1. What should I do when the filter no longer produces filtrate?
A drop in filter performance, and thereby reduced membrane permeability, can have a number of causes. So-called “scaling” or “fouling” can occur during ongoing operation. Both of these conditions can be reversed with basic cleaning and eliminated with chemical cleaning.
The membrane is delivered with a preservative impregnation of humectant. This makes the membrane storable in its delivered condition. Once the filter has been put into service, the glycerine washes out of the membrane and if, thereafter, the membrane is allowed to dry out it will lead to the membrane’s irreversible destruction. A drying out of the membrane after it has been put into service should be avoided.
2. What types of membrane fouling and blockage are there?
Fouling is a compressible coating on the membrane or in membrane pores which is caused by deposition and/or adsorption of organic and/or colloidal substances. The special use of membranes in MicroClear® filters, where pores are much smaller than micro-organisms, prevents the membrane from internal fouling. The potential for fouling is thus limited to the membrane’s surface and is therefore reversible. So-called “bio-fouling” is accelerated by EPS (extra-cellular polymeric substances). Membrane bio-reactors must be laid out such that EPS formations are suppressed. Please contact newterra GmbH about this, we will be glad to help you with the layout.
The cause of filter cake formation on the membrane must be eliminated at its source. Please contact the plant manufacturer or the filter producer. For example, filter cake can occur when aeration is shut off during filtration or when recommended filtration pressure is exceeded.
Scaling is the term used to describe the precipitation of inorganic water ingredients on and in the membrane. Scaling on and in MicroClear® filters can be reversed. Scaling, primarily caused by hardness ingredients like calcium and magnesium, can be eliminated by chemical cleaning in an acidic environment. Scaling generally occurs when water hardness is very high.
3. What is a basic cleaning?
A basic cleaning is the restoration of the “like new” condition by way of an established cleaning procedure. In three contiguously sequential steps, filters are basically cleaned by completely immersing them in different cleaning solutions. Typically the sequence is:
- a sodium hypochlorite solution 500 ppm (max. 1000 ppm calculated as active chlorine), for minimum of 8 hour
- citric acid, pH 2 for minimum of 4 hours In individual cases other membrane cleaners are also used (please contact Supratec ).
For basic cleaning, the filters are either lifted out of the activating basin (internal arrangement) or the membrane tank is completely flooded with the cleaning solution (external arrangement). For small residential plants, the onsite MicroClear® filter can be replaced with a clean filter and the removed filter is then thoroughly cleaned in the factory. If an operational problem occurs that has formed filter cake, mechanical cleaning is necessary prior to basic cleaning. The membranes built into our MicroClear® filters are accessible for mechanical cleaning. This is accomplished by fanning out the filter at the side opposite the permeate collector trough. The membrane can then be cleaned with the help of a very flexible, soft rubber spatula or simply a rubber glove. If the filter cake should prove to be hard to reach then even a flexible, soft rubber scraper can be used for cleaning. Wash off the membrane with clear water afterwards then perform a basic cleaning as described above.
4. What measures can be taken to minimise fouling?
- aeration cleaning
MicroClear® filters are continuously cleaned during operation as a consequence of positive aeration. The ventilators used produce a defined bubble size that, during their rise through the filter, create shear forces on the membrane. These shear forces remove particles from the membrane’s surface and carry them upward out of the filter.
The shear forces produced by the air bubbles can be increased with filtration pauses. The aeration cleaning phenomena explained above is reinforced during a pause since particles on the membrane’s surface are no longer additionally held in place by the normal suction pressure present during filtration. The typical operational cycle is 9 minutes of filtration and a 1 minute pause.
- backflushing without chemicals
Routine backflushing produces a cleaning effect for minimally contaminated filters. A membrane already blocked by fouling or scaling will generally only backflush through remaining unblocked membrane surface areas. It is not possible to alleviate fouling or scaling with only a pure permeate backflush process. Backflush with a chemically enriched permeate to remove blockage due to scaling (i.e. deposits of inorganic molecules, such as water hardeners, on and in the membrane).
How strong should the flow be for a backflush?
Filters may only be backflushed at a maximum pressure of 100 mbar. Backflushing at higher pressures can irreversibly damage the membrane’s functionality. The throughput for a backflush process is not decisive but rather the maximum backflush pressure. To simplify routine backflushing, a CIP (cleaning in place) tank can be connected to the MBR tanks. Please note that the maximum difference in height between the maximum CIP tank water level and the minimum MBR water level may never exceed 700 mm. This will prevent backflush pressure in excess of the 100 mbar maximum pressure limit. Backflush volumes should be set according to the table below.
- backflushing with chemicals
The permeate to be used for backflushing is collected in the CIP tank and, if necessary, enriched with chemicals there. The manufacturer is to be consulted about the concentration and reaction time for any chemicals used. The maximum pressure limit of 100 mbar is also applicable for backflushing with chemicals. Please observe the regulations for occupational safety when handling chemicals.
5. Does aeration cleaning remain turned on during a basic cleaning?
Aeration cleaning is temporarily turned off during basic cleaning as otherwise foaming can occur. Subject to consideration for occupational safety, aeration can be turned on for brief intervals if this is deemed necessary to achieve better circulation in the tank.
6. How often must the filter be thoroughly cleaned?
Basic cleaning of the filter should be done according to an established schedule. With consideration for water parameters and desired throughput, routine cleaning is to be carried out from the outset. This will ensure the performance of the plant over the long term. It is not reasonable to wait until the plant’s performance has dropped off to a minimum level. When permeability (L/m²hbar) drops below a value of 50, a basic chemical (and perhaps also a mechanical) cleaning of the filter is unavoidable.
At a design flux of 30 l/m²h for the MicroClear® filter, an average of 2 chemical cleanings per year will be necessary (depending on type of sewage and system design). If the plant is designed for half flux, the time between cleaning procedures is doubled. Thus a plant designed for operation at 15 L/m²h can indeed stay within design parameters with only one cleaning procedure per year.
7. Must the filter frame be removed from the MBR for basic cleaning?
If MBR filter frames are submerged in the biologic stage then they must be removed too. In external MBR arrangements, the filters can be treated in the MBR tank. Lifting the filters out of a tank or emptying a tank should take at least 5 min. For each single filter layer.
- MB2-series: 10 min.
- MB3- series 15 min.
- MB4- series 20 min. (Module must be seperated in to 2 parts)
Non observance will lead to a damage of the filters, because of exceeding the maximum backflush pressure.
8. What does maintenance cost?
The cost of maintaining a MicroClear® plant is easily estimated and surprisingly low. Since basic cleaning of the filter only takes place once or twice a year, and then with only low concentrations of conventional chemicals, the overhead (for example) of a plant with a throughput of 100 m³ per day is less than 100 Euro per year. Cleaning is usually completed within one day.
9. How big is the design flux with respect to the cleaning interval? Should chemical cleaning be made dependent on flux?
The graphic below illustrates how flux is influenced by the number of chemical cleanings.
After conclusion of the break-in phase, a plant laid out according to MicroClear® instructions achieves an almost continuous throughput that remains constant over a long period. If the membrane’s permeability drops below 50 l/m²hbar after a certain operational period, a chemical cleaning is necessary.
10. What happens when basic cleaning is not successful?
When basic cleaning followed by chemical cleaning fails to fully restore the filter’s original permeability, the cleaning process is to be repeated.
11. What capabilities does the membrane have?
The membrane built into MicroClear® MBR filters has a mean pore opening of about 0,03 μm. Bacteria, fungi, algae parasites and viruses are factors larger than this and therefore cannot pass through the membrane. The investigations of an independent institute have shown that bacteria are completely held back and the quantity of viruses passing through the membrane are reduced by 6 logarithmic decrements. Salts and small molecules pass through the membrane. Colloids and particles are held back very well.
12. Can MicroClear® reduce dissolved biochemical oxygen demand (BOD) and chemical oxygen demand (COD)?
Through filtration, the MicroClear® filter prevents the passage of particles that are analytically detected as COD or BOD. Dissolved freight detected as BOD or COD cannot be held back by the membrane. The principle of membrane bio-reactors relies on reducing COD and BOD detected organic freight through intensely fortified biological degrading processes. Dissolved COD and BOD should therefore be handled by the biological process.
13. Is the filter suitable for saline water?
Fundamentally, dissolved salts do not present a problem for the membrane. Salts pass through the membrane without a problem. However, large salt concentrations can impair the biological processes in the reactor. Potential influences should be discussed with the manufacturer.
14. How high is energy consumption? Energy consumption in Nm³/m² h?
Energy requirements are strongly dependent on the installed aggregates. Furthermore, the selected water level and other factors also effect energy consumption such that a generalised statement about energy consumption is difficult to make. Depending on efficiency and selected aggregates, air requirements can amount to about 0,28 Nm³/m² h.
15. What replacement parts are needed over 10 years?
Depending on application, plant size and water ingredients, the service life of filters can be up to 10 years. The aggregates used are of high quality and comply with German industrial standards.
16. How is trans-membrane pressure (TMP) calculated? Where is it measured?
Trans-membrane pressure (TMP) is measured at the permeate outlet and is subject to consideration for the height difference between the pressure measurement and the water level in the membrane tank. Since the water column in the MBR corresponds opposite to a permeate water column in the filters and lines, TMP is exclusively dependent on developed negative pressure caused by the permeate outlet and not whether or not the filter is positioned higher or lower in the membrane tank.
17. How are leaks detected? Is this possible with online measurement?
Online integrity testing of the membrane is only possible with particle counters and a great deal of overhead. Any measurable turbidity on the clear-water side presumes correspondingly substantial damage to the membrane. Small membrane impairments can therefore only be economically detected with a micro-biological test, for example, through evidence of pathogenic germs on the clear-water side.
18. Can membrane modules be placed directly next to one another?
A filter frame in the MBR must be surrounded by at least 500 mm of free-flowing water space within the reactor tank. Only then is it possible for necessary turbulence to develop in the reactor.
19. What is the minimum water depth above the filter?
A filter should always be covered with at least 200 mm of water. This precludes the possibility a filter could dry out and also ensures adequate turbulence for particle removal from the filter.
20. Can membranes be installed in basins with a depth of 6 m?
Basin depth has no influence on TMP (trans-membrane pressure) but does influence the amount of energy required to aerate the MBR filter. Furthermore, appropriate equipment should be available to remove filters for cleaning.
21. Why do MicroClear® filters achieve higher throughput than competitive products?
MicroClear® filters were developed with the goal of avoiding pressure losses within the filter and to equalise those pressure losses that do occur. MicroClear® filters are very different from competitive products in that MicroClear® filters have numerous permeate outlets on the filter’s face side. Furthermore, MicroClear® has chosen to forego additional material layers within the filter. As a consequence of these measures, and the characteristics of the filter plate, the filter’s entire surface area is utilised and the degassing of the entire filter is assured. The minimised pressure loss afforded by the plate design makes the difference.
22. What industrial applications are applicable to the MicroClear® filter system?
Basically the MicroClear® filter can be employed in all biological water clarification processes, including industrial applications, where readily degradable freight is to be eliminated. However, the MicroClear® filter has also proven itself in applications where contaminants are difficult to degrade, for example, the clarification of landfill seepage water. The fields of application for MicroClear® filters are very diverse and their full scope is not yet known. Surprise us with your ideas!
23. What does Supratec offer in addition to the filter?
We are able to offer you complete plug-and-play solutions for industrial and municipal plants. This typically includes the respective ventilation equipment. For compact and container plants we offer the full range of our know-how from over 15 years of MBR plant construction and operation by supplying one of our controllers for your plant.
24. What can the treated water be used for?
Grey-water and wastewater treated with MicroClear® technology is already being used around the world for toilet flushing, landscape irrigation/watering, vehicle washing and as general service water. Grey-water and wastewater treated by MicroClear® technology meets bathing water quality standards and offers further advantages. For example, odour generation and dye colouring in installations or on ceramics are practically eliminated because detectable nutrients are virtually nonexistent. The treated water’s quality is such that repeat germination is avoided.
25. How does gravity flow function?
Through the “principle of communicating tubes” a difference in height is put to use. This principle, in combination with appropriate outlet tube routing for the MBR beneath the water level, produces a water column which corresponds to the negative pressure for the permeate outlet. This is the simplest way to generate filtration pressure - and do it without maintenance or energy overhead.
26. How is the permeate pump controlled?
The pump outlet is operated by pressure control. The use of permeate pumps puts an even filtration pressure on the membrane. Depending on the state of the membrane, the filter delivers a corresponding throughput. Performance capability is therefore simple to comprehend and any cleaning work which becomes necessary can be scheduled in advance. If a blockage should occur, it is registered by virtue of decreasing performance and reported to the plant’s operating authority. Increasing filtration pressure beyond certain limits will intensify the cause of a blockage and tends to make the problem more critical.
27. How can permeate removal and backflushing be combined?
Permeate can routed through a backflush tank (CIP, cleaning in place). If the maximum level of this tank is arranged at a maximum of 700 mm above the minimum possible MBR water level, then backflushing can be accomplished by gravitation. Optionally, a chemical can be added during the backflush process.