Assignment II Design of Wastewater Discharge from the City of Gothenburg. Design of a Wastewater Discharge from the City of Gothenburg

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1 Assgnment II Desgn of Wastewater Dscharge from the Cty of Gothenburg Envronmental Hydraulcs Desgn of a Wastewater Dscharge from the Cty of Gothenburg The assgnment conssts of two parts. The frst part deals wth a proposed marne outfall arrangement for the wastewater dscharge from the cty of Göteborg nto the Skagerak coastal sea. Ths part focuses on the dffuser arrangement, the dluton of the jets n the stratfed coastal water, and the subsequent further dluton n the recevng water. The purpose of the second part s to use the concept of box models and retenton tmes for assessng concentratons of certan pollutants n a nearshore recevng water. 1

2 Part 1: Proposed Wastewater Tunnel nto Skagerak A 2 km long man tunnel below the bottom of Skagerak, should brng the wastewater to the outlet area va a dffuser arrangement consstng of a long ppelne wth nozzles at equdstant locatons. Dffuser wth Nozzle Arrangement The desgn wastewater flow rate s 6 m 3 /s. 2

3 Stratfed Recevng Water Two years of densty measurements (stably stratfed condtons) Densty devaton shown: = 1 + sea Densty devaton at a certan tme σ Determne: Objectve of Assgnment: Part 1 1. the length of the dffuser ppelne 2. the neutral densty level for dscharge n two dfferent cases (.e., dfferent vertcal densty dstrbutons). The frst case should correspond to a stuaton where the wastewater wll rse a consderable vertcal dstance. The second case should be a stuaton where the rse s small. 3. the vertcal thckness of the wastewater layer at the neutral level 4. the wdth of the wastewater plume 1 km downstream from the pont where the wastewater reached the neutral level 3

4 Jet Calculaton n a Stratfed Water Body Use an approxmate step-wse calculaton of the jet behavor. Dvde the water n a number of layers each wth the vertcal thckness Δy j. Perform calculatons of the jet n one layer at a tme. The calculaton start at the nozzle level and contnue untl the jet has reached ts neutral level. A modfed Froude densmetrc number s defned for each layer: F = U _ j= 1 y Δy g d r, j If the layer thckness s constant one obtans: F = U _ j= 1 g d r, j 4

5 The dluton at the jet axs s gven by: S m, y =.54 F( ) df.44 y f.5 F d S m,.38 y =.54 F( +.66) df 1.67 y f.5 F d The average densty n layer number n the recevng water s: _ r, = + r, r, 1 2 = at the nozzle level, = 1 after the frst layer etc. The densty m, of the water at the jet axs s: m, ( ΔS 1) r+ = ΔS _ m,, m, 1 m, Δ S = m, S S m, m, 1 S = 1, = m, m, The crteron for the neutral level s gven by: _ r, m, 5

6 Vertcal thckness h of the wastewater at the neutral level after homogenzaton: Q U h U h L S γ S rec rec = = (contnuty consderaton) m, Q = wastewater dscharge L = dffuser length U rec = water velocty n the recevng water at the neutral level S = dluton of the jet after homogenzaton S m, = dluton at the jet axs at the neutral level γ = correcton factor for the homogenzaton process at the neutral level (=2.). Horzontal spreadng of the wastewater plume at the neutral level Wastewater s transported horzontally n the downstream drecton due to the recevng water velocty at the neutral level. The spreadng n the vertcal drecton s very small due to the stratfcaton, whch strongly attenuates vertcal turbulence. Horzontal spreadng takes place causng a wdenng of the wastewater plume. 6

7 Analogy wth the horzontal spreadng of a plume from a pont source: x B = K K = B m s H, H ( / ) Urec B = wdth of the plume x = dstance from the pont source K H = horzontal dffuson coeffcent Part 1: Summary of Tasks I Determne the number of nozzles and the length of the dffuser on the bass of the followng data: Q = 6 m 3 /s, U and d accordng to Table, = 1 kg/m 3 In order to obtan a good dluton of the wastewater, try to fulfll the followng condtons: a hgh jet velocty at the nozzles (U > 2 m/s) a small dameter of the nozzles (d, mn ~.1 m) dffuser locaton at a large water depth a farly large dstance l nozzle between consecutve nozzles: ( ) l max 1 d and (.1.2) y nozzle neut Set y neut = 2 m above the nozzles 7

8 Part 1: Summary of Tasks II Each group should choose one set of data from the Table. On the bass of these data the followng parameters should be calculated: The neutral level for two densty stuatons n the recevng water. One stuaton should be characterzed by a small rse of the jet and the second one by a large rse of the jet. The dluton, S m, at the jet axs at the neutral level for the two chosen cases The vertcal thckness of the wastewater at the neutral level after homogenzaton The wdth of the wastewater plume 1 km downstream of the locaton where the wastewater reaches the neutral level. Table: Data for each group Group nr Intal jet velocty Water depth Nozzle da- Recevng water U (m/s) of nozzles (m) meter d (m) velocty U rec (m/s) The calculatons should be performed wth a constant layer thckness Δy = 2 m. 8

9 Part 2: Dscharge of Wastewater n a coastal recevng water Rver E Area of the recevng water: A = 1 km 2 Average depth: D av = 3 m Flow rate n the rver: Q = 5 m 3 /s. In order to assess the average flow rate E (m 3 /s) from the open sea to the recevng water, a tracer experment s performed. Rhodamne s mxed nstantaneously and smultaneously at dfferent locatons n the recevng water => well-mxed and homogeneous condtons preval n the recevng water. Tracer concentraton s measured as a functon of tme. Tme (days) Tracer conc c s 1 9 (kg/kg H 2 O) 1.2 (c s, )

10 Part 2: Summary of Tasks 1. Determne the water exchange E (m 3 /s) between the open sea and the recevng water from the tracer measurements. The decay coeffcent for the tracer s d = 1-7 s -1. There s no tracer n the rver water or n the open sea. 2. Assume that the wastewater contans.5 mg P/l and that the rver water contans.5 mg P/l. What wll the steady-state concentraton of phosphorus (P) be n the recevng water, f there s no degradaton of phosphorus? What wll the steady-state concentraton of phosphorus be, f t s degraded by the rate of 1 % per month? The concentraton of phosphorus n the open sea s neglgble. 3. What wll the steady-stated concentraton of phosphorus be n the recevng water, f the concentraton of phosphorus n the rver water s reduced 1 tmes? Perform the calculaton wth and wthout degradaton. 1