface waters become predominant in the outflow waters.
=
KEYWORDS
Gully pot; biochemical processes; solids; organic carbon; pH; redox potential; conductivity; dissolved oxygen.
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INTRODUCTION It is now widely recognised that the accumulation of sediments in urban sewer systems can lead to surcharging, flooding, premature overflow and receiving water pollution. The deposited sediment, when mixed with sanitary sewage in an oxygen-deficient combined sewer environment, provides an ideal incubation site for anaerobic biodegradation of any included organic materials. Subsequent scouring of the sewer invert during storm flow conditions can then yield high DOC as well as BOD, COD and NH 4-N loadings to the receiving water (Ellis, 1986). The majority of sediment enters the sewerage system from washoff of surface solids via roadside gully pots (or catch basins). However, there is considerable evidence to show that such inlet chambers are extremely poor sedimentation basins and retain little of the frnest. heavily contaminated particulate fractions (Sartor and Boyd, 1972; Morrison et al., 1988). Ideally. the larger and heavier solids should settle out in the gully chamber but a number of workers have demonstrated that overall trap efficiency is a direct function of inflow flow rates (Lager et al., 1977; Pratt et al .. 1987; Grottker, 1990). Inflow rates in excess of 3-41 s·1 have been shown to lead to significant disturbance. mixing and mobilisation of the inlet chamber contents (Morrison et aI., 1988) although it would also appear that particle heterogeneity, sediment age and depth are also factors influencing gully pot outflow loadings (Pratt and Adams. 1984; CIRlA. 1987; Clegg et al.• 1993; Butler and Karunaratne, 1994). A number of studies have categorised the composition and size distributions of gully pot sediments (Sartor and Boyd, 1972; Ellis and Harrop, 1984; CIRIA. 1987; Butler et al.• 1993). The total organic contents typically comprise 10-20% of the total solids weight with non-particulate material (litter, vegetation etc.) making up some 10% of the total sediment volume. Whilst the median sizes of basal gully pot sediment varies between I to 10 mm. the organic materials are typically comprised of particles less than 0.5 mm with up to 30% being associated with size fractions less than 0.06 mm. Even when the organic content is low «7%). the fact that the large majority of the total sediment COD is associated with the settleable fraction (Crabtree and Forster, 1989) might imply that the organic matter is bound to the smaller mineral size fractions. Previous papers have reported on gully pot mechanisms controlling metal speciation within the chamber and associated toxic outflows to the sewer system (Morrison et al., 1988, 1989); this paper will focus on the physical and biochemical processes occurring within gully pots following surface inflows which lead to sediment disturbance and pollutant mobilisation. EXPERIMENTAL The gully pot catchment chosen for this study is located at Chalmers University of Technology, Goteborg. Sweden and drains a 390 m2 surface. The catchment is almost completely impervious with drainage occurring only from parking areas, a road and a roadside kerb. The gully pot has a standing volume of 41.5 I and is situated adjacent to the University basement. The gully pot was cleaned before commencement of the study. As the basement is at a lower level it was convenient to dig a trench between the gully pot and the basement wall allowing electric cables and sampling tubes to connect between the two. The study was carried out over a two-month summer period between July and September. Sow measurements A gully pot flow measuring device was used to monitor the depth of road runoff passing through a 30· V• notch weir. A gully pot sampler to collect road runoff. was positioned in the gully pot chamber so that all the flow was channelled into the flume. How height was recorded and calibrated against flow volume passing through the V-notch.
The gully pot as a biochemical reactor
231
CQotinuous monjtorine system Conductivity. pH. dissolved oxygen concentration and redox potential were continuously monitored in the gully pot liquor. Gully pot liquor was pumped out of the gully pot, through an opening in the basement wall, via a strengthened 12 mm diameter polyethylene pipe and returned to the gully pot in the same manner after passing through the on-line measurement system. A water flow rate of 2.61 min-I allowed a water depth of 30 mm in the continuous system, and resulted in the gully pot liquor being recycled every 0.27 h under dry conditions. The continuous monitoring system consisted of a sealed PVC pipe of 50 mm diameter and 400 mm length. with an inlet of 13 mm diameter and an outlet of 25 mm diameter. Measurement electrodes were inserted, through holes in the pipe, to ensure sufficient immersion in the sampled gully pot liquor.
Road runoff entering the gully pot was sampled by a tipping bucket sampler located on the flow measuring device. The sampled water flowed under gravity through a polyethylene tube into the University basement where a step motor sampler produced a 1 I sample every three minutes. Gully pot liquor and outflow were both sampled manually through a 12 mm polyethylene pipe by gravity feed. Concentrations of dissolved and particulate organic carbon. dissolved calcium and suspended solids were analysed in the collected samples of road runoff. gully pot liquor and gully pot outflow. RESULTS AND DISCUSSION The contents of gully pots are subjected to a combination of biological and chemical processes during dry weather storage and. in addition, physical processes become important during periods of heavy rainfall. As a result a rapid change of gully pot contents may occur as a shock pulse of acidic, well-oxygenated rainfall enters the system via the road surface. Physical processes In this study, the impacts on the contents of a gully pot as a consequence of six storm events possessing different flow intensities and volumes have been assessed. The characteristics of each storm event including storm duration and length of the antecedent dry period are described in Table I. Two storm events (Storms 2 and 5) occurred after relatively long antecedent dry spells of 10 days. Storm 5 was the most intense storm with, at one stage, 2.7 mm of rain falling within a five minute period. Storm 2 was a long heavy storm which continued for 5.8 h with a maximum road runoff flow rate of 2.03 I s-I. This storm began with a low flow for the first hour of 0.05 I s-I, but had two intense rainfall periods during the third hour. Storm I was also an intense summer storm of two hours' duration with most of the flow volume occurring during the first half• hour. Storms 3, 4 and 6 were low intensity storms each with a short antecedent dry period.
The depression storage value for the catchment has been calculated from a plot of rainfall depth against total runoff volume for the two-month study period. The resulting linear relationship (correlation coefficient. 0.998) gave a value of 0.146 mm which represents the water storage threshold within the catchment resulting from surface ponding, water-particulate adhesion coefficients, evaporation/wind losses and infiltration capacity which must be exceeded before runoff enters the gully pot. This is a low value compared with the 0.13-\.5 mm range which has been observed in other European catchments (Ellis et aI., 1986; Falk and Niemczynowicz, 1978) but is not surprising given the small area and well-sealed nature of the contributing catchment. The gully pot outflow loadings for dissolved calcium. dissolved and particulate organic carbon and suspended solids for five of the storm events (Storms 2-6) are shown in Table 2 and can be compared to the hydrological characteristics in Table \. The measured loadings of organic material and solids show a positive relationship to the antecent dry period length. This suggests a continuous accumulation of organic material and suspended solids during the dry period if it can be assumed that an eqUilibrium between input
232
G. M. MORRISON et al.
and output is established for these parameters. Using a wet/dry vacuum cleaner to collect surface sediments from two sub-areas of the catchment it was estimated that a maximum total load of 17.2 kg of solids and 250g of particulate organic material was available for wash-off. Only a small proportion of these solids eventually appears in the gully pot outflow with 2-3% of the available surface solids being observed for Storms 2 and 5 and for the less intense storms this is reduced to less than 0.3%. A similar discrimination occurs for the particulate organic carbon with between 10 to 18% of the total available being monitored in the outflows during the higher intensity storms but only 0.6-3.0% appearing for Storms 3. 4 and 6. These results are consistent with those reported by Ashley and Crabtree (1992) in which solids washed into the gully pots were found to contain organic fractions of up to 40% and to possess a size composition in which 75% of particles were less than 250 Ilm in diameter. It is. therefore. clearly only the very fine solids which have been accumulated on the road surface, which have the ability to remain suspended in the storm water and to be transmitted directly into the gully pot system under normal flow conditions. Table I. Hydrological characteristics for the six monitored storm events Storm 1 2 3 4 5 6
Antecedent dry period, d
Storm duration, h
8
2.1 5.8 8.7 4.3 1.0 2.5
10 3 1 10 1
Rainfall, mm
18.5 2.9 4.6 9.7 6.5
Road runoff,l 1690 6930 1670 2160 3950 3990
Table 2. Gully pot outflow loadings of dissolved calcium. dissolved and particulate organic carbon and suspended solids Storm 2 3 4 5 6
Dissolved Ca, g
Dissolved organic C, g
Particulate organic C, g
Suspended solids, g
12.4 3.9 2.1 2.7 1.6
28.3 15.1 10.3 21.4 6.5
45.3 2.4 1.5 26.2 7.6
467 23 28 314 48
A major contnbutlon to the suspended sediment in the gully pot outflow during Storms 2 and 5 is derived from the gully pot basal sediment. During intense and large volume storm events a net loss of gully pot sediment occurs, while during less intense events the gully pot may gain sediment. For suspended solids the antecedent dry period relates only to road surface sediments so that when the gully pot contribution increases, the correlation (suspended solids with antecedent dry period) becomes weaker. The importance of the gully pot in contributing sediment is underlined in Storm 2 for which mass balance calculations show that the gully pot sediments produced 66.5% of the suspended solids in the outflow whereas for Storm 4 the contribution was only 11.3%. The results point to two processes which are important in controlling the loadings of organic material and suspended solids in gully pot outflow: I. Road surface sediment mobilisation which depends on both accumulation of fines and hydrodynamic mobilisation during the storm event. 2. Gully pot basal sediment mobilisation which only occurs during larger storm events (i.e. Storms 2 and 5. but not 3. 4 and 6). A threshold inflow volume/rolte therefore exists beyond which disturbance of the gully pot chamber reactor contents occurs.
The gully pot as a biochemical reactor
233
Plow,lIs
,
0.8 -
o.e~
• •
o ..... ~
•
•
•
•••
•
•
•
•
•• ••
• •
0.2-
••
• • ••
•
• oM.~~~"----~~~--"----~~~~ o
2
....
e
a
' 0 ' 2 ' .... ' e ' 8 20 22 2 ....
Tune, minutes
Dissolved oxygen. mg/l
pH
Redox. mV
200,...---------------------,8
-.
150
.:...~ -'~
Dissolved oxygen
....
8
100
\
50
,, \
Redox
o
o
7
2
4
e
8
10
12
......
14 1 e
_- - -Pit-· ,
8
20 22 24
5
4
TIme. minutes Figure 1. The effect of tncoming flow on redox potential. dissolved oxygen and pH of a gully pot liquor at the beginnmg of a 810nn event Biolo~ical
and chemical processes
The measured hydrogen ion concentration of the rainfall reaching the road surface during the study period was in the pH range 3.8·4.9. When this acidic rainfall reaches the road surface it is initially weakly buffered by road dust derived organic and inorganic salts. On entry to the gully pot the road runoff undergoes further buffering by the gully pot liquor which initially remains at the pH established between storms (Figure I). Subsequently the incoming road runoff reduces the pH to between 4.0 and 5.0. Mter the storm event both pH and conductivity show definite increases in the gully pot liquor (Figure 2) and is followed by an increase in dissolved calcium. These increases may be attributed to dissolution of cement in the concrete gully pot structure by the acidic gully pot liquor remaining after the storm event thL~
G. M. MORRISON tt al.
234
resulting in the release of calcium and carbonate species; this in tum leads to the observed changes in pH and conductivity. The relatively slow release of calcium ions compared with the initial rapid decrease in hydrogen ions on completion of the rainfall event is clearly illustrated in Figure 2. A closer examination of the data indicates that during the first 50 hours of the dry period the concentration of hydrogen ions decreases by 20 mg I-I and this is accompanied by an associated increase in calcium ions of 2 mg I-I. Dissolved oxygen levels decreased to 60-80% saturation after storm events and remained relatively constant over the subsequent dry period. Redox potential also remained relatively constant indicating that most of the in-gully-pot biological activity is restricted to the mobilisation of sediment, rather than within the gully pot liquor itself. pH
Conductivity, mS/m Dissolved Ca, mg/l 12~------------------------------------------'
•
10
•
a
pH
a 4
Conductivity 2
o~~~~~~~~~~~~~~~~~~~~
o
50
100
150
200
250
300
350
Time, h Figure 2. pH, conductivity and calcium variations in gully pot liquor during a dry period.
Gully pot interstitial sediment water mobilisation during storm events is indicated by changes in conductivity, dissolved oxygen, redox potential, ATP and dissolved organic carbon levels measured in the gully pot liquor. Interstitial waters were disturbed at the beginning of all recorded storm events and were indicated by a sharp drop in redox potential and dissolved oxygen concentration, with rapid recovery and return to normal conditions on further mixing with oxygenated road runoff (Figure I). Clegg et al. (1993) have observed variations in the redox potential of undisturbed gully pot sediments of between -50 and -450 m V indicating their capacity for contributing to the reducing conditions within the gully pot. For all the storms studied in this research programme, the minimum redox potential and dissolved oxygen concentrations recorded in the gully pot liquor were 46 mV and 6 mg I-I, respectively, with values between storms stabilising at around 250 mV and 9 mg I-I. The mechanism for redox potential and dissolved oxygen sag entails entrainment and exposure of the reduced basal sediments as overlying supernatant liquor is washed out. Further evidence for early sediment mobilisation is that dissolved organic carbon increases, by up to 41 mg I-I in Storm 2, due largely to the formation of sedimentary maturation products (Figure 3) while the road runoff contribution adds to the early flushing effect. CONCLUSIONS The results obtained in this experimental study indicate that flushing/cleaning of a gully pot is ideally required at 4-7 d intervals to provide maximum control of pollutant outflows to the sewer system. In the absence of such regular maintenance, gully pots can only be expected to act as effective grit chambers with
The gully pot as a biochemical reactor
235
rlimited contributions to the removal of persistent toxic pollutants which are washed from highway surfaces. Indeed. gully pots appear likely to accentuate storm water pollution problems through biochemical transfonnation and r~lease of pollutants from the sediment to the dissolved phase.
Flow,
Us
DOC, mg/l
2.5~-------------------------------------,50
DOC outflow
~---, -
.\
2
I-~"<\ ......;.. ~DOC I I I I
1.5 I I
, \ \ "
40
gullypot 30
I
20
1 I I
0.5
I
10
Time, minutes Figure 3. Variations of flow and dissolved organic material in Storm 2.
REFERENCES Ashley. R.M .• Crabtree. R.W. (1992). Sediment origins. deposition and build-up in combined sewer systems. Wat. Sci. Technol .• 25 (8).1-12. Butler. D .• Clark. P.• Payne. J. (1983). Management of sediment m drainage catchments. In: Proc. 6th /111. Conj. Urban Storm Drainage. MarsaJek. J•• Torno. H.C. (eds). Seapomt Publishing. Victoria. Canada. 736-741. Butler. D.• Karunaratne. S.P.H.G. (1995). The suspended solids trap efficiency of the roarlside gully pot Water Res.• 29. 719-729. CIRIA (1987). Sediment movement in combmed sewerage and stormwater drainage systems. Project Report No. J. Construction Industry Research and Information Association. London. Clegg. S .• Forster. c.F.. Crabtree. R.W. (1993). An examinauon into the ageing of gully pot sedtments. Environ. Technol .• 14. 453-461. Crabtree. R.W .• Forster. C.F. (1989). Preparation protocols for the analysIs of sewer sediment samples. Report ER 354E. Water Research Centre. Swindon. Ellis. J.B. (1986). POllutional aspects of urban runoff. In: Urban Runoff Pollution. Torno. H.C .• MarsaIek. J•• Desbordes. M. (eds). Springer-Verlag. Berlin. 1-38. Ellis. J.B .• Harrop. O. (1984). Variations in solids loading to roadSide gully pots. Sci. Tot. Environ.. 33. 203-211. Ellis. J.B .• Harrop. D.O .• Revitl, D.M. (1986). Hydrological controls of pollutant removal from highway surfaces. Water Res .• 20. 589-595. Falk. J.• Niemczynowicz. J. (1978). Characteristics of the above·ground runoff in sewered catchments. In: Urban Storm Drainage. Helliwell. P.R. (ed). Pen tech Press. Plymouth. 159-171. ReICher. I.. Prall, C.J .. Elliott. G.E.P. (1978). An assessment of the importance of roadside gully pots in determining the quality of stormwater runoff. In: Urban Storm Drainage. Helliwell. P.R. (ed). Pentech Press, Plymouth. 586-602. Groltker. M. (1990). Pollutant removal by gully pots in different catchment areas. Sci. Tot. Environ .• 93. 515-522. Lager. J.A .• Smith. W.O .• Tchoblanoglous. O. (1977). Suspended solids discharge from highway gully pots m a residenl1al catchment. Sc,. Tot. Environ .• 59. 355-364.
236
G. M. MORRISON et al.
Morrison, G.M.P .• Revill, O.M., Ellis. J.B., Svensson, G., Balmer, P. (1988). Transport mechanisms and processes for metal species in a gully pot system. Wattr Res.,ll, 1417·1427. Morrison. G.M.P., Revitt, O.M., Ellis. J .B. (1989). Sources and stonn loading variations of metal species in a gully pot catchment. Sci. TOI. Environ., 80. 267·278. Pratt, CJ., Adams, J.R.W. (1984). SedlDlentsupply and transtrussion in roadsiide gully pots, Sci. To/. Environ., JJ, 213·224. Pratt, CJ., EllIott, G.E.P., Fulcher. G.A. (1987). Suspended solids discharge from highway gully polS in a re5ldenual catchment. SCI. TOI. Environ .• 59, 355·364. Sartor. JD .. Boyd, G.B. (1972). Water pollution aspects of street surface contaminants. US EPA Report No. R2·72-08I, US Government Printing Office. Washington DC. Verbanck, M. (ed). (1993). Origin, occurrence and behaviour of sediments in sewer systems. Wat. Sci. Technol., 25 (8), 2J4 pp.