Microsoft Word - UK_SAF01_Submerged, aerated filter - Biological
treatment of Waste Water_09-06-201
Wastewater Treatment
Installation Instructions
Product information: As a result of many years of R&D and
extensive experience with polyethylene extrusion techniques,
EXPO-NET Danmark A/S has developed a structured filter media. The
media has proven extremely efficient in biological treatment of
domestic sewage, industrial wastewater and process water within
aquaculture industry. The media is made from the environmentally
friendly material polyethylene and consists of net tubes, which are
welded together to form a square block. The unique surface
structure of the many net tubes provides a large accessible surface
area for enhanced biological growth on the filter media.
Table 1: BIO-BLOK® media – overview
Type Application Surface
structure Area of
BIO-BLOK® 100 BOD Rough 70% 90% 67.5mm 54x54x55cm
BIO-BLOK® 125 BOD Rough 67% 89% 55mm 55x55x55cm
BIO-BLOK® 150 Ammonia/BOD Rough 64% 88% 55mm 55x55x55cm
BIO-BLOK® 200 Ammonia/BOD Rough 60% 82% 55mm 55x55x55cm
BIO-BLOK® 300 Ammonia/BOD Rough 51% 72% 36.6mm 55x25x55cm
Due to the natural characteristics of extruded products, all
measurements are approximate.
The filter media BIO-BLOK® is used for all kinds of biological
treatment of industrial wastewater, domestic sewage and process
water in the aquaculture.
Mode of Operation: Future construction, refurbishment or upgrading
of biological wastewater treatment plants is a matter of optimizing
living conditions for the micro-organisms. I.e., the bacteria etc.
must thrive on the substrate in order to work well and "do the
water treatment job". The surface of BIO-BLOK® filter media acts as
substrate for specialised bacterial strains and other
micro-organisms which in turn are able treat and degrade a wide
range of wastewater qualities. The treatment capacity of a
bioreactor generally depends on the quantity of bacteria that the
filter media can sustain. The larger the specific biologically
active surface area is the larger the bacterial population. When
micro-organisms (biological film or biofilm) grow on a level
surface, the specific surface area will remain constant even with
the establishment of a thicker biofilm.
BIO-BLOK® filter media
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The BIO-BLOK® media, however, is designed as a helical structure
with oval threads. If the threads become thicker due to growth in
the thickness of the biofilm, the specific biologically active
surface increases correspondingly. The principle is illustrated
below.
During high load periods on the filter media, the biofilm will grow
thicker and the specific biologically active surface of the filter
will increase considerably. Hence, the biological decomposition
rate increases. In practice, this means that wastewater treatment
plants constructed with BIO-BLOK® filter media do not only have
larger capacity but also have enhanced resilience and flexibility
towards fluctuations. Therefore, BIO-BLOK® reactors are able to
adjust to the overloads which typically occur in most wastewater
treatment plants.
Choice of BIO-BLOK® in connection with BOD reduction The bacteria
that reduce and decompose organic substances always develop a thick
biofilm. The thickness of the biofilm depends on how polluted the
water is and the thickness of the hydraulic surface of the filter
media in the actual system. The correct choice of BIO-BLOK® filter
media therefore depends on the accessible area in the net mesh
structure of the block when biofilm grows on the media. In
connection with reduction and decomposition of organic substances
(BOD), a biofilm thickness of approx. 2 mm usually develops and it
is this thickness that is used in connection with dimensioning. The
most effective types of BIO-BLOK® for this application are
BIO-BLOK® 100 and BIO-BLOK® 125, as these two types allow for
biofilm up to 4mm before the mesh hole clogs with biofilm. This
means that you have to calculate with the values mentioned in Table
2, when dimensioning filters for BOD reduction. If you are certain
that the biofilm will seldom grow to more than 2mm, you can
naturally choose other BIO-BLOK® filter media types and thus get a
smaller system or a higher decomposition and capacity. This means
that you can calculate with the values mentioned in Table 2, when
dimensioning filters for BOD reduction.
Choice of BIO-BLOK® in connection with ammonia reduction
Bacteria that reduce ammonia always develop a thin biofilm. The
precondition for ensuring these bacteria developing is that as much
BOD load as possible is removed; the reason for this is that the
nitrifying bacteria grow much slower than the BOD reducing
bacteria. As only a thin biofilm then develops, it is possible to
choose a BIO-BLOK® type with more mesh strings and
consequently
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smaller holes in the mesh “wall”. The most efficient types of
BIO-BLOK® for this application are BIO- BLOK® 150, BIO-BLOK® 200
and BIO-BLOK® 300. This means that you should calculate with the
values mentioned in Table 2 when dimensioning filters for BOD
reduction or nitrifying filters: Table 2: Specific biologically
active surface area for BIO-BLOK® filter media
Filter type Application Thickness of biofilm
1mm 2mm 3mm 4mm
BIO-BLOK® 200 Ammonia/BOD 312m2/m3 426m2/m3
BIO-BLOK® 300 Ammonia/BOD 360m2/m3 460m2/m3 560m2/m3
Due to the natural characteristics of extruded products, all
measurements are approximate.
Sludge Production Above Table 2 shows that when applying BIO-BLOK®
filter media, there will always be a big, active biomass in the
treatment plant with a relatively thick biofilm, depending on where
in the process the actual filter media is placed. This means that
the sludge age will be considerably higher than in activated sludge
systems, resulting in a significantly lower sludge production.
Estimated sludge production in these systems would be 0.1 – 0.33kg
SS/kg decomposed BOD. Since the sludge age in the BIO-BLOK® media
is high, it will also result in development of bacteria in the
biofilm, which can decompose organic matters that decompose slowly
such as medical residues etc., which is difficult in activated
sludge systems. By comparison, the sludge production in an
activated sludge plant is about 0.8kg SS/kg decomposed BOD.
Installation of BIO-BLOK® Filter Media: In submerged aerated
reactors BIO-BLOK® filter media is always installed with the net
tubes placed vertically.
BIO-BLOK® is easily cut into the desired shape with a saw since the
net tubes are welded together only at the tube ends. BIO-BLOK®
filter media can therefore be adjusted to any tank design.
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Bio-Reactor Design: It is an advantage for the biological processes
to divide the bio-reactor into compartments connected in series.
Depending on the size of the reactor this is normally between 3 and
8 compartments. In the compartments different bacterial cultures
will develop depending on the composition of the supplied waste
water. E.g. in the first compartments upstream, bacteria which
decompose organic matter will have preference. In the later
compartments nitrifying bacteria will normally dominate. Air supply
system and grating on which the BIO-BLOK® is positioned is
installed at the bottom of the compartments. Each compartment
should have separate air supply in order to adjust the quantity of
air supply for the individual compartment. The possibility to
aerate the filter media heavily is convenient from an operational
point of view and will also provide the means for easy backwash of
the media should this prove necessary. The height of filter media
may vary from two to many layers of BIO-BLOK®. As a rule of thumb,
BIO- BLOK® filter media installed in four layers, corresponding to
2.2 m, will provide the best operational economy for the air
blower. In order to make sure that the tube ends of the BIO-BLOK®
are positioned straight over each other, connection kits can be
used. Normally two connection kits are used in each joint/BIO-BLOK®
in the edge of the bioreactor.
Plan of bio-reaktor, example
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The top of the BIO-BLOK® is normally minimum 20cm below water
level.
Please note that the partition walls of the basin must be placed to
the bottom and placed above the water surface. In this way, the
waste water can achieve the best contact with the filter media. The
inlet and oulet should usually be placed in top and bottom as far
apart as possible. The size of the inlet and outlet should be about
twice of the main inlet size. The distance from the air diffusers
to the bottom of the BIO-BLOK® material depends on the type of air
diffusers (and the quantity of air) being used. It is important
that the bottom of the BIO-BLOK® is positioned at a distance where
the air bubbles are spread efficiently and distributed over as much
of the BIO-BLOK® surface as possible. The distance is typically
from 10 to 25cm.
Uplift Pressure and Weight: BIO-BLOK® filter media is made from the
environmentally friendly material polyethylene. The type of
BIO-BLOK® is decisive for the uplift pressure and the weight of the
filter media. The uplift pressure can be counteracted with a girder
system in order to keep the BIO-BLOK® units in place.
Water surface
The distance depends on the choice of air diffusers.
The filter height appears by using whole, vertically positioned
BIO-BLOK®. Standard height is 55cm. Can be produced in heights from
40 to 120cm as per agreement.
Bottom
Bottom grating
Top grating
Example of installation of BIO-BLOK® units by means of a bottom
grating and top girders.
Water inlet
Water outlet
Air diffusers
Weight/m3 BIO-BLOK® 100 62.00kg/m3 BIO-BLOK® 125 66.00kg/m3
BIO-BLOK® 150 69.00kg/m3 BIO-BLOK® 200 74.00kg/m3 BIO-BLOK® 300
100.00kg/m3 BIO-BLOK® Weight without Biofilm Coating:
Weight Weight/m3 BIO-BLOK® 100, 54 x 54 x 55 cm 7.43kg 46.00kg/m3
BIO-BLOK® 125, 55 x 55 x 55 cm 9.01kg 54.00kg/m3 BIO-BLOK® 150, 55
x 55 x 55 cm 9.97kg 60.00kg/m3 BIO-BLOK® 200, 55 x 55 x 55 cm
11.41kg 69.00kg/m3 BIO-BLOK® 300, 55 x 25 x 55 cm 8.03kg
106.00kg/m3 As it is a matter of extruded products, all
measurements are approximate.
In order to design and calculate the bottom grating system, it is
recommended to use a BIO-BLOK®
maximum weight (including biofilm) of 300kg/m3. This weight,
however, will only occur in situations where the BIO-BLOK® units
are clogged due to operational accidents and the water in the
reactor has to be pumped out for maintenance purposes. To optimise
the self-cleaning properties of the BIO-BLOK® media, the carrying
girders of the bottom grating system should not be so wide that
they cover a BIO-BLOK® tube opening (Ø 55mm). As top grating in
smaller reactors, 20 x 20cm steel grid fixed onto the side walls
can be used. In larger reactors, girders which further absorb the
upward forces (uplift pressure) can be used in combination with a
steel grid.
Air Diffusers: In most biological wastewater treatment plants air
diffusers frequently cause problems. Air diffusers are expensive
and periodical replacement is necessary, often with relatively
short intervals. In order to exploit the oxygen in the air as much
as possible air diffusers have been developed to produce micro-
bubbles. Today this type of air diffusers is successfully used in
traditional activated sludge systems. The same type of air
diffusers has also been applied in systems with fixed film
technology, however, without showing the same good results. The
reason for this is that micro-bubbles do not transport as much
water as desired upward through the filter media. Consequently, the
waste water does not get in contact with the bacteria on the filter
and treatment capacity is therefore reduced. It is well documented
in numerous wastewater treatment plants and aquaculture systems
that the phenomenon is reinforced in reactors based on fixed film
technology. These generally use approx. 30% less air when compared
to activated sludge systems. It is important that the BIO-BLOK®
media function optimally. In this context, choice of air diffuser
system must be emphasised. Choosing the wrong type of air diffuser
could prove expensive both to buy and to operate and air diffusers
have substantial influence on the treatment capacity of the BIO-
BLOK®.
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Stirring: It is extremely important that the waste water is
properly stirred so that the water gets in contact with the
bacteria on the filter media. Therefore, air diffusers developing
so big bubbles that this requirement is meet should always be
applied.
Examples of good stirring
Grating made of stainless steel and PVC air diffuser system
Example of Aromatic air diffuser system
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Content of Oxygen: The concentration of oxygen in an aerobic filter
should always be higher than 6mg/l. If the oxygen concentration is
under 4mg/l, the efficiency of the wastewater treatment plant will
deteriorate considerably.
Temperature: Temperature has a significant influence on biological
processes. Further, at temperatures above 35 °C, nitrification is
more difficult. At temperatures above 40 °C, aerobic heterotrophic
decomposition of organic matter and de- nitrification cannot take
place. Temperatures between 35 and 40 °C might cause big problems,
as the bacteria populations that cannot be decomposed thermophilic
will be completely inhibited.
The curves in fig. 1 and 2 describe Biological Oxygen Demand (BOD)
and Ammonia (NH4+) degradation rates with BIO-BLOK® media as a
function of temperature. The process water must not contain
critical levels of biologically toxic substances, and the BOD:N:P
ratio should be approx. 100:5:1.
pH: The pH value in the wastewater should be between 6.5 and 8.25.
At pH values below 6 and above 9 decomposition rates are
inhibited.
Fig.2: BIO-BLOK fixed film technology, NH4
reduction as a function of water temperature
0
0.2
0.4
0.6
0.8
1
1.2
0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25
Temperature (degrees Celsius)
reduction as a function of water temperature
0
5
10
15
20
25
0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25
Temperature (degrees Celsius)
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Decomposition in a Submerged, Aerated Filter Depends on the
Following: * Temperature of the waste water. * The biological
capacity of the waste water to decompose. * The residence time of
the waste water in the filter. * The biological accessible area of
the filter media per m3. * The oxygen stress in the waste water
(type and number of air diffusers). * The wastewater flow through
the filter. Depending on above, the following decomposition speeds
can be expected by a temperature of waste water from 15 – 25 o C: *
Decomposition of BOD7 without nitrification 10 – 15gr BOD/m2 x d *
Decomposition of BOD7 with nitrification 4 – 8gr BOD/m2 x d *
Decomposition of ammonia under oxygen limitation 1 – 2gr NH4-N/m2 x
d
Indicative Decomposition Speeds for Biological Processes: Water
Ammonia (> 3mg/l) BOD5 Temperature (gr/m2 x day) (gr/m2 x day) 0
0 0 2.5 0.15 3.9 5.0 0.30 7.8 7.5 0.40 10.4 10.0 0.50 13.0 12.5
0.60 15.0 15.0 0.70 17.0 17.5 0.80 18.5 20.0 0.90 20.0 25.0 1.00
22.3 Above figures show the decomposition speeds that, based on the
wastewater temperature, can be expected in the BIO-BLOK® filter
media provided that the optimum conditions are present in the
filter. We draw your attention to the fact that for ordinary waste
water, optimum biological decompositions rarely occur for a long
period of time. Therefore, depending on the physical and biological
conditions in the wastewater treatment system in question, a slower
biological decomposition can be expected in practice. The optimum
conditions for a well-functioning filter are as follows:
1. That the filter media has a large accessible surface on which
the actual bacteria can grow (m2 filter surface / m3 filter
media).
2. That the waste water has a sufficiently good contact to the
filter media, which can be created through good aeration or good
irrigation.
3. That there are no toxic matters or other matters in the supplied
waste water which might inhibit the biological decomposition.
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4. That the waste water, as required, has been pre-treated, i.e.
sedimentation, filtration or another form of pre-treatment.
5. That the volume of waste water corresponds to the volume for
which the treatment plant has been dimensioned.
6. That the waste water’s retention time in the treatment plant is
sufficient.
7. That the wastewater load on the biological filter is as uniform
as possible – i.e. there should be compensation basins in the plant
or the waste water should be recirculated.
8. That the factual waste water temperature is the temperature for
which the wastewater treatment plant has been dimensioned.
9. That the waste water contains the right ratio of organic
matters, nitrogen and phosphorus so that the wanted bacteria can
establish: (BOD : N : P = 100 : 5 : 1)
10. If the waste water contains big volumes of salt, this will
result in a considerably lower biological decomposition.
Nitrification: Nitrification is not easily achieved; many factors
have an important impact on the actual decomposition rate for the
specific effluent - * Temperature * Organic load * Composition of
the effluent * Level of oxygen in the effluent * The level of
ammonia in the effluent, which influences the decomposition per m2.
If the ammonia
level > 3mg/l, the decomposition will be higher per m2 surface
than if the ammonia level is < 3mg/l.
Speed of nitrification as function of the ammonia concentration in
the outlet in a clean nitrification trickling filter,
which follows biological treatment for removal of BOD5.
Georg Jensens Vej 5 – DK-9800 Hjørring – Phone: +45 98 92 21 22 –
E-mail:
[email protected] – www.expo-net.com
If it is a matter of ammonia reduction, the following additional
conditions also have to be fulfilled:
11. That the wastewater load on the biological nitrifying filter is
less than 10mg BOD / l.
12. That the content of oxygen in the waste water is as high as
possible, i.e. the content of oxygen has to be higher than
4mg/l.
All the above conditions have to be fulfilled in order to achieve
the previously mentioned indicative decompositions. If increased
decompositions are requested, more of the mentioned conditions have
to be improved considerably. If these decomposition figures should
be upgraded, it should be the owner or the project supervisors who
are responsible for doing this, because they have the ability of
changing and controlling the above 12 conditions.
Sludge Production: In submerged, aerated filters in which the
bio-reactor is constructed of minimum 4-6 compartments connected in
series, the following sludge production can be expected: Maximum
0.1 – 0.333kg SS/kg decomposed BOD. By comparison a sludge
production of approx. 0.8kg SS/kg decomposed BOD is estimated for
activated sludge systems.
09-06-2020