Download Fixed film processes

Lecturer: Dr. Hul Seingheng
By: Group 5 (I3 GCA)
Students : MAO Chanthy
MEAS Sophea
MENG Sophang
MORM Elen
OENG Seikleng
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Raw wastewaters which were overcharged
with organic and nitrogen pollution have long
been discharged directly to receiving water.
These substances can cause in a chronic state
of degradation. So wastewater treatment is
needed.
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One of the most popular types of treatment is
the biological one.
There are main families of wastewater
treatment process:
a. the fixed film processes,
b. the suspended growth processes.
2.1 Biofilm development
 The biofilm formation process
1. Bacterial adhesion
2. Adsorbed microorganisms grow, reproduce
and produce extracellular polymeric
substances, which frequently extend from
the cell forming a gelatinous matrix called a
biofilm.
 After some microorganisms have adhered to
a surface medium, they start growing
producing the biofilm, which is aerobic.
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A general one for all types of fixed film
processes found in wastewater treatment is
difficult to establish as many factors can be
considered:
+ type of medium
+ type of reactor
+ environmental conditions
+wastewater characteristics
+types of pollutants to be removed (C,N,P,
xenobiotics )
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The trickling filter which is very efficient with
respect to adhesion of bacteria was the first
fixed film process that was developed.
It is a packed bed reactor (2-3m deep) filled
with a medium consisting of 5-10cm stones
with a specific surface area of 40-100m2/m3.
It will be classified as low, standard ( or
intermediate) and high rate depend mainly on
the organic and hydraulic loading rate.
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Rotating biological contactor (RBCs) consist of a
series of circular plastic disk mounted on a
horizontal central shaft and distanced at 1.52.5cm
Normally 40% of the disks surface is submerged
and they are rotated in a tank containing the
wastewater.
Attached microorganisms rotate into the water,
where organic matter is adsorbed onto the
biofilm, and out of the wastewater, where the
oxygen necessary for the conversion of organic
matter is obtained by adsorption from the air.
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There are two main types of biological
aerated biofilters:
a. down flow biofilters
b. up flow biofilters
b. up flow biofilter
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For all processes, the continuous addition of
substrate and microorganisms present in the
wastewater encourage the formation of a
complex biofilm composed of bacteria, fungi,
protozoa, macro invertebrates (larvae,
worms) and sometimes algae.
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Bacteria are unicellular prokaryotic
organisms. They are the first to colonize the
biofilm for the fixed film treatment
processes.
These genera are found in the biofilm are:
Heterotrophic rod-shape organisms:
Zooglea, Pseudomonas, Chromobacter,
Achromobacter, Alcaligenes and
Flavobacteria.
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Filamentous bacteria: Beggiation, Thiotrix
and Spaerotilus. They are nitrifying organism
like Nitrosomonas and Nitrobacteria.
Role: They take part in the removal of the
soluble pollutant matter. But E. coli or
Nocardia spp. perform less well than others
in removal carbonaceous pollution.
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Fungi are the single or multi-cellular
eukaryotic organisms.
The most frequent fungi colonizing fixed film
processes are: Sepedonium spp.,
Subarromyces slendens, Ascoidea rubescens,
Fusarium aquaeductuum, Geotrichum
candidum and Trichosporon cutaneum.
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Role: They are important for purifier,
especially for the carbonaceous substrate.
However, the conversion rate is much higher
than for bacteria and it encourages a
significant production of biomass for a give
quantity of substrate.
Especially, it seems fungi have very specific
growth cycles based on the season.
Winter and early spring: there are maximum
number of fungi growth
Midsummer: They become rare
Protozoa are unicellular eukaryotic organisms
widespread in fixed film processes.
 They are divided into three branches:
-Mastigophora: phytomastigina and
zoomastigina
-Rhizopoda: heliozoan and amoebae
-Ciliophora: ciliated
 They mainly feed on organic particulate
matter, bacteria, algae or other protozoa.
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Several photosynthetic organisms, algae and
cyanobacteria can growth exposed to light at
the surface of biofilm.
Algea: single or multicellulaire
photosynthetic eukaryotes.
Cynobacteria (blue-green): unicellular
photosynthetic prokaryotes.
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The frequently found genera include:
Ulothrix, Phormidium, Anacystis, Euglena,
Stigeolonium, Chlorella and Oscillatoria.
Role: Algea and cynobacteria only play a
small role in fixed film processes. Some
species like Phormidium cyanobacterium,
develop into multilayers at the surface of the
trickling filter and decrease the efficiency of
the process.
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Metazoa are multicellulaire eukaryotic
organisms that encompass the phyla of
worms (rotifers, nematodes, gastrotricha and
oligochaetes) and arthropods (arachnids,
tardigrades, crustaacea, myriapod and
insects).
These predators and detrivivorous organisms
feed on the biofilm, embrittle it, encourage
loosening, decrease biomass volume and
accelerate mineralization.
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Their grazing capacity also helps avoid
clogging of trickling filters through biofilm
consumption.
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Some microorganism species, favoring or not
proper processes operation, will develop with
relation to various operational parameter
such as: substrate, nutrients, dissolved
oxygen, temperature, pH, and hydraulic
loading.
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It is important at this stage to make a
comparison between suspended and attached
growth process before discussion hybrid
systems.
The difference between the two type of
process : fixed film process seem to be less
effect than suspended growth process.
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Hybrid system can be defined by two types of
process :
-a biofilm reactor (usually a trickling filter)
-a biomass support system
2.5.1. System in series
 Secondary treatment process combining fixed
growth and suspended growth system in
series are popular, these process offer
simplicity of operation and process stability
of fixed film process with the high quality
effluent associated with suspended growth
process.
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Biomass support systems consist of
immersing various types of support media in
an activated sludge reactor to favor the
growth of fixed bacteria.
The support can be fixed in the reactor or can
consist of mobile media such as foam pads,
small carriers, etc.
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The main advantages of these systems are
improved nitrification and an increase in
sludge settleability.
One point that must always be remembered
when considering an activated sludge process
with biomass supports is that, because of a
higher biomass concentration, the volumetric
oxygen requirement will be greater in these
systems than in conventional activated
sludge.
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Growth of biofilms is a natural process that
has been engineered in many ways to treat
wastewater.
Fixed film process, developed mainly for the
treatment of municipal wastewaters, have
been presented here along with their
microbiology.
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Fluidized bed reactors (FBR) are a more
sophisticated version of the immersed mobile
biomass system presented earlier. Their main
advantages are:
-High removal efficiency of C and Ni
through large amounts of fixed biomass with
a low hydraulic rentention time
-No clogging
-Better oxygen transfer
-Reduce sludge production.
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Finally, the development of intensive
processes may be of interest, but there is a
growing need for the development of
extensive processes (low cost and low
operational requirements) for on site and
small community wastewater treatment.
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