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The water treatment plant of the town of Green, ON treats the waters of the Racoon River using an Actiflo® ballasted sedimentation system (see next page). Its key feature is the addition of microsand (MS) as a ballasting (weighing) agent that makes the flocs heavier so they settle much faster and can be removed in a much smaller clarifier. The system requires the use of a coagulant, the microsand and a polymer in the coagulation/flocculation stage. The clarifier's sludge (S) is processed through a hydrocyclone that recovers 99% of the microsand, so that it can be recycled. This process was selected because it can handle well large fluctuations in feed turbidity, these occur within the Racoon River after there is significant precipitation. The coagulant used in this treatment plant is a polymerized aluminium-based coagulant called PAS which has an aluminium concentration of 4.5 mass percent (i.e., 0.045 kg Al/ kg Total) and a density of 1320 kg/m3. The PAS dose is 26 g Al/m of Qin. The key reaction involving the coagulant is the formation of aluminium hydroxide precipitate and it is quantified by the following stoichiometric relationship. ΑΙ o Al(OH)3 The Actiflo process uses microsand (MS) (dparticle = 0.13mm, P part Ms = 2500 kg/m²), it is added (MMSNM) at a rate of 12 kg MS/d. The Actiflo system also uses and an anionic polymer as a flocculation aids (to bind the sand to the flocs), the polymer dose is 0.3 g/m²of QuN.
Note: If one measure TSS in the lab, microsand would show as TSS since the grains of microsand would be captured in the filtration step of the TSS analysis. In this problem consider TSS and microsand as two different components/contaminants. . . ASSUMPTIONS AND INFORMATION That unless otherwise indicated, that density of all streams is 1000 kg/m3 The coagulant does not contain solids. The coagulant stream does not contain microsand. The polymer stream contains no solids. The polymer stream does not contain microsand. The polymer does not create suspended solids. Assume that the total mass contribution of polymer stream (POL) is negligible. The new microsand stream (NM) it is dry, there is only dry sand. The recycled sand stream (RS) contains sand and some TSS. Assume that all of the aluminum (Al) added in the coagulant reacts via the above reaction which takes place in the flocculation tanks. No biological reactions take place. Assume that 1% of the Al(OH)3 formed is too small to be considered TSS, and it is not removed by the clarifier unit (i.e., it is carried in stream CE). The remaining 99% of Al(OH)3 formed are solids removed by stream S (i.e., considered part of TSS formed). The clarifier is a solids separations process, it does not separate dissolved species. The hydrocyclone is strictly a solids separation process, not a reactor. The hydrocyclone sends 1% of the incoming sand into stream HS. The hydrocyclone sends 90% of the incoming non-microsand TSS into stream HS. The DOC in the influent and effluents are considered to be dissolved substances (i.e., not suspended solids, so they do not count towards the TSS). The process achieves 45% DOC removal. . . . Name Formula Molecular Weight 55.85 Iron Fe Aluminum ΑΙ 27 Water H2O 18 Calcium carbonate CaCO3 100 Aluminum Hydroxide Al(OH)3 78.0
Coagulant (PAS) dose = 26 g Al/m² QIN 0.045 kg Al/kg total TSS=0 g/m² Microsand = 0 g/m p=1320 kg/m Polymer (P) dose = 0.3 g polymer / m² QIN Microsand = 0 g/m 3 TSS=0 g/m p=1500 kg/m 11 D Clarifier Clarifier Effluent (CE) TSS=0 g/m² Turbidity = 1 NTU Microsand = 0 g/m² Feed (IN) QIN = 6000 m3/d Turbidity = 200 NTU TSS=5 g/m Microsand = 0 g/m² NOM= 10 g TOC/m² Alk.= 65 g CaCO3/m3 pH= 6.5 Al= 0 g Al/m Flocculation Mixing Tanks Sludge (S) Ps=1400 kg/m Hydrocyclone New microsand (NM) 12 kg MS/d Recycled Sand (RS) p= 1600 kg/m3 MTotal = 8000 kg total/d Hydrocyclone Sludge (HS) QHS = ? TSS=0.7% p=1050 kg/m TASKS: a) Determine the volumetric flowrate the coagulant stream (i.e., Qpas) in m/d. Comments as to why the value is reasonable or not. (20 points) b) Determine the volumetric flowrate of stream HS (i.e., Qus) ( If you are unable to solve part (a) assume Qpas =1.0 m/d). (20 points) c) Determine the microsand concentration in stream B (i.e., CMSB) in g/m3. (If you are unable to solve part (b) assume Qhs = 70 m3/d). (10 points)