Focus on Aquapolo Ambiental S.A., São Paulo, Brazil
01 September 2011
Aquapolo is the largest wastewater reuse project in the Southern Hemisphere, and the fifth largest of its kind in the world. Upon completion, this facility will free up enough drinking water to continuously supply a population of 350,000 inhabitants, with the potential capacity to reach 600,000.
The need for advanced wastewater treatment and reuse is increasing in all parts of the world as the limited supply of freshwater is being rapidly depleted. This is especially true in the São Paulo region of Brazil, where there is a growing demand for high quality water for industrial processing as well as for clean drinking water.
The Aquapolo Ambiental water reuse venture was created to address new regulations to restrict industrial use of potable water in São Paulo. Aquapolo is the largest wastewater reuse project in the Southern Hemisphere, and the fifth largest of its kind in the world. Upon completion, this groundbreaking facility will free up enough drinking water to continuously supply a population of 350,000 inhabitants, with the potential capacity to reach 600,000.
Membrane bioreactor design has been identified as the most cost-effective solution to upgrade the existing wastewater treatment plant to meet São Paulo’s pressing demand for industrial reuse wastewater. The new Aquapolo facility will employ Koch Membrane Systems’ (KMS) PURON™ MBR technology as well as MegaMagnum® reverse osmosis (RO) membranes.
Construction started in April 2010; the water treatment portion of the project is following an aggressive schedule for start-up in November 2011.
Solving São Paulo’s Water Problem
The State of São Paulo, with close to 41 million people, is the world’s seventh most populous urban area and is considered the economic, financial, and technical hub of Brazil. The region contains nearly one-fourth of the country’s population but less than 2% of Brazil’s water.
Resources for drinking water are becoming increasingly scarce in São Paulo. The state government, recognising the importance of safeguarding drinking water for the region’s inhabitants, recently issued new regulations to restrict the industrial use of potable water, forcing factories to look for ways to reuse their wastewater, or obtain recycled water from another source.
Aquapolo Ambiental was born from a unique business structuring and project financing arrangement known as a Specific Purpose Partnership (SPP) between Foz do Brasil, the engineering division of Odebrecht Group, and Sabesp, a Brazilian state-owned utility that provides water and sewage services for residential, commercial, and industrial use in the municipalities of the state of São Paulo.
The SPP signed a 15-year finance contract with the Brazilian Development Bank (BNDES), a federal public company that provides long-term financing for private infrastructure projects that contribute to the development of the country, such as water and sewer systems. To build this project, the SPP will rely on the expertise of the Brazilian engineering and construction firm Construtora Norberto Odebrecht working under a three year Engineering, Procurement, and Construction contract.
The new treatment plant will be built on the grounds of the ABC Sewage Treatment Plant (ABC STP) of Sabesp, located on the boundary between São Paulo and São Caetano do Sul counties. Effluent from the ABC Plant will be treated and pumped to the Mauá Petrochemical Complex and sold to other factories in the region for use in industrial processes. The initial 2012/2014 phase will produce 56,160 m3/day (650 L/s) of reuse water, eventually reaching a capacity of 86,400 m3/day. Sixty-five percent of the plant output has already been sold under a 34-year contract to Quattor, a petrochemical company located within the Mauá petrochemical complex – Aquapolo’s target market. Other potential users are companies located along the 10-mile steel pipeline being built to carry water to the complex.
Feasibility Study Evaluates Options for Reuse
Water is used in virtually every industrial and manufacturing process. The recycled water used in industry for processes such as cooling, making steam,and cleaning requires removal of particular substances as well as a reduction of suspended solids and conductivity. Several critical discharge parameters of the current ABC STP operation seriously compromised the ability to reuse the existing effluent, including chemical oxygen demand (COD), biochemical oxygen demand (BOD), suspended solids, ammonium, total phosphorus and conductivity. The facility was built in the 1990s and has an installed capacity of 3000 L/s but it only operates at 50% of its design capacity.
KMS conducted a comprehensive feasibility study to identify the most cost effective strategy to upgrade the existing plant infrastructure. This study evaluated the amount of high quality water needed for reuse, which parts of the existing wastewater treatment structure could potentially be used in the upgraded facility, alternative system configurations, and the capital and operating costs of each configuration.
RO is typically the technology of choice for producing the quality of water needed for industrial reuse. RO involves forcing pressurised water through the membrane to remove salts and organic molecules, producing high quality water. In the case of the ABC STP facility, an evaluation of the current effluent quality and future water reuse targets determined that it was not possible to solely add RO directly after secondary clarifiers to meet reuse quality parameters.
The poor performance of the secondary clarifiers, suffering regularly from bulking sludge, was a major potential threat for any RO solution. Even the installation of either sand filtration or direct ultrafiltration (UF) would only move the problem upstream, but never eradicate it. Another point to address was the high ammonia level still present in the secondary effluent from the plant, varying between 10 and 25 mg/L NH4-N. As the current plant was only designed for carbonaceous removal, nitrification was impaired because the sludge retention time was too short, resulting in high ammonia effluent values. RO technology alone will not be able to meet strict reuse criteria of <1 mg/L NH4-N year average. While RO remained an essential part of the treatment process to reduce total dissolved solids and make the water clean enough for reuse, the RO system needed to be coupled with an effective pretreatment system.
It was concluded that an upgrade of the existing biological treatment was necessary. Four different potential upgrade scenarios were evaluated. All four options involved submerged MBR technology. Depending on raw water conductivity, which varies significantly seasonally, the water produced by the MBR could either be reused directly, or be blended with RO permeate to meet the conductivity target level.
The first three scenarios involved retrofitting existing concrete structures. Using this approach requires careful consideration of the current infrastructure and accommodation of future expansion. The fourth option considered was to make use of the existing plant operation and infrastructure and use the effluent of the secondary clarifier as feed water to the new MBR installation. It was clear that the nutrient deficient design of the existing plant would be a challenge biologically, but could be developed as a greenfield enhancement to the existing site operation without direct interaction with the site operation.
For this unique combination of existing conventional biological treatment with a new MBR system, the new MBR system was designated as a Tertiary Membrane Bioreactor (TMBR).
Pilot Study Evaluation
After a careful evaluation of each scenario, the KMS team concluded that adding a TMBR system to the existing plant infrastructure was the best solution for the new Aquapolo facility. In total, the Aquapolo project will encompass the construction of new pretreatment step, installation of chemical dosing system, and the construction of a 700 lps TMBR utilising 94,500 m2 of membrane area. The TMBR offers lower operating costs, lower process risk, greenfield design, and negligible interaction with current plant operations and equipment.
The TMBR is a polishing MBR process. It will treat water coming from the existing secondary clarifiers and further treat it in a newly built biological step using membranes as the final separation process. Some of the TMBR effluent will be further treated by RO membranes to meet conductivity goals for water reuse.
To validate the TMBR approach, a one-year pilot study was conducted to confirm the design parameters, optimise the system operation, and evaluate operating costs. A standard MBR pilot plant was used to exactly simulate the TMBR design concept. The membrane modules used were KMS hollow fibre submerged UF membranes. All flows were according to the feasibility design and Dissolved Oxygen, pH and mixed liquor suspended solids (MLSS) were closely controlled. The pilot was operated between 2 and 6 g/L MLSS and start up and daily plant operation was simulated. The feed flow was controlled to a fixed value and all maintenance cleaning and down time was reflected in the operational flux of the membranes. The pilot plant output was 775 L/h net flow. The biological system design was simulated with a hydraulic retention time of ~3 hours and a sludge retention time of between 12 and 20 days.
A number of operational tools were developed to stabilise the operation and improve the biomass quality. Up until seven months into the pilot study, primary feed was occasionally bypassed to the TMBR and occasional ethanol was dosed to the TMBR. This proved useful to stabilise the biological system, but more importantly to stabilise the nitrifier population and alkalinity balance. After that time, an additional operational tool was added in the form of ferric dosing. This had two advantages: first, the total residual phosphorus was precipitated out to meet the discharge criteria and secondly, the biomass became marginally more flocculated and easier to control for the nutrient removal processes. Using these operational tools, the target COD of 20 mg/L was achieved.
The TMBR pilot demonstrated that the submerged membranes benefited from the optimised biological system. Average flux rates of >25 L/m2h were achieved and daily maintenance cleaning was found to enhance the UF performance. Recovery cleaning returned the UF membrane to the original process permeability.
A standard RO pilot also was used to simulate the RO design concept of the proposed full scale system for 200 L/s feed water. The pilot trials were setup in three well-defined phases, a simple batch mode, a modified batch mode and a continuous mode. The RO system was able to produce an excellent water quality. The final water quality for reuse was achieved via blending the TMBR permeate and the RO permeate.
Together, both phases of this pilot study demonstrated a viable full scale solution for water reuse under difficult operational conditions.
The PURON Modules
KMS was chosen for the Aquapolo project for its technical support and biological design, the availability of MBR and RO pilot plants, its experience with water-recycling projects, and ability to meet an extremely accelerated delivery schedule. KMS scope of supply included sixty-three (63) PURON PSH1500 membrane modules, two MegaMagnum MM9 RO units, system design and controls, and after-market support and service.
The PURON module incorporates the use of a single header with reinforced hollow fibres that are fixed only at the bottom. The sealed upper end of the fibre is allowed to float freely. The free floating tip design eliminates the build-up of hair and fibrous materials that typically clog the upper ends of membrane fibres in MBR module designs that employ both top and bottom headers.
Solids and particulates, including bacteria, are retained by the membrane and remain on the outside, while permeate is drawn through the membrane to the inside of the fibres. The outside-to-inside flow pattern provides optimal solids management and a high flow-rate.
The Aquapolo facility will use the 1,500 m2 PURON module, which was specifically designed for large-scale MBR projects. Features such as an optimised permeate extraction manifold and air supply lines reduce the number of piping connections during installation. In addition to the PURON modules, the large diameter MegaMagnum RO elements will complete the system. Each of the MM9 systems chosen for Aquapolo has a flow capacity of 1.7 MGD/day and 6,500 m3/day. Each MegaMagnum spiral element used for the project contains 3,050 square feet of membrane surface area, compared to 400 square feet in commonly deployed 8-inch products.
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