Italiano
1. The Importance of Membrane Diffuser Aeration System Maintenance
Maintenance is crucial for the efficient and long-lasting performance of every aeration systems: appropriate maintenance extends the system’s lifespan and allows you to reduce energy costs. Regularly record the airflow, absorbed power, and operating pressure of the compressor(s) to monitor system performance. The supply pressure of the diffusers can vary greatly over time, depending on the type of treatment and the diffusers’ working point. As the system ages, the pressure follows a trend as shown in Table 1:
| Years of operation | % Increase in pressure loss due to aging | Diffuser pressure in cm H2O |
| 0 | 0% | 35,0 |
| 1 | 17% | 41,0 |
| 2 | 31% | 45,9 |
| 3 | 39% | 48,8 |
| 4 | 46% | 51,1 |
| 5 | 53% | 53,5 |
Table 1
The diffuser pressure (Pd) value is an average for the entire grid, obtained by subtracting the pressure head above the diffusers (Hw) from the manometric pressure (Pm), measured on the dropleg.
As shown in Figure 1 Pd= Pm-Hw
Figure 1
Please keep in mind that
- The diffuser pressure value is an average pressure for the grid fed by the dropleg, not the individual diffuser pressure;
- Distributed and concentrated losses in the conduits are ignored. These values are negligible if the pipes have been correctly sized.
- The losses measured during the system’s operation will be the sum of the losses due to aging, as indicated in Table 1, and those induced by sludge or salt deposits on the membranes. The latter can be reduced or eliminated with maintenance, but the previous are intrinsic to the diffuser materials. For instance, if after four years we measure a diffuser pressure of 60 cm H2O, maintenance can bring the pressure back to around 51 cm H2O but not below the pressure loss caused by aging.
2. Routine Inspection Activities
Frequency:
From every week to a month;
Activity:
Check for the leakage of liquid from the purge by opening the ball valve: if liquid is clear, it means that there are no leaks in the network, and what you see is moisture. A liquid of a color similar to that wastewater treated may indicate a leakage. If you intend to perform maintenance, do not stop the compressors to avoid further liquid entry until the tank is completely empty.
If the plant is equipped with the WTE “e-Purge”, you will have automatic turbidity control. An alarm will alert you to the presence of sludge in the liquid, a sign of a possible liquid entry into the network.
Perform a visual check of the basin surface to make sure that large air bubbles, a clear sign of a leak in the bottom network, do not appear.
3. Minimum periodic maintenance
Frequency:
Every 4-6 months or when the pressure has increased by 10% compared to the last pressure recorded at constant air flow.
Activity:
Perform the so-called “air bump” or “air shock”, a simple method to clean the surface of the diffusers: leave the valve of a grid dropleg open (assuming you more than one grid) and all the others closed. The diffusers of the running grid should not exceed two times the diffusers maximum admissible airflow. By doing this for a quarter of hour, you should note a significant pressure drop.
If you have a descaling liquid injection system such as the WTE “AirTech Cleaner”, in Figure 2 enter the descaler nebulized into the grid. Pay attention that this is a preventive maintenance: you must clean the membranes perforations from any deposits before they are completely clogged.
Figure 2
4. Main maintenance
Frequency
- At least once every two years;
- When the pressure increase is 50% higher than the expected pressure at constant air flow;
- If large bubbles appear on the surface of the tank;
Activity:
Empty the tank, making sure to keep the blowers running to avoid sludge accumulation on the diffuser membranes.
During the emptying, when the liquid level reaches the membrane surface, you can check and mark any leaks from the grid or diffusers on a sheet or take a picture.
Once the sludge has been removed from the grid and diffusers, check the condition of the membranes and other system components.
Clean the membrane surfaces mechanically with a pressure washer or brushes. It can be helpful to use a 5% solution of formic acid or acetic acid to remove calcium carbonate deposits or any oxides. A hydrochloric acid solution is equally effective: however, be careful with stainless steel components that are corroded by this acid.
With the grids provided by WTE, it is easy to clean the inside of the laterals: just remove the closing cap (see Figure 3) located at the end of each tube and introduce industrial water or tap water into the network.
Figure 3
In the unlucky event that you are not using a WTE aeration system, remove the last 2-3 diffusers from the diffuser tubes and fill the grid with water until it comes out of the holes left free by the diffusers you have removed, as shown in Figure 4.
Figura 4
It is also possible to open the end cap of the laterals and insert a flexible hose ending with a “poly pig”. Be careful that the water thrust does not damage the pipes, especially if they are in PVC and have been in use for decades.
Warning
Before working in the tank, decontaminate it to ensure the safety of those who will enter.
Remember that for safety reasons you must consider at least three operators: two at the bottom of the basin and one outside. There must be a system available for the recovery of a worker from the bottom of the tank, in the event of an accident or illness.
If your grid is made of PVC and is more than ten years old, it will be extremely fragile and can easily break even under minimal stress. Make arrangements with the grid supplier to have any replacement parts of the grid available.
5. Replacement of Diffusers
Frequency:
- Every 4 to 7 years, but we have cases where replacement was carried out after 10 years;
- When the pressure losses exceed 70% of the expected pressure at a defined flow;
- If the compressor enters in a “pumping” phase;
Activity:
Do not stock diffusers as if they were just any spare parts: the technology with which the membranes are made improves year by year, and the rubber is subject to natural aging even when the diffuser is not in operation. For this purpose, at WTE we always keep replacement diffusers or membranes in stock.
Replace all the diffusers or only the membranes of a section or an entire grid, and not just some within a grid as this affects the distribution of air flows between the diffusers. The air will tend to exit from the new diffusers which will have a lower pressure drop than the used ones.
If the connection between the diffuser and the tube is made with a “grommet” or a “patch”, check the tightness of it. If you are not sure about the tightness, wrap the male threaded connector of the diffuser with Teflon® tape. At WTE we do not use “grommets” or “patches”, but a threaded connection solvent welded to the lateral pipe and with the seat to accommodate an OR ring, thus avoiding the use of Teflon® film, and since it is a connection made with rigid PVC it is not subject to problems related to the aging of the elastomers with which “patches” or “grommets” are made.
We finally believe that a sensible public utilities manager uses products that can be replaced with others.
Having, for example, a ¾” female connection on the lateral guarantees the possibility of being able to use various types of disk diffusers in the future. A “proprietary” connection exclusively benefits the diffuser manufacturer as it binds the operator of the wwtp, for the entire life of the aeration system, to a single supplier.
Attention
WTE designs and supplies sustainable diffusers, in which the membrane can always be separated from the plastic that makes up the body of the diffuser. If the diffuser you are using contains PTFE a material included among those of the PFAS family, sconsult local authorities for their correct disposal.
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