Wastewater Treatment PDF

5. Wastewater Treatment 01/17/2025 1  Introduction The evolution of the field of wastewater treatment is the story of health and environmental concerns, especially as the size of cities increased. Methods of wast...

5. Wastewater Treatment 01/17/2025 1  Introduction The evolution of the field of wastewater treatment is the story of health and environmental concerns, especially as the size of cities increased. Methods of wastewater treatment were first developed in response to the concern for  public health and the adverse conditions caused by the discharge of wastewater to the environment 01/17/2025 2  Objectives of waste treatment processes: 1. Reduction of the organic content of wastewater (i.e., reduction of BOD) 2. Removal/reduction of trace organics that are recalcitrant to biodegradation and may be toxic or carcinogenic 3. Removal/reduction of toxic metals 4. Removal/reduction of nutrients (N, P) 5. Removal or inactivation of pathogenic microorganisms and parasites. 01/17/2025 3  Cont… 01/17/2025 Major contaminants in 4 wastewater  Composition of Domestic wastewater Domestic wastewater is a combination of human and animal excreta (feces and urine) and gray water resulting from washing, bathing, and cooking  People excrete 100–500 g wet weight of feces and between 1 and 1.3 L of urine per capita per day  Each person contributes 15–20 g BOD5/day 01/17/2025 5  Cont… Domestic wastewater is composed mainly proteins (40–60 percent), carbohydrates (25–50 percent), fats and oils (10 percent), urea derived from urine, and a large number of trace organic compounds that include pesticides, surfactants, phenols, and priority pollutants Nonmetals (As, Se), Metals (e.g., Cd, Hg, Pb), 01/17/2025 6  Cont…  Benzene compounds (e.g., benzene, ethylbenzene), and  Chlorinated compounds (e.g., chlorobenzene, tetrachloroethene, trichloroethene) 01/17/2025 7  Cont… Composition of Human Feces and Urine 01/17/2025 8  Cont… Three main tests are used for the determination of organic matter in wastewater:  BOD  COD  total organic carbon (TOC) Trace organics are detected and measured, using more sophisticated instruments such as gas chromatography and mass spectroscopy 01/17/2025 9  Cont… TOC It is determined via oxidation of the organic matter with heat and oxygen or chemical oxidants, followed by the measurement of the CO2 liberated with an infrared analyzer 01/17/2025 10  Overview of Wastewater Treatment Methods Treatment of wastewater :  Physical forces  Chemical and  Biological processes Unit operations: Treatment methods that rely on physical forces and include:  screening  sedimentation  filtration or flotation 01/17/2025 11  Cont… Unit processes: Treatment methods based on chemical and biological processes and include:  disinfection  Adsorption  Precipitation Biological unit processes involve microbial activity, which is responsible for organic matter degradation and removal of nutrients 01/17/2025 12  Wastewater treatment comprises the following steps: Primary treatment: The objective of primary treatment is to  remove a significant fraction of the suspended solids and floating material from the wastewater physical processes (unit operations) such as screening and sedimentation are used The suspended solids removed are organic in nature The floating material can include oil, grease, rags, etc.  These are removed as scum from the water surface in the tank 01/17/2025 13  Primary treatment mainly involves sedimentation or settling by gravity In some cases, sedimentation is enhanced by the addition of coagulation and/or flocculation agents. The process is called Enhanced Clarification, or Chemically Enhanced Primary Treatment (CEPT) 01/17/2025 14 Secondary treatment: The objectives of secondary treatment are to reduce the BOD and suspended solids of the effluent to acceptable levels Two major categories of biological treatment processes are (i) suspended growth and (ii) attached growth processes. In a suspended growth process,  the microorganisms are kept in suspension in a biological reactor by using a suitable mixing technique.  The microorganisms use the organic matter as food and convert them to new biological cells, energy, and waste matter 01/17/2025 15  In attached growth systems,  the microorganisms are attached to an inert medium, such as rock, gravel, synthetic media, etc, forming a biofilm.  As the wastewater comes in contact with and flows over the biofilm, the organic matter is removed by the microorganisms and degraded to produce an acceptable effluent. The common types of attached growth processes include trickling filters, bio- towers, and rotating biological contactors (RBC) 01/17/2025 16  The main advantages of the attached growth processes are:  Simplicity of operation.  Low energy requirement.  Low maintenance required.  Ability to handle shock loads.  Lower sludge production.  No problems with sludge bulking in secondary clarifiers.  Better sludge thickening properties. 01/17/2025 17  The disadvantages include:  Low efficiency at cold temperatures  Mass transfer and diffusion limitations can occur  Problems with biofilm maintenance due to excess sloughing  Higher BOD and solids concentration in the effluent  Odor problems can occur 01/17/2025 18  Secondary treatment Involves  Biological (e.g., activated sludge, trickling filter, oxidation ponds) and  chemical (e.g., disinfection) unit processes are used to treat wastewater  Nutrient removal 01/17/2025 19 01/17/2025 20  Cont… consists of biological degradation, in which the remaining suspended solids are decomposed by microorganisms and the number of pathogens is reduced In this stage the effluent from primary treatment may be pumped into  a trickling bed  an aeration tank  Lagoon A disinfection step is generally included at the end of the treatment 01/17/2025 21  Aerobic Treatment Process a. Trickling Filters The trickling or percolating filter was introduced in 1890 and is one of the earliest systems for biological waste treatment A trickling filter essentially converts soluble organic matter to biomass, which is further removed via settling in the final clarifier Removal of BOD by trickling filters is approximately 85 percent for low-rate filters and 65–75 percent for high-rate filters 01/17/2025 22  Cont… 01/17/2025 23  Cont… The biofilm forming on the surface of the filter media in trickling filters is called the zoogleal film  It is composed of bacteria, fungi, algae, protozoa, and other life forms Advantages: Trickling filters are attractive to small communities because of easy operation, low maintenance costs, and reliability They are used to treat toxic industrial effluents and are able to withstand shock loads of toxic inputs The sloughed biofilms can also be easily removed by sedimentation 01/17/2025 24  Cont… Disadvantages: High organic loading may lead to filter clogging as a result of excessive growth of slime bacteria in biofilms Excessive biofilm growth can also cause odor problems in trickling filters Clogging restricts air circulation, resulting in low availability of oxygen to biofilm microorganisms 01/17/2025 25  Cont… Zoogleal film formation on packing media in 01/17/2025 trickling filters 26  b) Rotating Biological Contactors (RBC) An RBC consists of a series of disks mounted on a horizontal shaft that rotate slowly in the wastewater The disks are approximately 40 percent submerged in wastewater Increased rotation improves oxygen transfer and enhances the contact between attached biomass and wastewater The advantages offered by RBC are  short residence time,  low operation and maintenance costs, and  production of a readily dewatered sludge that settles rapidly 01/17/2025 27  Rotating Biological 01/17/2025 Contactors 28  Cont… 1–4 mm-thick biofilm that is responsible for BOD removal Advantages: low cost, low maintenance, resistance to shock loads Disadvantages: filter clogging filter flies 01/17/2025 29  c) Activated Sludge Process a suspended-growth process This process consists essentially of an aerobic treatment that oxidizes organic matter to CO2 and H2O, NH4, and new cell biomass There are two main goals of the activated sludge system: 1. Oxidation of the biodegradable organic matter in the aeration tank (soluble organic matter is thus converted to new cell mass) 2. Flocculation, that is, the separation of the newly formed biomass from the treated effluent 01/17/2025 30  Cont… Types of conventional activated sludge processes Aeration tank: Aerobic oxidation of organic matter is carried out in this tank Primary effluent is introduced and mixed with return activated sludge (RAS) to form the mixed liquor recycling of a large portion of the biomass mean cell residence time (i.e., sludge age) much greater than the hydraulic retention time (the average time spent by the influent liquid in the aeration tank) large number of microorganisms that effectively oxidize organic compounds in a relatively short time 01/17/2025 31  Cont… 01/17/2025 32  Sedimentation tank This tank is used for the sedimentation of microbial flocs (sludge) produced during the oxidation phase in the aeration tank Some operational parameters commonly used in activated sludge process Mixed Liquor Suspended Solids (MLSS)  The content of the aeration tank in an activated sludge system is called mixed liquor  The MLSS is the total amount of suspended solids, including microorganisms, in the mixed liquor  It is determined by filtering an aliquot of mixed liquor, drying the filter at 105oC, and determining the weight of solids in the sample 01/17/2025 33  Cont… Mixed Liquor Volatile Suspended Solids (MLVSS) The organic portion of MLSS is represented by MLVSS, i.e., comprises nonmicrobial organic matter, as well as dead and live microorganisms and cellular debris The MLVSS is determined after heating of dried filtered samples at 600–650oC, and represents approximately 65–75 % of MLSS 01/17/2025 34  Food-to-Microorganism Ratio (F/M):  The F/M ratio indicates the organic load into the activated sludge system and is expressed in kilogram BOD per kilogram of MLSS per day  a measure of the amount of organic matter (food) entering the activated sludge process relative to the amount of microorganisms (biomass) in the system  The F/M ratio affects the growth rate, diversity, and activity of the microorganisms, as well as the quality of the treated effluent 01/17/2025 35  A high F/M ratio indicates that there is more food available for the microorganisms, which can lead to overgrowth, foaming, and high oxygen demand  The optimal F/M ratio depends on the type of activated sludge process, the design and operation parameters, and the environmental conditions  A low F/M ratio indicates that there is less food available for the microorganisms, which can lead to starvation, sludge bulking, and poor settling. 01/17/2025 36  Cont… Hydraulic Retention Time (HRT):  Hydraulic retention time is the average time spent by the influent liquid in the aeration tank  it is the reciprocal of the dilution rate D  where V = volume of the aeration tank; Q = flow rate of the influent wastewater into the aeration tank; and D = dilution rate 01/17/2025 37 Sludge Age: Sludge age is the mean residence time of microorganisms in the system or SRT – solids retention time of the reactor SRT is the most important design and operating parameter, as it affects  process performance,  aeration tank volume,  sludge production, and  oxygen requirements 01/17/2025 38 d) Oxidation pond/Stabilization pond An oxidation pond is a type of wastewater treatment that uses natural processes to remove pollutants from sewage or industrial effluents. It consists of a large, shallow pond that exposes the wastewater to sunlight, oxygen, and microorganisms such as bacteria and algae Are the oldest of the wastewater treatment systems There are four categories of oxidation ponds, which are often used in series:  aerobic ponds  anaerobic ponds  facultative ponds  aerated lagoons 01/17/2025 39  Wastewater stabilization/Oxidation ponds 01/17/2025 40  Aerobic ponds naturally mixed must be shallow (up to 1.5m)  they depend on penetration of light to stimulate algal growth that promotes subsequent oxygen generation The detention time of wastewater is generally 3 to 5 days 01/17/2025 41  Cont… 01/17/2025 42  Anaerobic ponds may be 1 to 10 m deep require a relatively long detention time of 20-50 do not require expensive mechanical aeration generate small amounts of sludge serve as a pretreatment step for high- BOD organic wastes rich in protein and fat (e.g., meat wastes) with a heavy concentration of suspended solids 01/17/2025 43  Cont… 01/17/2025 44  Facultative ponds are most common for domestic wastewater treatment Waste treatment is provided by both aerobic and anaerobic processes Facultative bacteria can grow either with or without oxygen.  They can switch between aerobic and anaerobic metabolism, depending on the availability of oxygen and other electron acceptors range in depth from 1 to 2.5 m subdivided in three layers:  an upper aerated zone,  a middle facultative zone, 01/17/2025  a lower anaerobic zone. 45  Cont… 01/17/2025 46  Aerated lagoons or ponds which are mechanically aerated may be 1-2 m deep have a detention time of less than 10 days treatment depends on the  aeration time  temperature,  type of wastewater For example, at 20oC, an aeration period of 5 days result in 85 % BOD removal 01/17/2025 47  Cont… 01/17/2025 48  Cont… Inactivation and/or removal of pathogens in oxidation ponds is controlled by  temperature  sunlight,  pH  Bacteriophage -a virus that is parasitic (reproduces itself) in bacteria  predation by other microorganisms  adsorption to or entrapment by settleable solids Indicator bacteria and pathogenic bacteria may be reduced by 90-99 % or more, depending on the retention time 01/17/2025 49  Tertiary Treatment Also known as advanced treatment The final stage of the treatment process This stage ensures that the treated water meets the highest standards for discharge or reuse involves a series of additional steps after secondary treatment to reduce further organics turbidity nitrogen phosphorus metals pathogens 01/17/2025 50  Cont… Involve some type of physicochemical treatment such as Coagulation Filtration Activated carbon adsorption of organics Reverse osmosis Additional disinfection 01/17/2025 51  Cont… Tertiary treatment of wastewater is practiced for additional protection of wildlife after discharge into rivers or lakes Even more commonly, it is performed when the wastewater is to be reused for irrigation (e.g., food crops, golf courses) recreational purposes (e.g., lakes, estuaries) drinking water 01/17/2025 52