Electrochemical removal of organics and oil from sawmill and ship effluents

ABSTRACT
The present study investigates the electrocoagulation treatment of two different wastewaters, namely sawmill wastewater and ship waste effluent, charged with organic matter. Monopolar electrode configuration was studied for both types of effluents at current intensity of 2.0 A for a total treatment time of 90 min. Soluble chemical oxygen demand (COD[subs]) removal was very low (12.5% to 13.6%) for sawmill effluent in comparison to 74.7% to 75.4% obtained for ship effluents. Thus, ship effluent was further examined in details for its treatment efficacy in terms of electrode configuration and type, current intensity, treatment time, and pH. It was observed that bipolar electrode configuration using the Al electrode at 0.3 A gave the highest COD[subs] removal of 77%. Effluent pH increased rapidly in the initial 20 min with a concomitant decrease in COD[subs] concentration. Electrocoagulated-flocculated ship effluent improved performance relative to simple flocculation with respective removals of 86% of turbidity, 56% of COD[subs]; 69% of total COD (COD[subt]), 90% of oil and grease, 94% of C[sub10]-C[sub50] hydrocarbons and 89% of biochemical oxygen demand (BOD[sub5]). Residual Al[sup3+] concentration in the solution followed a linear trend with treatment time. Meanwhile, the sludge production increased progressively during 60 min of treatment time. Total cost for treatment of ship effluent, including energy, electrodes, and sludge disposal fee is estimated between CAN$1.34 and CAN$2.40m[sup-3].
    Key words: sawmill effluent, ship effluent, electrochemical treatment, electrocoagulation, flocculation, COD removal, aluminium, iron.

INTRODUCTION
    Since the onset of industrialization, the volume of industrial wastewater to be treated has increased considerably, commensurate with the increase in population. Biological, physical, and chemical processes can be used for the treatment of these industrial effluents charged with organic compounds. Among the physical-chemical treatment methods used, coagulation-flocculation is the most commonly used method for the treatment of water and removal of organic and inorganic compounds (Massé and Masse 2000).
    Electrochemical techniques are gaining huge importance in the field of water treatment (Drogui et al. 2007). Electrocoagulation is an electrolytic technique derived from traditional chemical coagulation. The technique involves removal of soluble organics and inorganics and colloidal particles due to in situ production of floes of hydroxides obtained by anodic dissolution of soluble iron or aluminium electrodes (Lin et al. 2005; Kravets and Yang 2002), Several studies have reported the use of electrocoagulation for the removal of organic compounds resulting from textile industry wastewaters (Daneshvar et al. 2006), municipal wastewater (Pouet and Grasmick 1995), and restaurant wastewater (Chen et al, 2000).
    Among the evergrowing list of industries to meet the demands of the mounting population, sawmills and commercial ships (bilges) form an important constituent. The wastewater containing heavy oils found in the bilges of boats may require treatment before its discharge. Moreover, the accumulation of water in the bilges after transport and washings involves a volume of effluent contaminated with oil and grease (O&G) and other contaminants (insulation materials, paints, salts, and others), which must be treated before being released into surface waters. This type of wastewater is rather difficult to treat biologically as it contains refractory and toxic compounds, which may suppress the growth of microorganisms. Thus, the majority of oily effluents are treated by physico-chemical means. There are insufficient details on the quantitative values of these wastewaters in Canada and elsewhere (Tomaszewska et al. 2005).
    Landfilling with wood residues (chips, barks, and others) is commonplace in Quebec as it is the site of numerous sawmills. In fact, the production capacity of bark residues alone is approximately 1200 t per year in Quebec (Kantardjieff and Jones 2000). Generally, these residues are laid out in heaps on sites with facilities for recovery of rainwater. The leachates of the residues contain polymeric tannins (30% to 55%), carbohydrates (30% to 40%), phenolic compounds (10% to 20%) such as various organic acids, resinous compounds, and terpenoids (Frigon et al. 2003), The sawmill site leachate is moderately charged and contains toxic compounds in considerable concentrations (Frigon et al. 2003). Traditional aerobic and anaerobic treatment methods cannot cope with the environmental norms warranting alternative treatment methods for higher removal efficiency of different organic compounds.
    In this regard, the present article explores the possibility of utilization of an electrochemical treatment method; more specifically, electrocoagulation for removal of organics and oils from sawmill and ship effluents. The study will specifically look at the following parameters to assess the electrochemical treatment feasibility; (i) electrolytic cell configuration and electrode material; (ii) treatment time; (iii) initial pH; (iv) turbidity and organic compounds removal; (v) solids; and (vi) inoTganic compounds removal.

MATERIALS AND METHODS

ELECTROLYTIC CELLS AND EXPERIMENTAL PROCEDURE
    Electrochemical treatments were carried out in a batch electrolytic cell made of acrylic material (12 cm (width) × 12 cm (length) × 19 cm (depth)). The electrode sets (anode and cathode) consisted of eight parallel pieces of metal plates (10 cm wide × 11 cm high) each, having a surface area of 110 cm[sup2], situated 1.5 cm apart and submerged in the effluent. The electrodes were installed on a perforated acrylic plate placed 2 cm from the bottom of the cell. The width and length of the electrolytic cell (12 cm × 12 cm, respectively) were so maintained to minimize the dead zone and maintain a good contact between the electrolyte (effluent) and electrodes. The depth (19 cm) of the electrolytic cell was chosen because the electrodes should be entirely submerged in the electrolyte. Mixing in the cell was achieved by a Teflon-covered stirring bar installed between the perforated plate and the bottom of the cell. The electrolytic cell was gently agitated (between 50 and 100 r min[sup-1]) to avoid floe destruction. For all assays, a working volume of 1.7 L of effluent was used. Between two assays, the electrolytic cell (including the electrodes) was cleaned with 5% (v v[sup-1]) hydrochloric acid solution for at least 2 h and then rubbed with a sponge and rinsed with tap water. The anode and cathode sets were connected to the positive and negative outlets of a Xantrex DC power supply (model XFR40-70, Aca Tmetrix, Mississauga, Ont., Canada), respectively. Current was held constant for each run with a retention time of 90 min. Different cell arrangements, namely monopolar (MP) and bipolar (BP) were studied to determine the optimal configuration for organic matter removal. The MP electrode system consisted of eight pieces of mild steel or aluminium electrodes each, individually connected to the power supply. The BP electrode system consisted of eight pieces of mild steel or aluminium with the two outmost electrodes physically connected to the power supply; the six interspersed electrodes were operated as BP electrodes each, having a negative and positive area. Current intensities of 1.0, 1.5, 2.0, and 3.0 A were tested using the MP electrolytic cell. Current intensities of 0.3, 0.5, 1.0, and 1.5 A were tested using the BP electrolytic cell.
    Initially, several assays (simple assays without any replicate) were performed by testing different operating parameters, such as type of configuration (MP and BP), type of electrodes (Al and Fe), retention times, and initial pH to determine the best conditions (reduce cost and increase effectiveness) in treating ship effluent. Once the appropriate values of these parameters were determined, the best conditions were repeated in triplicate to verify the effectiveness and the reproducibility of the electrocoagulation process for the treatment of ship effluent. During the triplicates assays, several parameters (dissolved organic carbon (DOC), biochemical oxygen demand (BOD), oil and grease (O&G), hydrocarbons (C[sub10]-C[sub50]), total solids (TS), total suspended solids (TSS), volatile solids (VS) and sludge characteristics) were also analyzed.
    At the end of each test, pH and conductivity were measured and the treated effluent was carefully collected and subjected to settling for 18 h before filtration using Whatman No. 4 membranes (15-20 µm pore diameter, Whatman International Ltd, Maidstone, England) under vacuum, A volume of 20 mL of the filtrate containing 0.2% of sulphuric acid was stored at 4 °C until the analysis. The solid fraction (wet residue) was dried for 24 h and the amount of metallic sludge was measured.

EFFLUENTS
    Two types of effluents originating from different industries were used. Sawmill effluents were procured from a recent accumulation site of wood residues in the area of Abitibi (Quebec, Canada). The effluent was subjected to grit removal to eliminate larger particles. Oily effluent originating from ship bilges was obtained from an industry having stock reservoirs at Levis (Que., Canada). Sampling was directly carried out from the tanks, which primarily contained water from washings. All samples were collected in polypropylene bottles, shipped under cold conditions, and stored at 4 °C until use. The effluent was sampled at different times during this study and the initial characteristics varied with time. Table 1 shows the initial characteristics of the experimental effluents.

SAMPLING AND ANALYSIS
    To determine dissolved organic concentrations, the samples were first filtered on Whatman 934-AH membranes (1.5 µm pore diameter) under vacuum and were immediately acidified with 0.2% sulphuric acid and stored at 4°C for further analysis. This procedure was in conformity with APHA (1999) measuring the soluble fraction of chemical oxygen demand (COD[subs]). Meanwhile, total COD (COD[subt]) was determined without any initial filtration of samples. COD[subs] and COD[subt] concentrations were measured using the Hach COD method.
    The quality of the treated effluent was also measured in terms of pH, turbidity, conductivity, BOD[sub5], total O&G content, C[sub10]-C[sub50] hydrocarbons, TOC, TS, TSS, VS, VSS, nitrogen (TKN-N and NH[sub4][sup+]-N), total phosphorus (P[subtot], and metals (Al and Fe).
    The pH was determined using an Accumet Research pH-meter (model AR 25 Dual Channel pH/Ion meter, Fisher Scientific, Nepean, Ont., Canada) equipped with a double-junction Cole-Palmer electrode with Ag/AgCl reference cell (Cole Parmer Instrument, Anjou, Que., Canada). Turbidity (in terms of nephelometric turbidity unit, NTU) was obtained with a Hach turbidimeter (model 2100AN). Conductivity meter (Oakton, model 510) was used to determine the ionic conductivity of the effluent. Analyses of BOD[sub5], O&G and C[sub10]-C[sub50] hydrocarbons were carried out by Bodycote Laboratory (Québec, Que., Canada), accredited by the Department of the Environment of Quebec (standard methods: QC004-92, QC061-97, and QC083-97). The samples were extracted in hexane and analyzed by GC-FID, The concentration of hydrocarbons present in the samples was determined by comparing the total area of the group of peaks of n-C[sub10] to n-C[sub50] with area of the standard curves obtained under similar conditions. TOC was measured using a Shi-madzu apparatus (model TOC-5000A, Shimadzu Scientific Instruments Inc., Kyoto, Japan). TS (method 2540 B), TSS (method 2540 D), VS and VSS (method 2540 G) were determined in conformity with APHA (1999). Total Fe, Al, and P concentrations were measured after acid digestion (method MENVIQ.89.12/213.Mét 1.3 for liquid samples, and method 30301 for sludge samples (APHA 1999)). The analyses of Fe, Al and P[subtot] were carried out using ICP-AES (Varian, model Vista AX CCO Simultaneous ICP-AES, Palo Alto, Calif., USA). Total Kjeldahl nitrogen (TKN-N) was analyzed according to method 4500-N-org (APHA 1999). NH[sub4][sup+] was measured by flow injection analysis colorimetry with a QuickChem FIA+, 8000 Series+ apparatus (Lachat Instruments Inc., Loveland, Colo., USA) (method QuickChem 10-107-06-2-B (NH[sub4][sup+] and TKN)). Total C, N, and S were analyzed in solid samples by LECO apparatus (model HCNS-932, St. Joseph, Minn., USA).

ECONOMIC ASPECTS
    The economic study included chemical, electrode, and energy consumption and the cost of sludge disposal. The energy consumed was estimated at a cost of CAN$0.06 kWh[sup-1], The polymer (LPM 9511) was evaluated at a cost of CAN$5 kg[sup-1], whereas the electrolyte (Na[sub2]SO[sub4]) consumed was estimated at a cost of CAN$1.77 kg[sup-1], which corresponds to an industrial grade quality. The mild steel electrode consumed was estimated at a cost of CAN$228 t[sup-1], whereas a cost of CAN$1596 t[sup-1] was considered for aluminium electrode. The disposal costs for the residual sludge, including transportation and charges for waste disposal, were evaluated at CAN$60 t[sup-1] of residue by assuming that these residues were not considered as hazardous material. However, the disposal cost for residual sludge did not include the costs related to drying of the sludge. The total cost was evaluated in terms of Canadian dollars spent per cubic meter of treated effluent (CAN$ m[sup-3]).

RESULTS AND DISCUSSION
    Table 1 represents the characterization of sawmill and ship effluents showing the average values of several parameters (pH, conductivity, organic, inorganic, and solid concentrations). The comparison of these values with those from the limits of Québec City (for effluent discharge in the sewer) showed that O&G concentration for ship effluent was beyond the permissible level, whereas the pH of sawmill effluent was lower compared with the recommended limiting value. Thus, both effluents need to be treated before discharging in the sewage urban works.

ELECTROCOAGULATION PROCESS FOR TREATMENT OF SAWMILL AND SHIP EFFLUENTS
    The first series of tests were carried out for electrocoagulation treatment of the two effluents highly charged with organic matter. Thus, an electrolytic reactor made up of electrodes of iron or aluminium in MP configuration was used at a current intensity of 2.0 A for 90 min of treatment. Table 2 presents the initial and final conditions of each test as well as the removal of COD[subs] obtained during the effluent treatment originating from sawmills and ships. The initial pH of the effluents varied between 3.4 and 7.0, whereas the final pH ranged between 3.8 and 11.4. The energy consumption varied between 3.5 and 8.1 kWh m[sup-3], the consumption being higher for ship effluents. This was mainly due to the lower electrical conductivity of the ship effluent (approximately 673 µ.S cm[sup-1]) compared with sawmill effluent at an average value of 820 µ.S cm[sup-1]. With regard to the sludge production, it varied from 0.0 to 4.9 kg m[sup-3]. The sawmill effluent showed no sludge production. It was interesting to note that higher quantities of sludge were reported in the presence of aluminium electrodes at 4.9 kg m[sup-3], compared with 2.0 kg m[sup-3] during the use of iron electrodes for ship effluents. Hence, aluminium electrodes were more suited to anodic dissolution rather than iron electrodes.
    The energy costs varied between CAN$0.21 and CAN$0.49 m[sup-3] and were directly related to the electrical conductivity of treated raw waters. Lower conductivity of the ship effluent can result in significant energy costs. The disposal costs of the sludge varied between CAN$0.0 and CAN$0.3 m[sup-3]. The costs of theoretical consumption of electrodes (calculated according to Faraday's law) were CAN$0.73 m[sup-3] and CAN$1.65 m[sup-3] with iron and aluminium electrodes. In fact, the aluminium electrodes produced more metallic sludge because they were probably more conducive than the iron electrodes. Moreover, market cost of aluminium (pertaining to the manufacture of electrodes) is approximately seven times higher than that of the mild steel used in the manufacturing of iron electrodes (Chen 2004; Xu and Zhu 2004).
    Higher COD[subs] removals (74.7% and 75.4% for Al and Fe electrodes, respectively) were reported for ship effluents as compared with sawmill effluent. The lower COD[subs] removals (12.0% to 14.0%) were observed for sawmill effluent for which a weak variation in pH was observed and no sludge was recovered. The relatively weak efficiency was mainly attributed to a phenomenon of passiveness of the electrodes owing to phenolic compounds, such as phenol, catechol and guaiacol reported in significant concentrations (920 µ.g L[sup-1], 10 000 µ.g L[sup-1] and 780 µg L[sup-1], respectively) in sawmill effluent. These compounds limit anodic dissolution by forming a passive layer at the electrolyte-electrode interface.
    As a followup to these exploratory tests, it was possible to choose ship effluent for detailed electrocoagulation treatment studies from economical and environmental points of view. In light of these results, a detailed investigation (in term of costs and effectiveness) was carried out for ship effluent taking into account various parameters: type of electrodes (Fe and Al), electrolytic configuration (MP and BP), current intensity, treatment time, initial pH, and electrical conductivity.

ELECTROCOAGULATION TREATMENT OF SHIP EFFLUENTS

SELECTION OF ELECTROLYTIC CELL CONFIGURATION AND ELECTRODE MATERIAL
    When the BP and MP configurations of electrolytic systems were tested for ship effluents at different current intensities, percent COD removal represented a profile as shown in Fig. 1. The percentage removal of COD[subs] at various current intensities (0.3, 0.5, 1.0, and 1.5 A) for a period of 90 min by adopting either mild steel (Fe) or aluminium (Al) electrodes showed values ranging between 64% and 77%, For a given current intensity, the removal efficiency of the electrolytic cell (mild steel electrode) was slightly higher than the aluminium electrode. However, percent COD[subs] removal increased with the current intensity, irrespective of the type of electrodes. The enhanced COD[subs] removal was mainly accredited to co-precipitation of organic matter with metallic hydroxides. Moreover, metallic hydroxides in the electrocoagulation process act as good adsorbents for emulsified and colloidal dispersed oils (Saur et al. 1996; Calvo et al. 2003; Ibrahim et al. 2001).
    As current intensifies increased from 1.0 to 3.0 A, MP configuration showed COD[subs] removal between 59% and 77%. Meanwhile, the removal was higher for mild steel electrodes in comparison to the Al-based electrode system. In fact, the Fe electrode is usually preferred over the Al electrode due to the cost factor involved (Chen 2004; Mollah et al. 2001). Likewise, the BP electrolytic system possesses feasibility of setup for full scale application and only two electrodes are connected to the electric power source with no connection between the inner (BP) electrodes (Chen 2004) versus the MP system. Furthermore, an arrangement of the BP electrodes gives a simple physical setup, which facilitates ease of maintenance under practical application, Thus, mild steel electrode and BP configuration would be the best setup for treatment of ship effluents. Otherwise, the treated effluent using the aluminium electrode was found to be clearer than the electrocoagulated-effluent using the mild steel electrode. Thus, the Al-BP configuration has been considered as the optimal configuration for all future studies.

INFLUENCE OF TREATMENT TIME
    Figure 2 presents the rate of COD[subs] removal and effluent pH as a function of treatment time for the Al-BP configuration at 0.3 A. COD[subs] decreased more rapidly in the initial stage of the treatment and then decreased slightly from 20 to 90 min, COD[subs] decreased from 2000 mg L[sup-1] to 1300 mg L[sup-1] until 20 min and then the decrease was slower until 90 min. Initial higher COD[subs] removal was due to the formation of in situ floes, which provided surfaces for adsorption as well as coagulation of organic matter (Chen 2004). Likewise, the final pH also increased with the treatment time, ranging from 7.9 (obtained after the first 10 min) to 10.2 (measured after 90 min of treatment). The initial increase in final pH until the 20 min treatment time was ascribed to the formation of OH[sup-] ions at the cathode. The ions were produced in a significant way as the treatment time increased. From a 20 to 90 min period of treatment, the final pH stabilized around 10.0 due to the reduction of OH[sup-] in solution. In fact, at the beginning of the experiment, aluminium ions were produced by anodic dissolution and hydrolysis of aluminium leading to the formation of hydroxo-aquo complexes according to the reaction (1) (Jo-livet 1994):
    [1] [Al(H[sub2]O)[subN]][supz+] + hH[sub2]0 <=> [Al(OH)[subh](OH[sub2])[subN-h]][sup(z-h)] + hH[sub3]0[sup+]
    where N represents the number of molecules of water surrounding the metal in the coordination sphere, z is the valence of metal and h is the number of molecules of water reacting with the solvated-aluminium [Al(H[sub2]O)[subN]][supz+]. As the hydroxide ions (electron donor) are produced in a sufficient concentration, they react with the solvated-aluminium and lead to the formation of the precursor having a zero charge ([Al(OH)[subz](H[sub2]O)[subN-z]][sup0]), which is capable of condensing and forming the solid phase.
    [2] [Al(OH[sub2])[subN]][sup+] + zOH[sup-] <=> [Al(OH)[subz](OH[sub2])[subN-z]][sup0] + zH[sub2]o
    The rate of solid formation depends on the precursor concentration and as long as its concentration is low, the rate is quite insignificant. In fact, it took 20 min for the cell to produce enough ([Al(OH)[subz](H[sub2]O)[subN-z]][sup0]) and initiate the polymerization reaction. Moreover, sufficient concentration of metallic hydroxide particles were progressively produced, which initiated polymerization reactions, causing the formation of a white gelatinous precipitate illustrated by eq. [3] (Jolivet 1994):
    [3] Al(OH)[sub3] + Al(OH)[sub3] --> (OH)[sub2]Al-O-Al(OH)[sub2] + H[sub2]O
    It is worth noting that the electrolytic cell was gently stirred (between 50 and 100 r min[sup-1]) to avoid floc destruction. The appearance of polymeric complexes [Al[sub2](0)(OH)[sub4]] allowed removal of a relatively high amount of organic compounds (COD[subs]) present in solution within the first 20 min of experimental run. However, the polymeric complexes did not remain in solution and floated to the surface of the liquid due to the H[sub2] gas bubbles produced on the cathode electrodes. The bubbles attached to the flocs and solids were found to be moving up to the surface with bubbles. The solid formation and its subsequent separation from liquid allowed the change in pH to slow from 20 to 90 min and the pH reached a plateau.

INFLUENCE OF INITIAL PH
    The initial pH of ship effluents for a BP configuration for a treatment time of 60 min was adjusted to 4.0 using 5 mol L[sup-1] sulphuric acid solution and the results obtained are presented in Table 3. It is of interest to note that costs of acid were not taken into account in the analysis of the costs as it was not absolutely necessary to check the impact of pH on the performance of the system. The final pH increased with the initial pH. Metallic sludge produced at pH 4.0 was quite similar to that measured at pH 7.0. As the pH increases, aluminium hydroxide reacted with water to form aluminate ion (Al(OH)[sub4][sup-]), a dominant species above pH 9.0 (Holt et al. 2002).
    A COD[subs] removal of 54% was reported for an initial pH of 4.0 compared with 49% for an initial pH of 7.0. In fact, the solubility of aluminium hydroxides allowed lower COD[subs] removal at higher pH. Thus, when the initial pH was lowered to the acidic range, the formation of these hydroxides further enhanced the COD[subs] removal (Bektas et al. 2004).
    Although an initial acidification (initial pH of 4.0) involved an additional stage of treatment, it indeed makes it possible to gain nearly 6% of additional COD[subs] removal with a decrease in residual aluminium concentration by almost 90% (from 37.8 to 3.6 mg L[sup-1]). Moreover, the preliminary adjustment of pH makes it possible to decrease the importance of final neutralization before its release into the sewer. As seen in Table 3, the final pH can reach a value of 11.5 without initial adjustment, which is too high (permissive levels for pH range from 6.0 to 9.5). Thus, it is essential to adjust the initial pH of raw water before the suggested treatment can be carried out.

COUPLING ELECTROCOAGULATION AND FLOCCULATION FOR THE TREATMENT OF SHIP EFFLUENT
    Normally, the sludge obtained at the end of electrochemical treatment undergoes 18 h of sedimentation before filtration is carried out. It is thus necessary to add a stage of flocculation to facilitate recovery, to increase compactness, and to decrease settling time. To accomplish these goals, organic polymer (LPM 9511, a cationic polymer) needs to be added prior to the flocculation stage.

TURBIDITY AND ORGANIC COMPOUND REMOVAL
    Table 4 compares the raw effluent (untreated) and the effluent treated by electrocoagulation. By considering the two parameters, BOD[sub5] and O&G (parameters for which values are recommended by the Quebec municipality), it was interesting to note that electrocoagulation was effective for the reduction of the concentration of these compounds below the allowed limiting values (500 mg BOD[sub5] L[sup-1] and 150 mg O&G L[sup-1]). The residual concentrations of BOD[sub5] and O&G following the treatment were 17.7 and 80.0 mg L[sup-1], respectively. In fact, the turbidity decreased with the addition of another treatment train of flocculation as soluble organic matter was completely coagulated causing higher turbidity removal (85.6%). Moreover, the concentration of compounds in the form of hydrocarbon chains from 10 to 50 units (C[sub10]-C[sub50]) was also measured as a representation of oily composition of ship effluent. The results showed that the initial concentration of C[sub10]-C[sub50] decreased from 441 mgL[sup-1] to 25 mg L[sup-1] with a removal percentage of 94%. Thus, organic residues containing C[sub10]-C[sub50] molecules were removed in an effective manner. During the electrolysis step of the electrocoagulation process, the electro-generated Al cationic ion with high charge (+3) effectively neutralized the surface charge of the oil droplets. Ultimately, the destabilized oil droplets were adsorbed onto the highly dispersed aluminium hydroxide colloid formed between the electrogenerated Al[sup3+] or OH[sup-] (Cenkin and Belevstev 1985).
    The electrocoagulation also resulted in the simultaneous reduction of 56% of COD[subs] and 69% of COD[subt], whereas 70% of solids in the raw effluent are organic compounds. In comparison, the raw sample subjected to sedimentation without preliminary addition of flocculating agent resulted in 12% removal of COD[subs] and 32% removal of COD[subt]. Moreover, the residual turbidity of the raw sample decreased by about half (1250 NTU, 43% removal of initial turbidity) compared with the treated effluent (319 NTU) with removal efficiency of 86%. When the raw effluent was flocculated and settled, turbidity removal was improved (reaching 58% of reduction), while COD[subs] concentration was removed (62% of removal) compared with raw wastewater. With regard to COD[subt] removal, it changed to 43% after flocculation and sedimentation of the untreated effluent. Relatively higher COD, removal reported while flocculating untreated wastewater was due to the cationic capacity of the flocculant to aggregate suspended or dispersed colloids. The flocculation of colloidal particles facilitated sedimentation although the particle charges were not previously neutralized. The decrease of COD[subs] measured (62% of COD[subt] removal) in the supernatant of the untreated effluent was attributed to the sedimentation of a fraction of organic matter in the form of suspended solids. However, the sedimentation of the flocs allowed better clarification of the supernatant compared with sedimentation alone without addition of a flocculating agent. There is little information available on the treatment of ship effluent from literature sources. However, Karakulski et al. (1998) indicated that if there was no detergent in the effluent, final oil concentrations of less than 20 mg L[sup-1] can be obtained. Furthermore, Tomaszewska et al. (2005) showed with ultrafiltration that it was possible to completely eliminate turbidity from treated water and the final oil concentration was approximately 10 mg L[sup-1] (in our case, the final oil concentration and greases varied from 35 to 80 mg L[sup-1] according to the type of electrodes used). Meanwhile, wet air oxidation removed 90% of COD[subt] and 99.9% of O&G (Lopez Bernal et al. 1999). In conjunction with the parameters analyzed (BOD[sub5], O&G, COD[subs] and COD[subt]), O&G was one of the parameters with the highest removal rate (90% of O&G removed). This could be attributed to the hydrophobic capacity of oils and greases, which have an excellent affinity with the bubbles of H[sub2] released at the cathodes. In fact, the gas bubbles (H[sub2]) attach to oils and greases and the resulting complex (0&G)-H[sub2] accumulates on the surface of the liquid in the form of foam. Also, O&G can be removed by adsorption on the surface of metal hydroxides having a strong capacity for adsorption (Cenkin and Belevstev 1985), In addition, by considering the removal rates of COD[sub5] (56%), electrocoagulation offers the possibility of removing the dissolved organic compounds by complexation with the metal hydroxides or cathodic reduction. However, according to a 1973 regulation, the oil concentration of the water discharged into the oceans should not exceed 15 mg L[sup-1] and any higher residue must be retained on board to the port to transport it to a treatment plant (MARPOL 1973-78).
    Statistical analysis was performed for the results while applying optimal conditions of electrocoaogulation followed by flocculation (triplicate samples). For instance, the efficacy of COD, removal showed a mean value of 69.1% with a standard deviation of 1.0 (1.4% accuracy). The efficacy of BOD removal showed a mean value of 89.4% with a standard deviation of 4.4 (4.9% accuracy). In comparison, O&G removal showed a mean value of 90% with a standard deviation of 0.8 (0.89% accuracy). Electrocoagulation operated under the optimal conditions followed by flocculation involved a total cost of CAN$1.04 per cubic meter for treated ship effluents. This cost includes energy and electrode consumption, chemicals, and sludge disposal.

SOLIDS
    Table 5 presents solids (TS, TSS, and VS) removal in raw and electrocoagulated-flocculated effluent. Removal of TSS, TS, and VS were 31%, 26% and 44%, respectively. Although these percent removals were low, it was still significant suggesting active interaction between particles and metal hydroxides formed during electrocoagulation. However, during the recovery of sludge, certain fine particles of hydroxides resuspend in the solution, which can slightly affect the concentrations of TSS and TSS actually obtained. Thus, the re-suspension of hydroxides could increase TS and TSS concentration in the treated effluent, which could also affect the final turbidity.

INORGANIC COMPOUND REMOVAL
    Table 5 also presents the initial and final concentrations of some inorganic elements (TKN-N, NH[sub4][sup+]-N, P[subtot] total Al, and total Fe). In fact, Fig. 3 also presents the profile of total Al in the solution with treatment time. There was 16% to 17% removal of TKN-N, NH[sub4][sup+]-N and the treatment affected organic as well as ammonia nitrogen (which can be transformed into ammonia gas at basic pH and removed by degassing). Nitrites and nitrates were not measured during the study making it impossible to correlate their effect. However, an increase in nitrates and nitrites is generally ascribed to the biological process of nitrification (requiring dissolved oxygen) (Metcalf and Eddy 2003). There was a possibility that this concentration was lower in the case of ship effluent. In fact, the water remained stable in its long-term composition and BOD[sub5] concentration (representation of biodegradable matter), which rules out the presence of organic matter-biodegrading microorganisms.
    Residual Al in solution increased linearly with treatment time suggesting continuous release of Al[sup3+] ions for effective coagulation (Fig. 3). The coupling of electrocoagulation and flocculation caused an appreciable decrease (94% removal) in total phosphorus concentration. Likewise, the residual total aluminium removal was to the tune of 87% in spite of the dissolution of Al electrodes during electrolysis. In fact, phosphorus was effectively removed from oily effluent owing to phosphate co-precipitation with Al[sup3+] generated by anodic dissolution during electrolysis (see eq. [4]):
    [4] Al[sup3+] + H[sub2]P0[sup-][sub4] --> AlPO[sub4] + 2H[sup2+]
    AlPO[sub4] complex remained in the sludge and was removed from solution after fiocculation and sedimentation. Thus, electrocoagulation-flocculation was an effective treatment for removal of the majority of inorganic compounds.

METALLIC SLUDGE DEWATERING AND INORGANIC CONTENT
    Table 5 shows the volume fraction of sludge residues and filtrate, dryness (sludge total solids) of dewatered sludge, and content of inorganic elements (C, N, S, P, and Al). Sludge production profile with treatment time is also presented in Fig. 3. The amount of sludge increased linearly during the initial 60 min of treatment time and later slightly decreased. After saturation was reached, the sludge quantity did not increase despite the increase in total Al in solution, The volume fraction of sludge was measured following fiocculation by using a graduated cylinder. The filtrate fraction represents the water content of metallic sludge, The percentages presented in Table 5 were calculated by dividing the volume of sludge or filtrate by the total volume of treated effluent (1.7 L). The filterability of metallic sludge residues was evaluated after filtration of the fraction of sludge on the membrane under vacuum using the Whatman No. 4 filter paper (filtration time = 20 min). The dryness was reported to be 22%. Relatively high contents of Al (163 g Al kg[sup-1]) were measured for dewatered metallic sludge. The treatment concentrated total phosphorus to 76.9 P[subtot] g kg[sup-1] of dry residues. In addition, carbon content of 20%, nitrogen of 1.0%, and sulphur of 0.56% were measured. The sludge composition reflected that it was not hazardous and could be disposed of as municipal solids in sanitary landfills. Moreover, the dewatered sludge will meet class B1 criteria of sludge (USEPA 2000). In light of the results obtained in this study, ship effluent treatment gave promising results on removal of the organic matter. Moreover, the electrochemical treatment was effective in removing certain inorganic pollutants like phosphorus, while making it possible to clarify the final effluent, Furthermore, during electrocoagulation, the resulting effluent is not enriched with anions and salts, in comparison with chemical coagulation (using FeCl[sub3]), for example, which enriched the effluent with chloride ions. Electrocoagulation also has a practical advantage of producing in situ a coagulating agent, which should allow a reduction of the costs related to chemical transportation in large scale application. Likewise, this approach enables elimination of the constraints linked to the storage of chemical products.

CONCLUSIONS
    Electrochemical treatment (electrocoagulation) of ship and sawmill effluents led to following conclusions:
    * Electrocoagulation was more effective in removal of organic matter from ship effluent as compared with sawmill effluent leading to detailed studies on the former.
    * An Al-bipolar electrolytic cell operated at current intensity of 0.3 A with 60 min of treatment was found to be the best condition for treating ship effluent using the electrocoagulation process.
    * Sludge production was lower at pH 4.0 as compared with pH 7.0.
    * Coupling of electrocoagulation with flocculation resulted in higher turbidity and organics removal concomitant with inorganics removal and the final sludge meets class B1 criteria. Besides, electrocoagulation produces an in situ coagulating agent, which should contribute to reducing the operating costs related to chemical transportation in full scale application.
    * Electrochemical coagulation operated under the optimal conditions involves a total cost of CAN$1.04 per cubic meter of treated ship effluent. This cost includes energy and electrode consumption, chemicals, and metallic sludge disposal. It is worth noting that a reliable economical conclusion cannot be completely drawn from laboratory scale studies only. It requires a pilot plant study to better evaluate the economic aspects of electrocoagulation process (including energy cost, metallic residues disposal cost, and the cost required to built the electrochemical reactor).
ADDED MATERIAL
    P. Drogui,[sup2] M. Assetili, S.K. Brar, and J. Biais. Institut national de la recherche scientifique (INRS-Eau Terre et Environnement), Université du Québec, 490 rue de la Couronne, Québec City, QC G1K 9A9, Canada,
    H. Benmoussa. Centre de recherche industrielle du Québec (CRIQ), 333 rue Franquet, Sainte-Foy, QC. G1P 4C7, Canada. Written discussion of this article is welcomed and will be received by the Editor until 31 July 2009.
    1 A paper submitted to the Journal of Environmental Engineering and Science.
    2 Corresponding author (e-mail: patrick.drogui@ete.inrs.ca).

ACKNOWLEDGEMENTS
    Sincere thanks are extended to the Canada Research Chairs program and to the Natural Sciences and Engineering Research Council of Canada for their financial contribution to this research project.
    Résumé: La présente étude a porté sur le traitement par électrocoagulation d'effluents d'usine de sciage de bois et de cales de navire. Des rendements faibles d'élimination de la DCO sotub"ie (12,5 à 13,6%) ont été observés avec les effluents d'usine de sciage de bois en comparaison à ceux mesurés avec les effluents de cales de navire (74,7 à 77,0%). Le rendement d'élimination de la DCO soluble le plus élevé, soit 77 %, a été obtenu en configuration bipolaire en utilisant des électrodes d'Al, une intensité de courant de 0,3 A et un temps de rétention de 60 min. Une hausse rapide du pH couplée à une baisse de la DCO soluble a été notamment observée au cours des 20 premières minutes de traitement. La combinaison de l'électrocoagulation et de la floculation chimique a amélioré considérablement l'efficacité du traitement comparativement à la floculation seule et ce, avec les rendements d'élimination suivants: 86 % turbidité, 56 % DCO soluble, 69 % DCO totale, 90 % huiles et graisses, 94 % hydrocarbures (C[sub10]-C[sub50]) et 89 % DBQ[sub5].
    Mots-dés: effluent d'usine de sciage, effluent de cales de navire, traitement électrochimique, électrocoagulation, floculation, enlèvement de la DCO, aluminium, fer.
    Table 1. Initial characterization of industrial wastewater.

                                Industrial wastewaters
Parameters                           Sawmill effluent        Ship effluent   Permissive levels(FNa)
pH                                   3.41 +/- 0.03           7.09 +/- 0.10   6.0-9.5
Conductivity (µS cm[sup-1])         818 +/- 6               668 +/- 13      -
Turbidity (NTU)                      -                       -               -
COD[subs] (mg L[sup-1])                28 000 +/- 2700         2960 +/- 410    -
TOC (mg L[sup-1])                     9160                    468             -
BOD[sub5] (mg L[sup-1])                -                       167 +/- 49      500
O&G (mg L[sup-1])                     -                       800             150
TKN-N (mg L[sup-1])                   <DL(FNb)                125 +/- 2       -
TSS (mg L[sup-1])                     213 +/- 11              573 +/- 18      -
TS (mg L[sup-1])                      14 000 +/- 300          3090 +/- 200    -
VS (mg L[sup-1])                      11 700 +/- 300          1410 +/- 130    -

FOOTNOTES
a Québec City recommendations for effluent discharge in sewer (Ville de Qu6bec 2001).
b Detection limit.
    Note: NTU, nephelometric turbidity unit; COD, chemical oxygen demand; TOC, total organic carbon; BOD, biochemical oxygen demand; O&G, oil and grease; TKN, total Kjeldahl nitrogen; TSS, total suspended solids; TS, total soldis; VS, volatile solids.
    Table 2. Summary of electrocoagulation results for the two effluents in MP configuration. Current intensity = 2.0 A; Treatment time = 90 min.

                                        Effluents
Parameters                                   Sawmill                         Ship
Electrode type                               Fe              Al              Fe              Al
Average voltage (V)                          2.0             2.2             4.6             4.3
Initial conductivity (µS cm[sup-1])         820             823             673             685
Initial pH                                   3.42            3.44            7.00            7.00
Final pH                                     4.14            3.80            11.40           9.80
Initial COD[subs] (mg L[sup-1])                24 900          26 900          3910            2770
Final COD[subs] (mg L[sup-1])                  21 800          23 200          963             702
CODs removal (%  )                           12.5            13.6            75.4            74.7
Energy consumption (kWh m[sup-3])             3.53            3.88            8.12            7.50
Sludge production (kg m[sup-3])               0.00            0.00            2.00            4.94
Electrode dissolved (kg m[sup-3])             3.22            1.04            3.22            1.04
Energy cost (CAN$ m[sup-3])                   0.21            0.23            0.49            0.45
Sludge disposal cost (CAN$ m[sup-3])          0.00            0.00            0.12            0.30
Electrode cost (CAN$ m[sup-3])                0.73            1.65            0.73            1.65
Total cost (CAN$ m[sup-3])                    0.95            1.89            1.34            2.40

    Table 3. Effect of initial pH on the treatment of ship effluent by electrocoagulation. Current intensity = 0.3 A; BP cell configuration; Treatment time = 60 min.

                                Initial pH imposed
Parameters                           7.0             4.0
Final pH                             11.5            9.3
Sludge production (kg m[sup-3])       1.67            1.65
Total Al (mg L[sup-1])                37.8            3.6
Initial COD[subs] (mg L[sup-1])        1920            1750
Final COD[subs] (mg L[sup-1])          1020            802
COD[subs] removal (%  )               48.7            54.2

    Table 4. Turbidity and organic compound removal from ship effluent after sedimentation. Sedimentation time = 60 min; Organic polymer addition = 10 mg L[sup-1].

                                        Untreated effluent                              Electrocoagulated-flocculated
Parameters                                   Raw effluent            Flocculated effluent    effluent(FNa)
Initial pH                                   4.00                    4.02                    4.02 +/- 0.02
Final pH                                     -                       4.02                    9.25 +/- 0.02
Initial turbidity (mg L[sup-1])               2210 +/- 0              2210 +/- 0              2210 +/- 0
Final turbidity (mg L[sup-1])                 1250                    924                     319 +/- 42
Turbidity removal (%  )                      43.4                    58.2                    85.6 +/- 1.9
Initial COD[subs] (mg L[sup-1])                1810 +/- 200            1810 +/- 200            1810 +/- 200
Final COD[subs] (mg L[sup-1])                  1600                    697                     787 +/- 4
COD[subs] removal (%  )                       11.6                    61.5                    56.2 +/- 4.8
Initial COD[subt] (mg L[sup-1])                3400 +/- 50             3400 +/- 50             3400 +/- 50
Final COD[subt] (mg L[sup-1])                  2320                    1920                    1050 +/- 30
COD[subt] removal (%  )                       31.7                    43.2                    69.1 +/- 1.0
Initial O&G (mg L[sup-1])                                                                     800 +/- 0
Final O&G (mg L[sup-1])                       -                       -                       80.0 +/- 6.0
O&G removal (%  )                            -                       -                       90.0 +/- 0.8
Initial C[sub10]-C[sub50] (mg L[sup-1])                                                         441 +/- 19
Final C[sub10]-C[sub50] (mg L[sup-1])           -                       -                       25.0 +/- 5.6
C[sub10]-C[sub50] removal (%  )                                        -                       94.3 +/- 1.5
Initial BOD[sub5] (mg L[sup-1])                                                                167 +/- 49
Final BOD[sub5] (mg L[sup-1])                  -                       -                       17.7 +/- 2.1
BOD[sub5] removal (%  )                       -                       -                       89.4 +/- 4.4
Energy (CAN$ m[sup-3])                        -                       -                       0.20 +/- 0.00
Acid consumption (CANS m[sup-3])              -                       -                       0.04 +/- 0.00
Sludge disposal cost (CAN$ m[sup-3])          -                       -                       0.09 +/- 0.00
Polymer cost (CANS m[sup-3])                  -                       -                       0.05 +/- 0.00
Electrode cost (CAN$ m[sup-3])                -                       -                       0.66 +/- 0.01
Total cost (CAN$ m[sup-3])                    -                       -                       1.04 +/- 0.00

FOOTNOTES
a Current intensity = 0.3 A; BP cell configuration; Treatment time = 60 min.
    Table 5. Solid and inorganic compound removal from ship effluent and sludge dewatering.

                                        Ship effluent
Parameters                                   Raw                     Electrocoagulated-flocculated
Solids
TSS (mg L[sup-1])                             543 +/- 42              372 +/- 41
TS (mg L[sup-1])                              2280 +/- 120            1680 +/- 70
VS (mg L[sup-1])                              735 +/- 213             411 +/- 28
Inorganic elements
TKN-N (mg L[sup-1])                           109 +/- 10              91.0 +/- 9.0
NIW[sub4][sup+]-N (mg L[sup-1])                 85.3 +/- 5.2            72.1 +/- 1.
P[subtot] (mg L[sup-1])                        243 +/- 5               13.8 +/- 1.1
Total Al (mg L[sup-1])                        5.60 +/- 0.31           0.74 +/- 0.26
Sludge characteristics
Sludge fraction (%   v v[sup-1])              -                       17.9 +/- 2.1
Filtrate fraction (%   v v[sup-1])            -                       18.9 +/- 2.9
Sludge total solids (%   w w[sup-1])          -                       22.3 +/- 2.0
Carbon (C) content (%   w w[sup-1])           -                       20.0 +/- 1.7
Nitrogen (N) content (%   w w[sup-1])         -                       1.01 +/- 0.02
Sulfur (S) content (%   w w[sup-1])           -                       0.56 +/- 0.02
P[subtot] content (g kg[sup-1])                -                       76.9
Al content (g kg[sup-1])                      -                       163

Fig. 1. Soluble chemical oxygen demand removal at different current intensities using monopolar and bipolar electrode system configurations for ship effluents. Treatment time = 90 min.
    [Graph or Chart Omitted]
Fig. 2. Profiles of soluble chemical oxygen demand concentration and effluent pH as a function of time during electrocoagulation of ship effluent using Al-BP system (0.3 A).
    [Graph or Chart Omitted]
Fig. 3. Time profile of total Al and sludge production during electrocoagulation of ship effluent using Al-BP system (0.3 A).
    [Graph or Chart Omitted]

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