Abu Dhabi's Strategic Tunnel Enhancement Programme: Regional Odour Extraction System Approach

The Emirate of Abu Dhabi has experienced accelerated growth since the mid-1970's when its current sewerage system was designed. The Abu Dhabi Sewerage Services Company (ADSSC) is the service provider for sewerage services throughout the Emirate. Many of its existing sewerage infrastructure assets are overloaded and reaching the end of their intended life. ADSSC has embarked on a major capital improvement program (CIP). One of the key components the CIP is the Strategic Tunnel Enhancement Programme (STEP) which includes approximately 40 km of deep tunnel sewer (with internal diameters of 4.0, 5.0 and 5.5 m), over 50 km of micro-tunnelled link sewers, new main tunnel pumping station at the downstream end of the deep tunnel sewer, and decommissioning of 35 existing sewerage pumping stations.One of the key challenges faced by STEP is related to odour control. With the proposed tunnel route passing through some of the premier planned developments, the directive from higher authorities was clear; no odours and limited odour control facilities would be permitted. To accomplish the stated goal, a unique approach of a single (centralised) odour control system installed at the proposed downstream tunnel pumping station was proposed. Rigorous analysis utilizing computer based models confirmed the viability of this approach. However, other approaches including multiple satellite (localised or regional) odour extraction systems were considered.To better understand entrained air forces at vortex drops, and to confirm the preferred odour extraction approach, a physical modelling study was performed. Tests were conducted for both the centralised and regional extraction concepts. Results of the study completed in November 2011 concluded that a regional odour extraction system approach was preferred over a single (centralised) extraction approach to provide redundancy and provide the necessary additional capacity to maintain a negative pressure within the tunnel under all normal flow conditions.Multiple possibilities exist related to number and location of regional systems. More regional systems are beneficial as they result in greater redundancy, shorter distances for air movement (i.e.; greater zone of influence), and reduced potential for air short-circuiting. However, more regional systems also result in higher initial costs and greater O&M costs. In addition, constrained potential sites and specific sensitive developments greatly limit where and how many regional systems can be provided.Both a two regional systems approach and a three regional systems approach were considered. Each regional system would consist of multiple extraction fans, first stage biotowers, nutrient feed system, and final stage carbon polishing units. Regional systems would be designed to extract air directly out of the tunnel, retaining the concept of allowing drop structures to naturally ventilate link sewers and allowing the regional systems to extract the air collected within the tunnel for treatment. Both capital and operation and maintenance costs were estimated for the two approaches and a qualitative comparison conducted for each. The quantitative and qualitative analysis confirmed that a three regional extraction system approach was recommended with two additional connection points for future expansion.Proper sizing, appropriate treatment technology, careful siting, and effective operation will ensure the deep tunnel, drop structures, and link sewers are well ventilated and maintained at a negative pressure under all normal operating conditions and that emissions are controlled to prevent offsite odour impacts. By so doing, odours will be contained and removed, helping assure ADSSC's goal of no odours can be achieved.