Optimization of the Electrolysis Process Efficiency to Improve the Anaerobic Baffled Reactor Performance by Controlling the pH Value for Wastewater Treatment

Introduction
In this research, in order to improve the performance of conventional anaerobic baffled reactor reactors (ABR), the use of electrolysis process under optimum conditions by controlling the pH value and minimum electricity consumption on a laboratory scale has been investigated.
In anaerobic reactors, the pH value is strongly influenced by the quantity of carbon dioxide contained in the biogas. Significant variation of pH value and alkalinity occurs because of substrate influence, and acidic- alkaline compounds production during organic matters decomposition process. In this reactor, methanogenesis bacteria is very sensitive to changes in pH value and alkalinity. Therefore, maintenance of optimum operation conditions is mandatory. The suitable pH value for the anaerobic reactors performance is in the range of 6.8-7.2.
The alkalinity is initially in the form of bicarbonate. According to the reaction (1), it is in equilibrium with existing carbon dioxide in biogas, at a certain pH value.
OH - + CO2 ↔ HCO3 – (1)
For pH controlling in an electrochemical system using metal electrodes, electrolysis of water takes place by means of an electrical current to maintain load balancing. Water electrolysis occurrence results in oxygen and proton formation in the anode sector, also hydrogen and hydroxide in the cathode sector. Consequently, the pH value increases close to the cathode, while reducing pH value is observable in the anode sector. By the reducing pH value around the anode, the reaction (1) proceeds towards the production of carbon dioxide and hydroxide. After the power outage due to the low carbon dioxide solubility in accordance to Henry’s law, this reaction becomes irreversible. As a result, the main reason for pH increasing due to the electrolysis is the displacement of the bicarbonate balance and the release of carbon dioxide gas around the anode.
Various parameters impact electrolysis process, including electrode material, initial pH value, electric current density, electrolysis process duration, and distance between electrodes. In this study, in order to optimize the electrolysis process for pH recovery, these factors were investigated on a setup in laboratory scale.
Materials and methods
In order to improve the anaerobic baffled reactor performance, laboratory studies to investigate the electrolysis process effect on the pH value controlling were conducted.
Thus, several samples were taken from different chambers of the reactor and the effective parameters on electrolysis process were investigated by focusing on the pH value. Samples were affected by electrolysis using two identical electrodes of iron, stainless steel, copper, aluminum and brass with 12 cm length, 6 cm width and 1 mm thickness at different distances and different contact surfaces.
At each stage of the laboratory studies, in order to get closer to the real conditions during an organic shock, the initial pH value of the wastewater sample was adjusted to the range of 5.00 to 6.50 by using sulfuric acid. Also with conducting electricity, the capability of each electrode material were investigated. After a period of time required for pH recovery, the electric current was cut off and the pH value, concentration of released metals, electrolysis time and electrical current density were measured. All experiments were performed according to the standard methods.
In the experiments, synthetic wastewater (COD = 700±40 and TDS = 633±4 mg/L) was investigated. The wastewater was prepared using molasses, ammonium chloride (0.007 g/g COD) and potassium di hydrogen orthophosphate anhydrous (0.0006 g/g COD). The temperature and pH value of the wastewater were 45±1 ᵒC and 7.77±0.04, respectively.
Discussion of Results
Investigating the effect of electrodes material
At this stage, five types of iron, stainless steel, copper, aluminum and brass electrodes were tested under the same conditions, and the electrolysis time needed to revive a pH value was obtained. The results showed that the iron electrode can revive a pH unit of the sample in a shorter period of time and also a less electrical energy consumption. Furthermore, according to the results of the spectroscopic test and the inhibitory concentration for each material, it can be seen that copper and brass electrodes cause a release of copper metal more than the permitted limits. Therefore, they are unsuitable for use in this regard.
The results of the economic survey showed that the cost of iron electrode preparation is much lower than the others. Considering all aspects, the iron electrode is the most suitable material for using in the ABR reactor.
Investigating the effect of distance and contact surface of electrodes with wastewater
Under the same conditions, increasing the distance and the contact surface of electrodes can decrease and increase the electrical current, respectively. The reason is the direct relation between the electrical resistance of the solution with the distance of electrodes, its inverse relation with the contact surface of electrodes, and Ohm's law on electrolytes. These effects can neutralize each other, which is important in an economic point of view.
Investigating the effect of electrolysis time
By increasing the electrolysis time, the pH value also increases. While the rate of rising pH is decreasing. As time elapses, the increase of hydroxide and alkalinity occur according to reaction (1). As a result, due to the buffering properties, the resistance to pH changes increases and this reaction stops. By applying the electrical current density of 8 mA/cm2 to the iron electrodes, after passing 1.5 hours, the pH value reaches to 9.5. This suggests that by increasing the time, the efficiency of the system and the electric energy consumption will increase.
Investigating the effect of Electrical Current Density
The rate of metal dissolution in wastewater is a function of the electric current density. The increase in the current density causes an increase in the exchange of electrons, which in turn accelerates the electrolysis process. As a result, the efficiency of the electrolysis process increases for the pH recovery. In this study, based on the amount of pH recovery and electrical energy consumption in 1.5 hours, the range (8-11 mA/cm2) was selected as the best range for the current density in laboratory scale.
Investigating the effect of TDS
Concentration of total dissolved solids (TDS) is one of the parameters affecting the current density. Whenever the concentration of these substances is high, the number of charged particles, which are actually electron carriers will increase. Therefore, the electron transfer is facilitated and accelerated.
Investigating the effect of initial pH value
In order to establish acidic conditions for investigating ability of the electrolysis process with the aim of returning the pH value to neutral and alkaline conditions, initial pH value was determined in several steps in the range of 5.00, 5.50, 6.00 and 6.50. By performing the electrolysis process under optimum conditions, it was found that the lower range of the initial pH value will result in the easier pH recovery. The reason is an increasing in the dissolution of iron atoms by reducing the initial pH value.
Conclusions
Based on the results of previous researches regarding efficiency of electrolysis process to improve the performance of anaerobic reactors, in this study, optimization of the process was investigated on laboratory scale. Based on the obtained results, it can be concluded that the electrolysis process under optimum operation conditions in terms of operational and economic parameters is an appropriate option for upgrading the anaerobic baffled reactors performance.
Keywords: Wastewater treatment, Anaerobic baffled reactor, Electrolysis process, pH Adjustment, Electricity Consumption.