This invention relates to waste water/effluents treatment. This invention particularly
relates to treatment of effluents from the coke oven plants which contain very toxic
pollutants such as phenolic compounds, ammonia-nitrogen, cyanides and others.
BACKGROUND OF THE INVENTION
Blast furnace coke is produced in the coke oven batteries by carbonisation of
metallurgical coal. The coal is charged with 6-7% moisture; in addition it has inherent
moisture of around 1%. During process of carbonisation, this moisture along with tar and
by products processing water, generates a large volume (0.3 – 0.5 M3 effluent/T of coal
charge) of effluent which is toxic in nature. The effluent gets formed at various sources
and has different characteristics and volumes of generation. The mixed effluent is treated
in common effluent treatment plant in multiple stages in almost all major steel plants.
Biological treatment forms the major and core part of the treatment process. Out of
normative constituents, the major toxic parameters that fall under pollution control norms
are Phenolic compounds, Ammonia Nitrogen and Cyanide. Effluent treatment units were
built more than three decades ago when there was little available knowledge of biological
treatment process of Coke Oven effluent. Flow rates of individual units have also
increased over the years because of increased capacity utilization and capacity addition.
Therefore, most of the integrated steel plants in general find it difficult to achieve existing
norms.
Moreover, in aerobic nitrification process major toxic constituents Ammonia and Cyanide
are converted to nitrate, which although less toxic, is undesirable from sustainability
perspective for which norm exists for drinking water. Besides, over a period of three
decades, body of scientific and technical knowledge about biological wastewater
treatment and process control has also increased significantly. The integrated process
3
developed through detailed experimental work based on available scientific information
addresses all the issues from a holistic technical perspective.
Coke is the most important raw material fed into blast furnace in terms of its effect on
blast furnace operation and hot metal quality. A high quality coke is able to support a
smooth descent of the blast furnace burden with as little degradation as possible while
providing the lowest amount of impurities, highest thermal energy, highest metal
reduction, and optimum permeability for the flow of gaseous and molten products.
Introduction of high quality coke in blast furnace result in lower coke rate, higher
productivity and lower hot metal cost. In Integrated Steel Plants coke oven plant converts
coking coal into coke to provide coke of optimal quality which is an integral requirement
of Blast Furnace operation as described above.
Coke is a product of destructive distillation/pyrolysis of coal produced by heating coking
coal in the absence of air to remove volatile matter and moisture. Approximately 1.45-
1.60 tonnes of coking coal is consumed to produce one tonne of coke. During pyrolysis,
coke is heated to high temperatures and rapidly cooled with water and this releases
various coke oven gases. The condensed water from coke-oven gases consist of many
pollutants such as tar, ammonia, phenols, cyanide, naphthalene, light oil, and compounds
of sulphur. In order to prevent detrimental effects on environment, they need to be
removed from wastewater before being allowed to enter water systems.
Wastewater from coking process originates mainly from three sources: coal moisture,
water of decomposition, and process waters added during gas treatment and by product
recovery. The process water is the largest fraction responsible for total wastewater and
typically account for 60-85% of the total flow, which may range from 0.4-1 m3/ton
depending on the level of process water recycle. Major wastewater streams are
generated from cooling of coke oven gas during processing of ammonia, tar,
naphthalene, phenol, and light oil. Process wastewater may contain Benzene,
Biochemical Oxygen Demand, Chemical Oxygen Demand, Suspended Solids, Phenols,
Polycyclic Aromatic Hydrocarbons, Ammonia, Cyanides, and others.
At the high temperature of coke oven, substantially all the volatile matter in the coal is
evolved as a dense, brownish yellow gas. As this gas passes to a collecting main it is
scrubbed by sprays of flushing liquor which cool it to 75 to 90oC, condense most of the
4
high boiling constituents, and dissolve a large proportion of the water-soluble
compounds. The flushing liquor flows to a decanter where the heavy tar is separated;
part of the supernatant liquor is returned to the flushing system and part is sent to the
ammonia still. The weak ammonia liquor enters the top of a bubble-plate column where
the free ammonium salts are decomposed by steam. The liberated ammonia, hydrogen
sulfide, carbon dioxide, and hydrogen cyanide are returned to the coke oven gas main,
and the liquor which collects in the bottom of the column is pumped to a dephenolizer.
The dephenolized liquor is returned to the ammonia still where the fixed-ammonium salts
are decomposed. Consequently the ammonia, volatilized by steam, also sometimes gets
added to the effluent stream.
Formation of refractory pollutants like Cyanide
Hydrogen Cyanide is formed during carbonization of coal in coke ovens due to reaction of
ammonia with carbon from coke or with hydrocarbons in the CO gas.
C+NH3 = HCN + H2 (1)
C2H4 + 2NH3 = 2HCN + 4H2 (2)
CH4 + NH3 = HCN +3H2 (3)
The steam contained in Coke Oven Gas (COG) effects a partial hydrolysis of the HCN as
follows:
HCN + H2O = NH3 + CO (4)
The concentration of HCN in Coke Oven Gas depends on the following variables viz.
 Temperature of Carbonization: High temperature favours reaction 1-3 and lead to
higher HCN in COG.
 Height of coking chambers: Taller coke oven chambers gas flows through longer
layer of hot coke, which results in higher concentration of HCN in COG.
 Nitrogen content in coal: Higher Nitrogen content in charge coal results in higher
NH3 content in COG which also favours reactions 1-3.
5
 Moisture of coal: The coking of high moisture coal results in COG with higher steam
content favouring reaction 4 and reducing the HCN concentration in COG.
PRIOR ART
Abstracts of relevant patents:
United States Patent 3847807: REMOVAL OF CYANIDE AND COLOR BODIES FROM
COKE PLANT WASTEWATER
Waste waters, from a coke oven by-product plant, containing cyanides and objectionable
complex organic compounds are treated by a high density sludge process. The waste
waters are mixed with an aqueous high calcium lime slurry and a portion of the sludge
formed in the process, which portion is recycled in the process. The aqueous high
calcium lime slurry and the portion of recycled sludge are mixed for a time to obtain a
uniform mix. A solution containing iron values is added to the uniform mix. Iron cyanide
compounds and a portion of the complex organic compounds are precipitated. The
precipitate is flocculated and is passed to a settling tank wherein the precipitate settles
out to form a high density sludge;
United States Patent 5160632: CYANIDE REMOVAL FROM COKE OVEN WASH
WATERS
A process for removing cyanide anions from waste waters using halogen-free ferric salts
is disclosed. The process is particularly effective when treatment of cyanide waste waters
is done by adding ferric sulfate while adjusting pH of the waste waters with a halogenfree
acid, such as sulfuric acid, to a pH ranging from about 3.0 to 5.0. The ferric
ferricyanides formed are agglomerated and flocculated using synthetic polymers along
with whatever other dispersed or suspended solids may be present in the waste waters,
and this dense sludge is separated in a clarifier. Cyanide removal is at least 80%, relative
to initial cyanide concentrations, and preferably 90% removal or higher;
United States Patent Application 20150076061: COKING WASTEWATER TREATMENT
A process for treating coking wastewater contains the steps of passing the coking
wastewater in such an order through coagulation, particles removal, and ion-exchange
resin.
6
Non patent literature:
Biological treatment of Cyanides
Microbiological treatment of cyanide is the most economical process of cyanide
degradation (Desai and Ramakrishna 1998, Ebbs 2004). Biodegradation of cyanide is
dependent on different factors such as cyanide concentrations, presence of other toxic
compounds and their concentrations, pH, temperature, availability of nutrients, and
acclimation of microbes (Akcil and Mudder 2003). Biological treatment can be applied in
many configurations including in situ, aerobic and anaerobic, active and passive and
suspended and attached growth. Microorganisms convert cyanide into naturally occurring
compounds, including mineralization products using different metabolic enzymes (Fry
and Millar 1972, Ingvorsen et al. 1991, Meyers et al. 1991, Raybuck 1992, Stratford et al.
1994, Figueira et al. 1996). Under aerobic conditions the biodegradation of cyanides and
thiocyanate in wastewaters initially produces ammonia, which is converted to nitrite and
nitrate in the presence of nitrifying bacteria, whereas anaerobic biodegradation under
denitrification conditions produce nitrogen. Complete biodegradation of simple and metal
complexed cyanides and thiocyanate from mining wastewaters by various species of
Pseudomonas, Vibrionacas, and Enterobacteriashas been reported (Boucabeille et al.
1994). Most of the research reported bacteria and fungi as cyanide detoxifying
microorganisms, but only a few studies are available to describe the limited capability of
algae to degrade cyanides (Akcil and Mudder 2003, Gurbuz et al. 2004).
Significant advances have been reported in the use of plants for the phytoremediation
(Aksu et al. 1999) of cyanides and biodegradation of thiocyanate and metal–cyanide
complexes has also been reported.
The microorganisms involved in the cyanides and thiocyanate degradation usually
include a heterogeneous mixture of commonly found indigenous soil bacterial consortia
which have acclimatized to the toxic compounds due to prolonged exposure during the
treatment of these compounds (Mudder et al. 2001). Although cyanide is readily
degraded by anaerobic bacteria, thiocyanate is not. In addition, anaerobic biological
treatment is slower and more susceptible to toxic compounds present in the effluent. To
overcome the situation, use of an attached or suspended growth aerobic biological
treatment process is more preferable for thiocyanate removal (Campos et al. 2006).
7
There are a variety of attached and suspended growth processes being used for the
aerobic biological treatment of cyanide and thiocyanate. These include rotating biological
contactors, sequencing batch reactors (White and Schnabel 1998), packed beds,
biological filters, facultative lagoons, and activated sludge systems (Campos et al. 2006,
Patil and Paknikar 2000) .
Guiding factors for effective cyanide degradation
The success of biodegradation depends upon the presence of microbes with the
physiological and metabolic capabilities to degrade the pollutants in the contaminated
environment. Carbon has been identified as a limiting factor in the microbial degradation
of metal–cyanides, which may prevent the bioremediation of cyanides (Baxter and
Cummings 2006). The availability of oxygen is a significant factor, as oxygen is
consumed during several of the metabolic reactions in cyanide degrading pathways
(Raybuck 1992). Cyanide can be toxic to anaerobic bacteria, particularly methanogens.
Additional pollutants present at contaminated sites may also affect biodegradation. The
presence of high concentrations of co-contaminants can impact on cyanide degradation
by influencing the indigenous microbial population, by inhibiting the growth of particular
organisms (Kao et al. 2006, Baxter and Cummings 2006). Temperature is an important
parameter for determination of biodegradation rate. Cyanide degrading enzymes are
generally produced by mesophilic microorganisms (Fry and Millar 1972, Ingvorsen et al.
1991, Dumestre et al. 1997, Kao et al. 2003, Akcil et al. 2003). The pH also is an
important factor in the bioremediation of cyanides. The pH optima for bacterial and fungal
growth are typically 6–8 and 4–5, respectively, and cyanide degrading enzymes generally
have pH optima between 6 and 9, therefore, extremes of pH may have a significant effect
on biodegradation. However, Fusariumsolaniand mixed cultures of fungi, including F.
solani, Fusariumoxysporum, Trichodermapolysporum, Scytalidiumthermophilumand
Penicillin miczynski, were capable of degrading iron cyanides at pH 4 (Barclay et al.
1998). Cyanide conservation in fungal species is observed due to the activity of cyanide
hydratase (Ebbs 2004). The cyanide hydratase activity is induced by low concentrations
of cyanide in many fungi pathogenic to cyanogenic plants such as Stemphyliumlotiand
Gloeocercosporasorghi(Raybuck 1992). Fusarium sp. uses cyanide hydratase to
degrade cyanide (Dumestre et al. 1997). This pathway results in irreversible conversion
of cyanide into formamide and this ultimately gets converted into CO2 and NH3. This
pathway has also been reported in the bacterium Pseudomonas fluorescens(Kunz et al.
8
1992). Cyanide hydrates and cyanides are bacterial enzymes and have similarity at both
the amino acid and structural levels to nutrias and nitride hydrates enzymes (Fry and
Millar 1972). The biodegradation of organic cyanides (nitrites) has been reported by
enzymes nutrias or nitride hydrates coupled with amides (Raybuck 1992). Nitrilases and
nitride hydrates convert both aliphatic and aromatic nitrides to the corresponding acid or
amide, respectively, but show less substrate specificity than cyanide hydrates and
cyanides (Ebbs 2004). Nitride hydrates has been used by K. oxytoca for degradation of
various nitrile compounds (Kao et al. 2006). The presence of nitrite hydrates and
amidase has also been shown in RhodococcusrhodochrousJl, Rhodococcussp. N-774,
Brevibactenumsp. R312, Pseudomonas chlororaphisB23 and several other bacteria
(Raybuck 1992).
Advances in biodegradation of cyanide
Dursun et al. (1999) investigated degradation of ferrous (II) cyanide complex
(ferrocyanide) ions by free cells of P. fluorescens, in the presence of glucose and
dissolved oxygen. They also studied the degradation as a function of initial pH, initial
ferrocyanide and glucose concentrations and aeration rate in a batch fermenter. The
microorganism used the ferrocyanide ions as the sole source of nitrogen. They had found
that 79% cyanide removal efficiency was achieved with maximum biodegradation rate at
pH 5 and glucose concentration at 0.465 g/l (Dursun et al. 1999). Ackil et al. (2003) used
two strains of Pseudomonas sp. isolated from a copper mine for biodegradation of
cyanides at concentrations ranging from 100–400mg/l. They compared their studies with
chemical treatment methods and concluded that biological treatment methods are less
expensive and environmental friendly but as effective as chemical method. Kao et al.
(2006) reported cyanide degradation by K. oxytoca. They found that the resting cells
could degrade, but cell free extract was not able to degrade which happened as a result
of inactivation of nitrogenase (an oxygen-labial enzyme) caused by the oxygen exposure
after cell disruption. Babu et al. (1992) studied the biodegradation of cyanides, cyanides
and thiocyanates by Pseudomonas putida and found that cyanide compounds were used
as sole source of carbon and nitrogen. The end products of the degradation were NH3
and CO2 which caused reduction in pH of the medium. Immobilization has certain
advantages. Immobilization of cells improves degradation rate (Babu et al. 1992, Suh et
al. 1994, Kowalska et al. 1998, Dursun et al. 2000, Campos et al. 2006). It prevents
washing of cells and also it increases the cell density thereby increasing degradation
9
rate. The immobilization technique was also used by Kowalska et al.(1998). They used
ultrafiltration membranes made of polyacronitrile and carried out simultaneous
degradation of cyanides and phenol using Agrobacteriumradiobacter, Staphylococcus
seiuriandPseudomonas diminuta. The efficiency of phenol and cyanide biodegradation
was dependent on trans membrane pressure. The immobilization technique was earlier
studied by Babu et al. (1992) where immobilized cells of P. putidawere able to degrade
sodium cyanide as a sole source of carbon and nitrogen. Simultaneous adsorption and
biodegradation (SAB) is the latest development in cyanide removal (Dash et al. 2008).
They reported high removal efficiency using P. fluorescens immobilized on granular
activated carbon in SAB process. The degradation of cyanides has also been observed in
several fungal strains (Barclay et al. 1998). Fusariumsolani, Trichodermapolysporum, F.
oxysporum, Scytalidiumthermophilum, Penicilliummiczynskihave been isolated which are
able to grow on metallocyanide complex as a source of nitrogen (Dash et al. 2008).
Dumestre et al. (1997) examined degradation by Fusariumsolaniunder alkaline
conditions, i.e. at high pH of 9.2–10.7. This could be of interest as it explored the ability of
microorganisms to degrade cyanide under extreme conditions. Also at such high pH
limits the volatilization of cyanohydric acid which is formed during the reaction. Another
fungal strain Trichodermaspp. (Barclay et al. 1998, Ezzi and Lynch 2005) has been
isolated and studied for its degradation capabilities. Ezzi and Lynch (2005)found that this
species could use cyanide as a sole source of carbon and nitrogen. They also noticed
that the inclusion of glucose could improve degradation rate by three times. F.
oxysporum(Campos et al. 2006) was also found to possess good degrading capabilities
for cyanide and formamide. Campos et al. (2006) studied on packed bed reactor using
immobilized F. oxysporumon sodium alginate. Apart from degradation there also exist
biosorption (Patil and Paknikar 1999) processes in which microorganisms adsorb the
toxic compounds instead of degrading it. Several fungal species are there
(Aspergillusfumigatus, Aspergillusniger, Aureobasidiumpullulans,
Cladosporiumcladosporioides, Fusariummoniliforme, F. oxysporum, Mucorhiemalis),
which can act as biosorbents for cyanide compounds. Biodegradation and biosorption
process can be used in combination for removal of cyanides as they can be very efficient
as shown by Patil and Paknikar (1999). Iron(III) cyanide complex is known to get
adsorbed on Rhizopusarrhizuswhich is a filamentous fungus (Aksu et al. 1999). This
fungus can adsorb the complex at very highly alkaline conditions at pH 13 with very high
loading capacity of 612.2 mg/g of cyanide. A strain, Pseudomonas
pseudoalcaligenesCECT5344, isolated from the Guadalquivir river (Córdoba) used
10
several nitrogen sources including cyanide, cyanate, b-cyanoalanine, cyanacetamide and
nitroferricyanide under alkaline conditions, which prevents volatile HCN (pKa 9.2)
formation. CECT5344 is a cyanide-resistant strain which induces an alternative oxidase
and a siderophore-based mechanism for iron acquisition in presence of cyanide. Besides
bacteria and fungi, algae could be used for the degradation of cyanides. The
detoxification of cyanide by algae was examined by exposing cultured suspensions of
Arthrospiramaxima, Chlorella sp. and Scenedesmusobliquusat pH of 10.3. The removal
efficiency was 99%. This study has explored the use of algae in this field under extreme
conditions with high removal efficiency. A novel study has confirmed the use of plants in
the bioremediation of cyanide containing waste. Cyanide concentrations up to 125 mg/L
were readily degraded by the plant.
Combined treatment of Phenol, Cyanide, Nitrate and Ammonium Nitrogen
Biological treatment processes, such as single- or multi-stage conventional activated
sludge process (CAS) (Luthy and Tailor 1980, Cameron and Paul 2007), sequential batch
reactor (SBR) (Maranon et al. 2008), combined treatment using activated sludge and
SBR (Papadimitriou et al. 2006), biological fluidized-bed (Sutton et al. 1999), airlift
submerged bio-film reactor (Yun et al. 1998) anoxic/oxic (A/O) ( Lee and Park 1998),
and anaerobic–anoxic–oxic system (A1/A2/O) (Li et al. 2003, Wang et al. 2002, Qi et al.
2007, Cameron and Paul 2007) are mostly used for treating the coke oven effluent. The
effectiveness of the treatment processes is commonly assessed by the following
parameters- biological oxygen demand (BOD), chemical oxygen demand (COD), phenol,
cyanide, ammonia nitrogen (NH3-N), and total nitrogen (TN) concentrations in the treated
effluent. The anaerobic pretreatment stage followed by the denitrification stage present in
the A1/A2/O system and A/O system preferably removes the refractory polycyclic and
heterocyclic compounds (Li et al. 2003). Bioaugmentation of refractory organics
degrading microbes like Burkholderiapickettiiwith A1/A2/O system can improve the
performance of the combined system treating effluent containing higher concentration of
refractory organic compounds (Wang et al. 2002).
The treatment process in Zentralkokerie Saar, Dillingen (Germany) has two biological
steps: the first includes denitrification and organic matter removal which is carried out in
separated tanks. In the second step the nitrification takes place. In other existing plants,
such as Kaiserstuhl (Germany), Ser´emage (France) and Sidmar (Belgium), a prior
11
sedimentation step is followed by a biological treatment (combination of aerobic and
anoxic) and sludge settling and treatment. The main differences among these plants are
the aeration system (pure oxygen or air), the concentration of excess sludge, the type of
coagulant, and the use or not of dilution water. The possibility of performing the treatment
by means of a one-step activated sludge process was considered to verify whether most
of the pollutants contained in the wastewater (NH4
+-N, COD, phenols and SCN−) could
be efficiently removed. However, this might prove somewhat difficult due to possible
inhibition phenomena among the pollutants. Some authors found that phenol
concentrations of 5.6 mg/L inhibit nitrification by 75% (Yamagishi et al. 2001). However,
the sensitivity of ammonium oxidizers to various chemicals does not necessarily mean
that nitrifiers are absent in an activated sludge reactor treating substances that are
inhibitory to nitrification in the long term. The effective concentration of the inhibitor may
be lowered by adsorption, precipitation, chelation and biodegradation, and/or by
acclimation of nitrifying organisms to develop tolerance to the inhibitor. In fact, nitrification
activity has been found in laboratory activated sludge supplied with phenolic compounds
(Im et al. 2001). As a result of nitrification inhibition, simultaneous biological oxidation of
both organic and nitrogenous compounds with a single sludge system requires not only a
high HRT for the nitrification process to start once the organic matter has been
completely removed but also an excess SRT of 30 days to maintain a nitrifier population
(Yamagishi et al. 2001). A single sludge system also requires the involvement of other
microorganism which can degrade the inhibitor to the nitrifying microorganism, so that
nitrification may occur once these compounds have been removed. The effectiveness of
the reactor configuration was studied by comparing the performance of the CSTR and
SBR for the treatment of waste waters containing phenol and cyanides (Villaverde et al.
2000). An increased toxicity removal was observed in the SBR; however CSTR system
presented a lower ability for toxicity reduction of influent. Similar experiment was carried
out by using the semi-treated effluent in two stage treatment process. After ammonia
stripping operation the semi treated effluent was treated by two different types of reactors
to compare the performance of SBR and CSTR. The SBR showed better performance
over CSTR for treating phenol and cyanide present in the semi treated coke oven effluent
(Maranon et al. 2008). Experimental observation is also reported for a laboratory-scale,
anaerobic–anoxic–oxic submerged MBR (A1/A2/O-MBR) and its performance is
compared with the conventional anaerobic–anoxic–oxic activated sludge (A1/A2/O-CAS)
at varying loading rates with long-term operation. With a high loading rate the A1/A2/O12
MBR system showed better performance over A1/A2/O-CAS for degrading pollutant
present in the effluent (Zhao et al. 2009).
The attached cell systems are much more stable and may give better stability and
treatability of the toxic pollutants (Qi et al. 2007). A two stage system consisting of
aerobic rotating biological contactor followed by anoxic hybrid fluidized bed reactor can
be a good option. In the first stage, in RBC, phenol will be removed and cyanide will be
converted to ammonia and finally to nitrite and nitrate by the nitrifiers present in the biofilm.
In the second stage the produced nitrate will be converted to nitrogen gas by the
denitrifying bacteria.
Coke oven effluent is very toxic and requires treatment before discharge to the
waterbodies or soil. Literature shows that different technologies are available to treat the
major toxins present in the effluent. Anaerobic, aerobic, anoxic reaction conditions in
different reactor configurations shows different detoxification performances. Two /three
stage activated sludge process is most commonly used technique. But novel techniques
like A1/A2/O-MBR, nitrifying RBC followed by hybrid denitrify can give better performance
for complete mineralizing the toxins present in the effluent with a faster rate.
OBJECT OF THE INVENTION
The object of the invention is to provide for a sustainable biochemical process for the
treatment of waste water/effluents particularly obtained from the coke oven plants before
effluents discharge so as to avoid environmental pollution and also to follow strict
regulatory norms.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 is a graph showing variation in the COD of the Coke oven effluent as it passes
through the process;
Figure 2 is a graph showing phenol degradation in the process;
Figure 3 is a graph showing cyanide degradation at various stages in the effluent
treatment process;
13
Figure 4 is a graph showing ammonia oxidation at various stages in the process;
Figure 5 is a graph showing Nitrate concentration at various stages in the process;
Figure 6 is a graph showing variation in pH across the reactors.
DESCRIPTION OF THE INVENTION
According to the invention there is provided a sustainable three stage biochemical
process for the treatment of waste water/effluents which comprises aerobic (Stage 1)-
aerobic (Stage 2) and anoxic (Stage 3) degradation of toxic substances involving
nitrification and denitrification under controlled conditions using a set of culture
consortium that can withstand toxic effluent load of significant variance through
physicochemical process management where effluents at normative levels and nitrogen
are only discharged to the environment.
The invention is particularly made suitable for the treatment of waste water/effluents from
coke oven plants.
Following description will highlight the details of the development of the process including
establishment of different parameters and identification of different process conditions for
satisfactory operation of the process.
Culture acclimatization and development
The work was originally conducted using a 2-liter CSTR reactor in the batch mode. The
reactor was inoculated with the activated sludge. The anoxic hybrid reactor was also set
up in batch mode with synthetic wastewater containing known concentration of nitrates.
Once the 2L reactor acquired equilibrium and started giving desired results, a bigger
CSTR (4 L) was used. Two 4-Liter reactors were set up side-by-side. The acclimatized
sludge from the 2-liter reactor was used as the inoculums for the larger reactor.
Simultaneously, small (500ml) Shaker flask reactors were also set up to culture the
acclimatized sludge. A larger hybrid reactor with a capacity of 35 liters was constructed to
14
develop the bacterial consortium for denitrification. Synthetic wastewater was used in this
reactor in order to obtain maximum growth of microorganisms.
Initially, nitrification of the coke oven effluent was attempted in a 5-litre capacity Rotating
Biological Contactor. Later the reactor configuration was changed to a CSTR. The
denitrification was carried out in a hybrid reactor, where the denitrifying bacteria were
made to grow in the form of stable granules with good settling characteristics.
Consequently, process of developing acclimatized culture for treating coke oven waste
waters was carried out in 5 litres capacity batch reactors. The seeding was done using
return activated sludge procured from the Bokaro Steel Plant. Process studies were
carried out using undiluted effluent. After operating the batch reactors to ensure good
growth and substrate uptake by the microorganisms, the reactors were made continuous.
The anoxic reactor was seeded using active denitrifying culture.
Development of laboratory scale biological process
Various reactor configurations and biological growth mechanisms were explored for
degradation of raw untreated effluent. Based on results, Continuous Stirred Tank Reactor
(CSTR) was chosen for detailed study and 10L scale process was established. On
establishment of 10L scale process the reactor was upgraded to 50L scale. Initially due to
high level of inlet cyanide in some samples, consortium was wasted and reactor was
needed to have started afresh which resulted in delay in development of the process.
Consequently consortium was cultured afresh and process was established. The process
developed was able to degrade all toxic constituents below normative level within desired
residence time of 24 hrs.
The three-reactor set-up [Culturing of Micro-Organisms (MO) with Coke Oven Effluent]
was operated over a period of more than one year. Initially, each of the reactors were
operated in the batch mode and then made into a continuous process. Salient results are
as follows:
15
Pilot Plant experimental work
A 1,000 litres/day capacity Biological Wastewater treatment Process Pilot scale
simulation facility for aerobic-aerobic-anoxic biological treatment with integrated process
monitoring and control facility was established based on flow sheet developed after batch
scale experimental work.
Optimal Treatment Process development
Detailed studies were carried out in the pilot plant and a 3-stage sustainable process for
nitrification and denitrification of Coke Oven effluent has been developed to sustainably
and consistently achieve normative level before effluent discharge. The process is
capable of treating highly toxic effluents generated from Coke Oven and degrades
recalcitrant constituents, especially Cyanide and Ammonia below normative level. The
novelty of the process developed originates from the fact that the unique set of Culture
consortium, under identified process conditions can not only degrade these nitrogen
bearing toxic constituents below normative level, but also can break these compounds
and release nitrogenous component of these compounds into atmosphere as nitrogen
and thereby can render the process fully sustainable.
RESULTS
The inlet COD level varied from a low of 400 mg/l to a high of as much as 1200 mg/l. The
predominant compounds responsible for the COD were phenol and cyanide. In spite of
the large fluctuation in the inlet COD, the aerobic stages, operating at an HRT of 24
hours each, were able to take care of the organic load, reducing the COD of the treated
effluent from Reactor 2 to a value below 100 mg/L. Degradation of cyanide also
happened in the first two reactors. In spite of the cyanide concentration varying from 10
to 28 mg/l in the inlet raw effluent, the treated effluent from reactor 2 achieved less than
0.2 mg/l total cyanide.
RESULTS AND DISCUSSION ONLABORATORY SCALE PROCESS PERFORMANCE
The three-reactor set-up as explained above(Culturing of Micro-Organisms (MO) with
Coke Oven Effluent) was operated over a period of more than three months. Initially,
16
each of the reactors were operated in the batch mode and then made into a continuous
process. Sampling was done from various locations on the process. Reactors 1 and 2
were completely stirred reactors with 50 litres capacity each, whereas reactor 3 (the
anoxic hybrid reactor) had a total capacity of 65 litres. The detailed performance data of
these reactors are given in the graphs as shown in Figures.1 to 6 of the accompanying
drawings. Each graph gives an account of the fate of one of the major pollutants as it
passes through the process stages.
Figure.1 of the accompanying drawings shows the variation in the chemical oxygen
demand (COD) of the coke oven effluent, at various stages of the process. The inlet COD
varies from a low of 400 mg/l to a high of as much as 1200 mg/l. The predominant
compounds responsible for the COD are phenol and cyanide. Therefore, any increase in
phenol and cyanide concentrations of the influent will reflect directly in the inlet COD. A
major observation from the data is that, in spite of the large fluctuation in the inlet COD,
the aerobic stages (Reactors 1 and 2), operating at an HRT of 24 hours each, were able
to take care of the organic load, reducing the COD of the treated effluent from reactor 2
to a value below 100. The increase in COD seen from the effluent from reactor 3 is due to
the effect of the methanol added to this stage to serve as carbon source for the
heterotrophic denitrifies growing in reactor 3.
Figure 2 shows the fate of phenol as the effluent progresses through the three reactors.
It was seen that most of the phenol got degraded in the very first reactor, where chemoheterotrophic
aerobes are expected to dominate. Whatever phenol was left in the effluent
after this got completely removed in the second aerobic stage. Though the third reactor,
operating under anoxic conditions, was not meant for phenol removal, it can be seen that
on occasions where a small quantity of phenol is left over after the two aerobic stages, it
did get eliminated in the third reactor. This confirms that the denitrifiers growing in the
third reactor are capable of using phenol as its carbon source.
Degradation of cyanide also happens in the first two reactors as shown inFigure3 of the
accompanying drawings. Inspite of the cyanide concentration varying from 10 to 28 mg/l
in the inlet raw effluent, the treated effluent from reactor 2 has less than 0.2 mg/l total
cyanide. Often, it is below 0.1 mg/l levels. On the few occasions when there was a
cyanide spike in the treated effluent, it can be seen that there was a surge in the inlet
phenol concentration also. This indicates that a sudden increase in the phenol
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concentration could force the chemo-heterotrophs to prefer phenol over cyanide as the
carbon source. This also underlines the fact that unless the substantial phenol
degradation happens in the first two reactors, there is a possibility of higher cyanide in
the treated effluent. This was identified as an important parameter for the design of the
process control system for the process. Since the reactor volumes and HRTs cannot
undergo any change with time, the control has to use COD as the input variable. Since
COD analysis is too time consuming to be of use in a classical feed-back control loop,
control system design need to be based on residual dissolved oxygen concentration in
the two aerobic reactors. Since aeration is to be done using diffused air aerators, a
minimum dissolved oxygen concentration of 30% of the equilibrium DO concentration will
be used for the control system design. Any lowering of reactor DO levels is to be tackled
through an increases aeration rate, resulting in higher oxygen availability as well as
higher mixing.
Figures 4 and 5 give the data on ammonia and nitrate concentrations at various stages in
the process. About 50% of the influent ammonia gets removed in reactor 1. It should be
assumed that a majority of this is through assimilation into biomass by the chemoheterotrophy.
Some amount of ammonia gets converted into nitrate also, as seen in
Figure 5. The remaining ammonia gets converted into nitrate in the second aerobic stage.
This too is evident from the Figure 5. The reduction of all accumulated nitrate to
elemental nitrogen happens in the anoxic reactor (reactor 3). It can be seen that the
denitrifying stage (reactor 3) works very well. There is hardly any nitrate remaining in the
effluent leaving this stage.
Figure 6 shows the variation of pH from one stage to the next in the process train. It was
observed that the inlet pH (pH of coke oven effluent) was always above 9. The optimal
pH for the heterotrophs responsible for phenol oxidation is around 7.0, while that for
nitrifiers (autotrophs) responsible for ammonia as well as phenol oxidation is in the 7.5 –
8.5 range. The denitrifiers have pH optima in the 7.0-8.0 range. The above performance
was achieved without doing any initial adjustment to the fresh coke oven effluent. In
combination, It was possible to develop a robust sustainable process in laboratory scale.
Pilot plant experiments were carried out using wastewater samples collected from BSL,
Bokaro brought in a lot size of 5000 litres each. The design of experiments was
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developed using a “Greenfield Approach” i.e, without any experimental bias arising out of
pre-existing information available from experience of actual plant operating personnel.
There have been a select points of learning through batch scale experiments when
process control was not resorted to. These are:
 High F/M loading lead to foaming
 Protein release due to cell lysis and very high or very low F/M ratio lead to
significant pH excursion towards acidic range during batch culturing
 High Cyanide toxicity beyond a threshold level hinders culture growth
 Low BOD/COD Ratio hinders culture growth
 Significant pH fluctuation hinders culture growth
 Culture growth is significantly hindered in winter season
 Equilibrium Dissolved Oxygen level should never fall below the level of
cumulative oxygen transfer efficiency, but higher level of DO does not add
much value in terms of process efficiency
 Nutrient supplementation, especially phosphorous is essential for culture
growth
 Slow increase in volume is essential for culture sustenance; same is true for
continuous phase flow rate.
Highlights of successful batch-phase experimentation before the plant made continuous
are:
 Online pH control in batch phase was required for culture growth
 Dissolved Oxygen has been kept in a very close range
 A stable average MLSS in the range of 2800-3000 mg/l has been achieved in
first aerobic batch reactor
 A stable average MLSS in the range of 3000-3200 mg/l has been achieved in
second aerobic batch reactor
 Significant COD degradation ( < 100 mg/l) has been achieved in both batch
reactors
 Significant cyanide degradation achieved in First aerobic batch reactor
 Cyanide degradation below normative level ( < 0.1 mg/l) has been achieved in
second aerobic batch reactor
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 Ammoniacal Nitrogen (NH4-N) degradation below normative level has been
achieved in both aerobic batch reactors
 Degradation of phenolic compounds below normative level ( < 1 mg/l) has
been achieved in both the reactors.
The experimental aspects of plant scale trials have been:
Initially for a period of 10 days flow loop was run using actual effluent without any culture
to explore natural culture growth potential (as happens in normal sewage treatment
plant).
In second set of experiments, plant process was ‘simulated’ on “ as operated” basis i.e.,
mixture of wastewater and culture brought respectively from first and second aerobic
reactors of Bokaro Steel Plant was put into first and second aerobic reactors of pilot plant
and given adequate aeration.
In next set of experiments classic culture growth approach was followed in aerobic
reactors with seed culture brought from Bokaro Steel Plant. During batch experimentation
pH, Dissolved Oxygen, COD & cyanide level and sludge growth (through on-line
monitoring of MLSS) monitored at regular interval. pH and DO was controlled and sludge
recycling and wastage plan was optimized.
Key aspects of experimental design during studies are:
• During batch culturing once consistent median MLSS value of 3000 mg/l achieved
and uniform COD reduction attained, plant made continuous
• The plant made continuous in steps of 50 litres to attain full capacity;
• Number of days for each step decided based on culture growth pattern.
• Denitrification reactor started with synthetic effluent and once degradation
achieved was gradually replaced with actual effluent
• During continuous phase Dissolved Oxygen level in both the reactors maintained
at a median value of 3+ mg/l;
• pH in Reactor 1 maintained in a range of 7.0-7.5 and between 7.5-8.0 in Reactor
2
• No temperature adjustment carried out
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• It was ensured that Dissolved Oxygen level in aeration tanks do not fall below 2
mg/l for an extended period of time ( maximum 4 hrs ) through continuous power
supply.
The major experimental variables were:
• pH
• Dissolved Oxygen
• MLSS level
• Sludge Recycling
• Residence Time ( Flow rate )
Pilot Process has been developed and optimized. Sustained degradation of normative
constituents has been achieved with optimal process monitoring and control with
minimum 35 hours residence time.
MAJOR ACTION POINTS:
• External carbon source enhanced bacterial efficiency , but also enhances COD
• Consistently low BOD/COD ratio is a cause for concern; Therefore either
continuous artificial feed need to be provided or high BOD effluent ( e.g sewage )
need to be added
• Surge in inlet phenol is often responsible for surge in cyanide of outlet effluent
• Whenever Inlet Cyanide level ( after dilution ) has been kept below 15 mg/l ,
culture development has been faster;
• Inlet cyanide concentration cut-off level need to be maintained through pretreatment
• Whenever BOD/COD ratio has fallen below 0.4, culture development has been
very slow; So, for low BOD/COD ratio, simultaneous chemical treatment may be
required; otherwise the ratio should be used as a parameter for Flow
management/control system design
• pH variation can probably be used as surrogate monitor of bacterial activity
• There must be On-line pH control facility in plant
• Direct analyzers (e.g, MLSS ) have shown consistent results with very little
maintenance and therefore can be readily adopted by plant
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• It is probably possible to replace off-line BOD and COD monitoring with On-line
TOC monitoring with correlation developed in-situ
• Since COD analysis is too time consuming to be used in feedback control loop,
control system has to use residual DO concentration in two aerobic reactors as
major control parameter
• Even in controlled conditions performance gets affected with residence time of
less than 35 hours
• Continuous alkalinity maintenance need to be carried out in plant
From the above disclosure of the invention it is apparent that several additional
embodiments beyond those disclosed are possible which can be carried out by a person
skilled in the art and the same are included within the broad ambit of the invention
claimed herein.
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WE CLAIM:
1. A sustainable three stage bio-chemical process for the treatment of waste
water/effluents comprising aerobic ( Stage 1 )-aerobic ( Stage 2 )-and anoxic (Stage 3 )
degradation of toxic substances involving nitrification and denitrification under controlled
conditions using a set of culture consortium that can with stand toxic effluent load of
significant variance through physicochemical process management where effluents at
normative levels and nitrogen are only discharged to the environment.
2. A process as claimed in claim 1, wherein the aerobic reactor process Stage 1 and
2 and anoxic reactor process Stage 3 are carried out under individually identified optimal
operating and process conditions as herein described.
3. A process as claimed in claims 1 and 2, wherein inlet cut off conditions such as
herein described have been identified as control parameter.
4. A process as claimed in claims 1 to 3, wherein optimal mixed liquor suspended
solids (MLSS) range has been established as herein described.
5. A process as claimed in claims 1 to 4, wherein dissolved oxygen (DO)
maintenance range as herein described has been identified.
6. A process as claimed in claims 1 to 5, wherein nutrient requirements have been
identified as herein described.
7. A process as claimed in claims 1 to 6, wherein pH/alkalinity control range has
been identified as herein described.
8. A process as claimed in claims 1 to 7, wherein optimal residence time has been
identified and established as herein described.
9. A process as claimed in claim 1 to 8, wherein the waste water/effluent is from
Coke oven plant and contains phenolic compounds, ammonia- nitrogen, cyanides and
others.
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10. A process as claimed in claims 1 to 9 is a continuous one.