Wastewater treatment in Tomsk
Alexey Kondrashkin, 1/2006
Sometimes it seems that water resources of the Earth are unlimited, but in
case of population explosion and manufacturing forces increasing it becomes more
and more relevant to treat this resources carefully. Protection and rational use of it,
nowadays determine living conditions of all the humanity in future.
Rational use and protection of natural resources is one of the main points of
economical activity of Russia, Western Siberia and Tomsk region in particular.
The basic water source of Western Siberia is Tom River, which springs from
the western slope of Abakan mountain ridge and has a great number of inflows in
mountain area.
Because of many kinds of plants (metallurgical, coil ? coal, chemical) pollute
water in the basin of Tom, it happens that many various pollutants such as oil
products, phenols, heavy metals’ salts, organic substances etc. come into water.
That is why it is so important for the region to organize effective treatment plants,
able to clean sewage from the city and factories to harmless concentrations.
There are special rules called “Protection of surface water from waste water
pollution rules” which have the power of the law and determine the conditions of
sewage disposal to natural reservoir and initial data for calculation of necessary
cleaning range. These rules contain general order applying either for operation
plants, for plants, which are under reconstruction or still being built. These norms
regulate the quality of water. Composition of sewage is not determined but the
quality of water in the reservoir after mixing it with the wastewater, in other words
dilution of WW in reservoir is considered. These are general requirements for the
quality of water in reservoir after the WW has been mixed:
1. Increasing of content of suspended particles more than 0.25-0.75 mg/l is not
allowed.
2. There shouldn’t be any floating layers on the water surface, oil spoils or any
other admixture accumulations.
3. Water right after the chlorination or some time later and the meat of fish
from this reservoir shouldn’t have any strange smell, color or taste with
intensity more than 2 points.
4. The content of dissolved oxygen in the water shouldn’t be less than 4 mg/l.
5. The total biochemical consumption of oxygen in water at 200 C shouldn’t be
more than 3-6 mg/l.
6. There shouldn’t be any toxic substances in the water, presence of which can
directly or marginally cause harmful effect on human’s health, fishes or
microorganisms, which are fodder supplies for those fishes.
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7. The changes of active reaction of water in the reservoir by pH shouldn’t be
less than 6.5 and more than 8.5.
8. The temperature of water in a reservoir in summer after with WW mixing
shouldn’t be more than 200 C, in winter – not more than 50 C for those
reservoirs where fishes dwell. More than 280 C in summer and more than 80
C in winter for other reservoirs. The temperature of water shouldn’t increase
more than 50 C, and not more than 20C at spawning places.
After the resource-saving actions take place, there are still pollutions which
exceed the norms of Gossannadzor (governmental sanitary service) and Rybnadzor
(governmental fish service), that is why sink of WW without the precleaning is
prohibited.
There are three basic methods of WWT:
1.
2.
3.
Mechanical.
Biological.
Chemical.
Sedimentation tank – the main device of mechanical treatment of WW - is
used for moving away settled or floated to the surface solids. Make out primary
sedimentation tanks installed before biological treatment constructions and
secondary, which are used for the active sludge separation. For large particles
separation bar screens are used.
For heavy mineral admixtures separation sand traps are used. In case of
floatable pollutants, which has density less than density of water (oil, fats, resin
etc.) oil removers and dripping-pans are used.
Centrifugal and centripetal forces are used for intensification of sedimentation
of suspended particles from WW process in hydrocyclones. Rotation of WW in
hydrocyclone is realized by the tangential admission of WW to the cylindrical
body of the apparatus. Rotation of the stream cooperates small particles and
increases their size.
Coarsely dispersed particles can be separated by the filtration on the drum
gauzes (filters) or the microfilters.
To clean WW thoroughly use filters loaded with gravel and sand of various
size. WW water with coagulant goes through all the layers from the top to the
bottom.
The most commonly used method for difficultly filtrated suspensions
separation with the use of centrifugal forces is centrifugation. It is economically
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appropriate to use centrifuges for WW treatment when the sediment is valuable
and could be recuperate.
Biological treatment also takes place in natural conditions in soil and
reservoirs. If it is natural soil methods of biological treatment, WW is spilled on
special lots called irrigation and filtration fields. WW goes there through layers of
soils and is exposed oxidation by many various soil microorganisms.
Nowadays with so high level of industry development and, as a result, a great
amount of WW, natural methods of water purification in reservoirs are not enough
to deal with all these pollutions. De-oxygenation of water in reservoir can happen
and stop any life in it.
For sewage sterilization, which contains organic unstable pollutants, artificial
biological treatment is commonly used. This process undergoes in aerotanks.
Biological treatment of WW is based on decomposition of organic impurities by
bacteria living in activated sludge.
WW neutralization is realized by the processes of oxidation, as a result
chemical composition changes of WW, and organic matter mineralization. Final
products of oxidation are: for carbon – CO2, nitrogen – NO3-, sulfur – SO42-,
phosphorus – PO43-, hydrogen – H2O.
The principle for WW treatment in WWT plants is similar to natural methods.
But because of better aeration conditions, temperature, environment reaction,
phosphorus and nitrogen salts content for microorganisms, process of oxidation
goes much faster.
Some middle process between natural treatment in the reservoir and in the
aerotank is purification of WW in the biological ponds, which are called purifying
or aeration ponds.
Complex treatment plant (CTP) in Tomsk.
Planned production capacity of CTP is 146 000 m3/day. Nowadays it
production capacity is increased to 220 000 m3/day. This plant provides biological
treatment of WW of Tomsk.
WW which go into CTP contain:
-
Water, polluted with the chemicals from the plants and factories of Tomsk.
Storm flows from the plants and factories of Tomsk.
Economical sewages (?) from the plants and factories of Tomsk.
Economical and drainage sewages from biological treatment workshop (?).
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- Treated water (TW) from sludge tank and wash water from filtration station
of biological treatment workshop.
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Treated water fault into Tom river is realized by forcing manifold with
dispersing output, which provides 15 times sewages dilution with river water.
Length of dispersing output is 152 meters, the distance of pipes from the bottom of
the river is 1,5 m., the distance between outputs is 3,8 m. The amount of outputs is
40. In the place dispersing output is located, the cleaning of the right riverside (200
m) of Tom is executed. Riverside well is equipped on the dispersing output. It has
rough bars, valves and emergency bypass. Laboratory control for some substances
is accomplished.
As a result of CTP work we have biologically treated water corresponding to
“Protection of surface water from waste water pollution rules” and health norms.
Biochemical WW treatment method is based of the capacity of
microorganisms to decompose molecules of carbon containing organic impurities
into simply constituents – nitrogen containing compounds, water and carbonic acid
– which aren’t harmful for natural reservoir’s biocenose.
Biocenose of activated sludge contains many various kinds of bacteria:
Infusoria, Rotifera, Oligochaeta worms, flagellates and rhizopod, suctorial
infusorium, filamentous phage bacteria and aquatic plants etc.
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Water polluted with chemicals is diluted with economical sewage from the
plants and city to create suitable conditions for microorganisms activity. They need
organic substance as well as nitrogen, phosphorous and oxygen.
Treatment of waste water at Tomsk complex treatment plant
description.
There are 3 main stages of WW treatment process:
1.
2.
3.
Mechanical treatment of WW.
Biological treatment of WW.
Aftertreatment and disinfection of treated WW.
Mechanical treatment of waste water.
Chemically polluted water and economical sewage mixture get into
biochemical treatment workshop pressure-extinguisher (pos.1346), then it by a
gravity flow goes to a mechanical rake with bars, which are at the bars building
(pos.1347).
Big particles remain on the bars and periodically are taken away with
mechanical rake and moved by conveyor belt to a crusher. Crushed particles from
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crusher are washed away to a crushed garbage reservoir and then mixed with the
water before mechanical rake.
Refined from big particles WW then go to a three section aerated sand catcher
(pos.1348). As a result of velocity of a flow decrease to a 0.13-0.3 m/s, suspended
mechanical impurities (fraction of 0.5 mm and more) settle down to the sand trap
bottom and are washed away to a sludge tank.
Pumps provide water delivery to hydraulic elevator and water wash of
sediment. These pumps are installed at the machinery hall of bars building
(pos.1347).
Floating impurities get to a sand trap and concentrate at the outlet gate. Then
they are taken away to a floating impurities collector and then by gravity flow go
to pumps input and are pumped out to pressure-extinguisher (pos.1346).
WW after the sand trap goes trough Venturi flowmeter, and enters distributing
cup. Distributing cup is equipped with unsubmerged spillways. It divides sewage
into two flows. Each of which enters central disturbing device of primary
sedimentation tanks (pos.1350).
As the WW leaves disturbing (? distributing) device it gets to special space of
directing cylinder which provides better distribution of WW in sedimentation zone
of sedimentation tank. Through spillway purified water get into discharge camera
of sedimentation tank and then by the system of underground pipelines through
cameras OP-1 and OP-2 goes to biochemical treatment workshop. Sediment, which
has settled on the bottom of primary sedimentation tank is taken off by doublewing sludge remover to sludge collector situated in the center of a sedimentation
tank. From this sludge collector the sediment is removed by plunger pumps (NP28), which are located in the wet sludge pump station building.
Sludge is pumped through pressure pipeline (d200mm) to sludge storage. 24
hours collected sludge quantity is 20 m3, humidity is 9%.
Wet sludge pump station control is automatic, depending from the sediment
level in the sedimentation tank. Remote control (barring and stoppage) from
control pane is also possible. Control pane is located in the pump station building.
Substances floating on the sedimentation tank WW surface are taken off by
special device, which rotates together with the sludge remover frame. Gathered
substances and garbage are also put into sludge storage. Centrifugal pumps realize
fat pumping-out.
For better fat pumping out and to prevent crust formation on the surface of a
fat layer compressed air supply to fat collector is realized. Compressed air is
provided by blowing house (pos.1329).
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Fat pumping out operation is automatic and depends on fat substance lever in
the fat collector.
For sedimentation tanks evacuation and pump station pipelines washing
pumps are used.
Cleaning purposes water diversion is made from primary tank outlet pipe.
Pipes drainage water goes to pit and after that are moved away by a pump to
pressure pipeline sedimentation tanks evacuation.
Pump control is automatic depending on drainage water level in the pit.
Pump station is provided with economical-drinking water (d50mm) and water
for technical purposes (d25mm) for centrifugal pump packing compacting. WW
from sanitary devices is removed to fat collector. There is also sewerage system
(d150mm).
Waste Water Biochemical Treatment.
After the primary sedimentation tank water goes to mixer (pos.1325). Where
it is mixed with biogenic additions (ammonia superphosphate and sulphate), dirty
washing water, return water from filtration station, treated water for input flow
dilution and saturate with air oxygen from blowing house (pos.1329).
Biogenic additions are removed to mixer from service tank by pumps.
Biogenic solution preparation is realized in solution tank from where solution
is removed to another service tank. Solution tank sludge is removed to sludge
storage.
Mixer (pos.1325) has 2 sections and a system of perforated pipelines with air
in it. Air bubbles, floating to the surface provide intensive liquid mixing and
preliminary liquid air oxygen satiation (saturation).
From the mixer (pos.1325) mixture by gravity flow through pipeline
(d1200mm) goes to one of working sections of aerotank-mixer. (pos.1351) for
biochemical treatment.
Physically aerotank consists of:
1. Upper distributing channel, general for all aerotank sections.
2. Two sections of aerotank, each of which is divided by partitions into 4
corridors and equipped with distributing trough for adding of water to
working area of a section.
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3. Lower channel, general for two sections which gathers aerated water
mixed with activated sludge and distributes it among secondary
sedimentation tanks.
4. Airlift cell and sludge trough, general for two sections, which distributes
return activated sludge among sections of aerotank.
From upper channel waste water by gravity flow goes to disturbing
(distributing) trough of one of the working sections and through 8 holes in it pours
to working area of section (“b” and “c” corridors) where it mixes with the activated
sludge coming from “a” corridor.
Air coming from blowing house supercharger (pos.1329), provides intensive
mixture of WW with the sludge and air oxygen, necessary for microorganism’s
vital activity. Sludge mixture, intensively mixed with air, by gravity flow moves
one after another “b”, ”c” and ”d” corridors, and through “d” corridor spill way
goes to lower channel from where by pipeline (d1000mm) goes to one of working
secondary sedimentation tanks (pos.1353).
The optimal time for WW being in aerotank is 18 hours (? too long).
During this time microorganisms, which form activated sludge extract solved
impurities from WW and absorb colloid, fine-dispersed admixtures, decompose
complicated organic substances, use part of components for vital activity
(nutrition, growth, reproduction), secrete to WW substances (nitrogen
compositions, water, acid), harmless for natural reservoir, where WW after
treatment goes.
Mineral substances, solved in WW are not practically used by microorganisms
so they go through aerotank like “transit” but their concentration, especially excess
concentration exerts influence on oxidation capacity of activated sludge badly.
As a result of rapid stream velocity decreasing and density difference of
sludge mixture it happens that the activated sludge settles down at the bottom of
the sedimentation tank.
Biochemically treated water separated from activated sludge through outlying
circular overflowing trough gets out of sedimentation tank. Settled activated sludge
is moved away by pumps and through sludge camera by pipeline (DN 200mm)
goes to airlifting camera, from which it is taken away by airlift and moved to
sludge trough. Then it by gravity flow through drainages goes to “a” cameras of
working sections of aerotanks (pos.1351) where activated sludge is regenerated.
Regenerated activated sludge (with recovered oxidation capacity) by gravity flow
from “a” corridor goes to “b” corridor where it is mixed with WW for treating and
process repeats.
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Excess activated sludge resulting from microorganisms reproduction and
growth is removed from sludge trough through pipeline (DN 200mm) to sewage
system K-5 and then by sludge pipe to sludge store which is supposed to be filled
after 10 years. Here mechanical impurities separated after WW purification are
also stored.
Purification process is realized with the help of aerobic microorganisms that is
why oxygen presence is required.
For WW saturation with oxygen in mixer-aerotank (pos.1351) compressed air
from blowing house (pos.1329) is provided.
Saturation of liquid by oxygen effect depends on a diameter of bubbles of air,
going from aerators. The less the diameter - the higher the aeration effect.
Mixer-aerators (pos.1351) are equipped with aerators of trough type with filter
plates. Interstice size is 200 – 400 micrometers.
Atmospheric air is pressed to 0.6 atm. Then through collector AO (DN
1400mm) goes to mixer-aerotank where it is distributed among aerator channels.
From aerators air as small bubbles goes through WW, saturate it with oxygen
and intensively mixes it with activated sludge at the same time.
Activated sludge presents complex of microorganisms of different kinds (for
example bacteria, protozoa fungi etc.). The largest in number group is bacteria. By
physical configuration activated sludge looks like small flakes of brown color,
which consist of many multi-layer located or flocculated cells.
Bacteria accumulations in activated sludge surrounded with mucous layer are
called zooglea. Zooglea sludge has high fermentative (oxidation) activity and may
be compacted well.
Bacteria play major role in biochemical treatment of WW process. All the
other representatives of zooglea discharge auxiliary functions. Protozoa
microorganisms feed on dead bacteria and loosen activated sludge carrying out
sanitary functions.
Presence of protozoa microorganisms in sludge is indirect criterion of
activated sludge condition.
Possibility of living different kinds of bacteria (aerobic, anaerobic,
saprophytic and nitrifying etc.) together in activated sludge can be explained that
metabolites of one kind of bacteria are a food product for another.
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Cleavage product of organic substances by saprophytic bacteria is water,
carbon dioxide, nitrogen containing compounds etc.
Nitrogen containing substances are the food products for nitrifying bacteria
metabolites of which are nitrides and nitrates.
Aerobic nitrification stage happens if only organic substances absent, because
organic matter is inhibitor of nitrification process.
That is why if nitrates and nitrides appear in the treated water it is a sign that
oxidation process (organic matter fission) is over and as a result biological
treatment of WW is also finished.
Aftertreatment and disinfection of biochemically treated water.
Activated sludge is an amphoteric colloid and density of it differs from
density of water, treated water by sedimentation is managed to satisfy the quality
requirements. Activated sludge particles presence in treated water causes higher
value of BCO (biological consumption of oxygen), which is the major criteria of
purification efficiency.
That is why aftertreatment of biochemically treated water unit in
technological scheme is envisaged.
WW after secondary sedimentation tanks by gravity flow through pipeline
(DN 1400mm) goes to receiving tank of drum gauzes, and then to drum gauze
itself (pos.1357). Frame of drum gauze is equipped with filter elements, made from
metallic gauze with the cell size of 1mm. When water goes through drum big
particles of activated sludge stay on the gauze. From time to time they are washed
away and by gravity flow go to dirty washing water reservoir (V2000m3)
(pos.1361), from where is pumped to mixer (pos.1325). After the drum WW
collects at reinforced concrete reservoir in which metallic gauze is installed.
Through spill way water goes to receiving reservoir, one for all gauzes, from which
it is pumped through pipeline (DN 1000mm) to double-layered filters (pos.1362).
Going through sand-gravel loading of filters, water is separated from finedispersed particles of activated sludge and finally purified with suspended particles
content less than 10mg/l and Biological Consumption of Oxygen parameter not
more than 15 mg/l goes to aftertreated water reservoir, from where through
pipeline (DN 1230mm) is pumped to discharge to Tom river.
There is a pump for providing filters washing.
The water, after filters washing, contains sand particles, which are separated
in sand catcher (pos.1374) and are moved by hydraulic elevator to sludge store.
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Transporting water to hydraulic elevator is provided by pump. For devices
evacuation and providing hydroclassifier pump is also used.
For sand, loaded to double-layered filters (pos.1362), classification sand
hydroclassifier. For drainage water from machine hall to sewage system pumping
pump of type “gnome” is used.
Biochemically treated water WW before discharge to Tom River is
disinfected with chloric water.
Chloric water is prepared at a special workshop (pos.1331).
Liquid chlorine is weighted and after that it goes to chlorine evaporator. The
temperature in the evaporator is about 400C.
Right from evaporator gaseous chlorine goes to chlorinator. Here also goes
technical water from drum gauzes building (pos.1357). Prepared chloric water
from chlorinators is used for WW disinfections.
Purified and disinfected water goes straight to Tom River.
This water should correspond to requirements written in the tables below.
No.
Properties
Index
1 Floating admixtures There shouldn’t be any fat, oil products or any
admixture layer of the water surface
2 Color
Water shouldn’t have any unnatural color
3 Temperature
Water temperature shouldn’t be higher than the natural
temperature of the reservoir more than 50C. And not
more than 280C after mixing in summer, not more than
80C in winter.
4 PH Level
6,5-8,5
5 Dissolved oxygen
Not less than 6 mg/liter both in summer and in winter
periods
6 Koli-index*
Not more than 1000.
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Koli-index* – quantity index of feacal or food water pollution. Is determined
by quantity of bacteria, human’s bowels inhabitants (colon bacillus mainly) in one
liter or 1 kilogram of matter.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Waste water content Index
Suspended particles
BCO
Dry rest
Nitrates
Phosphates
Chlorides
Methanol
Urea
Sulphates
Ammonia nitrogen
Nitrites
Synthetic surface active
substances
Iron
Lead
Copper
Zinc
Formaldehyde
Aluminum
Chrome (+6)
Titan
Nickel
Benzol
Polyacrylamide
Oil products
Phenol
Toluol
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Permissible
concentration,
mg/l
Maximum
permissible
fault, g/hour
Maximum
permissible fault
tons/year
9,5
12,09
584
82,5
1,81
74,9
0,1
21,7
66,2
2,252
1,1
0,12
117800
149916
7241600
1023000
22444
928760
1240
269080
8208805
27924,8
13640
1488
762,85
970,83
46895,2
6624,8
145,34
6014,5
8,03
1742,51
5315,86
180,84
88,3
9,693
0,59
0,009
0,002
0,0063
0,124
0,069
0,01
0,078
0,0066
0,016
0,31
0,36
0,001
0,059
7316
111,6
24,8
78,12
1537,6
855,6
124
967,2
81,84
198,4
3844
4464
12,4
731,6
47,38
0,723
0,16
0,506
9,957
5,541
0,803
6,26
0,53
1,285
24,89
28,91
0,0803
4,74
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WWT –2005/6