Secondary settling tanks and sludge mass balancedex summer school 2011 Univ. Prof. Dr.-Ing F.W. Günthert Universität der Bundeswehr München Institut für Wasserwesen Siedlungswasserwirtschaft und Abfalltechnik Secondary Settling Tanks 1 SWA Structure F. W. Günthert Structure: • Glossary of terms • Tasks of secondary settling tanks • Settling velocity • Return activated sludge, settling behavior of biological sludge • Dimensioning of SST, surface area, depth • Planning aspects, inlet, outlet, sludge removal systems • Sludge load balancing • Conclusion, references Secondary Settling Tanks 2 SWA Glossary of terms Rectangular secondary settling tank (SST) F. W. Günthert (IAWQ Report No. 6, 1997) Secondary Settling Tanks 3 SWA Glossary of terms F. W. Günthert Centre feed, peripheral take-off, secondary settling tank (SST) (IAWQ Report No. 6, 1997) Secondary Settling Tanks 4 SWA Abbreviations and units F. W. Günthert List of abbreviations and units (IAWQ Report No. 6, 1997) Secondary Settling Tanks 5 SWA Interactions F.Basic principles . Günthert • Activated sludge process QWW SSAT SSBS QRS. SSRS SSEST • Mass balance over the process RS = SS ⋅ SS AT RS 1 + RS Secondary Settling Tanks 6 SWA . W. W.Basic principles -Tasks of secondary settling tanks F. Günthert • Separation of the activated sludge by settling dependent on: – flocculation in the inlet zone – hydraulic conditions in the settling tank – return sludge ratio (RS) – sludge removal procedure Thickening and removal of the activated sludge dependent on: – sludge volume index (SVI) – depth of the sludge layer – thickening time (tTh) – type of sludge removal system • Secondary Settling Tanks 7 SWA . h) – type of sludge removal system – return sludge facilities • Avoidance of denitrification and resolution of phosphorous by: – limitation of thickening time – sludge removal performance Secondary Settling Tanks 8 SWA .Basic principles -Tasks of secondary settling tanks F. Günthert • Intermediate storage at the beginning of stormwater flow periods dependent on: – hydraulic load (QWW. W. 02 to over 0.8 to 1.4 mg/l •CN by 0. This is dependent on the efficiency of AT and SST. Günthert The effluent is mainly caused by SS (activated sludge) and dissolved and colloidal matter. W.04 mg/l Secondary Settling Tanks 9 SWA .Effluent quality of SST F.3 to 1. A suspended solids concentration of 1 mg/l dry solids in the effluent of SST increases the concentration of: •CBOD by 0.1 mg/l •CP by 0.08 to 0.0 mg/l •CCOD by 0. Clarification F. Günthert Settling velocity depends on: • particle volume V [m³] • particle size [-] • density of particles [kg/m³] • density of fluid [kg/m³] • viscosity of fluid [kg/(m·s)] • flocculation [-] • flow conditions (Reynoldsnumber) [-] • flow stability (Froudenumber) [-] • Gravitational acceleration [m/s²] Secondary Settling Tanks 10 SWA . W. Settling velocity F. 1997 particle diameter d [cm] Secondary Settling Tanks 11 SWA . W. Günthert settling velocity v s [cm/s] ATV. Settling velocity F. Günthert flocculation zone hindered settling zone interim zone thickening zone settling behavior (settling zylinder 1l) Secondary Settling Tanks 12 SWA . W. W.Velocity distribution F. influenced by density currents Secondary Settling Tanks 13 SWA . Günthert Distribution of velocities. Settling velocity F.htot) Secondary Settling Tanks 14 SWA . temperature •inlet and outlet •density currents •sludge removal procedure •tank geometry (L:W. W. Ø. Günthert Settling tanks are influenced by: •different settleable particles •wind. 1997 Secondary Settling Tanks 15 SWA . Günthert effluent concentration SSEST specific sludge volume load KA 11.Effluent concentration of SST F. W. Günthert • Activated sludge process SSAT SSBS RAS.Return activated sludge (RAS) F. W. SSRS • Mass balance over the process RS = SS ⋅ SS AT RS 1 + RS Secondary Settling Tanks 16 SWA . Return activated sludge (RAS) F.75 * QWW. temperature •sludge removal system with fitted performance to the thickening time tth •sludge return flow (QRS < 0. W.h) Secondary Settling Tanks 17 SWA . Günthert Return activated sludge is determined by: •settling behavior of MLSS (SVI) •thickening time tth and thickening depth h4 of SST •short circuiting flow rate between inlet to the sludge hopper (design of inlet. performance of sludge removal system) wind. Günthert Effect of filamentous organism on floc structure: (a) pinpoint floc. 1997) Secondary Settling Tanks 18 SWA .Determinants of settling behaviour of biological sludge F. W. (IAWQ Report No. 6. a non-bulking well-settling but poorly flocculating sludge. Günthert Effect of filamentous organism on floc structure: (b) bulking sludge. 1997) Secondary Settling Tanks 19 SWA . W.Determinants of settling behaviour of biological sludge F. a poorly settling but excellently flocculating sludge. (IAWQ Report No. 6. (IAWQ Report No. 1997) Secondary Settling Tanks 20 SWA . Günthert Effect of filamentous organism on floc structure: (c) ideal sludge. W. 6.Determinants of settling behaviour of biological sludge F. well-settling and well-flocculating sludge. a non-bulking. Determinants of settling behavior of biological sludge F. W. 1997) Secondary Settling Tanks 21 SWA . Günthert Typical particle size distribution of activated sludge. (IAWQ Report No. 6. 1997) Secondary Settling Tanks 22 SWA . (IAWQ Report No. Günthert Percentage particle removal plotted against particle size distribution. W. showing that it is mainly a significant proportion of the smaller particles that escape with the effluent and that virtually all of the larger particles are retained and recirculated in the activated sludge system.Determinants of settling behaviour of biological sludge F. 6. Günthert The influence of underflow recycle ratio (R) on the short circuiting. (IAWQ Report No. 6. 1997) Secondary Settling Tanks 23 SWA . W.Short circuiting flow F. A 131 E. 2000) Secondary Settling Tanks 24 SWA . Günthert Suspended solids concentration in the bottom sludge dependent on the sludge volume index and the thickening time.Dimensioning of the secondary settling tank F. (ATV . W.DVWK . (ATV .A 131 E. to avoid denitrification and phosphorus resolution. W.DVWK .Dimensioning of the secondary settling tank F. 2000) Secondary Settling Tanks 25 SWA . Günthert Recommended thickening time dependent on the degree of wastewater treatment. Günthert Bottom sludge concentration SSBS 1000 3 = ⋅ t TH SVI tTH = 1.Return sludge concentration F. W.5 .DVWK .0 .2 h tTH = 1.5 to 0.5 h tTH = 2 .1.Design procedures . 2000) 26 Secondary Settling Tanks SWA .A 131 E.5 h) Thickening time without nitrification: with nitrification: with denitrification: Return sludge concentration SSRS ≈ 0.(2.7 · SSBS with suction facilities (ATV .7 · SSBS with scrapers SSRS ≈ 0. Günthert Approximate values for the mixed liquor suspended solids concentration in the biological reactor dependent on the sludge volume index for SSRS = 0.A 131 E.Approximate values of the MLSS F. W.7 * SSBS RS = SS ⋅ SS AT RS 1 + RS (ATV .DVWK . 2000) Secondary Settling Tanks 27 SWA . 2000) 28 SWA . W. process scheme.g.DVWK . SVI design of the secondary settling tank design of the biological reactor optimum matching interactions yes Secondary Settling Tanks no Variation SSAT (ATV .A 131 E. sludge age. flow rates and loads.Procedure of Dimensioning F. Günthert design criteria e. DVWK . W.A 131 E. Günthert For the design of secondary settling tanks the following are to be determined: •shape and dimensions of the secondary settling tanks •permitted sludge storage and thickening time •return sludge flow rate as well as its control •type and method of operation of the sludge removal system •arrangement and design of the inlet and outlet (ATV . 2000) Secondary Settling Tanks 29 SWA .Dimensioning of the secondary settling tank F. 2000) Secondary Settling Tanks 30 SWA .h for horizontal flow tanks – QRS ≤ 1.Design procedures .75 · QWW.EST ≤ 20 mg/L • 50 L/kg ≤ SVI ≤ 200 L/kg • Diluted sludge volume DSV ≤ 600 L/m3 • Return sludge rates – QRS ≤ 0.DVWK .0 kg/m3 (ATV . Günthert • Design based on peak wet weather flow QWW. W.0 · QWW.A 131 E.h • effluent suspended solids concentration XSS.application limits F.h for vertical flow tanks • Suspended solids concentration in the influent SSAT > 1. Dimensioning on the Basis of Experience F.DVWK . Günthert Standard values for the sludge volume index The respectively lower values for the sludge volume index (SVI) can be applied. W. if •primary settling is dispensed •a selector or an anaerobic mixing tank is placed upstream •the biological reactor is designed as a cascade (plug flow) (ATV . 2000) Secondary Settling Tanks 31 SWA .A 131 E. EST ≤ 20 mg/L • qSV ≤ 500 l/(m2 · h) for horizontal flow tanks • qSV ≤ 650 l/(m2 · h) for vertical flow tanks Optimization between qsv and tank depth should be undertaken Differentiating between horizontal and vertical flow by the ratio of vertical to horizontal components • • Secondary Settling Tanks 32 SWA .Design procedures .h qA qSV qA = SSEAT ⋅ SVI q SV = q A * DSV Sludge volume loading rate to achieve XSS.Surface area calculation F. W. Günthert • Surface area calculation A ST = • Q WW . h F. Günthert comp. 2000) Secondary Settling Tanks 33 SWA .Dimensioning of the secondary settling tank comp.DVWK . v Functional zones and depths of vertical flow (inverse cone) tanks.A 131 E. (ATV . W. A 131 E.Design procedures – surface area calculation F. (ATV . 2000) Secondary Settling Tanks 34 SWA . Günthert Permitted values for the transition area between predominantly horizontal and predominantly vertical flow secondary settling tanks.DVWK . W. and the thickening (or compaction) and sludge removal zone (h4). 6.This profile is accepted as standard for the depth design of SSTs with the ATV design procedures. W. the sludge storage zone (h3). 1997) Secondary Settling Tanks 35 SWA . the separation zone (h2). Günthert Typical solids concentration-depth profile in SSTs showing from the top down the clear water zone (h1). (IAWQ Report No.SST design procedures – depth calculation F. 2000) Secondary Settling Tanks 36 SWA . Günthert h1: clear water and return flow zone h2: separation and return flow zone h3: density flow and storage zone h4: thickening and sludge removal zone htot at 2/3 radius (ATV . W.Depth calculation F.DVWK .A 131 E.Design procedures . 2007) Secondary Settling Tanks 37 SWA .Design procedures . Günthert Clear water zone • safety zone with fixed depth h1 = 0. W.2 m 0.Depth calculation of h1 F.3 m (Jardin.5 m • with submerged outlet pipes: 0. A 131 E.Design procedures . Günthert Separation and return flow zone – detention time of 0.DVWK .5 ⋅ qA ⋅ (1 + RS) h2 = 1 − DSV / 1000 Secondary Settling Tanks SWA . 2000) 38 0.5 h for the maximum flow related to the free water volume – maximum flow: q A ⋅ (1 + RS ) – free water volume: free water volume 1− DSV / 1000 – zone depth: DSV30 (ATV .Depth calculation of h2 F. W. 2000) Secondary Settling Tanks 39 SWA .A 131 E. Günthert Density flow and storage zone – storage of sludge expelled from the activated sludge tank in 1.Design procedures .5 ⋅ 0. W.DVWK .5 h – sludge storage concentration value of 500 L/m3 – decrease of sludge concentration in the activated sludge tank of 30% is allowed 1.Depth calculation of h3 F.3 ⋅ qSV ⋅ (1 + RS) h3 = 500 (ATV . W. Günthert Thickening and sludge removal zone – thickening of the influent sludge load to the bottom sludge concentration – thickening time depending on the degree of wastewater treatment SSEAT ⋅ q A ⋅ (1 + RS) ⋅ t Th h4 = SSBS (ATV .Depth calculation of h4 F. 2000) Secondary Settling Tanks 40 SWA .DVWK .Design procedures .A 131 E. V = 3.8 m/h. htot = 4. D = 22 m.2 m/h • htot = 4 to 4.Depth calculation of htot F.4 m/h. D = 28 m.400 m3 41 Secondary Settling Tanks SWA . V = 3.3 m Influence of qSV on depth and volume (example) • qSV = 320 • qSV = 500 qA = 0. W.4 m.600 m3 qA = 1.Design procedures . Günthert Total depth at 2/3 radius (htot ≥ 3 m) h tot = h1 + h2 + h3 + h 4 Average design parameters for horizontal flow clarifiers • qSV = 400 to 500 l/(m2 · h) • qA = 1 to 1.0 m. htot = 3. h + QRS) < 10 cm/s discharge into the • (separation zone) • density flow and storage zone • (thickening and sludge removal zone) (beware of short circuiting and re-suspending thickened sludge at high SVI) flocculation essential for low effluent solids concentration • retention time at maximum flow (QWW.Planning aspects .Inlets F. Günthert Separation performance of clarifiers is influenced substantially by the inlet construction uniform distribution into the tank minimising potential and kinetic energy of the inflow • entry velocity at maximum flow (QWW. W.h + QRS) between 3 and 5 minutes • moderate G-values of 50 to 80 1/s Secondary Settling Tanks 42 SWA . Circular SSTs . W. 6. 1997) Secondary Settling Tanks 43 SWA . Günthert Typical unbaffled center feed inlet. (IAWQ Report No.Inlets F. 1997) Secondary Settling Tanks 44 SWA . W.Circular SSTs . 6.Inlets F. Günthert Typical arrangements of peripheral feed inlets. (IAWQ Report No. Inlets F. W. 2007) Secondary Settling Tanks 45 SWA . Günthert (Jardin.Planning aspects . A 131 E.Outlets F.DVWK .Planning aspects . Günthert • Effluent launders – – – – outboard weirs inboard weirs overflow rate < 10 m3/(m · h) overflow rate with fed on both sides < 6 m3/(m · h) • Submerged tubes – radially arranged – circularly arranged (ATV . 2000) Secondary Settling Tanks 46 SWA . W. Günthert (Jardin. 2007) Secondary Settling Tanks 47 SWA .Planning aspects . W.Outlets F. 4 to 0. W.DVWK .Design procedures Sludge removal design (circular tanks) F.6 m 72 to 144 m/h 1. Günthert Based on a sludge balance considering return sludge flow rate short-circuit flow rate 0.5 (ATV .A 131 E.8 · QRS Sludge removal in circular tanks hSR ⋅ a ⋅ vSR ⋅ DST QSR = 4 ⋅ f SR Typical values scraper height hSR: bridge velocity vSR: removal factor fSR: 0. 2000) 48 Secondary Settling Tanks SWA .4 to 0. Günthert QK * SSAT QSR * SSBS QRS * SSRS QRS * SSRS − QK * SSAT QSR > SSBS Secondary Settling Tanks 49 SWA . W.Sludge load balancing in SST QAT * SSAT F. DVWK . W.Dimensioning of the secondary settling tank Guidance values for the design of sludge scrapers. F.A 131. Günthert (ATV . 2000) Secondary Settling Tanks 50 SWA . 6. (IAWQ Report No. Günthert Scraper configurations studied in Germany. Type B is a logarithmic spiral with α constant at 45 °. 1997) Secondary Settling Tanks 51 SWA . and Types C and D are „windows shade“ type scrapers. Type A is the „Nierskratzer“ type where α1 > α2 . W.Circular SSTs F. W. 6. (IAWQ Report No. Günthert Sludge removal systems for rectangular SSTs: (a) blade scraper system. (b) flight scraper system.Rectangular SSTs F. 1997) Secondary Settling Tanks 52 SWA . Rectangular SSTs F. W. 1997) Secondary Settling Tanks 53 SWA . (IAWQ Report No. 6. Günthert Inlet with flocculation chamber and two paddles with horizontal axes. The sludge is withdrawn near the inlet and after one third of the tank length. W.7 : 1 – walls as smooth as possible Secondary Settling Tanks 54 SWA . Günthert • No sludge depositing – slope of the side walls at least 1.Planning aspects – Sludge Hoppers F. W.Conclusions F. Secondary Settling Tanks 55 SWA . Günthert • Sludge concentration in activated sludge tank depends on: – sludge characteristics (SVI) – sludge thickening in the clarifier (SSRS) – return sludge flow rate (RS) • Surface area of SST is determined by: – overflow rate (qA . • Secondary clarifiers designed by the A 131 are relatively deep compared to other design procedures.SOR) – sludge volume loading (qSV. SLR) • Tank depth consists of four zones. W.Conclusions F. • Proof of sludge removal performance by sludge load balance. Günthert • Inlet construction and outlet construction influence the performance of SST. Secondary Settling Tanks 56 SWA . Günthert. Design and Operation” IAWQ Scientific and technical Report No.W. F.A: McCorquodale. Günthert G.6. Modelling. P. L. Wuhlberg: “Secondary settling tanks – Theory. Barnhard. Hennef 2000. D.J.A: Ekama. Parker.References • F. J. Krebs. E. 1997. ISBN 3 – 935669-96-8 • Secondary Settling Tanks 57 SWA . W. ISBN 1 900222 03 5 ATV – DVWK –Standard A 131E : Dimensioning of Single-Stage Activated Sludge Plants GFA. guenthert@unibw. Dr. Prof.W.de Secondary Settling Tanks 58 SWA . Günthert Universität der Bundeswehr München Institut für Wasserwesen Siedlungswasserwirtschaft und Abfalltechnik wolfgang.Secondary settling tanks and sludge mass balance dex summer school 2011 Univ.-Ing F.