JPS637119B2 - - Google Patents

Description

TECHNICAL FIELD The present invention relates to environmental engineering, and in particular to a method and apparatus for the electrochemical treatment of sewage (or waste water), such as marine sewage, containing seawater with various salt contents.

BACKGROUND OF THE INVENTION The problem of biosphere pollution is one of the major problems facing the world today, and the problem of water pollution by sewage effluents occupies a particular place in biosphere pollution. With the increasing development of seafaring, the proportion of ship sewage in the total amount of sewage discharged into the ocean has increased rapidly. As a result, many countries have taken measures to prohibit ships without sewage treatment systems from sailing in their territorial waters.

When designing a ship's sewage treatment system, special attention must be paid to improving treatment reliability. This is because electrochemical treatment methods for sewage have now become widespread. However, due to the presence of seawater in the sewage being treated, hydroxides consisting mainly of anode-soluble metal hydroxides and magnesium hydroxide, providing metal-coagulant ions on the soluble electrode during the electrochemical treatment. Deposition of chemical compounds can be seen. These hydroxides coat the metal surfaces and block the interelectrode spaces, preventing liquid from flowing between the electrodes.

A method of treating sewage as disclosed in British Patent No. 1560730, Int. Cl. Provides cleaning of soluble electrode surfaces in electrolytic cells by. As the fluid flows through the electrolytic cell at a certain flow rate, these particles become suspended in the interelectrode space and scrape off the deposits from the electrode surfaces.

Although the method is simple and convenient at first glance, the fluid must flow at high velocities in order to suspend the particles, which makes it impossible to carry out the process under optimal conditions. . Moreover, when the electrolyzer is not in operation, no current flows through the electrodes and no fluid to be treated is supplied, leading to corrosion of the electrodes with hydroxide deposition on the soluble electrodes. When the electrolytic cell is left out of service for a long period of time, corrosion products of the soluble electrodes may fill the interelectrode space and cleaning of the electrode surface with abrasive particles may become impossible.

Thus, by the use of abrasive particles moving in the sewage stream under treatment, complete and reliable cleaning of the electrode surface cannot be achieved and the quality of the sewage treatment is therefore undesirable.

More advanced electrochemical sewage treatment methods are also known in the industry (USSR Inventor's Certificate No. 814881, Int.
Cl.CO2F1/12), this method flows the sewage to be treated into a diaphragm electrolyzer, the anolyte and catholyte are drained separately from the anode and cathode compartments, the catholyte is allowed to settle, and then the anolyte and catholyte are separated. It consists of mixing with a liquid.

However, it is quite difficult to settle the catholyte under conditions of pitching and rolling of the ship;
This is the main drawback of this method.

USSR Inventor Certificate No. 739004, Int.Cl.CO2F1/46
Also known in the prior art is a sewage treatment method disclosed in , which processes seawater-containing sewage in the cathode compartment of a diaphragm electrolyser, then separates the magnesium-containing sediment, and and directing the magnesium-containing fluid from the anode compartment to the cathode compartment.

Although this method can prevent excessive salt from depositing on the electrodes, a number of factors prevent it from treating sewage containing seawater with sufficient efficiency. These factors are as follows.

1. When treating sediment in the anode compartment of an electrolytic cell, the anode area must have a large volume to prevent the anode area from being blocked by sediment;
As a result, the distance between the anode and cathode becomes considerably large, and the voltage across the diaphragm cell reaches high values due to ohmic losses of the electrolyte. This results in increased power consumption for sewage treatment.

2. Repeated treatment of the sediment in the anode compartment by supplying more magnesium-containing fluid to the cathode compartment causes an increase in the concentration of magnesium ions in the sewage under treatment, and as the solution becomes saturated the magnesium salts These salts precipitate and contaminate purified sewage water.

3. When sewage is treated only in the cathode compartment of the electrolyser, 1/2 of the power consumption is not used directly for sewage treatment, but is used for dissolving the magnesium-containing sediment.

4 Separation of the magnesium-containing sediment requires a separate basin, thus increasing the size of the sewage treatment plant considerably, and under conditions of ship pitching, rolling and vibration the efficiency of the settling process is substantially reduced. Decrease.

Some of the aforementioned deficiencies are U.S. Pat. No. 4,188,278;
It is excluded in the method disclosed in Int.Cl. ² CO2B1/82.

This method of fluid treatment involves flowing a fluid through a region of varying potential into an electrolytic cell, and changing the direction of this varying potential, thereby reducing the acceleration and deceleration forces acting on the fluid along the fluid flow path. causing severe turbulence between the individual layers of the The method is carried out in a device that includes a pair of main electrodes and a number of auxiliary electrodes arranged between the main electrodes. The pair of main electrodes include a plurality of electrically connected bars arranged in one plane, and each of the auxiliary electrodes includes an electrically insulated bar. The main electrodes also create zones between themselves with different potentials, fluid flows through these zones with different potentials, and air bubbles are removed from the electrodes by an ultrasound source.

The power consumption of the apparatus implementing this method is also very high due to the ultrasonic vibrator, which serves to prevent salt deposition on the cathode surface of the electrode.

Additionally, when placing an electrode across a fluid stream,
The fibrous contaminants stick to the electrodes and it is impossible to remove them by ultrasound since the specific gravity of the fibrous contaminants and the specific gravity of the fluid under treatment are equal;
The flocs of coagulated contaminants are then destroyed by the severe turbulent area behind the electrode, which impedes the coagulation process of colloidal contaminants and ultimately affects the quality of sewage treatment. Furthermore, the turbulence of the sewage flow causes intense mixing of the electrode boundary layers of the fluid, and this intense mixing lowers the PH of these layers, resulting in the formation of salt deposits on the electrode surfaces. The sieve in the fluid streamline complicates the equipment for implementing this method, and blockage of the sieve affects the reliability of the equipment. Furthermore, mounting the electrodes on the housing wall requires providing an insulating space or making the housing wall of an insulating material, which makes the construction of the electrolytic cell considerably more complicated.

A more reliable method of electrochemical sewage treatment is disclosed in USSR Inventor's Certificate No. 808376, Int.Cl.CO2F1/46, in which the sewage is passed through a diaphragm electrolytic cell and the anolyte and catholyte are separated. It consists of discharging. Drain the purified sewer and return the catholyte to the flotation cell.

Sewage containing heavy metal ions is continuously treated with an electrocoagulator, flotation cell and filter, and then led to a diaphragm electrolyzer.

With this method of electrochemical sewage treatment, it is not possible to improve the sewage treatment due to the return of alkaline catholyte to the flotation cell, which causes an increase in the PH of the sewage under treatment.

A disadvantage of this treatment method is the low capacity since part of the flow has to be recycled.

Furthermore, when carrying out this method, the acidified purified water from the anode area of the electrolyzer is discharged overboard or into pipes, which can have a negative impact on coastal flora and fauna;
This results in increased corrosion of pipes.

Another disadvantage of this method is that it requires the use of a filter that allows small particles to pass through and is clogged by large particles, and requires retrieval of the filter.

OBJECTS AND STRUCTURE OF THE INVENTION The main object of the present invention is to arrange a sewage flow for treating anolyte and catholyte separately in the interelectrode space of a diaphragm electrolytic cell equipped with a soluble electrode and an insoluble electrode. The purpose is to eliminate the above-mentioned drawbacks.

Considering this main purpose, the sewage to be treated flows into a diaphragm electrolyzer equipped with soluble and insoluble electrodes, from which the anolyte and catholyte are separately discharged and further mixed, and the purified sewage is discharged. In a method for the electrochemical treatment of sewage, such as marine sewage, according to the invention, anolyte and catholyte are separately discharged from a diaphragm electrolyte cell equipped with insoluble electrodes, and then they are combined with soluble electrodes, in particular a diaphragm cell with an aluminum electrode, and anolyte and catholyte from the diaphragm cell with an insoluble electrode are respectively introduced into an anode compartment and a cathode compartment of the diaphragm cell with a soluble electrode, respectively. A parallel flow directed sewage treatment method is provided.

SUMMARY OF THE INVENTION AND DESCRIPTION OF OPERATIONS When the fluid to be treated is passed in parallel flow, the electrode boundary layer of the fluid does not mix with the remainder of the sewage stream being treated. This provides high alkalinity and acidity of the sewage flow to the electrode boundary area of the soluble electrode, thus preventing the formation of insoluble salt deposits during electrode dissolution.

Feed sewage at a pH of 1 to 1.5 units lower than the soluble metal hydrate ion production PH into the anode compartment of a diaphragm electrolyser with soluble electrodes and into the cathode compartment 1 to 1 to 1.5 units below the soluble metal hydrate ion production PH. It is desirable to supply sewage with a pH 1.5 units higher.

When the pH is at these values, aluminum hydroxide is thermodynamically unstable and dissolves, forming Al ³⁺ ions and AlO ₂ ⁻ ions, so that no deposits form in the interelectrode space.

The electrochemical treatment method for sewage will be schematically explained with reference to FIG.

The untreated sewage introduced via the inlet 1 of the diaphragm electrolyzer 2 with insoluble electrodes is divided into two streams. A diaphragm 3 made of glass fabric separates the anode compartment 4 and the cathode compartment 5.
An anode 6 and a cathode 7 made of graphite are placed in said compartment and are powered by a power source (not shown). Diaphragms with insoluble electrodes The anode and cathode compartments of the electrolytic cell are provided with an anolyte outlet 8 and a catholyte outlet 9, which are connected to a diaphragm with soluble electrodes 17 and 18, in particular made of aluminum, respectively. Inlet 1 of anode compartment 14 and cathode compartment 15 of electrolytic cell 16
2 and 13 and are connected by hoses 10 and 11. The soluble electrode is a diaphragm 1 made from glass fabric.
separated from each other by 9. The treated sewage stream exits the diaphragm cell via outlets 20 and 21 and is mixed in header 22, which then directs the sewage to a separator for clean water separation. When the sewage leaves the interelectrode space of an electrolytic cell with soluble electrodes, the sewage to be treated is neutralized and a hydroxide floc of soluble metals, especially aluminum, forms in the effluent, and this floc adsorbs pollutants from sewage.

The flocs of coagulant adsorbing contaminants are carried to the fluid surface by hydrogen and oxygen bubbles generated during hydrolysis, forming bubbles.

Examples Examples of the electrochemical sewage treatment method of the present invention are listed below.

Example 1 During electrolysis, a current was passed through the graphite and aluminum electrodes of the apparatus shown in FIG. The PH of the sewage in the anode compartment was 3-3.5 and the PH of the sewage in the cathode compartment was 10-11.2. The current supplied to the soluble and insoluble electrodes was 45A and 55A, respectively. The flow rate of sewage through the electrolyzer was 1000/hour.

The polarity of both electrode pairs in both electrolyzers was reversed every 30 minutes.

System Al ³⁺ at PH below 4.5 and above 9.1
Following the AlO ₂ ^-.3H ₂ O equilibrium, aluminum hydroxide is thermodynamically unstable and dissolves to form Al ³⁺ and AlO ₂ ^- ions. In the visual inspection,
No deposits were observed on the surfaces of the soluble and insoluble electrodes even after operating the cell for 950 hours. The PH value at the exit from the header is 8.8
It was ~7.6. The practicality of this method was checked by supplying current alternately to a diaphragm cell equipped with soluble and insoluble electrodes. When operating a diaphragm electrolytic cell with only soluble electrodes, the density is 5.
A current of mA/cm ² was passed through the aluminum electrode, and at the same time sewage with a pH of 7 to 8 was supplied.

The pH of the effluent from the anode compartment is 4.3
~4.5 of the effluent from the cathode compartment
PH was 9.1-9.3. The polarity was reversed every 30 minutes and a friable deposit formed on the electrode surface. Qualitative analysis of this deposit by infrared spectrophotometry revealed that
This deposit was found to be composed of aluminum hydroxide mixed with magnesium hydroxide. After 30-40 hours of operation, the deposits completely occluded the anode and cathode compartments. Upon completion of electrolysis, deposits consisting primarily of aluminum hydroxide were formed and the soluble aluminum electrode corroded.

When sewage is passed through a diaphragm cell with only insoluble electrodes (the diaphragm cell with soluble electrodes has been removed) at an electrolytic density of 4 mA/ ^cm2 , the pH of the sewage leaving the anode compartment is 2.9, and the pH of the sewage leaving the cathode compartment is 2.9. The pH was 11.1 and a white deposit formed on the cathode. As a result of the analysis of this deposit,
It was confirmed that it mainly consists of Mg(OH) ₂ . When the polarity was reversed, the deposits on the electrode surface dissolved.

Thus, when operating a diaphragm cell with only soluble electrodes, deposits formed on the surface of the aluminum electrodes and interfered with the electrochemical sewage treatment process. Deposits were also formed on the surface of the graphite electrode during electrolysis without reversing the polarity. This deposit was removed by reversing the polarity of the electrodes.

FIG. 2 shows an equilibrium diagram for the system Al-H ₂ O.
Line 1 is equilibrium Al ³⁺ Al ₂ O ₃・3H ₂ O and line 2 is equilibrium
Compatible with Al ₂ O ₃・3H ₂ OAlO ₂ ^- . From this diagram
It can be seen that Al ³⁺ ions and AlO ₂ ^- ions are thermodynamically stable at pHs below 4.5 and above 9.1.

When the PH value is 1 to 1.5 units lower and higher for the acidic and alkaline ranges, respectively, aluminum hydroxide is thermodynamically unstable and will not dissolve.
Generates Al ³⁺ ions and AlO ₂ ^- ions.

From the specific examples of the invention described above, it will be readily apparent to those skilled in the art that the main objects of the invention can be achieved within the scope of the claims. However, it will also be readily apparent that minor variations may be made in the operation of carrying out the inventive method of electrochemical treatment of marine sewage without departing from the scope of the invention. All of these variations are considered to be within the scope of the invention as claimed.

The advantages of the sewage treatment method of the present invention compared to known methods are as follows.

1. The electrolyzer capacity is increased by removing hydroxides (deposits) of anode soluble metals from the soluble electrode surface, resulting in improved efficiency.

2. The equipment for carrying out the method of the present invention does not require any special equipment for filtration, coagulation and recovery, so the cost of the equipment is very low.

Furthermore, this method does not cause corrosion of the equipment.

[Brief explanation of the drawing]

FIG. 1 shows a flowchart of the process according to the invention, and FIG. 2 shows an equilibrium diagram for the system Al ³⁺ AlO ₂ ^-.3H ₂ O. 1...Inlet, 2,16...Diaphragm electrolytic cell, 3,1
9...Diaphragm, 4,14...Anode compartment, 5,15...Cathode compartment, 6...
Anode, 7... cathode, 8... anolyte outlet, 9... catholyte outlet.

Claims (1)

[Claims] 1. Sewage to be treated is poured into a diaphragm electrolytic cell equipped with an insoluble electrode and a soluble electrode, the anolyte and catholyte are separately discharged from the diaphragm electrolytic cell and mixed, and the purified sewage is discharged. In the electrochemical treatment of sewage, such as marine sewage, the anolyte and catholyte are separately discharged from a diaphragm electrolyte cell equipped with insoluble electrodes, and these electrolytes are then combined with soluble electrodes, in particular aluminum electrodes. and wherein anolyte and catholyte from the membrane electrolyte cell with insoluble electrodes are conducted in parallel flow into an anode compartment and a cathode compartment, respectively, of the membrane electrolyte cell with soluble electrodes. The sewage treatment method, characterized in that: 2. Process according to claim 1, characterized in that the PH of the sewage fed to the anode compartment of the diaphragm electrolyzer with soluble electrodes is 1 to 1.5 units lower than the PH of the hydrated ion production of the soluble metal. 3. Claim 1 or 2, characterized in that the PH of the sewage supplied to the cathode compartment of the diaphragm electrolyzer equipped with soluble electrodes is 1 to 1.5 units higher than the PH of the hydrated ions of the soluble metal. the method of.