Method for recovering volatile fatty acid and water by coupling two-stage membrane distillation with alkali absorption

Abstract

The invention discloses a method for recovering volatile fatty acid and water by coupling two-stage membrane distillation with alkali absorption, and relates to the field of wastewater treatment and resource recovery, a two-stage membrane distillation and alkali absorption coupling system is adopted, and the system comprises a feeding chamber, an alkali liquor chamber and a water chamber which are sequentially arranged, the feeding chamber is separated from the alkali liquor chamber through a first hydrophobic microporous membrane, and the alkali liquor chamber is separated from the water chamber through a second hydrophobic microporous membrane; according to the method for recovering volatile fatty acid and water by coupling two-stage membrane distillation with alkali absorption, through the design of two-stage membrane distillation, primary migration of VFAs and water and alkali absorption and fixation of VFAs are realized by a first-stage first hydrophobic microporous membrane, and further separation of water from alkali liquor is realized by a second-stage second hydrophobic microporous membrane, so that high-power concentration of VFAs is realized in an alkali liquor chamber; and meanwhile, high-purity recycled water is obtained in the water chamber.

Description

Technical Field

[0001] This invention relates to wastewater treatment and resource recovery technologies, specifically to a method for recovering volatile fatty acids and water using a two-stage membrane distillation coupled with alkaline absorption. Background Technology

[0002] Municipal sludge and other organic wastes produce digestate rich in volatile fatty acids (VFAs, such as acetic acid, propionic acid, and butyric acid) during anaerobic digestion. These VFAs have high economic value in the food, pharmaceutical, and chemical industries, and have enormous potential for recycling. However, traditional VFA recovery technologies, such as distillation, extraction, and adsorption, generally suffer from poor selectivity, low separation efficiency, and low product purity and concentration. In particular, efficiently separating and concentrating low-concentration VFAs from large volumes of water while simultaneously recovering water resources remains a current technological challenge. Membrane distillation (MD) technology utilizes vapor pressure difference as the mass transfer driving force, theoretically enabling the separation of volatile and non-volatile components. However, in a single membrane distillation process, it is difficult to simultaneously prevent the migration of water when recovering VFAs, resulting in limited concentration efficiency and difficulty in obtaining high-purity VFA products. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for recovering volatile fatty acids and water through two-stage membrane distillation coupled with alkaline absorption. This method can achieve efficient separation and high concentration of volatile fatty acids from wastewater, while simultaneously recovering high-quality water. It has advantages such as high recovery rate, large concentration factor, stable operation, and high product purity.

[0004] To achieve the above objectives, the present invention provides the following technical solution: a method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption, which employs a two-stage membrane distillation and alkali absorption coupled system. The system includes a feed chamber, an alkali chamber, and a water chamber arranged sequentially. The feed chamber and the alkali chamber are separated by a first hydrophobic microporous membrane, and the alkali chamber and the water chamber are separated by a second hydrophobic microporous membrane.

[0005] The method includes the following steps:

[0006] S1. Wastewater containing volatile fatty acids is introduced into the feed chamber, alkaline absorbent is introduced into the alkaline chamber, and cold water or condensate is introduced into the water chamber.

[0007] S2. Maintain the temperature of the feed chamber higher than the temperature of the alkali chamber to form a first temperature gradient; maintain the temperature of the alkali chamber higher than the temperature of the water chamber to form a second temperature gradient;

[0008] S3. Driven by the vapor pressure difference generated by the first temperature gradient, volatile fatty acid vapor and water vapor in the feed chamber pass through the first hydrophobic microporous membrane into the alkaline solution chamber; the volatile fatty acid vapor is neutralized by the alkaline absorbent and converted into non-volatile fatty acid salts and captured, while the water vapor condenses in the alkaline solution chamber.

[0009] S4. Driven by the vapor pressure difference generated by the second temperature gradient, water vapor in the alkali chamber passes through the second hydrophobic microporous membrane into the water chamber and condenses, realizing the recovery of water, while volatile fatty acid salts are trapped in the alkali chamber due to their non-volatility.

[0010] S5. Collect the concentrated volatile fatty acid salt solution from the alkali chamber, collect the recovered water from the water chamber, and discharge the treated residual liquid from the feed chamber.

[0011] Furthermore, the alkaline absorbent is a sodium hydroxide solution or a potassium hydroxide solution with a concentration of 0.3 M to 3 M.

[0012] Furthermore, the concentration of the alkaline absorbent solution is 1 M.

[0013] Furthermore, the temperature of the feed chamber is 50°C to 70°C, the temperature of the alkali solution chamber is 30°C to 60°C, and the temperature of the water chamber is 5°C to 20°C.

[0014] Furthermore, the temperature of the feed chamber is 60°C, the temperature of the alkali solution chamber is 50°C, and the temperature of the water chamber is 10°C.

[0015] Furthermore, the first hydrophobic microporous membrane and the second hydrophobic microporous membrane are independently polytetrafluoroethylene membranes, polyvinylidene fluoride membranes, or polypropylene membranes, with pore sizes ranging from 0.1 μm to 1.0 μm.

[0016] Furthermore, the wastewater containing volatile fatty acids is anaerobic digestion wastewater, fermentation wastewater, or food processing wastewater.

[0017] Furthermore, the volatile fatty acids include one or more of acetic acid, propionic acid, butyric acid, valeric acid, and hexanoic acid.

[0018] Furthermore, the method operates in either continuous or intermittent mode.

[0019] A two-stage membrane distillation and alkali absorption coupled system for implementing the above-described method includes:

[0020] The shell is internally divided into a feed chamber, an alkali chamber, and a water chamber.

[0021] A first hydrophobic microporous membrane is disposed between the feed chamber and the alkali chamber;

[0022] A second hydrophobic microporous membrane is disposed between the alkali chamber and the water chamber;

[0023] A first temperature control and circulation device is used to maintain and circulate the wastewater in the feed chamber;

[0024] The second temperature control and circulation device is used to maintain and circulate the alkaline absorbent in the alkaline solution chamber;

[0025] The third temperature control and circulation device is used to maintain and circulate the cold water or condensate in the water chamber;

[0026] The first temperature control and circulation device is configured to make the temperature of the feed chamber higher than that of the alkali solution chamber, and the third temperature control and circulation device is configured to make the temperature of the water chamber lower than that of the alkali solution chamber.

[0027] Compared with existing technologies, the method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption provided by the present invention has the following beneficial effects:

[0028] Through a two-stage membrane distillation design, the first stage uses a first hydrophobic microporous membrane to achieve the initial migration of VFAs and water and the alkali absorption and fixation of VFAs. The second stage uses a second hydrophobic microporous membrane to achieve further separation of water from the alkali solution, thereby achieving high concentration of VFAs in the alkali solution chamber (concentration factor can reach more than 25 times, and up to 40 times under preferred conditions), while obtaining high-purity recycled water in the water chamber.

[0029] The alkaline absorbent can neutralize the migrating VFAs in real time, convert them into salts and enrich them, effectively preventing the migration of VFAs into the water chamber, ensuring a high recovery rate (up to 80% or more) and high selectivity of VFAs, and making VFAs virtually undetectable in the water chamber.

[0030] The system can recover more than 70% of the water while concentrating VFAs, achieving the dual goals of wastewater reduction and resource recovery.

[0031] When treating actual anaerobic digestion wastewater, the system can operate stably for a long time, demonstrating good anti-pollution ability and operational stability.

[0032] By adjusting parameters such as alkali concentration and temperature of each chamber, the concentration factor, recovery rate and water recovery rate of VFAs can be optimized to adapt to different water qualities and treatment requirements. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0034] Figure 1 This is a schematic diagram of the membrane distillation-alkali absorption coupling system (MD-AA system) used in an embodiment of the present invention, wherein... Figure 1 In diagram 'a', the structure of the membrane distillation-alkali absorption coupling (MD-AA) system is shown. Figure 1 In the image, b is a magnified view of a portion of the first and second hydrophobic microporous membranes. Figure 1 In the middle, c represents the principle of material migration and transformation.

[0035] Figure 2 This is a performance graph of the MD-AA system running for 22 hours in the example, where... Figure 2 In the figure, 'a' represents the change in solution volume and water recovery rate in each chamber. Figure 2 In the figure, b represents the change in VFA concentration and concentration rate. Figure 2 In the figure, c represents the change in total mass and recovery rate of VFAs. Figure 2 In the figure, d represents the concentration rate and recovery rate of each VFA component.

[0036] Figure 3 The graphs show the system performance under different operating conditions. Figure 3 In the figure, 'a' represents the recovery rates of water and VFAs at different alkaline temperatures. Figure 3 In the figure, b represents the concentration and concentration rate of VFAs at different alkaline temperatures. Figure 3 In this context, 'c' represents the recovery rates of water and VFAs at different alkali concentrations. Figure 3 In the figure, d represents the concentration of VFAs and the concentration rate at different alkali concentrations. Figure 3 In the figure, e represents the composition ratio of the recovered VFAs. Figure 3 In this context, f represents the concentration factor of each VFA component.

[0037] Figure 4 This is a long-term operating performance diagram of the system when treating actual anaerobic digestion wastewater, in which... Figure 4 In the diagram, 'a' represents a wastewater treatment process flow chart. Figure 4 In the figure, b represents the changes in water recovery rate and VFAs removal rate. Figure 4 In the figure, c represents the change in VFA concentration and concentration factor. Figure 4 In the figure, d represents the concentration of each recovered VFA component and the total recovery rate. Detailed Implementation

[0038] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0039] As attached Figure 1 To be continued Figure 4 As shown:

[0040] Example 1:

[0041] This invention provides a method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption. The method employs a two-stage membrane distillation and alkali absorption coupled system, which includes a feed chamber, an alkali chamber and a water chamber arranged sequentially. The feed chamber and the alkali chamber are separated by a first hydrophobic microporous membrane, and the alkali chamber and the water chamber are separated by a second hydrophobic microporous membrane.

[0042] The method includes the following steps:

[0043] S1. Wastewater containing volatile fatty acids is fed into the feed chamber, alkaline absorbent is fed into the alkali chamber, and cold water or condensate is fed into the water chamber.

[0044] S2. Maintain the temperature of the feed chamber higher than the temperature of the alkali chamber to form the first temperature gradient; maintain the temperature of the alkali chamber higher than the temperature of the water chamber to form the second temperature gradient.

[0045] S3. Driven by the vapor pressure difference generated by the first temperature gradient, volatile fatty acid vapor and water vapor in the feed chamber pass through the first hydrophobic microporous membrane into the alkaline solution chamber; the volatile fatty acid vapor is neutralized by the alkaline absorbent and converted into non-volatile fatty acid salts and captured, while the water vapor condenses in the alkaline solution chamber.

[0046] S4. Driven by the vapor pressure difference generated by the second temperature gradient, water vapor in the alkali chamber passes through the second hydrophobic microporous membrane into the water chamber and condenses, realizing the recovery of water, while volatile fatty acid salts are retained in the alkali chamber due to their non-volatility.

[0047] S5. Collect the concentrated volatile fatty acid salt solution from the alkali chamber, collect the recovered water from the water chamber, and discharge the treated residual liquid from the feed chamber.

[0048] Furthermore, the alkaline absorbent is a sodium hydroxide solution or a potassium hydroxide solution with a concentration of 0.3 M to 3 M.

[0049] Furthermore, the concentration of the alkaline absorption solution is 1 M.

[0050] Furthermore, the temperature of the feed chamber is 50°C to 70°C, the temperature of the alkali chamber is 30°C to 60°C, and the temperature of the water chamber is 5°C to 20°C.

[0051] Furthermore, the temperature of the feed chamber is 60°C, the temperature of the alkali solution chamber is 50°C, and the temperature of the water chamber is 10°C.

[0052] Furthermore, the first and second hydrophobic microporous membranes are independently polytetrafluoroethylene membranes, polyvinylidene fluoride membranes, or polypropylene membranes, with pore sizes ranging from 0.1 μm to 1.0 μm.

[0053] Furthermore, wastewater containing volatile fatty acids is anaerobic digestion wastewater, fermentation wastewater, or food processing wastewater.

[0054] Volatile fatty acids include one or more of acetic acid, propionic acid, butyric acid, valeric acid, and hexanoic acid.

[0055] The method runs in either continuous or intermittent mode.

[0056] A two-stage membrane distillation and alkali absorption coupled system for implementing the above method includes:

[0057] The shell is internally divided into a feed chamber, an alkali chamber, and a water chamber.

[0058] The first hydrophobic microporous membrane is disposed between the feed chamber and the alkali chamber;

[0059] The second hydrophobic microporous membrane is disposed between the alkali chamber and the water chamber;

[0060] The first temperature control and circulation device is used to maintain and circulate the wastewater in the feed chamber;

[0061] The second temperature control and circulation device is used to maintain and circulate the alkaline absorbent in the alkaline solution chamber;

[0062] The third temperature control and circulation device is used to maintain and circulate the cold water or condensate in the water chamber;

[0063] The first temperature control and circulation device is configured to make the temperature of the feed chamber higher than that of the alkali chamber, and the third temperature control and circulation device is configured to make the temperature of the water chamber lower than that of the alkali chamber.

[0064] System construction and basic performance testing

[0065] like Figure 1 As shown, a two-stage membrane distillation-alkali absorption coupled (MD-AA) reactor was constructed. The reactor consists of a feed chamber, an alkali chamber, and a water chamber, which are connected by two membranes with an effective area of ​​9 cm². 2 The three chambers are separated by a 3 cm × 3 cm hydrophobic polytetrafluoroethylene (PTFE) membrane (0.45 μm pore size). Each chamber is connected to an independent circulation tank via a peristaltic pump.

[0066] Prepare a simulated anaerobic digestion wastewater as feed: containing acetic acid (AA) 4000 mg / L, propionic acid (PA) 2000 mg / L, butyric acid (BA) 1000 mg / L, valeric acid (VA) 800 mg / L, and hexanoic acid (HA) 600 mg / L. Adjust the initial pH to 3.0 ± 0.3 with hydrochloric acid or sodium hydroxide, with a total volume of 250 mL. Inject 100 mL of 3 M sodium hydroxide (NaOH) solution into the alkali chamber. Inject 100 mL of deionized water into the water chamber.

[0067] Operating conditions: The inlet temperature of the feed chamber is controlled at 60℃, the inlet temperature of the water chamber at 10℃, and the inlet temperature of the alkali chamber at 50℃. The circulation flow rate of all three liquids is 75 mL / min. The system operates in batch mode for 22 hours.

[0068] The results are as follows Figure 2 As shown: After 22 hours of operation, the feed volume decreased from 250 mL to 103 mL, the water chamber volume increased from 100 mL to 249 mL, the alkali chamber volume remained relatively stable at around 100 mL, and the water recovery rate reached 60%. Figure 2 a). The total concentration of VFAs in the feed solution continuously decreased from 8.3 g / L to 1.6 g / L, while the concentration of VFAs in the alkali solution rapidly increased from 0 g / L to 16.3 g / L, with a concentration factor of approximately 10 times. Figure 2 (b) The total recovery of VFAs reached 76% after 22 hours ( Figure 2 (c) All VFA components (AA, PA, BA, VA, HA) exhibited highly efficient recovery and concentration, and no VFAs were detected in the water chamber. Figure 2 (d). The results show that the system can effectively separate and concentrate VFAs while recovering water.

[0069] Example 2: Study on the Influence of Key Operating Parameters

[0070] 2.1 Effect of alkaline solution temperature

[0071] With the feed temperature fixed at 60℃, the water chamber temperature at 10℃, and the concentration of the alkali solution (3 M NaOH) constant, experiments were conducted with the inlet temperature of the alkali solution chamber set at 60℃, 50℃, 40℃, and 30℃ respectively.

[0072] The results are as follows Figure 3 a and Figure 3 As shown in Figure b, as the alkali solution temperature decreases (i.e., the temperature difference in the first stage increases and the temperature difference in the second stage decreases), the VFAs recovery rate increases (from approximately 60% to approximately 80%). This is because the driving force of mass transfer in the first stage is enhanced. However, water recovery initially increases and then decreases, reaching its maximum at an alkali solution temperature of 50°C (approximately 60%). Simultaneously, the VFAs concentration rate (CR) also reaches its maximum at 50°C (approximately 10). This is because there is an optimal value for the second-stage temperature difference (between the alkali solution and the water chamber) (40°C in this embodiment); a difference that is too small or too large is detrimental to efficient water migration and the stability of the alkali solution volume.

[0073] 2.2 Effect of Alkali Concentration

[0074] With the feed temperature fixed at 60℃, the alkali solution temperature at 50℃, and the water chamber temperature at 10℃, experiments were conducted with alkali solution (NaOH) concentrations of 0.1 M, 0.3 M, 1 M, and 3 M, respectively.

[0075] The results are as follows Figure 3 c and Figure 3 As shown in Figure d, the alkali concentration significantly affects the system performance. When the alkali concentration is only 0.1 M, the neutralization capacity is weak, and the high osmotic pressure of the alkali solution leads to an excessively large second-stage water flux, causing a sharp decrease in the volume of the alkali chamber, making the experiment unsustainable, and resulting in the lowest water recovery rate and VFAs recovery rate. As the alkali concentration increases from 0.3 M to 3 M, the VFAs recovery rate continuously improves, exceeding 80% at both 1 M and 3 M. However, the concentration factor of VFAs in the alkali solution shows a trend of first increasing and then decreasing, reaching a maximum of approximately 40 times (corresponding to a concentration of approximately 27.4 g / L) at a 1 M alkali concentration. Figure 3 (d). This is because while a high alkali concentration (3 M) promotes the absorption of VFAs (high flux), it also increases the volume of the alkali chamber due to its strong hygroscopicity, thus diluting the VFA salt concentration. Therefore, considering the VFA recovery rate, concentration factor, and water recovery rate, a 1 M NaOH concentration exhibits a better balance in this embodiment.

[0076] Example 3: Actual wastewater treatment and long-term operational stability

[0077] The feed material was actual effluent from an anaerobic digester plant belonging to a drainage group in Beijing, after which suspended solids were removed by centrifugation (COD approximately 12.8 g / L). Long-term batch operation tests were conducted using an MD-AA system, with the feed (250 mL) replaced every 24 hours and the alkali solution (100 mL, 1 M NaOH) replaced every 96 hours. Fixed temperature conditions were maintained: feed 60℃, alkali solution 50℃, and water chamber 10℃.

[0078] The results are as follows Figure 4 As shown, the system exhibited good stability during continuous operation for 192 hours (8 cycles). Within each 24-hour cycle, the water chamber volume steadily increased, the feed volume decreased, and the alkali solution volume remained relatively stable (see Figure S23). Both the total VFA removal rate and water recovery rate gradually increased with operating time within each cycle. Figure 4 b). After 96 hours (4 cycles) of operation, the total concentration of VFAs in the recovered alkali solution exceeded 100 g / L, with a concentration factor of over 20 times. Figure 4 c in the middle Figure 4 (d). All VFA components (AA, PA, BA, HA, VA) were effectively recovered and concentrated. Figure 4(d) The VFAs recovery rate reaches 80% by the end of the first 24-hour cycle, and then fluctuates periodically due to the gradual saturation of the alkali solution. However, the system can maintain efficient operation by regularly replacing the alkali solution. Throughout the process, the quality of the recovered water is good, and no VFAs are detected. This indicates that the method of the present invention can still achieve high-purity, high-concentration, and high-recovery-rate VFAs recovery and efficient water reuse when treating actual wastewater with complex composition.

[0079] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkaline absorption, characterized in that, A dual-stage membrane distillation and alkali absorption coupling system is adopted. The system includes a feed chamber, an alkali chamber and a water chamber arranged in sequence. The feed chamber and the alkali chamber are separated by a first hydrophobic microporous membrane, and the alkali chamber and the water chamber are separated by a second hydrophobic microporous membrane. The method includes the following steps: S1. Wastewater containing volatile fatty acids is introduced into the feed chamber, alkaline absorbent is introduced into the alkaline chamber, and cold water or condensate is introduced into the water chamber. S2. Maintain the temperature of the feed chamber higher than the temperature of the alkali chamber to form a first temperature gradient; maintain the temperature of the alkali chamber higher than the temperature of the water chamber to form a second temperature gradient; S3. Driven by the vapor pressure difference generated by the first temperature gradient, volatile fatty acid vapor and water vapor in the feed chamber pass through the first hydrophobic microporous membrane into the alkaline solution chamber; the volatile fatty acid vapor is neutralized by the alkaline absorbent and converted into non-volatile fatty acid salts and captured, while the water vapor condenses in the alkaline solution chamber. S4. Driven by the vapor pressure difference generated by the second temperature gradient, water vapor in the alkali chamber passes through the second hydrophobic microporous membrane into the water chamber and condenses, realizing the recovery of water, while volatile fatty acid salts are trapped in the alkali chamber due to their non-volatility. S5. Collect the concentrated volatile fatty acid salt solution from the alkali chamber, collect the recovered water from the water chamber, and discharge the treated residual liquid from the feed chamber.

2. The method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption according to claim 1, characterized in that, The alkaline absorbent is a sodium hydroxide solution or a potassium hydroxide solution with a concentration of 0.3 M to 3 M.

3. The method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption according to claim 2, characterized in that, The concentration of the alkaline absorption solution is 1 M.

4. The method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption according to claim 1, characterized in that, The temperature of the feed chamber is 50°C to 70°C, the temperature of the alkali chamber is 30°C to 60°C, and the temperature of the water chamber is 5°C to 20°C.

5. The method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption according to claim 4, characterized in that, The temperature of the feed chamber is 60°C, the temperature of the alkali solution chamber is 50°C, and the temperature of the water chamber is 10°C.

6. The method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption according to claim 1, characterized in that, The first hydrophobic microporous membrane and the second hydrophobic microporous membrane are independently polytetrafluoroethylene membranes, polyvinylidene fluoride membranes, or polypropylene membranes, with pore sizes ranging from 0.1 μm to 1.0 μm.

7. The method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption according to claim 1, characterized in that, The wastewater containing volatile fatty acids is anaerobic digestion wastewater, fermentation wastewater, or food processing wastewater.

8. The method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption according to claim 7, characterized in that, The volatile fatty acids include one or more of acetic acid, propionic acid, butyric acid, valeric acid, and hexanoic acid.

9. The method for recovering volatile fatty acids and water by two-stage membrane distillation coupled with alkali absorption according to claim 1, characterized in that, The method operates in either continuous or intermittent mode.

10. A two-stage membrane distillation and alkali absorption coupling system for implementing the method according to any one of claims 1-9, characterized in that, include: The shell is internally divided into a feed chamber, an alkali chamber, and a water chamber. A first hydrophobic microporous membrane is disposed between the feed chamber and the alkali chamber; A second hydrophobic microporous membrane is disposed between the alkali chamber and the water chamber; A first temperature control and circulation device is used to maintain and circulate the wastewater in the feed chamber; The second temperature control and circulation device is used to maintain and circulate the alkaline absorbent in the alkaline solution chamber; The third temperature control and circulation device is used to maintain and circulate the cold water or condensate in the water chamber; The first temperature control and circulation device is configured to make the temperature of the feed chamber higher than that of the alkali solution chamber, and the third temperature control and circulation device is configured to make the temperature of the water chamber lower than that of the alkali solution chamber.

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