CIVE 432 Environmental Engineering - Chapter IV Wastewater Treatment PDF

CIVE 432 Environmental Engineering Chapter IV Wastewater Treatment Prepared by: May Mrad Fall 2023 10th November What is wastewater? The term Wastewater is used to describe waste material that includes industrial liquids waste, sewage water, commerc...

CIVE 432 Environmental Engineering Chapter IV Wastewater Treatment Prepared by: May Mrad Fall 2023 10th November What is wastewater? The term Wastewater is used to describe waste material that includes industrial liquids waste, sewage water, commercial, and agricultural runoff that is collected in towns and urban areas and treated at a wastewater treatment plant. 2 Source of Waste into Water Domestic wastewater from households Soaps and cleaning detergents Industrial discharge Agricultural runoff Municipal wastewater from communities (sewage) Dairy and industrial establishment Slaughterhouse waste, tannery waste, dairy waste etc… 3 Wastewater Treatment Why to treat wastewater Raw material to be conserved Clean water is a scarce commodity Nitrogen and phosphorus recovered used for crop growth Source of energy If wastewater is not treated Lower dissolved oxygen in streams Increases suspended solids or sediments in streams which in return increases turbidity 4 Wastewater Treatment Aim Wastewater treatment aims at reduction of BOD COD Eutrophication Prevent bio-magnification of toxic substance in food chain 5  Wastewater composition Wastewater contaminants include Suspended solids Biodegradable organics Pathogenic bacteria Nutrients like Nitrogen and Phosphorus 6 Typical Composition of Untreated Domestic Sewer 7 Composition of Industrial Sewer Examples of industrial wastewater concentrations for two conventional pollutants (BOD5and suspended solids) 8  U.S.EPA secondary treatment standards before discharging wastewater into natural water bodies 9 Wastewater Treatment Pre-treatment Primary Treatment Secondary Treatment Tertiary Treatment 10 Municipal Wastewater Treatment Systems 11 Pretreatment Purpose: Several devices are placed upstream of the primary treatment operation to provide protection to WWTP equipment. These devices have little effect in reducing BOD5. Unit operations of pretreatment Bar Racks Grit chamber Comminutors Equalization basin 12 Pretreatment Bar Racks or Bar Screens primary purpose is to remove large objects that would damage pumps, valves, or other mechanical equipment. Catches large objects that have gotten into sewer system such as bricks, bottles, piece of woods, plastic bags etc… 13 Pretreatment Grit Chamber sand, broken glass, silt, and pebbles are called grit. If these are not removed, they will abrade pumps and other mechanical devices. There are three basic types of grit-removal devices Velocity controlled: can be analyzed by classical law of sedimentation for discrete particles (Type I sedimentation). Stokes’ law may be used for design if the horizontal liquid velocity is maintained about 0.3 m/s Aerated grit chambers Constant –level short-term sedimentation basins 14 Pretreatment Velocity controlled grit chamber 15 16 Example Will a grit particle with a radius of 0.10 mm and a specific gravity of 2.65 be collected in a horizontal grit chamber that is 13.5 m in length if the average grit-chamber flow is 0.15 m3/s, the width of the chamber is 0.56 m, and the horizontal velocity is 0.25 m/s? The wastewater temperature is 22ºC. Assume the particle enters at the liquid surface 17 Solution From the table in the next slide: the density of water is 1,000 kg/m3and the dynamic viscosity is 0.955 ×10- 3Pa.s. –Assume laminar flow: the terminal settling velocity is determined using Stokes’s law 18 Solution 19 Solution 20 Pretreatment Comminutors devices used to grind wastewater solids (rags, paper, plastic) by revolving cutting bars. These devices are placed downstream of the grit chamber to protect cutting bars from abrasion. 21 Pretreatment Equalization: Wastewater does not flow into municipal wastewater at a constant rate; the flow rate varies from hour to hour, reflecting the living habits of the area served. The constantly changing amount and strength of wastewater to be treated makes efficient process operation difficult. The purpose of equalization basin is to dampen these variations so that the wastewater can be treated at a nearly constant flow rate. They are large basins that collect and store wastewater flow and from which the wastewater is pumped to the treatment plant at a constant rate. 22 Primary Treatment Purpose: Remove pollutants that will either settle or float. With the screening completed and the grit removed, the wastewater still contains light organic suspended solids, some of which can be removed by gravity settling in a sedimentation tank (primary sedimentation basin). Primary sedimentation basin: characterized by Type II settling. Tanks could be round or rectangular. Rectangular tanks are frequently chosen because of space constraints in some sites. Typically removes about 50-60% of suspended solids and 35% of BOD5 from raw sewage. Soluble pollutants are not removed. 23 Primary Treatment Design values for primary settling tanks: Rectangular tanks: Length to width ratio range from 3:1 to 5:1. These tanks are from 15 to 100 m in length and 3 to 24 m in width. Depth range from 3 to 5 m. Typical depth is 4 m. Circular tanks: diameter ranges from 3 to 60 m. Depth range from 3 to 5 m Overflow rate: at average flow the overflow rate ranges from 25 to 60 m3/m2.d. Overflow rate :under peak flow the overflow rate ranges from 80 to 120 m3/m2.d. Hydraulic detention time: ranges from 1.5 to 2.5 hours under average flow. Weir loading: for average flows less than 0.04 m3/s the weir loading should not exceed 120m3/d.m. For large flows the recommended weir loading rate is 190 m3/d.m. 24 Primary Treatment Primary Settling Tank 25 Primary Treatment Primary settling tanks 26 Example 2 Evaluate the following primary tank design with respect to detention time, overflow rate, and weir loading. Design data: Flow = 0.150 m3/s Influent SS = 280 mg/L Efficiency = 60% Length = 40.0 m Width = 10.0 m Liquid depth = 2.0 m Weir length = 75.0 m 27 Solution 28  Secondary Treatment Purpose: Remove soluble BOD that escapes primary treatment and to provide further removal of suspended solids (SS). Removes 85% of BOD5and SS. It doesn't remove significant amounts of nitrogen, phosphorous, or heavy metals. 29  Secondary Tank Secondary treatment is the portion of a sewage treatment sequence removing dissolved and colloidal compounds measured as biochemical oxygen demand Secondary treatment is traditionally applied to the liquid portion of sewage after primary treatment has removed settable solids and floating material Secondary treatment is typically performed by indigenous aquatic microorganism in a managed aerobic habitat Bacteria and protozoa consume biodegradable soluble organic contaminants 30 Secondary Treatment Basic ingredients needed for conventional aerobic secondary biological treatment Availability of many microorganisms Good contact between microorganisms and the organic material Availability of oxygen Maintenance of favorable conditions (e.g. temperature, sufficient time) Municipal secondary wastewater treatment systems Activated sludge sewage treatment systems Trickling filters Oxidation ponds (lagoons) Rotating biological contactors 31 Wastewater Microbiology Role of microorganisms: Stabilization of organic matter. They convert carbonaceous organic matter into various gases and into protoplasm (e.g. water, nucleic acids, monosaccharide, polysaccharide, proteins, and lipids) Protoplasm is itself organic and has a BOD. Therefore, for complete treatment the protoplasm has to be removed. Protoplasm has a specific gravity slightly greater than water; it can be removed by gravity settling (secondary settling). 32 Electron Acceptor Bacteria are also classified by their ability to utilize oxygen as a terminal electron acceptor in oxidation/reduction reactions. Aerobes Obligate aerobes: cannot survive in the absence of oxygen. Oxygen is used as the terminal electron acceptor (e.g. aerobic heterotrophs, nitrifiers) Facultative aerobes: can survive in the absence of oxygen; when oxygen is present, they use it as the terminal electron acceptor. In an aerobic environment, these organisms carry out aerobic respiration while they switch to anaerobic respiration in anaerobic conditions. Anaerobes: They use terminal electron acceptors such as CO2(e.g. methanogens), SO42- (e.g. sulfate reducers), NO3-and NO2-(denitrifiers) Obligate anaerobes: cannot survive in the presence of oxygen. Facultative anaerobes: they can grow in the absence or presence of oxygen. 33  Aerobes Vs. Anaerobes What is an obligate aerobe? Obligate Aerobes are the organisms that compulsorily require oxygen in their environment for growth and survival. What is an obligate anaerobe? Obligate anaerobes are organisms that do not require oxygen. In fact, oxygen is toxic for them, and these organisms can not tolerate oxygen in their environment. Instead, these organisms depend on fermentation or anaerobic respiration for generating energy for their growth and survival. 34 Microbes of Interest in Wastewater Treatment Bacteria: highest populations of microorganisms in wastewater treatment Fungi Algae Protozoa Rotifers 35 Decomposition of Waste Type of electron acceptor available determines the type of decomposition. Aerobic decomposition: molecular oxygen (O2) must be present as the terminal electron acceptor for the decomposition to proceed by aerobic oxidation. The end product of decomposition is carbon dioxide, water, and new cell material. Large amount of energy is released in aerobic oxidation resulting in relatively large production of new cells (biological sludge). Odor potential is low. - Anoxic decomposition: microorganisms use nitrate (NO3 ) as the terminal electron acceptor in the absence of molecular oxygen. Oxidation by this route is called denitrification. End product of denitrification are nitrogen gas, carbon dioxide, water, and new cells. Large amount of energy is released resulting in relatively large production of new cells (biological sludge). Odor potential is low. 36 Decomposition of Waste Anaerobic decomposition: molecular oxygen (O2) and nitrate (NO-3) must not be present as the terminal electron acceptors for the decomposition to proceed by anaerobic oxidation. Sulfate, carbon dioxide, and organic compounds that can be reduced serve as electron acceptors. The anaerobic decomposition of organic matter generally is considered to be a three-step process (figure 1). Anaerobic digestion yields carbon dioxide, methane, and water as the major end products. Small amount of energy is released during anaerobic digestion resulting in small production of new cells (biological sludge). Odor potential is high. 37 Bacterial Growth The following list summarizes the major requirements for growth that must be satisfied 1. A terminal electron acceptor 2. Macronutrients a. Carbon to build cells b. Nitrogen to build cells c. Phosphorous for ATP (energy carrier) and DNA 3. Micronutrients a. Trace metals b. Vitamins 4. Appropriate environment a. Moisture b. Temperature c. pH 38 Bacterial Growth in Pure Culture 39 Bacterial growth in Pure Culture Growth in Pure Culture Lag phase: when a microbial population is inoculated into a fresh medium, growth usually begins only after a period of time called the lag phase. Time is needed for synthesis of new enzymes that will carry out the reactions Exponential or log growth phase: since reproduction of bacteria is by binary fission, each cell divide to form two cells, each of which also divide to form two more cells, and so on. The increase in population follows a geometric progression: 1→ 2 →4 → 8 → 16 → 32, and so on. The population of bacteria (P) after the nth generation is given by the following expression n P = P0 (2) Where P0 is the initial population and n is the number of generations If we plot bacterial population growth on a log10 scale and time is plotted arithmetically (semilogarithmic graph), the plot will be a straight line of slope 𝑛 0.301 slope = 0.301 = (g is doubling time or generation time) 𝑡 𝑔 𝑡 where g = ; time required for the population to double in the exponential phase 40 𝑛 Bacterial Growth in Pure Culture Figure 3. Binary fission 41  Bacterial Growth in Pure Culture Bacterial Growth Rates. Since the relationship between time and number of cells is exponential instead of linear, plotting the cell concentration on a semi-log scale will standardize the data, giving the appearance of a linear relationship. Figure 4. Logarithmic growth 42 Bacterial Growth in Pure Culture Stationary phase: Assume a single bacterium with a 20-min generation time. A single bacterial cell weighs 10-12g. If this cells is allowed to grow exponentially for 48 h, then after 48 hours there will be 2.23 ×1043 cells which weighs 2.23 ×1031g or 2.23 ×1028 kg (4000 times the mass of earth). Obviously, this scenario is impossible. Typically, one or both of two things occurs to limit growth: (1) an essential substrate or nutrient is used up, or (2) some waste product(s) of the organisms accumulates in the medium to inhibit growth. Either way, exponential growth ceases, and the population reaches stationary phase (balance of death and reproduction). Death phase: bacteria die faster that they reproduce. 43 Bacterial growth in Mixed Culture In wastewater treatment, as in nature, pure cultures of microorganisms do not exist. Rather, a mixed cultures of microorganisms are found in wastewater treatment systems. In wastewater treatment it is convenient to measure biomass rather than numbers of organisms. The rate of expression of biomass increase is 𝑑𝑋 Rate of bacterial growth = rg = = μX 𝑑𝑡 Where: dX/dt= growth rate of biomass, mg/L.t μ = specific growth rate, t-1 X = concentration of biomass (microorganisms), (typically mg VSS/L) 44 Bacterial growth in Mixed Culture Because of the difficulty in direct measurement of µ; the specific growth rate is determined using Monod equation (Monod, 1949) Predict microorganism growth Where μm= maximum specific growth rate, t-1 S = concentration of limiting food in solution, (typically mg BOD5/L) Ks= half saturation constant, mg/L = concentration of limiting food when μ = 0.5 μm 45 Bacterial Growth in Mixed Culture Growth rate of biomass is Hyperbolic When S >> Ks; excess of limiting food; μ = μm zero-order in substrate S