Saline water and sludge produced during oil and gas exploration are usually stored on site (1). The current disposal of contaminated materials is inefficient and costly (2) and the remoteness of many sites complicates the disposal process as these sites are only accessible in winter (3). Storage of contaminated materials with no further treatment results in the destruction of vegetation and creation of bare areas as well as contamination of groundwater (2-6). However, the regulating bodies responsible for oil and gas operations in Canada are beginning to enforce clean ups to eliminate any potential environmental hazards (7). Therefore, an economically sustainable efficient technology is required to remove salts from contaminated soils and sludges associated with oil and gas production.
Sludge is composed of water, biological compounds (microbial cells and various cellular products), organic solids, metals and several other pollutants (8). The complex physiology of sludge offers great challenges in separation of hazardous compounds (9). The current techniques used for the treatment of contaminated soils include: (a) thermal conditioning (8), (10), (11), (b) chemical conditioning with additives such as lime (12), ferric chloride (13) and cyclodextrins (14), (c) thickening/dewatering using centrifugation (15), sedimentation (16), vacuum filtration (17), belt filtration (18) and drying beds (19), (d) composting using windrows (20), biopiles (21) and bioreactors (8) and (e) washing (22-24).
Washing is considered to be a relatively low-cost alternate to land disposal and has been used to treat soil/sludges containing organic and inorganic contaminants. In comparison to other sludge remediation techniques, washing has many advantages including: less time required, higher treatment efficiency, low requirements of chemicals/biological agents to treat pollutants in sludge and ease of operation (22). Soil washing is a physical/chemical treatment process which normally involves: (a) excavation of contaminated soil/sludge, (b) pretreatment of soil/sludge to remove large objects and oversized clods, (c) washing the soil/sludge with water (chemical extracts may be used in the wash water) to separate the contaminants and (d) recovering the clean soil/sludge fraction that can be redeposited on site (23). The removal efficiency depends on the cation exchange capacity (CEC) of the soil/sludge which is a measure of the soil/sludge ability to exchange cations. Cations are held by electrostatic forces on the soil/sludge particle surfaces to balance the negatively charged clays (24). A high CEC in soil indicates a stronger bond between the cations located on the surface of the soil/sludge (higher capacity to hold onto cations) and it is less likely that they will exchange with other cations.
However, the saline water resulting form the sludge washing must be treated. Methods for the demineralization of water include: distillation (25), membrane separation (microfiltration, ultrafiltration, nanofiltration and reverse osmosis) (26-27) and electrodialysis (28). Electrodialysis can be performed on saline water (containing [CaCl.sub.2], [MgCl.sub.2] and NaCl) using an ion-exchange column consisting of zeolite (29).
Zeolites are composed of [SiO.sub.4] tetrahedra in which each oxygen is shared between two tetrahedra, occasionally [Al.sup.3+] ions in place of [Si.sup.4+] (30). The chemical composition for various types of zeolites is shown in Table 1 (31). The ion-exchangeability is due to the presence of extra-frame cations, located in the regular array of channels and cages. Cations are bound to the lattice and to water molecules, which normally fill the zeolite micropores. Substitution is stoichiometric and unless partial or total exclusion occurs, the entire cation exchange capacity can be covered (32). Zeolites are commonly used in water softening processes to exchange [Ca.sup.2+] for [Na.sup.+] (29) and for the removal of radioactive and heavy metals (33-36).
[TABLE 1 OMITTED]
The primary aim of this study was to develop a treatment process for a salt-contaminated sludge associated with natural gas and oil production. The specific objectives were: (a) to develop an efficient salt removal process from saline sludge, (b) to develop a water remediation process for the saline wash water and (c) to combine the two processes into a mobile system for the onsite treatment of saline sludge.
MATERIALS AND METHODS
Experimental apparatus: A simple laboratory soil washing and water desalination system was constructed as shown in Figure 1. It consisted of five cylinders, the first (10 cm diameter and 100 cm height) was used for the clean water, the second (10 cm diameter and 25 cm height) was used for the contaminated sludge, the third (10 cm diameter and 100 cm height) was used for the collection of the brine water, the fourth (5 cm diameter and 25 cm height) was used as an ion-exchange column and the fifth (10 cm diameter and 25 cm height) was used for collection of desalinized water. The cylinders were constructed from Plexiglas pipes. The bottom of each cylinder was made of a Plexiglas circular plate which was glued into the bottom end of the pipe and fasten with four screws. The cylinders were connected by 1.5 cm diameter Tygon[R] tubing. Four 1.5 cm ball valves (Northeast Equipments Ltd., Halifax, Nova Scotia) and four pumps (Masterflex[R] L/S[R], Model # 77200-50, Cole-Parmer Instrument Company, Montreal, QC) were used to facilitate the movement of clean water into the sludge cylinder, the movement of saline water into the saline water collection cylinder, the movement of saline water into the ion-exchange column and the movement of desalinized water into the clean water collection tank. To prevent sediments from traveling through the tubing, a GeoTech[R] screen (GeoTech Environmental Inc., Tamarac, Florida) was placed at the bottom of the sludge cylinder. A GeoTech[R] screen (GeoTech Environmental Inc., Tamarac, Florida) was also placed at the bottom of the ion-exchange column before filling it with zeolite. A motor with speed controller (Dayton Electric MFG. Co., Chicago, Illinois) was used to mix the water/sludge mixture to enhance the transfer of salts from the sludge to the water. Another motor with speed controller (Dayton Electric MFG. Co., Chicago, Illinois) was also used to mix the wash water.
[FIGURE 1 OMITTED]
Sludge: Sludge samples were collected from a gas production field in British Columbia, Canada. The sludge was made of very fine sand and clay (37). The sludge characteristics are shown in Table 2. The analyses were performed at Maxxam Analytics Inc., Dartmouth, Nova Scotia. The total amount of measured and calculated salts are summarized in Table 3. The [CaCl.sub.2]:[MgCl.sub.2]:NaCl ratio was 1:1.16:32.86. The calculated total salt concentration was less than that obtained from the laboratory analysis by 1.2 % which is acceptable analytical error. The Electrical Conductivity (EC) is a measure of the solution ability to conduct electricity which is related to the concentration of ions and their electrical charges. The EC value was 24.2 dS/m indicating a very saline sludge. The SAR value was 35.1 which is higher than industrial limit of 12 indicating high level of sodicity. High soil SAR values (>6) can result in severe deterioration of soil permeability, rendering it unsuitable for vegetation growth (38). The Cation Exchange Capacity (CEC) is an important factor in determining the suitability of washing process for the removal of salts. The CEC for the saline sludge was 40.3 cmol [kg.sup.-1] which is considered to be high indicating that the sludge texture is clay to clay loam (24).
Table 2: Sludge characteristics. Item Value Industrial Limit (7) Soluble elements (mg (kg.sup.-1)) Calcium 95 N/A Magnesium 78 N/A Sodium 3400 N/A Chloride 5638.25 N/A Total salt 9211.25 -- Electrical 24.2 4 Conductivity (dS/m) Sodium 35.1 12 Absorption Ratio Cation 40.3 -- Exchange Capacity (c mol (kg.sup.-1)) PH 6.7 6.6-8.5 N/A = not applicable Table 3: Mass balance on soluble salts Salt Concentration (mg [kg.sup.-1]) NaCl 8642.71 Na 3400.00 Cl 5242.71 [CaCl.sub.2] 263.05 Ca 95.00 Cl 168.05 [MgCl.sub.2] 305.49 Mg 78.00 Cl 227.49 Total Lab Analysis 9211.25 Total Calculated 9323.00 Error (%) 1.2 [Na.sup.+] + [Cl.sup.-] [right arrow] Nacl (1) [Ca.sup.++] + 2[Cl.sup.-] [right arrow] [CaCl.sub.2] (2) [Mg.sup.++] + 2[Cl.sup.-] [right arrow] [MgCl.sub.2] (3) Na = 22.99 g [mol.sup.-1] Cl = 35.45 g [mol.sup.-1] Ca = 40.08 g [mol.sup.-1] Mg = 24.31 g [mol.sup.-1] Zeolite: The zeolite used as the ion-exchange medium is a Nova Scotian chabazite variety called Mountain Stronach. It was collected from Stronach Pit in Nova Scotia and had a Cation Exchange Capacity (CEC) of 144 cmol [kg.sup.-1]. The zeolite was washed with deionized water and then oven dryed at 105[degrees]C for 2 h. It was then kept in air-tight bags until used in the experiment.
Electrical conductivity standard curve: An electrical conductivity meter (Salinity Bridge, Model # 5500, Soil Moisture Equipment Corporation, Santa Barbara, CA) was used to measure the electrical conductivity in order to assess the transfer of salt from the sludge to the water during the sludge washing process and from the water to the zeolite (chabazite) during the water desalination process. The instrument was calibrated using standard solutions made of distilled deionized water and salts at a [CaCl.sub.2]: [MgCl.sub.2]: NaCl ratio of 1: 1.16: 32.86. The calibration curve is shown in...