CEMEXs Type N Masonry Cement, Type S Masonry Cement and Type M Masonry Cement are specially formulated and manufactured to produce masonry mortar. The masonry mortar is often used in brick,concrete blockandstone masonry construction; it is also used to produce stone plaster.
CEMEXs Masonry Cement consists of a mixture ofPortland or blended hydraulic cementand plasticizing materials (such as limestone, hydrated or hydraulic lime) together with other materials introduced to enhance one or more properties. These components are proportioned at the cement plant under controlled conditions to assure product consistency.
CEMEXs Masonry Cements are produced in Type N Masonry Cement, Type S Masonry Cement and Type M Masonry Cement strength levels for use in preparation of ASTM Specification C-270 Type N, M or Type S Masonry Mortar, respectively without any further additions.
Table 1 is a general guide for selection of mortar type. Other factors, such as type and absorption of masonry unit, climate and exposure, applicable building codes, and engineering requirements should also be considered.
CEMEXs Type N Masonry Cement, Type S Masonry Cement and Type M Masonry Cements are designed to be mixed with sand and water. The addition of hydrated lime or any other materials to a masonry cement mortar at the job site is not required or recommended.
Since Masonry Cement color is laboratory controlled and Masonry Cement offers the simplicity of the one bag system of batching, it is easier to achieve consistent color cement for a prefect appearance in the finished job.
CEMEXs Type N Masonry Cement, Type S Masonry Cement and Type M Masonry Cement are proportioned with sand meeting ASTM C-144, according to Table 4, and will produce mortar meeting the requirements of ASTM C-270 under the proportion specifications. Under the property requirements of ASTM C-270, however, cement-to-sand proportions for the job mixed mortar are to be in the range of 1:2 to 1:3, and the mortar must be pre-tested in the laboratory before the job begins.
Machine mixing should be used whenever possible. First, with mixer running, add most of the water and half the sand. Next, add the Masonry Cement and the rest of the sand. After one minute of continuous mixing, slowly add the rest of the water. Mixing should continue for at least three minutes; extending mixing up to five minutes improves mortar.
Good workmanship principles are required for successful application, including proper filling of head and bed joints, careful placement of units, appropriate tooling of the joint, modification of construction procedures and/or schedules to adapt to extreme weather conditionsand proper cleaning procedures.
Masonry joints should be tooled at the same degree of stiffness and moisture. If joints are tooled too early, excess water will be drawn to the surface, producing lighter joints. The joints will appear dark and discolored if tooling is done after stiffening has started.
Mortars exposed to hot winds and full sun will tend to lose workability due to the evaporation of water. Common sense precautions should be taken to protect the mortar such as shading the mixer, wetting mortar boards, covering wheelbarrows and tubs, and balancing mortar production to meet demand.
CEMEX, Inc. guarantees Broco Stucco Cement when shipped from our mill or terminals to meet the current requirements of ASTM C-1328, Standard Specification for Plastic (Stucco) Cement and ASTM C-91, Standard Specification for Masonry.
CEMEX warrants that the products identified are in accordance with the appropriate current ASTM and Federal Specifications. No one is authorized to make any modifications or addition to this warranty. CEMEX makes no warranty or representation, either expressed or implied with respect to this product and disclaims any implied warranty of merchantability or fitness for a particular purpose.
In no event shall CEMEX be liable for direct, indirect, special, incidental or consequential damages arising out of the use of this product, even if advised of the possibility of such damages. In no case shall CEMEXs liability exceed the purchase price of this product.
1. Dubovoy, V.S., and Ribar, J.W., Masonry Cement Mortars A Laboratory Investigation, Research and Development Bulletin RD0095, PCA Skokie, IL, 1990, 26pp. 2. Davison, J.I., Effect of Air-Entrainment on Durability of Cement-Lime Mortars, Durability of Building Materials, Elsevier Publishing Co., Amsterdam, 1981. 3. Zematis,W.L., Factors Affecting Performance of Unit Masonry Mortar, ACI Journal, Proc. Vol. 56, No. 6, American Concrete Institute, Detroit, MI 1959. 4. Ribar, J.W., Water Permeance of Masonry, a Laboratory Study, ASTM STP 778, ASTM 1982, pp 200-220. 5. "Cold Weather Construction" and "Hot Weather Construction", ACI 530.1/95, Section 1.8, pp. 5-12, ACI, 38800 Country Club Drive, Farmington Hills, MI 48331. 6. "Recommended Practices and Guide Specifications for Cold Weather Masonry Construction, International Masonry All Weather Council.
DEAR ALL CAN ANYBODY SUGGEST ME HOW TO CALCULATE THE MATERIAL RETENTION TIME IN A CLOSED CIRCUIT BALL MILL IN EACH COMPARTMENT? SIMILARLY FOR A VERTICAL MILL . WHAT IS THE USE TO CALCULATE THE SAME? IS IT HELPFUL FOR OPTIMISATION LIKE IN KILN ? PLEASE DO REPLY RAJ
You could do thatby measuring the material flowrate Q[tons/h] through the mill as well as the quantityM[tons] of material that accumulates within the mill. The retention time, also called residence time, is the ratio (M/Q) [h] .The flowrate Q could be measured from the production P[tons/h] and the circulating load C and given by Q[tons/h] = C P[tons/h] . The circulating load could be obtained by measuring the particle size distribution of the feed, the product and the reject, and by thenusing the Koulen method. (see this post)The material M accumulated within the mill could be observed by measuring the level of the material within the mill, deducting the volume occupied by the balls and assuming a reasonable value for the material density. Alternatively, you might empty the mill and weight what comes out. Another method uses fluorescein injection.See the Cement plant operations handbook B4.8 .See also page 33 3in this Mapei document .
You could do thatby measuring the material flowrate Q[tons/h] through the mill as well as the quantityM[tons] of material that accumulates within the mill. The retention time, also called residence time, is the ratio (M/Q) [h] .
The flowrate Q could be measured from the production P[tons/h] and the circulating load C and given by Q[tons/h] = C P[tons/h] . The circulating load could be obtained by measuring the particle size distribution of the feed, the product and the reject, and by thenusing the Koulen method. (see this post)
The material M accumulated within the mill could be observed by measuring the level of the material within the mill, deducting the volume occupied by the balls and assuming a reasonable value for the material density. Alternatively, you might empty the mill and weight what comes out.
Centralized water treatment plants are based on coagulation, flocculation and disinfection processes and found to be most cost-effective in treating large quantities of water. However, they entail large infrastructure costs which is difficult to raise in rural regions of developing countries and are generally installed using government funding. Hence, centralized treatment is available only in the metros of developing countries and mainly benefit the urban population. The transportation cost of water to the centralized treatment plant and from the treatment plant to the individual households is another major expense which limits its benefits to regions which are situated away from the treatment plant. Hence centralized treatment plants are generally installed near the freshwater resources (rivers or lakes) and benefit the people living closer to these water bodies.
The plant takes water from both the wastewater and potable water facilities at Hanover county and discharges only solids in the form of filter cakes from both the pretreatment and wastewater treatment plants.
The raw water pretreatment plant is designed principally for solids removal from the incoming Hanover county sewage effluent (grey water), backwash water and wastewater from the oily water collection system. Raw water enters a coagulation/flocculation chamber followed by a clarifier and dual media depth filters. Backwash water from the filters is periodically returned to the clarifier. Clarifier sludge is dosed with polymer before being thickened and then sent to the filter press for dewatering. The cake is sent to landfill and the recovered water returned to the clarifier.
Treated raw water is mixed with potable water and pumped to the boiler feedwater treatment system. The system is designed to remove 99% of the dissolved minerals and provide high-purity water to the boiler. The mixed water flows through a reverse osmosis plant operating at a recovery of 80% and an average salt rejection of 95%. Permeate from the RO mixes with product water from both the waste RO unit and the distillate from the brine evaporator/crystalliser situated in the wastewater treatment plant. The combined flow then enters a degasifier, to remove carbon dioxide, and a mixed bed dimineraliser. The mixed bed plant consists of two 100% capacity ion exchange vessels which remove the final 5% of the dissolved salts. The ion exchange beds process 2 200 000 gallons (8327 m3) before being regenerated. Waste from the process is pH adjusted and combined with the RO reject before being pumped to the wastewater treatment plant.
The wastewater treatment plant is designed to treat 250 gpm (56.8 m3 h1) of which 66% is recovered by the membrane processes and the rest through the brine evaporator/crystalliser unit (Fig. 5.6). The wastewater flow is generated by make-up RO reject (64%) (from make-up water plant), power block blowdown (22%) and mixed bed regenerate waste (14%). The combined wastewater flow initially passes through two 100% flow dual media anthracite/sand depth filters operating in a duty standby/backwash mode. Filter permeate is then treated in an EDR unit containing micron feed filters and three 50% capacity membrane stacks. Each parallel line contains 3 stacks in series consisting of 500 pairs of cation- and anion-selective membranes. The EDR unit is designed to recover 84% of the flow with the remaining 16% being sent to the brine tank. The EDR unit includes acid injection for pH control, anti-scalant and clean in place systems to control fouling. The three stages in each stack are operated at voltages of 299, 344, and 264 V with corresponding currents of 17, 11 and 4.8 amps respectively. The feed pump discharges at a pressure of 81 psi (5.6 bar) with a differential pressure across the stacks of 14 psi (0.96 bar) on the positive side and 18 psi (1.24 bar) on the negative.
The flow then enters a reverse osmosis plant containing three parallel streams designed at 50% flow enabling continuous operation. Each stream contains 24 cellulose acetate membranes arranged in a 4:2 array. The plant operates at an overall recovery of 75% and a salt rejection of 95%. Permeate is pumped to the demineralisation storage tank and reject is sent to the brine storage tank where it is mixed with the EDR reject.
Treatment of the brine is conducted in a vertical tube, falling film evaporator driven by vapour compression. Wastewater is pH adjusted to between 5.5 and 6 and then heated to boiling point and deaerated. Hot brine then enters the evaporator sump where it mixes with recirculating brine slurry which is pumped to the top of 2 inch (50.8 mm) heat transfer tubes. As the slurry falls a small portion of the water evaporates and condenses on the outside of the heat transfer tubes. The brine evaporator recovers 95% of the flow which is passed on to the demineralisation feed tank with a water quality of less than 10 ppm TDS. The 5% concentrated brine then enters a crystalliser where a further 95% of the remaining water is recovered. The stream is finally sent to a filter press and dewatered to a 20% moisture content sludge which is disposed of off site.
Automation of water treatment plant involves the control system opening and closing valves and starting and stopping equipment in predefined sequences to complete specific tasks or to provide the desired process plant output. To achieve these results the automation system relies on signals from correctly selected and placed instruments, devices such as actuators and motor control circuits and reliable control logic. The degree of automation to be used is fundamental to developing an automation system.
A functional design specification (FDS) must be prepared in detail setting out all the functional requirements for each item of equipment and the controls to be applied. The FDS is critical to achievement of a satisfactory monitoring and control system that meets the requirements. In the information technology industry the following terms are in common usage:
Software is the programming logic or set of instructions which designates the tasks to be carried out by a group of hardware. These tasks can range from starting a pump to calculating, from monitored process variables, the appropriate hardware actions to be taken.
Where two items of instrumentation equipment are required to operate as a system the interface between them must ensure proper data communication. When selecting components for a system, the designer must ensure that they will operate satisfactorily together.
I. Bendigo water treatment plant (BWTP). The 12.54 104 m3/day (33 MGD) BWTP has been producing drinking water for nearly 1 million people in central Victoria, Australia since 2002. It is one of the largest if not the largest MF plant in the world. The plant combines submerged microfiltration (CMF-S), ozonation and biological activated carbon (BAC) to treat a variable and difficult raw water. Raw (surface) water is pre-screened, and dosed with lime and carbon dioxide in a contact reactor to control alkalinity and corrosion. Next, water is dosed with a coagulant, liquid aluminium chlorohydrate (ACH) prior to entering the CMF-S plant to remove colour, some organic content, and dissolved metals. The coagulant dosage is typically 56 mg/l. The coagulant precipitate is removed by MF. The coagulant/CMF-S process removes up to 15% of the dissolved organic carbon.64
The CMF-S system consists of eight cells (six on-line and two on standby) each containing 576 submerged membrane modules. Membrane performance is monitored for turbidity and particle count. A daily pressure decay test is performed to check the integrity of the hollow fibres. The cells are backwashed as described earlier. The MF filtrate is pumped to the ozone/biologically active (BAC) system for disinfection and to reduce the original carbon level in the water, eliminating taste and odour compounds.
II. Orange County Water District (OCWD). OCWD located between Los Angeles and San Diego counties in southern California manages the groundwater basin that supplies about 3.0 108 m3 per year potable water to a population of more than 2 million. One of OCWD's best known project is Water Factory 21 (WF21), which protects groundwater from seawater intrusion by injecting up to 5.7 104 m3/day (15 MGD) of highly treated reclaimed water blended with deep-well water into four coastal aquifers. More than half of the injected water flows inland and augments potable water supplies.48 The injected water quality must exceed potable water quality to prevent plugging of the aquifer, and prevent degradation of groundwater quality.
WF21 is an advanced wastewater treatment (AWT) facility that reclaims secondary treatment effluent. In the beginning in 1975, the AWT plant consisted of lime clarification, ammonia stripping, recarbonation, multimedia filtration, granular activated carbon (GAC) filtration and chlorination. RO treatment was added in 1977 to reduce salts and organics in one-third of the flow stream. The 790 m3/h (5 MGD) RO plant consisted of 252 spiral-wound cellulose acetate membranes (20 cm nominal diameter) in a three-stage array (24:12:6). The removal of organic carbon by RO was greater than by GAC, and in 1985, ammonia-stripping towers were removed since RO reduced ammonia and nitrate concentrations by 80%. The WF21 treatment process flow schematic is shown in Figure 3.52.
In 1992, evaluation of MF pretreatment for replacing conventional pretreatment shown in Figure 3.52 (see also Figure 3.47) was initiated with a small pilot unit. The positive results of the pilot testing with Memcor/USFilter CMF pilot system led to the installation of a 114 m3/h (0.7 MGD) CMF demonstration system. The plant consisting of MF, RO and UV disinfection has been operated since 1997. The plant uses thin-film composite RO membranes producing RO permeate that meets all the requirements of the US National Primary Drinking Water Regulations, and reduces the concentration of TOC to less than 0.1 mg/l.
Based on the success of the CMF demonstration unit, a four module CMF-S pilot unit was commissioned in 1998. After one year of operation, the CMF-S demonstrated increased cleaning intervals while operating at flux equivalent to the CMF pilot and demonstration systems. In early 2000, the evaluation of the CMF-S technology was scaled up to a 36 m3/h (160 gpm), 32 module CMF-S demonstration system. Both systems were operated simultaneously for a year to compare the data. The data was used to design a 950 m3/h (6 MGD) CMF-S system, which was installed in 2004. This system provides injection water during the construction of the 26.5 104 m3/day (70 MGD) CMF-S system.
III. Industrial water treatment. In 1997, the first MF/RO plant was commissioned to reclaim secondary treatment effluent at the El Segundo recycling plant of the West Basin Municipal Water District (WBMWD) in southern California for injecting RO purified water into a seawater barrier to control seawater intrusion into groundwater as discussed in section 3.4. The injected water quality must exceed potable water quality to prevent plugging of the aquifer and prevent degradation of groundwater quality. The MF system was a Memcor/USFilter based CMF system. Subsequently, three more CMF systems were supplied by USFilter to supply re-purified water to three refineries as feed water for their boilers and cooling water. As part of Phase IV expansion, a CMF-S system was supplied by USFilter in 2005 for treating 1,840 m3/h (11.6 MGD) secondary treatment effluent.64
Similarly, a 190 m3/h (1.2 MGD) CMF-S membrane system was installed by USFilter in 2000 at a 1200 MW coal-fired power plant in UK when the well water source that provided feed water for the RO/IX high purity water system was shut down. The CMF-S system treats surface water and cooling tower blow-down water for the RO/IX deionisation system with turbidity less than 0.1 NTU, suspended solids at the detection limit of 1 ppm and SDI less than three. The MF membranes were oxidant-resistant PVDF.
Naturally occurring silt particles suspended in water are difficult to remove because they are very small, often colloidal in size, and possess negative charges, and are thusprevented from coming together to form large particles that could more readily be settled out. The removal of these particles by settling requires first that their charges be neutralized and second that the particles be encouraged to collide with each other. The charge neutralization is called coagulation, and the building of larger flocs from smaller particles is called flocculation.
A fairly simple but not altogether satisfactory explanation of coagulation is available in the double-layer model.Figure 7-2 is a representation of the static electric field surrounding the particle. The solid particle is negatively charged, and attracts positively charged ions counterions from the surrounding fluid. Some of these negative ions are so strongly attracted that they are virtually attached to the particle and travel with it, thereby forming a slippage plane. Around this inner layer is an outer layer of ions consisting mostly of positive ions, but they are less strongly attracted, are loosely attached, and can slip off. The charge on the particle as it moves through the fluid is the negative charge, diminished in part by the positive ions in the inner layer. The latter is called the zeta potential.
If the net negative charge is considered a repulsive charge, since the neighboring particles are also so charged, the charge may be pictured as in Fig. 7-3A. In addition to this repulsive charge, however, all particles carry an attractive electrostatic charge, van der Waals force, that is a function of the molecular structure of the particle. This attractive charge is also shown in Fig. 7-3A. The combination of these forces results in a net repulsive charge, an energy barrier, or energy hill, that prevents the particles from coming together. The objective of coagulation is to reduce this energy barrier to 0 so that the particles no longer repel each other. Adding trivalent cations to the water is one way to reduce the energy barrier. These ions are electrostatically attracted to the negatively charged particle and, because they are more positively charged, they displace the monovalent cations. The net negative charge, and thus the net repulsive force, is thereby reduced, as shown in Fig. 7-3B. Under this condition,the particles do not repel each other and, on colliding, stick together. A stable colloidal suspension can be destabilized in this way, and the larger particles will not remain suspended.
Figure 7-3. Reduction of the net charge on a particle as a result of the addition of trivalent counterions. (A) Particle carries net negative charge and van der Waals positive charge; energy barrier prevents coagulation. (B) Addition of trivalent cations reduces energy barrier, and coagulation is possible.
Alum (aluminum sulfate) is the usual source of trivalent cations in water treatment. Alum has an advantage in addition to its high positive charge: some fraction of the aluminum ions may form aluminum oxide and hydroxide by the reaction
These complexes are sticky and heavy and will greatly assist in the clarification of the water in the settling tank if the unstable colloidal particles can be made to come in contact with the floc. This process is enhanced through an operation known as flocculation.
A flocculator introduces velocity gradients into the water so that the particles in a fast-moving stream can catch up and collide with slow-moving particles. Such velocity gradients are usually introduced by rotating paddles, as shown in Fig. 7-4. The power required for moving a paddle through the water is
Time is also an important variable in flocculation, and the term Gt is often used in design, where t is the hydraulic retention time in the flocculation basin. Gt values are typically between 104and 105.
A water treatment plant is designed for 30 million gallons per day (mgd). The flocculator dimensions are length = 100 ft, width = 50 ft, depth = 16 ft. Revolving paddles attached to four horizontal shafts rotate at 1.7 rpm. Each shaft supports four paddles that are 6 in. wide and 48 in. long. Paddles are centered 6 ft from the shaft. Assume CD= 1.9, and the mean velocity of water is 35% of the paddle velocity. Find the velocity differential between the paddles and the water. At 50F, the density of water is 1.94 lb-s2/ft3and the viscosity is 2.73 105lb-s/ft2. Calculate the value of G and the time of flocculation (hydraulic retention time).
Materials. Portland cement used for mortar preparation was conforming to the specifications of ASTM type I. Fine aggregate was natural sand with 2.94 fineness modulus. SSA were obtained from sewage treatment plant of Pinedo (Valencia, Spain). Sikanol-M was used as plasticizer.
Apparatus and procedures. Samples of original SSA were ground using a laboratory ball-mill (Gabrielli Mill-2). SSA samples were introduced into the bottle-mill containing 98 balls of alumina (2 cm diameter) and were ground during 2.5, 5 and 10 minutes.
Mortar specimens cast in square prismatic mortar molds with internal dimensions of (4040160) mm were used. Preparation of mortars was carried out according to ASTM C-305 test (8), mixing 450 g. of Portland cement, 1350 g. of natural sand and 225 mL of water for control mortar and the rest of mortars replacing by mass a 15% of Portland cement by original or ground SSA. Mortars were put in a mold for obtaining specimens, which were stored in a moisture room (201C) for 24 hours. Afterwards the specimens were demoulded and cured by immersion in 401C water in order to activate the hydration process until testing at 3, 7, 14 and 28 days.
Mortars for workability studies were prepared according to ASTM C-305 (8), mixing 450 g. of Portland cement, 1350 g. of sand and varying water volumes between 200-225 mL for control mortar. The rest of mortars were prepared replacing growing percentages of Portland cement by ground SSA and workability test were developed following ASTM C-109 (9) test. Some tests were developed using a mortar plasticizer (Sikanol-M) in a 0.1% in weight respecting SSA + cement.
Freshly prepared mortars were placed into a conic mold which is centered on the flow table. Mortar was put on two layers and compacted with a wooden tamper (10 times). Afterwards, the mold was removed and the table was dropped 15 times (one per second). Flow table spread (FTS) was given as a mean of maximum and minimum diameters of the spread cone.
SSA obtained from water treatment plant was analyzed and the results obtained are presented in Table 1. From among these data can be emphasized the high concentration of sulfate in SSA (12.4 % expressed in SO3content). High concentration of sulfate are due, chemical reagent used in water treatment.
Workability (FTS). The influence of original and ground SSA on mortar workability has been studied. In Figure 1. Flow Table Spread (FTS) versus SSA grinding time is represented for mortars containing a 30% of SSA and 0.5 water cement ratio. In this figure is compared the platicizer influence on FTS. An initial decrease of workability is observed when a 30% of control mortar cement is replaced by original SSA (SSA 0), a more marked decrease is observed in mortar containing plasticizer. A different behavior is observed, for mortars with or without plasticizer, when SSA grinding time increases. Mortars containing plasticizer increase FTS when grinding time do. The most important increases is observed between SSA 0 and SSA 2.5. The absence of plasticizer shows a decrease of FTS when grinding time increase. In all cases workability of mortars containing plasticizer was higher than mortars without it.
In Figure 2. FTS versus volume of water for mortars containing a growing replacement of cement by ten minutes ground SSA and 0.1% (in weight) of plasticizer is represented. As could be expected, a increase of FTS is observed when water volume do, but this behavior is more pronounced when SSA percentage is slow (15 and 30%). Probably, the important adsorption of water on SSA particles surface determines the short increase of FTS when high SSA percentages (45 and 60%) are used.
Compressive Strength (Rc). Preliminary studies make clear, in first place, that SSA did not present autocementicious hardening, whereas, secondly, mixtures of Ca(OH)2 -SSA hardened in few days. This behavior indicated that SSA could present pozzolanic activity. The influence of original and ground SSA on mortars compressive strength has been studied. Mortars containing a 15% of ash and 0.5 water / (cement + SSA) ratio were cured at 40C and tested at 3,7,14 and 28 days (Figure 3.). No plasticizer was used. The results obtained showed higher Rc in short curing time (3 and 7 days) for control mortar (without ash). When curing time increases (14 to 28 days) mortars containing ash showed equal or higher Rc than control mortar. This fact confirm pozzolanic behaviour of SSA. No significatives differences were observed among mortars containing SSA with different grinding times.
Flexural Strength (Rf). The influence of original and ground SSA on flexural strength of mortars has been studied (Figure 4). The results obtained showed higher Rf for control mortar than ash mortars except for 28 days curing time that mortar containing 2.5 minutes ground SSA that gave same Rf than control mortar. No significative tendency between SSA grinding time and Rf is observed.
Activated carbon treatment at water treatment plants is typically installed to provide removal of natural organic compounds, taste and odor compounds, and synthetic organic chemicals. Activated carbon adsorption physically attaches gas or liquid phase molecules to the surface of the activated carbon. The commonly used adsorption processes include GAC and PAC. Activated carbon is an effective adsorbent because it is a highly porous material and provides a large surface area to which contaminants may adsorb.
Activated carbon is available as PAC and GAC. The latter is used in packed-bed columns downstream of non-GAC filtration or membrane processes to adsorb organic DBP precursors. GAC packed-bed columns can also provide adsorptive capacity for taste and odor-causing compounds as well as a multitude of microconstituents. Because this type of GAC column is used in an adsorptive mode, the GAC must be periodically regenerated or replaced (perhaps every 312 months) to retain the adsorptive capacity of the process.
When designing a drinking water treatment plant, one of the most crucial issues to be addressed is the presence of NOM. It reduces water quality in terms of color, odor, and taste, and increases the dose of coagulant required for treatment, which in turn leads to high volumes of sludge formation. Another problem resulting from the presence of NOM in water is bacterial proliferation in drinking water distribution systems.
The complex matrix of water, often containing humic and/or fulvic acids as well as different organic compounds coming from urban or industrial activities, is a major reason for difficulties that arise during NOM characterization. Despite this, it is important to know the fractions of NOM present in treated water to increase the efficiency of the treatment process. Yet, usually only 1030% of the species of NOM present in water are identified.
The most common technique for NOM removal from water is coagulation and flocculation followed by sedimentation and sand filtration. Yet, the coagulant dose strongly depends on the nature of the NOM present. Filtration, which is sometimes used as an alternative treatment method for NOM removal, often suffers from membrane fouling.
IE is another example of an alternative treatment for water that has a relatively high content of NOM. Depending on the nature of the water, 1040% of NOM was reported to remain unremoved after IE treatment; this was speculated to be composed of uncharged species. As various studies demonstrate, IE is more efficient than coagulation with alum for the elimination of charged species contained in NOM. IE is proposed to be an effective choice for their treatment, given its efficiency in treating the transphilic fraction of NOM (Bond etal., 2011).
Taking into account the fact that polar components of NOM can lead to high formations of DBPs, IE can be considered more beneficial than coagulation. Yet, industrialization of the IE process normally requires the use of IE resin in a packed bed, which is only possible at the polishing stages of treatment.
Since the development of magnetic ion exchange resin, known as MIEX, it has become possible to utilize IE resin in slurry mode prior to coagulation. Due to the presence of magnetic IOPs within the core of MIEX resin, the separation step is significantly simplified, so this resin can be utilized with different reactor designs. The high stability of this resin and the fact that it can be used without pretreatment makes MIEX an extremely promising option. It is not surprising that, currently, MIEX is the most studied ion exchanger for NOM removal from water. Already in 2001, the first potable water treatment plant using a MIEX-DOC process was launched in Australia. In this plant, the MIEX-DOC step was introduced prior to conventional treatment, and a significant improvement in water quality was observed. It should be noted that MIEX is efficient for NOM removal and the prevention of chlorinated and brominated DBP formation (Hsu and Singer, 2010). Yet, NOM removal efficiency is affected by the presence of other anions in water. Therefore, further studies should be conducted to optimize the performance of IE processes and decrease the costs for industrial-scale operations.
Engineers also have to protect the public from its members' own carelessness. The case of the woman trying to open a 2-L soda bottle by turning the aluminum cap the wrong way with a pipe wrench, and having the cap fly off and into her eye, is a famous example of unpredictable ignorance. She sued for damages and won, with the jury agreeing that the design engineers should have foreseen such an occurrence. (The new plastic caps have interrupted threads that cannot be stripped by turning in the wrong direction.)
In the design of water treatment plants, engineers are taught to design the plants so that it is easy to do the right thing, and very difficult to do the wrong thing. Pipes are color-coded, valves that should not be opened or closed are locked, and walking distances to areas of high operator maintenance are minimized and protected. This is called making the treatment plant operator proof. This is not a criticism exclusively applied to operators of bioreactors and other biotechnological projects and operations. In fact, such standard operating procedures (SOPs) are crucial in any operation that involves repeated actions and a flow of activities. Hospitals, laboratories, factories, schools, and other institutions rely on SOPs.7 When they are not followed, people's risks are increased. Biosystem engineers recognize that if something can be done incorrectly, sooner or later it will, and that it is their job to minimize such possibilities. That is, both risk and reliability are functions of time.
Risk is in part a function of time, given that time appears as a factor in the variables in the exposure equation (see Eqns (5.6) and (5.7) and Table5.5Eqn (5.6)Eqn (5.7)Table5.5 in Chapter 5). The more time one spends in contact with a substance, the greater is the exposure will be to that substance. In contrast, reliability is the extent to which something can be trusted. A system, process, or item is reliable so long as it performs the designed function under the specified conditions during a certain time period. In most engineering applications, reliability means that what is designed will not fail prematurely. Or, stated more positively, reliability is the mathematical expression of success; that is, reliability is the probability that a system that is in operation at time 0 (t0) will still be operating until the end of its designed life (time t=(tt)). As such, it is also a measure of the engineering accountability. People in neighborhoods near the biotechnological facility want to know if it will work and will not fail. This is especially true for those facilities that may affect the environment, such as landfills and power plants. Likewise, when environmental cleanup is being proposed, people want to know how certain the engineers are that the cleanup will be successful.
Time shows up again in the reliability engineering literature as the so-called hazard rate, i.e., the probability of a failure per unit time. Hazard rate may be a familiar term in environmental risk assessments, but many engineers may recognize it as a failure density, or f(t). This is a function of the likelihood that an adverse outcome will occur, but note that it is not a function of the severity of the outcome. The f(t) is not affected by whether the outcome is very severe (such as pancreatic cancer and loss of an entire species) or relatively benign (muscle soreness or minor leaf damage). The likelihood that something will fail at a given time interval can be found by integrating the hazard rate over a defined time interval:
Engineers must be humble, because everything they design will fail. We can improve reliability by extending the time (increasing tt), thereby making the system more resistant to failure. For example, proper engineering design of a landfill barrier can decrease the flow of contaminated water between the contents of the landfill and the surrounding aquifer, e.g.,a velocity of a few microns per decade. However, the barrier does not completely eliminate failure, i.e., R(t)=0; it simply protracts the time before the failure occurs (increases Tf) or reduces the severity of the failure.8
Although disclosure and labeling are absolutely necessary parts of reliability in bioengineering, they are wholly insufficient to prevent accidents. During the early 1970s, jet-powered airliners were replacing propeller aircraft. The fueling system at airports was not altered, and the same trucks fueled both types of craft. The nozzle fittings for both types of fuels were therefore the same. A tragic accident occurred near Atlanta, where jet fuel was mistakenly loaded into a Martin 404 propeller craft. The engines failed on takeoff, resulting in fatalities. A similar accident occurred in 1974 in Botswana with a DC-4 and again near Fairbanks, Alaska, with a DC-6 . The fuel delivery systems had to be modified so that it was impossible to put jet fuel into a propeller-driven airplane and vice versa. An example of how this can be done is the modification of the nozzles used in gasoline stations. The orifice in the gas tank of vehicles running on unleaded fuel is now too small to take the nozzles used for either leaded fuel or for diesel fuel . Byanalogy, bioengineers must recognize that no amount of signs or training could prevent such tragedies, only due diligence and awareness of factors that can lead to failure.
For example, as cement grinding progresses and grains become smaller, the attractive thermodynamic, physical, mechanical and chemical forces result in strong adhesion of particles. This causes agglomeration (or clumping) of cement particles. This limits the increase in specific surface area, and can result in coating of the mill internals, which causes a reduction in the rate of breakage.
Grinding additives contain surfactant chemicals, which absorb on the surface of the cement grains, neutralize the surface charges and shield against the inter-particle attractive forces. Reducing the effects of inter-particle attraction forces of cement grains helps to keep particles from clumping together. This reduces void filling and improves the dispersion of the feed to the separator, both of which lead to enhanced grinding efficiency.
There is an optimum powder filling level in the mill, relative to the media volume to ensure maximum grinding efficiency. This occurs when material fills approximately 85% of the media voids. However, mill operation at high circulating loads often results in a void filling somewhat above this amount.
Since additives reduce agglomeration, and consequent dry powder cohesion, flowability is improved and thus the mill filling level and retention time are lower when additives are present. As a result, the reduction in void filling is closer to the optimum level and this increases grinding efficiency. However, it's important to ensure that the decrease in void filling is not too excessive.
Separators operate by exerting a force balance on individual particles. Finer particles have higher surface-to-mass ratio and are collected as "fine product" by the separating airflow, while coarser particles have a higher mass-to-surface ratio and are collected as "coarse returns," due to gravity as well as the centrifugal forces of the separator mechanism.
The use of grinding aids increase separator efficiency. The lower bypass and lower coarse grade efficiency of the fine particles means less fines are returned to the mill. This reduces the negative influence on the overall fineness balance around the mill, improving overall grinding efficiency.
Non-ball mill systems such as vertical mills also benefit from dry dispersion of cement additives. Grinding additives assist with de-aeration in front of the grinding roll, which allows for smoother operation (less vibration) as well as better dispersion at the nozzle ring and in the separator, resulting in less fines in the grinding bed on the table.
Similarly, in a roll press, there is improved flow of feed material into the press due to better de-aeration of the feed, resulting in better draw-in between the rolls from the feed bin and less vibration. This brings about greater stability and higher feed rate.
During the last decade, semi-finish-grinding plants have been used more and more for the energy efficient grinding of high-quality cement. In 1999, it was found that by decreasing the ball charge filling ratio it was possible to lower the specific energy demand for grinding significantly.
It was obvious, too, that the L/D ratio influences the specific energy demand and the mill throughput as well. Therefore, a huge test program was carried with a semi-industrial ball mill, which was operated in closed circuit. The mass-specific surface area of the two feed materials (intermediate product) used were quite typical for industrial semi-finish grinding plants. The values were 2200 and 3000 cm2/g according to Blaine. The product finenesses were 3000 and 3800 cm2/g, respectively. The L/D ratio of the ball mill was varied in four steps of 1.75, 2.1, 2.79 and 3.49, and the ball charge filling ratio was varied in three steps of 15%, 20% and 25%. The experiments clearly indicated that the optimal L/D ratio and the optimal ball charge filling ratio are different for each feed fineness.
The influence of the ball charge grading on the specific energy demand, characterised by the average ball diameter, was tested by means of a discontinuous laboratory ball mill. The results showed that by using a finer ball grading the specific energy demand could be lowered considerably.
The obtained results can be explained well by theoretical considerations regarding the ruling stress intensity and the number of stress events. The stress intensity expressed as the power input per ball is dependent on the ball diameter to the third power and only slightly dependent on the inner diameter of the mill. The number of stresses can be characterised by the average retention time of the ground material inside the mill if the ball charge grading remains unchanged. The optimal retention time depends not only on the feed material and the desired comminution result but also on the ball charge filling ratio and particularly on the L/D ratio. On the basis of the present results and considerations, a specific optimisation of ball mills in semi-finish-grinding plants can be done.Get in Touch with Mechanic