Wastewater Treatment Chapter 11 Student Hrs Student Hrs BSE 1.306; noon BSE 1.306; 5:301 pm 6:30pm Lecture, Lecture, 4-5:15pm, MH 4-5:15pm, MH Quizzes open Friday Afternoon through 2.01.30 HW due 11:59 pm 2.01.30 Sunday 11:59pm M Tu W Th F Sat Sun 24-Mar 25-Mar 26-Mar 27-Mar 28-Mar 29-Mar 30-Mar topic 15 waste water waste water 31-Mar 1-Apr 2-Apr 3-Apr 4-Apr 5-Apr 6-Apr waste water 5 water treat 18 hydrology 7 water treat 7-Apr 8-Apr 9-Apr 10-Apr 11-Apr 12-Apr 13-Apr 19 review Midterm 14-Apr 15-Apr 16-Apr 17-Apr 18-Apr 19-Apr 20-Apr 20 hazardous rough draft waste 21 waste 8 wastewater You get the clicker points, but this is just for my information. How do you prefer to organize your class materials? A. Spiral notebook B. 3-ring binder C. iPad + Goodnotes D. iPad + other E. Other tablet What’s most important in that choice? A. Cost B. Lowest effort C. Most obvious solution D. How I was taught previously? • Previously • Water quality & drinking water treatment • Today • Wastewater treatment • Upstream & Downstream are relative Wastewater Contaminants • Suspended solids – can cause sludge deposits and anaerobic conditions in the environment • Biodegradable organics – can cause anaerobic conditions in the environment • Pathogens – transmit disease • Nutrients – can cause eutrophication • Heavy metals – toxic to biota and humans • Refractory organics – toxic to biota and humans • Dissolved solids – interfere with reuse Contaminants Removed During Wastewater Treatment • Sand, grit, debris • BOD - organic matter • Bacteria, viruses, protozoan parasites • Ammonia, nitrate • Phosphate • Suspended particles • Odor, color • Specific chemicals Characteristics & Options Mostly water (99.9%) with solids On site disposal system Dissolved solids (~50%) Usually septic tanks, mostly in rural area Insoluble solids (~50%) 25% of population uses on-site • Settable solids (settle out in 30 mins) • Suspended solids Municipal Treatment Pretreatment – removes materials that can cause operational problems, equalization optional Primary treatment – remove ~60% of solids and ~35% of BOD Secondary treatment – remove ~85% of BOD and solids Advanced treatment – varies: 95+ % of BOD and solids, N, P Big stuff Industrial Pretreatment • Extra steps before municipal system • Approach is to remove materials that will not be treated by municipal system • Local authority must monitor and regulate industrial discharges • Pretreatment requirements set by U.S. EPA Bar racks and grinders • Purpose • remove larger objects • grind larger objects • Solid material stored in hopper and sent to landfill • Mechanically or manually cleaned 10 Grit Chambers • Purpose: remove inert dense material, such as sand, broken glass, silt and pebbles • Avoid abrasion of pumps and other mechanical devices • Material is called “grit” 11 Grit Chamber Example • Design a grit chamber to remove sand particles (ρp = 2650 kg/m3) with a mean diameter of 0.21 mm. Assume the sand is spherical and the temperature of D the wastewater is 20oC. The wastewater flow is 10,000 m3/d. A velocity of 0.3 m/s will be automatically maintained, and the depth must be 1.5 times the width at maximum flow. inlet Outlet Terminal Velocity sand g ( ρ s − ρ )d 2 vs = 18µ L ≥ π·π· οΏ½ π£π£ π£π£π π https://doi.org/10.1039/D3VA00015J Municipal Treatment Pretreatment – removes materials that can cause operational problems, equalization optional Primary treatment – remove ~60% of solids and ~35% of BOD Secondary treatment – remove ~85% of BOD and solids Advanced treatment – varies: 95+ % of BOD and solids, N, P Equalization Example m3/s 0.7 After realizing how difficult it is to treat a variable flow, you decide to add an equalization basin as a pretreatment step. The typical daily flow parameters are given in the table. What time should you “start” the table? Flow Rate m3/s 0.6 0.5 0.4 0.3 0.2 0.1 0 0 5 10 15 Hour of Day 20 25 Time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Flow in 0.34 0.25 0.16 0.13 0.13 0.14 0.16 0.25 0.36 0.45 0.47 0.48 0.51 0.53 0.53 0.55 0.57 0.60 0.60 0.57 0.55 0.47 0.41 0.37 Average Flow 0.40 Accumulation = in - out Flow Rate Cumulative basin fill m3 0.7 7000.0 0.6 6000.0 5000.0 0.5 0.4 In 0.3 Out 0.2 4000.0 3000.0 2000.0 1000.0 0.1 0.0 0.0 -1000.0 Starting at 9am Starting at 9am Clicker Question What does municipal pre-treatment remove? A. Industrial toxins B. BOD C. Hardness D. Very large solids Municipal Treatment Pretreatment – removes materials that can cause operational problems, equalization optional Primary treatment – remove ~60% of solids and ~35% of BOD Secondary treatment – remove ~85% of BOD and solids Advanced treatment – varies: 95+ % of BOD and solids, N, P BOD & solids Primary Settling Basins Example Determine the detention time, overflow rate and weir loading for a primary sedimentation tank with the following design data. • Flow = 0.150 m3/s • Length = 40 m (effective) • Width = 10.0 m • Liquid depth = 2.0 m • Weir length = 75.0 m Clicker Question Which of these does sedimentation treat? A. Turbidity B. BOD C. Hardness D. Heavy metals Municipal Treatment Pretreatment – removes materials that can cause operational problems, equalization optional Primary treatment – remove ~60% of solids and ~35% of BOD BOD & solids Secondary treatment – remove ~85% of BOD and solids Advanced treatment – varies: 95+ % of BOD and solids, N, P Secondary Treatment • Provide BOD removal beyond what is achieved in primary treatment • removal of soluble BOD • additional removal of suspended solids • Use microorganisms to convert organic wastes into stabilized compounds - similar to self-purification process in streams Basic ingredients • Availability of high density of microorganisms • Good contact between organisms and wastes • Availability of wastes • Favorable temperature, pH, nutrients, carbon source (food) • No toxic chemicals present Activated Sludge • Process in which a mixture of wastewater and microorganisms (biological sludge) is agitated and aerated • Leads to oxidation of dissolved organics • After oxidation, separate sludge from wastewater Secondary Clarifier Example • You need to add secondary treatment at a WWTP to meet an effluent standard of 30.0 mg/L BOD5 and 30.0 mg/L suspended solids. • Design a completely mixed activated sludge system assuming the BOD5 of the suspended solids = 63% of the suspended solids concentration. • Estimate the required volume of the aeration tank. • Primary treatment effluent characteristics • Flow = 0.150 m3/s • BOD5 = 84.0 mg/L • Growth constants: • Ks = 100 mg/L BOD5 • μm = 2.5 day−1 • kd = 0.050 day−1 • Y = 0.50 mg VSS/mg BOD5 removed • X = 2000 MLVSS mg/L Food to microbe ratio πΉπΉ ππππ0 • = ππ ππππ • Control by wasting • High F/M means high wasting, short ππππ • Microbes saturated with food (poor treatment) • Low F/M means low wasting, long ππππ • Microbes starved • Typical F/M = 0.1-1 mg/mg-d Sludge Return • Too much food ο leftovers (in effluent) • Too little food ο microbes starve Overall mass balance can treat secondary treatment as everything inside the dashed line. Sludge return mass balance looks only at flows inside dashed line. Previously we calculated: So = 11.1 mg/L; V = 970 m3 We were given: Flow = 0.150 m3/s; X = 2000 MLVSS mg/L • Last time we found the volume of the aeration tank for an activated sludge system. Now let’s find the food to microbe ratio. Food Mass-Balance Food influent + food consumed= food effluent + food waste ππππ ππππ ππππ0 − ππ ππ πΎπΎπ π + ππ = ππ − πππ€π€ ππ + πππ€π€ ππ Y- yield coefficient = 0.4-0.8 mg VSS/mg BOD5 (growth parameter) VSS- volatile suspended solids Clicker Question I’m glad I studied mass balance so much for Exam 1 A. True B. Wasn’t worth it C. I already forgot it all D. I regretted not studying then and now Activated Sludge Activated Sludge Sludge Recycle Waste Stabilization Disposal Wastewater Polishing Discharge to River or Land Application 35 Clicker Question There’s only one way to draw a control volume for mass balance problems A. True B. False Times sludge age/mean cell residence time ππππ ππππ ππππ = ; πππ€π€ ππππ πππ€π€ ππππ + ππ−πππ€π€ ππππ Hydraulic detention time ππ π‘π‘0 = ππ πΎπΎπ π 1 + πΎπΎππ ππππ ππ = ππππ ππππ −πΎπΎππ − 1 ππππ ππ ππ0 − ππ ππ = π‘π‘0 1 + πΎπΎππ ππππ • Estimate the mass of sludge to be wasted each day (from the same plant) if MLSS = 1.43*MLVSS and the return flow is 5,000 m3/d Clicker Question Our “worker” microbes have a residence time that is _____ compared to the overall water. A. shorter B. the same C. longer Low-tech Secondary • Aerobic ponds • Facultative ponds • Anaerobic ponds • lagoons Aerobic ponds • Shallow ponds (<1 m deep) • Light penetrates to bottom • Active algal photosynthesis • Organic matter converted to CO2, NO3-, HSO4-, HPO42-, etc. 41 Example • Wastewater enters an aerobic lagoon at a BOD of 246.0 mg/L and a flowrate of 0.1 m3/s. The waste degrades at a rate of 0.40 d-1. Effluent from the lagoon must have a BOD of 23.9 mg/L. What is the necessary volume of the lagoon to meet these requirements? Give your answer in units of m3. Facultative ponds • Ponds 1 - 2.5 m deep • Long td = 30 - 180 d • not easily subject to upsets due to fluctuations in Q, loading • low capital, O&M costs Anaerobic Ponds • Primarily used as a pretreatment process for high strength, high temperature wastes • Can handle much higher loadings • 2 stage: • Acid fermentation: Organics → Org. acids • Methane fermentation Org. Acids → CH4 and CO2 Clicker Question Low tech secondary treatment works because A. They use the “technology” of existing life cycles B. Not everyone needs the water that clean C. Very high flow rates dilute contamination Wastewater Treatment Part 3 Are we done yet? • Not quite • Where does the stuff we removed go? Sludge Treatment Reduction Sludge Ash Thicken Condition Dewater Sanitary Landfill Stabilize Condition Dewater Soil Incorporation 48 Sludge Types • Bar Screens • Grit chambers • Primary sludge Not a true sludge, not a fluid. Can be drained easily and disposed of directly in a municipal landfill. • 3 to 8% solids • About 70% organic material • Secondary Sludge • 90% organic Activated Sludge Water in Clean(er) water out Solids Solids in rmvd Secondary Clarifier Sludge out Total sludge = solids + wetness Example • Using the following primary sedimentation tank data, determine the daily sludge production in m3/day. • Flow = 0.150 m3/s • Influent suspended solids conc. = 280 mg/L • Removal efficiency = 59% • Sludge concentration = 5% • Volatile solids = 60.0% • Specific gravity of volatile solids = 0.990 • Fixed solids =40.0% • Specific gravity of fixed solids = 2.65 Thickening Primary Sludge Secondary Sludge Gravity Thickening Further processing Flotation Sludge Treatment: Stabilization • Aerobic Digestion • Extension of activated sludge: aeration of sludge followed by sedimentation • Supernatant goes back to head of plant (high in BOD, TKN, total-P) • Anaerobic Digestion • 2 stage: acid fermentation followed by methane production • Advantages: • produce methane • do not add oxygen • As with aerobic digestion, supernatant goes to headworks Digesters (Stabilization) Anaerobic Digestion Sludge Treatment: Conditioning • Chemical Conditioning • Add lime, ferric chloride, or alum • Can also add polymers • Chemicals are added just prior to de-watering stage • Heat Treatment • High temperatures (175-230 oC) • High pressures (10 to 20 atmospheres) • Advantages • bound water is released and sludge is easily dewatered • Disadvantages • complex process • highly concentrated liquid stream Sludge Treatment: De-watering • Sludge Drying Beds • Simple • Low maintenance • Effected by climate • Most popular with small plants • Simple to operate and maintain • Sand drying beds – oldest and most common • Filtration • Cylindrical drum covered with filtration fabric • Apply vacuum to pull out water • Force out water by essentially squeezing water between two moving filter belts Continuous belt filter press Lower energy consumption than vacuum filtration No sludge pickup problems as with vacuum filtration Clicker Question After generating sludge, A. You have to dispose of it B. You can ignore it C. You try to increase it Volume Reduction • Incineration • Complete evaporation of water from sludge • Requires fuel • Solid material is inert • Exhaust air must be treated prior to discharge 58 • Wet Oxidation • Treated sludge is wet • Requires energy • Solid material is inert • Exhaust air must be treated prior to discharge Sludge Disposal • Method depends on RCRA regulations (40 CFR 503) • Land Spreading • lawns, gardens • agricultural land • forest land • golf courses and other public recreational areas • Municipal Solid Waste Landfill (Subtitle D) • Dedicated Land Disposal (DLD) • Utilization in other materials 59 Clicker Question Concern about sludge is A. Purely environmental B. Purely economic C. Both environmental and economic Land Application • Nutrients as a resource Secondary Treatment Flooding, channeling spray irrigation 61 Wetlands • Use of natural or artificial wetlands • Floating plants act as filters and support for bacteria Stormwater wetland, Lansing, MI Dispersed growth vs Fixed Growth • Dispersed Growth • Activated sludge • Oxidation ditches/ponds • Aerated lagoons, stabilization ponds • Fixed Growth • Trickling filters • Rotating Biological Contactors (RBCs) Trickling Filters • Rotating distribution arm sprays primary effluent over circular bed of rock or other coarse media • “Biofilm” develops on rocks and microorganisms degrade wastes as they flow past • Not a true filtering or sieving process • Material only provides surface on which bacteria to grow Rotating Biological Contactors (RBCs) • Consists of series of closely spaced discs mounted on a horizontal shaft and rotated while ~40% of each disc is submerged in wastewater • Discs: light-weight plastic • Slime is 1-3 mm in thickness on disc 66 Clicker Question Rotating biological contactors are a type of A. Dispersed Growth B. Fixed Growth C. Clarifier D. Sedimentation • The Martinez II Wastewater Treatment plant in Converse Texas services approximately 8,400 customers. Martinez II began operations in 1986 with an average daily flow of 2.12 MGD and a permitted flow of 3.5 MGD. Return Activated Sludge https://www.sariverauthority.org/services/utilities/ wastewater-treatment-plants/
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