JPS6120324B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for reusing small-scale wastewater, such as car wash wastewater and building wastewater, in a high-quality, efficient, inexpensive and stable manner, and to an improvement of the apparatus. Car wash wastewater is relatively difficult to treat as it contains inorganic substances such as earth and sand and carbon, as well as organic substances such as oils and fats, so conventional purification methods require a large site area for economic reasons.
In many cases, only the fats and oils were collected in a trap and then released. In addition, even in places where recycling facilities are being constructed, the addition of flocculant, mixing, flocculation, and sedimentation separation are performed under atmospheric pressure, and each step is performed in separate equipment, and then the supernatant liquid obtained is Since we use a pressure pump to pressurize the sand into a downward flow type sand filter tank under an operating pressure of 2 to 4 kg/ ^cm2 , a large installation area is required for adding, mixing, flocculating, and settling and separating the flocculant. Not only that, but also the excess area of the pressurized downward flow type sand filter tank requires about four times the amount of the upflow type sand filter tank, and accordingly, the amount of backwash water required for cleaning the sand sand tank is also about four times as much. It has the fatal disadvantage of consuming a large amount of water and wasting approximately 40 to 50% of the total amount of treated water. Backwash water needs to be combined with untreated wastewater and reprocessed, so if a large amount of backwash water is consumed, the amount of water to be treated increases accordingly, making it necessary to increase the size of each device. Furthermore, the larger the settling tank, the more suspended matter will float on the water surface.
The disadvantage is that the treated water becomes cloudy due to the floating phenomenon, is highly corrosive, and emits an unpleasant odor. In addition, there are treatment facilities that operate the entire equipment under pressure, but since they use a downward flow sand filter tank as mentioned above, they not only have the disadvantages mentioned above, but also have operating pressures of 2 kg/cm. Since the system is operated ^twice , water leakage occurs, running costs increase, a strong pressure-resistant structure is required, the price is high, and maintenance is difficult. Furthermore, automation is desirable when regenerating wastewater, but when automating conventional methods, it is necessary to detect the quality of the treated water after pretreatment (e.g. activated sludge treatment, coagulation, sedimentation, etc.). In most cases, expensive instruments such as turbidity meters and fluorometers that can automatically transmit signals are used, or suspended solids flow into the treated water and become cloudy and can only be detected with the naked eye. However, the cost of instruments for small-scale facilities was extremely high, making it difficult to put them into practical use. In particular, the profits from washing passenger cars are extremely small, which is a factor in suppressing construction costs, and under the current circumstances, it is difficult to spread the use of wastewater recycling facilities. In addition, gray water treatment from buildings is one of the types of domestic wastewater that is treated on a daily basis, and although it varies depending on the size of the residence or store, the wastewater volume generally varies from 20 to 300 m ³ /d, and biochemical oxygen Demand (BOD) 40-70ppm, suspended solids (SS) 20
~80ppm is high and raw urine is mixed in.
Shows large values of BOD 150-300ppm and SS 70-180ppm. When regenerating this wastewater, there are many examples of chemical injection using a combination of single devices after activated sludge treatment, but this has the same drawbacks as car wash wastewater, and it is unlikely that wastewater reclamation facilities will become widespread. I haven't reached it yet. In view of the current situation, the present invention provides an expensive turbidimeter,
By using a preliminary adsorption tank instead of instruments such as a fluorometer to detect when it is time to backwash the sand filter tank, treated water can be regenerated stably at low cost. The present invention provides a method and apparatus for regenerating high-quality treated water efficiently and inexpensively using an upflow type sand filter tank with extremely high quality. Next, the present invention will be explained in detail based on illustrated embodiments. In the figure, wastewater containing organic substances, suspended substances, etc. is stored in a wastewater storage tank 1, and after removing coarse solid substances with a screen 2 installed in this flow path, it is passed through a wastewater supply pump _P1 to a closed type rapid pump. In the middle of piping A until it reaches the coagulation tank 3, for example, polyaluminum chloride liquid is injected into the wastewater from pump _P2 ,
A neutralizing alkaline solution is injected from pump _{P 3} to raise the pH to approximately 6.9, and a polymer solution such as polyacrylamide is injected from pump P ₄ and introduced into the above-mentioned flocculation tank 3 through solenoid valve A. . In the flocculation tank 3, the wastewater is passed together with the flocculant between plates arranged in multiple stages at a flow rate of 0.4 to 1 m/sec for 1 to 2 minutes while being well mixed up and down, so that the collision between the suspended solids and the flocculant is repeated. After the flocs are produced, they are introduced through the opening of the flocculation tank 3 into a slow flocculation tank 4 where they are allowed to grow into the next larger floc. Slow flocculation tank 4
In this method, floc growth is promoted by passing the wastewater cross-flow between the multistage plates at a speed of 0.1 to 0.2 m/sec for 3 minutes to increase collisions. Next, the wastewater containing flocs is introduced into a closed sedimentation tank 5 through a piping hole, and is split into two at the inlet of the sedimentation tank 5, where it is allowed to settle through small holes in a porous launder (perforated plate for flow adjustment). It is desirable to supply it to tank 5. The porous launder has
A large number of small holes of 10 to 14 mm are opened, so that a uniform parallel flow is obtained with respect to the cross-sectional area of the sedimentation tank 5, and the floc does not collapse. In the sedimentation tank 5, the flow rate of the wastewater decreases rapidly, so that the wastewater settles on the bottom plate of the sedimentation tank 5 and is separated from the supernatant liquid. For the purpose of increasing the effective area of the settling tank 5 and promoting the settling of flocs, an inclined plate 6 may be provided in the tank. The lightweight flock is
Since the flow is directed toward the overflow port on the top surface, a baffle plate is installed halfway to allow the supernatant liquid to effectively and repeatedly collide with each other, and is installed between the above-mentioned inclined plates 6 to allow flocs to grow and settle easily. It is desirable to do so. When increasing the effective area Am ² of the inclined plate 6 to process the flow rate Qm ³ /hr, the apparent sedimentation rate Rm is Rm=Q/
Since it is expressed as A (m/hr), to operate using one wastewater supply pump _P1 , Rm = 0.4
The point at which the sedimentation separation operation is facilitated is when the speed is set at ~0.6 m/hr. The same effect can be obtained by using an annular multi-tube instead of the inclined plate. Preferably, the settling tank 5 is provided with a sensor 7 that detects the height of settled sludge. for example,
A sensor 7 that emits at a frequency of 3.75 MHz is installed at a desired height, and when the sludge reaches this height, it sends a signal to an ultrasonic transmitter 8, which causes the settling tank 5 to
By opening the solenoid valve B attached to the side wall of the sludge, the sludge can flow into the sludge storage tank 9 through the pipe C. In this case, the sensor 7 constantly detects the top surface of the sludge, so the sludge is kept below a certain water level and
It plays an important role in obtaining high-quality treated water by keeping it below the height of the inclined plate. Furthermore, it is desirable to provide the settling tank 5 with a pressure transmitter _PY . The internal pressure of settling tank 5 is 0.35 of the set pressure.
When the pressure reaches Kg/ ^cm2 , the internal pressure of the sedimentation tank can be reduced by opening the solenoid valve B and discharging the sludge.
It can be reduced to 0.1Kg/ ^cm2 . or
In order to ensure safety against pressure increases, a safety curved pipe 1 opened on the slab of the sedimentation tank 5 is required.
0, adjust its length and set the operating pressure to 0.35.
Kg/cm ² can be used as a safety valve. That is,
When the pressure suddenly increases, a buffering effect is performed in which the overflow water flows out into the pipe N and is automatically returned to the waste water storage tank 1, so that the safety of the settling tank 5 can be maintained. Sludge can also be discharged automatically simply by setting a timer. The overflow water from the sedimentation tank 5 has a uniform flow rate in a U-shaped overflow weir 11 with a porous opening plate provided at the outlet, and enters a closed pre-adsorption tank 12 through a piping E. The preliminary adsorption tank 12 includes a material 13 such as a polypropylene perforated plate that adsorbs residual suspended solids, etc.
are plate-shaped or cylindrical and provided in one or multiple stages. It is desirable to arrange several kinds of coarse porous materials made of polypropylene in series in a multi-stage stacking manner so that the porous materials become finer in order. When suspended solids and n-hexane extracts remain in the treated water from the sedimentation tank 5, the pressure increases rapidly to gradually clog the pores of the material 13. The pore diameter of material 13,
If the number of stages etc. are selected so that the water pressure in the preliminary adsorption tank 12 is 0.35 Kg/cm ² when the overefficiency decreases due to dirt in the sand filter tank 14, which will be described later, this preliminary adsorption tank 12 is equipped with a pressure transmitter Px. This functions to notify the time for backwashing of the sand filter tank 14. In this way, when the pressure of the treated water reaches 0.35Kg/ ^cm2 , the wastewater supply pump _P1 is automatically stopped by the action of the electromagnet due to the current signal emitted from the pressure transmitter Px, and the solenoid valve A is closed. . Therefore, the opening/closing part of the preliminary adsorption tank 12 is opened, the clogged material 13 is removed, and the sand filter tank 14 is backwashed, which will be described later. Further, the preliminary adsorption tank 12 is provided with a degassing pipe for removing air from the treated water. This is necessary in order to uniformly perform the process described below. At the bottom of the preliminary adsorption tank 12, there is a stand support 15 for supporting the material 13.
This prevents short circuits in the treated water, and also
This allows for close contact with 3. The treated water from the preliminary adsorption tank 12 is transferred to the preliminary adsorption tank 1.
It is press-fitted from the lower end of 2 through piping into the lower part of a rapid upward flow sand filter tank 14 whose upper part is open to atmospheric pressure. This upward flow sand filter tank 14 includes perforated plates 1 in order from the bottom layer.
6. A gravel layer 17, a wire mesh 18, and a sand bed 19 are loaded, and the inner wall thereof is made rough by applying a rough surface finish with severe irregularities of 2 to 3 mm, similar to the filling sand, and the treated water is Prevents excessive flow along smooth sidewalls. In the past, a large amount of water flowed unevenly along the side walls, creating a vortex between sand and water and reducing floor efficiency.However, by making the inner walls rough, a uniform upward flow was achieved. This will result in a significant improvement. Furthermore, a deaeration pipe is attached to the inner wall of the upward flow sand filter tank 14, for example, to the upper side of the gravel layer 17, for removing air bubbles from the treated water. Bubbles are inevitably generated inside the device for the following reasons, so sufficient deaeration must be carried out using a large number of tubes. In other words, the BDO source in the sludge and muddy water stored in the settling tank undergoes anaerobic and aerobic decomposition, and bubbles of carbon dioxide, carbon monoxide, methane gas, hydrogen sulfide, and other dissolved gases dissolved in the water are decomposed. Air bubbles may be generated due to air sucked in from pumps, pipe fittings, etc. In this case, the detergent in the wastewater causes significant foaming, which naturally results in a large amount of foam that is difficult to eliminate, which is a factor that inhibits the purifying action of conventional facilities. If bubbles are present in the process water, they will grow larger due to the pressure drop as they rise, creating cavities in the passages.
The latter water mainly passes through this cavity, creating a strong swirl of sand and water, reducing bed efficiency. In order to prevent this, a degassing pipe with a valve G is installed all around the inner wall to remove air bubbles from the treated water, prevent the formation of cavities, and equalize the water flow. In this way, the roughening of the inner wall and the installation of deaeration pipes play an important role, and by making the bed flow velocity uniform, even if the upward flow velocity is 800 to 900 m/day, it will be lower than the conventional 400 m/day. Since it is possible to stably obtain high-quality water that is not much different from that of a day, the efficiency is correspondingly higher and the sand bed area can be reduced. For the material of the sand bed, use manganese sand, zeolite sand, anthracite, etc. with an equivalent diameter of 0.6 mm and a uniformity coefficient of 1.4 or less. The bed packing can be single layer or multilayer as required. The treated water injected into the lower part of the upward flow sand filter tank 14 first passes through the perforated plate 16 that supports the gravel layer 17 and the gravel layer 17, and then passes through a wire mesh 18 to prevent the gravel layer 17 from mixing with the sand bed 19. The water passes through the sand bed 19 and becomes high quality treated water. The sludge settled in the lower part of the sand filter tank 14 is made to flow into the sludge storage tank 9 through a manual valve H and piping. Suspended substances remaining in the treated water gradually accumulate on the gravel layer and sand bed of the upward flow sand filter tank 14.
Since the excess efficiency decreases, it is necessary to backwash occasionally to remove suspended solids from the gravel layer and sand bed. This backwashing is started when the set pressure of the pressure transmitter P _x provided in the preliminary adsorption tank 12 reaches a suppressed pressure of 0.35 Kg/cm ² as described above. The reason why the timing of backwashing is determined based on the pressure change in the preliminary adsorption tank 12 is that the pressure change in the preliminary adsorption tank 12 due to clogging with sludge is larger than that in the sand filter tank 14. If the treated water is immediately passed through the sand filter tank 14 without using the preliminary adsorption tank 12, the water pressure in the sand filter tank 14 will increase due to clogging with sludge, but the degree of increase will be relatively small. Preliminary adsorption tank 1
2, it is necessary to increase the pressure change and detect the degree of clogging due to sludge. The backwash operation sequence is as follows. The set pressure of the pressure transmitter P _x (or pressure transmitter P _y when this breaks down) installed in the preliminary adsorption tank 12 is 0.35 kg/
When the regulated pressure of cm ² is reached, the waste water supply pump P ₁ is stopped, and after the solenoid valves A and E are closed, the solenoid valve F connected to the air pressure storage tank 20 is opened within 1 minute, and the end of the air pipe N is opened. Approximately 2Kg/cm ² from the stainless steel open pipe
The sand bed 19 is thoroughly cleaned by blowing in pressurized air for about 3 to 7 minutes. After continuing aeration for a predetermined period of time, close the solenoid valve F, open the solenoid valves C and D at the same time, operate the backwash pump _P5 , and pass the backwash water from the recycled water storage tank 21 through the piping. , the sand is sent to the lower part of the sand filter tank 14, and the gap between the gravel layer 17 and the wire mesh 18 is approximately
The sand bed 19 is thoroughly cleaned for 5 to 15 minutes by increasing the speed rapidly at a linear speed of 1450 m/day. Backwash pump
In addition to this, the action of _P5 is carried out through the pipe to the preliminary adsorption tank 12
By backwashing the material 13 inside, allowing the sludge to settle in the sedimentation tank 5 through the pipe 10, and allowing excess wastewater to flow out into the wastewater storage tank 1 through the pipe 10, the material regeneration action is automatically carried out. I devised it. The wastewater that has finished backwashing passes through the piping and piping connected to the overflow weir 22, flows out from the solenoid valve C to the opening weir 23, passes through the piping, passes through the sludge storage tank 9, or through the piping YO, and enters the wastewater storage tank 1. send. The reason why aeration is performed first during backwashing is to separate sand and sludge and facilitate washing with backwash water. The stainless steel open tube for aeration is desirably located upwardly from the wire mesh 18 by about 10 cm or more. In addition, when backwashing, valve G,
Piping 1 and 2 are open type. When the backwash is finished, the backwash pump _P5 is stopped, the solenoid valve D is closed, the solenoid valve E is opened, and the solenoid valve C is closed. After the backwashing is completed, the sand bed 19 of the sand filter tank 14 is
It will descend in ~13 minutes and return to its original height, so
Then, open solenoid valve A, and at the same time open the waste water supply pump.
Activate P ₁ and start the entire cycle of flocculation, sedimentation, adsorption, and filtration again. The treated water that has been purified after passing through the sand bed 19 of the upward flow sand filter tank 14 crosses the overflow weir 22, flows through the piping installed in the sand bed 19, passes through the solenoid valve E, and passes through the piping. The water is sent to the reused water storage tank 21 and stored there. Tap water or well water can be added to the reused water storage tank 21 for replenishment, and the treated water stored in this tank can be used repeatedly for car washing or for building toilets by adding chlorine water, etc. can be used. Example 1 Water quality of wastewater; n-hexane extractable content: 20~
160ppm, SS60~100ppm, BCD70~210ppm,
TS260~500ppm, DS200~400ppm, transparency 15~
20. Liquid properties: pale yellow and milky white with a foul odor, _p
H7.5~8.2, COD30~60ppm, detergent content 3~
7ppm, general bacteria count 2 x ¹⁰⁶ /cc, sedimentation rate of coarse SS 3m/hr, sedimentation rate of fine particles 0.7-1.2
m/hr. Baking soda is added to department store wastewater with the above water quality.
2ppm, polyaluminum chloride 15ppm, polyacrylic acid amide 1ppm and diatomaceous earth ^5-15ppm were added and flocculated as shown in the figure and in the above explanation.
The following results were obtained through coagulation, sedimentation separation, preliminary adsorption, and sand filtration under a pressure of 0.35 Kg/cm ² or less. Treated water quality: n-hexane extractable content 3-10ppm,
SS4~10ppm, BOD15~30ppm, DS140~
300ppm, synthetic detergent 2-5ppm, general bacteria count 2x
10 ² - 2 x 10 ³ pieces/cc, transparency 100 - 240, liquid properties: transparent and odorless, _pH 6.8, COD 10 - 15 ppm. By injecting chlorinated water, the number of general bacteria was reduced to 0 bacteria/cc, and the water quality satisfied all requirements for toilet water. Example 2 Water quality of wastewater; n-hexane extractable content: 10~
200ppm, SS60~250ppm, BCD40~110ppm,
TS300~500ppm, DS240~250ppm, synthetic detergent
0.7~2ppm, transparency 5~12, Fe, Mn total 1.2ppm,
Liquid properties: white milky to light brown, _pH 6~7,
COD15~120ppm. Car wash wastewater having the above water quality was treated in the same manner as shown in the figure and in the above explanation except for the following conditions, and when it reached 0.35Kg/cm ² , it was backwashed and the following treated water was treated under a pressure of 0.35Kg/cm ² or less. I got it. Processing conditions: flocculant, polyaluminum chloride 20~
25ppm (purity), NaOH 10-30ppm, polyacrylamide 0.7-1.0ppm, rapid flocculation time 1 minute, slow flocculation 2 minutes, sedimentation standing time 30 minutes, sedimentation speed 2.8 m/
hr, linear velocity in the preliminary adsorption tank 600 to 900 m/d, linear velocity in the sand filter tank 600 to 1000 m/d, sand bed manganese sand (diameter 0.6 mm, uniformity factor 1.4 or less), cylinder used 100 mm
φ, passing speed 3.3 to 5.45/min. Water quality of treated water: n-hexane 0.2-4ppm, SS4
~10ppm, BOD4~10ppm, TS210~220ppm,
DS206~210ppm, detergent 0.6~1.7ppm, transparency 100
~250, Fe, Mn total 0.05ppm, liquid properties: transparent, _p
H6.6~6.9, COD7~10ppm. Through such treatment, car wash waste water is significantly purified and can now be used satisfactorily as car wash water. Example 3 The following table shows the water quality before treatment, treatment conditions, water quality after treatment, and removal rate of car wash wastewater containing large amounts of wax and detergent. Note that the test sample was a white milky one with an initial transparency of 1 to 0.5. The other conditions were the same as those shown in the figure and the above explanation, and the test was carried out under a pressure of 0.35 Kg/cm ² or less, with backwashing performed when the pressure reached 0.35 Kg/cm ² .

【table】

[Table] As is clear from this table, even if the amount of flocculant is small, the treatment effect is great. Example 4 Removal of oil that does not agglomerate As a result of repeated tests, it was found that even when polyaluminum chloride, polyacrylamide, etc. are used, floating oil that does not agglomerate circulates and accumulates in large amounts, causing water pollution. occurs frequently. This is particularly likely to occur when waste water from car washes such as taxis, passenger cars, and trains is treated. In such a case, as shown in FIG. 2, a uniform protruding box 24 is provided on the top surface of the overflow side of the settling tank 5 to encourage oil floating on the water surface to be sent to the sludge storage tank 9 and to separate it from other sludge. Co-precipitate. Furthermore, it is desirable to vertically attach a floating object suppression plate 25 to the ceiling plate of the sedimentation tank 5 in order to promote the oil-collecting effect and remove floating objects. In this operation, especially when purifying wastewater with a high content of synthetic detergent, air bubbles are generated due to various causes, float to the surface, pass through the overflow weir 11, and enter the preliminary adsorption tank 12. causing clogging of material 13,
It plays a major role in preventing the defects that occur frequently in backwashing. That is, by using the floating matter suppression plate 25, 40,000 ppm of oil floating on the water surface is removed to the sludge storage tank 9.
When co-precipitated with other sludge, the n-hexane extract content of the treated water always drops below 20 ppm, and after subsequent treatment it becomes 0.2 to 1 ppm, making it easy to achieve safe, stable, and high-quality water. . Comparative Example When wastewater treatment was continued through flocculation, sedimentation separation, preliminary adsorption, and sand filtration in the same manner as in Example 2 without backwashing the sand bed in the sand filtration process, the pressure of pressure transmitter P _x gradually decreased. increased to 0.35Kg/cm ²
When the pressure exceeds 0.45 kg/cm ² , the accumulated material on the sand bed of the sand filter tank 14 becomes clogged and begins to rise and mix into the treated water. It was done. n-hexane extract 5~10ppm SS 10~15〃 BOD 10~15〃 TS 215~250〃 DS 210~220〃 COD 10~20〃 Transparency 30cm or less As is clear from the above description, the car wash wastewater of the present invention , a method for recycling small-scale wastewater such as miscellaneous wastewater from buildings, and its equipment include a preliminary adsorption treatment using a material for the treated water obtained by adding a flocculant to small-scale wastewater such as car wash wastewater, and subjecting it to coagulation and sedimentation separation. In this method, the pressure is detected so that the entire process is carried out under a pressure of 0.35 kg/cm ² or less, and the pressure is 0.35 kg/cm 2 or less.
Kg/cm ² is reached, the operation is automatically stopped, the clogging material in the preliminary adsorption step is removed, and the sand bed is backwashed in the pass step, and a closed flocculation tank, a closed sedimentation tank, A closed preliminary adsorption tank equipped with a pressure transmitter and a degassing pipe, as well as a part that can be opened and closed for exchanging materials, and a large upper part with a rough inner wall and a degassing pipe around the outer periphery of the gravel layer. It is a device consisting of an air pressure release type rapid upward flow filter tank, etc., and in particular uses an upward flow sand filter tank with extremely good flow efficiency, which has a degassing pipe and a rough inner wall, and also controls the backwashing period of the sand tank. It detects the pressure increase caused by the precise adsorption action of the material in the preliminary adsorption tank, and maintains the entire process at a low pressure of 0.35Kg/ ^cm2 , and backwashing is automatically performed to ensure high quality treated water. It has the function and effect of making it possible to supply efficiently and stably at low cost. In addition to car wash wastewater and building wastewater, the present invention can also be used in machine factories,
It can be widely applied to recycling treatment of oil-containing wastewater from chemical factories, etc.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, and FIG. 1 is an explanatory view thereof, and FIG. 2 is a sectional view showing another embodiment of the settling tank. 1... Wastewater storage tank, 2... Screen, 3... Rapid flocculation tank, 4... Slow flocculation tank, 5... Sedimentation tank, 6... Inclined plate,
7...Sensor, 8...Ultrasonic transmitter, 9...Sludge storage tank, 10...Safety and safety curved pipe, 11...Overflow weir, 12...
Preliminary adsorption tank, 13...material, 14...sand filter tank, 15
...Support stand, 16...Perforated plate, 17...Gravel layer, 18...
Wire mesh, 19... Sand bed, 20... Air pressure storage tank, 21
...Reused water storage tank, 22...Overflow weir, 23...Open weir,
24... Projection box, 25... Floating object suppression plate.

Claims (1)

[Scope of Claims] 1. Treated water obtained by adding a coagulant to small-scale wastewater such as car wash wastewater, coagulating it, and performing sedimentation separation is subjected to preliminary adsorption treatment with a material, and then the inner wall is roughened and desorbed. In the method of creating a uniform upward flow while preventing the generation of vortices by applying air,
In order to carry out the entire process under pressure of 0.35Kg/ ^cm2 or less, the pressure is constantly detected, and when the pressure reaches 0.35Kg/ ^cm2 , the wastewater supply is automatically stopped, and the sand bed is reversed in the overprocess. A method for reprocessing small-scale wastewater such as car wash wastewater, which involves washing. 2. A closed flocculation tank, a closed sedimentation tank, a closed pre-adsorption tank each equipped with a pressure transmitter and a degassing pipe, and with a part that can be opened and closed for exchanging materials, and an inner wall with a rough surface. , a small-scale wastewater regeneration treatment device for car wash wastewater and other small-scale wastewater, consisting of a rapid upward flow filter tank with a degassing pipe placed around the outer periphery of the gravel layer and a top atmospheric pressure open type.