Secondary Treatment
Overview
Dr. Doug McLaughlin
September 27, 2013
Objective of Secondary
Treatment
• Remove organic compounds from
wastewater using microorganisms
(primarily bacteria) in a controlled
environment
• Two main types of controlled
environments:
– Suspended growth
– Attached growth (“biofilm”)
WWTP Configurations Common to
the Pulp and Paper Industry
Waste sludge
Treated
effluent
Raw wastewater
Return sludge
Primary sludge
Waste sludge
Source: Fig. 11.4-1, NCASI Handbook of Environmental Regulations and Control
Microorganisms Can Be
• Aerobic
– Operate in the presence of oxygen, use
oxygen in cellular respiration.
• Anaerobic
– Operate in the absence of oxygen, do not
require oxygen and may be harmed by it.
• Facultative
– Operate in either environment, have both
aerobic and anaerobic respiration
mechanisms
Secondary Treatment in the U.S. Pulp
and Paper Industry
• Roughly 300 pulp/paper mills have onsite
secondary treatment
• Two main system types:
– ~1/3 use Activated Sludge Treatment (AST)
• Suspended growth, aerobic
– ~2/3 use Aerated Stabilization Basin (ASB) treatment
• Suspended growth, aerobic or facultative
– A few mills operate trickling filters (fixed film, aerobic)
– A few have anaerobic treatment (fixed film or
suspended)
Secondary Treatment Depends On
Microorganisms
• Types of microorganisms in surface and
wastewaters:
– Bacteria, fungi, algae, protozoa, viruses
• Bacteria are the workhorse for secondary
treatment
More About Bacteria
• Some general characteristics of bacteria
– Single-cell organisms
– Prokaryotic (without a true nucleus, simpler
cell structure)
– Cellular respiration involves the cytoplasmic
membrane
– Aerobic, anaerobic, and facultative types
– 0.2-3.0 µm size
Bacterial Aggregates in Aerobic
Secondary Treatment
• “Floc-forming” bacteria
– aggregations of individual bacterial cells into relatively
dense “floc” particles due to polysaccharide film
produced on cell wall under certain conditions
– Contributes to good settling characteristics
• “Filamentous” bacteria
– Form filaments of single cells attached end-to-end
– Having some filamentous bacteria is good - form a
“mesh” that improves filtering of colloidal solids,
improving effluent clarity
– Too many filaments leads to “sludge bulking” (poor
settling)
Other Important Microorganisms
for Secondary Treatment
• Protozoa and rotifers
– Motile, single cell organisms
– Consume bacteria as an energy source
– Eukaryotic (contain a true nucleus and more
complex cell structure)
– Mostly aerobic, some anaerobic protozoans
– Cellular respiration involves mitochondria
– 2-100 µm size
Protozoans and Rotifers in
Aerobic Secondary Treatment
• Protozoans and rotifers are considered
“higher life forms” in aerobic secondary
treatment
– both indicate adequate oxygen is present and
help keep dispersed bacteria in check,
improving effluent clarity
Simple Comparison:
Aerobic vs. Anaerobic Respiration
Successful treatment also requires proper temperatures, pH, nutrient availability,
sufficient reaction time.
Aerobic Secondary Treatment Reactor
A Simple Case – Single Pass CSTR
Q, Xo, So
X, S, V
Q, X, S
Q = flowrate, m3/day, mgd
X = active microorganism concentration, mg/L, lb/ft3
S = soluble substrate concentration, mg/L, g/m3, lb/ft3
V = reactor volume, gal, m3, cf
CSTR = Continuous Stirred Tank Reactor
Assumptions
• System operating at steady state
– Q, So, Xo, S, X, and V all constant
• Reactor is completely mixed
– Reactor concentrations = effluent concentrations
• Xo = 0
note: Active (substrate-removing) microbial concentration, X, is
approximated by the mixed liquor volatile suspended solids (MLVSS)
concentration.
Important Operating Parameters
•
•
•
•
Hydraulic Retention Time (HRT)
Solids Retention Time (SRT)
Oxygen requirements
Temperature
– Mesophilic range (~75⁰ F to ~105⁰ F)
– Thermophilic range (~115⁰ F to ~150⁰ F)
• pH