- Email: [email protected]
Analysis of hydraulic conditions and HRT on the basis of experiments and simulations on soil column
Desalination 246 (2009) 435–443
Analysis of hydraulic conditions and HRT on the basis of experiments and simulations on soil column Salah Jellali*, Talel Sediri, Hamadi Kallali, Makram Anane, Naceur Jedidi Laboratoire de Traitement et Recyclage des Eaux, Centre de Recherches et Technologies des Eaux, Technopole Borj Cedria. BP273, CP. 8020, Soliman, Tunisia Email: [email protected] Received 11 September 2007; revised 04 March 2008; accepted 11 March 2008
Abstract The hydraulic residence time (HRT) is a key parameter affecting the soil aquifer treatment process (SAT) efficiency as tertiary treatment of wastewater through infiltration in vadose zone. In order to maximize the HRT, a pulsed feeding technique of a salt solution (used as conservative tracer) was adopted to allow the increase of HRT and soil aeration between pulses. For this reason, several laboratory column experiments combined with the use of a numerical model was conducted. The optimal number of pulsations that meet with the maximum of the HRT and the temporal variation of the water saturation into the column was determined for different applied hydraulic heads. It varies between 10 and 12. The maximum corresponding water stock in the porous media column was evaluated to about 84%, 79% and 73%, respectively, for an applied hydraulic head of 96, 75 and 50 cm. The applied hydraulic head had an important effect on the HRT. In fact, this parameter varies from almost 6.5 to 10.8 h for an applied hydraulic head of 96 and 50 cm. The change in the permeability of the soil had also a significant effect on the HRT. Hence, HRT is decreased of about 9% when the soil permeability at saturation is multiplied by 2, and increases of about 11% when the permeability is two times lower. Keywords: Soil aquifer treatment; Hydraulic residence time; Numerical modelling
1. Introduction Infiltration–percolation is a process of urban wastewater treatment which uses the soil as a filtering material and a support for the purifying biomass. This process is two-folded use based on a principal treatment in small agglomerations *Corresponding author.
sanitation and a complementary treatment for aquifer artificial recharge. The objectives of the infiltration–percolation process consist on the elimination of the suspended solids, the biological oxidation of the organic matter and the nitrogen removal. The HRT of the infiltrating wastewater in the vadose zone fed by several pulses per day is an important parameter affecting the efficiency of the wastewater treatment in
Presented at the: MEDA WATER International Conference on Sustainable Water Management, Tunis, March 21–24 2007. 0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.03.065
436
S. Jellali et al. / Desalination 246 (2009) 435–443
order to maximize the aeration of the system. The oxygen renewal in the subsoil enhances the oxidation and degradation of organic substances [1–3]. This HRT depends on the operating parameters such as the hydraulic head, the number of flooding-drainage cycles per day, the feeding rate and the preferential pathways [4]. Brissaud et al. [5] realized sand column tests (1 m deep and 0.19 m diameter) in order to evaluate the impact of the number of wastewater sequences coming from a secondary effluent on its disinfection. They concluded that the fractionation of the daily hydraulic head influenced highly the disinfection rate through the HRT variation in the porous media column. In fact, when they applied only two sequences, the residence time was less than 1 h, whereas this value reaches about 17 h for 12 sequences. The HRT is directly linked to the presence of the oxygen in the porous media which is an important parameter since it generates the oxidation of the dissolved organic matter and oxidation of nitrogen compounds [6]. In general, the total oxygen demand (TOD) is defined as the oxygen mass consumption to achieve the oxidation of 1 l of influent [1]. The oxygen renewal is provided by two mechanisms: the air convection and the molecular diffusion. The convective flow depends on the variation of water stock in the filtrating bed, which can be maximized by the application of many cycles of operating and drying periods [7]. The diffusive flux depends on the soil water content, the hydraulic head and the number of daily water fractionation [8]. Generally, the process of infiltration–percolation is also alternated on operating and drying phases which has been demonstrated as an efficient way to avoid soil clogging, which is usually observed at the superficial layer as a “thin biomat” [9]. The duration of these periods depends essentially on the climatic conditions and the wastewater quality. In this work, we carried out experimental study on soil column and numerical simulations to evaluate the impact of the applied wastewater
pulsing (number and duration of pulsations) on its HRT in the vadose zone. The effect of changes in soil permeability was also evaluated. 2. Materials and methods The HRT of a salt solution was determined by experimental assays through a laboratory porous media column and a computer simulated numerical model (Hydrus, 1D). A full description of Hydrus 1D is reported by Simunek et al. [10]. The experimental assays were performed at lab scale using a plexiglass column (inner diameter of 0.2 m and a height of 1.7 m) as shown in Fig. 1. This column was instrumented with five moisture sensors placed at several depths in order to get moisture profile versus time. The inlet and outlet water volume and consequently the flow rates were continuously measured. The porous media used is a sandy soil sampled from the SAT pilot plant of Souhil wadi (Nabeul-Tunisia). Its main characteristics are summarized in Table 1. For the lab experiments, the total injected water quantity was fixed to 30 L/day, which corresponds 20 cm
30 cm 20 cm
Moisture sensor (MLX2) Wadi Souhil soil Gravel
30 cm 170 cm 150 cm
Synthetic salt solution (tracer)
30 cm
30 cm
30 cm Data logger Outflow
balance
Fig. 1. The used porous media column.
437
S. Jellali et al. / Desalination 246 (2009) 435–443 Table 1 Main characteristics of the used soil
7 × 10−5
Porosity (%)
Dispersivity (m)
d50 (mm)
25
0.002
0.28
to an applied hydraulic head of almost 96 cm/day. This applied hydraulic head was determined on the basis of the EPA recommendations [11]. It was generally applied in the wadi Souhil pilot station. This quantity was added to the column by equivalent several pulsations and the evolution of the applied hydraulic head was monitored each minute. The synthetic salt solution (sodium chloride) with a concentration of about 5 g/L, was injected instead of the second pulsation in order to have an approximate equilibrium state in the column after the first water sequence passage. Salt concentration was continuously assessed at the column outlet by an electrical conductivity measurement. Three column experiments were conducted respectively for 6, 8 and 10 pulsations. The evolution of the initial water content in the column was measured and registered versus depth and time by the moisture sensors. On the other hand, we used the Hydrus 1D numerical model for the simulation of water flow and transport in the vadose zone. It was used especially in order to simulate water flow and salt transport in the column for a relatively high number of water pulsations. It was firstly validated on the experimental results for six water pulsations. In a second step, this model was used to determine the effect of the number of water pulsations (6, 8, 10, 12, 14 and 16) for three applied hydraulic head (96, 75 and 50 cm) and the impact of the soil permeability variation on the HRT and water stock in the vadose zone. In order to simulate the water flow and salt transport in the unsaturated zone by the numerical model, the Souhil wadi soil was hydrodynamically characterized. To achieve this task, we used both a Büchner with a ceramic filter (for small pressures) and vacuum pressure chamber (for rel-
Specific Cation exchange surface (cm2/g) capacity (meq/100g) 200
1.8
atively high pressures). The Van Genuchten model [12] gives the variation of the water content (θ(h)) versus capillary pressure (hc) according to the following relationship:
θ ( h ) = θr +
θs − θ r ⎡1 + α hc n ⎤ ⎣ ⎦
(1)
m
where θr, θs are, respectively, the residual and saturated water, α, n and m are the Van Genuchten parameters. The best fitting between the experimental measures and Van Genuchten model [12] was observed for the following parameters (Fig. 2): α = 5 m–1; n = 4 and m = 0.75. The residual water saturation was estimated to about 9%. For the experimental and numerical methods, the HRT was determined using the following equations [4]: n
Mi M0
HRT = ∑ ti 1
(2) Experiment
400 Capillary pressure (cm)
Saturated permeability (m/s)
Van Genuchten model
300 200
100 0 0
20 40 60 80 Water saturation (%)
100
Fig. 2. Capillary pressure versus water saturation curve fitted according to the Van Genuchten model.
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Table 2 The soil configuration in the column retained for the simulations
12
Layer 1 permeability (m/s)
Layer 2 thickness (cm)
Layer 2 permeability (m/s)
Layer 3 thickness (cm)
5 × 10–5
93
6 × 10–5
45
where ti is the duration between the beginning of the traced sequence and the time observation i, Mi is the salt mass measured at the column outlet between the times i – 1 and i, M0 is the total injected salt mass, n is the number of the salt concentration measures at the end of the column. The salt mass measured at the column outlet between the instants i – 1 and i, was determined using the method of trapeze according to Eq. (2): Mi =
(Ci −1 + Ci ) (Vi − Vi −1 ) 2
(3)
where Ci−1, Vi−1, Ci and Vi are, respectively, the concentrations and the collected water volume at the column outlet at the times i – 1 and i. The specific flow rate (q (mL/min)) at the column outlet depends on the measured flow rate (Q (mL/min)) and the relative concentration (C/C0): q=Q
C C0
(4)
The water stock in the column was determined through the integration of the measured moisture profile using the trapeze method. The optimum number of water pulsations was determined using the results of the model, taking into account both the calculated HRT and the water stock in the column which affect significantly the aeration rate of the system [5].
Layer 3 Average water permeability infiltration (m/s) velocity (cm/min) 9 × 10–5
0.5
sequences feeding. This validation concerned the salt concentration breakthrough at the column outlet and the soil moisture profile at the sensors. The calibrated state corresponds to a configuration with three layers having different permeabilities (Table 2). The corresponding HRT was estimated to about 6.3 h which is comparable to the experimental one (5.8 h). We observe that for six water sequences feeding, the maximum salt restitution at the column outlet was registered during the sequence following the traced one (between 8 and 12 h). The comparison between the experimental and the numerical results shows a relatively good concordance (Fig. 3). The stock in the overall column reaches almost 100%, even for a short time (Fig. 4). This state means that the porous media in the column is fully water saturated. Consequently, the aeration of the system is very limited, which is not appreciable for the soil aquifer treatment 6
Salinity (g/l)
Layer 1 thickness (cm)
Simulation Experiment
4
2
0
3. Results and discussion
0
4
8 12 16 Time (hours)
20
24
3.1. Numerical model validation The numerical model validation was realized for the experiment of 96 cm/day and six water
Fig. 3. Comparison between the simulated and observed salt concentration at the column outlet for an applied hydraulic head of 96 cm and six water sequences feeding.
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S. Jellali et al. / Desalination 246 (2009) 435–443 12 Hydraulic residence time (hours)
Water stock (%)
100 80 60 40 20
Simulation Experiment
0 0
4
8 12 16 Time (hours)
20
8
4 H = 96 cm H = 75 cm H = 50 cm 0
24
0
5 10 15 20 Water sequences number
Fig. 4. Comparison between simulated and measured water stock in the column for an applied hydraulic head of 96 cm and six water sequences feeding.
Fig. 5. HRT evolution versus applied hydraulic head (H) and number of water sequences feeding.
operation. This relative high stock in the column was partially due to an effect of the bottom of the column where an important capillary fringe was observed. The simulated water flow at the column outlet was also in good concordance with the experimental one. However, a neglected difference persists (overestimation and underestimation) which to be related to the presence of air bubbles at the bottom of the column. The maximum simulated water flow rate was evaluated to about 110 mL/min, which is about 10% higher than the experimental one.
evolution in the column was determined using he numerical model Hydrus 1D. In real cases, the used hydraulic head depends on the wastewater quality and especially its availability in the case of small agglomerations. For the simulations, the water height levels chosen were 96, 75 and 50 cm. These wastewater quantities were added to the column with different water sequences feeding (6, 8, 10, 12, 14 and 16). For each applied hydraulic head, we observe that the maximum HRT (calculated using Eq. (1)) corresponded to a number of sequences of 12, except for 50 cm, where this maximum is related to 10 sequences (Fig. 5). Table 3 shows the impact of the applied hydraulic head and the number of water sequences feeding used on the HRT and the water stock in
3.2. Impact of the applied hydraulic head The impact of the applied hydraulic head on the HRT, the salt restitution and the water stock
Table 3 HRT and maximum water stock in the column evolution versus applied hydraulic head and the number of sequences feeding Number of water sequences Applied hydraulic head: H (cm) 96 75 50
HRT (h) Maximum water stock in the column (%) HRT (h) Maximum water stock in the column (%) HRT (h) Maximum water stock in the column (%)
6
8
10
12
14
16
6.26 98 7.80 89 10.78 78
6.33 90 7.82 82 10.84 74
6.40 85 7.83 80 10.84 73
6.48 82 7.84 77 10.75 72
6.46 81 7.79 76 10.68 73
6.44 80 7.77 76 10.58 73
S. Jellali et al. / Desalination 246 (2009) 435–443
Relative concentration
H = 96 cm; 6 sequences H = 75 cm; 6 sequences H = 50 cm; 6 sequences 1.0 0.8 0.6 0.4 0.2
H = 96 cm; 6 sequences H = 96 cm; 12 sequences H = 96 cm; 16 sequences 80 Specific flowrate (ml/min)
the column. It appears that the maximum HRT is quite depending on the level water height. In fact, it was almost 10.8 h for 50 cm and about 28% smaller for 75 cm. The lower HRT was observed for an applied hydraulic head of 96 cm; it was respectively about 40% and 17% lower than the ones corresponding to 50 and 75 cm (Fig. 5). The salt restitution at the column outlet depends on the applied hydraulic head. In fact for the same number of sequences feeding, the relative salt concentration was maximal for the highest used quantity (Fig. 6). The relative salt concentration was about 97% for an applied hydraulic head of 96 cm/day and it is only about 67% for 50 cm. Due to the difference on the water quantity applied per sequence, all the salt was collected at the column outlet between the second and the fourth sequences, and between the third and the sixth sequences, respectively, for an applied hydraulic heads of 96 cm/day and 50 cm/day. For the same hydraulic head, the restitution of the salt solution at the column outlet depends on the number of the water sequences feeding (Fig. 7). In fact, increasing the water sequences feeding decreases significantly the specific flow rate. For example, for an applied hydraulic head of 96 cm, the maximum specific flow rate was estimated at about 70 mL/min for six sequences and was less than 20 ml/min for 16 sequences (Fig. 7). This is due to the fact that when the num-
60 40 20 0 0
4
8 12 16 20 Time (hours)
24
Fig. 7. Impact of the number of water sequences feeding on the specific flow rate for an applied hydraulic head (H) of 96 cm/day.
ber of the water sequences increases, the flow rate and the salt concentration measured at the column outlet decrease. It might be due essentially to the thin water level which affects significantly the water flow in the column as reported in [1,9]. Two large peaks are observed for six sequences feeding, however three relatively small peaks are registered for 12 and 16 sequences. This is due to the temporal significant flow rate variation at the column outlet especially for the high sequences feeding (Fig. 8). A similar behaviour was H = 96 cm; 6 sequences H = 96 cm; 12 sequences H = 96 cm; 16 sequences 120
Flow rate (ml/min)
440
80
40
0 0
0.0 0
4
8 12 16 Time (hours)
20
24
Fig. 6. Salt restitution at the column outlet. Impact of the applied hydraulic head (H).
4
8 12 16 Time (hours)
20
24
Fig. 8. Impact of the number of water sequences feeding on the water flow rate for an applied hydraulic head (H) of 96 cm/day.
S. Jellali et al. / Desalination 246 (2009) 435–443
observed for the others applied hydraulic heads followed (75 and 50 cm). It is important to underline that the soil moisture was very dependent on both the applied hydraulic head and the number of water sequences. The minimum water stock in the column corresponded to the lowest applied hydraulic head and the highest water sequences. The maximum water stock in the column was observed for the highest hydraulic head and the lower water sequences number (Table 3). Indeed, for an applied hydraulic head of 96 cm/day and six water sequences feeding, the overall column were quasi-water saturated, and consequently the air stock in the column should be negligible to permit an efficient treatment of the pollution contained in wastewater. For 12 water sequences, the maximum water stock in the column varies from 82% to about 72%, respectively, for an applied hydraulic head of 96 and 50 cm. Due to the effect of the bottom of the column (extended capillary fringe), the initial water stock in the column (at the beginning of the simulations) was relatively high (more than 45%). This phenomenon explained the relative high value of stock in the column (Fig. 9), especially for the highest hydraulic head (96 cm) and the least water sequence feeding (6): the maximum water stock is more than 98% (Table 3). H=96 cm
H=75 cm
H=50 cm
Stock in the column (%)
100 Water phase 75 50
Air phase
25 0 0
4
8 12 16 Time (hours)
20
24
Fig. 9. Water stock evolution versus applied hydraulic head for 12 water sequences feeding.
441
According to Fig. 9, we observe that a quasiequilibrium state from the second water sequence. The maximum water stock in the overall column increases when increasing the applied hydraulic head. However the duration of the maximum value is inversely proportional to the applied hydraulic head. This duration was evaluated to about 0.56, 0.69 and 1.04 h, respectively, for 96, 75 and 50 cm applied hydraulic head. For a given water sequences feeding, the optimization of the HRT in the column should take into account the maximum water stock variation in the subsurface and its duration and in the same time the available wastewater quantity near the infiltration station. It is obvious, that when the hydraulic head decreases (small available wastewater), the HRT increases and the water stock in the vadose zone decreases permitting a better aeration of the subsoil and consequently a good degradation of the pollution contained in the wastewater. 3.3. Impact of the soil permeability The effect of the changes of the soil permeability on the HRT has been followed for the three applied hydraulic heads (50, 75 and 96 cm) on the basis of three configurations. The first one corresponds to the permeability retained for the numerical model calibration (cf. Section 3.1), the second one corresponds to a virtual soil matrix having a permeability which is two times lower and the third virtual soil matrix had a permeability two times greater. The following simulations were performed for 12 water sequences feeding, which corresponded to the optimal value found for an applied hydraulic heads of 96 and 75 cm. The results of the simulations show that the HRT is highly influenced by the soil permeability. For example, when the permeability was divided by a factor of 2, the HRT increases of about 13%, 11% and 9%, respectively, for an applied hydraulic head of 96, 75 and 50 cm. Whereas, when the permeability is double of the
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S. Jellali et al. / Desalination 246 (2009) 435–443
15 HRT (hours)
is so much higher (Fig. 10). Inversely, when the used permeability is two times higher, the maximal water stock in the column and its duration are lower (Fig. 11). Consequently, air stock in the column for this last configuration is more important and its renew is favoured. This phenomenon is very important for the pollution degradation in SAT systems [5]. A similar behaviour was observed for the other applied hydraulic heads.
K = 1/2 * K0 Wadi souhil soil : K = K0 K= 2 * K0
10 5
0 50 cm 75 cm 96 cm Applied hydraulic head
Fig. 10. Soil permeability effect on the hydraulic residence time (HRT).
Souhil wadi soil, the HRT decreases of almost 10%, 9% and 8%, respectively, for an applied hydraulic head of 96, 75 and 50 cm (Fig. 10). The effect of the permeability changes on the water stock in the column was also very important. The decrease of the permeability causes an increase of not only the maximum value of the water stock but also its duration. For example, for a water sequences number feeding of 12 and an applied hydraulic head of 75 cm, when the permeability is divided by a factor of 2, the maximum water stock increases of about 5%. Furthermore, the minimum water stock in the column at the beginning of the coming sequence K = K0
K=0.5*K0
Stock in the column (%)
100
K=2*K0
Water phase
75 Air phase
50 25 0 0
4
8 12 16 Time (hours)
20
24
Fig. 11. Water stock evolution versus soil permeability for an applied hydraulic head of 75 cm and 12 water sequences feeding.
4. Conclusions This study is based on the use of an experimental porous media columns and a numerical model, it shows that the HRT is very dependent on the applied hydraulic head height added and the number of water sequences feeding. Taking into account the calculated HRT values and the water stock evolution versus time, 10 or 12 water sequences feeding and 50 cm applied hydraulic head seem to be the best operating cycle. The corresponding HRT, which is about 10.8 h, maximize the contact time between the biomass and the wastewater percolating in the subsurface and consequently the dissolved pollution oxidation. The initial permeability of the porous media has also a significant effect on this parameter and consequently on the efficiency of the treatment. In fact, when the soil permeability is important, a rapid wastewater infiltration and consequently the renewal of the air phase which is mainly responsible for the pollution degradation are favoured. The specific conditions cited above, corresponding to the optimal HRT, should be applied in lab and in a second step in real sites. The clogging phenomena evolution of the superficial layer must be taken into account in the optimization of the overall management of the SAT process, especially for the determination of the infiltration and drying periods. References [1] F. Lefevre. Epuration des eaux usées urbaines par infiltration percolation, Etude expérimentale et
S. Jellali et al. / Desalination 246 (2009) 435–443
[2]
[3]
[4]
[5]
[6]
définition du procédé. Th. Doc. Montpellier II, 1988, 341 pp. J. Langmark, M.V. Storey, N.J. Ashbolt and T.A. Stenström, Artificial groundwater recharge treatment: biofilm activity and organic carbon removal performance, Water Res. 38 (2004) 740–748. J. Greskowiak, H. Prommer, G. Massmann, C.D. Johnston, G. Nützmann and A. Pekdger, The impact of variably saturated conditions on hydrogeochemical changes during artificial recharge of groundwater, Appl. Geochem., 20 (2005) 1409–1426. A. Wanko, R. Mose and A. Lienard, Distribution des temps de séjour en infiltration-percolation, Performance de deux types de matériaux, Tech. Sci. Méth., 4 (2004) 63–71. F. Brissaud, M. Salgot, A. Bancolé, C. Campos and M. Folch. Residence time distribution and disinfection of secondary effluents by infiltration-percolation, Wat. Sci. Tech., 40 (1999) 15–222. J.A. Guilloteau, Traitement des eaux résiduaires par infiltration-percolation, Performances, biomasse et renouvellement des gaz, Thèse de doctorat, Université Louis Pasteur, Strasbourg, 1992, 247 pp.
443
[7] F. Brissaud, F. Lefevre, C. Joseph, Z. Alamy and A. Landreau, Wastewater infiltration percolation for aquifer recharge or water reuse, IAHS Publ., 188 (1989) 433–455. [8] A. Schmitt. Modélisation de l’épuration par infiltration-percolation, Th. Doc. Montpellier II, 1989, 297 pp. [9] D.N.H Beach., J.E. McCray, K.S. Lowe and R.L. Siegrist, Temporal changes in hydraulic conductivity of sand porous media biofilters during waste water infiltration due to biomat formation, J. Hydrol., (2005) 1–14. [10] J. Simunek, M.Th. Van Genuchten and M. Sejna. The Hydrus 1D software package for simulating the one-dimensional movement of water, heat and multiple solutes in variably saturated media, Manual Use, 2005, 240 pp. [11] USEPA, Land treatment of municipal wastewater, Supplement on Rapid Infiltration and Overland Flow, 1984, 121 pp. [12] M. Th. Van Genuchten, A closed-form equation for predicting the hydraulic conductivity of unsaturated soils, Soil Sci. Soc. Am. J., 44 (1980) 892–898.
Analysis of hydraulic conditions and HRT on the basis of experiments and simulations on soil column Salah Jellali*, Talel Sediri, Hamadi Kallali, Makram Anane, Naceur Jedidi Laboratoire de Traitement et Recyclage des Eaux, Centre de Recherches et Technologies des Eaux, Technopole Borj Cedria. BP273, CP. 8020, Soliman, Tunisia Email: [email protected] Received 11 September 2007; revised 04 March 2008; accepted 11 March 2008
Abstract The hydraulic residence time (HRT) is a key parameter affecting the soil aquifer treatment process (SAT) efficiency as tertiary treatment of wastewater through infiltration in vadose zone. In order to maximize the HRT, a pulsed feeding technique of a salt solution (used as conservative tracer) was adopted to allow the increase of HRT and soil aeration between pulses. For this reason, several laboratory column experiments combined with the use of a numerical model was conducted. The optimal number of pulsations that meet with the maximum of the HRT and the temporal variation of the water saturation into the column was determined for different applied hydraulic heads. It varies between 10 and 12. The maximum corresponding water stock in the porous media column was evaluated to about 84%, 79% and 73%, respectively, for an applied hydraulic head of 96, 75 and 50 cm. The applied hydraulic head had an important effect on the HRT. In fact, this parameter varies from almost 6.5 to 10.8 h for an applied hydraulic head of 96 and 50 cm. The change in the permeability of the soil had also a significant effect on the HRT. Hence, HRT is decreased of about 9% when the soil permeability at saturation is multiplied by 2, and increases of about 11% when the permeability is two times lower. Keywords: Soil aquifer treatment; Hydraulic residence time; Numerical modelling
1. Introduction Infiltration–percolation is a process of urban wastewater treatment which uses the soil as a filtering material and a support for the purifying biomass. This process is two-folded use based on a principal treatment in small agglomerations *Corresponding author.
sanitation and a complementary treatment for aquifer artificial recharge. The objectives of the infiltration–percolation process consist on the elimination of the suspended solids, the biological oxidation of the organic matter and the nitrogen removal. The HRT of the infiltrating wastewater in the vadose zone fed by several pulses per day is an important parameter affecting the efficiency of the wastewater treatment in
Presented at the: MEDA WATER International Conference on Sustainable Water Management, Tunis, March 21–24 2007. 0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.03.065
436
S. Jellali et al. / Desalination 246 (2009) 435–443
order to maximize the aeration of the system. The oxygen renewal in the subsoil enhances the oxidation and degradation of organic substances [1–3]. This HRT depends on the operating parameters such as the hydraulic head, the number of flooding-drainage cycles per day, the feeding rate and the preferential pathways [4]. Brissaud et al. [5] realized sand column tests (1 m deep and 0.19 m diameter) in order to evaluate the impact of the number of wastewater sequences coming from a secondary effluent on its disinfection. They concluded that the fractionation of the daily hydraulic head influenced highly the disinfection rate through the HRT variation in the porous media column. In fact, when they applied only two sequences, the residence time was less than 1 h, whereas this value reaches about 17 h for 12 sequences. The HRT is directly linked to the presence of the oxygen in the porous media which is an important parameter since it generates the oxidation of the dissolved organic matter and oxidation of nitrogen compounds [6]. In general, the total oxygen demand (TOD) is defined as the oxygen mass consumption to achieve the oxidation of 1 l of influent [1]. The oxygen renewal is provided by two mechanisms: the air convection and the molecular diffusion. The convective flow depends on the variation of water stock in the filtrating bed, which can be maximized by the application of many cycles of operating and drying periods [7]. The diffusive flux depends on the soil water content, the hydraulic head and the number of daily water fractionation [8]. Generally, the process of infiltration–percolation is also alternated on operating and drying phases which has been demonstrated as an efficient way to avoid soil clogging, which is usually observed at the superficial layer as a “thin biomat” [9]. The duration of these periods depends essentially on the climatic conditions and the wastewater quality. In this work, we carried out experimental study on soil column and numerical simulations to evaluate the impact of the applied wastewater
pulsing (number and duration of pulsations) on its HRT in the vadose zone. The effect of changes in soil permeability was also evaluated. 2. Materials and methods The HRT of a salt solution was determined by experimental assays through a laboratory porous media column and a computer simulated numerical model (Hydrus, 1D). A full description of Hydrus 1D is reported by Simunek et al. [10]. The experimental assays were performed at lab scale using a plexiglass column (inner diameter of 0.2 m and a height of 1.7 m) as shown in Fig. 1. This column was instrumented with five moisture sensors placed at several depths in order to get moisture profile versus time. The inlet and outlet water volume and consequently the flow rates were continuously measured. The porous media used is a sandy soil sampled from the SAT pilot plant of Souhil wadi (Nabeul-Tunisia). Its main characteristics are summarized in Table 1. For the lab experiments, the total injected water quantity was fixed to 30 L/day, which corresponds 20 cm
30 cm 20 cm
Moisture sensor (MLX2) Wadi Souhil soil Gravel
30 cm 170 cm 150 cm
Synthetic salt solution (tracer)
30 cm
30 cm
30 cm Data logger Outflow
balance
Fig. 1. The used porous media column.
437
S. Jellali et al. / Desalination 246 (2009) 435–443 Table 1 Main characteristics of the used soil
7 × 10−5
Porosity (%)
Dispersivity (m)
d50 (mm)
25
0.002
0.28
to an applied hydraulic head of almost 96 cm/day. This applied hydraulic head was determined on the basis of the EPA recommendations [11]. It was generally applied in the wadi Souhil pilot station. This quantity was added to the column by equivalent several pulsations and the evolution of the applied hydraulic head was monitored each minute. The synthetic salt solution (sodium chloride) with a concentration of about 5 g/L, was injected instead of the second pulsation in order to have an approximate equilibrium state in the column after the first water sequence passage. Salt concentration was continuously assessed at the column outlet by an electrical conductivity measurement. Three column experiments were conducted respectively for 6, 8 and 10 pulsations. The evolution of the initial water content in the column was measured and registered versus depth and time by the moisture sensors. On the other hand, we used the Hydrus 1D numerical model for the simulation of water flow and transport in the vadose zone. It was used especially in order to simulate water flow and salt transport in the column for a relatively high number of water pulsations. It was firstly validated on the experimental results for six water pulsations. In a second step, this model was used to determine the effect of the number of water pulsations (6, 8, 10, 12, 14 and 16) for three applied hydraulic head (96, 75 and 50 cm) and the impact of the soil permeability variation on the HRT and water stock in the vadose zone. In order to simulate the water flow and salt transport in the unsaturated zone by the numerical model, the Souhil wadi soil was hydrodynamically characterized. To achieve this task, we used both a Büchner with a ceramic filter (for small pressures) and vacuum pressure chamber (for rel-
Specific Cation exchange surface (cm2/g) capacity (meq/100g) 200
1.8
atively high pressures). The Van Genuchten model [12] gives the variation of the water content (θ(h)) versus capillary pressure (hc) according to the following relationship:
θ ( h ) = θr +
θs − θ r ⎡1 + α hc n ⎤ ⎣ ⎦
(1)
m
where θr, θs are, respectively, the residual and saturated water, α, n and m are the Van Genuchten parameters. The best fitting between the experimental measures and Van Genuchten model [12] was observed for the following parameters (Fig. 2): α = 5 m–1; n = 4 and m = 0.75. The residual water saturation was estimated to about 9%. For the experimental and numerical methods, the HRT was determined using the following equations [4]: n
Mi M0
HRT = ∑ ti 1
(2) Experiment
400 Capillary pressure (cm)
Saturated permeability (m/s)
Van Genuchten model
300 200
100 0 0
20 40 60 80 Water saturation (%)
100
Fig. 2. Capillary pressure versus water saturation curve fitted according to the Van Genuchten model.
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Table 2 The soil configuration in the column retained for the simulations
12
Layer 1 permeability (m/s)
Layer 2 thickness (cm)
Layer 2 permeability (m/s)
Layer 3 thickness (cm)
5 × 10–5
93
6 × 10–5
45
where ti is the duration between the beginning of the traced sequence and the time observation i, Mi is the salt mass measured at the column outlet between the times i – 1 and i, M0 is the total injected salt mass, n is the number of the salt concentration measures at the end of the column. The salt mass measured at the column outlet between the instants i – 1 and i, was determined using the method of trapeze according to Eq. (2): Mi =
(Ci −1 + Ci ) (Vi − Vi −1 ) 2
(3)
where Ci−1, Vi−1, Ci and Vi are, respectively, the concentrations and the collected water volume at the column outlet at the times i – 1 and i. The specific flow rate (q (mL/min)) at the column outlet depends on the measured flow rate (Q (mL/min)) and the relative concentration (C/C0): q=Q
C C0
(4)
The water stock in the column was determined through the integration of the measured moisture profile using the trapeze method. The optimum number of water pulsations was determined using the results of the model, taking into account both the calculated HRT and the water stock in the column which affect significantly the aeration rate of the system [5].
Layer 3 Average water permeability infiltration (m/s) velocity (cm/min) 9 × 10–5
0.5
sequences feeding. This validation concerned the salt concentration breakthrough at the column outlet and the soil moisture profile at the sensors. The calibrated state corresponds to a configuration with three layers having different permeabilities (Table 2). The corresponding HRT was estimated to about 6.3 h which is comparable to the experimental one (5.8 h). We observe that for six water sequences feeding, the maximum salt restitution at the column outlet was registered during the sequence following the traced one (between 8 and 12 h). The comparison between the experimental and the numerical results shows a relatively good concordance (Fig. 3). The stock in the overall column reaches almost 100%, even for a short time (Fig. 4). This state means that the porous media in the column is fully water saturated. Consequently, the aeration of the system is very limited, which is not appreciable for the soil aquifer treatment 6
Salinity (g/l)
Layer 1 thickness (cm)
Simulation Experiment
4
2
0
3. Results and discussion
0
4
8 12 16 Time (hours)
20
24
3.1. Numerical model validation The numerical model validation was realized for the experiment of 96 cm/day and six water
Fig. 3. Comparison between the simulated and observed salt concentration at the column outlet for an applied hydraulic head of 96 cm and six water sequences feeding.
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S. Jellali et al. / Desalination 246 (2009) 435–443 12 Hydraulic residence time (hours)
Water stock (%)
100 80 60 40 20
Simulation Experiment
0 0
4
8 12 16 Time (hours)
20
8
4 H = 96 cm H = 75 cm H = 50 cm 0
24
0
5 10 15 20 Water sequences number
Fig. 4. Comparison between simulated and measured water stock in the column for an applied hydraulic head of 96 cm and six water sequences feeding.
Fig. 5. HRT evolution versus applied hydraulic head (H) and number of water sequences feeding.
operation. This relative high stock in the column was partially due to an effect of the bottom of the column where an important capillary fringe was observed. The simulated water flow at the column outlet was also in good concordance with the experimental one. However, a neglected difference persists (overestimation and underestimation) which to be related to the presence of air bubbles at the bottom of the column. The maximum simulated water flow rate was evaluated to about 110 mL/min, which is about 10% higher than the experimental one.
evolution in the column was determined using he numerical model Hydrus 1D. In real cases, the used hydraulic head depends on the wastewater quality and especially its availability in the case of small agglomerations. For the simulations, the water height levels chosen were 96, 75 and 50 cm. These wastewater quantities were added to the column with different water sequences feeding (6, 8, 10, 12, 14 and 16). For each applied hydraulic head, we observe that the maximum HRT (calculated using Eq. (1)) corresponded to a number of sequences of 12, except for 50 cm, where this maximum is related to 10 sequences (Fig. 5). Table 3 shows the impact of the applied hydraulic head and the number of water sequences feeding used on the HRT and the water stock in
3.2. Impact of the applied hydraulic head The impact of the applied hydraulic head on the HRT, the salt restitution and the water stock
Table 3 HRT and maximum water stock in the column evolution versus applied hydraulic head and the number of sequences feeding Number of water sequences Applied hydraulic head: H (cm) 96 75 50
HRT (h) Maximum water stock in the column (%) HRT (h) Maximum water stock in the column (%) HRT (h) Maximum water stock in the column (%)
6
8
10
12
14
16
6.26 98 7.80 89 10.78 78
6.33 90 7.82 82 10.84 74
6.40 85 7.83 80 10.84 73
6.48 82 7.84 77 10.75 72
6.46 81 7.79 76 10.68 73
6.44 80 7.77 76 10.58 73
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Relative concentration
H = 96 cm; 6 sequences H = 75 cm; 6 sequences H = 50 cm; 6 sequences 1.0 0.8 0.6 0.4 0.2
H = 96 cm; 6 sequences H = 96 cm; 12 sequences H = 96 cm; 16 sequences 80 Specific flowrate (ml/min)
the column. It appears that the maximum HRT is quite depending on the level water height. In fact, it was almost 10.8 h for 50 cm and about 28% smaller for 75 cm. The lower HRT was observed for an applied hydraulic head of 96 cm; it was respectively about 40% and 17% lower than the ones corresponding to 50 and 75 cm (Fig. 5). The salt restitution at the column outlet depends on the applied hydraulic head. In fact for the same number of sequences feeding, the relative salt concentration was maximal for the highest used quantity (Fig. 6). The relative salt concentration was about 97% for an applied hydraulic head of 96 cm/day and it is only about 67% for 50 cm. Due to the difference on the water quantity applied per sequence, all the salt was collected at the column outlet between the second and the fourth sequences, and between the third and the sixth sequences, respectively, for an applied hydraulic heads of 96 cm/day and 50 cm/day. For the same hydraulic head, the restitution of the salt solution at the column outlet depends on the number of the water sequences feeding (Fig. 7). In fact, increasing the water sequences feeding decreases significantly the specific flow rate. For example, for an applied hydraulic head of 96 cm, the maximum specific flow rate was estimated at about 70 mL/min for six sequences and was less than 20 ml/min for 16 sequences (Fig. 7). This is due to the fact that when the num-
60 40 20 0 0
4
8 12 16 20 Time (hours)
24
Fig. 7. Impact of the number of water sequences feeding on the specific flow rate for an applied hydraulic head (H) of 96 cm/day.
ber of the water sequences increases, the flow rate and the salt concentration measured at the column outlet decrease. It might be due essentially to the thin water level which affects significantly the water flow in the column as reported in [1,9]. Two large peaks are observed for six sequences feeding, however three relatively small peaks are registered for 12 and 16 sequences. This is due to the temporal significant flow rate variation at the column outlet especially for the high sequences feeding (Fig. 8). A similar behaviour was H = 96 cm; 6 sequences H = 96 cm; 12 sequences H = 96 cm; 16 sequences 120
Flow rate (ml/min)
440
80
40
0 0
0.0 0
4
8 12 16 Time (hours)
20
24
Fig. 6. Salt restitution at the column outlet. Impact of the applied hydraulic head (H).
4
8 12 16 Time (hours)
20
24
Fig. 8. Impact of the number of water sequences feeding on the water flow rate for an applied hydraulic head (H) of 96 cm/day.
S. Jellali et al. / Desalination 246 (2009) 435–443
observed for the others applied hydraulic heads followed (75 and 50 cm). It is important to underline that the soil moisture was very dependent on both the applied hydraulic head and the number of water sequences. The minimum water stock in the column corresponded to the lowest applied hydraulic head and the highest water sequences. The maximum water stock in the column was observed for the highest hydraulic head and the lower water sequences number (Table 3). Indeed, for an applied hydraulic head of 96 cm/day and six water sequences feeding, the overall column were quasi-water saturated, and consequently the air stock in the column should be negligible to permit an efficient treatment of the pollution contained in wastewater. For 12 water sequences, the maximum water stock in the column varies from 82% to about 72%, respectively, for an applied hydraulic head of 96 and 50 cm. Due to the effect of the bottom of the column (extended capillary fringe), the initial water stock in the column (at the beginning of the simulations) was relatively high (more than 45%). This phenomenon explained the relative high value of stock in the column (Fig. 9), especially for the highest hydraulic head (96 cm) and the least water sequence feeding (6): the maximum water stock is more than 98% (Table 3). H=96 cm
H=75 cm
H=50 cm
Stock in the column (%)
100 Water phase 75 50
Air phase
25 0 0
4
8 12 16 Time (hours)
20
24
Fig. 9. Water stock evolution versus applied hydraulic head for 12 water sequences feeding.
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According to Fig. 9, we observe that a quasiequilibrium state from the second water sequence. The maximum water stock in the overall column increases when increasing the applied hydraulic head. However the duration of the maximum value is inversely proportional to the applied hydraulic head. This duration was evaluated to about 0.56, 0.69 and 1.04 h, respectively, for 96, 75 and 50 cm applied hydraulic head. For a given water sequences feeding, the optimization of the HRT in the column should take into account the maximum water stock variation in the subsurface and its duration and in the same time the available wastewater quantity near the infiltration station. It is obvious, that when the hydraulic head decreases (small available wastewater), the HRT increases and the water stock in the vadose zone decreases permitting a better aeration of the subsoil and consequently a good degradation of the pollution contained in the wastewater. 3.3. Impact of the soil permeability The effect of the changes of the soil permeability on the HRT has been followed for the three applied hydraulic heads (50, 75 and 96 cm) on the basis of three configurations. The first one corresponds to the permeability retained for the numerical model calibration (cf. Section 3.1), the second one corresponds to a virtual soil matrix having a permeability which is two times lower and the third virtual soil matrix had a permeability two times greater. The following simulations were performed for 12 water sequences feeding, which corresponded to the optimal value found for an applied hydraulic heads of 96 and 75 cm. The results of the simulations show that the HRT is highly influenced by the soil permeability. For example, when the permeability was divided by a factor of 2, the HRT increases of about 13%, 11% and 9%, respectively, for an applied hydraulic head of 96, 75 and 50 cm. Whereas, when the permeability is double of the
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15 HRT (hours)
is so much higher (Fig. 10). Inversely, when the used permeability is two times higher, the maximal water stock in the column and its duration are lower (Fig. 11). Consequently, air stock in the column for this last configuration is more important and its renew is favoured. This phenomenon is very important for the pollution degradation in SAT systems [5]. A similar behaviour was observed for the other applied hydraulic heads.
K = 1/2 * K0 Wadi souhil soil : K = K0 K= 2 * K0
10 5
0 50 cm 75 cm 96 cm Applied hydraulic head
Fig. 10. Soil permeability effect on the hydraulic residence time (HRT).
Souhil wadi soil, the HRT decreases of almost 10%, 9% and 8%, respectively, for an applied hydraulic head of 96, 75 and 50 cm (Fig. 10). The effect of the permeability changes on the water stock in the column was also very important. The decrease of the permeability causes an increase of not only the maximum value of the water stock but also its duration. For example, for a water sequences number feeding of 12 and an applied hydraulic head of 75 cm, when the permeability is divided by a factor of 2, the maximum water stock increases of about 5%. Furthermore, the minimum water stock in the column at the beginning of the coming sequence K = K0
K=0.5*K0
Stock in the column (%)
100
K=2*K0
Water phase
75 Air phase
50 25 0 0
4
8 12 16 Time (hours)
20
24
Fig. 11. Water stock evolution versus soil permeability for an applied hydraulic head of 75 cm and 12 water sequences feeding.
4. Conclusions This study is based on the use of an experimental porous media columns and a numerical model, it shows that the HRT is very dependent on the applied hydraulic head height added and the number of water sequences feeding. Taking into account the calculated HRT values and the water stock evolution versus time, 10 or 12 water sequences feeding and 50 cm applied hydraulic head seem to be the best operating cycle. The corresponding HRT, which is about 10.8 h, maximize the contact time between the biomass and the wastewater percolating in the subsurface and consequently the dissolved pollution oxidation. The initial permeability of the porous media has also a significant effect on this parameter and consequently on the efficiency of the treatment. In fact, when the soil permeability is important, a rapid wastewater infiltration and consequently the renewal of the air phase which is mainly responsible for the pollution degradation are favoured. The specific conditions cited above, corresponding to the optimal HRT, should be applied in lab and in a second step in real sites. The clogging phenomena evolution of the superficial layer must be taken into account in the optimization of the overall management of the SAT process, especially for the determination of the infiltration and drying periods. References [1] F. Lefevre. Epuration des eaux usées urbaines par infiltration percolation, Etude expérimentale et
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[7] F. Brissaud, F. Lefevre, C. Joseph, Z. Alamy and A. Landreau, Wastewater infiltration percolation for aquifer recharge or water reuse, IAHS Publ., 188 (1989) 433–455. [8] A. Schmitt. Modélisation de l’épuration par infiltration-percolation, Th. Doc. Montpellier II, 1989, 297 pp. [9] D.N.H Beach., J.E. McCray, K.S. Lowe and R.L. Siegrist, Temporal changes in hydraulic conductivity of sand porous media biofilters during waste water infiltration due to biomat formation, J. Hydrol., (2005) 1–14. [10] J. Simunek, M.Th. Van Genuchten and M. Sejna. The Hydrus 1D software package for simulating the one-dimensional movement of water, heat and multiple solutes in variably saturated media, Manual Use, 2005, 240 pp. [11] USEPA, Land treatment of municipal wastewater, Supplement on Rapid Infiltration and Overland Flow, 1984, 121 pp. [12] M. Th. Van Genuchten, A closed-form equation for predicting the hydraulic conductivity of unsaturated soils, Soil Sci. Soc. Am. J., 44 (1980) 892–898.