Physical water/wastewater treatment processes Tentative schedule (I) • Week 1: Introduction • Week 2: Overview of water/wastewater treatment processes • Week 3: Major contaminants (Chemicals and pathogens) • Week 4: Preliminary treatment (Screen) • Week 5: Preliminary treatment (Grit Chamber) • Week 6: Sedimentation 1 • Week 7: Sedimentation 2 • Week 8: Mid-term Sedimentation Sedimentation • A solid-liquid separation utilizing gravitational settling to remove suspended solids • Water treatment – Plain settling of surface waters prior to treatment by a rapid sand filtration plant – Settling of coagulated and flocculated waters prior to rapid sand filtration – Settling of coagulated and flocculated waters in a lime-soda type softening plant – Settling of treated waters in an iron or manganese removal plant • Wastewater treatment – Grit or sand and silt removal – Suspended solids removal in primary clarifiers – Biological floc removal in activated sludge final clarifiers – Humus removal in trickling filter final clarifiers Water treatment plant (typical surface water) Wastewater treatment plant (typical municipal wastewater) Classification of settling • Based on the concentration of the particles and the ability of the particles to interact • Type 1: discrete, nonflocculent particles in a dilute suspension • Type 2: flocculent particles in a dilute suspension • Type 3: intermediate concentration of particles, considerable interparticle forces • Type 4: high concentation of particles, significant interparticle forces (compression) Type I settling(free setting) • The settling of discete and nonflocculent particle in a dilute suspension (e.g. settling of sand particles in grit chamber) • A free falling particle accelerates until the drag force (FD) equals the impelling force (FI). • When FD = FI, a particle falls with a constant velocity, Vs. Main design criteria • Settling velocity • Overflow rate • Detention time • Depth Settling velocity (derivation) Relationship between CD and Re Relationship between CD and Re • In Laminar flow regime 24 CD NRe • In transition flow regime 24 3 CD 0.34 NRe NRe • In turbulent flow regime CD = 0.4 Derivation of Stoke’s law • For laminar flow Example 1 • Calculate the Stoke’s settling velocity in water at 20 ⁰C of a spherical particle with a diameter of 10 μm and a density of 1.5 g/cm2. At that temperture, the density of water is 0.998 g/cm3 and the absolute viscosity is 1.002 X 10-2 g/cm*s Graphical solution for the settling velocity in different fluid regimes Figure 9.5 Main design criteria • Settling velocity • Overflow rate • Detention time • Depth Ideal rectangular settling basin Ideal circular settling basin Grit chambers (horizontal-velocity grit settling chamber (IV)) • The length of the chamber – The target: sand (0.2 mm diameter, 2.65 specific gravity 2.65, settling velocity (Vs) 21 mm/s) • t = h/Vs • L = Vt (V = horizontal velocity) Example 2 • The average flowrate at a small municipal wastewater treatment plant is 40,000 m3/d. the highest observed peak daily flowrate is 100,000 m3/d. Design rectangular clarifier with a channel width of 6 m. Use minimum of two clarifier. (Assume overflowrate is 40 m3/m2-d and a side water depth is 4 m). What are the detention time for average flow and peak flow? Type II settling • Definition: Settling of flocculent particles in a dilute suspension – Particles flocculate during settling – Particle size increases and settling velocity increases • Examples: primary settling of wastewaters, settling of chemically coagulated waters and wastewaters Water treatment plant (typical surface water) Wastewater treatment plant (typical municipal wastewater) Batch settling test (equipment) • To evaluate the settling characteristics • 130-205 mm diameter • Height at least equal to the depth of the proposed settling tank • Sampling ports at equal intervals Batch settling test (procedure) • Samples are removed at periodic time intervals and the suspended solids concentration are determined • The percent removal is calculated for each sample and plotted on a graph as a number versus time and depth of the collection • Interpolation are made between the plotted points and the curves of equal percent removal are drawn Batch settling test (application) • The oveflow rates, V0 – V0 = H/tc – Where, H = the height of the column – tc = the intercept of the Rc curve and x axis • The fraction of solids removed, RT, at time tc – RT = RC + H2/H(RD –RC) + H1/H(RE –RD) Example 3: Primary Clarifier • A primary clarifier is to be designed to treat an industrial wastewater having 320 mg/L suspended solids and a flow of 7570 m3/d. A batch settling test was performed using a 205-mm-diameter column that was 3.05 m long and had withdrawal ports every 0.61 m. The reduced data giving the percent removals are shown in Table 9.3. • Determine: 1. The design detention time and design surface loading rate if 65% of the suspended solids are to be removed. 2. The diameter and depth of the tank Example 1 Analysis 0 0.5 20% removal 1 30% removal 40% removal 1.5 50% removal Depth, m Depth, 60% removal 2 70% removal 2.5 0.0 50.0 100.0 3 Settling time, min Example 1 Example 1 Tentative schedule (I) • Week 1: Introduction • Week 2: Overview of water/wastewater treatment processes • Week 3: Major contaminants (Chemicals and pathogens) • Week 4: Preliminary treatment (Screen) • Week 5: Preliminary treatment (Grit Chamber) • Week 6: Sedimentation 1 • Week 7: Sedimentation 2 • Week 8: Mid-term .