waste water- coke plant

8/2/2019 Waste Water- Coke Plant

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COMBINED BIOLOGICAL TREATMENT OF SINTER PLANT WASTE WATER,

BLAST FURNACE GAS SCRUBBER WATER POLLUTED GROUNDWATER AND

COKE PLANT EFFLUENT

By Antoine van HoornEnvironmental Management Corus Staal

This paper was presented at a COMA Meeting in March 2005 held at the Corus Conference

Centre, Scunthorpe

1. The History of Treating Coke Plant Waste Water

The second coke plant at the Corus location IJmuiden was taken into operation in 1972. At aproduction rate of approximately 2 million tons of coke a year for both plants, about 65 m

3/h

of wastewater had to be treated.

From start up on the wastewater was handled according to the traditional and well knownactivated sludge process. So air was put in by very inefficient surface aerators and the sludgewas recycled over a settling basin for reuse. The biomass concentration in the aeration basinwas controlled at a constant level of 3 5 grams/litre.

The activated sludge plant was designed on the basis of experiences with treating domesticwastewater. Already some months after commissioning the plant it became clear that cokeplant effluent is very different from domestic sewage. Overall the removal efficiency wasvery poor. Most of the times only phenols, one of the major components of coke plantwastewater, were removed and not the rest of the COD and thiocyanate. Nitrification,converting ammonia or TKN into nitrate via specialised bacteria which are usually part of thebiomass, never occurred during that time.

In the eighties it became clear that phenol removal only was not sufficient anymore. TheDutch water authorities demanded that that the part of the COD that can be degraded bybiomass should be removed almost completely before it is permitted to discharge the treatedwastewater into the sea.

Due to the fact that there was no room available at coke plant #2 another less roomconsuming treatment system had to be developed. After extensive research including longterm pilot plant testing it was decided in 1988 to build a so called Oxitron fluidised bedsystem. By dissolving pure oxygen in a circulating water flow and keeping the water velocityin the reactors at a level that sand grains stayed in a kind of fluidised state it was possible togrow biomass on these sand grains and to remove almost all the COD from the wastewater.Under these conditionsit also seemed to be possible to grow nitrifying bacteria at the same time. Biomassconcentrations in this system went up to as high as 20 25 grams per litre. So the sameamount of biomass or sludge as in the traditional activated sludge plant could be achieved in amuch smaller volume.


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Figure 1: The fluidised bed installation at Coke Plant

Treating coke plant wastewater means that all the problems known from sewage works haveto be solved. From several coke plants all over the world was reported that bulking orfloating sludge, pin point flocks causing settling problems and high suspended solids in thetreated effluent are frequently observed. Probably due to the fact that coke plant effluentcontains toxic components the sludge is constantly under stress. One of the components thatcause these toxic effects is probably cyanide, but also other so far unknown substances can beresponsible for these phenomena. Anyway, several plants in the world have the sameproblems and need a lot off attention from plant personnel and often the results as forexample in removal efficiency are still not satisfying.

At the end of the nineties the water authorities concluded that some other waste water flowsfrom the Corus plant in IJmuiden did contain more COD and TKN than allowed. The quality

of the canal and the sea where the Corus wastewaters are discharged to is still not satisfyingand not in accordance with European standards. At the same time it was also found out thatthe problems at the fluidised bed plant could only be solved by investing a considerableamount of money. The problems basically concerned the sand biomass separation, a part ofthe process that during design was underestimated. In reality this part of the installationbecame a major problem to control the process and a bottle neck to continue. The fluidisedbed problems and the pressure from the water authorities directly led to the decision toreconsider the way Corus IJmuiden was treating the different waste water flows.

2. Different Waste Water Flows

In 1997 an inventory was made of the different waste water flows and it was concluded thatthe waste water coming from the gas scrubbers from the blast furnaces, the water comingfrom the flue gas scrubbers from the Sinter Plant end the polluted groundwater from CokePlant #1could be very well treated in combination with coke plant effluent.

2.1 Sinter Plant Waste Water

To prevent emissions to the air from SO2, heavy metals and other harmful components, the

flue gases from the Sinter Plant are cleaned with a patented process based on high pressurescrubbing. The process has been developed by Voest Alpine in Austria and is called Airfine.


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In the first scrubber SO2 is removed by spraying water in a recycle loop. The pH is regulated

by caustic soda addition. In the second step of the scrubber system very fine water droplets

are sprayed causing the fine particles to be wetted. These particles are together with thebottom flow of both scrubbers sent to the separate water treatment plant.

Figure 2: The high-pressure gas scrubber system at the SinterPlantIn this plant through pH-variations and the addition of different chemicals the heavy metalsare batch wise separated in two steps. In the first step the more harmful components like

mercury and cadmium are removed. The destination of the sludge from this step was

originally storage and possibly controlled dumping as chemical waste material. The rest ofthe heavy metals in the water coming from the first step is treated in a second batch process.

The sludge containing the more harmless metals is used again in the Sinter Plant. Up till nowthe separation in harmful and harmless waste material has not been a success. All the

produced sludge is for the time being put back into the Sinter Plant. For dewatering of the

sludge two filter presses are available.

Table 1 shows the average composition of water produced by the Airfine installation. The

COD and TKN concentrations are higher than is permitted by the water authorities. Metals ingeneral and lead in specific, make a direct discharge to surface water impossible.

2.2 Blast Furnace Gas Scrubber Water

The gas coming from the two blast furnaces is used for energy production.

It has to be cleaned before it is transported to the external power station. The gas is washed in

so called venturi or Bischoff scrubbers. Both furnaces have their own water circulation

system (650 and 850 m3/h respectively for BK#6 and BF#7). Solids are removed from the

water in a shared water treatment plant. To prevent built up of salts and scaling that can causeprecipitation on several critical places in the installation some water has to be extracted from

the circulation loops. Before the discharge of this water is permitted a separate treatment has

to take place to remove all the solids and the heavy metals present is this blow down.

NaOH

Demister

Quench

Stack

Flue gas

Settler

Neutralization

Gravelbedfilter

SewerRecycling

Filter press

Settler

Filter press

Heavy metal precipitation

Finescrubber

Recycling/immobilization


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Figure 3: Gas treatment at the Blast Furnaces

The settled sludge from the basins in the re-circulating flows and from the basin where theextra treatment takes place is separated through a 3 step hydro cyclone system. The fraction

with the smallest particles contains the most Zn. It is dewatered by a filter press and is stored

for future processing. Nowadays the Zn-concentration is still to low to extract it from thesludge.

The Zn-poor fraction of the sludge, this is the fraction with the more coarse particles, is

dewatered in a decanter centrifuge and is transported back to the Sinter Plant for reuse. The

recycle of scrubber water takes place via the basins 11 en 12. From this recycle about 140

m3/h is withdrawn to keep the risk of precipitation in the installation at a minimum. This

blow down is led to basin 13. Here are the particles separated that are left in the wastewater.

To keep the discharge of Zn as low as possible an addition of sodium sulphide takes place inbasin 13.

Any dissolved Zn will in this way precipitate as insoluble Zn-sulphide. The overflow from

basin 13 contains normally not more than 1 mg/l Zn.

untreated water

treated water

Water treatment

production of electricityfor iron production in BF

Energy production

Sinter plant temporarystorage

temporarystorage

BF-dust

gas

gas

G

G

BF gasexpansion

turbine

courseBF-dust

gas

Bisschofgas

scrubbers

Dust bag

explosionvalves

BIO 2000

Zn-poor

BF-dust

Zn-rich


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Figure 4: Waste water treatment at the blast furnace department

Table 1 (see below) shows the average composition of blow down water of the blast furnaces

gas scrubbers. Again COD and TKN concentrations are higher than permitted by the waterauthorities. Metals in general and zinc in specific, make a direct discharge to surface waterimpossible.

Despite the fact that a by-pass cooling tower is used in the circulation flow the temperature of

the blow down is fairly high. A separate small cooling tower is used to get the temperature at

levels acceptable for a biological treatment process.

2.3 Polluted Groundwater

At the end of World War 2 a RAF-pilot dropped a bomb by accident on the BTX-recovery of

Coke Plant #1. As a result of this action the ground water at this plant is highly polluted.

BTX concentrations of 5 10 mg/l were found. Local authorities demanded treatment of thiswater. After pilot plant testing it was concluded that biological treatment could clean this

water. For the next 25 years we have to pump up about 40 m3/h to neutralise the pollution.

The average composition is shown in the table below.

2.4 Coke Plant Wastewater

Coke oven gas is cleaned at coke plant #2 in a very traditional way. After cooling and tarseparation ammonia, H2S and BTX are removed from the gas. BTX is prepared for reuse inan installation at coke plant #1. Ammonia liquor coming from the scrubbers is treated in 2stripper columns, a combination of a so called de-acidifier and an ammonia stripper wherefree and fixed ammonia are removed at elevated pH-levels. For pH control caustic soda isdosed. The H2S-gas is converted to hydrogen sulphuric acid. The ammonia gas is oxidised in

this acid plant at the same time.

The gas treatment at coke plant #1 is a little different. Here ammonia sulphate is produced.The ammonia liquor from coke plant #1 is transported by pipeline to coke plant #2 for furthertreatment.

The steam consumption of the combined stripper and acidifier is 11 t/h; the amount ofammonia liquor is app. 60 m3/h and the amount of water coming from the H2S-scrubber app20 m

3/h. NaOH addition is 9 11 l/m

3waste water.

from BF#7 cooling tower

make-up water (WRK)

and gascondensate

from BF#6 MASH-water

and/or Na2S

basin 11 basin 12 basin 13

sludge to sludge to sludge to effluenthydrocyclones hydrocyclones hydrocyclones to Bio2000

clean water basin 11 clean water basin 12 clean water basin 13

to BF#7

to BF#6


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After stripping the average composition of the wastewater is as shown in the table below.

The main constituents are the high COD, TKN and cyanide concentrations where TKN stands

for Total Kjeldahl Nitrogen (see table 1). This is the total of ammonia nitrogen and possiblypresent organic nitrogen compounds. TSS stands for total suspended solids and is in majority

responsible for the PAH that normally will be found in coke plant effluent.

3. Developing a Combined Treatment Plant

Because all the above mentioned waste water flows contain COD and TKN that can be

removed biologically after extensive pilot plant testing it was decided to develop a combined

biological treatment system.

An extra advantage of the bio treatment is that a main part of the remaining heavy metals will

be absorbed by the bio sludge and the discharge will be even smaller then it is already.

After the pre-treatment at the blast furnaces and at the Sinter Plant the waste water is

transported to a new biological treatment plant at Coke Plant 2. Together with the wastewaterfrom both coke plants and the groundwater from coke plant #1 a total amount of

approximately 320 m3/h has to be handled in the new installation. The resulting cyanide

concentration in the mixture of wastewater is about 15 mg/l. For a biologically treatment thislevel can be harmful under certain process conditions.

Table 1: Composition of the different waste water flows

Flow Temp COD TKNCN-

totalTSS Zn metals Phenols CNS

m3/h C mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l

BF140-

15040-44 65-120

130-

1505-20 25-35 2-4 1-3 - -

SP 50-55 35-40 250-450200-

300- 10-25 - 0,3-0,5 - -

CP 80-90 30-35

3.000-

3.500

200-

300 20-60 20-50 - - 500-750

200-

250

GW 35-40 10-12 150-350100-

20010-20


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Nitrification and COD removal can take place simultaneously in the aerobic part of the

installation. The end products from this conversion are CO2, water and nitrate, NO3-.

Denitrification is the biological process where nitrate is converted by bacteria into nitrogen

gas. This process has to take place under anaerobic or anoxic conditions. So there should bea special part of the installation where dissolved oxygen concentrations are more or less equal

to zero. Denitrifying bacteria however need some COD as feed also. By putting in (part of)

the influent in the anoxic part of the installation and recycling nitrified wastewater, COD and

nitrate come together in this part. Because the denitrification takes place in the first part ofthe installation this is called predenitrification.

The denitrifying part of the biomass is the most sensitive to changes in process parameterslike temperature and pH. Also a lot of components that are biodegradable under aerobic

conditions are toxic for denitrifying bacteria.

4. The New Biological Treatment Plant

The heart of the new water treatment configuration is the biological treatment plant the socalled Bio 2000. This Carrousel type of installation was built in 1999 2000 and is quite

famous in Western Europe where it is applied quite often for the treatment of domesticwastewater.

The large basins with surface aerators and the relatively high velocities make the installationmore a complete mix reactor than a plug flow. The aerators are controlled by measuring

dissolved oxygen continuously and comparing it with a set point of 1,5 to 2 mg/l. Also pH is

controlled by adding caustic soda when the pH is lower than 6,8 or by adding sulphuric acidwhen the pH in the basin is higher than 7.4.

One of the major problems during the first years of operation was keeping the temperature in

the aeration basin low enough, that means lower than 33 C. Through extra cooling of the

water coming from the blast furnaces via a cooling tower and sometimes putting in some coldriver water this maximum temperature level was not exceeded since the summer of 2003.

Behind the BET-plant continuous backwashing sand filters were installed. Despite the fact

that the combination of wastewaters can easily be treated sometimes very fine biological

flocks are present in


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Figure 5: Floor-plan of the new Biological Treatment Plant

At the picture below you see the carrousel with the surface aerators installed in the blue

boxes. These boxes are isolated to prevent troubles with noise in the direct neighbourhood of

the installation.

The light blue columns in front are the six sand filters; in the back you will see the blast

furnaces nr 6 and 7.

Excess sludge is pumped to a thickener first. After that the sludge is dewatered and mixed

with the coal that goes into the ovens.

Figure 6: The new Biological Treatment Plant

Some dimensions: the aeration volume is 15.000 m3, this means a hydraulic retention time of

33 hours. The settling volume is 1500 m3. The diameter of the settling basin is 29 m. The

surface load is 5 m3/m

2/h.


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The sludge thickener is a basin with a diameter of 9,5 m and a volume of 250 m3. Average

sludge waste is 45 m3/day with 3 % dry solids. The sludge recirculation flow is max 640 m

3/h

(twice the amount of influent). Dosage of phosphoric acid is based on 2 mg/l phosphate in

effluent. That means app. 5 l/h 75% phosphoric acid has to be added.

5. Performance of the New Biological Treatment Plant

Commissioning the new treatment plant took about 10 weeks. In that period the influent flowswere gradually increased. To start up the biological processes sludge was transported by

truck from a sewage treatment works to introduce nitrifying bacteria and from the industrial

wastewater treatment plant from the company that is distilling the tar coming from the cokeplants. The latter sludge should be adapted more or less to the composition of our wastewater.

COD concentrations from the different waste water flows seemed to be very constant as can

be concluded from figure 7. When this concentration is changing this will only happen

gradually and over a longer period. Especially changes in coal mixture can influence COD

concentrations in coke plant effluent. But also changes in process can have this effect. Forexample changes in coking time. At present for coke plant #1 it is minimal 17 h and for coke

plant #2 19 hours.

TKN-concentrations on the other hand are fluctuating considerably (see figure 8). The

highest variations are observed in coke plant effluent. Roughly TKN concentrations are in therange of 100 300 mg/l. It seems like the ammonia strippers are not working as constant as

always was thought.

Figure 7: COD-concentrations incoming waste water

0

500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500

20

-ju

l-00

31

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12

-okt

-00

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/l

COD from BF

COD from BF

COD from CP1+2

COD influent Bio 2000

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kt-

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mg

/l

from Blast Furnacefrom Sinter Plantfrom Coke Plant 1+2


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Figure 8: TKN-concentrations incoming waste water

Dutch laws and regulations are in majority based on the European legislation. However,

every country in the European Community has the right to make their local laws andregulations stricter than necessary according to these European directives. For a long time

this was the policy of our government. Since last year our government is taking into account

the big changes that will be caused by the new Water Framework Directive. This new

directive can have a considerable impact on industry the coming years.

As a direct consequence the discharge permit for the bio treatment at coke plant 2 probably isnot more than temporarily. On the longer term, probably within a few years, the limits on

discharging wastewater will change. Especially the limited concentrations for COD, TKN,

suspended solids and the heavy metals that are linked with the suspended solids, will mostlikely become stricter. It is possible that under these circumstances a third treatment step like

activated carbon adsorption or a completely new treatment process will be necessary.

Nowadays the limits for the bio plant are as in table 2.

Table 2: waste water discharge limits

Maximum Average

Flow m3/h 400 340

TSS mg/l 60 30

COD mg/l 100 150

TKN mg/l 30 15

Phosphate mg/l 10 5

Cyanide mg/l 10 6

Thiocyanate mg/l 4 2

Cd mg/l 0.01

Hg mg/l 0.005

As mg/l 0.025

Cr+Cu+Pb+Ni+Zn mg/l 0.8

PAH mg/l 0.005

The performance of the installation is excellent. COD removal efficiency is almost alwaysover 85 % and TKN removal or nitrification is nearly 100 %. Concentrations are most of the

time within the limits set by the authorities (see figure 9).

Thiocyanate is together with phenols responsible for the high COD concentrations in coke

plant effluent. However phenols are very easily to degrade. Nowadays no samples are taken

anymore for analysing the phenol concentration because it is always lower than the detection

limit. From thiocyanate always a small fraction will not be removed. In the treated effluent

most of the times 1 3 mg/l of thiocyanate will be present. Cyanide itself is probably only fora small part degradable. It is also possible that removal of cyanide takes place by adsorption

on the bio sludge. However a small portion of the cyanide can not be biodegradable also.


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Figure 9: Flow, COD- and TSS-concentrations in treated waste water

Long term BOD tests proved that about 70 100 mg/l cannot be degraded biologically. Forcoke plant effluents only these values are as high as 370 mg/l. This part of the COD consists

of very persistent components. So far these components are not identified. The COD

concentrations in the treated effluent are about 100 mg/l. This means that almost all the

biodegradable COD is removed.

The average suspended solids concentration is 30 mg/l. Some water waste water coming

from the sulphuric acid plant is after neutralisation also transported to the discharge point ofthe bio treatment plant. It is possible that variations in suspended solids concentrations are a

direct consequence of this extra water because Ca-sulphate can be formed. So possible

gypsum particles are part of the suspended solids.

Up till now we did not succeed in keeping denitrifying bacteria in such a state that stabledenitrification over longer period occurred. The possible reason for that is the presence of

toxic components in the incoming coke plant wastewater.

From time to time the nitrification process becomes unstable. This happened frequently

especially in the first two years of operating the bio treatment plant. These instabilities results

firstly in higher nitrite levels and in a later stage in higher ammonia concentrations. When theconditions are not changed finally nitrification will come to a complete stop.

In the years 2001 and 2002 three major failures in nitrification efficiency occurred. No

explicit explanation could be found for these failures. There only are made a few suggestions

and possible causes afterwards. Toxification of the biomass resulting in inhibiting the

nitrification process could have been caused by one or more toxic compounds in one of theinfluent flows. Most likely this has been a cyanide complex that can be present in coke plant

effluent or blast furnace scrubber blow down.

Other reasons are more common like a biomass that was under stress and in that situation

overloaded. Too high food over micro organism levels can have a killing effect on bacteria.

A too high sludge load is often the reason for unhealthy sludge.

Sludge in a stressed condition also can be the result of amongst others higher temperatures

and large variations in wastewater composition.

0

50

100

150

200

250

300

350

1-jan

15-jan

29-jan

12-feb

26-feb

11-mrt

25-mrt

8-apr

22-apr

6-mei

20-mei

3-jun

17-jun

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29-jul

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26-aug

9-sep

23-sep

7-okt

21-okt

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18-nov

2-dec

16-dec

30-dec

m3/h

flow

mg/l COD

TSS


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It can take several weeks before the installation is working well again. In the IJmuiden

situation it is possible to discharge part of the waste water directly without treatment during

the time the biomass is recovering. This situation occurred once so far and was followed byextensive discussions with the water board resulting in a much higher pollution tax that had to

be paid.

TKN concentrations in the treated effluent are around 8,5 mg/l. The variation is about 6 mg/l.

This means that nitrification is complete (over 95 %). This is confirmed by the NH3

concentrations that normally are not exceeding the level of 2,4 mg/l (see figure 10). Thedifference between ammonia and TKN probably are non biodegradable organic nitrogen

compounds.

Figure 10: TKN- and NH3-concentrations in treated waste water

The nitrogen removal rate is high. The nitrate concentrations are around 480 mg/l.Sometimes a little nitrite is found in the effluent but under normal circumstances not more

than 0,2 0,4 mg/l.

Comparing the actual effluent data of 2004 with the limits from the discharge permit learnsthat COD and TKN are well below these limits. Also heavy metals are lower than allowed in

our permit.

CN and suspended solids are the only components that are not always in compliance.

Suspended solids concentrations can be influenced by the small wastewater flow from the

sulphuric acid plant as mentioned before. An investigation is on its way how to handle this

problem.

CN is frequently too high. Suggestion is to find a way to remove CN from coke oven effluent

separately and before biological treatment. All the alternatives found so far are too complex

or produce high amounts hard to settle sludge. Alternative methods consist of oxidation in

combination with Fe-addition.

6. Controlling the Process

One of the most important things in treating wastewater biologically is controlling the

process. There are several ways to do so.

0

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mg/l

AmmoniaNH3

0

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mg/lTotalKjeldahlNitrogenTKN

TKN

NH3


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1. By regular analyses of influent and influent. Shift personnel are taking samples of the

influent every day. With so called test kits COD en NH3 are analysed immediately.

These test kits are not accurate, but the results are satisfying enough to see anyessential changes. From the effluent samples are taken on a continuous base using

special sampling devices. These devices are connected to the flow meter so that is itis possible to take volume proportional samples. Every three days a sample collected

during from 24 hours, is analysed by the chemical department. Besides COD and

TKN also the other components are analysed.

2. pH, temperature, Dissolved Oxygen and NH3 are analysed continuously. pH is

controlled between 6,8 and 7.2, DO set-point is 1,5 2,0 mg/l, temperature in theaeration basin should be not higher than 32 C. The ammonia monitor is the most

important one. If ammonia is increasing actions have to be taken. Departments that

are delivering wastewater are contacted to see if nothing has changed in thereprocesses and for example the oxygen input has to be increased. All monitoring is

done by the central control room.

3. The sludge concentration is kept on a level of 4,5 to 5 grams/l. Every other day the

sludge concentration is analysed and excess sludge is wasted based on the results of

these analyses.

4. Every couple of months, app. 4 times a year the sludge is examined by anexperienced microbiologist from an external company. From the microscopic view

and the changes that are observed information gets available about the health of the

sludge.

5. By measuring the respiration rate of the sludge.

Tests are done with an instrument from Strathkelvin in Glasgow. The primary goal is

controlling sludge health by measuring respiration every day.

A secondary goal is recognizing toxic components in influent and inhibition of the biological

processes. The tests done in the second half of 2004 did result in purchasing the Strathtox-model in Feb 2005.

At the moment implementation in day-to-day-operation is taken place. Regular tests are done

by shift personnel.

waste water- coke plant

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