Using Membrane Bio-Reactor (MBR) technology helped Dubai Sports City widen the scope for wastewater re-use within the development
The wastewater plant tour ended inside the Reverse Osmosis (RO) room with Majd Hamdallah, Head of Operations, Eagle Electromechanical Company heading towards his colleague, who held in his hands a large glass tumbler filled with clear water. About time, I murmured to myself, as the long traipse through the plant's innards had left me with a gnawing thirst.
Taking the tumbler in one hand, Hamdallah gestured at its contents with the other and said, "We treat a portion of the treated sewage effluent (TSE) to drinking water quality that meets the Dubai Electricity and Water Authority (DEWA) standards by using RO." He then brought it up and proceeded to take a few swigs from it.
Hamdallah greeted my incredulous look with a big grin. "There is no better way to prove a point than walking the talk," he said, which in this case, was how the superior product output of Membrane Bioreactor (MBR) technology increased the scope for wastewater re-use than conventional treatment methods. From somewhere, a voice whispered, "It's your turn now."
This exchange had taken place inside the premises of the largest operating membrane bio-reactor (MBR) wastewater treatment plant (WWTP) in the Middle East, designed, built and operated by Eagle Electromechanical Company. The 25,000 m3/day WWTP, located in the Dubai Sports City development in Dubailand, stands out from its peers in several ways. It also represents a milestone in engineering excellence for the company, which has always practised localisation and designed its own wastewater treatment systems instead of relying on outside designs, as intended by Mohamed Hijaz, Eagle's founder and General Manager.
"All the things you are going to see inside the plant today were designed in-house by our engineers," stressed a visibly proud Hamdallah at the beginning of the tour. The Dubai Sports City WWTP takes Eagle's wastewater operating capacity in the region to over 100,000m3/day in Dubai alone.
Key highlights
A major highlight of the Dubai Sport City WWTP is that it is completely enclosed, while occupying an incredibly small footprint of 3,200 square metres for its capacity. Eagle designed the plant in a manner which makes it inconspicuous in terms of appearance noise as well as odour because it is based next to a sensitive location. "The sports academy is located only 60 metres away from the plant site. Hence, the client's brief was strict - the WWTP shouldn't be visual eyesore nor should there be any complaint about odour," explained Hamdallah. Another highlight is the RO system where a portion of the TSE (10%) is treated to DEWA drinking water quality standards for use as make-up water in Dubai Sports City's 84,000m3 canal project, also designed by Eagle.
The project's targets played an important role in the choice of MBR technology and RO plant for the Dubai Sports City WWTP. The targets were as follows:
a) Receive and treat raw sewage collected by Dubai Sports City sewerage network
b) Treat rejected canal filtration plant backwash waste stream
c) Polish the TSE supplied by Dubai Municipality
d) Supply TSE to irrigation systems within the development
d) Supply TSE for fire fighting network within the development
e) Supply of RO treated high quality water (800m3/day) to top up the sports canal
"We had to produce high quality TSE for irrigation with BOD less than 5mg/L, TSS less than 5mg/L, ammonia less than 1mg/L (<0.5 mg/L achievable), Total Nitrogen less than 10mg/L (3mg/L achievable in warm climate) and Total Phosphorous less than < 0.5 mg/L (0.1mg/L achievable). We also had to produce DEWA-quality water to top up the sports canal with nil BOD, TN, TP, Ammonia and total dissolved solids (TDS) less than 250-ppm. This sweet water was necessary to control the TDS in the canal water due to surface evaporation losses. After taking into consideration all these requirements as well as the area designated for the WWTP, we felt that MBR technology was the optimal solution," said Hamdallah.
The construction of the Dubai Sports City WWTP started in June 2006 and got completed in April 2009. The total built up area (including the 25,000m3 capacity irrigation tank, roads and pumping stations for irrigation and fire fighting) is 9,000 square metres. The average daily capacity of the plant is 25,000m3, with a maximum daily capacity of 30,000m3. The plant has three streams of 8,600m3/day capacity each.
Blending in
The engineering structure of the WWTP is completely enclosed within a building, ensuring that it architecturally blends in with the surroundings while preventing odour emissions. Efficient utilisation of space was accomplished in two ways - first, by eliminating sedimentary tanks because separation of solids from treated water takes place in the MBR. In conventional activated sludge processes, the same is achieved by sedimentation in the secondary clarifier tank, which takes up
a considerable amount of space.
"MBR eliminates the secondary clarifier and sand filters associated with conventional treatment, although the biological process remains the same. In normal extended aeration, we have 24 hours retention time; for MBR plants, we only need 6-8 hours retention time," said Hamdallah.
Second, flow balancing was conducted within the biological reaction tanks, negating the need for a separate balancing facility. This has the additional advantage of eliminating odour problems as the raw sewage is not retained within tanks upstream. "At Eagle, we prefer to do equalisation for the peak flow inside the process instead of doing it before the pre-treatment to eliminate odour emissions," explained Hamdallah.
Critical pre-treatment
Effective pre-treatment is critical to protect the membranes from damage and to maximise membrane life. Hence, special attention is paid to upstream screening in MBR plants to protect the membranes from the hair and fibrous material that can clog or damage them. The major components of the screening system at the Sports City WWTP are coarse screening, grit removal and fine screening.
The inlet head works at the WWTP has four fine screens (three duty/one standby) with 1-mm omni-directional openings, preceded by three 6-mm coarse screens (two duty/one stand-by) and grit cum grease removal system with grit classifier. "If you neglect pre-treatment, you will end up with more cleaning, more use of chemicals and more maintenance. A well designed screening stage ensure that you don't any bypass or overflow of unscreened wastewater making its way into the biological stage," explained Hamdallah.
During the coarse screening stage, raw sewage is transferred into the inlet area, where two step screens are provided. According to the set points of level sensor, when the sewage reaches a certain level, the step screen starts working and moves large waste particles like plastic sheets and metals to the screw conveyor, which transfers the debris to the compactor works. At the compactor, the liquid is discharged to the drain system while the dried discharge is sent to the discharge bin. Soda ash dosing is done at the inlet channel according to the pH set points for maintaining correct pH level.
The screened sewage from the inlet channel is passed through the grit removal tank, where the constant action of the agitator moves the sand particles downwards. The sand particles are removed by the pumps at the bottom of the tank and transferred to the sand classifier.
After grit screening, the sewage passes to the fine screen area. Hamdallah pointed out that generally for fine screening, hollow fibre manufacturers specify 1-mm omni-directional punch hole screens while flat sheet manufacturers specify 3-mm screens.
"Based on local experience with different projects, we concluded that both hollow fibre and flat sheet membranes require 1-mm punch-hole screens," he said. The screened sewage is transferred to the fine screen chamber (sewage lifting chamber), where three pumps lift it to the inlet chamber where it is distributed to three phases of anoxic tanks. "When water level reaches the first start level of the float switch, the first pump starts pumping; when it reaches the second level, the second pump starts pumping. Three additional float switches have been provided for dry run protection," said Hamdallah.
Inside the bio-reactor
The liquid sewage from inlet channel is transferred to three anoxic tanks, which is provided with three mixers for proper mixing of incoming screened sewage. The anoxic zones provide biological de-nitrification. Each pre-anoxic compartment is equipped with submersible mixers to ensure adequate mixing in each compartment.
The mixed liquid sewage is passed to three oxic tanks (aeration tanks). Aeration is provided in the aerobic lanes by fine bubble diffused aeration grids to achieve nitrification and oxidation of organic matter (BOD5). Adequate oxygen is required for the proper growth of bacteria in the oxic tanks. A group of blowers discharge air into one common process aeration manifold, which delivers air to the drop legs/diffusers installed in the aeration tanks.
The treated sewage then passes to the post-anoxic compartment/ Balancing Tank through overflow opening. "Before the membranes, we have a balancing tank for equalisation, which is also used as a second anoxic tank," said Hamdallah.
The Balancing Tank is equipped with submersible mixers to ensure adequate mixing. Four submersible balancing transfer pumps transfer the mixed liquor from the Balancing Tank to the membrane feed channel. This arrangement allows the biological trains and membrane trains to operate independently.
The mixed liquid from membrane distribution channel passes through membrane trains. (A membrane train is a treatment unit consisting of multiple 'cassettes' that are joined together and connected to a common permeate pump). The membranes at the Sports City WWTP are ZeeWeed Hollow Fibre Ultrafiltration membranes, supplied by GE Water & Process Technologies. The configuration consists of a total of 30 membrane cassettes, five cassettes per train. There six membrane tanks. In the Returned Activated Sludge (RAS) chamber, mixed liquor from each membrane tank overflows a full-width weir at the end of the tank into mixed liquor recirculation.
In the sludge digestion tanks, two decanting arms are provided. Aeration for the tank is provided with the help of coarse bubble diffusers at the bottom of the tanks. Two overflow pipes transfer the overflow liquid back to coarse screen.
"Because we were close to a sensitive area, we preferred aerobic digesters so that we have more control and more stabilised sludge," said Hamdallah. The sludge settles at the bottom of the tanks. It is then pumped to the sludge decanting centrifuge system, where the separation takes place.
Basically, the feed enters a horizontal cylindrical bowl through a stationary inlet tube and is accelerated smoothly by an inlet distributor. The centrifugal force that stems from the rotation causes sedimentation of the solids on the wall of the bowl. The cake leaves the bowl into the casing and the solid discharge is transferred to discharge bin. "The design requirement was to achieve 20% dry solid content but we are actually getting up to 25%," said Hamdallah.
Storage and disinfection
Permeate from the membrane tanks is transferred to the TSE or irrigation tank with the help of six permeate pumps. "We use permeate reversible pumps that not only pumps the permeate feed to the TSE tank, but also drives the back-pulse mode for cleaning the membranes," said Hamdallah. During the automatic back-pulse mode of operation, the membranes are flushed from the inside periodically for a set duration. This ensures that that the surface and pores of the membrane are kept clean, which in turn reduces fouling potential. The water used to back pulse the membranes is the permeate stored in the back-pulse tank.
Chlorine dosing is carried out in the main header when the permeate is being transferred to the irrigation tank for disinfection purposes. The chlorine is properly mixed with the aid of static mixer provided in the header pipe. "As the effluent is stored within irrigation tanks, there is a chance of bacterial re-growth. Thus, in order to inhibit this, residual chlorine is necessary to safeguard the health of people which may come into contact with the irrigation water. We have a higher dose to keep the chlorine content always at a minimum 0.5 mg/L," said Hamdallah. However, the use of MBR also helps reduce the burden on the final disinfection system because the membranes physically disinfect the water by restricting the passage of bacteria and even viruses.
The pump room has a total of four irrigation pumps, two fire pumps, one jockey pump and two booster pumps. "Instead of potable water, we are using the high quality water obtained after RO processing for fire fighting. As per Dubai Civil Defence regulations, we also have diesel generator for powering the pumps if there is a failure," noted Hamdallah.
The treated water in the back pulse tank is treated through RO to remove salts and other impurities. "Because the TSE used in the RO is of a very high quality, we have achieved recovery rate of 84%," claimed Hamdallah. The capacity of the RO plant is 1000m3/day.
High level of automation
Eagle has relied on automation for smooth operation of the WWTP. The irrigation pump starts and stops on the basis of the flow and pressure set- points of the Programmable Logic Controller (PLC). Electrical power for all drives and equipments in the WWTP is provided by MCC panels. In fact, there are three major MCC rooms for the different areas of the WWTP. The MCC panels also control the functionality of the components via programmed PLC system. All operational activities within the WWTP is monitored and logged by two SCADA systems, one dedicated to the membrane unit and other for the entire plant. Hamdallah said, "The high level of automation ensures superior feedback and control, enabling the operators to monitor and manage any changes which may present themselves and respond accordingly. It supplements a robust process which ensures that the MBR generates the same effluent quality, irrespective of flow and load variations."
Dealing with low flow
While walking to the RO plant, Hamdallah pointed out that the WWTP receives 10,000m3/day of raw sewage, 95% of which is supplied by sewage tankers. He said, "Generally, in new developments, the requirement for irrigation is much higher than the produced sewage, which is low due to the initial low occupancy levels. In this case, we had to compensate the difference through tankers."
However, tanker sewage poses its own challenges, like more septic characteristics than normal sewage. "For example, the BOD for normal sewage is 180-200 mg/L, while for tanker sewage, it is around 300-350 mg/L. Therefore, the plant had to deal with stronger sewage than normal, and it has been very successful in this regard," said Hamdallah.
He pointed out that Eagle's focus on indigenous design and knowledge transfer has played a pivotal role in the Sports City plant's success, from operational as well as environmental standpoints, with its aesthetic value a big bonus. "Eagle succeeded in this great challenge of building a large capacity plant inside a closed building on such a small footprint. Moreover, we are providing a high quality product of irrigation and potable water grade, whilst meeting the best practices of odour control, visual impact and sludge treatment." said Hamdallah. He also sought to highlight MBR as the best technology to promote and sustain wastewater re-use in this region, while pointing out Eagle itself has 16 MBR installations in the region, nine of them running. "This is probably the largest number of operational plants in the Middle East," claimed Hamdallah, adding that Eagle is committed to creating a green environment for the present and future generations by designing and operating wastewater treatment plants aimed at waste reduction and water re-use.
We reached the RO plant room, and Hamdallah moved towards his colleague holding a large glass tumbler....
Post-script: As you can see, I decided to walk the talk myself.
By Anoop K Menon
© H2O 2010
