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Infrastructure & Design

Innovative Commercial and Technical Solutions for CSG Produced Water Treatment Project
The coal seam gas (CSG) exploration and appraisal activities by Santos in Narrabri, New South Wales (NSW), Australia, results in produced formation water (PFW) that is stored in specially constructed double lined ponds. The ponds also store legacy brine from desalination of PFW from previous exploration activities. Santos is seeking to treat this water for beneficial reuse in irrigation, dust suppression, construction and drilling, and emergency firefighting purposes.
Santos have outsourced this requirement to Osmoflo under a hybridized build, own and operate model, which includes the design and delivery of a water and brine treatment plant (WBTP), and specialist operation and maintenance services for 3 years initially, with a possible extension to 5 years.
The WBTP is containerized to allow demobilization and future possible re-deployment, and to find an appropriate balance between the often competing drivers of the projects technical objectives, and the commercial requirements of delayed and minimized capital expenditure. Key features include:
  • Provide 1 MLD irrespective of the very wide range of water qualities and chemistries, in order to maximize disposal opportunities via irrigation
  • Concentrate the PFW salinity in excess of typical brackish and seawater desalination system in order to provide maximum 'ullage' in the fixed volume ponds.
  • Dewater the solids from filter backwashing for off-site disposal
  • Strict environmental performance requirements for noise levels and absolute containment of all PFW, permeate and brine.
The PFW and legacy brine has a wide salinity range of 18,000 to 51,000 mg/L and algae counts up to 8.8 million cells/mL. These need to be desalinated to produce irrigation quality treated water of salinity <600 mg/L. Osmoflo has selected disc filtration followed by a microfiltration (MF) system as pretreatment prior to desalination through reverse osmosis (RO). The disc filters and MF have proven successful in treating other CSG produced formation water as well as water with high algal counts. The RO system has been designed with a two stage configuration with an interstage booster pump to allow variable water recovery from 55 to 70 per cent depending on the PFW salinity. The brine from the RO system is further concentrated through Osmoflo’s Brine Squeezer (OBS) process which is an innovative, cost-effective, ultra-high recovery RO process. The Brine Squeezer process is designed to concentrate the brine to a salinity >90,000 mg/L in order to minimize the brine volumes. The final concentrated brine will be stored in the ponds.

Project Solution

Full Performance Risk Contract
The tailored commercial structure is a first of its kind for the CSG industry. It has been centered around a fit-for-purpose WBTP, where the design, construction and operation must comply with all applicable Australian Standards, good water industry practice and any related project approvals from the Government. The payment structure has two principle components:
1) Operating Lease for the Facility (off-balance sheet as Osmoflo retains ownership and risk over the term). This payment has ‘fixed’ charges under the operational lease and for the O&M services, and a ‘variable’ charge based on actual throughput.
2) Lump-sum component for the Associated WTP Infrastructure – which is mainly the fixed or buried infrastructure. Osmoflo constructs and owns the Associated WTP Infrastructure until Practical Completion date and then transfers ownership to Santos on payment of the lump-sum fee.

Water and Brine Treatment Plant
Pond Transfer and Feedwater
Each pond will have a containerized pump station to transfer the water from the pond to the WBTP. The container will be bunded internally so that any leakage or spillage from flanged connections and pumps will discharge back into the pond. The container also consists of the pond return pipework to allow discharge of washwater or final brine (via manual valve selection) to each cell from the WBTP.
The HDPE pump suction line extends to 27-29 m into the cell from the pond embankment and is attached with polyethylene pipe floats to allow it to float with the pond water level. At the end of the suction pipe the intake strainer drops 1 m below the surface to minimize algae concentration in the raw water. Similarly, the return pipeline extends approximately 136 m into the cell and floats on the pond surface.
The HDPE pipework from the pond to the WBTP bunded area will be PN16 (>6 times more than the maximum operating pressure) and there will be no flanged connections or air release connections in order to mitigate risks of leakage to the environment.

Site Facility
The WBTP site (refer to Figure 1) will be located on a new earthworks pad and the treatment process areas will be fully bunded to contain any spillage or leakage of PFW, brine or chemicals and allow controlled discharge of rain water. The bunding is achieved with geosynthetic clay liners on top of compacted sandy clay and is at 800 mm height at the top level of the bund. The bund surface will have a rubble finish. The chemical storage and loading area on the southern side of the bund will be sealed with 40 mm asphalt surfacing with heavy duty binder. Gradient within the bund is towards closed sumps so that any overland flow, including rain water, can be tested and confirmed to be harmless before release.
Water storage tanks within the bund will be heavy duty polyethylene tanks with multiple modes of tank level detection to minimize the risk of overflow. These include tank level transmitter with high level shutdown interlock, high level switch in the tank and a high level switch in a sealed (from rain) drum where the tank overflow is directed into.
The majority of equipment is containerized, which will minimize on site construction. All containers will have drip drains where all drain and sample lines are directed into. The drip drains will be piped to sumps to prevent uncontrolled spillage into the bund.
Other major infrastructure outside of the bund includes generators and a treated water storage tank that is a 5,000 kL bolted modular panel tank with plastic liner. Power generation is an important part of WBTP as power is a major part of the operating costs. It has been designed with three generators. The 1400 kW generator is a containerized reciprocating gas engine that supplies all the necessary power with spare capacity. It uses fuel gas from Santos’ CSG operations to significantly reduce the operating costs of the WBTP. The 656 kW and 240 kW diesel generators are backup generators.


Figure 1 - WBTP Layout

Water Treatment
Osmoflo's extensive experience of membrane filtration and RO treatment of CSG produced water was a key to ensuring robust and flexible treatment design with high level of automation. The high algae counts in the pond water ensure the need for a robust pretreatment process and could be compared to the issues of dealing with algal blooms in seawater RO desalination. Laboratory analyses showed that the algae are of small particle size mostly <150 m. The legacy brine had the highest counts (up to 8.8 million cells/mL), the PFW in the pond has high counts (mostly <1 million cells/mL) and the PFW water in the pipeline feeding the ponds had the lowest counts (<20,000 cells/mL).
Osmoflo wanted to keep the treatment processes simple including pretreatment and decided to reduce the algae counts of the feedwater by blending the sources of available water. This facilitated selection of membrane filtration as pretreatment without the need for more extensive pretreatment such as dissolved air flotation. Pall microfiltration (MF) equipment was selected as it had a track record in treating similar produced water and that it could treat up to 400,000 cells/mL of algae at reasonable membrane fluxes. Arkal disc filters were selected to be coarse filtration for the MF as they had also had a track record. Pretreatment design was finalized to incorporate two trains of 100 m disc filters operating at lower velocities (with ability to reduce the velocity by 25 per cent by installing more pods) followed by MF operating at <40 L/m2/h flux.
The high algae counts with the high organic levels combined with the fact that CSG produced water tended to have high biofouling potential meant the pretreatment design needed to include for disinfection. A provision for shock dosing of three different biocides has been included to mitigate biological fouling in the RO process. These include isothiazolinone, Kuriverter IK-110 (Kurita Industries) and DBNPA (2,2-dibromo-3-nitrilopropionamide) and ensures that the ‘bugs’ do not become tolerant to one particular biocide. In addition, manual shock chlorination of the pond transfer pipelines to disinfection these pipeline has been provisioned.
The feedwater blending also has the benefit of reducing the TDS variation between the PFW and legacy brine. The RO system, called Primary RO (PRO) is designed as a two stage concentrator with interstage booster to allow flexibility in treating the variable feed water TDS and temperature and ensure the highest recovery possible while distributing the membrane flux properly to minimize fouling. The PRO recovery will vary between 55 and 70 per cent for the blended feed water. The PRO system consists of two trains for some level of redundancy similar to the pretreatment.
Treated water from the PRO (called PRO permeate), blended with the permeate from a polishing brackish water RO (BWRO) system that desalinates the Brine Squeezer permeate is dosed with calcium chloride. The calcium chloride increases the calcium in the blended permeate and ensures the sodium adsorption ratio (SAR) or sodicity is <5 and suitable for irrigation.

Brine Treatment
It is expected that the PRO will produce a brine TDS of 70,000 mg/L while contractual obligations require >90,000 mg/L for the final brine TDS. One of Osmoflo’s key strength in securing this contract was the use of the in-house developed and patented Brine Squeezer process to reduce the RO brine reject volumes cost effectively. It has been demonstrated in two applications to increase the overall plant recovery 92-95 per cent from 70-75 per cent resulting in only 5-8 per cent brine to dispose of. The Brine Squeezer (OBS) process incorporates many existing and available equipment and learned techniques to manage and mitigate fouling and scaling of increasingly concentrated brine streams - with some innovative features and techniques including:
  • Use of unique combination of membrane properties
  • Use of sacrificial coating (when required) to manage fouling and scaling
  • Use of innovative cleaning strategies to effectively clean foulants/scales
Other simple features to manage fouling and scaling include operation at high crossflows, low fluxes and higher ionic strength.

Figure 2 - Schematic of the Brine Squeezer Process

The OBS is expected to produce a brine concentrate 100,000-120,000 mg/L and in blending with the small amount of PFW or legacy brine remaining in the ponds the final brine TDS of >90,000 mg/L is able to be achieved. The higher final brine TDS allows for more cost-effective further concentration and salt recovery as part of the CSG industry’s vision for beneficial reuse of salts and solids disposal offsite.
The OBS system has the highest level of automation, intelligently adjusts operating parameters for the higher fouling and scaling conditions and automatically cleans the membrane. Frequent CIPs are expected and the membranes are used as a semi-consumable as they are operating outside the normal boundary of RO. The innovative hot water CIP technique on the OBS will easily remove biofouling, organic fouling and some of the scaling. The unique sacrificial coating will only be used to enhance scale removal and reduce CIP chemicals.

Solids Dewatering
DAF and centrifuge were selected for solids dewatering due to proven tracking records in algae laden water and flexibility in handling variations in flows and solids concentration. The jar testing correlated that the use of filtered feedwater (same ionic species as the PFW or legacy brine) for backwashing the MF (which forms the majority of the solids to be dewatered) will cost >$200,000 per month in chemicals in the dewatering process. This was costly and a more cost-effective design of using the permeate from the OBS for MF backwashing was developed. The significant lower TDS and alkalinity of the OBS permeate are expected to significant reduce the chemical consumption from extensive field experience.
The DAF design incorporates a conservative detention time of approximately 30 mins for coagulation to ensure adequate reaction time since experience indicated algae are one of the more difficult types of solids to coagulate. The hydraulic loading of the whole DAF has been sized conservatively so that there is a 40 per cent contingency over the normal feed flow.
The centrifuge has also been sized conservatively to treat the DAF sludge under 16 hours and incorporates an automatic greasing which will reduce maintenance frequency and effort. It is designed to produce a sludge cake of 18-20 per cent dry solids that is suitable for disposal off site via trucking.

Conclusions
Santos' Narrabri Gas Project requires treatment of PFW and legacy brine stored in ponds at Leewood. Through open communications and a partnership approach, a custom hybridized build, own and operate model has been agreed to manage produced water at Leewood for 3 years initially, with a possible extension to 5 years. This commercial solution is a first for the CSG industry and will deliver a win-win outcome for both Santos and Osmoflo.
Osmoflo have developed robust and cost-effective technical solutions for the WBTP that have taken into account the technical requirements of the feed water constituents, final brine concentration and solids removal. These include:
  • Pond pump stations and associated pipework to mitigate pond liner damage and provide for leakage containment.
  • WBTP is fully bunded with any potential spillage from tank overflows and container drain adequately managed in order to mitigate uncontrolled discharge of produced water, brine or chemicals.
  • Single stage membrane pretreatment and Primary RO is designed as the "workhorse" of the treatment process that is robust and flexible in managing variations in the highly variable feedwater
  • Innovative Brine Squeezer that has state-of-the-art fouling and scaling mitigation strategies to concentrate the brine in excess of 90,000 mg/L.s
  • Conservative sizing of the solids dewatering system to produce spadeable sludge for offsite disposal