Al Wathba 2 and Allahamah. Wastewater Treatment Plants

Transcription

1 Al Wathba 2 and Allahamah Wastewater Treatment Plants

2 Treatment Capacity Flow: Al Wathba 2: 300,000 m³/day (Tolerance flow: +15%) Allahamah: 130,000 m³/day (Tolerance flow: +15%) Population Equivalent: Al Wathba 2 (Abu Dhabi): 1 million Allahamah (Al Ain): 650,000 Challenge... The Abu Dhabi Emirate is the largest of the seven emirates in the United Arab Emirates. It covers 90% of the nation s total land area. The main cities of the Abu Dhabi Emirate are the capital Abu Dhabi and Al Ain. Abu Dhabi has one of the fastest growing population and economies in the world. As a consequence of the rapid growth, the Emirate faces two major challenges in the sewage sector: (1) to provide sufficient treatment capacity to treat the municipal sewage, and (2) to meet the increasing water demand for industry and agriculture. The Abu Dhabi Economic Vision 2030 establishes a longterm strategy for achieving economic growth and highest environmental standards, taking into account the growing sewerage and recycled water infrastructure demands in Abu Dhabi and Al Ain. QATAR Abu Dhabi SAUDI ARABIA UNITED ARAB EMIRATES IRAN Al Ain OMAN

3 Al Wathba 2 Plant Solution... Abu Dhabi Sewerage Services Company (ADSSC) was established in June 2005 to collect and treat sewage discharged from all residential and commercial customers in the Emirate of Abu Dhabi, distribute the recycled water to consumers and safely dispose of the bio-solids. In order to meet the increasing demand for sewage treatment capacities, ADSSC entered into a sewage treatment agreement with a Special Purpose Company (SPC) for the procurement of two Sewage Treatment Plants (STPs) with a capacity of 300,000 m 3 /d (Al Wathba 2 Plant) for Abu Dhabi and 130,000 m 3 /d (Allahamah Plant) for Al Ain. Both STPs have been developed under a Build, Operate, Own, and Transfer (BOOT) scheme. The project includes the finance, design, engineering, procurement, testing, commissioning, and operation for a period of 25 years of both STPs. The Abu Dhabi Water and Electricity Authority (ADWEA) is the main shareholder of the SPC. ADWEA holds 60% of the SPC s shares. The remaining 40% of the shares are held by a consortium of Veolia Water and Besix. Both Veolia Water and Besix are global market leaders in the wastewater business and have provided worldclass services for several decades. The sewage is being treated in both STPs to very high standards, in order to allow for a 100% utilization of the recycled water for irrigation or industrial purposes. The utilization of recycled water saves millions of cubic metres of desalinated potable water. The SPC is fully committed to protect the environment at all times. Therefore, an environmental management system was implemented to monitor environmental impacts throughout the lifecycle of the project including the construction phase. Strong emphasis has been laid on minimizing the carbon footprint and optimizing the electricity consumption of the STPs. More technical solutions to further reduce the carbon footprint are being studied (reuse of biogas and biosolids).

4 The sewage to the Al Wathba 2 STP is conveyed by a 1.6 m diameter pipeline. The length of the pipeline, which runs from the Mafraq STP to the Al Wathba 2 STP is 14 km. It reaches the Al Wathba 2 STP at a depth of 18 m below ground level. Any large objects in the wastewater are removed in the inlet chamber. Coarse screens then further extract all objects greater than 50 mm in size. WATER STREAM Inlet Pumping Station Submersible pumps in the inlet pumping station 1 lift the sewage approx. 23 m into the headwork from where the sewage gravitates through the STP. Preliminary treatment At the headworks the sewage flows through fine screens. The fine screens remove all particles with a size larger than 6 mm. From there the sewage is delivered to grit and grease removal tanks 2, where sand, grit, stones and broken glass settle down in the tanks. Bottom scrapers collect the settled matter and deliver it to containers for disposal. Surface skimmers remove oil and grease from the sewage. tanks from where it is pumped to the sludge treatment building 9. Secondary treatment Secondary treatment consists of removing the majority of BOD 5 and ammonia from the wastewater. It comprises two processes: activated sludge process 4 and clarification 6. Activated sludge process The activated sludge process is a biological process that utilizes microorganisms to convert organic and certain inorganic matter from wastewater into cell mass. It aims at removing dissolved organic matter and nitrogen from the sewage. This is achieved by the use of specific microorganisms which operate in sequential aerobic and anoxic conditions to eat up the organic matter in the wastewater. Powerful turbo blowers inject air in fine bubbles at the bottom of the activated sludge tanks to expedite the biological process by providing oxygen to the microorganisms. The wastewater is then passed to the de-aerator/ splitter tank 5, where it is degasified by removing the air bubbles. From there the wastewater is delivered to the clarifiers. Primary treatment Once the sewage has passed the preliminary treatment, it is delivered to three primary settlers 3, which remove most of the suspended solids from the sewage. For this purpose, Veolia Water s Multiflo TM treatment system is used. This patented system takes advantage of a low counter current flow along inclined plates called lamellas which results in an optimised settlement process that also provides a self cleaning effect on the lamellas. Mechanically driven scrapers continually move the collected sludge towards a hopper in the base of the Clarification Secondary clarifiers help to separate the solids from the liquid phase of the mixed liquor, and remove biological sludge from the floor. The biological sludge is collected at the bottom of the clarifiers and re-circulated back to the inlet of the activated sludge tanks. This stabilizes the concentration of the microorganisms in the activated sludge process.

5 Al Wathba 2 plant s wastewater treatment process: water, sludge and odour streams Tertiary treatment The tertiary treatment is the last wastewater treatment process within the STP. It ensures that the wastewater is treated so that it complies with regulatory standards for the use of treated effluent for irrigation purposes. The tertiary treatment includes two steps: filtration and disinfection 7. Dual media filter (pumice stone and sand) are used for the filtration process. They remove any residual suspended solids with a size greater than 10 µm. The filtered treated effluent is then disinfected by injecting sodium hypochlorite. The sodium hypochlorite is generated onsite by electrochlorination units 8 from a mix of salt and water. Once the wastewater has passed through all treatment stages, it meets regulatory standards and can be considered as recycled. The recycled water is then pumped to a reservoir with a capacity of 50,000 m 3 from where it is distributed to consumers to be used as water for irrigation purposes in Abu Dhabi.

6 The same treatment processes as described for the Al Wathba 2 STP applies also to the Allahamah STP. Main Water parameters (after treatment) Parameters Contractual Value (mg/l) Value achieved (mg/l) BOD Suspended Solids 10 5 Ammonia Nitrogen 2 0.5

7 SLUDGE STREAM The sludge line handles the following: (1) primary sludge from the primary settlers (2) scum from the de-aerator/splitter and the clarifiers and (3) excess biological sludge from the clarifiers. Thickening Primary sludge (coming from the primary settlers) and excess biological sludge (coming from the clarifiers) undergo a thickening process. The water content of the sludge is reduced by adding polymers to create flocculated particles and applying rotating strength in the drum thickeners 9. The thickened sludge is then pumped to the Digesters. Anaerobic digestion Dewatering From the secondary digester, digested sludge passes through centrifuges 9 in order to achieve a dry solids content of 24%. Polymer is injected to the sludge upstream of the centrifuges to create flocculated particles and improve drainage. Drying The dewatered sludge is transported by trucks to the open sludge solar drying beds 12. These are equipped with mechanical equipment which moves back and forward to aerate the sludge and speed up the drying process. Subject to the weather condition, this process takes around 30 days. At the end of this process the dry solids content is more than 85%. ODOUR STREAM This step aims at stabilizing the sludge. Volatile organic matter is reduced by utilising anaerobic bacteria in the absence of oxygen. This is a mesophilic digestion process in which the sludge is fermented in digesters at a temperature of approx. 36 C and mechanically homogenized. This process lasts for a minimum period of 15 days to ensure optimal reduction of the volatile organic matter. The biogas that is generated has a high proportion of methane and is partly used to heat the digester during the cooler season. The sludge from the primary digesters 10 gravitates to the secondary digesters 11. They serve as buffer tanks and further remove volatile organic matter and degas the sludge before it is dewatered. Nitrogenous and sulphurous compounds cause odorous nuisance and have to be treated. The hydrogen sulphide gas (H 2 S) is the main odorous gas to be treated in the STPs. The fouled air is collected from each structure where H 2 S is generated ( 1, 2, 3, 4 and 9 ) and conveyed through ventilation pipes to the odour treatment building 13. The odour treatment consists of both biological and chemical treatments. The fouled air flows from the bottom to the top of each biological and chemical scrubber to be treated. Chemical treatment consists of spraying consecutively acid and alkaline solutions (a mix of soda and bleach) to remove nitrogenous and sulphurous compounds.

8 Al Wathba Veolia Besix Waste Water-PJSC P.O. Box Abu Dhabi, United Arab Emirates Tel: +971(0) Website: Photo credits: Veolia Photo Library/Stéphane Lavoué; Al Wathba Veolia Besix Waste Water Company Photo Library/Philippe Paulissen Conception and realisation: Avec des Mots - Illustrations: Comillus/Jean-Luc Guérin. December 2012