Settling curves of pollutants in storm water

Transcription

1 Settling curves of pollutants in storm water E.R.T. de Graaf 1 *, E.J. Baars 1 and J. Kluck 2 1 Waternet, PO Box 94370, 1090 GJ Amsterdam, The Netherlands 2 Tauw bv, PO Box 133, 7400 AC Deventer, The Netherlands *Corresponding author, erno.de.graaf@waternet.nl ABSTRACT This paper presents the upflow settling tube; a new method to determine the settling velocities of particle bound pollutants. The upflow settling tube is an accurate method requiring relative small amounts of sludge. The method has been used to investigate the settling velocity of the sludge captured in a settling facility for storm water in Amsterdam. The research was initiated because Amsterdam wanted to gather background information define effective design rules for settling facilities. Solid particles with a low settling velocity contain slightly higher concentrations of pollutants than solid particles with a high settling velocity. However this study shows the pollutants bind to particles with a large variety of settling velocities. The research helps to design settling facilities: e.g. it gives insight in the effectiveness (in pollution reduction) of increasing a settling facility. KEYWORDS Storm water; pollutants; settling facilities; settling velocities; settling curves INTRODUCTION In Amsterdam more than 70% of the city has a separate sewer system. The majority of the storm water runs without treatment into the urban channels and water ways. The storm water spills pollute the receiving water with among others PAHs and heavy metals. In the early nineties Amsterdam started a research into the effectiveness of a prototype settling facility. Amsterdam wants to gather background information to define effective design rules for facilities to reduce the pollution of surface water. This design rules will be based upon the functioning of the facility and more insight in the settling characteristics of pollutants in storm water. This paper presents the results of a monitoring program and a new method to measure the settling velocities of particle bound pollutants. Settling velocity of pollutants In order to design settling facilities, in the first place the velocity at which the pollutants settle should be known. Parts of the pollutants are bound to all kind of particles with good or bad settling characteristics. The rest of the pollutants is dissolved or exists in the pure state (e.g. oil drops). Next, the amount of pollutants in the storm water should be known to estimate how much has to be retained in order to obtain a good water quality in the receiving waters. In practice the amount of pollutants and the settling velocities are mostly unknown. The design of settling facilities in the Netherlands is generally based on rough assumptions, which lack a sufficient sound basis. De Graaf et al. 1

2 Wentink and Boogaard (2002) summarised former research on the amount of pollutants being dissolved or bound to solid particles. Also some research is available on the particles size distribution in storm water. This, combined with a relation of particles size and pollutants connected to particles, gives more insight, but introduces the problem that the settling velocities does not depend on particle size only. The direct relation between settling velocity and the percentage of pollutant settling at that velocity would be a better start for the design of settling facilities. Some methods have been developed to determine this settling velocity of solid particles and pollutants. Most of them are based on settling tubes filled with a stagnant, non flowing mixture of water and sludge. For example the VICAS (France), VICTOR and VICPOL protocols (Gromaire et. al., 2007). However the results of these methods are sensitive to the homogeneity of the initial suspension as shown by Torres and Bertrand- Krajewski (2007). This article presents a new method which is less sensitive to initial conditions. Case study The prototype settling pond was constructed in Amsterdam in the early nineties. A separate sewer system spills the storm water of a 12.5 ha catchment into a pond. The catchment contains houses, some busy crossroads and a square with many copper tracks of the local public trolley transport. A small part of the pond was separated from the rest by sheet piles, see Figure 1. A storm water outlet spills in the separated part. At the far end the water can flow out the settling facility. The settling facility is 5 m wide, 75 m long and 2 m deep. In 2002 research into the quantity and quality of the sludge layer revealed almost 300 m 3 of accumulated sludge (captured over 10 years). This was far more than expected. The sludge layer contained high concentrations of zinc and copper. According to Dutch standards the sludge layer had to be classified as highly contaminated soil (class 4+). Outside the settling facility the sludge appeared far less contaminated (class 2). Therefore the settling facility was concluded to be an effective way of trapping pollutants. Figure 1. Settling facility in the Julianapark. 2 Settling curves of pollutants in storm water

3 2007 RESEARCH In august 2007 Waternet (the water management organisation in Amsterdam) measured the sludge layer and took samples of the deposited sediment. Figure 2 shows the sampling locations in and around the Julianapark settling facility: The thickness of the sludge layer has been measured each 0,5 m along R1, R2, R3 and R5; The thickness of the sludge layer has been measured each 3 m from inflow to outflow perpendicular to R1, R2, R3, R4 and R5; At O1 and O2 samples of the sludge were taken from the pond floor to determine the amount of pollution outside the settling facility; At R1, R2, R3 and R5 small samples of the sludge were taken to determine the pollution captured in the settling facility and the grain size distribution; At R1, R3 and R5 large samples (ca. 10 litre) of the sludge were taken to determine the settling velocity distribution of pollutants. When performing the settling tube tests 10 litres of sludge appeared more than sufficient. Eventually only one litre was used. Figure 2. Sample locations. METHOD The method described in this paper is based on continuous upward flow and was developed by J. Kluck. The separation of a sludge sample in the required fractions is done in an upflow settling tube (Figure 3). De Graaf et al. 3

4 Upflow settling tube By pass Reservoir with lighter particles Sludge sample shutter Flow meter tap water Figure 3. Flow scheme settling tube. pump The settling tube has a diameter of 15 cm and a height of 60 cm (effective settling height 45 cm or more depending on the discharge) From the bottom we enter tap water to create a predefined upward water flow velocity. At the top the water and the lightest sludge is drained to a reservoir of about 100 litres. At this stage the sludge is added at the bottom of the tube using a plastic hose-pipe. We remove the hose carefully to prevent unwanted perturbations. Next we remove a shutter from the bottom and open a valve to create a predefined flow velocity. The sludge sample is separated in two fractions by the uniform flow. The particles with a settling velocity smaller than the upward flow velocity will be transported to the large reservoir. The particles with settling velocities larger than the upward flow velocity remain in the settling tube. Next the procedure is repeated with a higher flow velocity in order to create the next fraction. After the splitting in fractions the suspended solids in the 100 litre reservoirs are left alone to settle. The deposited sediments from the bottom of the reservoirs is analysed to determine the concentrations of a large variety of pollutants. The settling tube test results in dry matter characterised by a settling velocity between x m/h and y m/h. In this study we separated the following fractions: 0-2 m/h, 2-5 m/h, 5-10 m/h, m/h and >20 m/h. The analysis of the fractions results in a concentration of pollutants (mg/kg dry matter). Combining these results produces the percentage of pollutants bound to solid particles with a certain settling velocity or higher. This settling tube allows to create relatively large fractions, so that several pollutants can be analysed for each fraction. In order to find several times 10 grams of dry matter in each subsample we used about 1 kg of sludge (with about 10% dry matter). The time needed for each separation depends on the flow velocity. Particles from the bottom of the column will have to be transported upwards over 60 cm. Particles with a settling velocity equal to the flow velocity will not flow upwards nor settle. Particles with a settling velocity almost equal to the flow velocity will move upwards very slowly. Each separation has been continued long enough to be sure that all particles wit a settling velocity less than 4 Settling curves of pollutants in storm water

5 90% of the flow velocity are removed. At a flow velocity of 1 m/hour, and a settling height of 60 cm, the separation takes 6 hours. For smaller velocities we advise to reduce the height and diameter of the settling tube. A drawback of the settling tube is the fact that at low flow velocities (in order to split at a low settling velocity) the upflow procedure takes several hours. Furthermore, collection of all the settlable particles from the large reservoir takes even longer. This can take up to 24 hours for the smallest settling velocities. RESULTS AND DISCUSSION Figure 4 shows the distribution of dry matter over the different settling velocities. At the cross section half way the settling facility (R3) the settling characteristics of solid particles (dry matter in the figure) are better than at the cross section at the inflow (R1). This remarkable result was also found in 2002, when Boogaard and Baars (2004) found relative more large particles further away from the inflow. It might be that small storm events bring relative fine materials close to the inflow, while during large storm events large flow velocities transport large particles to the end of the settling facility. However this hypotheses is hard to test. With 750 m 3 of volume in the settling facility, and assuming plug flow through the facility a storm event of 6 mm can be stored in the facility (while pushing out the water which was inside). At R3 pollutants seem to be binding more to particles with bad settling characteristics. At (R1) a remarkable high concentration of all pollutants was found in the fraction settling velocities > 20 m/h. The concentration of PAHs was even higher (9600 mg/kg) and is not shown in the graphs. This is probably caused by a drop of tar or oil. At both locations a remarkable large part of the dry matter settles at velocities between 10 and 20 m/h. Pollutants in mg/kg >20m/h Location R1 (near inflow) 10-20m/h 5-10m/h 2-5m/h 0-2m/h Dry matter Zinc 50% Lead 45% Copper 40% 35% 30% 25% 20% 15% 10% 5% 0% Dry matter (% of total) Pollutants in mg/kg >20m/h Location R3 (middle) 10-20m/h 5-10m/h 2-5m/h 0-2m/h Dry matter Zinc Lead 50% Copper 45% PAHs (EPA) 40% 35% 30% 25% 20% 15% 10% 5% 0% Dry matter (% of total) Figure 4. Concentrations of copper, zinc, lead and PAHs (left axis) and fractions (right axis). Figure 5 shows the measured relation between settling velocity and pollutants in the sludge. The figure presents the percentages of the pollutants settling at the value on the x-axis or at higher settling velocities. Notice that each step at the x-axis corresponds to more or less doubling the size of the settling facility. For location R3 it appears that about 75% of the bound copper is connected to particles with settling velocities of 5 m/hour or higher. Designing on a lower settling velocity e.g. 2 m/hour results only in a minor increase of the pollutants retained. On the other hand the distribution at R1 indicates that only 60% of the bound copper is connected to particles with a settling velocity of 5 m/h or higher. To our surprise the results for all three types of heavy metals are almost identical. De Graaf et al. 5

6 Location R1 (near inflow) Location R3 (middle) 100% 100% 80% 80% 60% 60% >20m/h >10 m/h > 5 m/h > 2 m/h Zinc Lead Copper Dry matter >0 m/h 40% 20% 0% >20m/h >10 m/h > 5 m/h Zinc (Zn) Lead (Pb) Copper (Cu) Sum PAHs (EPA) Dry matter > 2 m/h >0 m/h 40% 20% 0% % settling at velocity [m/h] % settling at velocity [m/h] Figure 5. Removal efficiency for (bound) pollutants. Evaluation In 2007 the sludge of the Julianapark has been analysed. The water was not analysed. Therefore, only the pollutants which did settle in the settling pond have been analysed. Dissolved pollutants or very light pollutants possibly flowed through the settling facility to the rest of the pond. But because of the size and shape of the settling facility, plug flow is likely and therefore it is likely that most water has stayed long in the facility, allowing also light particles to settle. Wentink and Boogaard (2002) revealed that the majority of PAHs and heavy metals in storm water is connected to solid particles. On average 72% of the heavy metals bind to solid particles in surface water. For PAHs this is even 86%. Although the samples do not contain the actual inflow of suspended sediment they should represent the inflow accurately: The surface load So represents the capacity of the settling facility. At 5 mm/h (14 l/s/ha) and a runoff coefficient off 100% the surface load is 1.7 m/h. The majority of the precipitation has an intensity lower than 5 mm/h. Concerning the small surface load of the settling facility we assume that the majority of the suspended solids settled in the settling facility. Q S o = in m/h Q = the discharge through the settling facility. A = the surface A area off the settling facility (length * width). Regarding the thickness of the sludge layer (figure 6) the majority of solid particles is indeed captured in the settling facility. The sludge layer has a volume of 350 m 3. This matches 89 m 3 /year; 7 m 3 /ha/year and assuming 700 mm effective runoff/year this leads to 200 mg/l solid particles in runoff. Boogaard and Lemmen (2007) studied a long list of Dutch studies. The concentrations of suspended solids in this studies were in the same order of magnitude or smaller than 200 mg/l. Assuming plug flow in the settling facility of 750 m3, a storm event of about 6 mm can be stored in the facility (while pushing out water from a former rain event). The water from a small storm events (e.g. 3 mm of rainfall) will stay in the part near the inlet, allowing all particles (with low and high settling velocities) to settle there. 6 Settling curves of pollutants in storm water

7 Sludgelayer [cm] 3,5 2 Width [m] 0, Length [m] Figure 6. Thickness of the sludge layer (captured over 4 years). 0 In figure 6 the thin sludge layer at the inflow (left), caused by the relative high flow velocities of the inflow, is clearly visible. Uncertainties The settling facility. The sludge layer accumulated over 4 years. We know very little about the processes in the settling facility during this 4 years. The input of atmospheric deposition, the growth of algae, other biological processes and the possible agglomeration of solid particles are unknown. The upflow settling tube. Wall effects (Ristow, 1997) might have effected the results. However the solid particles are very small compared tot the diameter of the settling tube so wall effects can be neglected. Visual inspections of the flow showed that turbidity effects only occurred in the lowest 15 cm of the upflow settling tube. Because of this the turbidity effects did not significantly influence the results. The influence of agglomeration and disintegration of solid particles is unknown. No washing effect was found during the tests. Dissolving of a small part of the bound pollutants in the tap water was expected, but did not happen. Analysis of the water in the 100 litre reservoirs showed concentrations of pollutants comparable to tap water. The average concentration of pollutants in the fractions matches the concentrations in the original sludge. For future research it would be interesting to compare the results of stagnant settling tube methods (e.g. VICAS ) and the upflow settling tube to see if the different methods yield the same results. Design rule There is no part of the settling curve where quick wins can be found. E.g. by decreasing the design settling velocity from x to y m/h the removal efficiency doesn t increase dramatically. This also means there is no point at which further enlargement of the settling facility is useless. However no more than approximately 70 or 80% of the pollutants can be trapped in settling facilities because the last 20 / 30% occurs in the dissolved state. For nutrients should be counted on even lower removal efficiencies. But we showed that for these samples that 60% to 80% of the settleable pollutants settle at a settling velocity of 5 m/hour or more. For 2 m/hour the range would be 75% to 85%. When De Graaf et al. 7

8 accounting for a dissolved fraction of 25%, these percentage drop to 45% - 60% for 5 m/hour and 55% - 64% for 2 m/hour. These small differences between 5 an 2 m/hour require more than a double sized settling facility! Based on this research designing rules can t be drawn up. Two factors determine the required removal efficiency: Environmental / hygienic goals and the carrying capacity of the receiving waters. The design settling velocity depends on local suspended soil characteristics. More locations should be tested to increase the knowledge of variations in local sludge characteristics. In 2008 Waternet starts a new research determining the suspended soil characteristics of another settling facility. CONCLUSION The upflow settling tube is an accurate test to determine the settling curves if the sludge input represents the suspended inflow. Only small quantities of sludge are needed. The results of an upflow settling tube test provide a sound basis for design rules of settling facilities. Solid particles with a slow settling velocity contain slightly higher concentrations of pollutants than fast settling solid particles. In the Julianapark in Amsterdam at a design settling velocity of 5 m/h an estimated 60% to 80% of the bound pollutants can be captured. Designing settling facilities should be based on local suspended soil characteristics and carrying capacity of the receiving waters. In four years the settling facility in the Julianapark captured a thick layer of sludge. Inside the settling facility the concentrations of pollutant are far higher than in other parts of the pond. The settling facility is therefore an effective measure to abate the pollutant load to receiving waters. ACKNOWLEDGEMENT The authors like to thank Khaled Sa ad and for operating the upflow settling tube and Waternet for financing this research. REFERENCES Boogaard, F.C. and Baars, E.J. (2004). Settling facility works well (in dutch). In: H2O nr , The Netherlands Boogaard F.C. and Lemmen G.B. (2007), The facts about the quality of runoff: manual (in dutch), Stowa rapportnummer , Utrecht, The Netherlands Gromaire M.-C., Saad M. and Chebbo G. (2007). Settling velocity grading of particle bound pollutants Evaluation of settling column tests. In: M. van der Meulen, J.A.E. ten Veldhuis and R.P.S. Schilperoort (eds.), SPN5 conference proceedings, pp Ristow G.H. (1997). Wall correction factor for sinking cylinders in fluids. Fysical review, vol. 55, nr. 3, pp Torres A. and Bertrand-Krajewski J.-L. (2007). Evaluation of uncertainties in settling velocities of particles in urban stormwater runoff. In: M. van der Meulen, J.A.E. ten Veldhuis and R.P.S. Schilperoort (eds.), SPN5 conference proceedings, pp Wentink R. and Boogaard F.C. (2002). Appearance of pollutants in runoff (in dutch), Tauw bv, rapport R FCB-D01-U, Utrecht, The Netherlands 8 Settling curves of pollutants in storm water