A continuous-flow urban sewage treatment device and method for preventing sludge bulking

Abstract

This invention relates to a continuous flow urban wastewater treatment device and method for preventing sludge bulking. It can reduce the risk of excessive proliferation of filamentous bacteria, enhance the sludge granulation degree in the continuous flow system, and improve sludge settling performance. The technical solution is to first use some of the organic matter in the wastewater for denitrification and phosphorus release in the pre-anoxic tank, and then further consume the remaining organic matter in the wastewater through denitrification in the intermediate anoxic tank. This enriches polyphosphate-accumulating bacteria and reduces the organic matter concentration in the aerobic tank to inhibit the growth of filamentous bacteria. The aerobic tanks are operated in series to improve the reaction rate and reduce aeration energy consumption. Polyphosphate-accumulating bacteria are enriched in sludge with excellent settling properties, which can also promote sludge granulation and further improve sludge settling performance.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a continuous flow urban wastewater treatment device and method for preventing sludge bulking. Background Technology

[0002] The activated sludge process is currently the most widely used technology in urban wastewater treatment, offering convenient operation and management as well as reliable treatment results. However, when factors such as influent, dissolved oxygen, and organic load cause an imbalance in the bacterial species ratio in the activated sludge, filamentous bacteria are prone to over-proliferation, deteriorating sludge settling performance and thus affecting the stability of treatment results. Especially during cold seasons, the specific proliferation rate of filamentous bacteria exceeds that of bacterial flocs, leading to deterioration of sludge settling performance and, in severe cases, even causing sludge overflow in the secondary settling tank and system collapse.

[0003] Since the discovery of aerobic granular sludge, it has become possible to improve sludge settling properties while enhancing treatment efficiency. However, existing aerobic granular sludge is mainly used in intermittent flow systems. Given that both granular sludge and filamentous bacteria require organic matter from the incoming water for growth, creating granular conditions in continuous flow systems could not only reduce the risk of excessive filamentous bacteria proliferation but also help leverage the high treatment efficiency of granular sludge. Therefore, enhancing the degree of sludge granulation in continuous flow systems holds promise for improving the treatment effect of municipal wastewater while simultaneously improving sludge settling performance. Summary of the Invention

[0004] In view of the above situation and to overcome the defects of the prior art, the purpose of this invention is to provide a continuous flow urban wastewater treatment device and method for preventing sludge bulking. First, in the pre-anoxic tank, some of the organic matter in the wastewater is used for denitrification and phosphorus release. Then, in the intermediate anoxic tank, the remaining organic matter in the wastewater is further consumed through denitrification, thereby ensuring that the concentration of organic matter available to filamentous bacteria in the aerobic tank is minimized, while polyphosphate-accumulating bacteria are enriched to increase the density and granularity of the sludge.

[0005] One of the technical solutions provided by this invention is a continuous flow urban wastewater treatment method for preventing sludge bulking, which utilizes a continuous flow urban wastewater treatment device for preventing sludge bulking and then proceeds according to the following steps: S1: Inoculate activated sludge from the municipal wastewater treatment plant into a continuous flow municipal wastewater treatment device to prevent sludge bulking, so that the sludge concentration reaches 3000-5000 mg / L; S2: Control the incoming water to enter the bottom of the first pre-anoxic tank and the second pre-anoxic tank at the same flow rate. The first normally closed solenoid valve and the second normally closed solenoid valve open and close alternately. When the first normally closed solenoid valve and the second normally closed solenoid valve are open, the first agitator and the second agitator in the first pre-anoxic tank and the second pre-anoxic tank are also opened at the same time. When the first normally closed solenoid valve and the second normally closed solenoid valve are closed, the first agitator and the second agitator in the first pre-anoxic tank and the second pre-anoxic tank are also closed at the same time. S3: After being treated in the first and second pre-anoxic tanks, the urban sewage enters the anoxic tank. The third stirring paddle in the anoxic tank is continuously turned on to further treat the urban sewage. S4: Urban sewage treated in the anoxic tank enters the first aerobic tank for further treatment. Aeration is supplied to the first aerobic tank through aeration pipes. The dissolved oxygen concentration in the first aerobic tank is controlled at 1.0-2.0 mg / L. S5: Urban sewage treated in the first aerobic tank enters the second aerobic tank for further treatment. Aeration is supplied to the second aerobic tank through aeration pipes. The settled sludge after screening in the vertical flow screening tank is returned to the first and second pre-anoxic tanks through a return pump and sludge return pipes. The dissolved oxygen concentration in the second aerobic tank is controlled at 1.0-2.0 mg / L. The upward flow velocity of the built-in vertical flow screening tank is controlled at 1.0-5.0 m / h. The sludge return ratio is controlled at 50-200%.

[0006] Furthermore, in S2, the hydraulic retention time (HRT) of the pre-anoxic tank is controlled between 30 and 120 minutes.

[0007] Furthermore, in S2, the opening time of the first normally closed solenoid valve and the second normally closed solenoid valve is controlled to be between 30 and 60 minutes.

[0008] Furthermore, in S3, the HRT of the anoxic tank is controlled between 60 and 240 min.

[0009] Furthermore, in S4, the HRT of the first aerobic tank is controlled at 240-480 min.

[0010] Furthermore, in S5, the HRT of the second aerobic tank is controlled between 120 and 480 min.

[0011] The second technical solution provided by this invention is a continuous flow urban wastewater treatment system for preventing sludge bulking, comprising a pre-anoxic tank, an anoxic tank, and an aerobic tank. A first and a second pre-anoxic tank are respectively equipped with a first and a second stirring paddle. The first pre-anoxic tank is connected to the second pre-anoxic tank via an inlet pipe. The first and second pre-anoxic tanks are connected to the inlet of the anoxic tank via a first outlet pipe. An stirring paddle is provided on the anoxic tank. The outlet of the anoxic tank is connected to the inlet of the first aerobic tank via a second outlet pipe. The outlet of the first aerobic tank is connected to the inlet of the second aerobic tank via a third outlet pipe. The first and second aerobic tanks are respectively connected to an aerator via aeration pipes. A vertical flow screening tank is provided inside the second aerobic tank, and the vertical flow screening tank is connected to both the first and second pre-anoxic tanks via sludge return pipes.

[0012] Furthermore, a first inlet check valve is installed on the inlet pipe near the first pre-anoxic tank, and a second inlet check valve is installed on the inlet pipe near the second pre-anoxic tank.

[0013] Furthermore, a first normally closed solenoid valve is installed on the sludge return pipe near the first pre-anoxic tank, and a second normally closed solenoid valve is installed on the sludge return pipe near the second pre-anoxic tank.

[0014] Furthermore, the first pre-anoxic tank and the second pre-anoxic tank operate in parallel, while the anoxic tank, the first aerobic tank, and the second aerobic tank operate in series.

[0015] Furthermore, a return pump is installed on the sludge return pipeline.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) In the pre-anoxic tank with stirring operation, denitrifying bacteria use organic matter in urban sewage to reduce nitrates in the mixed liquor of returned sludge, thereby creating a good anaerobic phosphorus release environment; 2) In the pre-anoxic tank where stirring has stopped, the sludge with good settling properties at the bottom preferentially utilizes the organic matter in the incoming water to fully release phosphorus under anaerobic conditions, thereby promoting the accumulation of polyphosphate-accumulating bacteria in the sludge with excellent settling properties. 3) The remaining organic matter in the pre-anoxic tank is further utilized by the denitrification process in the intermediate anoxic tank, which not only enriches polyphosphate-accumulating bacteria but also minimizes the concentration of organic matter entering the aerobic tank and inhibits the growth of filamentous bacteria in the aerobic tank. 4) The aerobic tanks are connected in series to increase the reaction rate and reduce aeration energy consumption; 5) Enriching polyphosphate-accumulating bacteria in sludge with good settling properties promotes sludge granulation and further improves sludge settling properties. Attached Figure Description

[0017] Figure 1This is a schematic diagram of the structure of the continuous flow urban wastewater treatment device for preventing sludge bulking according to the present invention.

[0018] In the diagram, the components are: 1. First pre-anoxic tank; 1.1. First stirring paddle; 1.2. First normally closed solenoid valve; 1.3. First inlet check valve; 2. Second pre-anoxic tank; 2.2. Second normally closed solenoid valve; 2.3. Second inlet check valve; 3. Anoxic tank; 3.1. Third stirring paddle; 4. First aerobic tank; 4.1. Aeration pump; 4.2. Aeration pipe; 5. Second aerobic tank; 6. Vertical flow screening tank; 6.1. Sludge return pump; 6.2. Sludge return pipe; 7. Inlet pipe; 8.1. First outlet pipe; 8.2. Second outlet pipe; 8.3. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] This invention relates to a continuous flow urban wastewater treatment method for preventing sludge bulking, which utilizes a continuous flow urban wastewater treatment device for preventing sludge bulking and then proceeds according to the following steps: S1: Inoculate activated sludge from the municipal wastewater treatment plant into a continuous flow municipal wastewater treatment device to prevent sludge bulking, so that the sludge concentration reaches 3000-5000 mg / L; S2: Control the incoming water to simultaneously enter the bottom of the first pre-anoxic tank 1 and the second pre-anoxic tank 2 at the same flow rate. The first normally closed solenoid valve 1.2 and the second normally closed solenoid valve 2.2 are opened alternately. When the first normally closed solenoid valve 1.2 and the second normally closed solenoid valve 2.2 are open, the first agitator 1.1 and the second agitator 2.1 in the first pre-anoxic tank 1 and the second pre-anoxic tank 2 are also opened at the same time. When the first normally closed solenoid valve 1.2 and the second normally closed solenoid valve 2.2 are closed, the first agitator 1.1 and the second agitator 2.1 in the first pre-anoxic tank 1 and the second pre-anoxic tank 2 are also closed at the same time. S3: After being treated by the first pre-anoxic tank 1 and the second pre-anoxic tank 2, the urban sewage enters the anoxic tank 3. The third stirring paddle 3.1 of the anoxic tank 3 is continuously turned on to further treat the urban sewage. S4: Urban sewage treated in anoxic tank 3 enters the first aerobic tank 4 for further treatment. Aeration is supplied to the first aerobic tank 4 through aeration pipe 4.2. The dissolved oxygen concentration in the first aerobic tank 4 is controlled at 1.0-2.0 mg / L. S5: The municipal sewage treated in the first aerobic tank 4 enters the second aerobic tank 5 for further treatment. Aeration is supplied to the second aerobic tank 5 through aeration pipe 4.2. The settled sludge after being screened in the vertical flow screening tank 6 enters the first pre-anoxic tank 1 and the second pre-anoxic tank 2 again through the return pump 6.1 and sludge return pipe 6.2. The dissolved oxygen concentration in the second aerobic tank 5 is controlled at 1.0-2.0 mg / L; the upward flow velocity of the built-in vertical flow screening tank 6 is controlled at 1.0-5.0 m / h; and the sludge return ratio is controlled at 50-200%.

[0021] Furthermore, in S2, the hydraulic retention time (HRT) of the pre-anoxic tank is controlled between 30 and 120 minutes.

[0022] Furthermore, in S2, the opening time of the first normally closed solenoid valve 1.2 and the second normally closed solenoid valve 2.2 is controlled to be 30-60 minutes.

[0023] Furthermore, in S3, the HRT of the anoxic pool 3 is controlled between 60 and 240 min.

[0024] Furthermore, in S4, the HRT of the first aerobic tank 4 is controlled at 240-480 min.

[0025] Furthermore, in S5, the HRT of the second aerobic tank 5 is controlled at 120-480 min.

[0026] Depend on Figure 1 The proposed continuous flow urban wastewater treatment device for preventing sludge bulking includes a pre-anoxic tank, an anoxic tank, and an aerobic tank. The first pre-anoxic tank 1 and the second pre-anoxic tank 2 are equipped with a first agitator 1.1 and a second agitator 2.1, respectively. The first pre-anoxic tank 1 is connected to the second pre-anoxic tank 2 via an inlet pipe 7. The first pre-anoxic tank 1 and the second pre-anoxic tank 2 are connected to the inlet of an anoxic tank 3 via a first outlet pipe 8.1. The anoxic tank 3 is equipped with an agitator 3.1. The outlet of tank 3 is connected to the inlet of the first aerobic tank 4 via the second outlet pipe 8.2. The outlet of the first aerobic tank 4 is connected to the inlet of the second aerobic tank 5 via the third outlet pipe 8.3. The first aerobic tank 4 and the second aerobic tank 5 are connected to the aerator 4.1 via the aeration pipe 4.2. The second aerobic tank 5 is equipped with a vertical flow screening tank 6. The vertical flow screening tank 6 is connected to the first pre-anoxic tank 1 and the second pre-anoxic tank 2 via the sludge return pipe 6.2.

[0027] Furthermore, a first inlet check valve 1.3 is installed on the inlet pipe 7 near the first pre-anoxic tank 1, and a second inlet check valve 2.3 is installed on the inlet pipe 7 near the second pre-anoxic tank 2.

[0028] Furthermore, a first normally closed solenoid valve 1.2 is installed on the sludge return pipe 6.2 near the first pre-anoxic tank 1, and a second normally closed solenoid valve 2.2 is installed on the sludge return pipe 6.2 near the second pre-anoxic tank 2.

[0029] Furthermore, the first pre-anoxic tank 1 and the second pre-anoxic tank 2 operate in parallel, while the anoxic tank 3, the first aerobic tank 4, and the second aerobic tank 5 operate in series.

[0030] Furthermore, a return pump 6.1 is installed on the sludge return pipe 6.2.

[0031] The wastewater treatment process of this invention in this device is as follows: Wastewater first enters two parallel pre-anoxic tanks. These two pre-anoxic tanks alternately receive sludge returned from the secondary sedimentation tank, and the agitator is activated only when receiving externally returned sludge. In the pre-anoxic tank with the agitator activated, activated sludge utilizes the carbon source in the wastewater as an electron donor for denitrification. In the pre-anoxic tank with the agitator deactivated, polyphosphate-accumulating bacteria absorb the carbon source in the wastewater and release phosphorus. The effluent from both pre-anoxic tanks flows into an intermediate anoxic tank to fully utilize the carbon source in the wastewater for denitrification. Subsequently, the effluent from the intermediate anoxic tank enters an aerobic tank for nitrification and phosphorus absorption, and is discharged after sludge-water separation in the secondary sedimentation tank. This invention can enrich polyphosphate-accumulating bacteria to increase sludge density, and the intermediate anoxic tank can maximize the consumption of organic matter required for the growth of filamentous bacteria in the aerobic tank. By enriching polyphosphate-accumulating bacteria and reducing the organic matter required for the aerobic growth of filamentous bacteria, sludge settling properties are improved, and this has been fully proven in practice. The following are relevant embodiments. Example 1

[0032] This embodiment uses the influent from a domestic wastewater treatment plant in Wenxian County as the raw water. This plant employs the traditional AAO process. The specific influent water quality is as follows: COD concentration 150-320 mg / L. -1 The concentration of NH4⁺-N is 20-32 mg•L. -1 TN concentration is 25-40 mg•L -1 The TP concentration was 2.0-4.0 mg•L. -1 The test system is as follows: Figure 1 As shown, each reactor is made of plastic tubs. The two pre-anoxic tanks are each 50L in volume, the intermediate anoxic tank is 50L in volume, the primary aerobic tank is 200L in volume, the secondary aerobic tank is 100L in volume, and the vertical flow screening tank is 10L in volume. The total effective volume is 450L.

[0033] The specific activation and control steps are as follows: 1. Start-up system: Inoculate the activated sludge from Wenxian Wastewater Treatment Plant into the entire device to achieve a sludge concentration of 3000-5000 mg / L; 2. Control System: 2.1 The incoming water is controlled to simultaneously enter the bottom of the first pre-anoxic tank 1 and the second pre-anoxic tank 2 at the same flow rate, and the HRT of the pre-anoxic tank is controlled at 60 minutes; normally closed solenoid valves 1.2 / 2.2 are alternately opened and closed, and the opening time of each solenoid valve is controlled at 60 minutes. When normally closed solenoid valves 1.2 / 2.2 are open, the agitators 1.1 / 2.1 in pre-anoxic tank 1 / 2 are also opened simultaneously. When normally closed solenoid valves 1.2 / 2.2 are closed, the agitators 1.1 / 2.1 in pre-anoxic tank 1 / 2 are also closed simultaneously. 2.2 The agitator of the intermediate anoxic tank 3 is continuously turned on and operated; the HRT of the intermediate anoxic tank 3 is controlled at 60 min; 2.3 The HRT of aerobic tank 4 is controlled at 240 min; the dissolved oxygen concentration of aerobic tank 4 is controlled at 1.0-2.0 mg / L; 2.4 The HRT of aerobic tank 5 is controlled at 120 min; the dissolved oxygen concentration of aerobic tank 5 is controlled at 1.0-2.0 mg / L; 2.5 The upward flow velocity of the vertical flow screening tank 6 is controlled at 1.0-5.0 m / h; the sludge return ratio is controlled at 50-200%.

[0034] The test results showed that during the entire operation period from early September to early December, the SVI of the wastewater treatment plant was [data missing]. 30 The value fluctuated between 80-160 mL / g, especially with slight growth of filamentous fungi as temperatures dropped in October. (SVI) 30 The value consistently exceeded 100 mL / g. Meanwhile, under the same influent conditions, the SVI in a continuous flow municipal wastewater treatment plant designed to prevent sludge bulking... 30 The value was consistently below 100 mL / g, and its sludge settling performance was consistently better than that of wastewater treatment plants during the same period, with an average SVI of [missing value]. 30 The value remained between 50-70 mL / g for a long period, and no filamentous fungal growth was observed under a microscope.

[0035] Regarding sludge particle size, the average volumetric particle size of activated sludge in wastewater treatment plants is concentrated in the range of 25-40 μm, while the average volumetric particle size of sludge in continuous flow urban wastewater treatment devices that prevent sludge bulking gradually increases from 25 μm to 60 μm. Microscopic observation shows that the sludge structure is relatively denser, and even some aerobic granular sludge seeds have appeared.

[0036] In biological phosphorus removal, the specific phosphorus release rate has been consistently increasing, from an initial 0.1 mg PO4. 3- -P / gMLSS was gradually increased to 0.3 mgPO4 3- -P / gMLSS. The TP concentration in the effluent remained below 0.5 mg / L throughout the period.

[0037] In summary, the pre-anoxic tank of this invention creates conditions for denitrification, anaerobic phosphorus release, and polyphosphate accumulation through two operating modes: stirring and stopping. The remaining organic matter in the tank further participates in denitrification in the intermediate anoxic tank, which not only enriches polyphosphate-accumulating bacteria but also reduces the organic matter concentration in the aerobic tank to inhibit the growth of filamentous bacteria. The aerobic tanks are operated in series, which can improve the reaction rate and reduce aeration energy consumption. The polyphosphate-accumulating bacteria are enriched in sludge with excellent settling properties, which can also promote sludge granulation and further improve sludge settling performance.

Claims

1. A continuous-flow municipal sewage treatment method for preventing sludge bulking, characterized by, Utilize a continuous flow municipal wastewater treatment device that prevents sludge bulking, and then proceed with the following steps: S1: Inoculate activated sludge from the municipal wastewater treatment plant into a continuous flow municipal wastewater treatment device to prevent sludge bulking, so that the sludge concentration reaches 3000-5000 mg / L; S2: Control the incoming water to enter the bottom of the first pre-anoxic tank (1) and the second pre-anoxic tank (2) at the same flow rate. The first normally closed solenoid valve (1.2) and the second normally closed solenoid valve (2.2) are opened alternately. When the first normally closed solenoid valve (1.2) and the second normally closed solenoid valve (2.2) are opened, the first agitator (1.1) and the second agitator (2.1) in the first pre-anoxic tank (1) and the second pre-anoxic tank (2) are also opened at the same time. When the first normally closed solenoid valve (1.2) and the second normally closed solenoid valve (2.2) are closed, the first agitator (1.1) and the second agitator (2.1) in the first pre-anoxic tank (1) and the second pre-anoxic tank (2) are also closed at the same time. S3: After being treated by the first pre-anoxic tank (1) and the second pre-anoxic tank (2), the urban sewage enters the anoxic tank (3). The third stirring paddle (3.1) of the anoxic tank (3) is continuously turned on to further treat the urban sewage. S4: Urban sewage treated in the anoxic tank (3) enters the first aerobic tank (4) for further treatment. Aeration is supplied to the first aerobic tank (4) through the aeration pipe (4.2). The dissolved oxygen concentration in the first aerobic tank (4) is controlled at 1.0-2.0 mg / L. S5: The urban sewage treated in the first aerobic tank (4) enters the second aerobic tank (5) for further treatment. Aeration is supplied to the second aerobic tank (5) through the aeration pipe (4.2). The sludge after being screened by the vertical flow screening tank (6) enters the first pre-anoxic tank (1) and the second pre-anoxic tank (2) again through the return pump (6). (1) and the sludge return pipe (6.2). The dissolved oxygen concentration in the second aerobic tank (5) is controlled at 1.0-2.0 mg / L; the upward flow velocity of the built-in vertical flow screening tank (6) is controlled at 1.0-5.0 m / h; and the sludge return ratio is controlled at 50-200%.

2. The continuous-flow municipal sewage treatment method for preventing sludge bulking according to claim 1, characterized by, In S2, the hydraulic retention time (HRT) of the pre-anoxic tank is controlled between 30 and 120 minutes.

3. The continuous-flow municipal sewage treatment method for preventing sludge bulking according to claim 1, characterized by, In S2, the opening time of the first normally closed solenoid valve (1.2) and the second normally closed solenoid valve (2.2) is controlled to be 30-60 minutes.

4. The continuous-flow municipal sewage treatment method for preventing sludge bulking according to claim 1, characterized by, In S3, the HRT of the anoxic pool (3) is controlled at 60-240 min.

5. The continuous-flow municipal sewage treatment method for preventing sludge bulking according to claim 1, characterized by, In S4, the HRT of the first aerobic tank (4) is controlled at 240-480 min.

6. The continuous-flow municipal sewage treatment method for preventing sludge bulking according to claim 1, characterized by, In S5, the HRT of the second aerobic tank (5) is controlled at 120-480 min.

7. The continuous flow municipal sewage treatment for preventing sludge bulking as claimed in claim 1, wherein The system includes a pre-anoxic tank, an anoxic tank, and an aerobic tank. The first pre-anoxic tank (1) and the second pre-anoxic tank (2) are respectively equipped with a first stirring paddle (1.1) and a second stirring paddle (2.1). The first pre-anoxic tank (1) is connected to the second pre-anoxic tank (2) via an inlet pipe (7). The first pre-anoxic tank (1) and the second pre-anoxic tank (2) are connected to the inlet of the anoxic tank (3) via a first outlet pipe (8.1). The anoxic tank (3) is equipped with a stirring paddle (3.1). The outlet of the anoxic tank (3) is connected to the second outlet pipe. The channel (8.2) is connected to the inlet of the first aerobic tank (4). The outlet of the first aerobic tank (4) is connected to the inlet of the second aerobic tank (5) via the third outlet pipe (8.3). The first aerobic tank (4) and the second aerobic tank (5) are connected to the aerator (4.1) via the aeration pipe (4.2). The second aerobic tank (5) is equipped with a vertical flow screening tank (6). The vertical flow screening tank (6) is connected to the first pre-anoxic tank (1) and the second pre-anoxic tank (2) via the sludge return pipe (6.2).

8. The continuous flow municipal sewage treatment for preventing sludge bulking according to claim 7, characterized by, A first inlet check valve (1.3) is installed on the inlet pipe (7) near the first pre-anoxic pool (1), and a second inlet check valve (2.3) is installed on the inlet pipe (7) near the second pre-anoxic pool (2).

9. The continuous flow municipal sewage treatment for preventing sludge bulking according to claim 7, wherein A first normally closed solenoid valve (1.2) is installed on the sludge return pipe (6.2) near the first pre-anoxic tank (1), and a second normally closed solenoid valve (2.2) is installed on the sludge return pipe (6.2) near the second pre-anoxic tank (2).

10. The continuous flow municipal sewage treatment for preventing sludge bulking according to claim 7, wherein The first pre-anoxic tank (1) and the second pre-anoxic tank (2) operate in parallel, while the anoxic tank (3), the first aerobic tank (4), and the second aerobic tank (5) operate in series; a return pump (6.1) is installed on the sludge return pipeline (6.2).

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