Wastewater Treatment Notes PDF

WASTEWATER TREATMENT WATER POLLUTION ‒ Defined as the presence in water of impurities in such quantity and of such nature as to impair the use of the water for a stated purpose. ‒ Water pollution is any chemical, physical or biological change in the quality of water that has a harmful effect on any...

WASTEWATER TREATMENT WATER POLLUTION ‒ Defined as the presence in water of impurities in such quantity and of such nature as to impair the use of the water for a stated purpose. ‒ Water pollution is any chemical, physical or biological change in the quality of water that has a harmful effect on any living thing that drinks or uses or lives (in) it. When humans drink polluted water, it often has serious effects on their health COMPONENTS OF WASTEWATER FLOWS The components that make up the wastewater flow from a community depend on the type of collection system used and may include the following: 1) Domestic (also called sanitary) wastewater. ‒ wastewater discharged from residences and from commercial, institutional, and similar facilities. 2) Industrial wastewater ‒ wastewater in which industrial wastes predominate. 3) Infiltration/Inflow ‒ water that enters the sewer system through indirect and direct means. Infiltration is extraneous water that enters the sewer system through leaking joints, cracks and breaks, or porous walls. Inflow is storm water that enters the sewer system from storm drain connections (catch basins), roof leaders, foundation and basement drain, or through manhole covers. 4) Storm water ‒ Runoff resulting from rainfall and snowmelt. TYPES OF SEWER SYSTEMS 1) Sanitary sewer system – wastewater flows in sanitary sewers consist of major components: (a) domestic wastewater (b) industrial wastewater (c) infiltration/inflow 2) Storm sewer system 3) Combined water system CONTAMINANTS OF CONCERN IN WASTEWATER TREATMENT CONTAMINANTS Suspended Solids Biodegradable Organics REASON FOR IMPORTANCE Suspended solids can lead to the development of sludge deposits and anaerobic conditions when untreated wastewater is discharged in the aquatic environment Composed principally of proteins, carbohydrates, and fats, biodegradable organics are measured most commonly in terms of BOD (biochemical oxygen demand) and COD (chemical oxygen demand). If discharged untreated to the environment, their biological Wastewater Treatment│ 1 Prepared by: Engr. MAAbellera stabilization can lead to the depletion of natural oxygen resources and to the development of septic conditions. Pathogens Communicable diseases can be transmitted by the pathogenic organisms in wastewater. Nutrients Both nitrogen and phosphorus, along with carbon, are essential nutrients for growth. When discharged to the aquatic environment, these nutrients can lead to the growth of undesirable aquatic life. When discharged in excessive amounts on land, they can also lead to the pollution of groundwater. Priority Pollutants Organic and inorganic compounds selected on the basis of their known or suspected carcinogenicity, mutagenicity, teratogenicity, or high acute toxicity. Many of these compounds are found in wastewater. Refractory Organics These organics tend to resist conventional methods of wastewater treatment. Typical examples include surfactants, phenols, and agricultural pesticides. Heavy Metals Heavy metals are usually added to wastewater from commercial and industrial activities and may have to be removed if the wastewater is to be reused. Dissolved Inorganics Inorganic constituents such as calcium, sodium and sulfate are added to the original domestic water supply as a result of water use and may have to be removed if the wastewater is to be reused. Wastewater Treatment Treatment Objectives: 1) Removal of suspended and floatable material 2) Treatment of biodegradable organics 3) Elimination of pathogenic organisms 4) Removal of toxic compounds such as refractory organics and heavy metals 5) Removal of nutrients DIVISION OF WASTEWATER TREATMENT SYSTEMS 1) Preliminary Wastewater Treatment The removal of large solids to prevent damage to the remainder of the unit operations. Examples of preliminary treatment are screening and comminution for the removal of debris and rags, grit removal for the elimination of coarse suspended matter that may cause wear or clogging of equipment, and flotation for the removal of large quantities of oil and grease. 2) Primary Wastewater Treatment Removal of a portion of suspended solids and organic matter by settling or sedimentation. Primary treatment systems are usually physical processes. The effluent from a primary treatment will ordinarily contain considerable organic matter and will have a relatively high BOD. Removes about 60% of the solids and about 30% of BOD. Wastewater Treatment│ 2 Prepared by: Engr. MAAbellera 3) Secondary Wastewater Treatment Treatment which is directed principally for the removal of biodegradable organics and suspended solids. Includes biological treatment by activated sludge, fixed – film reactors, lagoons, and pond systems. 4) Tertiary Wastewater Treatment Polishing of secondary effluent. This is primarily the removal of nutrients, Nitrogen and Phosphorus. Removal of constituents such as toxic compounds, increased amounts of organic material and suspended solids are also included here. Also, removal of ions and salts through ion exchange and reverse osmosis processes are under this treatment. 5) Solids Treatment and Disposal The collection, stabilization and subsequent disposal of the solids removed by other processes. CLASSIFICATION OF WASTEWATER TREATMENT METHODS 1) Physical Unit Operations – the application of physical unit operations such as screening, mixing, flocculation, sedimentation, flotation, filtration and gas transfer. 2) Chemical Unit Processes – treatment methods in which the removal or conversion of contaminants is brought about by the addition of chemicals or by other chemical reactions known as chemical unit processes. Precipitation, adsorption, and disinfection are the most common examples used in wastewater treatment. Chemical precipitation is accomplished by producing a chemical precipitate that will settle. Adsorption involves the removal of specific compounds from the wastewater on solid surfaces using the forces of attraction between bodies. 3) Biological Unit Processes – treatment is brought about by biological activity in what is known as biological process. Biological treatment is used primarily to remove the biodegradable organic substances (colloidal or dissolved) in wastewater. Biological treatment is also used to remove nutrients in wastewater. A. PRELIMINARY TREATMENT A.1) Screening ‒ The first operation performed on incoming wastewater for the purpose of removing materials that might damage equipment or hinder further treatment. Screening devices are used to remove coarse solids from wastewater. Coarse solids consist of sticks, rags, boards, and other large objects that find their way to the wastewater collection systems. Removal of these materials protects pumps and other mechanical equipment and prevents clogging of valves and other appurtenances in the wastewater plant. Classification of screens: a) Coarse screens – usually consist of vertical bars spaced 1 or more centimeters apart and inclined away from the incoming flow. Solids retained by the bars are usually removed by manual raking in small plants while mechanical cleaner units are used in larger plants. Wastewater Treatment│ 3 Prepared by: Engr. MAAbellera b) Fine screens – consist of woven – wire or cloth perforated plates mounted on a rotating disk or drum partially submerged in the flow or on a traveling belt. Fine screens should be mechanically cleaned on a continuous basis. Cleaning Method: a) Mechanically – cleaned bar screens (must be in angles) 1) Bar racks – 3 to 4 inches apart, filter out larger particles 2) Bar screen – 0.5 to 1.5 cm apart to filter out smaller particles b) Manually cleaned bar screens ‒ A straight channel should be provided a few meters ahead of the screen to ensure good distribution of flow across the screen and flow velocity should not exceed 1 m/s with 0.3 m/s considered as good design. Clean bars and screen result in a head loss of less than 0.1 m. Disposal of Screenings: Screenings are coated with organic material of very objectionable nature and should be promptly disposed to prevent health hazard. Oftentimes, the screenings are stabilized with lime before disposal. 1. Removal by hauling to disposal areas (landfilling) 2. Disposal by burial on the plant site (small installation only) 3. Incineration either alone or in combination with sludge and grit (large installations only) 4. Disposal with municipal solid wastes 5. Grinding and returning to the wastewater flow (for food trimmings and other organic wastes) ‒ Screening is inefficient when there is backflow of influent wastewater. This may result to septic shock loading with H2S being produced. (In 2 hours, fermentation in wastewater can begin) A.2) Comminution ‒ Screenings are sometimes shredded and returned to the wastewater flow. A hammermill device is most often used for this purpose. The shredder or comminutor is located across Wastewater Treatment│ 4 Prepared by: Engr. MAAbellera flow path and intercept the coarse solids and shred them to approximately 8 mm in size. These solids remain water. ‒ A comminutor device called a barminutor uses a certical bar screen with a cutting head that travels up and down the rack bars, shredding the intercepted material. Channel design for comminutors is similar to that for screens. A.3) Grit Removal ‒ Grits are not biodegradable and occupies valuable space in digesters. It is therefore desirable to separate them from the organic suspended solids. The most common grit in municipal wastewaters are solids such as pebbles, sand, silt, eggshells, glass, and metal fragments. They are abrasive in nature and will cause accelerated wear on pumps and sludge handling equipment with which it comes in contact with. ‒ Grit removal facilities basically consist of an enlarged channel area where reduced flow velocities allow grit to settle out. ‒ One is known as the channel-type horizontal-flow grit chamber. Grit chambers are designed to remove discrete particles with diameters of 0.20 mm and specific gravity of 2.65. In channel-type horizontal flow grit chamber, it is important to maintain a horizontal velocity at approximately 0.3 m/s. A 25% increase may result in washout of grit while a 25% reduction may result in the retention of non-target organics. ‒ Another type is the aerated grit chambers (used in larger treatment plants). Compressed air is injected to keep lighter organic material in suspension while the heavier grit falls to the bottom. Roll velocity rather than horizontal velocity serves to separate the non-target organics from the grit. Adjustments of air quantities provides settling control. Aeration in this type of grit chamber is usually extended from 15-20 minutes when elimination of the noxious gases from the wastewater is desired. ‒ Vortex – type grit chamber uses vortex flow pattern. A rotating turbine controls velocity. The grit settles by gravity into the hopper in one revolution of the basin’s contents. Wastewater Treatment│ 5 Prepared by: Engr. MAAbellera Purposes of Grit Chamber: ‒ Protect moving mechanical equipment from abrasion and accompanying abnormal wear ‒ Reduce formation of heavy deposits in pipelines, channels, and conduits ‒ Reduce the frequency of digester cleaning caused by excessive accumulation of grit Flow measurement ‒ The most common devices used are Parshall flumes and Palmer-Bowlus flumes. These devices are essentially open channel venturi meters. Fluid Velocities ‒ Sewer: 0.6 m/s; grit removal = 0.3 m/s; primary treatment = fraction of 0.3 m/s B. PRIMARY TREATMENT Sedimentation ‒ The separation from water, by gravitational settling, of suspended particles that are heavier than water. Sedimentation and settling are used interchangeably. Likewise, a sedimentation basin may also be referred to as sedimentation tank, settling basin, or settling tank. The primary purpose of sedimentation is to produce a clarified effluent simultaneous with production of concentrated sludge that can easily be handled and treated. Factors affecting settling tank efficiency: 1) Types of solids 2) Age of wastewater 3) Rate of solids flow 4) Cleanliness 5) Mechanical condition of the tank Removal efficiency of sedimentation: • Settleable solids (90 – 95%) • Suspended solids (50-65%) • BOD (20-35%) • Colloidal solids (not removed unless coagulants are added) Flow Distribution: 1) Inlet Structure – slows down the flow and distributes it evenly, kinetic energy is dissipated 2) Short circuiting – major problem in the design of sedimentation tanks, water has preferred route (tends to follow shorter route) Wastewater Treatment│ 6 Prepared by: Engr. MAAbellera C. SECONDARY TREATMENT ‒ Usually consists of biological conversion of dissolved and colloidal organics into biomass that can subsequently be removed by sedimentation. Contact between microorganisms and the organics is optimized by suspending the biomass in the wastewater or by passing the wastewater over a film of biomass attached to solid surfaces. BIOLOGICAL WASTEWATER TREATMENT ‒ In biological treatment, microorganisms use the organics in wastewater as food supply and convert them into biological cells or biomass. Wastewater contains a wide variety of organics and therefore needs a wide variety of organisms or a mixed culture for complete treatment. The different cultures will utilize the food source most suitable for their metabolism. Most mixed cultures also will contain grazers (organisms that prey on other species). The newly created biomass must be removed from the wastewater to complete the treatment process. ‒ The microorganisms involved in wastewater treatment are essentially the same as those that degrade organic material in natural freshwater systems. However, the processes are not just allowed to proceed in their natural fashion but are carefully controlled in engineered reactors to optimize both the rate and completeness of organic removal. As an effect, removal efficiencies that would be affected over a period of days in natural systems are accomplished in a period of hours in engineered systems. Phases: • Lag phase – acclimation period • Log – growth / exponential phase – growth is at logarithmic rate • Stationary phase – growth of some microorganisms is offset by death of some (food becomes limiting) • Endogenous phase – microorganism metabolizes their own protoplasm, others lyse and add to food supply (food scarcity continues and eventually microorganism dies) Factors affecting rate of biomass production and food utilization 1) Temperature – rate constants increase with temperature with the range of 0 – 55°C with a corresponding increase in biomass production and food utilization. Increases in reaction rates approximately follow the Van’t Hoff – Arrhenius rule of doubling with every 10°C increase in temperature up to a maximum temperature. Excessive heat denatures the enzymes and can destroy the organism. 2) pH – enzyme systems have a fairly narrow range of tolerance. Microorganisms that degrade wastewater organics function best near neutral pH with a tolerance range from about pH 6 to pH 9. 3) Toxins, salt concentration and oxidants – toxicants poison the microorganism, salt concentrations interfere with internal-external pressure relationships and oxidants destroy enzyme and cell materials. Wastewater Treatment│ 7 Prepared by: Engr. MAAbellera 4) Nutrients deficiency in incoming wastewater 5) DO level – needs air to maintain growth ‒ Microorganisms are capable of adjusting to a wide range of environmental factors provided changes occur gradually. Sudden changes such as rapid drop in pH or a slug of salt, may do irreparable damage to the culture. Design of biological systems requires knowledge and understanding of: 1) The biological principles 2) Kinetics of metabolism necessary to control the 3) Principles of mass balance environment in the 4) Physical operations reactor Two Types of Biological Growth/Culture used in Wastewater Treatment 1) Suspended Cultures/Growth – will include activated sludge, ponds, and lagoons. ‒ The microorganisms are suspended in the wastewater either as single cells or as clusters of cells called flocs. They are thus surrounded by the wastewater which contains their food and other essential elements and nutrients necessary for their growth and reproduction. Three types are completely mixed without sludge recycle, completely mixed with sludge recycle, and plug flow with sludge recycle. 1.1) Activated sludge process – the process derives its name from the fact that settled sludge containing live or active microorganisms is returned to the reactor to increase the available biomass and speed up in the reactions. The activated-sludge process is thus a suspended – culture process with sludge returns and may be either a completely mixed or a plug-flow process. The process is aerobic with oxygen being supplied by dissolution from entrained air. Common Variations of Activated Sludge Process a) Step Aeration – influent addition at intermediate points provides more uniform BOD removal throughout the reactor tank. Wastewater Treatment│ 8 Prepared by: Engr. MAAbellera b) Tapered Aeration – air is added in proportion to BOD c) Contact Stabilization – biomass adsorbs organics in contact basin d) Pure – oxygen activated sludge – oxygen added under pressure keeps dissolved oxygen level high Wastewater Treatment│ 9 Prepared by: Engr. MAAbellera e) Oxidation ditch f) High rate – short detention time, high food to microorganism ratio in aerator to maintain culture in log – growth phase g) Extended Aeration – long detention time, low F/M ratio in aerator to maintain culture in endogenous phase Wastewater Treatment│ 10 Prepared by: Engr. MAAbellera Design Parameters/ Operational Parameters in Activated Sludge Process 1) F/M (food to microorganism ratio) – measures organic loading ▪ Extended Aeration – When this ratio is low (little food for a lot of microorganisms), usually the aeration period is long (retention time is long) and the microorganisms make maximum use of the food available resulting to a high degree of treatment. Little biomass is produced hence, little or no waste activated sludge to dispose. ▪ High Rate – when F/M ratio is high (more food for few microorganism), aeration period is very short (smaller tank requirement) but treatment efficiency is lower. 2) MLSS (mixed liquor suspended solids) – represents the suspended solids in the reactor. It is useful in the determination of F/M since sometimes microorganisms are expressed in terms of suspended solids. 3) MCRT/SRT (mean cell residence time/solids retention time or sludge age) – represents the average time in which the microorganisms stay in the reactor or the average time in which the solids stay in the reactor. 4) HRT (hydraulic retention time) – the average time in which the liquid remain in the system. 5) DO (dissolved oxygen) – concentration of DO is very important in aerobic processes because microorganisms need oxygen for their metabolism. Optimum value is 1.5 – 2.5 mg/L. For DO level of 4 (max), the aeration is said to be high, for DO level of 5 and 6, it is said to be over aerated. Over aeration will provoke undesirable microorganisms and may result to filamentous bulking 6) SVI (sludge volume index) – describes the settleability of activated sludge. ❖ Filamentous Bulking – becomes a problem in secondary clarifiers. When the solids in the secondary clarifier are very difficult to settle, the sludge is said to be bulking sludge. This condition is characterized by a biomass which is comprised of almost totally filamentous microorganisms. The success or failure of an activated sludge system often depends on the performance of the final clarifier. Causes of Poor Settling due to Filamentous Bulking Operational Causes 1) Wrong DO (low) 2) Insufficient nutrients 3) Widely varying organic waste loading 4) Wrong F/M ratios (low) 5) Insufficient soluble BOD gradient Wastewater characteristics 1) Fluctuations in flow and strength 2) Fluctuations in pH, temperature, staleness 3) Deficiencies in nutrients in incoming ww 4) High concentrations of heavy metals Operating causes of Non – filamentous Organisms 1) Over aeration 2) Improper organic loading 3) Presence of toxic substances Wastewater Treatment│ 11 Prepared by: Engr. MAAbellera Cures 1) 2) 3) 4) 5) Change or adjust F/M ratio Change or adjust DO level in the aeration tank Dosing with H2O2 to kill the filamentous m.o. Pre – treat ww to remove toxins, check nutrients and maintain temperature and pH Check ww characteristics and process loading Aeration Techniques: 1) Use of Air Diffusers – used in plug flow. Compressed air is injected. ▪ Fine bubble diffusers – more efficient because of larger surface area per volume of air. (bubble diameter of 2 to 2.5 mm) larger energy requirement for greater compression (lesser head loss). Also, compressed air must be filtered before diffusion to remove particulates that would cause clogging. ▪ Coarse bubble diffusers – (bubble diameter is up to 25 mm), requires less maintenance and lower head loss, but poorer oxygen transfer. 2) Mechanical Aerators – used in completely mixed reactors. Mechanical mixers are used to stir the contents violently enough to entrain and distribute air through the liquid. This produces turbulence at the air-liquid interface and this turbulence entrains air into the liquid. It may consist of highspeed impellers that add large quantities of air to relatively small quantities of water. Mixing is by velocity gradient. Brush types of aerators are used to provide both aeration and momentum to wastewater in the oxidation ditch. 1.2) Ponds and Lagoons ‒ A wastewater pond, alternatively known as stabilization pond, oxidation pond or sewage lagoon, is a shallow earthen basin in which wastewater is retained long enough for natural purification processes to provide the necessary degree of treatment. At least part of the system must be aerobic to produce an acceptable effluent. Although some oxygen is provided by diffusion from air, the bulk oxygen in ponds is provided by photosynthesis. Lagoons are distinguished from ponds in that oxygen for lagoons is provided by artificial aeration. ‒ Shallow ponds in which dissolved oxygen is present at all depths are called aerobic ponds. Most frequently used as additional treatment processes, aerobic ponds are often referred to as polishing or tertiary ponds. Deep ponds in which oxygen is absent except for a relatively thin surface layer are called anaerobic ponds. Anaerobic ponds can be used for partial treatment of strong organic wastewater but must be followed by some aerobic treatment to produce acceptable products. Under favorable conditions, facultative ponds, in which both aerobic and anaerobic zones exist may be used as the total treatment system for municipal Wastewater Treatment│ 12 Prepared by: Engr. MAAbellera wastewater. Facultative ponds and lagoons are assumed to be completely mixed reactors without biomass recycling. 2) Attached Growth – trickling filters – classical attached biomass system (round), biotowers (rectangular, square and taller) and Rotating biological contactors (RBC) ‒ Organisms present: heterotrophic (mostly) and facultative bacteria (predominant), fungi and protozoa (abundant), algae (present where there is light), animals such as rotifers, sludge worms, insect larvae, snails etc. ‒ Biomass growth: the organisms attach themselves to the medium and grow into dense films of a viscous, jelly like nature. Wastewater passes over this film in thin sheets with dissolved organics passing into the biofilm due to the concentration gradients within the film. The suspended solids and colloids are retained on the sticky surfaces where they decompose into soluble products. 2.1) Trickling Filter – uses randomly packed solid medium usually fist size rocks. The medium is stationary, and the wastewater is passed over the biofilm in intermittent doses. ▪ Primary clarifier is needed in this reactor to avoid clogging of media. To increase treatment efficiency, multistage, high-rate filters are designed to meet secondary effluent standards. ▪ Packings: when packed with stones, height is limited to 3 m; when packed with plastics, height is up to 6 – 8 m. ▪ Features: simple, low operating costs ▪ Disadvantage: odor problem, vector problem (flies, mosquitoes, moths etc), pretreatment of ww is required (primary sedimentation), not suitable for degradation of suspended organic matter, clogging problem may occur (if this occurs, there is a need to remove all media to remove clogs) Wastewater Treatment│ 13 Prepared by: Engr. MAAbellera 2.2) Biotowers – uses modular synthetic media of high porosity and low weight and this enables a vertical arrangement of medium several meters high. ▪ Basically, these are deep trickling filters. The medium is stationary, and the wastewater is passed over the biofilm in intermittent doses. Primary clarifiers maybe omitted but there is a need to grind the solids in the ww to sufficiently small sizes prior to application to avoid clogging of void spaces. ▪ Packings: light weight flat PVC sheets in alternating patterns (vertical stacking), height is 6 – 8m. ▪ Advantages: porosity and nature of packing allow greater loading rates and eliminate clogging problems. Increased ventilation minimizes odor problems under most operating conditions, compact nature of the reactor allows for economical housing for operation in severe climates. ▪ Disadvantages: relatively high pumping cost required by large recycle and head loss through deep bed. 2.3) Rotating Biological Contactors (RBC) – uses rotating disks partially submerged in the ww. The medium moves the biofilm alternately through water and air thus also maintains aerobic condition. Primary clarifiers may be omitted in this process. ▪ The medium consists of plastic sheets ranging from 2 – 4 in diameter. One module consists of each shaft full of disk along with its tank and rotating device. Several modules may be arranged in parallel and or in series to meet the flow and treatment requirements. The disks are submerged in the ww to about 40% of their diameter and are rotated (rotational speed ranges from 1-2 rpm) by power supplied to the shaft. ▪ Disadvantages: lack of documented operating experience, high capital cost, and sensitivity to temperature. Covers must be provided to protect media from damage by elements and from excessive algal growth. Wastewater Treatment│ 14 Prepared by: Engr. MAAbellera D. TERTIARY TREATMENT 1. Chlorination and dechlorination – disinfection to kill pathogens and removal of excess chlorine. If secondary treatment is sufficient in treating the ww, chlorination becomes its tertiary treatment. 2. Filtration – to remove residual suspended solids (through rapid sand filters) 3. Oxidation ponds – to polish BOD before discharge to water courses 4. Use of activated carbon to reduce BOD 5. Nutrients removal TERTIARY TREATMENT – NUTRIENTS REMOVAL A. Nitrogen ‒ In the Philippines, where almost all of the lakes, rivers and estuaries are undergoing various stages of eutrophication, the Philippine Effluent Standards (DAO 35) does not have nitrogen as a regulated parameter. Unfortunately, industrial discharges contribute significantly to the nutrient load of a receiving body water. Equally disturbing are the domestic discharges from the household who either have poorly designed septic tank or do not have a tank at all, making the heavily clogged canals the carrier of their sewage waste. ‒ Sources of Nitrogen Natural sources or transport mechanisms of nitrogen substances include atmospheric precipitation, dust fall, non – urban and non – agricultural run – off and biological fixation. Nitrogen measured in precipitation is most often a result of both soluble and particulate nitrogen forms scrubbed from the atmosphere. Natural components would include nitrogen oxides fixed by lightning and emitted from volcanic eruptions, wind – blown dust originating from natural areas and ammonia released from decaying animal and plant matter. Sources of nitrogen related to human activity include untreated and treated domestic sewage and industrial wastes, leachates, atmospheric deposition and surface run – off. 1) Domestic waste – untreated sewage flowing from municipal collection systems typically contains 20 – 85 mg/L of total Nitrogen. 2) Industrial wastewater – industries contributing to nitrogen discharges include fertilizer manufacturing, paper and pulp industries, mining and metal ore processing, and food processing industries. 3) Landfill leachates – a survey of leachate characterization studies for many landfills shows ammonia values 0 – 1160 mg/L and nitrite and nitrate nitrogen of 0.2 – 10.3 mg/L. 4) Atmospheric deposition – inorganic or particulate nitrogen and mineralized nitrogen that settles by gravity. 5) Surface run – off – fertilizers from farmlands, leakages from landfills, leakages from failing sanitary sewers and septic systems etc. ‒ Effects of Nitrogen Discharges Excessive accumulation of various forms of nitrogen in surface and groundwater can lead to adverse ecological and human health effects. One of the major effects has been the direct and indirect depletion of dissolved oxygen in receiving waters. Instream nitrification directly consumes oxygen while bio-stimulation of aquatic plant growth lowers oxygen indirectly when a plant dies and undergoes bacterial decomposition. Wastewater Treatment│ 15 Prepared by: Engr. MAAbellera Other impacts can be of major importance in particular situations. These include ammonia toxicity to aquatic life, adverse public health effects and a reduction in the suitability of water for re-use. ‒ A major problem in the field of water pollution is eutrophication which is defined as excessive plant growth and algal blooms resulting from over - fertilization of rivers, lakes and estuaries. Eutrophication can result in a deterioration in the appearance of previously clear waters, odor problems from decomposing plant growth and a lower DO level, which can adversely affect the respiration of fish, benthic aquatic animals and attached bottom plant growth. TWO STEPS IN NITROGEN REMOVAL FROM TREATED WASTEWATER a) Nitrification ‒ This is the biological oxidation of ammonium. This is done in two steps, first from the nitrite form then to the nitrate form. Two specific chemoautotrophic bacterial genera are involved, using inorganic carbon as their source for cellular carbon. Nitrosomonas Nitrobacter + NH4 + O2 → NO2 + O2 → NO3Ammonium Nitrite Nitrate b) Denitrification ‒ This is the biological reduction of nitrate to nitrogen gas. This can proceed through several steps in the biochemical pathway, with the ultimate production of nitrogen gas. A fairly broad range of hetrotrophic bacteria are involved in the process, requiring an organic carbon source for energy. NO3- + organic carbon → NO2- + organic carbon → N2 + CO2 + H2O ‒ It is important to note that if both oxygen and nitrate are present, the bacteria will typically prefer the consumption of oxygen in the oxidation of the organic matter because it yields more energy. Thus for denitrification to proceed, anoxic condition (nitrate without oxygen) must exist, although this is not strictly the case for all bacteria. • • Anaerobic - Condition in which free and dissolved oxygen is unavailable. Requiring or not destroyed by the absence of air or free oxygen. Anoxic - condition in which oxygen is available in the combined form only; there is no free oxygen. Anoxic sections in an activated sludge plant may be used for denitrification. B) Phosphorus Removal ‒ Phosphorus is an ubiquitous constituent of municipal wastewater, averaging around 10 mg/L in most cases. ‒ Forms: originally bound phosphorus (body and food wastes, released as orthophosphates when decomposed), polyphosphates (from synthetic detergents), and orthophosphates (hydrolyzed polyphosphates). Wastewater Treatment│ 16 Prepared by: Engr. MAAbellera TWO STEPS IN PHOSPHORUS REMOVAL FROM TREATED WASTEWATER a) Chemical Method of P – Removal (Chemical Precipitation) a.1) Metal addition ‒ Alum or Al2(SO4)3 Al2(SO4)3 + 2HPO42- → 2AlPO4 (ppt) + 2H+ + 3SO42‒ Iron or FeCl3 FeCl3 + HPO42- → FePO4 (ppt) + H+ + 3Clb.1) Lime addition ‒ Ca(OH)2 5Ca(OH)2 + 3HPO42- → Ca5(PO4)3OH (ppt) + 3H2O + 6OH▪ The addition of lime also provides pH adjustment necessary for the process. The reaction requires a pH at least 9.0 for significant phosphorus removal. ▪ Note: Phosphorus removal can be incorporated into primary or secondary treatment or separate as a tertiary process. Selection of the point depends on efficiency requirements, ww characteristics and the type of secondary treatment employed. Common point of addition/ application of chemicals is in the final clarifier (2° clarifier) b) Biological P – Removal ▪ Phosphorus is utilized by microorganisms for cell maintenance, synthesis, and energy transport. Microorganisms also store phosphorus for subsequent use. ▪ Biological methods of phosphorus removal rely on the fact that microorganisms when stressed, they release phosphorus. The microorganisms can be stressed by cutting off their supply of oxygen (anoxic condition) and fooling them into thinking that all is lost, and they will surely die! When this anoxic condition is followed by a sudden reintroduction of oxygen, the cells will store phosphorus in their cellular material exceeding their normal need. The Phosphorus can therefore be removed by settling in a clarifier. ▪ Biological phosphorus removal is accomplished by sequencing and producing the appropriate environmental conditions in the reactors. Phosphorus is released from cells under anoxic/anaerobic conditions and luxury uptake of phosphorus by the microorganisms takes place under aerobic/oxic conditions. ▪ Acinetobacter is one of the primary organisms responsible for removal of phosphorus. These organisms utilize the volatile fatty acids (VFAs) in the influent wastewater under anaerobic conditions by releasing stored phosphorus. Combined Biological Nutrients Removal (BNR) – Removal of Nitrogen and Phosphorus 1. A2/O Process Wastewater Treatment│ 17 Prepared by: Engr. MAAbellera 2. 5 – Stage Bardenpho Process 3. UCT (University of Cape Town) Process 4. VIP (Virginia Initiative Plant) Wastewater Treatment│ 18 Prepared by: Engr. MAAbellera