Environmental engineering. SHORT ANSWER theory questions. Doable on one page or
ID: 1741443 • Letter: E
Question
Environmental engineering.
SHORT ANSWER theory questions. Doable on one page or two.
a- What is the main objective of the primary and secondary wastewater treatment.
b- Name two types of material for geomembrane liners
c- What is the difference between municipal and hazardous waste landfill liners?
d- List four recyclable components of MSW e- Name two sludge dewatering technologies
f- What type of sedimentation occurs in primary clarifies and in gravity thickeners?
g- Name two types media used in trickling filters?
h- Name two types each for suspended growth and attached growth secondary treatment
i- What are the two key factors for successful secondary wastewater treatment?
j- Name two compostable solid waste water components
Explanation / Answer
section A
Primary treatment
The objective of primary treatment is the removal of settleable organic and inorganic solids by sedimentation, and the removal of materials that will float (scum) by skimming. Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD5), 50 to 70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation but colloidal and dissolved constituents are not affected. The effluent from primary sedimentation units is referred to as primary effluent. primary effluent from three sewage treatment plants in California along with data on the raw wastewaters. In many industrialized countries, primary treatment is the minimum level of preapplication treatment required for wastewater irrigation. It may be considered sufficient treatment if the wastewater is used to irrigate crops that are not consumed by humans or to irrigate orchards, vineyards, and some processed food crops. However, to prevent potential nuisance conditions in storage or flow-equalizing reservoirs, some form of secondary treatment is normally required in these countries, even in the case of nonfood crop irrigation. It may be possible to use at least a portion of primary effluent for irrigation if off-line storage is provided. Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units. Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units. In large sewage treatment plants (> 7600 m3/d in the US), primary sludge is most commonly processed biologically by anaerobic digestion. In the digestion process, anaerobic and facultative bacteria metabolize the organic material in sludge (see Example 3), thereby reducing the volume requiring ultimate disposal, making the sludge stable (nonputrescible) and improving its dewatering characteristics. Digestion is carried out in covered tanks (anaerobic digesters), typically 7 to 14 m deep. The residence time in a digester may vary from a minimum of about 10 days for high-rate digesters (wellmixed and heated) to 60 days or more in standard-rate digesters. Gas containing about 60 to 65% methane is produced during digestion and can be recovered as an energy source. In small sewage treatment plants, sludge is processed in a variety of ways including: aerobic digestion, storage in sludge lagoons, direct application to sludge drying beds, in-process storage (as in stabilization ponds), and land application.
Secondary treatment
The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids. In most cases, secondary treatment follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes. Aerobic biological treatment (see Box) is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic endproducts (principally CO2, NH3, and H2O). Several aerobic biological processes are used for secondary treatment differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter. High-rate biological processes are characterized by relatively small reactor volumes and high concentrations of microorganisms compared with low rate processes. Consequently, the growth rate of new organisms is much greater in high-rate systems because of the well controlled environment. The microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary effluent. The sedimentation tanks used in secondary treatment, often referred to as secondary clarifiers, operate in the same basic manner as the primary clarifiers described previously. The biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing. Common high-rate processes include the activated sludge processes, trickling filters or biofilters, oxidation ditches, and rotating biological contactors (RBC). A combination of two of these processes in series (e.g., biofilter followed by activated sludge) is sometimes used to treat municipal wastewater containing a high concentration of organic material from industrial sources. i. Activated Sludge In the activated sludge process, the dispersed-growth reactor is an aeration tank or basin containing a suspension of the wastewater and microorganisms, the mixed liquor. The contents of the aeration tank are mixed vigorously by aeration devices which also supply oxygen to the biological suspension . Aeration devices commonly used include submerged diffusers that release compressed air and mechanical surface aerators that introduce air by agitating the liquid surface. Hydraulic retention time in the aeration tanks usually ranges from 3 to 8 hours but can be higher with high BOD5 wastewaters. Following the aeration step, the microorganisms are separated from the liquid by sedimentation and the clarified liquid is secondary effluent. A portion of the biological sludge is recycled to the aeration basin to maintain a high mixed-liquor suspended solids (MLSS) level. The remainder is removed from the process and sent to sludge processing to maintain a relatively constant concentration of microorganisms in the system. Several variations of the basic activated sludge process, such as extended aeration and oxidation ditches, are in common use, but the principles are similar. ii. Trickling Filters A trickling filter or biofilter consists of a basin or tower filled with support media such as stones, plastic shapes, or wooden slats. Wastewater is applied intermittently, or sometimes continuously, over the media. Microorganisms become attached to the media and form a biological layer or fixed film. Organic matter in the wastewater diffuses into the film, where it is metabolized. Oxygen is normally supplied to the film by the natural flow of air either up or down through the media, depending on the relative temperatures of the wastewater and ambient air. Forced air can also be supplied by blowers but this is rarely necessary. The thickness of the biofilm increases as new organisms grow. Periodically, portions of the film 'slough off the media. The sloughed material is separated from the liquid in a secondary clarifier and discharged to sludge processing. Clarified liquid from the secondary clarifier is the secondary effluent and a portion is often recycled to the biofilter to improve hydraulic distribution of the wastewater over the filter. iii. Rotating Biological Contactors Rotating biological contactors (RBCs) are fixed-film reactors similar to biofilters in that organisms are attached to support media. In the case of the RBC, the support media are slowly rotating discs that are partially submerged in flowing wastewater in the reactor. Oxygen is supplied to the attached biofilm from the air when the film is out of the water and from the liquid when submerged, since oxygen is transferred to the wastewater by surface turbulence created by the discs' rotation. Sloughed pieces of biofilm are removed in the same manner described for biofilters. High-rate biological treatment processes, in combination with primary sedimentation, typically remove 85 % of the BOD5 and SS originally present in the raw wastewater and some of the heavy metals. Activated sludge generally produces an effluent of slightly higher quality, in terms of these constituents, than biofilters or RBCs. When coupled with a disinfection step, these processes can provide substantial but not complete removal of bacteria and virus. However, they remove very little phosphorus, nitrogen, nonbiodegradable organics, or dissolved minerals
Section b
Geomembranes are also called flexible membrane liners (FML). These liners are constructed from various plastic materials, including polyvinyl chloride (PVC) and high-density polyethylene (HDPE). The preferred material for use in MSW and secure landfills is HDPE. This material is strong, resistant to most chemicals, and is considered to be impermeable to water. Therefore, HDPE minimizes the transfer of leachate from the landfill to the environment. The thickness of geomembranes used in landfill liner construction is regulated by federal and state laws. In Ohio, HDPE geomembranes must have a minimum thickness of 0.060 inches for use in MSW landfills (OAC 3745-27-08) Geomembranes dominate the sales of geosynthetic products, at 1.8 billion USD per year worldwide, which is 35% of the market.[2] The US market is currently divided between HDPE, LLDPE, fPP, PVC, CSPE-R, EPDM-R and others (such as EIA-R), and can be summarized as follows:[citation needed] (Note that M m2 refers to millions of square meters.) ? high-density polyethylene (HDPE) ~ 35% or 105 M m2 ? linear low-density polyethylene (LLDPE) ~ 25% or 75 M m2 ? polyvinyl chloride (PVC) ~ 25% or 75 M m2 ? flexible polypropylene (fPP) ~ 10% or 30 M m2 ? chlorosulfonated polyethylene (CSPE) ~ 2% or 6 M m2 ? ethylene propylene diene terpolymer (EPDM) ~ 3% or 9 M m2
SECTION C
Types of Waste A hazardous waste may be in solid, semi-solid, liquid or gaseous form. According to the EPA, the hazardous waste can be classified into listed wastes (source-specific wastes, non-specific source wastes and unused chemical products), characteristic wastes (toxic wastes, ignitable wastes, reactive wastes and corrosive wastes) universal wastes (batteries, pesticides, mercury-containing equipment and bulbs) and mixed wastes. Municipal solid waste consists of paper, yard waste, metals, food, glass, wood, plastic and miscellaneous materials. Problems Solid waste generation is escalating with the increasing population. Disposal of solid waste in landfills is detrimental to the environment, as it can pollute the surrounding air and water. Toxic gases such as methane and carbon dioxide are formed when the waste in landfills decomposes. People living near landfills are susceptible to lung cancer, bladder cancer and leukemia. Incineration of solid waste releases toxic air pollutants, such as dioxins, which are carcinogenic and may cause birth defects. Nuclear waste is hazardous and can remain radioactive for long periods of time, thereby affecting the environment and human health. Improper hazardous waste disposal from industries causes health problems in nearby communities. The presence of cancer-causing metal arsenic and toxic metal toluene may cause memory loss, hearing loss and various other conditions. Disposal Disposal options for hazardous waste are landfills, incineration, land treatment units and injection wells. Other alternatives include recycling and reducing the use of hazardous waste. Landfill is the most prevalent disposal option for solid waste. In addition, solid waste is also burned at extremely high temperatures to reduce the waste volume. Alternative techniques of disposing solid waste include recycling and composting. Potential Uses Hazardous waste containing metal particles and ash are sent to metal recovery facilities where metal can be recovered from them. According to Science Daily, a new technology involves the recovery of uranium from the ashes of radioactive garbage to be recycled back into nuclear fuel. Recycling solid waste materials such as paper, plastic, glass, metal and rubber, transforms the old products into new ones by mechanical or chemical methods. Heat is generated during incineration of solid waste, which could be used to heat water. The steam thus produced could be used to drive turbines to generate electricity. Regulation The EPA has clear regulations on how to dispose hazardous and solid wastes. Special precautions need to be taken to dispose hazardous waste both in solid and liquid forms. EPA mandates the combustion or incineration of hazardous waste when possible. For waste in liquid form, underground injection wells should be used. For solid waste disposal, EPA has guidelines on how to design landfills, where to locate them and how to maintain them.
Section D
Typical Sludge Dewatering techniques are the following:
1. Gravity Thickening 2 Centrifuges 3 Belt Presses
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.