Basic Environmental Technology - Wastewater Treatment PDF

Basic Environmental Technology Water Supply, Waste Management, and Pollution Control SIXTH EDITION CHAPTER 10 Wastewater...

Basic Environmental Technology Water Supply, Waste Management, and Pollution Control SIXTH EDITION CHAPTER 10 Wastewater Treatment and Disposal Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Legislation and Standards: Early effort at the federal level to guide the nation’s clean water strategy: Water Quality Act of 1965 : Strengthened by the Federal Water Pollution Control Act Amendments of 1972. Goal: encourage individual states to clean up surface waters to the extent that they would once again be “swimmable and fishable.” The Federal Water Pollution Control Act was amended again in 1977. The focus of U.S. Environmental Protection Agency (EPA) priorities was redefined; and enforcement powers were strengthened. The Federal Water Pollution Control Act has been referred to as the Clean Water Act (CWA). Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Legislation and Standards: The strategy of reducing pollution from point sources is implemented by the National Pollution Discharge Elimination System (NPDES). All municipal or industrial treatment facilities that discharge wastewater effluents must obtain an NPDES Discharge Permit from the EPA or a delegated state agency. The NPDES permits state the allowable amounts of specific pollutants that a particular facility can discharge into the environment. Implementation of the NPDES permit program effectively established a system of effluent standards for water pollution control. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved FIGURE 10-1 Overview of a centralized wastewater treatment system. Screening, grit removal, and sedimentation (settling) are primary treatment processes. Secondary treatment usually involves biological processes and additional settling. Not all sewage treatment plants require tertiary (or advanced) treatment. Primary Treatment Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved  Effluent limits are based on the best conventional technology, which includes a combination of primary, secondary, and sometimes tertiary treatment. Most treatment plants in the United States are required to use some form of secondary treatment, which removes at least 85 percent of BOD and suspended solids from the wastewater. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Treatment Efficiency Treatment efficiency can be defined as the ratio of the amount of pollutants removed to the amount of pollutants in the raw wastewater. PIN = concentration of pollutant flowing into the treatment system POUT = concentration of pollutant flowing out of the system Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : Raw sewage flowing into a treatment plant (the plant influent) has a BOD5 value of 200 mg/L. What is the maximum concentration of BOD5 allowed in the treated sewage discharge (the plant effluent) if the required treatment efficiency is 85 percent? If the flow rate is 5 mgd, how many pounds of BOD will be discharged per day? Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : Raw sewage flowing into a treatment plant (the plant influent) has a BOD5 value of 200 mg/L. What is the maximum concentration of BOD5 allowed in the treated sewage discharge (the plant effluent) if the required treatment efficiency is 85 percent? If the flow rate is 5 mgd, how many pounds of BOD will be discharged per day? Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved  Pretreatment of Industrial Wastewater Wastewater discharged by industry often contains toxic chemicals, such as cyanide from electroplating processes and lead from battery manufacturing plants. Pre-treatment: the removal of pollutants from industrial sewage before it is discharged into a municipal sewerage system to reduce its toxicity’ Two sets of rules are now in effect under the National Pretreatment Program. Categorical pretreatment standards Prohibited discharge standards, Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Pretreatment of Industrial Wastewater Categorical pretreatment standards are industry-specific; they mandate different requirements for each type of industry. For example, there is a categorical standard for the iron and steel industry that limits the ammonia and cyanide discharged by any firm in that industry into a municipal sewerage system. Prohibited discharge standards, are substance-specific; they prohibit any discharge to sewer systems of certain types of wastes from all sources. For example, the discharge of any wastewater with pollutants that can create a fire hazard or explosion in the sewage system is not allowed. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Primary Treatment Wastewater treatment processes can be categorized as biological, chemical, or physical (i.e., mechanical). Primary treatment refers to the first group of unit processes to remove pollutants from wastewater at a treatment plant. These are all physical processes. ( screening, comminution, and grit removal) Untreated or raw wastewater usually flows continuously, under the force of gravity. Preliminary treatment systems typically include: Screens ( removing large floating objects), Comminutors (a mechanical cutting or shredding device) grit chambers ( remove sand and other inert gritty material from the sewage) Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Primary Treatment Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Sedimentation After preliminary treatment, the wastewater still contains suspended organic solids that can be removed by sedimentation. Settling tanks that receive sewage after grit removal are called primary clarifiers. The combination of preliminary processes and gravity settling is called primary treatment. To achieve BOD and TSS removal efficiencies of at least 85 percent, as now required for all sewage treatment plants by the CWA, at least one additional treatment process must follow primary treatment. Generally, this next step is characterized as secondary treatment Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : A primary clarifier has an average influent TSS (total suspended solids) concentration of 250 mg/L. If its TSS removal efficiency is expected to be 60 percent, what is the expected average effluent TSS concentration? Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : A primary clarifier has an average influent TSS (total suspended solids) concentration of 250 mg/L. If its TSS removal efficiency is expected to be 60 percent, what is the expected average effluent TSS concentration? Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Secondary Treatment To remove the suspended solids that did not settle out in the primary tanks and the dissolved BOD that is unaffected by physical treatment. Generally, 85 percent BOD and TSS removal efficiency and represents the minimum degree of treatment required in most cases. In the United States, secondary treatment systems almost always comprise biological processes Biological treatment of sewage involves the use of microorganisms. The microbes, including bacteria and protozoa, consume the organic pollutants as food. They metabolize the biodegradable organics, converting them into carbon dioxide, water, and energy for their growth and reproduction. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved A biological sewage treatment system must provide the microorganisms with a comfortable home. They must be provided with: Enough oxygen, Adequate contact with the organic material in the sewage, Suitable temperatures, and other favorable conditions. Two of the most common biological treatment systems: Trickling filter: fixed-growth system Microbes remain fixed or attached to a surface while the wastewater flows over that surface to provide contact with the organics Activated sludge process: suspended-growth system, Microbes are thoroughly mixed and suspended in the wastewater rather than attached to a particular surface. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved  Trickling filter. Trickling filter consists basically of a layer or bed of crushed rock about 2 m (6 ft) deep. It is usually circular and may be built as large as 60 m (200 ft) in diameter. The primary effluent is sprayed over the surface of the crushed stone bed and trickles downward through the bed to an underdrain system. The trickling filter effluent is collected in the underdrain system and then conveyed to a sedimentation tank called a secondary clarifier. The secondary clarifier, is similar to the primary clarifier, although there are differences in detention time, overflow rate, weir loading, and other details. To Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved  To maintain a relatively uniform flow rate through the trickling filter and to keep the distributor arm rotating even during periods of low sewage flow, some of the wastewater may be Cutaway view of a trickling filter. recirculated. In other words, a portion of the effluent is pumped back to the trickling filter inlet so that it will pass through the bed of stones more than once Recirculation of flow through a trickling filter. The rate of sewage flow applied to the filter is the sum of the influent flow rate and the recirculated flow rate. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Recirculation The amount of recirculation can vary. It is characterized by a recirculation ratio, which is the ratio of recycled flow to the raw wastewater flow. The recirculation ratio, R, is generally in the range of 0.0 to 3.0. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : A 2-m-deep trickling filter with a diameter of 18 m is operated with a recirculation ratio of 1.5. The raw wastewater flow rate is 2.5 ML/d, and the 5- day BOD of the raw sewage is 210 mg/L. Assuming that the primary tank BOD removal efficiency is 30 percent, compute the hydraulic load and the organic load on the trickling filter. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : A 2-m-deep trickling filter with a diameter of 18 m is operated with a recirculation ratio of 1.5. The raw wastewater flow rate is 2.5 ML/d, and the 5- day BOD of the raw sewage is 210 mg/L. Assuming that the primary tank BOD removal efficiency is 30 percent, compute the hydraulic load and the organic load on the trickling filter. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : A 2-m-deep trickling filter with a diameter of 18 m is operated with a recirculation ratio of 1.5. The raw wastewater flow rate is 2.5 ML/d, and the 5- day BOD of the raw sewage is 210 mg/L. Assuming that the primary tank BOD removal efficiency is 30 percent, compute the hydraulic load and the organic load on the trickling filter. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : A 2-m-deep trickling filter with a diameter of 18 m is operated with a recirculation ratio of 1.5. The raw wastewater flow rate is 2.5 ML/d, and the 5- day BOD of the raw sewage is 210 mg/L. Assuming that the primary tank BOD removal efficiency is 30 percent, compute the hydraulic load and the organic load on the trickling filter. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : A 2-m-deep trickling filter with a diameter of 18 m is operated with a recirculation ratio of 1.5. The raw wastewater flow rate is 2.5 ML/d, and the 5- day BOD of the raw sewage is 210 mg/L. Assuming that the primary tank BOD removal efficiency is 30 percent, compute the hydraulic load and the organic load on the trickling filter. The primary effluent BOD can be computed with: Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Efficiency Generally, the BOD removal efficiency increases with: Decreasing organic load, Increasing recirculation, Increasing temperature. For example: With no recirculation (R = 0) and a temperature of 20°C, a typical trickling filter will have an efficiency of about 60 percent when the organic load is about 2 kg/m3. But if the organic load is 0.5 kg/m3 at the same conditions of recirculation and temperature, the efficiency will be 75 percent. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example: The BOD removal efficiency of a trickling filter system is 79 percent, and the efficiency of the primary treatment that precedes it is 35 percent. If the raw BOD is 200 mg/L, what is the effluent BOD? Is the treatment plant providing an efficiency that meets the requirement for secondary treatment? 35 percent of the raw BOD was recently removed by primary settling. Thus, 65 percent or 0.65 х 200 = 130 mg/L remains as BOD in the primary effluent. But 79 percent of the BOD entering the trickling filter is removed, leaving 21 percent or 0.21 х 130 = 27 mg/L in the secondary effluent. The overall plant efficiency is therefore computed as: This is greater than 85 percent, so the treatment plant is providing secondary treatment. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example: The BOD removal efficiency of a trickling filter system is 79 percent, and the efficiency of the primary treatment that precedes it is 35 percent. If the raw BOD is 200 mg/L, what is the effluent BOD? Is the treatment plant providing an efficiency that meets the requirement for secondary treatment? 35 percent of the raw BOD was recently removed by primary settling. Thus, how much remains as BOD in the primary effluent? 65 percent or 0.65 х 200 = 130 mg/L But 79 percent of the BOD entering the trickling filter is removed, what is the secondary effluent? leaving 21 percent or 0.21 х 130 = 27 mg/L in the secondary effluent. The overall plant efficiency is therefore computed as: This is greater than 85 percent, so the treatment plant is providing secondary treatment. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved A biological sewage treatment system must provide the microorganisms with a comfortable home. They must be provided with: Enough oxygen, Adequate contact with the organic material in the sewage, Suitable temperatures, and other favorable conditions. Two of the most common biological treatment systems: Trickling filter: fixed-growth system Microbes remain fixed or attached to a surface while the wastewater flows over that surface to provide contact with the organics Activated sludge process: suspended-growth system, Microbes are thoroughly mixed and suspended in the wastewater rather than attached to a particular surface. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Activated Sludge Treatment (a) Flow diagram of a conventional activated sludge treatment system. (b) View of a typical rectangular aeration tank. (c) A group of circular secondary clarifiers. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved  Activated Sludge Treatment The basic components of an activated sludge sewage treatment system include an aeration tank and a secondary settling basin or clarifier. Primary effluent is mixed with settled solids recycled from the secondary clarifier and then introduced into the aeration tank. Compressed air is injected continuously into the mixture through porous diffusers along one side of the tank. The aerobic microorganisms in the tank grow and multiply, forming an active suspension of biological solids called activated sludge. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Activated Sludge Treatment In the aeration tank, microorganisms consume the dissolved organic pollutants as food. The aerobic microorganisms in the tank grow and multiply, forming an active suspension of biological solids called activated sludge. The combination of the activated sludge and wastewater in the aeration tank is called the mixed liquor. In the basic or conventional activated sludge treatment system, a tank detention time of about 6 h is required for thorough stabilization of most of the organics in the mixed liquor. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved  Activated Sludge Treatment After about 6 h of aeration, the mixed liquor flows to the secondary clarifier: The activated sludge solids settle out by gravity. The clarified water near the surface, called the supernatant, is discharged over an effluent weir. The settled sludge is pumped out from the bottom of the tank. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved  Activated Sludge Treatment The settled sludge is in an active state. In other words, the microbes are well acclimated to the wastewater and, given the opportunity, will readily absorb and decompose more organics by their metabolism. It is not possible to recycle or return all the sludge to the aeration tank. The excess sludge, called waste-activated sludge, must eventually be treated and disposed of (along with sludge from the primary tanks). Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Food-to-Microorganism (F/M) ratio An important factor used in the design and operation of activated sludge systems The food is measured in kilograms (pounds) of BOD added to the tank per day. Because the suspended solids in the mixed liquor consist mostly of living microorganisms, the suspended solids concentration is used as a measure of the number of microorganisms in the tank. This concentration is called the mixed liquor suspended solids (MLSS). Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Food-to-Microorganism (F/M) ratio The F/M ratio: indicator of the organic load on the system with respect to the amount of biological solids in the tank. For conventional aeration tanks, the ratio is in the range of 0.2 to 0.5. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example: An activated sludge tank is 30 m long and 10 m wide and has an SWD of 4 m. The wastewater flow is 4.0 ML/day, and the raw 5-day BOD is 200 mg/L. The MLSS concentration is 2000 mg/L. Compute the food-to-microorganism ratio for the system. A conventional activated sludge aeration tank is preceded by primary treatment. Assuming that 35 percent of the raw BOD is removed in the primary clarifier, 65 percent of the BOD will be applied to the aeration tank: - The ratio is in the range of 0.2 to 0.5. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : A conventional aeration tank is to treat a flow of 800,000 gpd of primary effluent with a BOD of 125 ppm. The MLSS concentration is to be maintained at 1800 ppm, and a food-to-microorganism ratio of 0.4 is specified. Compute the required volume of the aeration tank. If the side water depth is to be 15 ft and the tank length is to be three times its width, how long should the tank be? volume = length х width х depth, or V = L х W х SWD, the length is three times the width, or L = 3W, volume is expressed as: Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Example : A conventional aeration tank is to treat a flow of 800,000 gpd of primary effluent with a BOD of 125 ppm. The MLSS concentration is to be maintained at 1800 ppm, and a food-to-microorganism ratio of 0.4 is specified. Compute the required volume of the aeration tank. If the side water depth is to be 15 ft and the tank length is to be three times its width, how long should the tank be? volume = length х width х depth, or V = L х W х SWD, the length is three times the width, or L = 3W, volume is expressed as: Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved Activated Sludge Treatment Sludge Settling In the activated sludge process, the organic pollutants are absorbed by microorganisms in an aeration tank. These microorganisms essentially are activated sludge. But without proper clarification or separation (gravity settling ) of the sludge from the liquid portion of the mixed liquor, the treatment process will not be effective at all. If the sludge does not settle fast enough, some of it will be carried over the effluent weirs of the clarifier and cause pollution of the receiving body of water. Basic Environmental Technology, Sixth Edition Copyright © 2015 by Pearson Education, Inc. Jerry A. Nathanson | Richard A. Schneider All Rights Reserved