Constructing one of India's largest single location Effluent Treatment Plants

Construction of one of India's largest single location Effluent Treatment Plants (at Sukinda, Odisha)

ABSTRACT

Tata Steel operates India’s one of the largest chromite mines at the Sukinda Valley in Odisha producing chrome ore which is subsequently converted it to Ferro Chrome and sold to customers across the world. A large quantity of water, generated during mining and due to rainfall, needs to be handled during the mining operations. Chrome Ore mainly contains trivalent chromium oxide and a very small fraction of hexavalent di-chromate. Water coming in contact with chromium ore preferentially leaches out soluble hexavalent chromium from the ore body, as a result, water from the mine contains 0.2 – 4 mg/l of hexavalent chromium against a safe limit of 0.05 mg/l for human consumption; requiring all water to be treated before its release from the mines. Thus, we have set up an Effluent Treatment Plant at Sukinda with a capacity of ~108 million litres/day.  This is the largest such plant in the region, and possibly, one of India's largest single location Effluent Treatment Plants.  

INTRODUCTION

Describing Sukinda and importance to FAM

Sukinda Valley, known for its high-grade chromite deposits, is located in the eastern state of Odisha, India. This valley contains 99% of India’s chromite deposit. The ultramafic mass occurs sporadically over an area of 420 sq. km around Sukinda(Fig-1).  The deposit was first proved by geologists of Tata Steel in 1949,   followed by intensive exploration. Tata Steel’s Sukinda Chromite Mines, with a mine extending over 406 Ha, is one of the largest chrome mines in India. The Chrome ore mined at Sukinda has enabled Tata Steel to be one of the larger chrome alloy players in India.

Fig: 1 Tata Steel’s Chromite Mine at Sukinda

Chrome Ore

Chrome ore occurs as Chromite, which is chromium oxide, and is essentially in the form of un-weathered, hard, compact, fine-grained dark grey lumpy ore or as a weathered, and loosely bonded, brown-black, friable ore in ultra-basic host rock(Fig-2). Chromite contains a large proportion of stable trivalent oxide of Chromium with a small fraction in the unstable hexavalent state.

Fig: 2 Massive, Un-weathered Lumpy Ore and Weathered Friable Ore

Hexavalent Chromium

While trivalent compounds of chromium are not soluble in water, hexavalent chromium compounds are. Water coming in contact with the chromium ore leaches out soluble hexavalent chromium from ore body. Both mine water and surface runoff have 0.2-4 mg/l of hexavalent chromium against the safe limit of 0.05 mg/l for human consumption.

Hexavalent chromium (Cr+6) is considered a human carcinogen with genotoxic properties. Hexavalent chromium can cause the following diseases:

• Ingestion of hexavalent chromium-contaminated water causes irritation and ulcers in the stomach and the intestines
• Contact with hexavalent chromium (in the form of dust or dissolved in water) with soft mucous tissues of the eyes and the nose can lead to irritation and ulceration
• Exposure to liquids/water contaminated with hexavalent chromium causes allergic skin reactions

 

There is no evidence of elevated levels of these diseases (compared to the national and the state average) in the valley. Another study, by Utkal Polyclinic, has also shown a lower incidence of skin diseases in the Sukinda Valley, which, due to the allergic effects of Cr+6 on skin, is contrary to expectations. This is probably due to the low levels of Cr+6 found naturally (and in Sukinda).

 

Water Management at Sukinda Mines

The Sukinda Valley experiences about 110 cm to 180 cm of rainfall annually, of which eighty percent (80%) occurs during the monsoon season i.e. between June and September. Owing to this highly uneven distribution of rain, the weather in the Sukinda Valley ranges from extremely dry to extremely wet.

 

The major portion of the rain goes as surface run-off and flows through the garland drains, that have been made around the quarries and dumps. The flow carries silt and dry vegetation with it, apart from picking up hexavalent chromium as it trickles down the chrome rich quarries and dumps. These drains also channel the water pumped out during mining operations.  (Fig-3)

Fig – 3 : Map showing water discharge circuit, garland drains and ETP Locations

History of Hexavalent Chromium Management at Sukinda Mines

Hexavalent Chromium in the Sukinda Valley water was first detected in the mid-1990s. Tata Steel pioneered the efforts to mitigate the ill effects of hexavalent chromium by collaborating with India’s premier environmental research institute, NEERI, Nagpur and set up three Effluent Treatment Plants (ETPs) based on technology co-developed with NEERI. Initially, spent pickle liquor, from Tata Steel’s old Sheet Mills was used for reducing hexavalent chromium; and later a process using FeSO was established. (Table – 1) 

 

 

IMPETUS TO UPGRADE

While the pioneering efforts of Tata Steel in setting up Effluent Treatment Plants were well acknowledged, there was a growing pressure to upgrade the (over ten year old) ETPs, both internally and from the Government agencies, due to the following reasons :

1. Partly in order to mitigate environmental effects due to its mining operations and partly to meet statutory obligations, Tata Steel has been conducting a very successful afforestation program around the Sukinda and Kalarangiatta areas. Other miners at Sukinda too have made some efforts in afforestation which has resulted in good rainfall.

 

1. Tata Steel’s Sukinda Chromite Mine is already one of the deepest Open Cast Chromite Mines in India. As open cast mining is reaching the ultimate pit bottom, Tata Steel has long been contemplating starting Underground Mining at Sukinda. This will further increase the quantity of water that needs to be handled.

 

1. The ETPs set up in early 2000’s were possibly state of art at that time. However, with facilities available for automation, online monitoring etc., and an increased understanding of water treatment methods, the ETPs seem to be extremely maintenance and manpower intensive today, and technologically obsolete.

 

1. The State Pollution Control Board, Odisha (OSPCB), hired IIT Kharagpur to conduct a study on possible solutions to the issue of water pollution. The study recommended that a common Effluent Treatment Plant be set up by OSPCB treating all water before reaching the Damsala Nallah.

 However, the common ETP proposal was a non-starter because :

1. It was complicated, and not easy to execute on the ground; primarily due to requirement of channelling all the water from the valley to one location, which would be challenging due to (i) the large undulating contour of the Sukinda Valley and (ii) the vast area to be covered.
2. The common ETP would require a large amount of capital expenditure. Sharing of the capital expenditure between different mine owners was an extremely contentious issue. Also, land for the ETP and right of way for the drainage would have been bottlenecks.
3. Operations of the Common ETP would be challenging too, given the different operating philosophies, that different mine owners have.

Due to the above issues, it was decided that all the mines would set up their own ETPs individually based on the conceptual design provided by IIT, Kharagpur to treat water generated from their mines (including surface runoff) inside their leases.

CHOOSING THE RIGHT TECHNOLOGY AND EXECUTION STRATEGY

1. Evaluation of various techniques to treat Cr+6 and why we chose FeSO technology

There are many solutions to eliminate hexavalent chromium from water. Some technologies are well established and in use commercially for specific situations. There are also some innovative solutions that have been proven experimentally. A summary of the available technologies, and the reasons for the selection of the FeSO technology is summarized in the table below (Table 2) :

Tata Steel, along with CLRI, has also developed a Herbal Treatment process using Terminalia Chebula, an organic product, for Cr(VI) removal in chromite concentrates; but the process is not suitable for treatment of Cr+6 in mine effluent, given the volume of water to be treated, the slow reaction rate & the cost of Terminalia Chebula(ref. 2)

1. ETP Process Design : Optimization of the treatment

Before designing the process, extensive jar tests were carried out on the effluent water to allow for :

1. a) Highly efficient and fast reaction for the reduction of hexavalent chromium Cr+6 to Cr+3 .
2. b) Rapid flocculation of precipitated Cr+3 compounds to reduce residence time in the Clariflocculator, while enabling control of TSS within statutory limits.

Both the above are necessary to increase the throughput of the ETP and enable treatment of a large volume of water in a short time. As a result of the jar tests, we have included three more facilities in the ETP, which were not in the original design, namely :

1. Acid Dozing of the raw effluent in a flash mixer to bring down the pH before reaction with FeSO since the FeSO reaction is most efficient at a low pH. Also, because of the efficient reaction at low pH, the consumption of FeSO and the amount of sludge generated can be substantially reduced.

 

1. Stirring arrangement in the flash mixer and a reaction channel to allow for complete reduction of hexavalent chromium.

 

• pH correction using an alkali before dozing with a polyelectrolyte, to ensure complete reaction, as polyelectrolyte reaction needs a neutral pH, along with a stirring arrangement. The alkali recommended for pH reduction is NaOH but for various reasons we are using Ca(OH).

 

1. Sizing of the Effluent Treatment Plant

To determine the most suitable size for the effluent treatment plant, we needed to determine the volume of both mine water (water pumped out from the mines during operation) and the surface run off. The determination of mine water volume was simpler, due to ready data available from which a correlation between mine production and water volume could be obtained(ref. 3). The volume of mine water then was approximated from the long term plan for mining, inclusive of the planned shift to Underground Mining. Determining the surface run-off was more difficult. For this, we studied the water flow pattern and the meteorological data over the last seven years for the region. The maximum rainfall over 24 hours in the last ten years formed the basis for the calculation of surface run-off volume. The most likely maximum volume of water that would need to be treated, thus determined, became the basis of determining the size of the Effluent Treatment Plant. This resulted in us recommending the setting up of an ETP capable of treating 4500 m3/hr; by far the largest ETP in the region. Other miner owners arrived at far smaller ETPs, but subsequent events proved that we were correct. A year(and a rainy season) later, many of the neighbouring mines are already expanding the capacities of their much smaller ETPs.

1. Specifications for the Effluent Plant Output

We took a decision that the Effluent Treatment Plant output would not only meet the current specifications for treated effluents in non-urbanized areas, but in order to be future ready, meet the specifications for treated effluents in both urban areas and the likely stricter norms for treated effluent that are likely to be imposed in the future. Thus the plant has been designed such that the output has less than 0.01 mg/l of Cr+6 against a norm of 0.05 mg/l and meets the stricter TSS standard of < 10mg/l (drinking water specifications) against a norm of < 100 mg/l (norms for treated effluents in non-urbanized areas). We also took a decision to treat both surface run off water and mine water in same way, which none of the other mines in Sukinda planned to do. This decision proved to be fortuitous, as the Pollution Control Board has now asked all mine owners to treat both surface run off and mine water and many mine owners have had to look at augmenting the capacity of their ETPs.

Fig. 4 gives the capacity and guaranteed output water parameters of the Effluent Treatment Plant at Sukinda (Fig 4)

Fig - 4 : Capacity and Guaranteed Output Water Parameters of the Effluent Treatment Plant at Sukinda

1. Modular ETP

The wide variation in the quantity of surface water to be treated between the monsoon months of June-Sept (where over eighty per-cent of the rainfall takes place) and the very dry months in winter (Nov-Dec) and peak summer(Apr-May) posed its own challenges. Instead of making a single large 4500m3/hr Effluent Treatment Plant, which would unnecessarily increase operations cost in the dry period, we decided to make the ETP in three modules of 1500m3/hr capacity each (Fig 5).

Fig - 5 : Block Diagram of the Modular Effluent Treatment Plant at Sukinda (1500 m3/hr X 3 modules)

1. Locating the ETP

Building an ETP of 108 million litres/day capacity is a huge challenge due to lack of sufficient available space in Tata Steel’s Sukinda Chromite Mines leasehold area. The site for the ETP was selected considering two main aspects, namely

1. The selected location would have to be at the lowest point of the mine so that all the water (mine water and surface run-off) could be channelized at the minimum cost using gravity.

 

1. There would need to be adequate space at the site so that a 108 million litres / day ETP can be constructed at the site.

The best candidate (though with many shortcomings) for location of the said ETP was at the south-west boundary of the lease, the area used for despatches and truck parking. Also, the main despatch road ran through almost the middle of the selected area. Thus we needed to first relocate the truck parking yard and re-route the main despatch road, a big task by itself.

Due to the topography of the mine, some of the water would naturally flow to the north east extremity of the mine, near the temple. Initially it was decided that we would need to construct one of the ETP modules (1,500m3/hr) at the temple end and two ETP modules (2 x 1,500 m3/hr) at the south west end(also known as the main site).

In a measure of further optimization, along with our Engineering Consultants, TCE, Jamshedpur and execution partners, M/s EFFWA Infra and Research, Mumbai, the Engineering & Projects team at Sukinda redesigned the layout of the modular ETPs such that all three clariflocculators could be located next to each other at the main site with two collection tanks, one at the temple end and the other at the main site; connected by a system of pumps and underground piping(Fig 6).

Fig - 6 : 3D View of the Modular Effluent Treatment Plant at Sukinda (1500 m3/hr X 3 modules)

1. Project Cost Optimization

In line with our tradition and philosophy of implementing the best environmental protection measures and to remain a benchmark in the industry, we increased the capacity of the Effluent Treatment Plant, with a concomitant increase in capital cost. We brainstormed to lower the cost of the project, resulting in many innovative solutions, such as :

• ETP at a Single Location : We had initially proposed construction of two separate ETPs at both extremities of the mine. Every unit (pumps, motors, transformers, RCC tanks, clariflocculators etc) would thus need to be constructed in both locations(albeit of a smaller capacity). Making the ETP at one location, and the pumping of the effluent collected to the main ETP site; resulted in substantial savings of about Rs XX crores.

 

• Free supply of steel : We changed the terms of the contract to include free supply of re-bars, which, being a product made by Tata Steel, was available to us at transfer prices (lower than market prices due to lower taxes) and also gave us the advantage of assured quality, By this we saved over Rs XX crores in the project cost.

 

• Transmission of raw effluent partly through existing drains instead of through a underground steel pipe : Initially we had planned to send the pumped raw effluent through a steel pipe. However, on examination of the topography we found a natural slope from mid-way enabling us to use the existing network of garland drains(after repairing them and increasing their carrying capacity) to transmit the raw effluent. This resulted in a saving of Rs XX crores.

 

• Optimization of the steel structures, by evaluating the soil bearing capacity, for each location : Initially we had taken a single measurement of the soil bearing capacity(SBC) at the centre of the proposed ETP location. Because the SBC value was low, the structures were designed with higher reinforcement. However, during excavation, it was found that the area, being partly refilled, had widely varying SBCs. SBC values for each structure were measured and the structures were re-designed, resulting in a substantial saving in construction cost, about Rs XX crores.

 

• Changing the sludge disposal method : Initially we had planned to handle dry sludge (less than 30% moisture) from the centrifuge using a conveyor to the sludge yard from where it would be re-handled. However, the conveyor was taking up a large amount of space that we could be ill afford.  To optimize space and improve subsequent operations, we re-designed the sludge handling system so that the dry sludge falls directly into buckets from which it will be lifted by the Placer Dumper for disposal. This has led to a reduction in cost by Rs XX crores. 

 

These cost reduction measures, taken up through a DMAIC project(ref. 5), resulted in lowering the cost by Rs XX crores  

 

CHALLENGES DURING THE EXECUTION

The State Pollution Control board had given us a deadline of 31-Dec-2014 for setting up facilities to treat mine water and 30-Jun-2015 for setting up of the complete ETP. This was a seemingly impossible deadline, considering that we had to start from scratch, from choosing the technology, the execution partner, the engineering consultants, getting CapEx approvals and executing the project within the stiff timeline of one and a half years.

Getting the CapEx approval and post techno-commercial negotiations to choose the execution partner and engineering consultants, took close to six months. To meet the challenge of meeting the almost impossible deadlines, we decided to :

 

• Initially concentrate on meeting the target of treating mine water by completing one module of the ETP by 31-Dec-2014.
• Completion of the second module of the ETP by 30-Jun-2015 to treat the surface run-off.
• Completion of the third module (to treat water from underground mining/increased capacity in open- cast) and other finishing jobs post-meeting the above deadlines.

1. Keeping the Project within timelines

To keep the progress on track, we used the CCPM method along with Weekly Review Meetings at the local level, a weekly report which was circulated right up to the VP(Raw Materials), and a detailed monthly review with the design team.

A major challenge has been working through two very heavy monsoon periods, one at the start of the project, where a lot of excavation was involved, and one towards the end, where the heavy rains have notched up the difficulty of safe and timely work by several degrees.

To address the issue of the almost impossible timelines, we also had to start civil construction before completion of detailed design and engineering. To ensure that this did not affect the project, sequencing of drawing approvals, accuracy in design and detailed engineering etc. were ensured by close coordination between the Project Team, the Engineering Consultants, and the Execution Partners.

1. Quality Checks

A system of field quality audits during project execution was established, along with the help of the engineering consultants, for various parameters, as shown in the table below (Table 4) :

Table – 4 : Summary of  Quality Checks during the Construction of the ETP

Many facilities were set up to ease construction and improve construction quality as shown in table below (Table 5) :

 

 Table – 5 : Various facilities set up during the construction of the ETP

1. Safety Challenges during Execution

   

1.  Making the Site Safe from Normal Operations The best candidate (though with many short-comings) for the location of the ETP was at the south-west boundary of the lease, the area used for despatches and truck parking. However, the main despatch road ran almost through the middle of the selected area. Thus we needed to first relocate the truck parking yard and re-route the main despatch road, a big task in itself. This helped in ensuring that the construction site was kept separate from normal operations and greatly increased safety during construction.
2.  Weather Proofing : A major challenge has been working through two very heavy monsoon periods, where the heavy rains notched up the difficulty of safe and timely working by several degrees. To ensure safe working we ensured that no foundation work (excavation, making of columns etc), electrical work (HT cabling etc.) or erection work was scheduled during the monsoon period.
3. Similarly, we ensured that during the very hot period from mid-March to mid-June, when heat induced incidents are common, we scheduled all heavy work for the early mornings and late evenings, and by taking precautions of ensuring adequate lighting and separate gangs of workmen and supervisors, sometimes, working through the night, instead of during the day. 
4. Lack of space, mentioned earlier, has also meant that the work that could have been carried out in parallel at any other site, per-force has had to be sequenced because of safety and other execution concerns.

 

1. Site Safety

Safety is not negotiable, and we at Tata Steel, through years of training, efforts and learning from incidents have made safe working almost second nature. This is also true for most of our contractors and partners. However, our execution partner M/s Effwa Infra & Research and their sub-contractors were working with Tata Steel for the very first time. We were also unable to ensure adequate training on construction activities, since the training program at Sukinda is tuned more to sae working in mines. Hence, we had to jointly develop safe working SOPs, HIRA and training of workmen during the execution of the job. Inadequate planning and sequencing of jobs is a major cause for onsite incidents. Timely and regular safety audit in the initial stages by external teams, helped us greatly in identifying hazards(ref 4)  Post the audits, we developed many safety protocols which were implemented in letter & spirit.

Adopting Safe Construction Practices : The severe restriction in space and other difficulties led us to adopt safe and at times unique construction practices, a few examples being highlighted below : 

1. Pump House area– This area is highly space-constrained. Drawings for pump house were approved after the completion of construction of the Clariflocculator-1. Due to the pump house being almost 3 meters lower than the Clariflocculator, an almost vertical cut needed to be made for the raft of the pump house, where the shear resistance angle of the soil was ~ 10o < Φ ≤ 35o . Hence, we made sheet piles to stabilize the slope before excavation of the pump house structure.
2. Deep Excavation– The tank structures involved very deep excavation, of over 4 meters below the ground. Due to constraints of space the slope angle was greater than the angle of shear of the soil. We used slope stabilizing nets and shoring to stabilize the slopes.
3. Ground Water Seepage– The deep excavation resulted in constant ingress of ground water in the excavated pits. Thus constant pumping of the water while casting needed to be done, which was risky as well as complex. Protocols developed especially for such situations ensured close coordination between the pumping and casting gangs and safe working in these risky situations.
4. As there was a High Tension Line running close to the project site, we shifted the 11kV line with proper shutdown planning with the help of CESU to ensure that we could work safely.

 

UNIQUE FEATURES OF THE EFFLUENT TREATMENT PLANT

 The Effluent Treatment Plant under construction at Sukinda has many unique features(Table-6) :

 

 Table – 6 : Unique Features of the Effluent Treatment Plant at Tata Steel Sukinda Chromite Mines

 24/7 real-time monitoring of the input raw effluent and output treated water for Cr+6, pH and TSS through online monitors installed at both input (raw effluent) and output (treated water). This will prevent any inadequately treated effluent from leaving the mine and give warning signals if the treated output water quality is not up to the mark.

 The ETP is highly automated, with a feedback mechanism. Thus the dozing of chemicals (acid, FeSO, alkali, and flocculants) is automated through a system of PLC based controllers, based on the input raw effluent and the output water quality.

 Automated backwash arrangements for the pressure sand filters to ensure that the filters do not choke.

 Real Time Monitoring of Data

For real-time monitoring of data, we have set up a data communication system that captures real-time information from the analysers for Cr+6, TSS and pH installed at the outlet in a server and transmit the data thus captured automatically to OPCB/ CPCB server on real time basis. The schematic of data transmission is shown in Fig-7 and the output screen in Fig-8.

Fig - 7 : Schematic Diagram of Capturing & Transmitting Data for Real Time Monitoring

Fig - 8 :  Result of Online Monitoring of Treated Effluent

- the effluent is well within the specified limits

Thus the output water quality data are available both internally through a dedicated web page and can be transmitted to the Pollution Control Board on a real time basis.

A photograph of the Effluent Treatment Plant is shown in Fig-9 (with more photos at the end of this article).

Fig - 9  : The Effluent Treatment Plant (View of Clariflocculator #1)

CONCLUSIONS

The success of the ETP Project can be summarized to be as a result of the following :

1. Vendor selection only after an intense technical evaluation of the capability of the vendor and not on commercial considerations alone.

 

1. A strict focus on time lines and cost at all levels with frequent high level reviews and support to the project team

 

• Over-riding concerns of safety and quality with mechanisms for frequent checks (preferably external to project team)

 

1. Deep understanding and cooperation between all agencies working on the project, which developed during the course of the execution and was necessary to cope up with unforeseen challenges that can crop up at any time : and need to be resolved collaboratively.

WAY FORWARD

The output water quality post treatment at the Effluent Treatment Plant is better than the water available in the local Nullah, giving us the opportunity to use it as an input for our Water Treatment Plant(WTP) and in various other places, like dust suppression, gardening etc.(Fig-10). The benefits of this are:

Good Quality Water: During the monsoon season water flowing through Domsala river has very high TSS. The output water from the ETP is already treated and thus a better input to the WTP than the water from the Nallah.

 Cost Saving : Apart from substantial cost saving in pumping from the Nallah which is over 3 km away, the chemical consumption at the WTP will substantially reduce, due to the consistent and better input water quality, reducing the cost of treatment as well.

 Towards Zero Discharge : As per the Pollution Control Act, an industry should ideally have ZERO discharge. Thus reusing the water from the ETP is one step towards achieving zero discharge.

Fig - 10  :  Schematic Diagram of our plan to use the ETP discharge as an input to our WTP

References

1. Rama Murthy Y (Dr) et.al, IFA/ABP/389/2013, Development of Process for Water Treatment at Chrome Ore Beneficiation Plant, Sukinda, Jun-14.

 

1. Kapure, Gajanan et.al., Application of Terminalia Chebula for Removal of Hexavalent. ISIJ International, Vol. 48. 2008.

 

1. Internal Report on Water Quality & Runoff Management at Sukinda Chromite Mine, 2012.

 

1. Internal Safety Audit Report, Pravin Srivastava & Anirban Mukherjee, Sept-2014.

 

1. ASPIRE/DMAIC/2829 “Reducing Cost of Construction of Effluent Treatment Plant at Sukinda Chromite Mines”, Apr-15.