Struvite Formation in Leachate Recirculation Pipes of ...

DOI: 10.5276/JSWTM.2012.291

STRUVITE FORMATION IN LEACHATE RECIRCULATION PIPES OF BIOREACTOR LANDFILLS M.K.P.T.Somathilake Department of Civil Engineering, University of Calgary 2500 University Drive NW, Calgary, AB, T2N 1N4 CANADA Email: [email protected]

J.P.A. Hettiaratchi, Professor Department of Civil Engineering, University of Calgary 2500, University Drive NW, Calgary, AB, T2N 1N4 CANADA Email: [email protected]

ABSTRACT A primary operational issue associated with bioreactor landfills is the potential clogging of leachate recirculation pipes due to the formation of deposits. Studies on calcium carbonate formation have been performed in the past. Struvite (MgNH4PO4.6H2O) is a chemical precipitate reported as a widespread problem in the wastewater industry. This study presents results from laboratory experiments conducted to investigate the possibility of Struvite scale formation in leachate recirculation pipes. Two types of pipes, HDPE (High-Density Polyethylene) and PVC (Polyvinyl Chloride), were tested under three different flow rates using simulated leachate. Significant reduction in concentrations of magnesium, total phosphorous, and ammoniumnitrogen were observed when simulated leachate was passed through the test pipes. The crystal structure and composition of the deposits after each experimental run were monitored and found to closely match that of Struvite. The highest scale deposition was observed for the low flow rate, irrespective of the pipe material. HDPE pipe exhibited less accumulation during high and medium flow rates than the PVC pipe. In the case of the low flow rate, the highest accumulation was in the HDPE pipe. Keywords: bioreactor landfills, leachate recirculation, pipe clogging, Struvite

INTRODUCTION Recent research has shown that landfills could be operated as bioreactor landfills with reduced short term and long term environmental impacts. Bioreactor landfills incorporate leachate recirculation to increase the moisture content of the waste mass to enhance the rate of waste degradation and thereby decrease the life of a landfill (Lozecznik, 2006; Reinhart et al., 2002; Townsend et al., 1996; Al-Yousfi and Pohland, 1998). Leachate recirculation refers to the collection

of leachate from bottom of the landfill and then pumping it back into the landfill waste mass. A leachate recirculation system may consist of horizontal pipes/trenches which are located at various depths within the landfill cell. A leachate recirculation trench is composed of a perforated leachate pipe surrounded by a granular material. Leachate may be transmitted from the top of the landfill to horizontal pipes within these trenches through vertically positioned pipes. Due to the ability to control the leachate application rates and distribution points by using horizontal trenches, a good moisture dis-

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tribution can be achieved making this an effective leachate recirculation technique (Reinhart et al., 2002; Rowe et al., 2000). Also, the use of horizontal pipes minimizes the exposure of leachate to the environment. Some of the other methods that have been used for leachate recirculation include surface irrigation or spray irrigation, surface ponds, and prewetting of waste. The pipe systems used within these landfills are required to operate as designed for long periods of time (at least decades). However, foreign materials can clog the granular material of the collection system (Rowe et al., 2000; VanGulck and Rowe, 2004; Cook et al., 2005; Fleming et al., 1999) and the pipes (and perforations) used in recirculation systems (Manning, 2000; Maliva et al., 2000). These foreign materials can be forms of microbial mass, chemical precipitates, and clay/silty material (McBean et al., 1995). Clogging can impair the performance of these systems to the point that they no longer perform to their design function and decrease their lifespan. A clogged recirculation pipe will experience a reduction in hydraulic performance for transmitting leachate. The clog accumulation in recirculation pipes due to suspended materials and microbial mass can be easily avoided by adopting design modifications to the leachate collection system, Studies on calcium carbonate formation have been performed in the past (Rowe et al., 2000). Struvite (MgNH4PO4.6H2O) is another chemical precipitate, reported as a widespread problem in the wastewater treatment industry (Parsons and Doyle, 2004; Ohlinger et al., 1998; Mohajit et al., 1989; Borgerding, 1972). These deposits may be formed anywhere in the leachate recirculation system and is extremely difficult to remove. However, research on potential clogging due to Struvite formation in horizontal leachate recirculation pipes is lacking. The primary focus of this paper is to present results from a study conducted to evaluate the clogging potential of leachate recirculation pipes due to Struvite formation.

Theoretical considerations









1/ 3

Where, S r = supersaturation ratio

The following equation describes the dissolution of a salt in water,

Aa Bb ( s )  aA z  ( aq)  bB z  ( aq) According to Le Corre (2006), the solubility product Ksp for the above equation is: Ksp = [Az+] a. [Bz-] b

(1)

(2)

Where, [Az+] and [Bz-] are the total concentrations of ions in solution -z+, z- are the valences of the considered ions. The dissolution equation for Struvite is, MgNH4PO4.6H2O → Mg2+ + NH4++ PO43Therefore, the solubility product becomes,







K sp  Mg 2 . NH 4 . PO43



(3)

For this calculation, pH, ion activity, and ionic strength have not been taken into account. Hence, Ksp value has been expressed as an activity solubility product named as Kso which takes into consideration the effects of pH, ionic strength and ion activity. According to Le Corre(2006) and Ali (2005),













K SO   NH  NH 4   Mg 2 Mg 2   PO 3 PO43 (4) 4

To explain Struvite precipitation in pipes, it is necessary to understand the fundamental concepts of precipitate formation from solutions. Therefore, a brief description, including relevant equations, is given below. The theoretical concepts were used during the analysis of results generated from the experimental program. Supersaturation of a solution is the governing parameter for crystallization. When the value of supersaturation ratio is greater than one, Struvite crystals should precipitate (Kofina & Koutsoukos, 2005). According to Ohlinger et al., (1999), the supersaturation ratio for Struvite can be calculated as,

 Mg 2 NH 4 PO43  Sr    K SO  

{C}= γ [C] (Le Corre, 2006) where, γ is the activity coeffiC C cient of the ionic specie C and [C] its total concentration

4

Where γi is the activity coefficient of the ionic specie i Bulk fluid ionic strength (I), is used to determine the activity coefficient of each ion in solution. The activity coefficient has been calculated from the approximation of Debye-Hückel equation (Doyle et al, (2002); Mullin, (1961))

 I 1/ 2   log  i  AZ i2   0.3I 1/ 2  1  I 

(5)

Where, γ-activity coefficient Z- Valency of the ion 0 A- Debye-Huckel constant (0.509 at 25 c) (Doyle et al, 2002; Ali 2005) Ionic strength

-14 K SO = solubility product (5.49*10 )

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Flow rate calculations

Qlab  Qlandfill 

Since the flow pattern in the pipe is subject to viscous and inertia forces only, Reynolds number is selected as the criterion for similarity. Other properties of the fluid, such as elasticity and surface tension, as well as gravity forces, do not affect the flow pattern (Giles, 1956). Reynolds Number, R 

vd 

(6)

Where; -3  = Density of the fluid (kgm ) -1

V = Velocity of the fluid (ms ) d = Diameter of the pipe (m) -2  = Dynamic fluid viscosity (Nsm ) By equating the Reynolds number of the field landfill leachate recirculation pipes and the laboratory leachate recirculation pipes, the following equations are developed.

(Vd ) landfill  (Vd ) lab

(7)

From equation (7) and rearranging variables,

Vlandfill Vlab



d lab d landfill

From the continuity equation, Q  AV Where; Q = Volumetric flow rate

(8)

(9)

V = Velocity of the fluid flowing through the pipe A = Inside cross section area of the pipe

(11)

MATERIALS AND EXPERIMENTAL METHODS Materials Since the primary aim was to determine the Struvite (MgNH4PO4.6H2O) precipitation potential, the synthetic +2 + leachate used contained only the ions of Mg , NH4 and PO4 3 . To determine the concentrations of these ions in synthetic leachate, actual leachate characteristic data were reviewed from various young landfills as reported by Zaloum and Abbott, (1997); Borzacconi et al., (1999); Inanc et al., (2000); Im et al., (2001); Tatsi et al., (2003) and Calli et al., (2005). Typical landfill leachate contains several other ions which are capable of forming complexes other than Struvite by reac+2 + -3 tions with Mg , NH4 and PO4 reducing the possibility of Struvite precipitation. Therefore, the selected synthetic leachate will give the maximum Struvite scale mass that can be precipitated on pipe walls. The pH of the synthetic leachate was adjusted using 5M +2 NaOH of USP/NF grade (98% assay) solution. Mg , total P, and NH4-N concentrations were prepared using stock solutions of Magnesium Chloride Hexahydrate (MgCl2.6H2O) USP/FCC grade (min. 99-101% assay) from Fisher Scientific, Ammonium Dihydrogen Phosphate (NH4H2PO4) ACS grade (min. 99.0% assay) from VWR international and Ammonium Chloride (NH4Cl) USP/FCC grade (min. 99-101% assay) from Fisher Scientific.

Experimental set-up

By comparing equation (8) with equation (9), the following equation is developed.      Q 2 d     4  landfill d   lab d landfill      Q 2  d     4  lab 

d lab d landfill

(10)

All the variables in equation (10) are known except the flow rate in the laboratory piping system. Therefore, by rearranging the variables, the following equation is developed.

A schematic diagram of the experimental apparatus is presented in Figure 1. Two different pipe materials, HDPE and PVC, having an internal diameter of 21.7mm and a length of 60cm, respectively, were used to represent the horizontal leachate recirculation pipes in a landfill bioreactor cell. The PVC pipe was machined to have the same surface roughness and internal diameter as the HDPE pipe. The pipes were connected in parallel for the experiment. One end of each pipe was connected to a vertical PVC pipe which was connected to the upper reservoir (see Figure 1). The other end was connected to a horizontal PVC pipe which was connected to the lower reservoir. The volume of reservoirs was 65L.The effluent and influent ends contained valves to control the flow rate through the pipe system. The discharge end of the pipe system was connected to a submersible pump, to convey leachate back to the upstream reservoir. The effluent end was turned upward to maintain a full flow inside the two horizontal pipes that were being tested.


293

FIGURE 1 Schematic diagram of laboratory apparatus (not to scale)

Experimental procedure

total influent volume was 50L. After about one week of leachate recirculation, the solution was replaced in an attempt to maintain approximately the same chemical characteristics of leachate throughout the experimental run. Three different flow rates (high, medium, and low) were tested to assess the effect of flow rate on Struvite scale formation. The following flow rates were used in the laboratory pipe system: from day 1 to day 73; 1.5-1.7L/min (medium), from day 73 to day 159; 1.9-2.1L/min (high), from day 159 to 236 days; 1.1-1.3L/min (low). The flow rates used in the experiment were within the leachate injection rate range being used at the Calgary biocell (Stantec, 2004). The Calgary biocell is an innovative bioreactor landfill operated sequentially in anaerobic and aerobic modes. The current experimental program is a laboratory scale simulation of the processes occurring within the Calgary biocell. The typical trench diameters used at the Calgary biocell were between 50 and 150mm (Stantec, 2004). In order to simulate actual landfill leachate recirculation conditions, the simulated leachate solution was passed through the pipe system without cleaning the scale deposited during each flow rate. The laboratory pipe system was operated at an ambient temperature of 270C.

Leachate and scale composition analysis

The test solution was passed through the pipe specimens at the desired flow rates for approximately seven days. The

Medium flow rate (mag: 1000x)

Characteristics of the synthetic leachate was analyzed

High flow rate (mag: 1786x)

Low flow rate (mag: 1100x) FIGURE 2 ESEM images of the scale formed for the three flow rates

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60 Medium flow rate High flow rate Low flow rate Theoretical wt (%)

50

wt (%)

40 30 20 10 0 N

O

Mg

P

Element

FIGURE 3 Composition analysis of the scale formed for the three flow rates according to the following chemical processes for the concentrations of Mg+2, NH4-N, total Phosphorous, and pH. They are the key chemical parameters associated with the occurrence of Struvite. The pH value was tested daily and Mg+2, NH4-N, and total P concentrations were tested on a weekly basis at the time of replenishing the system with a new solution. About 500mL, i.e. 1% of total volume, was taken from the effluent end of the pipe for chemical analysis. The pH was measured using an Accumet portable pH meter AP 61 (Fisher Scientific) and it was equipped with the appropriate electric probe. Mg+2 concentrations were analyzed using Unicam 969 Atomic Absorption (AA) Spectrometer with SOLAAR32 windows operating software. Total P analysis was measured using HACH method no.8190, (total acid persulfate digestion EPA approved method). NH4-N concentration was tested by HACH method no. 8038 (Nessler method, EPA approved), and it was calculated for ammonium concentration. Samples were stored according to the standard methods for each experiment. At the end of each experimental run for each flow rate, the two parallel pipes were removed from the system to analyze the scale material that has accumulated inside the pipes and to measure the scale thickness. Chemical composition and crystal analysis of the scale was assessed with Philips XL30 Environmental Scanning Electron Microscope (ESEM), equipped with scanning control with EDAX software to obtain the energy dispersive x-ray spectroscopy (EDS). From the ESEM and EDS analyses, a qualitative elemental composition of the scale material was obtained. The scale sample was covered with a gold-palladium thin film deposited by Technics sputtering system Model Hammer ІІ for the ESEM and EDS analysis. The thickness of the pipe was analyzed using Scanco Medical Xtream Computer Tomography (CT) scanner. The software that was used to analyze the thickness was Image Processing Language V5.00c by Scanco Medical. The scale thickness was measured for a length of 135mm for each pipe material and separately for the influent side and effluent side.

RESULTS AND DISCUSSION Scale composition and morphology The scale material represented orthorhombic crystal structure for all tested flow rates. A similar structure for Struvite crystals was identified by Wang et al., (2005); Ali (2005); Doyle et al., (2002). Furthermore, quantitative analysis was performed to assess the scale composition. Though the actual proportions slightly differed from stoichiometric proportions, it can be stated that the scale deposits are Struvite crystals.

Effect of flow rate on scale formation and leachate characteristics changes The changes in Magnesium and NH4-N concentrations with time and flow rates are shown in Figures 4 and 5, respectively. The measured concentrations were almost constant at the low flow rate. This may be caused by the constant rate of scale formation as a result of low turbulence of the feed solution. There was no noticeable variation of NH4-N concentration with time at all flow rates. A possible reason is that the mass of nitrogen contribution to the chemical reaction was low due to its low atomic mass. Furthermore, the influent concentration of nitrogen is much higher when compared with the other chemicals used.

Effect of flow rate and pipe materials on scale thickness According to Walker and Sheikholeslami (2003), velocity affects scale formation in two ways. Fluid flow promotes the transportation of ions to the pipe wall and promotes crystallization at the wall. Diffusion of soluble species towards the solid surface would enhance crystallization at the surface. However, removal rate is increased with higher velocities because of higher shear rate at the liquid solid interface. There are more chances of forming precipitates with lower


295

FIGURE 4 Mg concentration vs elapsed time +2

FIGURE 5 NH4-N concentration vs elapsed time velocities since at lower velocities there is more time for the reactions to occur, before the fluid exits the pipe. Since this study is conducted under isothermal conditions, the variation in precipitation rate can only be due to the velocity gradient. Results obtained from the experimental program showed

a)

b)

that highest scale formation occurred at the low flow rate which agrees with the above explanation. Scale formation in HDPE pipe The scale thickness along the 135mm length in the influ-

c)

d)

FIGURE 6 CT scanning images of cross sections - medium flow rate a) PVC influent end b) PVC effluent end c) HDPE influent end d) HDPE effluent end

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a)

b)

c)

d)

FIGURE 7 CT scanning images of cross sections – high flow rate a) HDPE influent end b) HDPE effluent end c) PVC influent end d) PVC effluent end

a)

b)

c)

d)

FIGURE 8 CT scanning images of cross sections – low flow rate a) PVC influent end b) PVC effluent end c) HDPE influent end d) HDPE effluent end

ent side of HDPE pipe was the highest (see Figure 9). In comparison, the amount of scale formation was much lower at medium and high flow rates. Scale formation was only observed within the first 20mm length along the influent end, and thereafter there was no measurable scale along the pipe. Even though the accumulated thickness was zero at medium and high flow rates, the measured thickness was around 1mm throughout the length of the pipe at the low flow rate at the influent side. Similarly, when considering the scale thickness along the HDPE pipe on the effluent side, the highest scale thickness was observed at the low flow rate. Although there was no deposition observed at high flow rate and a very low amount during medium flow rate, at low flow rates, a scale thickness of more than 0.6mm was observed along the first 25mm from the effluent end. However, the scale thickness decreased to 0.2mm at the effluent end. Scale formation in PVC pipe The results for the PVC pipe were similar to those observed with HDPE pipe, although magnitudes were different. A steady increase of the scale thickness was observed with increasing pipe length at the low flow rate on the influent side. At the other flow rates, the thickness of scale deposit

was a constant along the length, with an exception at high flow rates. At the high flow rate, there was no measurable scale in the first 2/3 of the effluent side, but, there was considerable scale in the latter 1/3 of the effluent side. The experimental results for both types of pipes show that more scale formation had occurred during the low flow rate, irrespective of the pipe material. According to Liu et al (2006), scale formation on hard surfaces is controlled by the adhesion of species to the surface and cohesion between elements of the material. The adhesion between the surface and the scale depends upon the Vander-waals forces, electrostatic forces and contact area effect. The greater the area the greater will be the total attractive forces, but in this study the inside pipe area was same for both materials. The forces between the deposits depend on the nature of the material; i.e. deposits may be covalently bonded or held together physically. Results show that the forces between the scale particles are higher in HDPE pipe since it depends upon the nature of the material. The adhesion between the surface and the scale is higher in PVC pipe material since the Vander-waals forces and electrostatic forces are higher in PVC pipe surface than the HDPE pipe surfaces due to their material characteristics.


297

Scale Thickness(mm)

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

20

40

60

80

100

120

140

Pipe Length(mm) Medium Flow Rate

High Flow Rate

Low Flow Rate

Scale Thickness(mm)

FIGURE 9 Scale thicknesses along HDPE pipe - influent side

0.8 0.6 0.4 0.2 0 0

20

40

60

80

100

120

140

Pipe Length(mm) Medium Flow Rate

High Flow Rate

Low Flow Rate

FIGURE 10 Scale thicknesses along PVC pipe - influent side

CONCLUSIONS The scale material analysis carried out after each experimental run showed that the crystal structure and composition of the deposits were similar to that of Struvite. The highest scale deposition was observed for the low flow rate, irrespective of the pipe material. In this stage, more scale thickness was observed on the influent side of the HDPE pipe whereas in the PVC pipe more scale thickness was formed towards the middle. The average scale thickness was approximately 400 times more for the low flow rate than the thickness formed during the medium and high flow rates in the HDPE pipe. On the other hand, the accumulated average scale thickness for the low flow rate was about 3 and 30 times more than the medium and high flow rates in the PVC pipe, respectively. Thus, leachate should be permeated at a higher flow rate through the recirculation pipes to minimize Struvite scale deposition. HDPE pipe material showed minimum scale accumulations during high and medium flow rates than PVC pipe. Scale accumulations were about 9 and 90 times lower. For low flow rate, scale accumulation was about 1.5 times greater in HDPE pipe than in PVC pipe. Therefore, if HDPE pipes

298

are used in leachate recirculation systems with high flow rates, clogging from Struvite scale formation would be minimized.

REFERENCES Ali, M.I. (2005). “Struvite crystallization from nutrient rich wastewater” Doctor of Philosophy Thesis, James Cook University. Al-Yousfi, A.B. & F.G. Pohland, (1998). “Strategies for simulation, design, and management of solid wastes disposal sites as landfill bioreactors” Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management, Volume 2, No. 1, pp. 13-21. Borgerding, J. (1972). “Phosphate deposits in digestion systems” Journal of Water Pollution Control Federation, Volume 44, No. 5, pp. 813-819. Borzacconi, L., I. Lopez, M. Ohanian, and M. Vinas, (1999). “Anaerobic-aerobic treatment of municipal solid waste leachate” Environmental Technology, Volume 20, pp. 211-217. Calli, B., B. Mertoglu, and B. Inanc, (2005). “Landfill lea-


VOLUME 38, NO. 4

NOVEMBER 2012

chate management in Istanbul: Applications and alternatives” Chemosphere, Volume 59, No. 6, pp. 819-829. Cooke, A.J., R.K. Rowe, and B.E. Rittmann, (2005). “Modelling species fate and porous media effects for landfill leachate flow” Canadian Geotechnical Journal, Volume 42, No. 1116, pp. 1132. Doyle, J.D. and S.A. Parsons, (2002). “Struvite formation, control and recovery” Water Research, Volume 36, No. 16, pp. 3925-3940. Fleming, I.R., R.K. Rowe, and D.R. Cullimore, (1999). “Field observations of clogging in a landfill leachate collection system” Canadian Geotechnical Journal, Volume 36, No. 4, pp. 289-296. Giles, R.V. (1956).” Theory and problems of hydraulics and fluid mechanics” Schaum Pub. Co., New York. Haydar, M.M. and M.V. Khire, (2005). “Leachate recirculation using horizontal trenches in bioreactor landfills” Journal of Geotechnical and Geoenvironmental Engineering, Volume 131, No. 7, pp. 837-847. Im, J.H., H.J. Woo, M.W. Choi, K.B. Han, and C.W. Kim, (2001). “Simultaneous organic and nitrogen removal from municipal landfill leachate using an anaerobic-aerobic system” Water Res., Volume 34, pp. 2403-2410. Inanc, B., B. Calli, and A. Saatci, (2000). “Characterization and anaerobic treatment of sanitory landfill leachate in Istanbul” Water Science Technology, Volume 41, pp. 223230. Le Corre, K.S. (2006). “Understanding struvite crystallisation and recovery” Doctor of Philosophy thesis” Cranfield University. Liu, W., P.J. Fryer, Z. Zang, Q. Zhao, and Y. Liu, (2006). “Identification of cohesive and adhesive effects in the cleaning of food fouling deposits” Innovative Food Science and Emerging Technologies, Volume 7, pp. 263269. Lozecznik, S. (2006).” Hydraulic design, operation and clogging of leachate injection pipes in bioreactor landfills” Master of Science thesis, University of Manitoba. Maliva, R.G., T.M. Missimer, K.C. Leo, R.A. Statom, C. Dupraz, M. Lynn, and J.A.D. Dickson, (2000). “Unusual calcite stromatolites and pisoids from a landfill leachate collection system” Geology, Volume 28, No. 10, pp. 931934. Manning, D.A.C. (2000). “Carbonates and oxalates in sediments and landfill: Monitors of death and decay in natural and artificial systems” Journal of the Geological Society, Volume 157, pp. 229-238. McBean, E.A., F.A. Rovers & G.J. Farquhar (1995). “Solid

waste landfill engineering and design” Prentice Hall PTR, New Jersey. Mohajit, K.K.B, E.P. Taiganides, and B.C. Yap (1989). “Struvite deposits in pipe and aerators” Biological Wastes, Volume 30, pp. 133-147. Mullin, J.W. (1961).” Crystallization” Butterworths, London. Ohlinger, K.N., T.M. Young, and E.D. Schroeder, (1998). “Predicting struvite formation in digestion” Water Research, Volume 32, No. 12, pp. 3607-3614. Ohlinger, K.N., T.M. Young, and E.D. Schroeder, (1999). “Kinetics effects on preferential struvite accumulation in wastewater” Journal of Environmental Engineering, Volume 125, No. 8, pp. 730-737. Parsons, S.A. and J.D. Doyle, (2004). “Struvite scale formation and control” Water Science and Technology, Volume 49, No. 2, pp. 177-182. Reinhart, D.R., P.T. McCreanor, and T. Townsend, (2002). “The bioreactor landfill: Its status and future” Waste Management and Research, Volume 20, No. 2, pp. 172-186. Rowe, R.K., M.D. Armstrong, and D.R. Cullimore, (2000), “Mass loading and the rate of clogging due to municipal solid waste leachate” Canadian Geotechnical Journal, Volume 37, pp. 355-370. Stantec Inc. (2004). “Design bases documents for sustainable landfill biocell- Shepard landfill facility.” Tatsi, A.A., A.I. Zouboulis, K.A. Matis, and P. Samaras, (2003). “Coagulation-floculation pretreatment of sanitory landfill leachates” Chemosphere, Volume 53, pp. 737744. Townsend, T.G., W.L. Miller, H. Lee, and J.F.K. Earle, (1996). “Acceleration of landfill stabilization using leachate recycle” Journal of Environmental Engineering, Volume 122, No. 4, pp. 263-268. VanGulck, J.F. and R.K. Rowe, (2004). “Evaluation of clog formation with time in columns permeated with synthetic landfill leachate” Journal of Contaminant Hydrology, Volume 75, pp. 115-139. Walker, P. and R. Sheikholeslami, (2003). “Assessment of the effect of velocity and residence time in CaSO4 precipitating flow reaction” Chemical Engineering Science, Volume 58, No. 5, pp. 3807-3816. Wang, J., J.G. Burken, X. Zhang, and R. Surampalli, (2005). “Engineered struvite precipitation: Impacts of componention molar ratios and pH” Journal of Environmental Engineering, Volume 131, No. 10, pp. 1433-1440. Zaloum, R. and M. Abbott, (1997). “Anaerobic pretreatment improves single sequencing batch reactor treatment of landfill leachates” Water Science and Technology.


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