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Feasibility of Recharging Reclaimed Wastewater to the Coastal Aquifers of Perth, Western Australia
0957–5820/06/$30.00+0.00 # 2006 Institution of Chemical Engineers Trans IChemE, Part B, July 2006 Process Safety and Environmental Protection, 84(B4): 237– 246
www.icheme.org/psep doi: 10.1205/psep.05202
FEASIBILITY OF RECHARGING RECLAIMED WASTEWATER TO THE COASTAL AQUIFERS OF PERTH, WESTERN AUSTRALIA Q. LI1 , B. HARRIS2, C. AYDOGAN1, M. ANG1 and M. TADE1 1 Department of Chemical Engineering, Curtin University of Technology, Perth, Australia Division of Resources and Environment, Curtin University of Technology, Perth, Australia
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erth has a dry climate with most rainfall confined to winter season. By recharging treated wastewater into suitable aquifers along the coastal margin of Perth, a range of benefits may be achieved including: (1) prevention of saline water intrusion; (2) restoration of groundwater levels; (3) water banking and withdrawal to balance seasonal differences in water supply; (4) improvements in water quality; and (5) flexibility in aquifer management. The technical key points to implement this wastewater reuse strategy are in the hydrogeological sustainability, groundwater contamination, and treated wastewater quality. Recent developments in wastewater treatment technology have made it possible to convert sewage into fair quality/potable water. With the available data on Perth groundwater, a cost-effective wastewater treatment process is designed, which is intended to fully utilize the geopurification function. Numerical analysis is performed to investigate the impact of the reclaimed water injection on Perth basin’s hydrogeology and chemical, biological content in subsurface formations. Deep aquifer injection and shallow aquifer injection are both considered. Keywords: aquifer storage and recharge; wastewater treatment; groundwater; contamination; numerical analysis.
INTRODUCTION
The water consumption of humankind has become unprecedentedly high. Today’s water consumption in developed world is more than six folds of the consumption in the 1950s (EPA, 2004). In 2001 – 2002, the annual water consumption in Perth was 202 825 ML, and the total volume of wastewater collected was 101 891 ML through Water Corporation (Office of Water Regulation, 2002). Water Corporation has nine wastewater treatment plants (WWTP) in Perth, with the three major ones: Beenyup, Subiaco, Woodman Point processing 90% of the collected wastewater. In 2001, the Subiaco WWTP processed 58 ML day21 of wastewater (Water Corporation, 2001). By 2040, it is estimated that the volume of wastewater produced by Perth’s inhabitants will triple. In Perth, in general, only 3 –5% of treated wastewater is recycled, while the WA water authority is targeting at recycling 20% of treated wastewater by 2012. Currently more than 100 gigalitres of wastewater after secondary wastewater treatment is discharged to the ocean from the major metropolitan wastewater treatment plants in Perth per year. Faced with the two scenarios, ‘closing the loop’ seems a natural solution to save our depleted freshwater resource. A great deal of wastewater reuse strategies have emerged over the last two decades, and some of them have even been put in practice in a few world-leading regions, such as in Singapore, Berlin, and Orange County in USA. In this
Only 3.5% of the world’s water supply is freshwater and more than two-thirds of that is frozen. About 0.7% of the total earth’s water supply is freshwater available as either surface water or groundwater; groundwater accounts for two-thirds of this amount. The National Land and Water Resources Audit found that 30% of Australia’s 538 recognized groundwater management units are either close to or overused when compared with their estimated sustainable yield. Western Australia (WA) consumes twice as much groundwater as the surface water (farmweb, 2004). Western Australia has about 1.9 million inhabitants in total, with 1.4 million population narrowly focused on the southwest coastal margin around its capital Perth. The volume of groundwater abstracted from the superficial, Mirrabooka, Leederville and Yarragadee aquifers in 2001/2002 was 575 GL, and was estimated at up to 640 GL in 2003/2004. With the continuing population growth and declining rainfall, it is foreseeable that a higher demand for water could seriously stress Perth’s groundwater reserves. Correspondence to: Dr Q. Li, Department of Chemical Engineering, Curtin University of Technology, Perth, WA 6845, Australia. E-mail: [email protected]
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paper, we will focus on one of the promising solutions to rescue the groundwater resources and relieve water shortages by recharging treated wastewater (storm water run-off, sewage and industrial wastewater) into the groundwater system, i.e., the aquifer storage and recovery (ASR). A number of technical benefits may be achieved by recharging reclaimed wastewater into suitable aquifers, including: (1) restoration of groundwater levels; (2) improved water quality; (3) water banking and withdrawal balancing seasonal differences in water supply; (4) prevention of saline water intrusion at coastal margin. Economically, ASR is evaluated as potentially the least-cost option for alternative water supply (Bessler, 1993), because it requires cheap physical infrastructure, and uses the natural distribution systems. Orange County of California has set a successful example of ASR water reuse scheme (gwrsystem, 2004a, b). Studies have also been carried out in South Australia, the driest state in Australia, since last a few years (Gerges, 2000). Some aquifers have large attenuation capacities. They naturally reduce organic and microorganism contaminants (Rugge et al., 1994). Among the three possible attenuation mechanisms: sorption, dilution and degradation, the biogeochemical degradation is believed to be the dominant process (Rugge et al., 1994; Ying et al., 2003). In the current practice of ASR, prior to injection, the wastewater undergoes an extensive water purification process; typically consisting of microfiltration (MF), reverse osmosis (RO), ultraviolet light (UV) and hydrogen peroxide treatment (gwrsystem, 2004b). After treatment, the reclaimed water meets or surpasses all drinking water standards (gwrsystem, 2004b). However, this level of treatment often represents an over-treatment for groundwater replenishment. Injecting the ‘over-clean water’ could jeopardize the native chemical and biological constitutions in the groundwater system, thereafter, lead to massive, irreversible clogging problems. Moreover, under-utilized natural geopurification also reduces the economical competitiveness of ASR. The significant problems with recharging wastewater are (1) the cost effective treatment of large volumes of wastewater; (2) the effective and sustainable management of the host aquifer system; and (3) health, safety and environmental risk control. They are all particularly site-specific, determined by the biogeochemical properties of the native aquifers. In this study, we investigate a wastewater treatment strategy by integrating the aquifer purification function as a treatment unit. A preliminary feasibility study based on numerical computation is carried out to investigate the ASR impact on hydrogeological sustainability and contamination risk. WASTEWATER TREATMENT AND AQUIFER INJECTION PROCESS DESIGN Conventional WWTPs often supply two-steps treatment, with primary treatment to screen out big objects, and the secondary treatment to remove biological nutrients. Table 1 presents a typical Perth area wastewater quality after secondary treatment. A typical complete wastewater treatment process, as shown in Figure 1, prior to aquifer injection producing drinkable quality water, implies a high cost and limited yield. Reverse osmosis (RO) is capable of eliminate 99% of most ions and most organics over 150 molecule weight, however, it requires
13.8 – 69 bar transmembrane pressure. Moreover, the use of UV photocatalytic oxidation usually suggests a small throughput. Recently a new process is designed by Cui et al. (2004), which removes carbon and nitrogen simultaneously using an anoxic-aerobic circulating fluidized bed biological reactor (CFBBR), as shown in Figure 2. Such a process alone can replace both the secondary and tertiary treatment units in a conventional design. Typical data of CFBBR treated water quality is tabulated in Table 2. This novel process represents a highly integrated wastewater treatment method with high efficiency and high throughput. It provides a promising future for wastewater treatment industry by offering a significant cost and space reduction. The aquifers in the Perth area that we considered are the shallow/superficial and deep/Yaragadee. The shallow aquifers are generally unconfined, and less than 60 m from the surface. The deep/Yaragadee aquifers are confined or semi-confined at a depth generally below 500 m. The Leederville aquifer is semi-confined, lying between superficial and Yaragadee aquifers. In the Perth metropolitan area, the groundwater quality is potable to near potable. The average movement in shallow aquifers is about 30 m year21; while in deep aquifers, the maximum movement is less than 1 m year21. However, the groundwater movement is very site-specific. Both injection and abstraction wells can alter groundwater movements. The cost of drilling a well into a deep aquifer can be 100 times (Au$1 million) more expensive than a well in a shallow aquifer (a few thousands Au$). Most abstraction wells in Perth area are drawing water from shallow aquifers. Consequently, we can design an ASR strategy with considerations of both shallow and deep aquifer recharges, as illustrated in Figure 3: since the groundwater in shallow aquifers is dynamic and often drawn for domestic usage, such as reticulation, the water quality needs to match the drinkable water standard to comply with the safety, health and environmental regulations. Hence, nanofiltration, UV and hydrogen peroxide treatments should be added after CFBBR process, prior to the injection into the superficial aquifer. This shallow aquifer injection will result in a sustained water table level, improved groundwater quality, as well as a utilization of natural water
Table 1. Wastewater quality after secondary treatment (Subiaco WWTP, 2004). Variable
Concentration
pH Alkalinity BOD Suspended solids Total dissolved solids (mg L21) Total kjeldahl nitrogen (mg L21) NO3 – N (mg L21) Total nitrogen (mg L21) Phosphate–phosphorus (mg L21) Total phosphorus (mg L21) Sodium (mg L21) Calcium (mg L21) Magnesium (mg L21) 30-minute chlorine demand 24-h chlorine demand Thermotolerant coliforms (CFU/100 mL)
7.1 77 15 24 827 4 17 20 7 8 175 30 8.5 10 16 640 –240 000
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Figure 1. A conventional wastewater treatment process with tertiary and advanced treatments.
distribution systems. Less treated wastewater, as the end product of CFBBR, can be injected into deep aquifer, and harvested after 1 or 2 years retention time. The long residence time in deep aquifers allows a full utilization of geopurification function and the natural water storage system. In particular, this deep aquifer injection/abstraction can help balance the seasonal difference in water supply by injecting in winter seasons and abstracting in summers. NUMERICAL ANALYSIS ON ASR FEASIBILITY IN PERTH Model Set-up Aquifer modelling is carried out using the finite element model code FEFLOWTM, which specialises in modelling of flow and transport processes in porous media under saturated and unsaturated conditions. The finite element method employed for the 3-D modelling requires
discretization of the model domain into a mesh of triangular elements that are connected by nodes. Solutions are obtained by interpolation between nodes. The model is discretized into more than 20 000 nodes and has very dense discretization to less than 2 m proximal to injection or pumping bore locations. A model domain is set to the North of the Swan River, as shown in Figure 4. It is bounded to the east by Gnangara Mound, to the west by Indian Ocean, and to the south by the Swan River. The northern boundary is set at 35 km north to the Swan River. The calibrated numerical model is designed to be highly flexible, with the minimum number of degrees of freedom required to reasonably represent the overall character in shallow and deep groundwater systems. Such a model allows rapid testing of the general impacts of large numbers possibility pumping and injection bore configurations in a practical time frame.
Figure 2. Schematic diagram of the circulating fluidized bed biological reactor (Cui et al., 2004).
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LI et al. Table 2. Municipal wastewater quality after CFBBR treatment (Cui et al., 2004). V. HRT ¼ 2.0 h (feed) Parameter TCOD (mg L21) SCOD (mg L21) NH3 – N (mg L21) NO3 – N (mg L21) NO2 – N (mg L21) TSS (mg L21) VSS (mg L21) PO4 –P (mg L21) TKN TBOD SBOD TPO4 –P
V. HRT ¼ 2.0 h (effluent)
Range (min–max)
Average (Avg. + SD) (n)
Range (min–max)
Average (Avg. + SD) (n)
172 –420 14–308 20.2–50.5 0.3 –1.2 0–0.06 20–302 18–222 3.7 –5.6 21.9–58.5 140 –384 10–264 5.1 –7.8
262 + 58 (19) 110 + 91 (19) 24 + 8 (19) 0.74 + 0.25 (19) 0.013 + 0.015 (19) 156 + 67 (19) 135 + 54 (19) 4.4 + 0.46 (17) 30.7 + 13 (19) 207 + 87 (8) 80 + 81 (8) 6.3 + 0.82 (8)
4–51 0–28 0–4.7 2.71–12.9 0.001–0.3 0–24 0–20 0.97–3.23 1.02–6.3 4–18 0–9 1.66–2.9
18 + 14 (19) 10 + 7 (19) 1.3 + 1.5 (19) 7 + 3 (19) 0.07 + 0.06 (19) 10 + 7 (19) 8 + 6 (19) 2 + 1 (17) 1.8 + 2 (19) 10 + 6 (8) 4.5 + 3 (8) 2 + 0.5 (8)
Note: HRT refers to ‘hydraulic retention time’; numbers within parentheses represent the total number of samples.
Parameters for Perth area regarding groundwater system, and transport reactions of trichloroethylene (TCE) and Escherichia coli (E. coli) are obtained from literatures and communications with local water authorities. Table 3 contains some typical values that we have used in the simulation. Hydrogeological Perspective For the deep aquifer seven deep injection bores spaces at two kilometre intervals inject 2500 KL day21 over an injection zone of 500 m at the top of the deep aquifer. The interval transmissivity is equivalent to approximately 200 m2 day21. Figure 4(a) shows the steady state (i.e., no injection or pumping) hydraulic heads for the deep aquifer. Figure 4(b) shows the hydraulic heads after pumping a total pumping of 17.5 ML day21 (6.3 GL year21) of treated municipal waste water through the seven deep injection bores for a period 5 years. The average maximum increase in hydraulic head, along the line of bores is less than 4 m. The cone of influence around the bore remains relatively symmetrical and expands to width of approximately 8– 10 km. The injected water remains symmetrical about the bores. The symmetry reflects both the moderate to low hydraulic conductivities in the sandstone and the very low throughflow velocity in the deep aquifer (i.e., less than 1 m year21). The consequence of the above is that the injected water can be readily recovered by converting any of the injection bores to pumping bores. For Perth’s northern shallow aquifers we simulate a distribution system consisting of 18 injection bores and
18 pumping bores. The injection bores are located on a line 2.6 km inland from coast and are spaced at 1 km intervals. The recovery bores are set on a line, 2.7 km inland from the injection bore. The pumping and injection bores, pump or inject at a rate of 1000 KL day21. The total pumping and injecting rate is 18 ML day21 or (6.6 GL year21). Figure 5 shows the difference between steady state model and the model with pumping/injection for 5 years. After 5 years we can see that maximum average water level increases by less than 3 m. The small elevation in water level relates to the very high permeability of the shallow aquifer. Figure 7 shows a vertical section perpendicular to the coastline. It shows a simulation of salinity distribution after 50 years’ abstraction from a long line wells located 1 km away from the coastline. The wells for the simulation are spaced at 1 km intervals and abstraction rate for the wells is 1000 m3 per day. For the shallow aquifer the injection and pumping bore configuration is designed to prevent any chance of saline intrusion. In practice the configuration would be designed to address the existing saline intrusion problems in Perth, as demonstrated by our FEFLOW simulation results in Figures 6 –9. Figure 6 shows the calibrated salinity distribution at a typical Perth coastal area, under the natural groundwater hydraulic and diffusion mechanism without any abstraction or injection. The left boundary of the graph represents the depth from sea level at the coastal line, the scale at the bottom indicates the distance from the coastal line. It can be seen that the saline water has intruded 1 km into the on-shore groundwater system,
Figure 3. A double-cycle ASR system in conjunction with wastewater treatment strategies.
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Figure 4. The distribution of hydraulic head in deep aquifers within the model boundaries: (a) before injection, (b) after injection.
Table 3. Estimated parameters used in the simulation. Estimated flow parameters Parameter Hydraulic conductivity (m s21) Storativity Storage compressibility Porosity
Superficial aquifer 1E-4 (Davidson, 1995) 0.2 (Cargeeg et al., 1987) — 0.2 (Davidson, 1995)
Deep aquifer 0.2E-4 (Davidson, 1995) — 0.005 (Davidson, 1995) 0.2 (Davidson, 1995)
Estimated transport parameters for E. coli and TCE Parameter E. coli Sorption coefficient (Henry) 0.21 (Pang et al., 2003) 1E-10 (Johnston, 1983) Diffusivity (m2 s21) Longitudinal dispersivity (m) 7.9 (Prommer, 2005) Transverse dispersivity (m) 0.9 (Barber et al., 1991) 3.74E-6 (Banning et al., 2003) First order decay rate (s21)
TCE 0.12 (Benker et al., 1998) 1E-10 (Johnston, 1983) 7.9 (Prommer, 2005) 0.9 (Barber et al., 1991) 8E-9 (Wu et al., 2005)
Estimated parameters for saltwater intrusion (different scale) 2E-4 Hydraulic conductivity (m s21) Porosity 0.2 Sorption coefficient (Henry) 0 8E-6 Diffusivity (m2 s21) Longitudinal dispersivity (m) 1 Transverse dispersivity (m) 1 0 First order decay rate (s21) Density ratio 0.02 Note: The estimated flow parameters shown in the table are indicative for the values used in the numerical simulation.
which is analogous to Perth today’s groundwater situation. Figures 8 and 9 show that by inserting the injection bores (injection rate 1000 m3 per day) between the coastal line and abstraction bores, it effectively generates a salinity reduction after 1 year and significantly pushes back the high salinity water after 10 years. In addition, the shallow aquifer bore configuration would also be designed to manage water levels for the shallow aquifer in the most efficient way so that there is no chance of flooding or aquifer depletion. The ASR process would become an active aquifer management tool to balance the significant seasonal differences in rainfall in Perth. Contamination Risk Assessments
Figure 5. The altered water level in shallow aquifer as a result of injection and abstraction.
Under a specific range of conditions the ASR process may act to purify the recharged reclaimed water. Alternatively, it may result in irreversible aquifer contamination or clogging of the well. Therefore, the hydro-biochemical and hydrogeological impacts of injecting large volumes of reclaimed water into a formation need to be established prior to any substantial capital investment. This routinely involves numerical modelling and simulations of the
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Figure 6. Vertical section perpendicular to the coastline showing typical salinity distribution without pumping or injection. (Note: the left axis of the graph represents simulated depth to the impermeable clay basement of the superficial aquifer). The unit of salinity is mg L21.
groundwater systems. The challenge is to specify the correct parameters and algorithms to realistically simulate the transport reactions that occur beneath the earth. This requires a thorough understanding of the functions and kinetics of the biogeochemical degradations that occur within specific aquifers, and the evolution of aquifer attenuation capacities. In this study, we chose tricholoroethylene (TCE) and E. coli to demonstrate the typical temporal and spatial reactive transport of the organic and pathogenic contaminants. The relevant parameters are taken from published work as shown in Table 3. TCE TCE is a colourless liquid which is often used as a solvent for cleaning metal parts. Drinking or breathing high levels of TCE may cause nervous system effects, liver and lung damage and possibly death. TCE has been found in underground water sources and many surface waters as a result of the manufacture, use, and disposal of
the chemical. The EPA has set a maximum contaminant level for TCE in drinking water at 0.005 mg L21. In deep aquifer injection (2500 KL day21 well21), if a well injects TCE contaminated water at a concentration of 1 mg L21, TCE will be transported along by the injected water flow with some adsorbed by the aquifer matrix and some decayed over the time. Figure 10 shows the bird view of the TCE distribution over the area after 5 years: when the contaminated water is under constant injection, the radius of contaminated area is about 90– 135 m around the injection well; when it is under a pulse injection, the maximum TCE concentration (0.016 mg L21) occurs at about 60 m away from the well, and the radius of the affected area is about 110 m. When this contaminated water is constantly injected, its concentration distribution along the observation line SE (shown in Figure 6) as a function of time and distance is illustrated in Figure 11. If the contamination is a short-term spill, as a pulse injection, the simulation result shows the TCE concentration distribution on the SE line as in Figure 12.
Figure 7. Simulation of salinity distribution after 50 years abstraction from wells 1000 km away from the coastline pumping at the rate of 1000 m3 per day (wells screens are placed between 40 to 50 m below ground level). The unit of salinity is mg L21.
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Figure 8. Salinity distribution following the 50 years’ abstraction (1000 m3 per day) then one year of injection at the rate of 1000 m3 per day with the injection bores placed between the coastal line and the abstraction bores. The unit of salinity is mg L21.
When the contamination occurs in the operation of shallow aquifer injection, TCE plume is transported much faster and further. Figure 13 shows the temporal and spatial distribution of TCE on the SE line over 2 years span under constant contamination; while the TCE distribution under pulse contamination is similar to the scenario in deep aquifer. The maximum distance that TCE could be transported from the shallow aquifer injection well in two years time is around 320 m. As expected, the simulation results show that a constant injection of TCE contaminated water causes a wide range, long-term pollution in the groundwater system, despite the natural attenuation function of the subsurface formations. This represents the worst-case scenario, simulating an extreme case where no proper regulation is in place upon reclaimed water quality control for groundwater recharge. In reality, most likely, accidents occur in pulse injection fashion due to equipment faults or human errors. However, by identifying the sorption coefficient, first-order decay rate of TCE, as well as the hydroconductivity parameters in the particular aquifer matrix, we can simulate the plume transport under
different conditions. With the simulation results, the location and spacing of wells and other pertinent infrastructure design, and safe operation guidance can be rationally provided. E. coli Microbiologically contaminated water may also cause many serious diseases. Most of them are due to pathogenic bacteria excreted by people suffering from or carrying the disease. E. coli is widely adopted as a sensitive indicator organism to test the presence of pathogenic faecal bacteria in water, since E. coli is a normal inhabitant of the human intestine and excreted in large numbers. Thus, its presence in water indicates human excretal contamination with high probability in the presence of pathogenic faecal bacteria. World Health Organization specifies in their Drinking Water Quality Guidelines that the presence of E. coli in drinking water should be 0 (Tebbutt, 1998). Similar numerical analysis is conducted for E. coli contamination. As shown in Table 3, E. coli has a higher tendency than TCE to be adsorbed onto the aquifer matrix, and the first order decay rate of E. coli is three orders of
Figure 9. Salinity distribution following 50 years’ abstraction (1000 m3 per day) the 10 years of injection at the rate of 1000 m3 per day with the injection bores placed between the coastal line and the abstraction bores. The unit of salinity is mg L21.
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Figure 10. A bird view of TCE concentration distribution around the deep aquifer injection well after 5 years, colour flood contour shows under constant contamination, while lined contour shows under pulse contamination. The observation points along SE line are shown.
magnitude higher than TCE. Figure 14 shows the E. coli temporal and spatial distribution along SE line in deep aquifer injection with constant contamination at a number concentration of 600 CFU/100 mL; while Figure 15 shows the distribution in deep aquifer injection with pulse contamination. The maximum distance that E. coli could travel is about 14– 15 m away from the injection well after 2 years. In shallow aquifer injection, the consequences of the constant contamination and pulse contamination are shown in Figures 16 and 17, respectively. The simulation results show that E. coli could travel about 35 m away from the well as the furthest after 2 years.
Figure 11. The temporal and spatial distribution of TCE in deep aquifer along the SE observation line under constant contamination.
Figure 12. The temporal and spatial distribution of TCE along the SE observation line in deep aquifer resulted from pulse contamination.
However, this simulation outcome appears as an overoptimistic scenario in E. coli transport. The field studies of Lewis et al. (1982) have shown that bacteria can travel several hundred metres in aquifers, which is an order of magnitude further than the above simulation prediction. It was identified that bacteria transport is often in the form of colloid transport (Ryan and Elimelech, 1996). It can be described mathematically as (Elimelech et al., 1995): @C 3 1f ¼ ah0 C @x 4 ac
(1)
where C is the number concentration of colloids of a specific size; x is the horizontal distance; ac is the radius of the grains comprising the aquifer; f is the porosity of
Figure 13. The temporal and spatial distribution of TCE along the SE observation line in shallow aquifer under constant contamination.
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Figure 14. The temporal and spatial distribution of E. coli along the SE observation line in deep aquifer under constant contamination.
Figure 16. The temporal and spatial distribution of E. coli along the SE observation line in shallow aquifer under constant contamination.
the aquifer; a is the collision efficiency; h0 is the dimensionless capture rate of colloids under favourable chemical conditions. Solving the differential equation, we obtain (Elimelech et al., 1995):
where LT is the travel distance from the source point; CL is the number concentration of colloids at LT; while C0 is the number concentration at the source point.
The challenge in using these equations lies in the determination of the a and h0, as they are controlled by a wide range of variables including aquifer grain size, shape, surface properties, electrical charge and packing behaviour, as well as colloidal particle size, shape, surface properties, electrical charge and travel velocity in groundwater. Studies are ongoing towards a rationalized determination of these parameters (Foppen et al., 2005). Therefore, if micro-organisms and other compounds which could be in presence in colloidal form are considered, extra care should be taken when conducting the reactive-transport simulations. Equations (1) and (2) should be employed if a and h0 could be determined.
Figure 15. The temporal and spatial distribution of E. coli along the SE observation line in deep aquifer under pulse contamination.
Figure 17. The temporal and spatial distribution of E. coli along the SE observation line in shallow aquifer under pulse contamination.
LT ¼ ln (CL =C0 )
4ac 3(1 f )ah0
(2)
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Banning, N., Toze, S. and Mee, B.J., 2003, Persistence of biofilm-associated Echerichia coli and Pseudomonas aeruginosa in groundwater and treated effluent in a laboratory system, Microbiology, 149: 47 –55.
The manuscript was received 19 August 2005 and accepted for publication after revision 21 April 2006.
Remarks The computation work is relatively inexpensive: each simulation run without considering contamination takes about 5– 10 minutes to complete; while the contamination risk simulation could take up to 1 h. These numerical results can be used for risk assessment and drawing guidelines for aquifer management. By knowing the reclaimed water quality, the hydrogeology properties, and the reactive transport parameters, we can design the well location, spacing, injection rate, abstraction rate, residence time, contaminant-free zones based on the simulation findings. The strategy can also work in the opposite sequence: when we understand the interactions between the subsurface formations and guest chemicals and micro-organisms (this information can be translated into the right sorption rates and decay rates for solutes, and collision efficiency, capture rate for colloidal suspensions) and the allowed residence time in the aquifers, we can design the reclaimed water quality and use the most cost-effective wastewater treatment processes in accordance. This quantitative numerical analysis will assist us to obtain a holistic view of the entire ASR process, to achieve a cost-effective water reuse strategy, and to ensure a safe operation in compliance with the stringent health and environment regulations. CONCLUSIONS This work presents a preliminary feasibility study of recharging reclaimed water to the coastal aquifers in Perth. We proposed a double-cycle reclaimed water injection strategy, incorporating a new efficient and effective wastewater treatment unit: CFBBR. Numerical analysis using FEFLOW was carried out to study the impact of the artificial injection and abstraction on the hydrogeological sustainability and contamination risk in both deep aquifers and shallow aquifers, as well as on the prevention of saline water intrusion in the shallow aquifers. By designing reclaimed water distribution systems that allows injection and abstraction simultaneously into both deep and shallow aquifers, we create the flexibility to: (1) manage groundwater levels in Perth aquifers; (2) efficiently match the wastewater treatment processes to local aquifers’ geophysichemical properties; (3) proactively store and retrieve water to relieve the water supply imbalance caused by short and long-term climatic variations; (4) selectively improve groundwater quality, particularly along the coastal and river margins. It is also shown that the temporal and spatial distributions of the chemical and biological contaminants can be numerically modelled and computed. The results are useful for the ASR process design and wastewater treatment design. More work is underway to refine this numerical model and to obtain site-specific data.
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www.icheme.org/psep doi: 10.1205/psep.05202
FEASIBILITY OF RECHARGING RECLAIMED WASTEWATER TO THE COASTAL AQUIFERS OF PERTH, WESTERN AUSTRALIA Q. LI1 , B. HARRIS2, C. AYDOGAN1, M. ANG1 and M. TADE1 1 Department of Chemical Engineering, Curtin University of Technology, Perth, Australia Division of Resources and Environment, Curtin University of Technology, Perth, Australia
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erth has a dry climate with most rainfall confined to winter season. By recharging treated wastewater into suitable aquifers along the coastal margin of Perth, a range of benefits may be achieved including: (1) prevention of saline water intrusion; (2) restoration of groundwater levels; (3) water banking and withdrawal to balance seasonal differences in water supply; (4) improvements in water quality; and (5) flexibility in aquifer management. The technical key points to implement this wastewater reuse strategy are in the hydrogeological sustainability, groundwater contamination, and treated wastewater quality. Recent developments in wastewater treatment technology have made it possible to convert sewage into fair quality/potable water. With the available data on Perth groundwater, a cost-effective wastewater treatment process is designed, which is intended to fully utilize the geopurification function. Numerical analysis is performed to investigate the impact of the reclaimed water injection on Perth basin’s hydrogeology and chemical, biological content in subsurface formations. Deep aquifer injection and shallow aquifer injection are both considered. Keywords: aquifer storage and recharge; wastewater treatment; groundwater; contamination; numerical analysis.
INTRODUCTION
The water consumption of humankind has become unprecedentedly high. Today’s water consumption in developed world is more than six folds of the consumption in the 1950s (EPA, 2004). In 2001 – 2002, the annual water consumption in Perth was 202 825 ML, and the total volume of wastewater collected was 101 891 ML through Water Corporation (Office of Water Regulation, 2002). Water Corporation has nine wastewater treatment plants (WWTP) in Perth, with the three major ones: Beenyup, Subiaco, Woodman Point processing 90% of the collected wastewater. In 2001, the Subiaco WWTP processed 58 ML day21 of wastewater (Water Corporation, 2001). By 2040, it is estimated that the volume of wastewater produced by Perth’s inhabitants will triple. In Perth, in general, only 3 –5% of treated wastewater is recycled, while the WA water authority is targeting at recycling 20% of treated wastewater by 2012. Currently more than 100 gigalitres of wastewater after secondary wastewater treatment is discharged to the ocean from the major metropolitan wastewater treatment plants in Perth per year. Faced with the two scenarios, ‘closing the loop’ seems a natural solution to save our depleted freshwater resource. A great deal of wastewater reuse strategies have emerged over the last two decades, and some of them have even been put in practice in a few world-leading regions, such as in Singapore, Berlin, and Orange County in USA. In this
Only 3.5% of the world’s water supply is freshwater and more than two-thirds of that is frozen. About 0.7% of the total earth’s water supply is freshwater available as either surface water or groundwater; groundwater accounts for two-thirds of this amount. The National Land and Water Resources Audit found that 30% of Australia’s 538 recognized groundwater management units are either close to or overused when compared with their estimated sustainable yield. Western Australia (WA) consumes twice as much groundwater as the surface water (farmweb, 2004). Western Australia has about 1.9 million inhabitants in total, with 1.4 million population narrowly focused on the southwest coastal margin around its capital Perth. The volume of groundwater abstracted from the superficial, Mirrabooka, Leederville and Yarragadee aquifers in 2001/2002 was 575 GL, and was estimated at up to 640 GL in 2003/2004. With the continuing population growth and declining rainfall, it is foreseeable that a higher demand for water could seriously stress Perth’s groundwater reserves. Correspondence to: Dr Q. Li, Department of Chemical Engineering, Curtin University of Technology, Perth, WA 6845, Australia. E-mail: [email protected]
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paper, we will focus on one of the promising solutions to rescue the groundwater resources and relieve water shortages by recharging treated wastewater (storm water run-off, sewage and industrial wastewater) into the groundwater system, i.e., the aquifer storage and recovery (ASR). A number of technical benefits may be achieved by recharging reclaimed wastewater into suitable aquifers, including: (1) restoration of groundwater levels; (2) improved water quality; (3) water banking and withdrawal balancing seasonal differences in water supply; (4) prevention of saline water intrusion at coastal margin. Economically, ASR is evaluated as potentially the least-cost option for alternative water supply (Bessler, 1993), because it requires cheap physical infrastructure, and uses the natural distribution systems. Orange County of California has set a successful example of ASR water reuse scheme (gwrsystem, 2004a, b). Studies have also been carried out in South Australia, the driest state in Australia, since last a few years (Gerges, 2000). Some aquifers have large attenuation capacities. They naturally reduce organic and microorganism contaminants (Rugge et al., 1994). Among the three possible attenuation mechanisms: sorption, dilution and degradation, the biogeochemical degradation is believed to be the dominant process (Rugge et al., 1994; Ying et al., 2003). In the current practice of ASR, prior to injection, the wastewater undergoes an extensive water purification process; typically consisting of microfiltration (MF), reverse osmosis (RO), ultraviolet light (UV) and hydrogen peroxide treatment (gwrsystem, 2004b). After treatment, the reclaimed water meets or surpasses all drinking water standards (gwrsystem, 2004b). However, this level of treatment often represents an over-treatment for groundwater replenishment. Injecting the ‘over-clean water’ could jeopardize the native chemical and biological constitutions in the groundwater system, thereafter, lead to massive, irreversible clogging problems. Moreover, under-utilized natural geopurification also reduces the economical competitiveness of ASR. The significant problems with recharging wastewater are (1) the cost effective treatment of large volumes of wastewater; (2) the effective and sustainable management of the host aquifer system; and (3) health, safety and environmental risk control. They are all particularly site-specific, determined by the biogeochemical properties of the native aquifers. In this study, we investigate a wastewater treatment strategy by integrating the aquifer purification function as a treatment unit. A preliminary feasibility study based on numerical computation is carried out to investigate the ASR impact on hydrogeological sustainability and contamination risk. WASTEWATER TREATMENT AND AQUIFER INJECTION PROCESS DESIGN Conventional WWTPs often supply two-steps treatment, with primary treatment to screen out big objects, and the secondary treatment to remove biological nutrients. Table 1 presents a typical Perth area wastewater quality after secondary treatment. A typical complete wastewater treatment process, as shown in Figure 1, prior to aquifer injection producing drinkable quality water, implies a high cost and limited yield. Reverse osmosis (RO) is capable of eliminate 99% of most ions and most organics over 150 molecule weight, however, it requires
13.8 – 69 bar transmembrane pressure. Moreover, the use of UV photocatalytic oxidation usually suggests a small throughput. Recently a new process is designed by Cui et al. (2004), which removes carbon and nitrogen simultaneously using an anoxic-aerobic circulating fluidized bed biological reactor (CFBBR), as shown in Figure 2. Such a process alone can replace both the secondary and tertiary treatment units in a conventional design. Typical data of CFBBR treated water quality is tabulated in Table 2. This novel process represents a highly integrated wastewater treatment method with high efficiency and high throughput. It provides a promising future for wastewater treatment industry by offering a significant cost and space reduction. The aquifers in the Perth area that we considered are the shallow/superficial and deep/Yaragadee. The shallow aquifers are generally unconfined, and less than 60 m from the surface. The deep/Yaragadee aquifers are confined or semi-confined at a depth generally below 500 m. The Leederville aquifer is semi-confined, lying between superficial and Yaragadee aquifers. In the Perth metropolitan area, the groundwater quality is potable to near potable. The average movement in shallow aquifers is about 30 m year21; while in deep aquifers, the maximum movement is less than 1 m year21. However, the groundwater movement is very site-specific. Both injection and abstraction wells can alter groundwater movements. The cost of drilling a well into a deep aquifer can be 100 times (Au$1 million) more expensive than a well in a shallow aquifer (a few thousands Au$). Most abstraction wells in Perth area are drawing water from shallow aquifers. Consequently, we can design an ASR strategy with considerations of both shallow and deep aquifer recharges, as illustrated in Figure 3: since the groundwater in shallow aquifers is dynamic and often drawn for domestic usage, such as reticulation, the water quality needs to match the drinkable water standard to comply with the safety, health and environmental regulations. Hence, nanofiltration, UV and hydrogen peroxide treatments should be added after CFBBR process, prior to the injection into the superficial aquifer. This shallow aquifer injection will result in a sustained water table level, improved groundwater quality, as well as a utilization of natural water
Table 1. Wastewater quality after secondary treatment (Subiaco WWTP, 2004). Variable
Concentration
pH Alkalinity BOD Suspended solids Total dissolved solids (mg L21) Total kjeldahl nitrogen (mg L21) NO3 – N (mg L21) Total nitrogen (mg L21) Phosphate–phosphorus (mg L21) Total phosphorus (mg L21) Sodium (mg L21) Calcium (mg L21) Magnesium (mg L21) 30-minute chlorine demand 24-h chlorine demand Thermotolerant coliforms (CFU/100 mL)
7.1 77 15 24 827 4 17 20 7 8 175 30 8.5 10 16 640 –240 000
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Figure 1. A conventional wastewater treatment process with tertiary and advanced treatments.
distribution systems. Less treated wastewater, as the end product of CFBBR, can be injected into deep aquifer, and harvested after 1 or 2 years retention time. The long residence time in deep aquifers allows a full utilization of geopurification function and the natural water storage system. In particular, this deep aquifer injection/abstraction can help balance the seasonal difference in water supply by injecting in winter seasons and abstracting in summers. NUMERICAL ANALYSIS ON ASR FEASIBILITY IN PERTH Model Set-up Aquifer modelling is carried out using the finite element model code FEFLOWTM, which specialises in modelling of flow and transport processes in porous media under saturated and unsaturated conditions. The finite element method employed for the 3-D modelling requires
discretization of the model domain into a mesh of triangular elements that are connected by nodes. Solutions are obtained by interpolation between nodes. The model is discretized into more than 20 000 nodes and has very dense discretization to less than 2 m proximal to injection or pumping bore locations. A model domain is set to the North of the Swan River, as shown in Figure 4. It is bounded to the east by Gnangara Mound, to the west by Indian Ocean, and to the south by the Swan River. The northern boundary is set at 35 km north to the Swan River. The calibrated numerical model is designed to be highly flexible, with the minimum number of degrees of freedom required to reasonably represent the overall character in shallow and deep groundwater systems. Such a model allows rapid testing of the general impacts of large numbers possibility pumping and injection bore configurations in a practical time frame.
Figure 2. Schematic diagram of the circulating fluidized bed biological reactor (Cui et al., 2004).
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V. HRT ¼ 2.0 h (effluent)
Range (min–max)
Average (Avg. + SD) (n)
Range (min–max)
Average (Avg. + SD) (n)
172 –420 14–308 20.2–50.5 0.3 –1.2 0–0.06 20–302 18–222 3.7 –5.6 21.9–58.5 140 –384 10–264 5.1 –7.8
262 + 58 (19) 110 + 91 (19) 24 + 8 (19) 0.74 + 0.25 (19) 0.013 + 0.015 (19) 156 + 67 (19) 135 + 54 (19) 4.4 + 0.46 (17) 30.7 + 13 (19) 207 + 87 (8) 80 + 81 (8) 6.3 + 0.82 (8)
4–51 0–28 0–4.7 2.71–12.9 0.001–0.3 0–24 0–20 0.97–3.23 1.02–6.3 4–18 0–9 1.66–2.9
18 + 14 (19) 10 + 7 (19) 1.3 + 1.5 (19) 7 + 3 (19) 0.07 + 0.06 (19) 10 + 7 (19) 8 + 6 (19) 2 + 1 (17) 1.8 + 2 (19) 10 + 6 (8) 4.5 + 3 (8) 2 + 0.5 (8)
Note: HRT refers to ‘hydraulic retention time’; numbers within parentheses represent the total number of samples.
Parameters for Perth area regarding groundwater system, and transport reactions of trichloroethylene (TCE) and Escherichia coli (E. coli) are obtained from literatures and communications with local water authorities. Table 3 contains some typical values that we have used in the simulation. Hydrogeological Perspective For the deep aquifer seven deep injection bores spaces at two kilometre intervals inject 2500 KL day21 over an injection zone of 500 m at the top of the deep aquifer. The interval transmissivity is equivalent to approximately 200 m2 day21. Figure 4(a) shows the steady state (i.e., no injection or pumping) hydraulic heads for the deep aquifer. Figure 4(b) shows the hydraulic heads after pumping a total pumping of 17.5 ML day21 (6.3 GL year21) of treated municipal waste water through the seven deep injection bores for a period 5 years. The average maximum increase in hydraulic head, along the line of bores is less than 4 m. The cone of influence around the bore remains relatively symmetrical and expands to width of approximately 8– 10 km. The injected water remains symmetrical about the bores. The symmetry reflects both the moderate to low hydraulic conductivities in the sandstone and the very low throughflow velocity in the deep aquifer (i.e., less than 1 m year21). The consequence of the above is that the injected water can be readily recovered by converting any of the injection bores to pumping bores. For Perth’s northern shallow aquifers we simulate a distribution system consisting of 18 injection bores and
18 pumping bores. The injection bores are located on a line 2.6 km inland from coast and are spaced at 1 km intervals. The recovery bores are set on a line, 2.7 km inland from the injection bore. The pumping and injection bores, pump or inject at a rate of 1000 KL day21. The total pumping and injecting rate is 18 ML day21 or (6.6 GL year21). Figure 5 shows the difference between steady state model and the model with pumping/injection for 5 years. After 5 years we can see that maximum average water level increases by less than 3 m. The small elevation in water level relates to the very high permeability of the shallow aquifer. Figure 7 shows a vertical section perpendicular to the coastline. It shows a simulation of salinity distribution after 50 years’ abstraction from a long line wells located 1 km away from the coastline. The wells for the simulation are spaced at 1 km intervals and abstraction rate for the wells is 1000 m3 per day. For the shallow aquifer the injection and pumping bore configuration is designed to prevent any chance of saline intrusion. In practice the configuration would be designed to address the existing saline intrusion problems in Perth, as demonstrated by our FEFLOW simulation results in Figures 6 –9. Figure 6 shows the calibrated salinity distribution at a typical Perth coastal area, under the natural groundwater hydraulic and diffusion mechanism without any abstraction or injection. The left boundary of the graph represents the depth from sea level at the coastal line, the scale at the bottom indicates the distance from the coastal line. It can be seen that the saline water has intruded 1 km into the on-shore groundwater system,
Figure 3. A double-cycle ASR system in conjunction with wastewater treatment strategies.
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Figure 4. The distribution of hydraulic head in deep aquifers within the model boundaries: (a) before injection, (b) after injection.
Table 3. Estimated parameters used in the simulation. Estimated flow parameters Parameter Hydraulic conductivity (m s21) Storativity Storage compressibility Porosity
Superficial aquifer 1E-4 (Davidson, 1995) 0.2 (Cargeeg et al., 1987) — 0.2 (Davidson, 1995)
Deep aquifer 0.2E-4 (Davidson, 1995) — 0.005 (Davidson, 1995) 0.2 (Davidson, 1995)
Estimated transport parameters for E. coli and TCE Parameter E. coli Sorption coefficient (Henry) 0.21 (Pang et al., 2003) 1E-10 (Johnston, 1983) Diffusivity (m2 s21) Longitudinal dispersivity (m) 7.9 (Prommer, 2005) Transverse dispersivity (m) 0.9 (Barber et al., 1991) 3.74E-6 (Banning et al., 2003) First order decay rate (s21)
TCE 0.12 (Benker et al., 1998) 1E-10 (Johnston, 1983) 7.9 (Prommer, 2005) 0.9 (Barber et al., 1991) 8E-9 (Wu et al., 2005)
Estimated parameters for saltwater intrusion (different scale) 2E-4 Hydraulic conductivity (m s21) Porosity 0.2 Sorption coefficient (Henry) 0 8E-6 Diffusivity (m2 s21) Longitudinal dispersivity (m) 1 Transverse dispersivity (m) 1 0 First order decay rate (s21) Density ratio 0.02 Note: The estimated flow parameters shown in the table are indicative for the values used in the numerical simulation.
which is analogous to Perth today’s groundwater situation. Figures 8 and 9 show that by inserting the injection bores (injection rate 1000 m3 per day) between the coastal line and abstraction bores, it effectively generates a salinity reduction after 1 year and significantly pushes back the high salinity water after 10 years. In addition, the shallow aquifer bore configuration would also be designed to manage water levels for the shallow aquifer in the most efficient way so that there is no chance of flooding or aquifer depletion. The ASR process would become an active aquifer management tool to balance the significant seasonal differences in rainfall in Perth. Contamination Risk Assessments
Figure 5. The altered water level in shallow aquifer as a result of injection and abstraction.
Under a specific range of conditions the ASR process may act to purify the recharged reclaimed water. Alternatively, it may result in irreversible aquifer contamination or clogging of the well. Therefore, the hydro-biochemical and hydrogeological impacts of injecting large volumes of reclaimed water into a formation need to be established prior to any substantial capital investment. This routinely involves numerical modelling and simulations of the
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Figure 6. Vertical section perpendicular to the coastline showing typical salinity distribution without pumping or injection. (Note: the left axis of the graph represents simulated depth to the impermeable clay basement of the superficial aquifer). The unit of salinity is mg L21.
groundwater systems. The challenge is to specify the correct parameters and algorithms to realistically simulate the transport reactions that occur beneath the earth. This requires a thorough understanding of the functions and kinetics of the biogeochemical degradations that occur within specific aquifers, and the evolution of aquifer attenuation capacities. In this study, we chose tricholoroethylene (TCE) and E. coli to demonstrate the typical temporal and spatial reactive transport of the organic and pathogenic contaminants. The relevant parameters are taken from published work as shown in Table 3. TCE TCE is a colourless liquid which is often used as a solvent for cleaning metal parts. Drinking or breathing high levels of TCE may cause nervous system effects, liver and lung damage and possibly death. TCE has been found in underground water sources and many surface waters as a result of the manufacture, use, and disposal of
the chemical. The EPA has set a maximum contaminant level for TCE in drinking water at 0.005 mg L21. In deep aquifer injection (2500 KL day21 well21), if a well injects TCE contaminated water at a concentration of 1 mg L21, TCE will be transported along by the injected water flow with some adsorbed by the aquifer matrix and some decayed over the time. Figure 10 shows the bird view of the TCE distribution over the area after 5 years: when the contaminated water is under constant injection, the radius of contaminated area is about 90– 135 m around the injection well; when it is under a pulse injection, the maximum TCE concentration (0.016 mg L21) occurs at about 60 m away from the well, and the radius of the affected area is about 110 m. When this contaminated water is constantly injected, its concentration distribution along the observation line SE (shown in Figure 6) as a function of time and distance is illustrated in Figure 11. If the contamination is a short-term spill, as a pulse injection, the simulation result shows the TCE concentration distribution on the SE line as in Figure 12.
Figure 7. Simulation of salinity distribution after 50 years abstraction from wells 1000 km away from the coastline pumping at the rate of 1000 m3 per day (wells screens are placed between 40 to 50 m below ground level). The unit of salinity is mg L21.
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Figure 8. Salinity distribution following the 50 years’ abstraction (1000 m3 per day) then one year of injection at the rate of 1000 m3 per day with the injection bores placed between the coastal line and the abstraction bores. The unit of salinity is mg L21.
When the contamination occurs in the operation of shallow aquifer injection, TCE plume is transported much faster and further. Figure 13 shows the temporal and spatial distribution of TCE on the SE line over 2 years span under constant contamination; while the TCE distribution under pulse contamination is similar to the scenario in deep aquifer. The maximum distance that TCE could be transported from the shallow aquifer injection well in two years time is around 320 m. As expected, the simulation results show that a constant injection of TCE contaminated water causes a wide range, long-term pollution in the groundwater system, despite the natural attenuation function of the subsurface formations. This represents the worst-case scenario, simulating an extreme case where no proper regulation is in place upon reclaimed water quality control for groundwater recharge. In reality, most likely, accidents occur in pulse injection fashion due to equipment faults or human errors. However, by identifying the sorption coefficient, first-order decay rate of TCE, as well as the hydroconductivity parameters in the particular aquifer matrix, we can simulate the plume transport under
different conditions. With the simulation results, the location and spacing of wells and other pertinent infrastructure design, and safe operation guidance can be rationally provided. E. coli Microbiologically contaminated water may also cause many serious diseases. Most of them are due to pathogenic bacteria excreted by people suffering from or carrying the disease. E. coli is widely adopted as a sensitive indicator organism to test the presence of pathogenic faecal bacteria in water, since E. coli is a normal inhabitant of the human intestine and excreted in large numbers. Thus, its presence in water indicates human excretal contamination with high probability in the presence of pathogenic faecal bacteria. World Health Organization specifies in their Drinking Water Quality Guidelines that the presence of E. coli in drinking water should be 0 (Tebbutt, 1998). Similar numerical analysis is conducted for E. coli contamination. As shown in Table 3, E. coli has a higher tendency than TCE to be adsorbed onto the aquifer matrix, and the first order decay rate of E. coli is three orders of
Figure 9. Salinity distribution following 50 years’ abstraction (1000 m3 per day) the 10 years of injection at the rate of 1000 m3 per day with the injection bores placed between the coastal line and the abstraction bores. The unit of salinity is mg L21.
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Figure 10. A bird view of TCE concentration distribution around the deep aquifer injection well after 5 years, colour flood contour shows under constant contamination, while lined contour shows under pulse contamination. The observation points along SE line are shown.
magnitude higher than TCE. Figure 14 shows the E. coli temporal and spatial distribution along SE line in deep aquifer injection with constant contamination at a number concentration of 600 CFU/100 mL; while Figure 15 shows the distribution in deep aquifer injection with pulse contamination. The maximum distance that E. coli could travel is about 14– 15 m away from the injection well after 2 years. In shallow aquifer injection, the consequences of the constant contamination and pulse contamination are shown in Figures 16 and 17, respectively. The simulation results show that E. coli could travel about 35 m away from the well as the furthest after 2 years.
Figure 11. The temporal and spatial distribution of TCE in deep aquifer along the SE observation line under constant contamination.
Figure 12. The temporal and spatial distribution of TCE along the SE observation line in deep aquifer resulted from pulse contamination.
However, this simulation outcome appears as an overoptimistic scenario in E. coli transport. The field studies of Lewis et al. (1982) have shown that bacteria can travel several hundred metres in aquifers, which is an order of magnitude further than the above simulation prediction. It was identified that bacteria transport is often in the form of colloid transport (Ryan and Elimelech, 1996). It can be described mathematically as (Elimelech et al., 1995): @C 3 1f ¼ ah0 C @x 4 ac
(1)
where C is the number concentration of colloids of a specific size; x is the horizontal distance; ac is the radius of the grains comprising the aquifer; f is the porosity of
Figure 13. The temporal and spatial distribution of TCE along the SE observation line in shallow aquifer under constant contamination.
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Figure 14. The temporal and spatial distribution of E. coli along the SE observation line in deep aquifer under constant contamination.
Figure 16. The temporal and spatial distribution of E. coli along the SE observation line in shallow aquifer under constant contamination.
the aquifer; a is the collision efficiency; h0 is the dimensionless capture rate of colloids under favourable chemical conditions. Solving the differential equation, we obtain (Elimelech et al., 1995):
where LT is the travel distance from the source point; CL is the number concentration of colloids at LT; while C0 is the number concentration at the source point.
The challenge in using these equations lies in the determination of the a and h0, as they are controlled by a wide range of variables including aquifer grain size, shape, surface properties, electrical charge and packing behaviour, as well as colloidal particle size, shape, surface properties, electrical charge and travel velocity in groundwater. Studies are ongoing towards a rationalized determination of these parameters (Foppen et al., 2005). Therefore, if micro-organisms and other compounds which could be in presence in colloidal form are considered, extra care should be taken when conducting the reactive-transport simulations. Equations (1) and (2) should be employed if a and h0 could be determined.
Figure 15. The temporal and spatial distribution of E. coli along the SE observation line in deep aquifer under pulse contamination.
Figure 17. The temporal and spatial distribution of E. coli along the SE observation line in shallow aquifer under pulse contamination.
LT ¼ ln (CL =C0 )
4ac 3(1 f )ah0
(2)
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The manuscript was received 19 August 2005 and accepted for publication after revision 21 April 2006.
Remarks The computation work is relatively inexpensive: each simulation run without considering contamination takes about 5– 10 minutes to complete; while the contamination risk simulation could take up to 1 h. These numerical results can be used for risk assessment and drawing guidelines for aquifer management. By knowing the reclaimed water quality, the hydrogeology properties, and the reactive transport parameters, we can design the well location, spacing, injection rate, abstraction rate, residence time, contaminant-free zones based on the simulation findings. The strategy can also work in the opposite sequence: when we understand the interactions between the subsurface formations and guest chemicals and micro-organisms (this information can be translated into the right sorption rates and decay rates for solutes, and collision efficiency, capture rate for colloidal suspensions) and the allowed residence time in the aquifers, we can design the reclaimed water quality and use the most cost-effective wastewater treatment processes in accordance. This quantitative numerical analysis will assist us to obtain a holistic view of the entire ASR process, to achieve a cost-effective water reuse strategy, and to ensure a safe operation in compliance with the stringent health and environment regulations. CONCLUSIONS This work presents a preliminary feasibility study of recharging reclaimed water to the coastal aquifers in Perth. We proposed a double-cycle reclaimed water injection strategy, incorporating a new efficient and effective wastewater treatment unit: CFBBR. Numerical analysis using FEFLOW was carried out to study the impact of the artificial injection and abstraction on the hydrogeological sustainability and contamination risk in both deep aquifers and shallow aquifers, as well as on the prevention of saline water intrusion in the shallow aquifers. By designing reclaimed water distribution systems that allows injection and abstraction simultaneously into both deep and shallow aquifers, we create the flexibility to: (1) manage groundwater levels in Perth aquifers; (2) efficiently match the wastewater treatment processes to local aquifers’ geophysichemical properties; (3) proactively store and retrieve water to relieve the water supply imbalance caused by short and long-term climatic variations; (4) selectively improve groundwater quality, particularly along the coastal and river margins. It is also shown that the temporal and spatial distributions of the chemical and biological contaminants can be numerically modelled and computed. The results are useful for the ASR process design and wastewater treatment design. More work is underway to refine this numerical model and to obtain site-specific data.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2006, 84(B4): 237– 246