The Author is Director at Enigma Process Technologies Corp in Muntinlupa, Philippines. His company provides engineered solutions for laundry waste, hospital waste, animal blood, animal waste, palm oil mill waste etc. → See also:
The application in which membranes are supposed to be used should be clearly described and in case experience is already available from any past operation of membranes they should be mentioned.
The main topics are:
Based on the above the design, construction, material of membrane and application data can be evaluated and prepared for a first decision.
It goes without saying that any experience with membranes in the same application before is of great help since the fact and figures based on the records during the previous operation of membranes is of utmost importance and shortens the time that is required to take a decision tremendously. Additionally the gained experience can trigger a further in-depth study of possible design and construction changes, if not to say improvements, special material selection or new design/construction in general.
Experience should by no means be underestimated in such a decision finding mission.
As mentioned, membranes represent quite an investment. Thus the decision to use membranes for a specific application must consider many aspects which are related with the filtration result, the operation and the work force that is involved as well as maintenance and replacement costs and additional equipment required for the operation of membranes.
The monetary value of a membrane can easily exceed the value of a standard filter element three to six times. There are applications where the filtration result achieved with membranes does not exceed the dry solid content achieved with conventional filter elements and the savings is only to find in the range of filtration time which in some cases might not be of any importance at all. It also has to be considered that in some areas the issue labor costs does not play an important role at all or that the operation skill required does not exist and therefore not recommendable to use membranes at all, unless proper steps are taken to find a remedy for the existing obstacle.
Some decisions are merely based on a non-monetary aspect others strictly on monetary issues. As such an evaluation can be objective or subjective either on the manufacturers or on the end user´s side. Nevertheless a clear understanding should be found between the two parties to avoid any disappointment at a later stage.
The objective part deals with the fact and figures of the process.
Still easy to calculate are the costs for the investment, the installation and the operation. Rather difficult is the calculation of the savings or the monetary benefits. They are as follows:
Waste Disposal Costs
Whether an existing liquid-solid separation system other than filter press-membrane combination is available or whether an existing conventional arrangement filter press-filter element will be retrofitted the result achieved with the selection of membranes is obvious and can be seen in the higher dry solids achieved and as such in lower costs for the disposal of cake due to less volume and weight to be land filled or incinerated. The detailed costing depends on the individual case and costs for transportation, distance, pre-treatment costs, incineration costs and any additional fees prescribed by the authorities or the communities.
The current costs have to be determined and compared with the savings as well as with the investment. The decision to install membranes surely increases the investment but with increasing investment the savings are increasing in a ratio, which exceeds the investment, costs tremendously.
In case of a conversion from an existing liquid-solid separation system other then filter presses to a filter press - membrane configuration a test filtration is advisable to underline any decision pro such system.
Thermal Energy Costs
Thermal energy costs can apply in wastewater treatment as well as in product filtration where the cake has to be additionally dried before any further processing.
It actually does not require any explanation, less energy costs for the evaporation of liquid equals less BTU for the incineration.
Here the calculation of the monetary advantages is rather simple and the production manager in charge should have the figures off hand.
Operation Labor Costs
In case of a retrofitting of an existing filter press - conventional filter element configuration into filter press - membrane system, i.e. replacing plate and frames or chamber plates the savings are to be found in many areas:
Less time for the handling of the same quantity of sludge with equal or even higher result in the dry solids. The same volume per day will require less hours of operation. The decision is to expect that the volume per day will be increased. The savings are obvious.
The membrane system is to increase the dry solid content. Costs for cake removal are decreased, the removal time is faster, no additional time required to scrap off the cake from the filter cloth and also less volume to handle downstream of the filter press.
Even more effective is the cake removal compared with plate and frames.
Not to mention the rather messy affair of cake removal on a plate and frame configuration, the time to scrap off remaining cake from the cloth and the danger of damaging filter cloth is entirely eliminated.
Wash Media Costs
The savings of wash media only refers to those applications where the cake has to be washed prior to any disposal. The main aim is to replace the mother liquid in the cake with water or any other wash media. This can be the removal of soluble salts or other liquids, which is done by rinsing some sort of leaching. These applications are found in the dyestuff, mining and chemical industries.
Regardless which industry it is it is obvious that the savings in wash media costs are tremendous underlined by the following arguments:
Application of a low-pressure squeeze before and/or during the wash step avoids that the cake becomes receptive to channeling during the wash process.
Application of a low-pressure squeeze before and/or during the wash step leaves less mother liquid in the cake that has to be washed.
Application of the final squeeze results in recovery of additional wash media.
Disposal Costs for Wash Media
Both cases are counting, the disposal of wash media into the municipal sewer based on costs per cubic meter or the required additional and separate treatment of the wash media by means of a separate treatment plant due to the nature of the wash media and the non-permission to disposal at such stage.
Increase in Capacity
The demand for increased capacity usually results in an investment in an additional filter press.
By using membranes this investment can be abolished and savings are easily calculated. In case of a full membrane application combined with a new filter press the membranes represent approximately 50 % of the total investment.
Further increasing capacity results are due to reduced filtration time.
The result as mentioned earlier is:
Less time for the same volume or same time with more volume.
Achievement of Higher Yield
In the application of palm oil the usually known dry solid content in liquid-solid separation is replaced by the terminology YIELD. In the separation of olein and stearin the results normally range from 76-80 % yield. Each percent of yield depending on the country represents an amount that cannot be neglected.
With the right selection of membranes and the right selection of design and construction as well as material a valuable contribution to a maximum yield can be given.
In terms of yield special production goals towards quality of olein have to be taken into consideration whereby the high yield might not be so decisive but the afore mentioned does apply in the majority of the palm oil applications.
Higher Dry Solid Content
The advantages by producing a higher dry solid content are actually the main issue of the membrane application combined with the fact that in such cases where membranes are used the filtration time or cycle time is tremendously reduced at the same time. In the majority of cases both higher dry solid content and reduced cycle time goes hand in hand.
Nevertheless many aspects have to be taken into consideration, the main issues were listed above but additionally floor space, feed pumps, plumbing, conveyors, labor etc are further topics that need to be watched.
Lastly the combination of a mixed pack, i.e. chamber plate/membrane plate combination should be investigated and might be in some cases the better choice with savings of approximately 25 % in comparison with a pure full membrane pack.
Special attention however is here required when the type of membrane is selected.
The non-monetary evaluation is not objective, calculations of advantages versus disadvantages; savings etc. are not possible to the extent as they are possible for the monetary evaluation.
The following are to be considered as subjective:
Higher quality in the end product can lead to advantages over the competitors and can also lead to increasing sales price. For example the volume of wash media to remove soluble salts in the product during filtration might be above permitted level. By designing the right membrane system the limited volume could be accommodated which would result in a better quality of product.
Possibility to meet increasing environmental control refers to the fact that existing laws from the individual governments re disposal of hazardous waste, regulations for landfill etc are still based on other systems than filter press - membrane configurations. Under the existing rules and regulations are many other systems to find which judged by the achievement of dry solids are far exceeded by the application of membranes. The investment in membranes would therefore not only reduce the possibility of an installation of a new system in case the rules and regulations are changed again it would also give ample safety in terms of forthcoming changes of rules and regulations since the indicated standards by the law are more than met.
Flexibility within the process is important in case more than one product or within one product various parameters have to be covered which a well-designed membrane, which meets all the requirements, can easily do.
III and IV are summarized in the following breakdown. For construction evaluation the explanations are categorized with III (n) and for application evaluation with IV (n).
In a further table the results out of the evaluation are summarized and form the basis for the later decision on the type of membrane to be selected.
The indicated figures III (n) and IV (n) can refer to more than one category.
III-1 III-2
List chemicals with concentration in the sludge, the solids and the temperature.
The use of the chemical resistance list is necessary; especially then when no previous experience is available.
Basic guidelines to be considered are:
Any traces of manganese or copper and cobalt require copper stabilized Polypropylene.
Chlorine oxidizes Polypropylene and turns it brittle.
EP contains rubber and Polypropylene. Both chemical resistance characteristics apply.
For rubber membranes the body plates can be of different material, like Polypropylene, Cast Iron, Aluminum, therefore the influence of the sludge on the base plate material has to be investigated.
Temperature is decisive for the design of the base plate or rather web thickness including the thickness of the membrane.
III-1 III-6
Is the filter press to be used for more than one application and if so, check on the chemical resistance for the additional products.
As the individual product for the application is examined in terms of chemical resistance of the membrane material, the filtration process etc. the same has to be done with any additional product. After this step is done it has to be elaborated whether all the criteria are met with the chosen membrane or whether it is not advisable to use a separate filter press - membrane installation for the additional products.
III-1 III-2
Check on the chemicals that are used as wash media and the temperature.
The chemical resistance based on the wash media is not prevailing it may be very important to know the difference in temperature for the process as well as for the washing. According to these findings the material for the membrane has to be selected.
III-2 III-4 III-10 IV-13
Is previous experience available and what are the pressures for feeding, squeezing etc.
Feed pressure and temperature is important to know to determine the design and the construction of the membrane as well as the body plate of the membrane. The main aim of the membrane is to utilize low feed pressure resulting in high flow rate, use higher pressure for squeezing and then to continue with the next filtration.
There are limited cases where the feeding is done at 10 bar followed by a squeezing of 14 bar. These limited cases should be considered when the base plate of the membrane is designed and when the material for the base plate is selected.
In case of retrofitting the maximum allowable pressure of the filter press has to be considered. There are cases where even with low pressure filter presses the application of membranes at low-pressure operations are recommendable and the investment in a new filter press can be avoided.
Out of balance pressures have to be considered in view of the design and construction of the base plate/web thickness.
PVDF is a material, which does not only apply for higher temperatures and shows a superior chemical resistance but also offers a higher physical strength, which is to be considered when high pressures are involved. The basic high material costs of PVDF however have to be evaluated.
III-2 III-4
Does the temperature around the filter press change.
This section might be exaggerated but there are cases where the temperature surrounding the filter press does change and consequently has an influence, not on the application but on the material of the membrane.
Mobile filter presses for example being exposed to outside temperatures do have to use a selection a membrane material which can resist low temperatures for example in winter time. In that case EP material or rubber are the correct material to chose over Polypropylene, which might become brittle. But possible oils in sludge have to be examined.
III-3
Is previous experience available with the various materials for membranes for this specific application?
Apart from the possibly existing experience always check on the piping at customers´ side. If in the piping system Teflon gaskets are used the use of gaskets in the membrane has to be re-considered. If the customer is using stainless steel pipes the decision whether to use a base plate of mild steel has to be revised since there might a reason for using stainless steel pipes.
III-8 III-9
Is previous experience available with replaceable or one-piece membranes?
If the performance of one-piece membranes has been without any failure a change to replaceable membranes is not advisable. If failures of the one-piece membrane were in the range of above 10% and were not related to chemical attack the recommendation to use replaceable membrane is a logical step. It is also recommendable to use replaceable membranes in case of a high amount of cycles per day and high wear and tear on the membrane material.
III-4
Which cycle time and how many cycles per day are expected.
Where filtrations are not frequently done a replaceable membrane is totally out of question, however, important where many filtrations at short cycle times are the case.
In applications which due to the temperature would not allow the selection of a certain material for the membrane it might be acceptable to still use this material provided the cycle time is short and the filtration not frequently done.
III-4
Is the filter press a fixed installation or a mobile one?
The best solution is a one-piece membrane but is has to be taken into consideration that due to surrounding temperatures other material than Polypropylene is recommendable. In case of many applications i.e. different products the use of a replaceable membrane might be taken into consideration but it also has to be observed that the maintenance of a replaceable membrane is not exactly what the end user wishes, i.e. replacement of gaskets, cleaning of the membranes due to product getting in between the membrane and the base plate and corrosion of metallic components like center feed ring.
III-5
Is previous experience available with any leakage problems?
If the membrane in case of a decision for a replaceable one is not properly maintained leakages might occur.
These leakages can occur when solids get between the membrane and the base plate. There is also the possibility that squeeze liquid might enter into the filtrate holes.
III-6
Which metallic materials are acceptable under consideration of the chemical resistance list?
Again proper examination of possible chemical attacks on the material has to be observed. In some cases it might be advisable to use hasteloy locking rings.
III-8
Is previous experience available with the selection of materials?
III-8 III-9
Is previous experience available with design and construction of the base plate/ web thickness?
III-10 IV-8
In case of retrofit, how many plates can be inserted, what is the maximum pressure of the filter press and which total weight of filter plate pack is permissible.
The technical specification of the filter press to be retrofitted has to be carefully examined. In some cases the use of membranes might be totally out of the question due to the maximum pressure of the filter press in other cases the possibility to use membranes might be within the permissible range and can contribute to tremendous improvement in the filtration result even eliminating the need for a replacement of the filter press.
IV-1
What is the approximate solids concentration of the slurry?
This data is already important for the selection of the infeed system whether corner feed or center feed.
More than that, specific gravity is important to be considered when it deviates from that of water.
The diameter of the corner feed is smaller than the diameter of the center feed. Thus slurries with high solids are more likely to block the corner feed than the center feed. Furthermore products that have a tendency to crystallize can easily contribute to uneven cake built-up in the chambers since the crystallization can block the flow of the sludge into the chambers since the infeed slot between two chambers and considering the thickness of filter cloth and assuming no filter cloth locking device is used is too small. Some chambers might be overfilled some under filled a situation, which will cause plate bending, or even breakage of the base plate of the membrane.
In general for slurries above 5 % by weight center feed should be used.
IV-2
Description of the physical nature of the solids, i.e. granular, crystalline, compressible, firm, slimy, porous etc
In case the sludge contains fibrous solids center feed would be the right choice.
Compressible and slimy products are related to a thin cake and have a tendency to clog the filter cloth. A mixed pack should be avoided except when the combination consists of membrane and filter plate (flush plate).
Also higher feeding pressure is required and/or longer filtration time. The possibility of “out-of balance pressure” is given and the most preferred solution would be center feed and design of membranes with stay bosses as support.
Also for center feed are to be considered those slurries with a high viscosity. Another argument for center feed is the possible “out-of balance” situation for solids with a high settling rate.
IV-3
What is the flow rate of the slurry?
Decisive for the selection of corner feed or center feed is the cross sectional area of the feed opening as related to the speed due to the flow rate. The flow rate through the feed channel, which is in the beginning at maximum speed, has to be checked. It is recommendable to use center feed when the flow rate is high in order to avoid possible pressure shocks, which can be easily caused in case of corner feed and at high speed.
Low flow rates and flow rates in the medium range are the basis for either center feed or corner feed.
IV-4
What is the cake thickness before squeeze and after squeeze?
To know the cake thickness before and after squeezing is important for the selection of membranes or mixed pack. Membranes can handle any kind of cake thickness i.e. with the combination of other filter elements i.e. filter plates or chamber plates.
A possible combination is a standard membrane cake thickness with a chamber plate having a cake thickness lower than the membrane.
IV-5 IV-6
Do the solids require a washing. What is the aim of washing?
There might be a number of arguments for one or the other system, like corner feed for washing, no stay bosses etc but these arguments mainly go back to previous experience or developed philosophy. Basically there is no problem at all to use center feed for washing. The advantage of membranes for washing is to be found in the fact that due to pre-squeezing possibilities a channeling of the wash media is avoided. Pre-squeezing also avoids the excessive consumption of wash media and lastly the final squeeze increases the amount of wash media recovery. The decision whether corner feed or center feed is difficult without considering all other aspects that lead to the decision on the type and design of membrane to be selected.
IV-6
Is there any experience available pertaining to washing?
IV-7 IV-11 IV-12
Is there any experience in connection with the selection of the infeed whether corner feed or center feed?
The same question was raised before but as per the following table the answer falls under a different category, which forms only one part of a list of categories that are to be taken into account. Customers´ information on experience, the economic, interchangeability within the filter press installations and experience in similar application are all accounting for an easier and confident selection of the right membrane.
In another example the frequent exchange of a barrel neck cloth speaks pro center feed since the infeed diameter is bigger and therefore the installation of the cloth much faster.
IV-9
Which air supply is available for the squeezing system?
For the selection of squeeze media the basic decision is that pressure above 7 bar applies water as squeeze media and below air as squeeze media. But also the costs have to be considered. Air is compressible and as such safety has to be looked at closely. Water is for sure the squeeze media, which provides safety. Any design of the squeeze media supply has to follow the operation manual of the membrane filter elements to eliminate any pressure shocks. An internal squeeze system is no doubt a clean solution however causes problems when any leakage has to be detected. Special attention has to be paid to the installation of the filter cloth since the openings for the squeeze system on the filter elements cannot be blocked. Any squeeze system with water should be accompanied by a vacuum pump to evacuate the membrane after completion of the squeezing step. This evacuation prepares the membrane for the next filtration and minimizes also pressure shocks that could possible occur from the feed pump. Water also has the advantage over air in terms of costs and can be re-used. The flow can be controlled correctly since water is not compressible. The same refers to pressure and release of water into the squeeze system.
In spite of all expectations it is theoretically possible that the hydraulic fails to function and releases the closing force during he squeezing step in such a situation again water is the safest way.
The possibility to squeeze with product, i.e. squeezing with olein in palm oil application and its advantages should not be excluded. Also important in case of squeezing with water the temperature is important for the selection of the material for the base plate, i.e. homopolymer or copolymer. Temperature of water for squeezing below 0 degree celcius has to be taken into consideration.
IV-11 IV-12
Is there any condition of the application that might affect the design of the membranes?
| Membrane | PP | PP | EP | EP | |
| Web Design | PP One-piece | PP Detachable | PP One-piece | PP Detachable | |
| Criteria | Comments | ||||
| III-1 Chemical | |||||
| III-2 Physical | |||||
| III-3 Experience | |||||
| III-4 Service Conditions | |||||
| III-5 Leakage | No contamination | ||||
| III-6 Metals | Chemical resistance | ||||
| III-7 Economics | Cost effective | ||||
| III-8 Special Notes | |||||
| III-9 Special Notes | |||||
| III-10 Filter Press | Mobile, fixed | ||||
| Overall |
| Membrane | Rubber | Rubber | Rubber | PVDF | |
| Web Design | PP Detachable | AL Detachable | ST Detachable | PVDF One-piece | |
| Criteria | Comments | ||||
| III-1 Chemical | |||||
| III-2 Physical | |||||
| III-3 Experience | |||||
| III-4 Service Conditions | |||||
| III-5 Leakage | No contamination | ||||
| III-6 Metals | Chemical resistance | ||||
| III-7 Economics | Cost effective | ||||
| III-8 Special Notes | |||||
| III-9 Special Notes | |||||
| III-10 Filter Press | Mobile, fixed | ||||
| Overall |
| Corner Feed | Center Feed | Air Squeezing | Water Squeezing | Product Squeezing | Comments | ||||||
| Criteria Data | Mixed Pack | Full Pack | Mixed Pack | Full Pack | Int. | Ext | Int. | Ext. | Int. | Ext. | |
| IV-1 Solids % | |||||||||||
| IV-2 Solids nature | |||||||||||
| IV-3 Flow rate | |||||||||||
| IV-4 Cake Thickness | |||||||||||
| IV-5 Washing | |||||||||||
| IV-6 Washing Experience | |||||||||||
| IV-7 Application Experience | |||||||||||
| IV-8 Squeeze Pressure | |||||||||||
| IV-9 Air Supply Pressure | |||||||||||
| IV-10 Economics | |||||||||||
| IV-11 Special Conditions | |||||||||||
| IV-12 Special Conditions | |||||||||||
| Overall | |||||||||||
| Abbreviation Keys | |
|---|---|
| NA | Not Applicable |
| NR | Not Requested |
| R | Requested |
| A | Applicable |
| P | Preferred |
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