3/12/2012 1 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 3 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."
Transcript
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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
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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."
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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.
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“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
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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
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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.
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Definition
• Sludge particles produced in wastewater by
growing organisms in aeration tanks where
dissolved oxygen is added
• The process involves “Oxidation”
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Oxidation
• Adding Oxygen, removing Hydrogen, or
removing electrons from an element or a
compound.
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Typical Activated Sludge process diagram 10
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Why use Activated Sludge?
• Why not a Trickling Filter, RBC, or Lagoon?
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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)
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Key to Activated Sludge
• Process control:
controlling the size and shape of the floc particles
controlling their settling rates
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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
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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
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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…
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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…
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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
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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.
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a few definitions…
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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.
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Sludge Age Definitions
• High-rate: 0.5-2.0 days
• Conventional: 3.5-7 days
• Extended Aeration: 10 days or more (small plants)
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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
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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)
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Nitrification
• The conversion of nitrogen matter into nitrates by bacteria.
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Denitrification
• A biological process by which nitrate is converted to nitrogen gas.
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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.
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Bulking Sludge
• 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.
Looks like clouds billowing in
clarifier
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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.
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MLSS
• The milligrams of suspended solids per liter of mixed liquor
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SVI
• Sludge Volume Index
• 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.
• Enough food to keep the bacteria alive, hungry, and healthy– Organic loading rate, lbs of BOD/day/1,000 ft3
– Food to microorganism ratio (F/M)
• Food = lbs of BOD/day
• Floc = lbs of MLVSS or lbs of MLSS
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Healthy = Nutrition
Organic solids contain:
– Carbon
– Hydrogen
– Oxygen
– Nitrogen
– Phosphorus
– Sulfur
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100:5:1:0.5
100 lbs BOD
to
5 lbs Nitrogen
to
1 lb Phosphorus
to
0.5 lbs Iron
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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
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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
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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???
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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
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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…?
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Quick Guide
Problem Visual MLVSS F/M MCRT RR Settling Protozoa
