Wastewater Treatment Plant Microorganisms

Floc forming bacteria

(saprophytes): primarily facultative

heterotrophs, soil and aquatic

genera

Pseudomonas, Achromobacter,

Flavobacterium, Alcaligenes,

Arthrobacter, Zooglea, Acinetobacter,

Citromonas, Bacillus

Nitrifying bacteria: ammonia

oxidizing bacteria (AOB) and

nitrite oxidizing bacteria (NOB)

Nitrosomonas, Nitrobacter,

Nitrospirillum

Predators: protozoa, rotifers,

nematodes

Vorticella, Aspicidica, Paramedium

Nuisance bacteria and eucaryotes:

bulking, foaming, overgrazing

Nocardia, Microthrix, Sphaerotilus,

fungi, snails

Specialty populations Phosphate accumulating organisms

(PAO), algae (lagoons)

Other Viruses (bacteriophage), yeast,

pathogens (Campylobacter, E. coli,

Salmonella, Giardia, Cryptosporidium)

Single cell or small

aggregates in cultures.

Can degrade

contaminants, but not

efficiently separated or

recycled.

Flocs contain many

strains of bacteria with

multiple morphologies

Settling and recycle

selects for aggregation.

Stable aggregates usual

are a mix of types

Flocs also contain protozoa

(stalked and free-swimming),

rotifers, nematodes.

Extracellular polysaccharide

binds cells, debris and retains

nutrients and metabolites.

Can be barrier to O2 diffusion

Protozoan grazers are an

important component of the

floc ecosystem. Indicate

oxygen availability,

contribute to decay of

particulate matter,

consumption and recycling

of biomass COD.

Changes in bacteria

population and floc

morphology produce

process upsets.

Proliferation of

filamentous bacteria

results in bulking sludge.

Filamentous

biomass after

30 minutes

settling

After

treatment with

NaOCl,

filaments

killed

selectively 

Flocs dominated by

polymer producing

bacteria (Zooglea),

Nocardia, and Thiothrix

also can cause process

upsets such as foaming

when aerated.

Foaming can top aeration

tanks and interfere with

uniform processing. Foam

in the final clarifier is

carried over weirs causing

excess solids in

discharged water.

Biofilms are aggregates of

bacteria and other

microorganisms attached to

an inert surface. Appear to be

a mat in micrographs. But…..

Biofilms are actually

quite 3-dimensional

with pores of varying

size up to large

channels that allow

advective as well as

diffusive transport of

substrate and nutrients

and particles.

TCE-degrading consortia 

growing attached to iron sulfide

particle. Consortia are more

resistant to shocks and

facilitate product utilization

 Certain growth

conditions (“feast-

famine”) result in bacteria

storage of polymers (PHB)

for use when no external

substrate available.

Metabolism used in

biological PO4 removal.