Conceptual/experience-based design of suspended solids removal systems

Transcription

1 Conceptual/experience-base esign of suspene solis removal systems The Importance of Suspene Solis Removal The importance an impact of suspene solis is more than it appears. Suspene solis is a collective name for ifferent materials in water which have a common feature, which is that they are in the soli phase. The suspene solis in water coul be compose of ifferent material, which coul be organic or inorganic. Therefore, if the material of a specific suspene soli is organic, by removing that suspene soli, the organic content of the water will ecrease. On the other han, if the suspene solis are soli particles of Fe(OH) 3, then by removing that, the total iron content of the water will ecrease. The impact of high suspene solis in water epens not only on their concentration an particle sizes, but also on the nature of the suspene solis. For example, if the suspene solis are in the form of san, then a stream of this water woul be abrasive, while a stream containing suspene solis in the form of magnesium hyroxie, which is a gelatinous suspene soli, woul not be abrasive an coul even ecrease the abrasiveness of other suspene solis (e.g. san) by embracing them. Organic suspene solis make a stream a time sensitive stream; it coul quickly become septic an pungent. It coul also be problematic if the stream is to be irecte to heat transfer equipment. It is not unusual to see a fraction of organic suspene solis being converte to gas (as a VOC/volatile organic compoun) an ecreasing the heat transfer rate by attaching bubbles to heat transfer areas. The main problems of suspene solis are: 1. Soli settlement an reuce capacity of equipment an pipes, or interference with their operation. The egree of particle settlement epens on the ensity of particles but is usually problematic when the particle sizes are larger than 200 m (for naturally occurring san an clay particles). A high concentration of the solis worsens this problem. 2. Plugging an clogging of mechanical systems: If a piece of equipment has a clearance (a narrow passageway), usually particles of a size greater than the clearance - an even own to two thirs of the size of the clearance - coul cause plugging in the system. Generally speaking, when the size of the particles goes beyon 1 to 4 mm, non-conventional centrifugal pumps shoul be consiere. A high concentration of solis worsens this problem. On the other han, positive isplacement pumps are more sensitive to suspene solis. That is why permanent suction strainers are more popular for them than for centrifugal pumps. 3. Erosion: The suspene solis concentration an the size an material type of the suspene solis can make a slurry either more or less abrasive. There coul be cases where even a 10% slurry isn t 2014 Mohamma Toghraei engeu.ca Page 1

2 consiere as an abrasive slurry because the suspene solis are not very har, but rather are in the form of a gelatinous soli. 4. Waste chemicals: If there is a chemical injection point before the suspene solis removal operation, a portion of the chemical will inevitably be waste through the remove suspene solis. One famous example of this is the chlorination of water for the purpose of isinfection. Chlorination in the presence of suspene solis is never complete because chlorine cannot be very effective on the microorganisms which are hien in the particles. Reporting Suspene Solis in Water In general, there are three aspects of suspene solis which coul be reporte: 1. Suspene solis concentration 2. Suspene solis size an size istribution 3. Suspene solis material -Suspene solis concentration Usually particles in the normal range are reporte in weight per volume of water: for example, mg/l or ppm. In very irty applications an in the case of very large suspene solis (more than a few centimetres) it makes more sense to report the solis content of water in volume of solis per unit volume of liqui; for example, the solis content of non-urban storm water coul be 0.1 m 3 of solis per m 3 of water. However, reporting in volume per volume has avantages for equipment selection too. Generally, the suspene solis tolerance of equipment such as centrifugal pumps is reporte in volume/volume units, since the volume is what a piece of equipment can see, an part of it is occupie by the suspene solis. In very high quality water applications like the pharmaceutical or microchip manufacturing inustries, in which TSS (total suspene solis) nees to be very low, it is reporte as the number of particles per unit volume of water. For example, the water quality specification in the pharmaceutical inustry coul be 2 particles per 100 ml of ultra-pure water. -Suspene solis size Although a water sample with a single particle size can be mae in the lab, in reality a range of particle sizes are foun in natural or inustrial waters. This size istribution generally follows a bell-shape curve (possibly skewe), with one peak. A bell-shape curve with two peaks can sometimes be interprete as a stream which was generate by two or more recently mixe streams. -Suspene solis material The suspene solis material is not usually of importance. This is because the removal methos for suspene solis are generally non-material-specific. Possibly the only important material characteristic of suspene solis is whether they are organic or inorganic Mohamma Toghraei engeu.ca Page 2

3 Table 1 shows ifferent characteristics of organic vs. inorganic suspene solis in water. Table 1: The features of organic an inorganic suspene solis Inorganic Suspene Solis Gravity separation is mainly via seimentation Easier to trap in filters because there are more solis The remove suspene solis can be of beneficial use for builing material Organic Suspene Solis Gravity separation coul be via seimentation or flotation The remove suspene solis can be use for lan applications The remove suspene solis can quickly become septic (oorous, etc.) TSS Concentration vs. Particle Size In the lab, one can make water samples with ifferent concentrations of suspene solis an ifferent particle sizes. But in natural systems, usually particle size an suspene solis concentration have some correlation. Roughly speaking, the larger the concentration of suspene solis (in weight per volume), the larger the particle size. Suspene Solis Removal Theoretically, to remove particles with a wie range of sizes in orer to prouce a liqui stream free of any particles, it is enough to install one strainer sheet with a pore size smaller than smallest particle size. Actually, epening on the Soliity of particles, the pore size coul be larger than the smallest particle size up to 50 or 60%. However, this recommenation is not the best solution from a technical or economic stanpoint. By implementing this, we woul have a system which woul nee frequent cleaning an, consequently, a great eal of attention. A better solution is to arrange a series of strainers with ifferent pore sizes; starting from the biggest pore size an graually ecreasing to the smallest pore size (Figure -1). Figure 1: Theoretical Removal of total suspene solis 2014 Mohamma Toghraei engeu.ca Page 3

4 This concept can be implemente in the following preliminary esign to optimise it to a more practical esign: 1. One variation of this arrangement is to replace the last few strainers with a epth filter. As is to be expecte, a strainer is a surface filter which removes all particles larger than its pore size. While, a epth filter removes particles bigger than its pore size an, surprisingly, all other smaller particles own to a certain size! In other wors, a epth filter is NOT a 3 version of a surface filter (or strainer) from an application viewpoint. Their uties are completely ifferent an they are actually complementary to strainers. In practice, a epth filter is place at the en (or near the en) of a suspen solis removal string. This means that upstream of the epth filter, there coul be one or more strainers. In theory, only one epth filter coul be installe to remove all of the particles (bigger an smaller than its pore size ) own to e.g. 2 micrometers; however, because of this unique capability of being able to remove particle sizes even smaller than its pore size, in practice, particles that are bigger than the pore size of the epth filter are remove by strainers as much as possible upstream of the epth filter. This is in orer to offloa the big particles from the epth filter using less sophisticate systems (strainers), as large particles ecrease the operation cycle of epth filters. Usually a epth filter removes particles in the range of 2 to 50 micrometers. In the above iscussion, whenever a epth filter was mentione, pore size was use loosely because pore size is well-efine for strainers an fixe meia filters, but not for loose meia filters. This is mainly because of the complication of efining pore size for loose meia epth filters. In such cases, meia size is use instea to specify a epth filter. 2. For particle sizes larger than 100 µm, with concentrations of more than 1%, an if the flow rate is high (say, more than 200 m 3 /h), a more attractive option than a strainer is seimentation. Seimentation is not popular for particles smaller than 50 µm because it takes a long time. Although there is no theoretical limit, but the largest particles which can practically be remove in a seimentation basin are aroun 5000 µm (5 mm). This is because the particles remove en up in the sluge on the bottom of the seimentation basin, an if they are larger than 5 mm, they cannot be hanle/ conveye easily by conventional centrifugal pumps. Technically, a seimentation basin can be use even for particles larger than 5 mm, but this is not economically very attractive. 3. Although up to this point the iscussion has claime that a epth filter can remove particles of a given size an lower, in reality epth filters with conventional esigns cannot effectively remove particles smaller than 2 µm. If this resiual suspene solis content is still problematic, it shoul be remove via surface filtration Mohamma Toghraei engeu.ca Page 4

5 Base the above concepts, a generic string for suspene solis removal starts with one or more surface filters (strainers), then a epth filter, an occasionally one or more surface filters at the en(figure 2). FLOW Strainers Can be replace by seimentation Can be replace by epth filtration Strainer/surface filtration Figure 2: A practical approach for removing total suspene solis In the following examples, the use of the above concept is shown: Example 1: Obtaining water suitable for an RO (Reverse Osmosis) system from a grounwater source: The use of a multimeia san filter followe by a cartrige filter is popular. This is because the TSS of grounwater is usually less than 50 mg/l an a multimeia san filter will remove all the TSS own to 10 micrometers. Because the RO membrane is sensitive to particle sizes of about 10 micrometers, another filter - a surface-type cartrige filter, which works base on the mechanism of straining - is use at the en. Example 2: Obtaining water suitable for a biological treatment system from raw municipal wastewater: Municipal wastewater can have suspene solis of up to 15 cm in size. Therefore, at the beginning of the string, one or more sets of strainers shoul be installe. If the particle size istribution shows that one set of strainers (fine screen type) can hanle the particle content suitably, one set is enough. However, if the installation of one set of strainer leas to too frequent cleaning (in manual cleaning types, more than once per 8 hours) or mechanical cleaning that is too complicate (in the case of mechanically cleane strainers), two sets of strainers - one coarse strainer (bar rack) an the other fine (fine screen) - shoul be installe. After that, once the size of the remaining particles in the wastewater is less than roughly 5 millimetres, seimentation is use (the equipment for this is calle a primary clarifier). Water from this clarifier has less than a few hunre ppm of TSS an is reay to go the next step, which is a bioreactor. Suspene culture bioreactors can tolerate up to a few hunres of ppm of TSS, so no further upstream TSS removal is neee. Example 3: Obtaining ultrapure water from grounwater: Again, here we can start with a multimeia filter to rop the suspene solis particle size to less than 2 micrometers an then, because the 2014 Mohamma Toghraei engeu.ca Page 5

6 intention is to generate ultrapure water with an almost zero particle count, two cartrige filters with pore sizes of 0.45 micrometers an 0.2 micrometers can be use in series. Filtration Mechanisms The filtration operations - epth filtration an sieving can be accomplishe via ifferent mechanisms (Table 2). While all the mechanisms are available for epth filtration, not all of them are applicable in a straining operation. The only mechanism that works uring straining is sieving. Sieving is a mechanism through which a particle is remove by trapping in the pores of a given meia. Inertial impaction is when a particle collies with, an resies on the surface of filter meia, an is thereby remove from the stream. When a particle is graspe by the surface of a filter meia, the mechanism is calle interception. In seimentation, particles are seimente on the surface of the filter meia ue to gravitational force. iffusion occurs when a particle is so small that Brownian motion causes it to strike the surface of filter meia. In liqui filtration with a low liqui velocity, inertial impaction can be consiere as a negligible mechanism with regars to particle removal. Table 2: ifferent suspene Straining epth Filtration solis removal mechanisms Sieving Inertial impaction Not important in liqui filtration Interception X Seimentation X iffusion X From a simplistic point of view, when particles approach a strainer or filter, one of three things can happen: trapping on the surface of strainer/filter, trapping insie the filter meia, an escape. These three events are shown in the Table 3 with some rules of thumb which preict their behaviour. All numbers in the table are approximate an can be change for ifferent filtration rates. They are state here only to give a general unerstaning of filtration Mohamma Toghraei engeu.ca Page 6

7 Table 3: Behaviour a typical suspene solis in straining an filtration operation Straining Filtration Schematic Trapping on surface Trapping in epth > (1.0 to 1.5) epening on the soliity of the suspene soli Not applicable > (0.1 to 1.5) epening on the soliity of the suspene soli (0.05 to 0.15) < < (0.3 to 1.5) epening on the soliity of the suspene soli 2014 Mohamma Toghraei engeu.ca Page 7

8 Escape Removal mechanism < (1.0 to 1.5) epening on the soliity of the suspene soli Straining < (0.05 to 0.15) epening on the soliity of the suspene soli an flux; < 1-2 micrometer => 100% escape Inertia, Seimentation, iffusion Some Aspects of Filtration Equipment Number of filter vessels One important parameter in filtration systems is the number of filters. In epth filtration, by choosing a bigger vessel (with a larger volume of meia), we can have fewer vessels an less piping an valves (lower capital cost), but more frequent backwashing is neee, which means a lower on-stream factor for the filter. On the other han, by increasing the number of filter vessels, the capital investment (the expense of fabrication an installation) will be higher but the operating cost will be lower. Therefore, an economically optimal point for the number of filters can be foun. Kawamura 1 state that the number of filters in potable water treatment systems is (after unit conversion): N= Q 0.5 /10.46 Equation - 1 Where: Q : fee flow to filters in m 3 /h N : number of filters However, the author believes that in inustrial facilities, the number of pressure filters is better calculate with this formula: N= Q 0.5 /K Equation - 2 Where: Q: fee flow to filters in m 3 /h K: ranges between 10 to 26 an is typically aroun 20 N: number of filters Surface Filtration or Cake Filtration? 2014 Mohamma Toghraei engeu.ca Page 8

9 Theoretically, cake filtration can be one using the same equipment as a surface filter by proviing more space for trappe solis. This is because in both cases the pore size is smaller than the particle sizes an the particles will be trappe on the surface of the filter. However, cake filtration is usually one using specific equipment esigne for this purpose. In this equipment, trappe suspene solis are present in such high quantities that they buil up a thick layer of particles (cake) on the surface of the surface filter; this built-up cake has a goo porosity to work as seconary filter, an, when it is time, the cake can be remove from the filter easily. If the cake which is generate oes not have goo porosity an etachment properties, aing a filtration ai as boy fee an/or pre-coat to the stream can help. Having sai that, usually cake filtration is not use in water an wastewater treatment because the concentrations of suspene solis in those streams are so low that cake filtration is not necessary. However, cake filtration can be use in process inustries or for waste streams from suspene solis removal operations, which have higher suspene solis concentration. Aitionally, cake filtration has the avantage that the remove solis from cake filtration are in the form of a wet cake, whereas from filtration an seimentation this is a watery stream with less than 30% suspene solis content. There are many ifferent systems for suspene solis removal in inustry but here a conceptual approach is use to escribe the applications of the workhorses of the inustry, which are strainers, (epth) filters an clarifiers. (1) Kawamura S., Integrate esign an of water treatment facilities 2n e. New York: John Wiley & Sons. MOHAMMA TOGHRAEI, P.Eng. is an instructor an consultant with Engrowth Training (engeu.ca) base in Calgary, Alberta. He has over 20 years of experience in the fiel of inustrial water treatment. His main expertise is in the treatment of wastewater from oil an petrochemical complexes. He receive a B.Sc. in chemical engineering from Isfahan University of Technology an an M.Sc. in environmental engineering from Tehran University, both in Iran. He is also a member of APEGA (Association of Professional Engineers an Geoscientists of Alberta). He can be reache at Mohamma Toghraei engeu.ca Page 9