If there are uncertainties about the question, do not hesitate to ask the supervisor.

Transcription

1 Delft University of Technology Faculty of Civil Engineering and Geosciences Subfaculty Civil Engineering Section Sanitary Engineering CT447 Drinking water I Technology Date : 4 November 5 Time : 4.7. h There are 7 times 4 TrueFalse statements of equal weight. In all statements it is important to show insight in the influence of process parameters on the treatment step in consideration, in order to be able to optimise design and operation. During the examination a A4tje with own notes may NOT be used. If there are uncertainties about the question, do not hesitate to ask the supervisor. Motivate your answer and check if the answer is complete and practical. Overview of questions. Aeration. Filtration 3. Coagulation/flocculation 4. Softening 5. Adsorption 6. Sedimentation 7. Membrane filtration

2 General data Table Atom mass of important elements in water chemistry. Element Atom mass Element Atom mass H C N O F Na Mg Al P S Cl K Ca Mn Fe As Pb 3 35, Table Dynamic and kinematic viscosity as function of temperature. Temperature [ C] Dynamic viscosity [ 3 Pa s] Kinematic viscosity [ 6 m /s] ,79,5,3,5,,9,8,79,5,3,5,,9,8 Relevant formulas in water chemistry bij T = C CO + H O <> H 3 O + + HCO 3 HCO 3 + H O <> H 3 O + + CO 3 CaCO 3 + CO + H O <> Ca + + HCO 3 K = 3,44 7 pk = 6,46 K = 3,5 pk =,48 K a = 4, 5 NH 3 + H O <> NH OH NH O + H O <> NO 3 + H 3 O + CH O + O <> CO + H O H 3 O + + CaCO 3 <> Ca + + HCO 3 + H O 5CH O + 4NO 3 <> N + 4HCO 3 + CO + 3H O FeS <> Fe + + S SO 4 + CH O <> H S + HCO 3 S + H 3 O + <> H S + H O CH O <> CH 4 + CO NaOH + Ca + + HCO 3 <> CaCO 3 + Na + + H O Ca(OH) + Ca + + HCO 3 <> CaCO 3 + H O 4Fe + + O + H O + 8OH <> 4Fe 3+ + OH Fe OH <> Fe(OH) 3 Mn + + O + 4OH <> MnO + H O

3 Table 3 k D values for different gasses as function of temperature. k D C C C Nitrogen Oxygen Methane Carbon dioxide Hydrogen sulfide Tetrachloroethene Trichloroethene Chloroform,3,49,55,7 4,69,9,39,43,3 3,65 3,38 4, 9,6,6,33,34,94,87,88,39 5,7 Universal gas constant R = 8,34 J/(K.mol) Table 4 Composition of air in atmosphere (T = C, p = 5 Pa) Gas Volume percentage [%] Nitrogen Oxygen Carbon dioxide Argon Rest gasses 78,,95,3,93,

4 Relevant formulae p c = MW R T dt Ql cec RQ = K = Q cc w K = expk t ( ) K = 3 k k D exp k t + RQ D + RQ K 5 = kd + + k t RQ H I = = 8 dc = k ( c s c ) s K = + k t K = 4 n K = (k) k kd RQ D exp k t RQ k RQ D exp k t ν ( p ) v ν ( ).8 3 L g p d f w H max = ( p ) L ρ ρ ρw c BV c = + exp k EBCT c q Q K w ( TMD Δπ) J = = A ν e ρ mem R T c i zi π = MWi cp Ret = c P G = μ V Q s = B L Re = v R ν f p v H = 3 L x q max = = K c m Q T BV = = V.8. e 3.8 g p d e n s T EBCT P + P ΔP f c hydr TMD = P p = P f Pp Q γ = Q p f J δ β = exp D i P = ρ g Q Δ H p o r = ( p ) + sdp s v Fr = gr B H λ R = τ = ρ w vs B + H 8 e

5 . Aeration and gas transfer. Aeration of groundwater is mainly meant to remove carbon dioxide, methane and ammonia.. From theory it can be concluded that removal of carbon dioxide by cascades is independent of fall height..3 Removal of carbon dioxide is 5% per cascade step. Then removal of 5 cascade steps in series is 75%..4 When ph is 6.46 (at temperature C) and HCO 3 is mmol/l then CO concentration is mmol/l.. Filtration. At ground water pumping station Ridderkerk of HydronZH the first filters are dryfilters. This is probably because of the high methane concentration in the influent water.. At pumping station Scheveningen of DZH mainly iron is removed. The filtration velocity is 5 m/h, the sand grains have a diameter of mm, the porosity is 4%, the bed height is.5 m and the water temperature is C. Then the clean bed resistance of the filterbed is 8 cm..3 For the filters mentioned in statement. the backwash velocity has to be 4 m/h to reach % expansion (assuming a sand density of 65 kg/m 3 )..4 The maximum pore filling of the filters mentioned in statement. is 8% and the density of the flocs is 5 kg/m 3. Then.6 kg of iron flocs can be stored in m Coagulation and flocculation 3. Sweep coagulation is normally applied when suspended solids concentration in influent water is high. 3. After coagulation and floc formation the water is normally pumped to a floc removal device. 3.3 When water temperature drops from o C to o C, the power input in the floc formation unit should increase with a factor During coagulation the ph of the water will drop, making the water more acid. 4. Softening 4. At groundwater pumping station Ridderkerk lime is used for softening. The reason for that is the low buffering capacity of the incoming water. 4. At groundwater pumping station Ridderkerk split treatment is not applied. One of the reasons could be that the magnesium concentration in the incoming water is high. 4.3 At Amsterdam water Supply the incoming calcium concentration is mmol/l. Reduction of calcium to.5 mmol/l is necessary. In a pellet reactor the maximum softening depth is.5 mmol calcium/l. The bypass ratio can thus be 5%. 4.4 At Amsterdam Water Supply, the pressure loss over a expanded pellet bed of 5 cm is 4 cm. The density of the pellets is 7 kg/m 3. Then the porosity of the expanded bed is 53%.

6 5. Adsorption 5. At groundwater pumping station Ridderkerk GAC is applied. In the past the 4 filters were operated in a Pseudo moving bed mode (x filters). Nowadays the 4 filters are operated in parallel. This most probably because of the higher removal efficiency of the parallel operated filters. 5. A filter of 5 m 3 is loaded with an influent concentration of μg/l atrazin and a flow of 5 m 3 /h. The density of the carbon is 5 kg/m 3. The maximum saturation of the carbon is determined by the following Freundlich constants: n=.5 and K = 3 (g/kg) (m 3 /g) n. The maximum amount of Atrazin stored in the filter (assuming a steep adsorption front) is thus kg. 5.3 The number of Bedvolumes of the filter of statement 5. is 36, the run time of the filter is 45 days. 5.4 When the influent concentration of Atrazin increases to 4 μg/l, the maximum amount of Atrazin stored in the filter of statement 5. will increase with a factor Sedimentation 6. The removal efficiency of a horizontal flow tank with the dimensions (L x W x H) x 4 *3 m 3 that treats a flow of 86.4 m 3 /h with the settling characteristics of tabe 6. is 5%. Table 6.. The cumulative frequency distribution of the settling velocities is given in the table below. S [mm/s],,,4,6 P [%] 33,3 66,6 6. The flow in the horizontal flow settling tank mentioned in statement 6. is stable and turbulent (temperature is C). 6.3 By placing a vertical baffle in the flow direction in the tank, turbulence decreases and stability increases. 6.4 The length of the outlet gutter of a horizontal flow settling tank should be equal to the width of the settling tank. 7. Membrane filtration 7. For the treatment of backwash water ultrafilters are applied in order to remove Natural organic matter (NOM). 7. At Heineken factory, water from DZH is chosen (instead of water from HydronZH) to feed the Spiral Wound Reverse Osmosis installation, because of its low calcium and silica content. This is mainly to avoid scaling of the membranes. 7.3 At Heineken factory the Spiral Wound Reverse Osmosis plant is partially bypassed. This is mainly because of the sodium content. Assuming % retention of sodium, an influent concentration of 36 mg/l and a required sodium content in the Heineken beer of mg/l the bypass ratio is 8%. 7.4 At Heineken factory, the Spiral Wound Reverse Osmosis installation consist of a christmas tree configuration in three stages. In these configurations the recovery of all the three stages is normally the same.

7 CT447 Drinking water I Technology Date : 4 November 5 Time : 4.7. h Answers of exam. Aeration and gas transfer.. False, ammonia is not removed during aeration.. True, because of the high Kdvalue of carbon dioxide, removal is determined by mass balance and not by kinetics and removal is thus independent of fall height..3. False. k=.5 and K=(k) 5 = True. ph=pk+log(hco3/co) = > = = log(hco3/co) => CO= mmol/l.. Filtration.. False. The first filters are dryfilters and this is because of the high ammonia concentration in the influent water. ν ( p) v e 6 (.4) 5/36.. False. H = 8 L = 8.5 =. m 3 3 g p d e 3 ^.8 g ( p)3 d / False. pe =.5; v = (8.65 ) = 56m / h.8.8 3ν ( p).4. True. Total pore volume for flocs is.4*.8 =.3 m 3 /m 3. mass of flocs is.3*5=.6 kg/m 3 3. Coagulation and flocculation 3. False. Sweep coagulation is normally applied when suspended solids concentration in influent water is low. 3. False. Pumping will breakup the flocs, so the water will flow by gravity. 3.3 True. To keep the same Gvalue, the power input should compensate for the higher viscosity. 3.5 True. Because of the formation of iron hydroxide flocs the water becomes more acid. 4. Softening 4. False. Using lime more HCO 3 is used than when caustic soda is used and thus the buffering capacity will be lowered more. Low buffering capacity of the incoming water thus is not the reason. 4. True. Because of high magnesium concentrations spilt treatment cannot be applied. Softening only removes calcium till a maximum of.5 mmol/l 4. True..5*.5+*.5=.5 mmol/l 4.3 True. H=.7*(p)*L => p= Adsorption 5. False. The removal efficiency of Pseudo moving bed filters is better, but probably there was a hydraulic problem. 5. True. The total amount of atrazine stored is maximally q=3*5*5*..5 = kg/filter 5.3 True. T=BV*V/Q=36*5/(5*4)=45 days 5.4 True. When atrazin concentration increases with factor the loading capacity increases with factor n =.5 =.4.

8 6. Sedimentation 6. False. The surface loading s o =Q/A=44/8=.8m/h=.3mm/s. P =5%. The removal in a horizontal flow tank however is higher than 5%, in this case 75%. 6. False. v =86.4//36=.m/s. R=/. Re=4 and Fr=3.4 e7. => turbulent and unstable. 6.3 True. R becomes.75 => Re = 5 and Fr = 5.4 e False. Because of resuspension the outlet device should have a length several times times the width of the tank. 7. Membrane filtration 7. False. Ultrafilters do not remove dissolved organic mattter and back wash water normally does not contain organic matter. 7. True. Calcium and silica can give scaling of the membranes (mainly when the concentrations are high due to concentration polarization and in the later stages of the membrane filtration installation) which affect the recovery. 7.3 True..8*36= mg/l 7.4 False. The recovery in later stages is lower, because the feed pressure in the later stages is lower because of higher hydraulic and osmotic pressure losses.