CIV235 Long Answer Q's
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 discharge point a low DO flow and high BODu are mixed instantaneously -higher BODu causes deoxygenation of the water so DO decreases -as BODu is used up the deoxygenation rate decreases causing the sag curve -river gains new atmospheric oxygen and DO rises to DO saturation level
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 .
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 scale on pipes and other domestic appliances. It is caused by multi-valent cations in water. Carbonate hardness happens when Ca2+, Mg2+ 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 Ca2+, Mg2+ associated with other ions, Cl- , NO3 - , SO4 2- , solid residue not removed by heating.
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
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.
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 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 -all waste taken to treatment plant except when there is too much rain. Separate just sewage. Separate system consist of two pipe system, one for sewage water, another for storm water. These normally entail a more steady delivery (rate and strength wise) to the waterwater treatment plan. Stormwater is thus not not treated. Separate systems normally guarantee a better flooding performance than combined systems.
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
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
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.
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.
What is the aim of disinfection in the water treatment cycle, state 6 characteristics of a good disinfecting agent. 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 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. 1/ 2 ( ) V P G µ = P G µV 2 = 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.
What is meant by dry weather flow, how is it calculated? What is the daily flowrate 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 l/day P=population G=average daily consumption per head l/h/d E=Industrial & commercial discharges to sewer l/d
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 (OfWat) - economic regulator, sets customer prices every 5 years Drinking Water Inspectorate - ensures safe drinking water & monitors compliance with regulations.
Describe the global water cycle, list all the important processes and explain why this is considered a closed system as regards water.
Hydrologic cycle components: -precipitation -evaporation -transpiration -surface run-off (overland flow, water network run off) -infiltration or percolation -interflow (soil moisture, vadose zone) -base flow, groundwater flow (aquifers) It is a closed cycle because water changes location (air, land surface, vegetation storage, soil moisture, groundwater, surface water bodies, glaciers, ice caps) and phases (solid, liquid, vapor) but the total mass remains constant. The whole cycle is powered by energy from the sun.
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
Explain what is meant by the "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
List two nutrients of major concern for a natural receiving water body.
Nitrogen and Phosphorous
Explain the difference between CBOD and NBOD, use a sketch
Oxygen demand only from carbon-containing compounds Oxygen demand only from nitrogen-containing compounds
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)
Explain the difference between point sources and non-point sources of pollution.
Point sources: localized releases of pollutants Eg. combined sewer overflows, outflow from treatment works, storm sewer overflow, and any single identifiable locations of pollution release. Non-point sources: pollutants are released over larger areas typically at lower strength level. Non-point source are run-off from catchment surfaces, agricultural land, industrial estates. Location of release is typically more difficult to identify for non-point sources. This makes it difficult to attribute responsibility for the release of pollution.
Why are suspended sediments problematic in receiving waters?
Reduce light levels (high turbidity), carry other pollutants both impact on ecology
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
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.
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 any external factors which may impact on the reliable running of this system.
Temperature can influence decay rates, and rate of biomass growth, variation in biomass influent concentration
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 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. Determined in lab tests where the concentration of dissolved oxygen in samples is measured over time. Tests are run biochemically, and at 20C. Normally measurements are made at 5 days. BOD5
Coagulation occurs in a "Flash Mixer" and is characterised by rapid mixing, and flocculation in a flocculator which is characterised by slow mixing. In both cases the strength of the mixing is defined by G and the Camp Number. Explain what the physical reasoning is behind these two variables and list typical values for coagulation and flocculation.
The degree of mixing is measured by Velocity Gradient (G). G, mixing intensity (the velocity gradient), G=dv/dy, s-1
Explain the process and the reasons for using lime soda for softening water.
The purpose is to precipitate Ca2+ and Mg2+ ions and remove the precipitate before releasing the water. Ca2+ + CO3 2- to CaCO3 Mg2+ + 2OHto Mg(OH)2 Lime (CaO ) dosing increase hydroxyl ions (OH- ), increase pH cause the carbonate buffer system to shift to right, thus provide CO3 2- for precipitation of Ca2+ ions
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 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 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 the urban water cycle, focus closely on the parts of the cycle that require intervention by Civil Engineers.
Urban water sources are stream, lakes, reservoirs and aquifers. Water is transferred from these sources to storage and treatment facilities, then transferred to end users via distributions system. After use, whether domestic or industrial, wastewater is collected via sewers (combined or separate) and then sent to treatment. Ultimately treated water is released back to either surface water or groundwater. This cycle is not quite a close one, because of pipe leakages, overflows when systems fail, infiltration/exfiltration. Potential for losses is significant.
What is meant by water stress and how this can affect different parts of the world.
Water stress occurs (a) when the demand for water exceeds the available amount during a certain period or (b) when poor quality restricts water use. Water stress causes deterioration of fresh water resources in terms of quantity (aquifer overexploitation, dry rivers, etc.) and quality (eutrophication, organic matter pollution, saline intrusion, etc.).
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
Define turbidity, describe how it is often measured in a laboratory environment
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