Activated Sludge Base Notes: for student DMACC 2012 WAT307 Activated Sludge Intro A brief introduction designed to provide basic knowledge of Activated Sludge including some process control basics What is Activated Sludge? Biomass produced in raw or settled wastewater (primary effluent) by the growth of organisms in aeration tanks in the presence of dissolved oxygen The term "activated" comes from the fact that the particles are teeming with bacteria, and protozoa Activated sludge can be defined as "a mixture of microorganisms which contact and digest bio-degradable materials (food) from wastewater." 3 1
What is Activated Sludge? Activated sludge is microorganisms. The Activated sludge process is a biological process. To properly control the activated sludge process, you must properly control the growth of microorganism. This involves controlling the items which may affect those microorganisms. 4 Activated Sludge Term is derived from ww being mixed with air/oxygen for some time. A BROWN floc develops Floc contains microorganisms (bugs) & other material Bugs use much of the suspended and dissolved material as food (BOD) Bugs are AEROBIC (mostly) and require oxygen to function 5 A little Activated Sludge history Roots of activated sludge were planted in 1882 Simple aeration of wastewater led to the discovery that COMPLETE STABILIZATION of BOD and NITRIFICATION of all Ammonia to NitrAte occurred with 40 days of aeration 6 2
Purpose of Activated Sludge Oxidize/consume/convert/break down organics in wastewater To remove as many of the suspended & dissolved solids as possible leaving an effluent quality that is high enough to not hinder the receiving stream. Can be used to treat RAW wastewater, but most commonly treats SETTLED wastewater. 7 Definition Sludge particles produced in wastewater by growing organisms in aeration tanks where dissolved oxygen is added The process involves Oxidation 8 Oxidation Adding Oxygen, removing Hydrogen, or removing electrons from an element or a compound. 9 3
Typical Activated Sludge process diagram 10 Why use Activated Sludge? Why not a Trickling Filter, RBC, or Lagoon? 11 Typical Efficiencies: Trickling Filter: 80-85% BOD loading rates 5-25lb/d BOD/1000cuft RBC 80-95% BOD loading 2.5-4lb/d BOD/1000 cu.ft Lagoon est. 80-90% BOD? loading rate for facultative max 25lb/d/ac Conv. Act. Sldg. 95-99% can load much higher (more BOD) 12 4
Key to Activated Sludge Process control: controlling the size and shape of the floc particles controlling their settling rates 13 Process Control Can t Happen If: Poor or no sampling Poor or limited lab and field testing Operator has little time Operator isn t educated You fail to use data along with your senses and experience 14 Excellent Operation Will Occur If: You know what you are doing You have the time and resources to do it You use valid data A lot of math Frequent sampling and testing Network with experienced operators 15 5
So what impacts how well it works? D.O. Mixing ph Temperature Nutrients Size and shape of floc particles Age of floc particles Clarification process Other 16 Activated Sludge operators should be familiar with these terms: OUR! ORP Sludge Age F:M ratio MCRT SRT RAS WAS Nitrification Denitrification Floc! Filamentous bacteria Ciliates Rotifers Floc Operating parameters MLSS MLVSS Many others 17 Test Your Knowledge 1. Which of the following is an indicator of low dissolved-oxygen conditions? a. Low ph. b. Clear effluent. c. Dark-gray or black activated sludge, often associated with a putrid odor. d. Foaming. e. D.O. meter reads 4.0 in aeration basin 18 6
Test Your Knowledge 2. Which of the following conditions would most likely result in stiff, white, billowing foam on the biological reactor? a. Low MLSS. b. High DO. c. High MCRT. d. Low F:M. 19 a few definitions 20 Sludge Age In the activated sludge process, a measure of the length of time a particle of suspended solids has been undergoing aeration, expressed in day. It is usually computed by dividing the weight of the suspended solids in the aeration tank by the weight of excess activated sludge discharged from the system per day. 21 7
Sludge Age Definitions High-rate: 0.5-2.0 days Conventional: 3.5-7 days Extended Aeration: 10 days or more (small plants) 22 F:M Ratio Ratio of food (BOD) to number of microorganisms (bugs) Controlled by WASTING and by RETURN RATE (RAS) Common range is 0.2-0.5 23 MCRT Mean Cell Residence Time Average time a bug spends in the treatment process Controlled by WASTING Formula: Lbs under air (lbs SS WAS/d + lbs SS in Effl/d) 24 8
25 Nitrification The conversion of nitrogen matter into nitrates by bacteria. 26 Denitrification A biological process by which nitrate is converted to nitrogen gas. 27 9
Bulking Sludge A phenomenon that occurs in activated sludge plants whereby the sludge occupies excessive volumes and will not concentrate readily. This condition refers to a decrease in the ability of the sludge to settle and consequent loss over the settling tank weir. 28 Bulking Sludge Looks like clouds billowing in clarifier Bulking in activated sludge aeration tanks is caused mainly by excess suspended solids (SS) content. Sludge bulking in the final settling tank of an activated sludge plant may be caused by improper balance of the BOD load, SS concentration in the mixed liquor, or the amount of air used in aeration. 29 30 10
RAS Activated return sludge is normally returned continuously to the aeration tank. Recycling of activated sludge back to the aeration tank provides bacteria for incoming wastewater. It should be brown in color with no obnoxious odor and is often also returned in small portions to the primary settling tanks to aid sedimentation. 31 MLSS The milligrams of suspended solids per liter of mixed liquor 32 Sludge Volume Index SVI To determine what the return sludge pumping rate should be and to get some idea of sludge settling characteristics. SVI is a commonly used indices and involves simple lab tests and calculations. 33 11
WAS Waste Activated Sludge 34 Process Variations Conventional Complete Mix Extended Aeration High Rate Step Feed Plug Flow Nit/Denit/BNR Tapered Aeration SBR VLR Oxidation Ditch Biolac MBBR Other? 35 Design Criteria Type OLR DT MCRT F/M Return Sludge Effic. Convent. 30 40 6 8 diffus. 9 12 mech. 6 15 0.2 0.5 10 30% 90 95% Tapered 30 40 6 8 6 15 0.2 0.5 10 30% 90 95% Step Feed 40 60 4 6 6 15 0.2 0.5 High Rate 100 2 3.5 3 2.0 3.5 50 400% 50-400% 50 75% 50 75% Extended Aeration 15 25 > 24 20 30 0.05 0.2 100% 75 36 85% 12
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46 Key to Activated Sludge Make sure the right bacteria can thrive Key factors Food Contact time Environmental conditions Shock loads 47 Food Enough food to keep the bacteria alive, hungry, and healthy Organic loading rate, lbs of BOD/day/1,000 ft 3 Food to microorganism ratio (F/M) Food = lbs of BOD/day Floc = lbs of MLVSS or lbs of MLSS 48 16
Healthy = Nutrition Organic solids contain: Carbon Hydrogen Oxygen Nitrogen Phosphorus Sulfur 49 100:5:1:0.5 100 lbs BOD to 5 lbs Nitrogen to 1 lb Phosphorus to 0.5 lbs Iron 50 Nitrogen By weight, the organics should contain at least 20% nitrogen Bacteria require more nitrogen than fungi do Wastes low in nitrogen (high in carbohydrates) give fungi the competitive edge over bacteria and may take over Supply additional nitrogen to get a healthy bacterial population 51 17
Contact Time There must be enough contact time between the bacteria in the return sludge and the organics in the influent for the bacteria to assimilate the organics 52 Proper Operation 1. D.O. 2. MLVSS 3. Bacteria (MCRT, SVI) 4. F/M 5. No shock loading and overloading 6. What do YOU SEE??? 53 Things you should notice every day Color of mixed liquor Color, texture and amount of foam Turbulence/mix pattern D.O. RAS and WAS rates Sludge blanket depth Sludge blanket appearance 54 18
Operator/field tests D.O. ph MLSS/TSS (if automatic monitoring) Solids level/blanket depth (how do you measure this?) 30-minute+ settleometer (Let s see how it s done) Your ideas? 55 56 Quick Guide Problem Visual MLVSS F/M MCRT RR Settling Protozoa Too Young White Foam Low High Low High Slow Amoeba, Flagellates Too Little Light Low High Low High Slow Amoeba, Flagellates Too Old Dark Foam High Low High Low Fast Rotifers, Nematodes Too Much Dark High Low High Low Slow (hindered) Rotifers, Nematodes 57 19
Quick Guide Problem F/M OLR MLVSS MCRT Settling Too Young High High Low Low Slow Too Little High High Low Low Slow Too Old Low Low High High Fast Too Much Low Low High High Slow (hindered) 58 20