Common Disinfectant

The purpose of disinfection is to eliminate microorganisms or at least reduce their numbers to a “satisfactory” level. Some disinfectants are effective in killing or inactivating specific types of microorganisms and others are effective against all types. It is therefore essential to know the identity of the target microorganisms to be destroyed. However, selection of disinfectants depends not only on their effectiveness, but also on their corrosiveness and other hazards related to their handling.

The types of chemicals used for disinfection of health-care waste are mostly aldehydes, chlorine compounds, ammonium salts, and phenolic compounds. The characteristics of these common types of disinfectant as chemical disinfectant are discussed below :

Formaldehyde (HCHO)

Application

Inactivating effect against all microorganisms, including bacteria, viruses, and bacterial spores; may be applied to dry, solid waste, in combination with steam at 80 ⁰C. Contact time: 45 minutes.

Physical and chemical properties

Gas at ambient temperature; flammable and explosive in mixtures with air at concentrations of 7–73%; reactive at ambient temperature; polymerizes at temperatures below 80 ⁰C. Formalin is a 37% solution of formaldehyde. Formaldehyde odour threshold: 0.1–1ppm.

Health hazards

WHO guideline value for the general public: 0.1ppm. WHO guideline value for occupational exposure: 1 ppm for 5 minutes, with no more than 8 peaks in one working period (of up to 8 hours). Irritant effects may be experienced at concentrations of 1–3ppm upwards; exposure to concentrations above 10ppm may result in severe irritation of eyes or respiratory tract. Occupational safety limit: 1ppm in the USA. Formaldehyde has been classified as a probable human carcinogen by the International Agency for Research on Cancer; all precautions should therefore be taken to avoid inhalation of this compound during handling. NIOSH IDLH: 20ppm.1

Protective measures

Gloves and protective eye glasses should be worn during handling of formaldehyde to protect skin and eyes; in case of skin contact, the affected area should be rinsed abundantly with water.

Corrosiveness

Formalin is slightly corrosive to most metals except stainless steel and aluminium; it should be stored in stainless steel, aluminium, or polyethylene containers, in well ventilated, leakage-proof rooms.

Fire

Firefighters should wear breathing masks when tackling fires involving formaldehyde.

Comments

Formaldehyde is suitable for use as a chemical disinfectant only in situations in which a high level of chemical safety can be maintained.

Ethylene oxide (CH2OCH2)

Application

Inactivating effect against all microorganisms, including bacteria, viruses, and bacterial spores; disinfection of solid waste at temperatures of 37–55 ⁰C, at 60–80% humidity, for 4–12 hours.

Physical and chemical properties

Gas at temperatures above 10 ⁰C; flammable and explosive in mixtures with air at concentrations of 3% and above; very reactive at ambient temperature; soluble in water and most organic solvents. Odour threshold: 320–700ppm.

Health hazards

Liquid ethylene oxide and aqueous solutions are extremely irritant to skin and eyes; occupational safety limit: 1–5 ppm (depending on the country). Ethylene oxide has been classified as a human carcinogen by the International Agency for Research on Cancer; all precautions should therefore be taken to avoid inhalation of this compound during handling. NIOSH IDLH: 800ppm.

Protective measures

Gloves and protective eye glasses should be worn during handling of ethylene oxide to protect skin and eyes; in case of skin contact, the affected area should be rinsed abundantly with water; in case of eye contact, the eyes should be rinsed abundantly with water for at least 15 minutes, followed by medical examination; immediate hospital attention is needed in case of inhalation or ingestion; continuous monitoring of ethylene oxide should be performed.

Corrosiveness

Ethylene oxide is corrosive to rubber and plastics but not to metal; it is usually stored in pressurized metal containers, in liquid form, under high-pressure nitrogen gas.

Fire

Ethylene oxide fires are very difficult to stop; in case of fire, gas inflow should be stopped; CO2 or powder extinguishers should be used; firefighters should wear protective masks.

Comments

The use of ethylene oxide is not recommended because of significant related health hazards.

Glutaraldehyde (CHO-(CH2)3-CHO)

Application

Active against both bacteria and parasite eggs. Available in 25–50% aqueous solutions; should be used as 2% aqueous solution with acetate buffer. Contact times: 5 minutes for disinfection of medical equipment; 10 hours to kill spores. For waste, operating parameters should be adjusted on the basis of bacteriological tests.

Physical and chemical properties

Liquid; very reactive; non-flammable. Addition of methanol allows for long-term conservation.

Health hazards

Concentrated solutions are irritant to eyes and skin; occupational safety limit depends on the country (e.g. 0.2ppm or 0.7mg/m³ in France).

Protective measures

Gloves and protective eye glasses should be worn during handling of glutaraldehyde to protect skin and eyes; in case of skin contact the affected area should be rinsed abundantly with water; in case of eye contact, the eyes should be rinsed abundantly with water for at least 15 minutes, followed by medical examination.

Corrosiveness

Aqueous solutions of glutaraldehyde are corrosive to most metals; usually stored in stainless steel containers, steel containers lined with phenolic resins, or reinforced polyethylene containers, in well ventilated, leakage-proof rooms.

Comments

Glutaraldehyde is suitable for use as a chemical disinfectant only in situations in which a high level of chemical safety can be maintained. Glutaraldehyde waste should never be discharged in sewers; it may be neutralized through careful addition of ammonia or sodium bisulfite; it may also be incinerated after mixing with a flammable solvent.

Sodium hypochlorite (NaOCl)

Application

Active against most bacteria, viruses, and spores; not effective for disinfection of liquids with high organic content such as blood or stools; widely used for treatment of wastewater. For waste, operating parameters should be adjusted on the basis of bacteriological tests.

Physical and chemical properties

Available as aqueous solution with 2–12% of active chlorine; at ambient temperature slowly decomposes into sodium chlorate, sodium chloride, and oxygen; solutions of low concentration are more stable; solutions should be protected from light which accelerates decomposition; reacts with acids to produce hazardous chlorine gas.

Health hazards

Irritant to skin, eyes, and respiratory tract; toxic.

Protective measures

Gloves and protective eye glasses should be worn during handling of sodium hypochlorite to protect skin and eyes; in case of eye contact, the eyes should be rinsed abundantly with water.

Corrosiveness

Aqueous solutions are corrosive to metals; usually stored in plastic containers in well ventilated, dark, and leakage-proof rooms; should be stored separately from acids.

Comments

Sodium hypochlorite may be widely used because of relatively mild health hazards. Unused solutions should be reduced with sodium bisulfite or sodium thiosulfate and neutralized with acids before discharge into sewers. Large quantities of concentrated solutions should be treated as hazardous chemical waste.

Chlorine dioxide (ClO2)

Application

Active against most bacteria, viruses, and spores; widely used, for instance in drinking-water preparation, sanitation, and wastewater treatment.

Physical and chemical properties

Reddish-yellow gas at ambient temperature; explosion limit: >10% in air; will react with water or steam to produce corrosive fumes of hydrochloric acid.

Health hazards

Irritant to skin, eyes, and respiratory tract; toxic. NIOSH IDLH: 5ppm.

Protective measures

In case of eye contact, eyes should be rinsed abundantly with water; contaminated areas of the body should be washed with soap and water.

Corrosiveness

Containers of chlorine dioxide should be stored in well ventilated and leakage-proof rooms.

Chemical Disinfection of Waste

Chemical disinfection, used routinely in health care to kill microorganisms on medical equipment and on floors and walls, is now being extended to the treatment of health-care waste. Chemicals are added to waste to kill or inactivate the pathogens it contains; this treatment usually results in disinfection rather than sterilization. Chemical disinfection is most suitable for treating liquid waste such as blood, urine, stools, or hospital sewage. However, solid—and even highly hazardous— health-care wastes, including microbiological cultures, sharps, etc., may also be disinfected chemically, with the following limitations:

• Shredding and/or milling of waste is usually necessary before disinfection; the shredder is often the weak point in the treatment chain, being subject to frequent mechanical failure or breakdown.

• Powerful disinfectants are required, which are themselves also hazardous and should be used only by well trained and adequately protected personnel.

• Disinfection efficiency depends on operational conditions.

• Only the surface of intact solid waste will be disinfected.

Human body parts and animal carcasses should not normally be disinfected chemically. If alternative facilities for disposal are not readily available, however, they may be shredded and then subjected to chemical disinfection. In planning the use of chemical disinfection, requirements for the eventual disposal of the residues should be carefully considered; improper disposal could give rise to serious environmental problems.

Microbial resistance to disinfectants has been investigated and it is possible to list the major groups of microorganisms from most to least resistant as follows: bacterial spores—mycobacteria—hydrophilic viruses—lipophilic viruses—vegetative fungi and fungal spores—vegetative bacteria. A disinfectant known to be effective against a particular group of microorganisms will also be effective against all the groups that are less resistant. Most parasites, such as Giardia and Cryptosporidium spp., are significantly resistant to disinfection and are usually rated between the mycobacteria and the viruses.

The effectiveness of disinfection is estimated from the survival rates of indicator organisms in standard microbiological tests.

At present, chemical disinfection of health-care waste is limited in industrialized countries. However, it is an attractive option for developing countries, particularly for treating highly infectious physiological fluids, such as patients’ stools in case of cholera outbreaks.

Chemical disinfection is usually carried out on hospital premises. Recently, however, commercial, self-contained, and fully automatic systems have been developed for health-care waste treatment and are being operated in industrial zones. The disinfected waste may be disposed of as non-risk health-care waste, but the chemical disinfectants may create serious environmental problems in case of leakage or after disposal.

Chemical disinfection of hospital sewage requires less powerful—and

less hazardous—chemicals.

Operational considerations

The speed and efficiency of chemical disinfection will depend on operational conditions, including the following:

• the kind of chemical used;

• the amount of chemical used;

• the contact time between disinfectant and waste;

• the extent of contact between disinfectant and waste;

• the organic load of the waste;

• operating temperature, humidity, pH, etc.

Shredding of waste before disinfection

Shredding of solid health-care waste before disinfection is essential for the following reasons:

• to increase the extent of contact between waste and disinfectant by

increasing the surface area and eliminating any enclosed spaces;

• to render any body parts unrecognizable to avoid any adverse visual

impact on disposal;

• to reduce the volume of waste.

Water is usually added during shredding; it prevents excessive warming and facilitates subsequent contact with the disinfectant. Excess water may have to be treated, e.g. by chemical disinfection.

Rotating-blade shredders are used most commonly, and consist of blades attached to two wheels that rotate in opposite directions. The presence of an excessive proportion of sharps in waste may cause deterioration of the shredder.

Shredding of waste before disinfection plus subsequent compacting can reduce the original waste volume by 60–90%.

Types of chemical disinfectants

The aim of disinfection is to eliminate microorganisms or at least reduce their numbers to a “satisfactory” level. Some disinfectants are effective in killing or inactivating specific types of microorganisms and others are effective against all types. It is therefore essential to know the identity of the target microorganisms to be destroyed. However, selection of disinfectants depends not only on their effectiveness, but also on their corrosiveness and other hazards related to their handling.

The types of chemicals used for disinfection of health-care waste are mostly aldehydes, chlorine compounds, ammonium salts, and phenolic compounds. The use of ethylene oxide is no longer recommended for waste treatment because of the significant hazards related to its handling. However, it has been used in the past and may still be in use in some places.

The use of ozone (O3) for disinfection of waste is currently being investigated. This disinfectant is strong and relatively safe.

Most of the disinfectants described here are stable for at least 5 years and—with the exception of sodium hypochlorite—remain effective for 6–12 months after opening of the container.

Powerful disinfectants are often hazardous and toxic; many are harmful to skin and mucous membranes. Users should therefore wear protective clothes, including gloves and protective eye glasses or goggles. Disinfectants are also aggressive to certain building materials and should be handled and stored accordingly.

Small amounts of disinfectants can be discharged into sewers without pretreatment, provided that there is an adequate sewage-treatment process; large amounts of disinfectants should never be discharged into sewers. No disinfectants should be discharged into natural water bodies.

Non-tectonic Surface Deformation

Non-tectonic phenomena that can result in surface deformation at a facility site include 

1) Glacially-induced faulting,

2) Growth faulting,

3) Collapse due to underground voids such as found in Karst terrain, and

4) Subsidence due to extensive fluid withdrawal.

 

Glacially-induced faults

Glacially-induced faults generally do not represent a significant displacement hazard because the conditions that created them are no longer present. If residual stresses from Pleistocene glaciations are still present, these features should be investigated with respect to their relationship to current in-situ stresses.

Growth faults

Large, naturally occurring growth faults, as found in the coastal plain of Texas and Louisiana, can pose a surface displacement hazard even though offset most likely occurs at a slower rate than that of tectonic faults, they shall be identified and avoided in siting, and their displacements shall also be monitored. Some growth faults and antithetic faults related to growth faults are not easily identified; therefore, investigations of the potential amount and character of displacement shall be undertaken in regions where growth faults are known to be present.

Underground cavities

The existence of cavities in some geological materials (e.g. limestone, gypsum, anhydrite, etc.,) may lead to ground collapse. If collapse features are present, they shall be considered and investigated with respect to their potential for causing deformation of the facility site and, if so, whether engineered stabilization measures are feasible.

Earthing System

The object of an earthing system is to provide a system of conductors, as nearly as possible at a uniform and a zero, or earth, potential. The purpose of this is to ensure that, in general, all parts of the equipment and installation other than live parts shall be at earth potential, thus ensuring that persons coming in contact with these parts shall also be at earth potential at all times.

Circuit and system earthing

The purpose of circuit and system earthing is to limit excessive voltage from line surges, from cross-overs with higher voltage lines, or form lightning, and to keep non current carrying enclosures and equipment at zero potential with respect to earth. Earthing the system helps facilitates the opening of overcurrent protection devise in case of earth faults.

Earthing asspciated with current carrying conductor is normally essential for the security of the system and is generally known as system earthing, while earthing of non-current carrying metal work and conductor is essential for the safety of human life, animals, and property and it is generally known as equipment earthing.

The earthing arrangements shall be such that:

The value of resistance from the consumer’s main earthing terminal to the earthed point of the supply, or to earth, is in accordance with the protective and functional requirements of the installation, and expected to be continuously effective,

- Earth fault currents and earth leakage currents likely to occur are carried without danger, particularlyfrom the point of view of thermal, thermomechanical and electromechanical stresses.

Precaution shall be taken against the risk of damage to other metallic parts through electrolysis.

Where a number of installations have separate earthing arrangements, protective conductors running between any two of the separate installations shall either be capable of carrying the maximum fault current likely to flow through them, or be earthed within one installation only and insulated from the earthing arrangements of any other installation. In the latter circumstances, if the protective conductor forms part of a cable, the protective conductor shall be earthed only in the installation containing the associated protective device.

Methods of earthing

The usual method of earthing is to join the exposed metal work

Structural Selection Criteria for Coastal Area

The placement of engineered structures on or near the coastline must be contemplated with extreme care. In general, alteration of the natural coastline comes with an associated environmental penalty. Hard (structures made of stone, steel, concrete, etc.) or soft (beach nourishment, sediment filled bags, etc.) engineering structures can alter many physical properties of the beach to often induce undesired effects.

These alterations of natural processes can take the form of increased reflectivity to incident waves, interruption of longshore currents and resulting longshore sediment transport, alteration of incident wave patterns and the generation of abnormal underwater topography.

In the selection and design of coastal structures, there are a diverse set of criteria that need to be considered. Structural stability criteria and functional performance criteria encompass two areas of primary concern for selection and evaluation of coastal structures.

Structural stability criteria are usually associated with extreme environmental conditions, which may cause severe damage to, or failure of a coastal structure. These stability criteria are, therefore, related to episodic events in the environmental (severe storms, hurricanes, earthquakes) and are often evaluated on the basis of risk of encounter probabilities. A simple method for evaluating the likelihood of encountering an extreme environmental event is to calculate the encounter probability (Ep) as

Where T_R = the return period

L = the design life of the structure (see Borgman, 1963)

The greatest limitation to structural stability criteria selection is the need for a long-term data base on critical environmental variables sufficient enough to determine reasonable return periods for extreme events. For example coastal wave data for U.S. coasts is geographically sparse and in most locations where it exists the period of collection is in the order of 10 years. Since most coastal structures have a design life well in excess of 10 years, stability criteria selection often relies on extrapolation of time limited data or statistical modeling of environmental processes.

Functional performance criteria are generally related to the desired effects of a coastal structure. These criteria are usually provided as specifications for design such as the maximum acceptable wave height inside a harbor breakwater system or minimum number of years for the protective lifetime of a beach nourishment fill project. Functional performance criteria are most often subject to compromise because of initial costs.

Seismic Slope Stability

Surveys of earth dam performance during earthquakes suggest that embankments constructed of materials that are not vulnerable to severe strength loss as a result of earthquake shaking generally perform well during earthquakes [Seed et al., 1978]. The potentially catastrophic consequences of an earth embankment material that undergoes severe strength loss during earthquake shaking is demonstrated by the near failure of the Lower San Fernando dam as a result of the 1971 San Fernando earthquake [Seed et al., 1975]. The center and upstream sections of the dam slid into the reservoir because a large section within the dam liquefied. The slide movement left only 1.5 m of earth fill above the reservoir level. Fortunately, the reservoir level was 11 m below the original crest at the time of the earthquake. Still, because of the precarious condition of the dam after the main shock, 80,000 people living downstream of the dam were ordered to evacuate until the reservoir could be lowered to a safe elevation.

Therefore, Considerable attention has been focused over the last few decades on developing procedures to analyzethe seismic performance of earth embankments [e.g., Newmark, 1965; Seed, 1979; Marcuson et al., 1992]. The first issue that must be addressed is an evaluation of the potential of the materials comprising the earth structure to lose significant strength under cyclic earthquake loading. Saturated cohesionless materials (gravels, sands, and nonplastic silts) that are in a loose state are prime candidates for liquefaction and hence significant strength loss. Experience has shown that cohesionless materials placed by the hydraulic fill method are especially vulnerable to severe strength loss as a result of strong shaking. A modified version of the Seed and Idriss [1982] simplified method has been employed to evaluate the liquefaction potential of cohesionless soils in earth slopes and dams [see Seed and Harder, 1990]. Certain types of clayey materials have also been shown to lose significant strength as a result of cyclic loading. If clayey materials have a small percentage of clay-sized particles, low liquid limits, and high water contents, the material’s cyclic loading characteristics should be determined by cyclic laboratory testing [Seed and Idriss, 1982].

The embankment, however, may undergo some level of permanent deformation as a result of the earthquake shaking. With well-built earth embankments experiencing moderate earthquakes, the magnitude of permanent seismic deformations should be small, but marginally stable earth embankments experiencing major earthquakes may undergo large deformations that may jeopardize the structure’s integrity. Simplified procedures have been developed to evaluate the potential for seismic instability and seismically induced permanent deformations [e.g., Seed, 1979; Makdisi and Seed, 1978], and these procedures can be used to evaluate the seismic performance of earthen structures and natural slopes.

In pseudostatic slope stability analyses, a factor of safety against failure is computed using a static limit equilibrium stability procedure in which a pseudostatic, horizontal inertial force, which represents the destabilizing effects of the earthquake, is applied to the potential sliding mass. The horizontal inertial force is expressed as the product of a seismic coefficient, k, and the weight, W, of the potential sliding mass (Fig. 1) If the factor of safety approaches unity, the embankment is considered unsafe. Since the seismic coefficient designates the horizontal force to be used in the stability analysis, its selection is crucial. The selection of the seismic coefficient must be coordinated with the selection of the dynamic material strengths and minimum factor of safety, however, as these parameters work together to achieve a satisfactory design. For earth embankments, case histories are available which have guided the selection of these parameters. For example, Seed [1979] recommends using appropriate dynamic material strengths, a seismic coefficient of 0.15, and a minimum factor of safety of 1.15 to ensure that an embankment composed of materials that do not undergo severe strength loss performs satisfactorily during a major earthquake.

A significant limitation of the pseudostatic approach is that the horizontal force, representing the effects of an earthquake, is constant and acts in only one direction. With dynamically applied loads, the force may be applied for only a few tenths of a second before the direction of motion is reversed. The result of these transient forces will be a series of displacement pulses rather than complete failure of the slope. Normally, a certain amount of limiting displacement during an earthquake event is considered tolerable. Hence, if conservative strength properties are selected and the seismic coefficient represents the maximum disturbing force (i.e., maximum shear stress induced by the earthquake on the potential sliding surface; see Seed and Martin, 1966), a factor of safety of one is likely to ensure adequate seismic performance. Other conservative combinations of these parameters could be developed, but their use must be evaluated in the context that their use in analysis ensures a design that performs well during earthquakes (i.e., tolerable deformations).

Seismically induced permanent deformations are generally calculated using a Newmark-type procedure. The Newmark [1965] analysis assumes that relative slope movements would be initiated when the inertial force on a potential sliding mass was large enough to overcome the yield resistance along the slip surface, and these movements would stop when the inertial force decreased below the yield resistance and the velocities of the ground and sliding mass coincided. The yield acceleration is defined as the average acceleration producing a horizontal inertial force on a potential sliding mass which gives a factor of safety of one and can be calculated as the seismic coefficient in pseudostatic slope stability analyses that produces a safety factor of one. By integrating the equivalent average acceleration [see Bray et al., 1993] acting on the sliding surface in excess of this yield acceleration as a function of time, the displacement of the slide mass can be estimated. A commonly used procedure for calculating seismically induced permanent deformations has been developed by Franklin and Chang [1977] and computer programs are available [e.g., Pyke and Beikae, 1991]. Simplified chart solutions have been developed by Makdisi and Seed [1978] for earth embankments.

An emerging area of concern in many regions of the world is the seismic performance of waste fills. For example, recent U.S. federal regulations (40 CFR 258-USEPA 1991: Subtitle D) require that municipal solid waste landfills located in seismic impact zones be designed to resist earthquake hazards. Since these designated seismic impact zones encompass nearly half of the continental U.S., these regulations are having a pronounced impact on the design of new landfills and the lateral expansion of existing landfills. The results of a comprehensive study of the effects of the characteristics of the waste fill, subsurface soils, and earthquake ground motions are presented in Bray et al. [1993]. The investigators found that the seismic loading strongly depends on the dynamic properties and height of the waste fill. General design considerations regarding the seismic stability of solid waste landfills are discussed in Repetto et al. [1993].

Preparation and Regeneration of Activated Carbon

The crystals of diamond or graphite or as fullerene spheres are generally available forms of Pure carbon. The atoms in graphite are arranged as sheets of hexagons and the atoms in diamond are arranged as tetrahedra. Activated carbon grains consist of random arrays of microcrystalline graphite. Such grains can be made from many different organic substances, but various grades of coal are most commonly used, because the product is hard, dense, and easy to handle.

The raw material is first carbonized at about 500°C in a nonoxidizing atmosphere. This produces a char that contains some residual organic matter and many small graphite crystals. The char is then subjected to a slow oxidation in air at about 500°C or steam or carbon dioxide at 800 to 950°C. The preferred activation atmospheres are steam and carbon dioxide, because the oxidation reactions are endothermic and more easily controlled. The slow oxidation removes residual organic matter and small graphite crystals and produces a network of microscopic pores. The higher temperature promotes the formation of larger graphite crystals, which reduces the randomness of their arrangement.

It is almost always economical to recover and regenerate spent carbon, unless the quantities are very small. This is true even of powdered carbon, if it can be easily separated from the process stream. The economic break point for on-site regeneration depends on carbon usage and varies somewhere between 500 and 2000 lb/day (Snoeyink, 1990). Larger amounts should be shipped to commercial regeneration plants.

Spent carbon can be regenerated as it was made by heating in rotary kiln, fluidized bed, multiple hearth, or infrared furnaces (von Dreusche, 1981; McGinnis, 1981). In multiple hearth furnaces, the required heat is supplied by the hot combustion gases of a fuel. The fuel is oxidized at only small excess air to minimize the amount of oxygen fed to the furnace. In infrared furnaces, heating is supplied by electrical heating elements. This method of heating provides somewhat better control of oxygen concentration in the furnace, but some oxygen enters with the carbon through the feedlock.

The usual regeneration stages in any furnace are as follows (McGinnis, 1981; Snoeyink, 1990):

• Drying stage — usually operated at about 200 to 700°C, depending on furnace type

• Pyrolysis stage — usually operated at about 500 to 800°C

• Coke oxidation stage — usually operated at about 700°C for water treatment carbon and at about
900°C for wastewater treatment carbons (Culp and Clark, 1983)

The pyrolysis stage vaporizes and cokes the adsorbed organics. The vaporized material diffuses into the hot gases in the furnace and either reacts with them or is discharged as part of the furnace stack gas. The coked material is oxidized by water vapor or by carbon dioxide, and the oxidation products exit in the furnace stack gases:

For water treatment carbons, the pyrolysis and activation stages are operated at temperatures below those required to graphitize the adsorbate char. Consequently, the newly formed char is more reactive than the original, largely graphitized activated carbon, and it is selectively removed by the oxidation process. Wastewater treatment carbons are oxidized at temperatures that graphitize as well as oxidize the adsorbate char. Significant amounts of the original carbon are also oxidized at this temperature.

Regeneration losses are generally on the order of 5 to 10% by weight. This is due mostly to spillage and other handling and transport losses, not to furnace oxidation. There is usually a reduction in grain size, which affects the hydraulics of fixed bed adsorbers and the adsorption rates. However, the adsorptive capacity of the original carbon is little changed (McGinnis, 1981).

CONDITIONS FOR LIQUEFACTION

Liquefaction

The term liquefaction refers to the physical change that occurs when certain soils are shaken and transformed from solid ground capable of supporting a structure to a quicksand-like liquid with a greatly reduced ability to bear the weight of a building.

Liquefaction hazard

Liquefaction poses a real, identifiable hazard to structures built on the ground or buried beneath the surface. Damage to buildings caused by liquefaction can result in structural collapse and loss of life or injuries. Loss of the soil capacity to support the weight of a structure can have disastrous effects.

The damaging effects of liquefaction were dramatically displayed during the October 17, 1989 Loma Prieta earthquake. This magnitude 7.1 event triggered soil liquefaction over a wide area, but particularly in the Marina district of San Francisco about 50 miles from the epicenter. Many buildings were damaged or destroyed due to foundation failure and thousands of homes were left without gas or water when buried utility lines ruptured due to liquefaction-induced lateral spreading. Natural gas from ruptured gas lines ignited and consumed a block-wide area. Firefighting was hampered because of the disabled water system.

Factors to occur liquefaction

Three critical factors must be present for sediments to be prone to liquefaction. The sediment must be

1) saturated with ground water,

2)composed of sand or silt-sized particles, and

3) compacted fairly loose.

For liquefaction to occur, all three factors must be present at the same time; for example, neither a loosely compacted, dry sand, or a saturated, densely compacted sand would be prone to liquefaction because one of the three critical liquefaction elements is missing. The Liquefaction Potential Map for Salt Lake County shows that the most liquefaction-prone areas (the High and Moderate areas) are located along the valley floor, tributary stream channels, and near the Great Salt Lake. Soils in foothill areas are generally less susceptible to liquefaction because they are coarser, and not saturated by shallow groundwater

Ground Water

Sediments must be saturated with ground water in order to liquefy during an earthquake. A shallow “perched” water table will contribute to liquefaction conditions and should not be disregarded or confused with deeper water levels recorded in culinary water well logs. Fluctuations in the shallow ground water level will also affect liquefaction conditions. Seasonal or cyclical “wet “periods often cause ground water levels to rise, saturating shallower sediments, and perhaps increasing the liquefaction potential in an area.

Grain Size

The size of the sediment particles controls the size of the pore spaces. This is critical in clay and fine silt grains (those less than 1/32 of an inch in diameter) because, although water can fill the small pore spaces, the flow of water between pores becomes so restricted that liquefaction becomes difficult. Gravel particles (larger than 1/5 of an inch in diameter) pose a different situation. Due to the much larger mass of the grains and generally higher porosity, the great intensity and duration of ground shaking that is required to induce liquefaction rarely occurs, except in the largest earthquakes. Generally, only sands and coarse silts combine the optimum grain mass and pore-space geometry to liquefy, given the intensity of shaking expected in a moderate to large Wasatch Front earthquake. The sands and silts must also be relatively “clean” for liquefaction to occur. This means that liquefaction is most likely to occur in sands and coarse silts with a uniform grain size. A clayey-sand, for example, would have a reduced liquefaction potential because the clay-sized particles fit between the sand grains, “tightening” up the framework and increasing soil cohesion.

Soil Density

Loose compaction of the soil also contributes to the liquefaction potential. The more densely the grains are compacted in the framework, the greater the earthquake-shaking intensity, or acceleration, needed to raise pore pressures enough to shift the grains. It is unlikely that a typical Wasatch Front earthquake could provide sufficient shaking to induce liquefaction in very densely compacted soils.

Soil density generally increases with the age and depth of deposits. Sediments tend to compact over time and with burial, increasing their density. Historically, liquefaction has been observed mainly in sediments less than 45 feet below the ground surface.

Cyclic Loading of Soil During Earthquakes

The information about the cyclic loading of the soil during an earthquake is essential for any evaluation of liquefaction and potential ground failure. The ground motions used to represent the cyclic loading are applied for the initial liquefaction hazard assessment and, if necessary, in the design of mitigation strategies involving ground treatment. Fundamentally, the best measure of cyclic demand would account for the intensity, duration, and to a lesser extent the frequency content of the input motions. These three aspects of loading are explicitly accounted for using energy-based concepts, wherein the cyclic energy per unit volume of soil can be calculated from time-histories of particle motion or stress-strain plots. Although energy methods have been successfully applied to laboratory data and field case studies (Sunisakul 2004) these methods have not been widely adopted in engineering practice. Instead, the most commonly employed methods of analysis relate the intensity of shaking to either the horizontal acceleration or cyclic stress in the soil layer of interest, and the duration of the motions through simple, empirical magnitude-dependent scaling factors (Youd et al 2001, Seed et al 2003, Idriss and Boulanger, 2004). The characteristics of the horizontal ground motions within the soil column (i.e. acceleration, stress, strain time histories) are often computed using dynamic soil response models such as SHAKE (Schnabel et al 1972), ProShake 2004, DESRA (Lee and Finn 1978) and SUMDES (Li et al 2000). The input, or bedrock, ground motions required for these numerical models are selected on the basis of their similarity to target motions established using empirical ground motion relationships that account for factors such as the style of faulting, earthquake magnitude, source-to-site distance, and rock stiffness. Recorded ground motions can be easily obtained from on-line catalogs (web sites for CGS, COSMOS, MCEER, PEER, and USGS ground motion catalogs are provided in the reference list under Strong Motion Databases).

The characteristics of the bedrock, or firm soil, motions at a specific site will depend on several geologic and geographic variables. These include

1) The regional tectonic environment,


2) The seismicity of the region,


3) The proximity of the site to active faults, and


4) on the exposure interval of interest in design (e.g. 500, 1,000, or 2,500 year motions).

A complete seismic hazard evaluation for ground motions at a site must address both the spatial and temporal occurrence of earthquakes. In Oregon, the primary seismic sources are associated with the Cascadia Subduction Zone (interface or “mega-thrust,” and intraplate or intra-slab earthquakes), shallow crustal events, and to a lesser degree seismicity related to volcanic activity. The locations of each of these earthquake sources have been determined, or in some cases estimated, using an array of geologic and geophysical methods of field investigation, in situ imaging, and numerical modeling. The faults have been mapped (Geomatrix 1995, USGS 2004b, c) using the latest input and consensus from the geoscience community. The geoscience community acknowledges that the current understanding of seismicity rates in Oregon is incomplete due to factors such as the short historic record of earthquakes, the relatively long recurrence interval between events, and environmental controls that obliterate the geomorphic expression of most faults. To account for this source of uncertainty most seismic hazard analyses incorporate spatially random, “areal sources” to the collective hazard in the region. Once the locations of all of the regional sources have been established the occurrence of earthquakes as a function of time and magnitude (i.e. the rate of seismicity) is required.


The full characterization of source locations and the rate of occurrence of earthquakes is the basis of the seismic hazard evaluation. These primary factors, along with the location of the site relative to the faults, are used to estimate the characteristics of the ground motions utilized in analysis. The temporal occurrence of earthquakes along a specific fault defines the rate of seismicity associated with that source. Of primary interest in seismic analysis and design is the aggregate seismicity for all sources that may impact the structure of interest. This requires that the rate of seismicity for all sources is estimated. The recurrence interval of the maximum credible earthquake along a specific fault is often based on its slip rate and the rupture area required for an event of that magnitude (McGuire 2004). The likelihood of ground motions of a certain level is then a function of the rate of seismicity and the length of the time over which the observation is made. The time interval is referred to alternatively as the exposure time, mean return time, or recurrence interval, and it is specified for each project based on the importance of the structure. The exposure times associated with these designations are 500 and 1,000 years, respectively. The intensity and duration of the ground motions at a given site will be different for these two return periods, and the liquefaction hazard will clearly reflect these differences.

Design Criteria for Hydraulic Analysis and Design of Bridges

Structures with a centerline span of 20 feet or more can be defined as Bridges. But, Design criteria for structures designed hydraulically irrespective of span length are treated as bridges in this blog. The following general criteria shall be used in the hydraulic analysis and design of bridge:

a) The final design selection should consider the maximum backwater allowed by the National Flood

Insurance Program (NFIP) unless exceeding the limit can be justified by special hydraulic conditions;

b) For sites outside of Federal Emergency Management Agency (FEMA) regulation, the backwater shall not cause increased flood damage to property upstream of the crossing;

c) The final design should not significantly alter the existing flow distribution in the floodplai

d) The "crest-vertical curve profile" is the preferred highway bridge crossing profile when allowing for embankment overtopping at a lower discharge and for adequate deck drainage;

f) Sag vertical curves can cause deck drainage to pond and ice up on the bridges and should be avoided;

g) Horizontal curve transitions cause water to flow across lanes and should not be located on a bridge because of icing and hydroplaning problems;

h) Clearance or freeboard should be provided between the low girder and the design water surface to allow for the passage of ice and debris;

i) The design capacity of any bridge will be the flow that will pass through the bridge with adequate freeboard and without roadway overtopping;

j) Estimate all degradation and aggradation plus contraction scour and local scour for the design year and for the 500-year event. Indicate the total scour envelope with a continuous line drawn such that the structural designer may adequately design substructure components. Scour depths are to be estimated with consideration of the local geology;

k) Velocities through the structure(s) will not damage either the highway facility or increase damages to adjacent property;

l) Pier spacing and orientation, and abutment location shall be designed to minimize flow disruption and

potential scour. Bridge piers should not be placed in the main channel area;

m) Foundation design and/or scour countermeasures shall be made to avoid failure by scour. Typically, substructure components are designed to avoid failure by scour;

n) Although appropriate in some debris prone streams, connecting a discrete pier column to a debris-deflecting wall can significantly increase scour depths if the channel alignment ever shifts. A debris-deflecting wall can also greatly increase the stiffness of a pier that reduces the number of available design options. More preferably, a long span bridge design reduces the number of piers and therefore, reduces the benefits derived from debris deflecting walls. It is now often more efficient for a designer to simply design a pier (and if necessary the superstructure) for increased stream loads due to debris;

o) When two or more bridges are constructed in parallel over a channel, care should be taken to align the piers and to provide streamlined grading and protection for abutments. This abutment grading is to minimize expansion or contraction of flow between the two bridges;

p) Commercial mining of sands and gravel in streams is common because the material is clean and well graded and the stream replenishes the supply. Borrow pits, either upstream or downstream of a highway-stream crossing, can cause or aggravate scour at the bridge. This fact should be considered when calculating bridge scour, and should be estimated by sediment transport modeling;

q) Disruption of ecosystems is to be minimized. Consideration is to be given to the preservation of valuable characteristics that are unique to the floodplain and stream;

r) Economic analysis of the design shall include complete life cycle costs and benefits. Factors that should be considered are construction, maintenance, operation, as well as any potential liabilities;

s) Adequate right-of-way shall be provided upstream and downstream of structure for maintenance operation.