Integrated bioreactor system and bioreactor method

Abstract

The present invention relates to an integrated bioreactor system and a bioreactor method, and belongs to the field of domestic sewage treatment. The system includes a water collection unit and a sewage treatment unit; the water collection unit has a first water inlet, a first water outlet and an exhaust hole; the sewage treatment unit includes a filler area; the water collection unit is connected to the sewage treatment unit by a pipe in the form of a communicating vessel. The method collects and pre-treats sewage, and the pre-treated sewage is introduced into the sewage treatment unit through a communicating vessel. After further treatment, the tail water is intermittently discharged from the sewage treatment unit by a siphon method and dissolved oxygen is introduced. The system and method use the principle of a communicating vessel combined with a siphon to control the sewage to be effectively purified in different ways and stages, and has the advantages of simple maintenance, good decontamination effect, zero energy consumption, and low cost. At the same time, by adjusting different siphon high points, the efficient operation of the sewage treatment unit during the freezing period is achieved, which is suitable for large-scale promotion in rural areas without water collection pipelines.

Description

Technical Field

[0001] The invention belongs to the field of domestic sewage treatment, and in particular relates to an integrated biological reaction system and a biological reaction method. Background Art

[0002] Improper rural sewage treatment directly endangers the safety of the rural ecological environment, leads to environmental problems such as eutrophication of lakes and rivers, and is not conducive to the progress of ecological environment construction.

[0003] Rural sewage discharge points are scattered, but the terrain in most rural areas varies greatly, and it is impossible to collect sewage through pipe networks and treat it uniformly. Therefore, modular sewage treatment facilities are more in line with the current status of sewage treatment in most rural areas. The most widely used modular facilities are septic tanks and septic tanks. Among them, septic tanks are single anaerobic reactions, which have a strong odor and cannot meet the requirements for sewage discharge; septic tanks greatly improve the efficiency of sewage treatment, but have disadvantages such as high cost and energy consumption. At the same time, the low temperature environment during the freezing period in the north leads to low sewage treatment efficiency, and even paralysis of sewage treatment facilities.

[0004] In order to solve the above problems, the present invention is proposed in view of the actual situation that the quality and quantity of rural sewage fluctuate greatly. Summary of the invention

[0005] In order to solve the above technical problems, the present invention proposes an integrated bioreactor system and bioreactor method, which realizes the regular collection and treatment of sewage, introduces dissolved oxygen under zero energy consumption conditions, has a good treatment effect, and realizes efficient purification of rural domestic sewage. Its technical solution is as follows:

[0006] The integrated bioreactor system includes a water collection unit and a sewage treatment unit.

[0007] The water collection unit has a first water inlet, a first water outlet and an exhaust hole for pre-treating sewage; the sewage treatment unit includes a filler area, and the upper surface of the filler area can be selectively planted or raised; the water collection unit is connected to the sewage treatment unit through a communicating pipe. The sewage is pre-treated by the water collection unit and then enters the sewage treatment unit through the communicating pipe for further treatment.

[0008] A second water inlet is arranged at the lower part of the sewage treatment unit and is connected with the first water outlet through the communicating vessel pipeline; a second water outlet is arranged at the upper part of the sewage treatment unit on the side opposite to the second water inlet.

[0009] Preferably, the upper end surfaces of the water collection unit and the sewage treatment unit are on the ground or keep a certain distance from the ground. The first water inlet and the air vent are generally arranged on the ground at the upper end of the water collection unit.

[0010] Furthermore, the second water outlet is connected to a siphon unit; preferably, the high point of the siphon unit is higher than the second water outlet; more preferably, the high point of the siphon unit includes a first high point and a second high point controlled by a valve in parallel, the first high point is higher than the second high point, the first high point is 8-10 cm away from the upper end surface of the sewage treatment unit, and the second high point is higher than the second water outlet. In the non-freezing period, the highest water level can be on the ground surface, and the first high point can be controlled for use; in the winter freezing period, the second high point can be controlled for use.

[0011] Furthermore, the height of the first water outlet is located between the second water inlet and the second water outlet.

[0012] The sewage from the first water inlet continuously enters the water collection unit. When the sewage height exceeds the first water outlet, the sewage flows into the sewage treatment unit through the connecting pipe. After that, when the water levels in the water collection unit and the sewage treatment unit are the same, the water levels of the two units rise synchronously due to the principle of the communicating vessel. When the water level in the sewage treatment unit reaches the high point of the siphon unit, a siphon phenomenon occurs, and the water in the sewage treatment unit is quickly pumped away, and the water level drops from the high point to the second water outlet. At the same time, oxygen is introduced from the upper surface of the sewage treatment unit through the gravel gap. At the same time, due to the principle of the communicating vessel, the sewage in the water collection unit will be replenished into the sewage treatment unit, and the water levels of the two units are the same and between the high point and the second water outlet, and so on.

[0013] Preferably, the depth of the sewage treatment unit relative to the upper end surface of the sewage treatment unit is 0.8-1.2m; the depth of the water collection unit relative to the ground is 1.3-2m; the first highest point of the siphon unit is 8-10cm away from the upper end surface of the sewage treatment unit; the second water outlet is 30-40cm away from the upper end surface of the sewage treatment unit, and the first water outlet is 35-45cm away from the upper end surface of the sewage treatment unit.

[0014] Furthermore, the filler in the filler area includes fillers with large specific surface areas such as crushed stone, ceramsite or activated carbon; preferably, the filler particle size is 5-30 mm. More preferably, in the filler area, the filler particle size in the lower part is larger than the filler particle size in the upper part. Further preferably, the filler particle size in the filler area above the second water outlet is about 5-10 mm, so as to prevent the filler from entering the pipe or valve of the siphon unit and causing blockage; the filler particle size in the filler area below the second water outlet is about 10-30 mm.

[0015] Furthermore, the water collection unit is provided with a partition in the vertical direction, dividing the water collection unit into a water inlet area and a pre-treatment area. The first water inlet is located in the water inlet area, and its upper end may be covered with an upper cover, with a larger opening to facilitate water inlet; the upper end of the pre-treatment area may also be covered with an upper cover, and the exhaust hole is located on the upper cover at the upper end of the pre-treatment area, and is generally a small hole of about 1 cm. The water collection unit is used to store sewage and pre-treat the sewage at the same time.

[0016] Optionally, the water collecting unit is cylindrical.

[0017] Furthermore, the pre-treatment zone is filled with a composite filler; preferably, the pre-treatment zone is provided with an anaerobic zone and a facultative anaerobic zone from bottom to top, which are used to treat the sewage in sections to achieve partial degradation of pollutants; at the same time, suspended matter in the water body is precipitated to prevent the suspended matter from directly entering the sewage treatment unit and clogging the filler in the sewage treatment unit.

[0018] Preferably, the pre-treatment zone is provided with a plurality of intersecting baffles inclined upward to prevent the sludge from being turned upside down during the floating process of methane produced by anaerobic microorganisms.

[0019] The present invention also provides a bioreaction method, comprising the following steps:

[0020] S1. Intermittently discharge sewage into the water collection unit for pre-treatment;

[0021] S2. The pre-treated sewage in S1 is introduced into the sewage treatment unit through a connecting vessel;

[0022] S3. The sewage introduced into the sewage treatment unit in S2 is intermittently discharged from the sewage treatment unit by siphoning and the dissolved oxygen is introduced into the system;

[0023] The siphon method includes using the first high point for drainage during the non-freezing period and using the second high point for drainage during the freezing period.

[0024] Furthermore, the pre-treatment includes anaerobic and facultative anaerobic reactions to achieve partial degradation of pollutants and sedimentation of suspended matter in the water body.

[0025] Furthermore, the sewage treatment unit performs physical adsorption and anaerobic-facultative anaerobic-aerobic microbial purification on the pre-treated sewage, and at the same time, planting or animal husbandry can be carried out on the surface corresponding to the sewage treatment unit.

[0026] The integrated bioreactor system and bioreactor method of the present invention realizes regular collection and treatment of sewage by setting a water collection unit and a sewage treatment unit connected by a communicating vessel, and controls the sewage entering the water collection unit and the sewage treatment unit by a siphon to effectively purify the sewage in different ways and stages, and introduces dissolved oxygen under zero energy consumption conditions. It has the advantages of simple maintenance, good purification effect, and landscape effect, and alleviates the pressure on land occupation during the construction of septic tanks, and is suitable for large-scale promotion in rural areas. At the same time, by adjusting different siphon high points, the sewage treatment unit can be operated efficiently during the freezing period, and has strong cold resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 is a schematic structural diagram of the integrated bioreactor system of Example 1;

[0029] Figure 2 This is a microscopic picture of the composite filler loaded in the pre-treatment zone described in Example 1.

[0030] Description of reference numerals:

[0031] 1-water collection unit; 2-sewage treatment unit; 3-first water inlet; 4-exhaust hole; 5-partition; 6-baffle; 7-first water outlet; 8-second water inlet; 9-upper surface; 10-second water outlet; 11-first high point; 12-connecting pipe; 13-siphon unit; 14-second high point. DETAILED DESCRIPTION

[0032] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0033] In the description of the present invention, it is to be understood that the terms “center”, “longitudinal”, “lateral”, “length”, “width”, “thickness”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, “clockwise”, “counterclockwise”, “axial”, “radial”, “circumferential”, etc., indicating orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0034] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "multiple" and "several" mean two or more, unless otherwise clearly and specifically defined.

[0035] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0036] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0037] Example 1

[0038] Integrated bioreactor systems, such as Figure 1 As shown, it includes a water collection unit 1 and a sewage treatment unit 2.

[0039] The water collection unit 1 has a first water inlet 3, a first water outlet 7 and an exhaust hole 4 for pre-treating sewage; the sewage treatment unit 2 includes a filler area, and the upper surface layer 9 of the filler area can be selectively planted or raised, and in this embodiment, plants are planted; the water collection unit 1 is connected to the sewage treatment unit 2 by a connecting pipe 12. The sewage is pre-treated by the water collection unit 1, and then enters the sewage treatment unit 2 through the connecting pipe 12 for further treatment.

[0040] A second water inlet 8 is provided at the lower part of the sewage treatment unit 2 and is connected to the first water outlet 7 via a connecting pipe 12 ; a second water outlet 10 is provided at the upper part of the sewage treatment unit 2 on the side opposite to the second water inlet 8 .

[0041] In this embodiment, the upper end surfaces of the water collection unit 1 and the sewage treatment unit 2 are both on the ground.

[0042] The second water outlet 10 is connected to a siphon unit 13; the high point of the siphon unit 13 is higher than the second water outlet 10; in this embodiment, the high point includes a first high point 11 and a second high point 14 controlled by a valve in parallel, the first high point 11 is higher than the second high point 14, the first high point 11 is 10 cm from the ground, and the second high point 14 is higher than the second water outlet. In the non-freezing period, the highest water level can be on the ground surface, and the corresponding valve can be opened to control the use of the first high point 11; in the winter freezing period, the corresponding valve can be opened to control the use of the second high point 14, and the water level can be 30 cm below the ground surface.

[0043] The height of the first water outlet 7 is located between the second water inlet 8 and the second water outlet 10 .

[0044] The filler in the packing area includes gravel; in this embodiment, the particle size of the gravel is about 5-30 mm, wherein the particle size of the gravel in the packing area near and above the second water outlet 10 is about 5-10 mm, which can prevent the gravel from entering the pipe or valve of the siphon unit 13 and causing blockage; the particle size of the gravel in the packing area below the second water outlet 10 is about 10-30 mm. There are no special requirements for gravel, and it can be obtained locally. In other embodiments, functional materials such as ceramsite or activated carbon can be used as fillers to improve the treatment effect of wastewater. The upper surface layer 9 can be planted with crops such as cabbage, corn, and sorghum.

[0045] The water collection unit 1 is provided with a partition 5 in the vertical direction, which divides the water collection unit 1 into a water inlet area and a pre-treatment area. The first water inlet 3 is located in the water inlet area. The upper surface of the water inlet area can be covered with an upper cover, and the opening is relatively large, which is convenient for water inlet. The upper surface of the pre-treatment area can also be covered with an upper cover. The exhaust hole 4 is located on the upper cover of the pre-treatment area, which is a small hole of about 1 cm. The water collection unit 1 is used to store sewage and pre-treat the sewage at the same time. In this embodiment, the water collection unit 1 is cylindrical.

[0046] The pre-treatment zone is filled with composite fillers, especially composite fillers with large specific surface areas. In this embodiment, Figure 2 As shown in the figure, the structure of the combined packing is to change the plastic disc buckle into a double-ring large plastic ring, press the aldehyde fiber or polyester yarn on the ring ring, so that the fiber bundle is evenly distributed; the inner ring is a snowflake-shaped plastic branch, which can not only hang the film, but also effectively cut the bubbles, improve the transfer rate and utilization rate of oxygen, so that the water and air biofilm can be fully exchanged, and the organic matter in the water can be efficiently treated.

[0047] The pre-treatment zone is provided with an anaerobic zone and a facultative anaerobic zone from bottom to top, which are used to treat the sewage in sections to achieve partial degradation of pollutants; at the same time, the suspended matter in the water body is settled to prevent the suspended matter from directly entering the sewage treatment unit 2 and clogging the filler in the sewage treatment unit 2.

[0048] As a preferred embodiment, the pre-treatment zone is provided with four intersecting baffles 6 inclined upward to prevent the sludge from being turned upside down during the floating process of methane produced by anaerobic microorganisms.

[0049] The biological reaction method of this embodiment comprises the following steps:

[0050] S1. Pre-treat the sewage;

[0051] Sewage intermittently enters the water inlet area of ​​the water collection unit 1 from the first water inlet, and enters the pretreatment area from the water inlet area, undergoing anaerobic and facultative anaerobic reactions respectively to achieve partial degradation of pollutants and sedimentation of suspended solids in the water body.

[0052] S2. The pre-treated sewage in S1 is introduced into the sewage treatment unit through a connecting vessel;

[0053] When the height of the pre-treated sewage exceeds the first water outlet 7, the sewage flows into the sewage treatment unit 2 through the communicating pipe 12. Subsequently, when the water levels in the water collection unit 1 and the sewage treatment unit 2 are the same, the water levels of the two rise synchronously due to the communicating pipe principle. Since plants are planted on the upper surface of the sewage treatment unit 2, the sewage treatment unit 2 performs physical adsorption, anaerobic-facultative anaerobic-aerobic microbial purification and other treatments on the pre-treated sewage.

[0054] S3. The sewage introduced into the sewage treatment unit 2 in S2 is intermittently discharged from the sewage treatment unit 2 by siphoning and the dissolved oxygen is introduced into the system;

[0055] The siphon method includes using the first high point 11 to drain water in the non-freezing period and using the second high point 14 to drain water in the freezing period. In the freezing period, the surface of the sewage treatment unit freezes. At this time, the valve of the second high point 14 is opened, and the surface ice layer and the air play a role of insulation, so that the system still works, overcoming the problem of poor operation effect during the freezing period in the north.

[0056] When the water level in the sewage treatment unit 2 reaches the high point of the siphon unit 13, a siphon phenomenon occurs, the water in the sewage treatment unit 2 is quickly pumped away, and oxygen is introduced from the upper surface of the sewage treatment unit 2 through the gaps between the gravel, and the water level drops from the high point to the second water outlet 10. At the same time, because of the principle of the communicating vessel, the sewage in the water collection unit 1 will be replenished into the sewage treatment unit 2, and the water levels of the two are the same and between the high point and the second water outlet 10, and so on. Among them, the high point includes a first high point 11 and a second high point 14 in parallel controlled by a valve, and the first high point 11 or the second high point 14 can be selected according to the freezing situation.

[0057] Since the liquid levels of the water collection unit 1 and the sewage treatment unit 2 are the same, the sewage in the water collection unit 1 reciprocates between the high point of the siphon unit 13 and the second water outlet 10, undergoes anaerobic digestion reaction below the second water outlet 10, and undergoes aerobic / facultative aerobic reaction between the high point of the siphon unit 13 and the second water outlet 10, thereby achieving efficient operation of the sewage.

[0058] In this embodiment, combined with the rural domestic water consumption, a family of three is calculated as one household, and the average household water consumption is 134L / d. The volume of the water collection unit 1 should be 402L. Multiplying by the coefficient 1.2, the actual design volume of the water collection unit 1 can be 482L. The depth of the water collection unit 1 relative to the ground is 1.5m, and the diameter is 64cm; the depth of the sewage treatment unit 2 relative to the ground is 1m; the first high point 11 of the siphon unit 13 is 10cm from the ground; the second water outlet 10 is 35cm from the ground, and the first water outlet 7 is 40cm from the ground. The hydraulic retention time is 2.5d, and the area of ​​the sewage treatment unit 2 is 1m*1m. This working condition can meet all areas with an average temperature of >-10℃ in January and an average temperature of >18℃ in July, as well as most areas with an average temperature of <-10℃ in January and an average temperature of <18℃ in July.

[0059] Comparative Example 1

[0060] The system structure is basically the same as that in Example 1, except that the siphon unit in Example 1 is not provided, and the sewage flows continuously without a siphon effect.

[0061] Comparative Example 2

[0062] The system structure is basically the same as that in Example 1, except that the first water outlet of the water collection unit is lowered to a height flush with the second water inlet of the sewage treatment unit in the horizontal direction. After the siphon effect occurs, all the water in the sewage treatment unit flows away.

[0063] By comparing the treatment effects of Example 1 and Comparative Examples 1-2, as shown in Table 1, the COD removal rate, ammonia nitrogen removal rate and total nitrogen removal rate of the treated water obtained by the sewage treatment system and method of Example 1 are greatly improved compared with those of Comparative Examples 1 and 2.

[0064] Table 1 Sewage treatment test data

[0065] project COD removal rate / % Ammonia nitrogen removal rate / % Total nitrogen removal rate / % Example 1 86.54 89.17 92.84 Comparative Example 1 63.54 81.57 86.48 Comparative Example 2 64.71 80.59 83.64

[0066] Those skilled in the art will understand that Figure 1 The structure shown in the figure is only a structural diagram of the part related to the technical solution of the present invention, and does not constitute a limitation on the remaining structures and components to which the solution of the present application is applied. The specific structure and components may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0067] Please note that the technical features of the above embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of the technical features in the above embodiments are not described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. The above embodiments only express several implementation methods of the present application, and their descriptions are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, without departing from the concept of the present application, several variations and improvements can be made, which all belong to the scope of protection of the present application. Therefore, the scope of protection of the patent in this application shall be based on the attached claims.

Claims

1. An integrated bioreactor system, characterized in that: include: A water collection unit, having a first water inlet, a first water outlet and an exhaust hole, for collecting sewage and pre-treating it; A sewage treatment unit, including a fill area, wherein the upper surface of the fill area can be selectively planted or raised with livestock; The water collection unit is connected to the sewage treatment unit through a pipe in the form of a connecting vessel; A second water inlet is provided at the lower part of the sewage treatment unit, which is connected to the first water outlet through the communicating vessel; a second water outlet is provided at the upper part of the sewage treatment unit on the side opposite to the second water inlet; The upper end surfaces of the water collection unit and the sewage treatment unit are on the ground or keep a certain distance from the ground; The second water outlet is connected to a siphon unit; The high point of the siphon unit is higher than the second water outlet; The high point of the siphon unit includes a first high point and a second high point controlled by a valve in parallel, the first high point is higher than the second high point, the first high point is 8-10 cm away from the upper end surface of the sewage treatment unit, and the second high point is higher than the second water outlet; The height of the first water outlet is between the second water inlet and the second water outlet.

2. The integrated bioreactor system according to claim 1, characterized in that: The depth of the sewage treatment unit is 0.8-1.2 m, and the depth of the water collection unit is 1.3-2 m.

3. The integrated bioreactor system according to claim 2, characterized in that: The second water outlet is 30-40 cm away from the upper end surface of the sewage treatment unit, and the first water outlet is 35-45 cm away from the upper end surface of the sewage treatment unit.

4. The integrated bioreactor system according to any one of claims 1 to 3, characterized in that: The filler in the filler zone includes crushed stone, ceramsite or activated carbon.

5. The integrated bioreactor system according to claim 4, characterized in that: The particle size of the filler is 5-30 mm.

6. The integrated bioreactor system according to claim 5, characterized in that: In the packing area, the particle size of the packing in the lower part is larger than that of the packing in the upper part.

7. The integrated bioreactor system according to claim 6, characterized in that: The filler particle size in the filler area above the second water outlet is 5-10 mm, and the filler particle size in the filler area below the second water outlet is 10-30 mm.

8. The integrated bioreactor system according to claim 1, characterized in that: The water collection unit is provided with a partition in the vertical direction, dividing the water collection unit into a water inlet area and a pre-treatment area. The first water inlet is located in the water inlet area, and the exhaust hole is located at the upper end of the pre-treatment area.

9. The integrated bioreactor system according to claim 8, characterized in that: The pre-treatment zone is filled with composite fillers.

10. The integrated bioreactor system according to claim 9, characterized in that: The pre-treatment zone is provided with an anaerobic zone and a facultative anaerobic zone from bottom to top.

11. The integrated bioreactor system according to claim 10, characterized in that: The pre-treatment zone is provided with a plurality of cross-shaped baffles inclined upward to prevent the sludge from being turned upside down during the floating process of methane produced by anaerobic microorganisms.

12. A bioreaction method using the integrated bioreaction system according to any one of claims 1 to 11, characterized in that: The following steps are involved: S1. Intermittently discharge sewage into the water collection unit for pre-treatment; S2. The pre-treated sewage in S1 is introduced into the sewage treatment unit through a connecting vessel; S3. The sewage introduced into the sewage treatment unit in S2 is intermittently discharged from the sewage treatment unit by siphoning and dissolved oxygen is introduced into the system; The siphon method includes using the first high point for drainage during the non-freezing period and using the second high point for drainage during the freezing period.

13. The bioreaction method according to claim 12, characterized in that: The pretreatment includes anaerobic and facultative anaerobic reactions to achieve partial degradation of pollutants and sedimentation of suspended solids in the water body; The sewage treatment unit performs physical adsorption and anaerobic-facultative anaerobic-aerobic microbial purification on the pre-treated sewage, and at the same time, planting or animal husbandry can be carried out on the surface corresponding to the sewage treatment unit.

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