7415331
Description
Flashcards by Billie Santos, updated more than 1 year ago
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Created by Billie Santos
over 7 years ago
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| Question | Answer |
| What is meant by water stress and how do this vary across different parts of the world? | Water stress occurs when the demand for water exceeds the available amount during a certain period or when poor quality restricts its use. Water stress causes deterioration of fresh water resources in terms of quantity (aquifer over-exploitation, dry rivers, etc.) and quality (eutrophication, organic matter pollution, saline intrusion, etc.). Focus on quality rather than just quantity. |
| Describe the urban water cycle, focus closely on the parts of the cycle that require intervention by Civil Engineers | Takes water from natural water body (surface/ground water) , storage-treatment, transfer to end user via distributions system, use, water polluted, collected via sewers (combined/separate) then treatment and release back to either surface water or groundwater. Leakage, overflows when system fails, infiltration/exfiltration means that this system is not really closed – there is the potential for losses. |
| Explain how the provision of water and wastewater services are regulated in the UK. List the regulators and their functions. | Environment Agency (looks after environmental impact, abstraction and discharge licences, groundwater protection), Office for Water Regulation – economic regulator, reasonable rate of return, sets customer prices every 5 years, decides on levels of investment; Drinking Water Inspectorate – ensures safe drinking water, licences processes equipment, monitors compliance with regulations. |
| Explain the key differences between a combined and separate sewer system, and the key advantages and disadvantages of each system. | Combined sewer systems – sewage and rainfall, separate just sewage. Combined, all waste taken to treatment plant except when there is too much rain, sewage volume and strength variable at WwTP entry. Separate; two piped systems in steady of one, steady deliver of constant rate and strength to WwTP, stormwater system does not treat stormwater. Normally better flooding performance that combined system. |
| Explain the difference between point sources and non-point sources of pollution. | The distinction between point sources and non –point sources of pollution lies in the spatial area over which contaminant originate. In the case of point sources, contaminant release is concentrated in spatially contained areas, say in the order of tens of square meters. Conversely, non-point sources cover much larger areas, say in the order of square kilometers, and contaminant loading per surface area is much smaller. |
| Provide example of point sources and non-point sources of pollution. | Looking at urban water cycle; point sources, combined sewer overflows, outflow from treatment works, storm sewer overflow, single identifiable locations of release of pollution; non-point source run-off from catchment surfaces, agricultural land, industrial estates etc., no identifiable location of release. This makes it difficult to attribute responsibility for the release of pollution. |
| List two nutrients of major concern for a natural receiving water body. | Nitrogen and phosphorous. |
| Define biochemical oxygen demand, describe the procedure for determining BOD, in particular the temperature and duration used in the testing procedure. | The amount of oxygen required to oxidize any organic matter present in the water biochemically, 20C and normally 5 days. |
| Explain the difference between CBOD and NBOD, use a sketch. | CBOD: Oxygen demand only from carbon-containing compounds. NBOD: Oxygen demand only from nitrogen-containing compounds |
| Why are suspended sediments problematic in receiving waters? | Reduce light levels, carry other pollutants both impact on ecology. |
| Explain what is meant by “emerging pollutants” and why they are causing particular concern. | Nanoparticles – large specific surface area very reactive, difficult to control Pharmaceuticals - complex molecules, difficult to oxidize, more common in domestic sewage aging population Pesticides – complex molecules, difficult to oxidize, accumulate in environment |
| Explain, using equations and sketches the concept of “instantaneous” mixing when applied to a control volume. | Idea is to look at mass conservation, there is no degradation of DO, or BOD as it all happens at one point in time. Add up the products of a concentration and a flux in a basic continuity equation. Examples for DO and L(ultimate BOD) given above. |
| Explain using a sketch the impact of BOD on a natural river. | |
| Describe what is meant by the “DO sag”. Explain what processes cause this phenomenon. | After the discharge point the DO levels decrease as the flow with a low DO and high L are mixed “instantaneously” with the river. As you move downstream, the higher L value causes de-oxygenation of the water and the DO level drops. The rate of deoxygenation reduces as the BOD is used up. At the same time the river gains new oxygen from the atmosphere this is at a rate determined by the river flow velocity and water depth. If the depth and velocity are constant with distance so is the reoxygenation rate. Eventually the net effect is for the DO level in the river to rise towards the saturation level. The “sag “ is the part of the DO curve with distance that drops to a minimum and then rises again. |
| What physical processes control the value of the coefficients that describes the deoxygenation rate and the re-aeration rate in natural rivers. | De-oxygenation rate is dependent on the amount of ultimate BOD available at time t, and the rate constant is dependent on a bed activity coefficient and water temperature. The re-aeration rate is dependent on the water depth and velocity. |
| If the wastewater is treated to a higher level before discharge would you expect the critical point to move upstream or downstream from its original position? | tc is larger and distance is longer. |
| List and explain the purpose of the various processes that you would expect to see within a water treatment plant. | Screening - removes coarse debris Coagulation – chemicals are added to the raw water and rapidly mixed to cause particles to floc Flocculation – water mixed carefuly to build up flocs of a particular requirement Sedimentation/Flotation – flocs are sedimented out either due to self-weight settlement or air flotation tanks Filtration – rapid or slow sand fliters used to remove remaining particulate matter. Disinfection – used to kill or inactivate bacteria Storage – potable water stored until it can be released into the water distribution network |
| List typical water quality parameters used to characterise raw water. | Physical: colour/temperature, odour and taste, TSS, conductivity Chemical: hardness, nitrite and nitrate, chlorine, F, As, metals (Cu, Pb, Hg), Toxic Organic Compounds, pH Microbiological: Variety of bacteria, main test one is Escherichia Coli (E. coli) |
| What is pH, what does it represent? | Concentration of H+, measured on an inverse log scale, pH = -log10([H+]), pH = 7 means [H+] = 10-7 mol H+/L, low value acidic, large value alkaline |
| Define turbidity, describe how it is often measured in a laboratory environment. | Definition: the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through a sample of water, An indicator of water clarity. In lab, measure transmission of light (at a set wavelength) through a standard cuvette, colloidal particles scatter light, units NTU from standard solution. |
| Define total suspended solids, describe how it is often measured in a laboratory. | The amount of matter suspended in the water, measured using the mass retained on a dried paper filter. |
| Define Hardness, what is the impact of this parameter on supplied potable water. | Definition: the concentrations of all multivalent metallic cations in solution, major cations : [Ca2+], [Mg2+], often expressed as CaCO3 mg/L equivalent. Total Hardness is equal to Carbonate hardness + Noncarbonate hardness, Carbonate hardness : hardness that is less than or equal to the carbonate alkalinity. Noncarbonate hardness : hardness that exceed the carbonate alkalinity, [Ca2+], [Mg2+] associated with Cl-, NO3- (permanent hardness) Above about 150mg/L starts to be noticed by consumers and causes the build up of unwanted solids in domestic appliances. |
| Explain the difference between coagulation and floculation. | Coagulation uses the addition of chemicals to alter the surface charge of the particles that contribute to colour and turbidity so that the particles adhere to one another and are capable of settling by gravity. Flocculation provides the hydraulic conditions (e.g. gentle agitation) necessary for optimal flocculation growth so that sedimentation can occur more successfully. Coagulation is used with flocculation to remove particles which cannot be removed by sedimentation or filtration alone (due to size and density). |
| Explain how coagulation works, how does the use of alum help to cause coagulation? | Coagulants used in water treatment are often aluminium and ferric salts. These positively charged multivalent ions neutralise the naturally occurring negatively charged particles, thus allowing the particles to aggregate. Alum is an aluminium salt. |
| Describe the physical differences between Type I, Type II and Type III settling. | Type I settling – dilute suspensions, discrete particle settling, particles act independently and so settling velocity is key parameter. Type II and III settling - concentrated suspensions particle to particle interactions are significant, particles collide adhere and so change their settling characteristics, typically flocs settle more quickly. |
| Describe how a dissolved air flotation tank operates and why it is advantageous to use such a system compared to a Type I settling tank. | Dissolved Air Flotation dissolves air in the water under pressure. When the pressure is released the water becomes supersaturated with air and millions of small bubbles form. These bubbles attach to any particles in the water causing their density to become less than that of water, particles also collect together. The particles then rapidly float to the surface making them easily available for collection and removal, leaving the water underneath. Smaller foot print than settlement tank, bale to remove smaller particles for the same size of tank. Coagulation chemical costs are reduced as it can remove smaller flocs. |
| Define Hardness in potable water, what is meant by Carbonate Hardness and NonCarbonate Hardness. | A term often used to characterize the ability of a water to cause solid residue. It causes scaleon pipes and other domestic appliances. It is caused by multi-valent cations in water. Carbonate hardness happens when Ca(2+), Mg(2+) is associated with HCO3-()) , CO3(2-) sometimes called temporary hardness as the solid residue can be removed by heating the water. Noncarbonate hardness is caused when Ca(2+), Mg(2+) associated with other ions, Cl(-), NO(3-), SO4(2-), solid residue not removed by heating. |
| Explain the five different processes that can occur in filtration. | Straining – relies on small pore size to trap small particles, sedimentation relies on the slow flow velocity between the pores for the particles to settle on to the surface of the filter grains, interception particles moving through the porous media hit the filter grains, flocculation the slow flow field between grains encourages particles to collide and flocculate, biofilm growth on media traps particles – slow filters only. |
| Explain how a rapid sand filter works and is operated, describe typical loading rates for a rapid sand filter and explain why they are higher than for a more traditional slow sand filter | A rapid sand filter often has two layers of filter material, sand and anthracite. This combination means that the filter presents a range pf pore sizes to the water, largest first. This allows the filter to conduct more water for the sane head and it takes longer for the filter to become blocked. The rapid sand filter is also backwashed around once a day. This means that the filter material is disturbed and agitated by water flow back through the filter, this removes the fine particulates. After back washing the filter beds settles naturally with the different materials being sorted due to their settling velocities. Loading rates for rapid sand filters are of the order of 120-235 m3/day/m2. Slow sand filter rates are of the order of 2.5 to 7.6 m3/day/m2. |
| What is the aim of disinfection in the water treatment cycle, state 6 characteristics of a good disinfectant. List typically used disinfecting agents. | Disinfection operations are aimed at killing or making inactive pathogenic microorganisms. Ideal disinfectant is economic, toxic to pathogens, not toxic to humans, rapid acting, provides residual protection. Chlorine, chloramine, hypochlorite salts, ozone. |
| Describe the treatment processes commonly found in waste water treatment plants in the UK. List all the important processes and explain why each process is important. | Initial screening using a coarse bar screen to remove large debris that may damage plant later on in the process. An equalization basin is used to cope with the diurnal pattern of dry weather flow, storm flow and variable industrial outputs to provide a steadier flow and COD to plant. Primary settling to remove settleable BOD, biological treatment to transform soluble BOD to biomass, secondary settling to remove biomass, advanced treatment processes to remove phosphorus and any remaining nitrogen based products. |
| What is meant by dry weather flow, how is it calculated? What is the daily flow rate used in the design of wastewater treatment plants in the UK? | Dry weather flow is the mean rate of flow in “dry” weather (not more than 2.5mm rain in 24 hours) over a 24hour period. DWF = PG + I + E Formula A = DWF+2E+1360P – Combined sewer system Formula A = 3DWF – Separate sewer system |
| Name and describe the typical pollutants that wastewater treatment plants aim to remove. | Suspended solids – can cause sludge deposits and anaerobic conditions in the environment. Biodegradable organics – can cause anaerobic conditions in the environment. Pathogens – transmit disease. Nutrients – can cause eutrophication. Heavy metals – toxicity to biota and humans .Refractory organics – toxicity to biota and humans. Dissolved solids – interfere with reuse |
| Describe the three types of settlement processes found in wastewater treatment plants. Clearly explain the differences between the processes. | Type I (Discrete sedimentation): Occurs in dilute suspensions, particles which have very little interaction with each other as they settle. Type II (Floculant Sedimentation) particles flocculate together as they settle. Simple stokes type equation for particles settling velocities cannot be used. Laboratory tests needed to estimate mean settling velocity. Type III (Hindered settlement) High concentrated particles tend to settle as a zone, or ‘blanket’ Again no Stokes law, no adequate mathematics to type III; There are three zones separated by interface: clarified zone, intermediate zone and sludge compression zone. |
| Describe any external factors which may impact on the reliable running of a mixed aerated sludge system. | Temperature can influence decay rates, and rate of biomass growth, variation in biomass influent concentration. |
| Describe two different systems that can be used to aerate wastewater. List the advantages and disadvantages of each system. | Diffused air systems - compressed air introduced at base of tank, provides oxygen and mixes bacteria and flocs, 90% BOD removal, expensive uses high energy levels. Mechanical aeration systems – disturbs water surfaces to gather air from atmosphere, less energy intensive but less productive does not provide such large amounts of oxygen. |
| List the advantages and disadvantages of the activated sludge process. | Advantages Smaller area of land No filter flies Small head loss through works Disadvantages High running costs as use large amount of power Requires skilled operation Large quantity of secondary sludge produced Can cause bulking and foaming |
| List the advantages and disadvantages of trickling filters. | Advantages Less energy needed that activated sludge process; low running costs; little attention, simpler operation; no bulking sludge problems, better sludge thickening withstand shock toxic loads BOD5 removal efficiency: 80 – 90%. Disadvantages of trickling filters Poorer effluent quality that activated sludge process; sensitive to low temperature; large area of land required, lots of insects. |
| List typical processes used to de-water sludge and explain how the processes work. | Thickening- Settling or centrifugation Digestion -Microbial process often with no added oxygen anerobic digestion Stabilization of solids, removal of pathogens, production of methane Takes 2-3 weeks in large covered tanks (30-40oC). Conditioning- Addition of alum, ferric chloride, lime to aggregate solids Dewatering to remove water- Air drying, spreading basins, centrifugation, vacuum filtration |
| Describe the global water cycle, list all the important processes and explain why this is considered a closed system as regards water. Discuss the availability of freshwater for use. | The global water cycle consists of the journey that water takes as it moves from the land to the sky and back again. It occurs as a results of the three fundamental physicals processes of evaporation, condensation and precipitation, and is powered by energy from the sun. Key components of the cycle include: evaporation (from sea, lakes, streams, glaciers, ice caps, soil moisture, etc), precipitation (rain, snow, ..), infiltration in the subsurface, runoff (e.g. in rivers and other surface water bodies), transpiration (storage) by vegetation. It is a closed cycle in the sense the global amount of water circulating is constant. However, freshwater only accounts for about 3% of total, and thus constitute a limited resource especially if one considers the geographical distribution of water demand and water availability. |
| When designing a sedimentation tank, what impact does the following have: Larger particles compared with smaller particles, same density, and Type I settling; | Larger particles would require a smaller tank as they have a higher settling velocity. |
| When designing a sedimentation tank, what impact does the following have: A 3-meter deep tank compared with a 5-meter deep tank. | Substantially there is no difference as the settling behavior depends on the ratio between flow rate and plan area. (Surface loading rate). |
| When designing a sedimentation tank, what impact does the following have: Mineral particles as opposed to organic particles of the same particle size | Mineral particles are likely to have a higher density than organic matter thus they will settle faster, thus requiring as smaller tank. |
| When designing a sedimentation tank, what impact does the following have: Particles with a single settling velocity, as compared to particles with a distribution of settling velocities with a mean value equal to the particles with a single settling velocity. | A bigger tank will be necessary for the non uniform particle size distribution, because the design must be such that the particles the particle with the smaller settling velocity are captured. |
| When designing a sedimentation tank, what impact does the following have: Different water temperatures, e.g. 5°C vs. 20°C | Since viscosity is temperature dependent, and decreases with increasing temperature, the settling velocity at 20°C is higher than at 5°C. Therefore a smaller tank is required for T=20 C than for T=5 C. |
| Ewden Water Treatment Works is connected to two impounding reservoirs. Explain the purpose of the two reservoirs. What characteristics of the raw water does the treatment plant at Ewden focus on? Explain the processes used to deal with these characteristics. | One impounding reservoirs is used to supply raw water to the treatment plant. The other is a compensation reservoir used to maintain a minimum flow in the River Don to comply with ecological constraints. The two main characteristics are colour and bacterial contamination. Colour coagulation and flocculation are applied (twice! DAF first and then rapid sand filters). Magnetic ion exchange filters are used at entry to plant. Colour is a main issue at Ewden. Disinfection deals with removal of bacteria, algae, and pathogens. |
| What quantities of solids and BOD are removed in primary, secondary and tertiary treatment? | Primary treatment – remove ~60% of solids and ~35% of BOD Secondary treatment – remove ~85% of BOD and solids Tertiary treatment – varies: 95+ % of BOD and solids, N, P |
| Describe the main features to be incorporated into the design of primary sedimentation tanks to achieve efficient solids removal. Give examples of two types of tank and state typical value of suspended solids and BOD5 reductions. | Tanks designed as that the particles given time to fall to bottom of tank within distance from inlet to outlet. Disturb flow as little as possible at inlet and outlet, efficient sludge removal, scrapers on floor of tank, scan boards. It includes rectangular and circular tank. Rectangular tank: flat floor, sludge scraped into hoppers near inlet pipe, depth 1.8‐3.0 m, length:width: 3‐4:1. Circular tank: sludge hopper at centre below inlet pipe, overflow weir around perimeter, diameter 10‐30 m, wall 2‐3m, floor slope greater than rectangular tanks, 7.5‐15 |
| Design criteria for Primary sedimentaion | weir overflow rate: 230m3/m/day at DWF max. surface loading rate: 30m3/m2/day at 3DWF 6 hours detention time 4-8 tanks. |
| Design criteria for Secondary sedimentation | weir overflow rate: 150-300 m3/m/day at DWF (230) max. surface loading rate: 30-60 m3/m2/day at 3DWF (4) 1.5 hours detention time 4-8 tanks. |
| Give some possible solutions to water stress. | Transport water over long distance, from water abundant regions to water stress region; Virtual water trade (Instead of transporting water itself, import food and industrial goods which produced from water abundant region. The weight of traded goods is 1/100th-1/1000th of the weight of water required to produce the goods. However, the economy of water stressed regions may be too weak to afford such trade. Advanced technology,e.g.more efficient way to produce food and goods, desalination; Management; treat wastewater discharges to environment, recycle water, e.g. 80% water used in industrial sectors in Japan is recycled. |
| List conventional water pollutants. | Pathogenic organisms Organic matter Nutrients (e.g. N, P) Salts Suspended solids Toxic and Hazardous Substances |
| List emerging pollutants. | Colloids: nanomaterials - Pharmaceutcals: antbiotics, ant-inflammatory drugs… - Endocrine disrupting chemicals: steroids and hormones… |
| What is meant by oxygen demand? | Definition:the amount of oxygen required to oxidize an oxygen demanding pollutant. Indirect measurement of the amount of organic (or carbon-containing) pollution in a water body. |
| List sources of oxygen demanding material. | Municipal waste Agricultural/animal waste Material in storm water Leachate Industrial wastes |
| What two things effect the amount of molecular oxygen dissolved in water (DO)? | Temperature Salinity |
| What is BOD and why is it useful? | Biochemical oxygen demand is the amount of oxygen required to oxidize any organic matter present in the water biochemically. It's an indirect measure of the organic concentration in the water. BOD is directly related to oxygen consumption which reflects the impact of organic matter released into environment. |
| What are the standard conditions for BOD analysis? | 20 degreesC darkness to prevent algae from producing oxygen Excess of nutrients for microorganisms (no limit on growth or stabilization) Essential nutrients ( N,P,K,Fe) and bacterial seed may be added. BOD5-5days B)Du-20 to 30 days |
| What does 'L' represent? | 'L' is the oxygen equivalent of the concentration of the organic material. |
| What is ThOD and how is it determined? | The amount of oxygen required to oxidize a known compound fully to CO2 and H20. ThOD can calculate the amount o oxygen required to oxidize the ammonia present in the water or waste water. 1. Write and balance the chemical oxidation reaction with end products of CO2 and H20. 2. Calculate ThOD based on the mass of the chemical elements in the water. |
| What is COD and why is it useful? | The equivalent amount of oxygen required to oxidize any organic matter in a water sample by means of a strong chemical oxidizing agent.Similar to BOD but is determined by using a strong chemical oxidizing agent to calculate the oxygen demand. Ratio of BOD5/COD is used to indicate treatability of wastewater; - Domestic wastewater: BOD5/COD=0.4-0.65; - Industrial wastewater: BOD5/COD>0.3, - Difficult to use biological treatment:BOD5/COD<0.3. |
| What are the advantages and disadvantages of COD analysis? | Advantages – Relatively fast compared with the BOD analysis (3 hours vs 5 days) – Better repeatability that BOD test 8 Disadvantages – COD analysis does not approximate the conditions of a stream or other water body. |
| Describe the stages behind eutrophication. | N or P in natural water bodies is rare so this controls microbial population. Human activities release excess N/P to environment which exceeds ecosystem loading limit. It promotes excessive growth of algae and cyanobacteria. This blocks the light. As algae decay, it causes the depletion of oxygen, which suffocates fish and shellfish. |
| Whats are total dissolved solids? | Salts that do not evaporate. |
| What are the sources/concentration influences of salts? | Minerals irrigation industrial discharges highway runoff sea water intrusion |
| Why are salts a bad influence on the water cycle? | Salts limit the use of drinking water and cause crop damage/soil poisoning. |
| What are suspended solids and how are they measured? | SS are organic and inorganic particles in the water that do not settle readily. Fall velocity, depends on particle density and size. Measured by filtering a water sample, drying and weighing the filter paper. |
| List some toxic and hazardous substances. | Volatile organic compounds Hydrophobic organic compounds Surfactants Petroleum Additives Pesticides Heavy metals Other inorganic elements Acids/bases Oxidants Chlorination byproducts Combustion byproducts |
| What are the 3 processes in the transformation of BOD in a river? | Mixing De-oxygenation Re-oxygenation |
| How do you develop the DO sag curve? | 1. Determine the initial conditions 2. Determine the de-oxygenation rate from BOD test and stream geometry 3. Determine the re-aeration rate from stream geometry 4. Calculate the DO deficit as a function of time 5. Calculate the time and deficit at the critical point. |
| What are the primary objectives of water treatment? | 1. remove suspended material (turbidity) and colour 2. eliminate pathogenic organisms. 3. remove hardness and other minerals. |
| What are the characteristics of GW? | Constant compostition High mineral content Low turbidity Low colour Low or no dissolved oxygen High hardness High Fe, Mn |
| What are the characteristics of surface water? | Variable composition Low mineral content high turbidity Coloured Dissolved oxygen present Low hardness taste and odour |
| What is turbidity, how is it used and how is it measured? | Definition: the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through a sample of water. An indicator of water clarity. In the field, use a Secchi disk . In lab, measure transmission of light through a standard curvette. |
| Why is pH an important part of water quality? | pH controls the chemical form (species) of many compounds. Low pH leads to faster dissolution of surrounding minerals; releases potentially toxic elements. Changes biodiversity. High pH can increase concentration of ammonia, toxic to fish; increased precipitation of metals. |
| What is water hardness? | The concentrations of multivalent metallic cations in solution. Total=carbonate+noncarbonate |
| What is E.coli used to measure and why? | E. coli is used as an indicator of water quality: normal inhabitant of intestines of many animals. May implicate other pathogens. |
| Describe the coliform test. | Most Probable Number (MPN). MPN is coliform-group organisms per unit volume of sample water. Expressed as the number of organisms per 100 mL off sample water. • Series dilution, count colony numbers, and refer to MPN table. • Estimate a statistical range of the number of coliform. |
| Describe the 4 water quality parameters | Physical: physical characteristics are associated with the appearance of water, its colour/turbidity, temperature, taste and odour. Chemical: chemical characteristics are identified by observing reactions, hardness (nitrate and nitrie), chloring, F, As, Cu, Pb, Hg, toxic organic compounds. Microbiological: Microbes in water are very important to human health. (E coli) Radiological: Radiological factors are considered where contact with radioactive materials is possible. |
| What type of tank is used for coagulation? | Coagulation is performed in a rapid-mix tank: The objective of a rapid-mix tank is to destabilize the colloids. Depending on the coagulant type and dosage of coagulant the rapidmix tank provides to a reactor. |
| What type of tank is used for floculation? | Flocculation is performed in a slow- mix tank: The objective is to bring the particles into contact so that they can collide, stick together and grow to size that will readily settle. |
| List some typical coagulants. | Aluminium sulfate (Filter Alum) Ferric chloride Ferrous/ferric sulfate Polyaluminium chloride (PAC) |
| Why are coagulants needed? | The positively charged, usually trivalent, cations neutralise the naturally occuring negatively charged particles allowing them to aggregate. |
| Describe the 3 flocculation mechanisms. | Brownian motion: for relatively small particles which follow random motion and collides with other particles. Differential settling: particles with different settling velocities in the vertical alignment collides when one overtakes the other. Mechanical mixing: Using different types of mechanical mixers to promote particles contacts and thus their agglomeration. |
| What are diluted suspensions? | For type 1 settling, the particles act independently. Discrete particle settling. (Non-flocculent) |
| What are concentrated suspensions? | For type 2 and 3 the particle - particle interactions are significant. Particles may collide and stick together to form flocs which settle more quickly. |
| What is dissolved air flotation? | DAF is a technique to promote flotation by dissolving air in the water under pressure. When the pressure is released the water becomes supersaturated with air as millions of small bubbles form. These bubbles attach to any particles in the water causing their density to become less than that of water. The particles then will rapidly float to the surface which make them available for collection and removal, leaving the clarified water behind. |
| What are the dissolved air flotation key parameters? | Air:solid ratio (0.005-0.06 ml/mg) at 20C. Hydraulic loading rate: a measurement of the volume of effluent applied per unit effective surface area per unit time. Typical solid loading: mass per unit area per unit time. Operation pressure: 3-7 bar |
| What are the advantages of dissolved air flotation? | Initial construction costs less than larger seIling basins. • Further savings are achieved when the plant is enclosed in a building as less area is required for DAF. • Chemical costs are reduced as flotation in general requires less chemical pre-treatment to form a light, small floc. |
| What is water softening? | The process of removing mostly Ca2+ and Mg2+ ions. |
| How can you easily identify hard water? | Soap scum More soap needed when washing Scaling on pipes Valves stick together due tp carbonate crystals Stains on plumbing fixtures |
| What is alkalinity? | Represents the ability of water to neutralise acid; a measure of buffering capacity or acid neatralizing capacity. Typically measured by titration with a strong acid. |
| Describe the process of filtration. | Filtration is a process for separating suspended or colloidal impurities from water by passing it through a porous medium, usually a sand bed. |
| What are the 5 mechanisms of filtration? | Straining: particles larger than the pore space of the filtering medium are strained out. Sedimentation: particles settle on the filtering medium within the filter. Interception: particles that move on the streamline are removed as they come in contact with the surface of the filtering medium. Flocculation Biofilm growth: reduction of the pore volume. |
| Slow sand filtration | Water with suspended/coloidal material is placed on to of sand filter bed. Pores get clogged in top layer-hydraulic performance degrades progressively. Top 75mm of sand is periodically removed and the filter is re-started. |
| Rapid sand filtration | 50-100 times higher loading rates than slow sand filtration. Media are coarser and layered. Filters are operated and then backwashed. Sandgrains are pushed apart and filtrated particles can be removed. |
| Describe the back wash process. | As particles are removed, filter becomes clogged, head losses increase, and turbidity increases. Backwash is done about once a day, and takes 10-15 min. The system has to be designed appropriately to be able to handle flow with one filter out of service. Backwashing is accomplished by forcing water (and sometimes air) up from the clear well (washwater) back through the filter. Particles in the filter become suspended, and thus let the trapped particles to be released. Backwash water is sent back to the inlet and retreated. Filter particles settle back in layers due to their settling velocities. |
| What are the considerations when choosing a site for a WwTP? | Adjacent to river or stream to allow effluent to be discharged. A low elevation to allow maximum length of gravity sewer and exploit gravity flow in works. Away from houses (odour). To serve as large areas as possible: reduce pumping in network and optimise energy use in treatment processes. |
| Whats the maximum amount of water to the treatment works? | 6* dry weather flow |
| Whats the maximum flow that can receive full treatment? | 3*dry weather flow |
| What is the suspended growth process? (activated sludge) | The suspended-growth microorganisms are used to clean wastewater. The microorganisms are activated by an input of oxygen. |
| What is the attached growth process? (aerobic biofilm) | The microorganisms responsible for the conversion of organic matters and nutrients are attached to an inert packing material. |
| 2 treatment methods for the activated sludge process | Plug-flow reactor Completely mixed batch reactor |
| 2 treatment methods for the aerobic biofilm process | Trickling filters Rotating biological contractors |
| Why are wetlands used as a biological treatment method? | Floating plant act as filters and support for bacteria. |
| How to choose mode of BOD decomposition? | If BOD5<500 mg/L,use aerobic decomposition, because decomposition is rapid, efficient, and has low odour potential. If BOD5>1000 mg/L, aerobic decomposition is not suitable because of the difficulty in supplying enough oxygen. In this case, pure-oxygen aeration or aerated lagoons (oxidation ponds)are used. |
| What are the effects of temperature in the biological treatment phases? | Temperature affects: meganisms; gtabolic activities of the microoras transfer rate therefore aeration efficiency; settling characteristics of the biological solids. |
| Describe the activated sludge process. | The organic content of sewage is reduced by mixing the wastewater with a biologically active sludge in the presence of an adequate supply of dissolved oxygen. The organic removal process includes floc absorption and metabolism. Removal occurs as biomass coagulate and precipitate in secondary sedimentation tank. Sludge can then be recycled to feed the aeration tank. |
| What happens in the aeration tank? (activated sludge) | The organics in the waste are metabolised to produce end products and new biomass. Mixing must be adequate to prevent the sedimentation of microorganisms and promote the mixing of oxygen, sewage, microorganisms and nutrients. Oxygen must be mixed in continuously or semi-continuously through aeration. |
| What happens in the settling tank? (activated sludge) | Biomass and other suspended solids are separated from the treated water. a portion of the clarifier underflow is returned to the aeration tank; the remainder is discarded from the clarifier and sent out for further processing. |
| What is sludge age? | Thetac is also called Sludge Age, and represents the average time the activated-sludge solids remain in the system. If the sludge is not able (does not have enough time) to reproduce itself before being washed out if the system, failure will result. Higher sludge ages cause the sludge to undergo more endogenous decay |
| What is sludge bulking? | The sludge bulking takes place when the biological floc in the aeration tank has poor settling characteristics. This leads to high effluent suspended solids concentrations and poor performance. Generally, the overgrowth of the filamentous microorganisms can result in the sludge bulking. |
| What is foaming? | Some bacteria, such as Nocardia and Microthrix, which have hydrophobic cell surfaces, when they overgrow, can attach to air bubble surfaces, and stabilize the bubbles to cause foam on the water surface of the aeration tank. |
| What are the Advantages and Disadvantages of Activated sludge process? | Advantages • Smaller area of land required • No filter flies • Small head loss through works Disadvantages • High running (large power demand) • Requires skilled operation • Large quantity of secondary sludge produced • Can cause bulking and foaming |
| Describe the biofilm process. | Organic matter and nutrients are attached to an inert packing material. • Biofilm is composed of millions of microbes (bacteria, fungi, algae and protozoa) that accumulate on surfaces of various substrata in aqueous environment. • Microbes grow on medium or substratum surface can serve as a physical support biofilm formation. |
| What is the structure of biofilm? | Biofilms are highly heterogeneous; • Substrate (BOD/COD), O2 and nutrient diffuse across boundary layer and enter into biofilm; • Reactions are diffusion-limited: rate is limited by how much material diffuses through. |
| What are trickling filters? | Rotating distribution arm sprays primary effluent over circular bed of rock or other coarse media; Air circulates in pores between bed particles; “Biofilm” develops on bed particles and microbes degrade waste materials as they flow past Organisms slough off in clumps when film gets too thick. |
| List the advantages and disadvantages of trickling filters. | Advantages of percolating filters • Less energy needed; low running costs; • Little attention, simpler operation; • no bulking sludge problems, better sludge thickening; Ability to withstand shock toxic loads. BOD5 removal efficiency: 80 – 90%. Disadvantages of percolating filters Poorer effluent quality; Sensitive to low temperature; Large area of land required, lots of insects around. |
| What sort of problems can we face when using trickling filters? | Effluent contains much dark brown suspended matter (humus sludge), which requires settlement in humus tanks before discharge. Filters contain large numbers of scouring organisms- worms, flies, insect larvae etc. which feed upon biological film. During winter, these organisms retreat into bed of filter, and filter becomes choked. This is called “ponding”, and is also caused by overloading filter dosing with wastewaters containing too many suspended soilds. |
| How can the problems with trickling filters be overcome? | - forking the bed surface – flushing the bed with plant effluent – dosing chlorine – slowing down distributor speed – change to alternating double filtration (A.D F.) or re-circulation. |
| Why is tertiary treatment needed? | Sometimes a high quality effluent is required. (Low BOD, low fecal coliform, low nutrients). This ensures the protection of sensitive water bodies or drinking water supplies. |
| What are the 4 steps to sludge processing? | 1. Thickening (Settling or centrifugation) 2. Digestion (Microbial processes to stabilise solids and remove pathogens. Produces methane) 3. Conditioning (Addition of alum, ferric chloride, lime to aggregate solids) 4. Dewatering (Air drying, spreading basins, centrifugation, vacuum filtration to remove water) |
| Aerobic or anaerobic digestion? (stabilisation) | Aerobic-similar to activated sludge. Accomplished by aeration of sludge, followed by sedimentation. Supernatant goes back to head of plant. Treated sludge is 3% solids. Anaerobic-acid fermentation plus methane production. Produces energy from methane. |
| How can sludge be thickened? | Flotation- increases solids content from 0.5-1% to 3-6%. Especially effective on activated sludge. Gravity thickening - best with primary sludge. Increases solids from 1-3% to 10%. |
| What is sludge conditioning? | Chemical conditioning: addition of lime, ferric chloride, alum or polymers. Chemicals are added prior to the de-watering stage. Heat treatment: 175-230C, 10-20 atm. Bound water is released and sludge is easily dewatered. It's a complex process and highly concentrated liquid stream is produced. |
| What is the sludge de-watering process? | Sludge drying bed- simple and low maintenance cost. Affected by climate change. Filtration- application of vacuum to extract water. Water is forced out by squeezing the sludge between two moving filter belts. |
| How can we reduce the volume of sludge? | Incineration: complete evaporation of water from sludge. Solid material is inert. Fuel powered and the exhaust air must be treated prior to discharge. Wet oxidation: Treated sludge is wet. Solid material is inert. Requires energy and exhaust air must be treated prior to discharge. |
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