Backwashing involves pausing the filtration process and pumping an external washing solution (water or cleaning solution) from the permeate side to the retentate side.

Every membrane bundle is tested. It means that we test potting integrity (quality) as well. This is a rather tricky task as e.g. standard P60 membrane bundle has roughly one running kilometer of fiber and the test must identify the largest pore or to make sure that the largest pore diameter is below prescribed value. The bubble point test is used.

The basic idea is simple - The evaluation of the minimal differential pressure at which a steady stream of air bubbles goes from the largest pore of a hollow fiber membrane. The bubble point test is a well known and trusted method. For details see e.g. [1]

However pores generated by dry stretching have slot like form

The corresponding theory used to evaluate, for details see e.g. [2], is based on a significant simplification namely that the pore shape is round. There is no a simple relation between real slot like pore and the bubble points. The bubble point results are affected by e.g. surface roughness, which could be comparable with the dimensions of the pores etc. Moreover the minimal rejected particle diameters cannot be deduced from the bubble point or similar parameters. However, our long term experience indicates that the bubble point test results are reliable way how to quantify quality of membrane bundles and modules. However, modules are tested differently namely by pressure decay.

The relations between pore diameters and the parameters of the rejected particles is very complex. It is heavily affected e.g. by electric charge of the membrane surface and particles. In other words it means that filtration tests are inevitable.

We guarantee that the bubble point of a membrane bundle will be higher than an agreed value usually 3.3 Bar. Measurements will be done using the integrity test stand in IPA. The pressure will be kept constant for 10 seconds. The pressure measurements will be done with one percent accuracy.

It is possible to increase the pressure limit from 3.3 up to 4 Bars.

[1] Rod Cousins: Membrane technology, The University of Strathclyde, Glasgow

[2] DIN 58355-2, Membrane filter, Teil 2: Prufung des Blasendrucks

Method of cleaning surfaces of vessels, pipework and membranes without dismantling.

Alkaline

Alkaline solutions remove organic fouling. Prepare a 1-2% cleaning solution (e.g. NaOH). Heat the solution up to 40 to 45°C and fill the system with it. Let the solution circulate in the operating mode for about 30 to 45 minutes without removing any filtrate. Switch off the system and let the solution sit in the system for about 15 to 20 minutes. Then restart the system and let the solution circulate for an additional 10 to 15 minutes in the normal operating mode. Drain the system and flush it with warm water for 5 to 10 minutes.

Acid

Acid solutions are used to remove inorganic fouling and for neutralising the alkaline cleaning solution. Prepare 1-2% citric (or acetic) acid solution. Heat the solution up to 40 to 45°C, feed it into the filter system and let it circulate in normal operating mode for about 25 to 30 minutes. Switch off the system and let the solution sit in the system for about 5 to 10 minutes. Then restart the system and let the solution circulate for an additional 5 to 10 minutes in normal operating mode. Drain the system and flush it with warm water for 5 to 10 minutes.

Biocides

After long shut periods, the module can be disinfected using a suitable disinfectant (ZSanitat). First, flush the system for about 5 to 10 minutes with warm water. Then, fill a 1% solution of the disinfectant into the system. let it circulate for 20 minutes. Finally, flush the system with clean water until a neutral pH value is reached.

Warning: Oxidizing agents are not recommended because may cause damage and fast ageing of membranes.

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Mechanical energy supplied by the air bubbles will loosen suspended solids on the membrane surface and prolong filtration without additional chemical cleaning. Due to the energy conservation, air pulsation is recommended. 

Modules consist of: PVCu, stainless steel 316L, George Fischer fittings, PP, PE, O-rings, glue and potting material.

When module is working, the limit part is membrane, temperature should not go above 40°C. If this is problem, contact us.

Modules can take lower temperatures (below 4°C), for example while transporting, but remember PVCu becomes brittle.

Membranes exposed to higher temperatures will degrade faster. Generally any prolonged storage in temperatures above 50°C will accelerate degradation, especially in case of surface treated membranes.  

Self-priming pump can be replaced by jet pump. This is especially advantage in small systems or when foam is created in permeate. When dealing with foam it is advised to connect cyclone before the pump, to protect the pump.

Suspended silica (SiO2)                 Filterable

Soluble silica (SiO2)                      Does not foul membranes

Microcrystalline silica (SiO2)           Causes irreversible pore plugging

Non-ionic silica (SiO2)                   pH <7 causes irreversible pore plugging pH > 7 stable or aggregates

Generally when pressure increases or permeate flow decreases by 15%.

see: !!!Chemical cleaning!!!!

Some particles (A) coming into contact with the HF surface plugs the pores. Some smaller particles (B) form a deposit inside the pores on their whole length. The particles "A" causes the reduction of active membrane surface. The particles "B" causes the restriction of the free pore volume.(Remark: the active membrane surface is the free pore area. The initial total pores area is related to the membrane area by the surface (not volume) porosity.) The fraction of "A" particles creates the cake layer in the second step and very intensively reducing the permeate flux.

The quantity of blocked pores may be reduced by backflushing, that is the pressure application from the permeate side of HF to the feed side. The applied pressure should be higher than the filtration pressure. The very rapid backflushing is termed backshocking or backpulsing and is very effective procedure to cleaning of pores walls and pores mouths.

Backflushing is method for removal fouling by pumping the permeate back to the retentate side.

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The backflushing and backshocking is done with permeate, gas, air or cleaning solutions. Application of gas phase for cleaning of hydrophobic membrane can be a risky: the gas expelling hydrophilic liquid from some pores so that these pore are dried and therefore active membrane surface area is reduced.

If the cleaning solutions are used for backshocking or backpulsing then to clean up of pores mouths and the pores wall is utilized both a chemical energy and a kinetic energy of flowing liquid. If is used the permeate or potable water then is utilized only a kinetic energy of flowing liquid and the blocking particles are as though drifted or discharged. The magnitude of the kinetic energy is dependent among others on the velocity of flowing liquid. It is useful if the starting edge of short backpressure pulses of liquid is very sharp and therefore is created the steep energetic pulse.

Such like backpulse may be generated periodically or semi continuously by using of diaphragm pressure vessel. In those pressure devices a diaphragm separates the vessel space in a gas and liquid spaces. The liquid space may be filled with the cleaning solution or water. The gas space of diaphragm vessel may be connected to source of pressure gas. The velocity of gas space gassing is high and therefore the increase in his pressure is also rapid. The diaphragm transmits a change of gas pressure very quickly and therefore the pressure the incompressible liquid in liquid space will be exploded at the time when will be increased the pressure in the gas space of diaphragm vessel. Therefore the liquid may be choppy ejected from liquid space of pressure vessel.

If the efflux outlet in liquid space of diaphramg vessel is connected to permeate side of HF module then the hydraulic pulse is transmitted inside the HF membranes. That is why the pressure inside the HF increased and walls of HF slightly expanded. The portion of compressed liquid flowed through the pores in walls of HF at the same time. Therefore the liquid momentum (eventually its chemical energy) and motion of HF walls participate in the cleaning of HF membranes.

It is more economical to keep membranes clean instead of cleaning them when they get fouled.

In order of fouling intensity:

1. Air cleaning – continual cleaning by pulsed air

2. Backwash – permeate or clean water is pressed (max pressure 300 kPa) inside membranes to defoul them

3. Chemical cleaning – possible chemical solutions: firmy Ecochem

Cleaning procedure should be tested/optimized on pilot unit, because it differs project form project.

It is important to keep in mind that:

1. The filamentary, sharp and rough particles must be removed from inflow.

2. If the flow inside of module (in between the bundles) is insufficient then suspended solids accumulate and their concentration can be several times higher the average concentration in a tank.

3. Because the area of compartment´ s shell is large the membrane module is repeated stressed by liquid streaming. Therefore if the module is immersed in the aerated or in the intensive mixed tank then must be fixed firmly to the tank structure.

4. The cleaning procedure must be optimized on an ad hoc basis. The estimation of the cleaning plan for CIP and the backwashing operations are the targets of pilot tests.

We recommend the P3 - ULTRASIL (Henkel - Ecolabe) for application in the cleaning procedure.

The membranes cleaning is based on the high energy values dissipated repeatedly within a short period of time, which results in relatively lower power consumption compared to other methods utilizing cross-flow and continuous/intermittent two-phase gas/liquid flow. The rapid increase of the velocity of the liquid to be filtered after initialization of the gas pulse leads to the effective washing off of the solids deposited on the membrane surface. The cleaning method can also be conveniently implemented if the membrane filtration process needs to be interrupted for chemical cleaning of the membrane surface (CIP).

The process of cleaning is characterized by its technological flexibility which enables the setting of a wide range of substantially different cleaning procedure parameters, including the pressure of air, the pulse length, the interval between pulses, the number of pulses per cluster and the interval between clusters. These parameters must be adjusted individually.

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In modules only frames can be replaced on site. Frames can be fixed only at the manufacture facilities.

Disassemble module and replace the damaged frame. It is possible to blind the defected frame by placing blinding disc in PVCu union of permeate piping.

If membrane fibres dries out (even when PSP membranes are used) they must be re-wetted before immersing.

When single fibre is damaged: carefully shorten it, about 10 cm from the closest potting and blind the fibre by making a knot at it.

TMP value which corresponds to critical flux is called critical pressure. The critical flux is defined as the mean flux for which the first deposit appears on the membrane. The critical pressure depends on numerous parameters such as suspension properties, surface interaction or hydrodynamic conditions.

Operational pressure drop has to be lower than critical pressure drop and operational pressure has to be lower than water vapor pressure at operating temperature.

UV light, oxidizing agents (chlorine compounds, H₂O₂…), cause fast aging of polypropylene hollow fibers. That means hollow fibers tolerate oxidizing agents in limited concentration, at low temperature and for short duration of time.

The pore size of membrane P60 is below 0,1 micro.

• Typical turbidity below 0,1 NTU

• SDI (Silt Density Index) below 3

• Suspended solids concentration below 0,5 mg/l

• E.coli reduction 6 log

Native hydrophobic membranes or used (and dry) single primered membranes can be re-wetted before the next use. Membrane modules have to be fully immersed in alcohol solution for instant use or membrane wetting agent (ZWet) for later use.

Alcohol wetting -  membrane modules have to be fully submerged in the isopropanol or ethanol (at least 60% concentration) solution. Wetting solution should be filled in the inner space of the modules. Depending on the module size and complexity, make sure that all air from the membranes and module housing has been released. The wetting solution is then flushed with clean water.

ZWet wetting - membrane modules have to be fully submerged into the ZWet solution for at least 2 hours. Air from the inside of the module will escape slowly, so modules will have the tendency to float. Drain the solution and dry the module (flowing dry air). The membrane modules are prepared for storage. Before first use, flush modules with clean water.

Aeration is used to oxygenate solutions by steady inflow of small air bubbles, where aim is to maximize contact of air with solution. Where in air cleaning process small amount of large bubbles are used to shake the membrane fibres. Aim is to transfer as much kinetic energy to the membranes as possible. Intensive/impulsive movement shakes off the fouling mater and establishes desired flow.

Continuous air cleaning consist of “keeping it clean” and “intensive cleaning” phases. Usually “keeping it clean” phase is 1 hour long and consists of series of 1 second pulse of 4 bar and 1 minute pause. Following “keeping it clean” phase consists of 1 second pulse of 4 bar and 1 second pause for 1 minute. These two phases alternates while membrane modules are operational.There are basically three reasons for aeration, namely:

• Saturation of water with oxygen (bio-aeration)
• Homogenize MLSS (mixed liquor suspended solids)
• Air cleaning of outside surfaces of submerged membranes

Saturation and homogenization have their own algorithms and are not described here. Mechanical de fouling (whipping) can be done by any gas; however, the most frequently used is air. A common sense characteristic of this de-fouling method is that fibres are accelerated in a way similar to whipping by pulses of high-pressure air injected in between membrane bundles. Let us call this procedure – IMPULSING.

The two-phase gas-liquid flow causes violent agitation of the membrane fibres and removes solids deposited on the membrane surface. A typical gas-pulsation cycle consists of a short high-frequency pulsation cycle and a relatively longer low-frequency pulsation cycle. An optimal trade-off between energy consumption on degree of de fouling is essential. The de fouling is indirectly represented by e.g. trans-membrane pressure required for a given flux. The frequency of gas pulses and their time intervals can be changed to achieve optimum cleaning effect at minimal gas consumption.

The cleaning is achieved by application of gas pulses duration greater than 0.1 second, injected either periodically or with non-constant periodicity.

C (mg/l, ppm) is the concentration of active (available) chlorine in solution. It is the sum of gas chlorine concentration and the chlorine in form of chloride anion, hypochlorite anion and undissociated hypochlorite. T (h) is the contact time of HF membrane with chlorinated solution. The product C×T is the rating of HF oxidizing tolerance.

The chlorine resistance of polypropylene hollow fiber membranes is 200 ppm and the chlorine tolerance is below 100 ppm × h.

NaOCl can be dosed for disinfections and cleaning of the membranes. Typical frequency is once a week. It is advised to dose the chemical in a lower back pulse flow than normal in order to flush the chemicals in the membrane fibres in a more 'controlled' way. Back pulse flow should for instance be 20 m³/h instead of 30 m³/h.

The equilibrium constant K = [HClO][H][Cl^-] / [Cl₂(aq)] [H₂O] in the hypochlorite solution has value 10 ^-3.4 at 25 °C.

If log [H₂O] = 1.7 then log [Cl₂(aq)] = 1.7 - pH + log ( [HClO] [Cl^-] ). The dissociation constant value for hypochlorite acid is 10 ^-7.5 .

We believe that in the approximately neutral solutions we tested the tolerance of PP HF to hypochlorite. For the determination of chlorine tolerance limit we must measure at lower pH value.

(The certain difficulty makes the equilibrium Cl2 (aq) = Cl2 (g) , log K = -1.2 and the photodissociation 2 HClO = 2 H+ + 2 Cl^- + O₂ .)

Polypropylene membrane treated by PSP process doesn’t have to be wetted prior to its first use. The persistent single primering (PSP) of polypropylene HF membranes is performed in the stabilized ternary mixture of surfactants and wax. The components of this mixture are biodegradable.

Prior to use of membrane modules with PSP hollow fibres must be the modules immersed in the water for least three hours. The possible foaming may be removed by spraying with an alcohol (approximate dose 5 g/m²) or soap water.

If the membrane is dry out after use then its hydrophilic property disappears and the fibres acquired the water repelling properties. For the next use is necessary re-wetting.

Module construction and potting is pH resistant, so membrane pH tolerance has to be used. For polypropylene pH tolerance is 2 up to 11.

Rapid decrease in permeates quality means defective bundles are in the system. Special diagnostic system can be obtained. Other option is to alternate in blinding of permeate connection of individual modules and monitoring permeate quality. To identify defective frame module has to be disassembled and tested separately.

Calculation differs according to: operational temperature, turbidity of inflow, type of inflow, method of cleaning. Pilot unit should be used, but general calculation could be made (TMP - trans membrane pressure which must be below critical TMP): 

• Filtration of activated sludge from 15l/m² h TMP

• Pretreated surface water from 30 to 60 15 l/m² h TMP

• Outlet from secondary sedimentation tank from 15 to 25 l/m² h TMP

With lower temperatures the viscosity is decreasing so fluxes will decrease as well. From above freezing up to 60ºC.

Below 5ºC PVCu becomes brittle (careful while transporting).

FAQ contactors1 text

Module 10x1   3.04 dm³

Module 10x2   5.23 dm³

Module 10x5   12.48 dm³

 Values are calculated.

To optimize operational parameters (cleaning procedures, energy consumption, filtration surface…) pilot unit should be in operation adequate time.

Contact us, we are open to various types of pilot testing and have wide membrane application knowledge.

Ask for samples.

Energy consumption consists of energy for self-priming pump and compressor (air cleaning). Self-priming pump is selected according operating pressure drop and compressor according selected cleaning procedure.