Electrocoagulation for spent coolant from machinery industry

Spent coolant is considered as one of the most polluting industrial wastes and causes environmental problems. It mostly contains high non-biodegradable organic carbon and oil contents; the biodegradability index was very low at 0.04, which is dif ﬁ cult to be effectively treated by common treatment systems. Electrocoagulation (EC) was proposed for a pre-treatment of coolant. The laboratory-scale of EC reactor was developed with Al electrodes and 10 mm of interelectrodes. The ef ﬁ ciency of the EC reactor on chemical oxygen demand (COD) removal was investigated at various current densities and electrolysis times. The highest current density of 50 mA/cm 2 induced a short electrolysis time of 10 min to reach the steady state of approximately 65% COD removal. When lower current densities of 20 – 40 mA/cm 2 were supplied to the EC reactor, COD removal ef ﬁ ciency of 65% can be achieved at longer electrolysis times. According to the speci ﬁ c energy consumption and sludge production, the optimal condition for spent coolant treatment was the current density of 20 mA/cm 2 and electrolysis time of 30 min in which a COD removal of ef ﬁ ciency of 68% was obtained, 0.88 kWh/kg-COD of the speci ﬁ c energy consumption and 0.04 kg/kg-COD of the sludge production.

cement industry as an alternative fuel in furnaces. In the meantime, the illegal disposal of spent coolant to lakes and rivers is also found in several areas, which causes serious environmental issues affecting human and aquatic lives (Marfe & Stefano ).
A large volume of spent coolant is generated in the machinery manufacturing industry which contains high non-biodegradable organic carbon, oil and metals contaminations. The concentrations are as follows: biochemical oxygen demand (BOD; refer to biodegradable organic carbon) of 5,200 mg/L, chemical oxygen demand (COD; refer to both biodegradable and non-biodegradable organic carbon) of 122,000 mg/L, oil and grease were 6,800 mg/L, and Fe was 32 mg/L. The above pollutant concentrations exceed the effluent standard limit of the Industrial Estate Authority of Thailand to be further treated in central wastewater treatment plants (PCD ). According to a pre-treatment of chemical precipitation, four coagulants of Al 2 (SO 4 ) 3 , AlCl 3 , Fe 2 (SO 4 ) 3 and FeCl 3 were suggested for the spent coolant in a previous study (Hilal et al. ).
The pre-treatment can decrease the total organic carbon (TOC) content from 44,200 mg/L in the initial concentration to 5,300 mg/L. When the spent coolant was treated by the pre-treatment and together with nanofiltration, the lowest TOC concentration of 3,800 mg/L was found (∼91% removal efficiency). Further, a pilot plant included peat-bed filtration, and demulsification centrifugation and ultrafiltration were proposed for the spent coolant treatment (Benito et al. ). According to the plant design, an efficient COD removal of 90% was observed and the oil and grease content was effectively removed from 22,400 mg/L in the spent coolant to 30 mg/L in the final effluent.
Membrane technology is known to be a reliable process for oily wastewater treatment via pressure driven and pore size selection. The process is a direct realization of oil and water separation with no phase change. Abadi et al. () employed a tubular ceramic microfiltration system for a typical oily wastewater. The TOC removal efficiency was greater than 95% and the treated water contained a low oil and grease content of 4 mg/L. Salahi et al. () employed a sheet nano-porous membrane in order to treat oily wastewater from a petroleum refinery. The nano-porous membrane was an efficient system to treat the wastewater and the treated water was able to be reused for agriculture; the efficiencies of oil and grease content, COD and BOD reached 99.9,80.3 and 76.9% respectively. However,Yu et al. () suggested that multi-membranes such as ultrafiltration and reverse osmosis were required for oily wastewater treatment, or a single membrane combined with other traditional methods. The membrane system is compact, easy to operate and cost-effective for a high membrane area per unit volume, however the cleaning system is required to periodically clean the membrane and restore the membrane flux.
Another treatment technology of electrochemicals is a potential newly developed technology for water and wastewater treatment. Among the electrochemical technologies of electrodeposition, electrocoagulation, electroflotation and electrooxidation, the electrocoagulation in which aluminium (Al), iron (Fe) or hybrid of Al/Fe was used as electrodes has been widely used in various applications. The objective of this study is to evaluate the performance of a developed electrocoagulation (EC) reactor to treat the spent coolant from a machinery manufacturing industry. The four monopolars of Al electrodes were parallel in the EC reactor with an interelectrode distance of 10 mm.
The effects of current density and electrolysis time, which were the most important operating factors, were studied to optimize the COD removal efficiency and the specific energy consumption. The floating scum (sludge) which was a by-product from the treatment process was measured and characterized. According to optimal conditions, the experiments were further conducted at various pH values and NaCl additions.

METHODOLOGY Spent coolant
The spent coolant was collected from a machinery manufac-

Electrocoagulation (EC) reactor
The experimental setup is shown in Figure 1. The laboratoryscale EC reactor was made from a 5-mm acrylic sheet with a dimension of 5 cm (width) × 6 cm (length) × 12.5 cm (height) and a working volume of 0.2 L. Four parallel monopolar electrodes were made from an Al sheet with the dimensions of 0.1 × 4 × 12.5 cm. The total effective electrode was 30 cm 2 and the interelectrode distance was 1 cm.
The EC reactor was placed on a magnetic stirrer. A DC power supply (2 kW, 0-400 V, 0-5 A) was used to provide the electricity current to the EC reactor. The experiments were operated at a room temperature of ∼25 C, mixing speed of 150 rpm and NaCl concentration of 5 g/L.
At the end of the experiments, the sludge was removed and the dry weight was measured. Then, the liquid was measured providing the remaining organic carbon as the COD value, which commonly represents the organic carbon content in wastewater.

Experimental design
In this study, the optimization of experimental conditions for treating the spent coolant from the machinery manufacturing industry was conducted under the developed EC reactor. The COD removal efficiency and the specific energy consumption were two parameters to evaluate the effectiveness of the system. The experiments were operated under various current densities (10-50 mA/cm 2 ) and electrolysis times (2-60 min). Both variables extensively affected the overall performance, reaction rate and energy consumption.

Analytical techniques
The efficiency of the EC reactor for the spent coolant treatment was evaluated by the reduction of the COD value, which is commonly referred to as the organic carbon content of the wastewater. The COD value was measured by using a COD analyzer (AL200 COD Vario, Aqualytic). The other parameters, such as BOD, SS, TDS and oil and grease content, were measured by an environmental consultant company; the methods of measurement are presented in Table 1. The pH value was detected by using a pH meter (Eutech Instruments). The COD removal efficiency and the specific energy consumption were calculated using Equations (1) and (2) where U is the voltage (in V), I is the applied current (in A), t is the reaction time (in hours) and V is the treated volume (in L).

Optimization of experimental condition
The In the meantime, a reduction of pH was observed during the electrocoagulation process. The pH sharply decreased from 10.5 to 8.6 when the current density of 50 mA/cm 2 was supplied to the EC reactor for 10 min, whereas the pH was slightly decreased to 9.5-9.8 at the lower current densities. When the steady state of COD removal was At >30 min of the electrolysis time, the current density had no effects on improving the COD removal efficiency.
The COD removal efficiency was in the range of 65-70% at current densities of 20-50 mA/cm 2 . However, the lowest current density of 10 mA/cm 2 was insufficient to effectively treat the spent coolant; the COD removal efficiency was found to be only 58% at the steady state. This is because at the low current density, small quantities and particles of aluminium hydroxide were generated, which do not allow efficient adsorption of the destabilized pollutants. The biodegradability index (BOD 5 /COD) in the treated water was evaluated at the electrolysis time of 30 min in which the steady state of the spent coolant treatment was observed.
The results show that the biodegradability index rose with increasing current densities (see Figure 3). The biodegradability index was 0.09 at the current density of 20 mA/cm 2 , and reached 0.20 at the highest current density of 50 mA/cm 2 . It can be seen that the concentration of COD was stable whereas the BOD concentration rose with the current densities. This phenomenon shows that more nonbiodegradable organic carbon was decomposed to readily biodegradable organic carbon when the higher current density was supplied. The performance of the EC reactor to treat the spent coolant was not enhanced at long electrolysis times (>30 min), however the biodegradable organic carbon content increased in the treated water, and could be possibly further treated by traditional treatment plants.
The operating cost of the electrocoagulation process is dominated by energy consumption. To optimize the operating condition for the industry site, the COD removal efficiency and the specific energy consumption was plotted (see Figure 4). The specific energy consumption was increased by current densities and electrolysis times. The   Under optimal conditions, the experiments were further studied at various pH values and no NaCl addition. As the pH increased from 3 to 7, the COD removal efficiency was increased from 52 to 70% (Figure 5(a)). The COD removal efficiency decreased to 64-68% at the higher pH

Sludge characterization
At the end of the experiments, the sludge was collected and measured as a dry weight. The results in Figure 6 show that the amount of sludge generated was continuously increasing by current densities and electrolysis times. This is because numerous Al ions were produced at the high current density The water quality of treated water was measured and is summarized in Table 1. The treated water was neutralized to pH of 7.7 with no acid and chemical addition after the treatment. The electrocoagulation process also improved other parameters of water quality; the BOD and Fe was effectively reduced by 92-95% and the SS was reduced by 66%. On the other hand, the TDS concentration was increased from 11,500 mg/L in the spent coolant to 25,300 mg/L. The increasing TDS caused the addition of NaCl for maintaining the good performance of the EC reactor. It can be seen that not only the organic carbon was removed, but the electrocoagulation also removed other pollutants such as BOD, SS and Fe. This study indicated that the EC reactor using Figure 6 | Amount of sludge from electrocoagulation at various current densities and electrolysis times.