Deep Sewerage Tunnel Ventilation & Odor Control Design and Operational Experiences in Hot Climate Conditions

Several countries in the Middle East have experienced accelerated growth since the mid-1970's when their current sewerage systems were designed. The Abu Dhabi Sewerage Services Company (ADSSC) is the service provider for sewerage services throughout the Emirate of Abu Dhabi. Many of its existing sewerage infrastructure assets were overloaded and reaching the end of their intended life. ADSSC embarked on a major capital improvement program (CIP). One of the key components of the CIP was the construction of a deep tunnel which includes approximately 40 kilometers (km), 25 miles of deep tunnel sewer (with internal diameters of 4.0, 5.0 and 5.5 meters; which equal 13, 16 and 18 feet, respectively), as well as over 50 km (30 miles) of micro-tunnelled link sewers, a large new main tunnel pumping station at the downstream end of the deep tunnel sewer (largest PS in the world) and the decommissioning of 35 existing sewerage pumping stations. One of the key challenges faced by the implementation of the deep tunnel was related to tunnel ventilation and odor control. With the proposed tunnel route passing through some of the premier developed areas, the directive from higher authorities was clear; no odors and limited odor control facilities would be permitted. To accomplish the stated goal, a unique approach of several (regional) odor control systems were designed and installed along the new deep tunnel and at the downstream tunnel pumping station. Proper sizing, appropriate treatment technology, careful siting, and effective operation were required to ensure the deep tunnel, drop structures, and link sewers were well ventilated and maintained at a negative pressure under all normal operating conditions and that emissions were controlled to prevent offsite odor impacts. By so doing, odors would be contained and removed, helping assure ADSSC's goal of no odors would be achieved. Each regional odor control system consists of multiple extraction fans, first stage biotowers, a treated effluent water feed system, and final stage carbon polishing units. The regional odor control systems were 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. Given the size and complexity of this massive investment, the project design team elected to proceed with a physical model study during the design stage. The model study aimed to validate and refine certain design concepts associated with the drop shaft structures with an emphasis on hydraulic performance and the proposed odor control approach. In addition, CFD modelling of vortices and sewer hydraulics was performed to optimize the design. Moreover, liquid and headspace sampling of the sewer network was undertaken during the design stage as well as during the first years of operation when the Deep Tunnel Sewer was slowly taken online. A detailed analysis study was conducted a few years after initial start-up and commissioning for the purpose of quantifying performance and identifying areas of improvement. To address the challenges associated with analyzing the complexities associated with the existing tunnel and link sewers, a sewer process computer model was completed for simulating the various operating conditions and gaining understanding of possible root causes for some odor generation and fugitive emissions. The results of all measurements and the modelling were documented and analyzed, and some key findings from the study included: The main odor hotspots were identified:

Inadequate ventilation from the deep tunnel due to operational issues at the regional systems

Highly septic sewage entering the deep tunnel at several locations

Surcharging of local network pipe in some areas

Best fit solutions for each odor hotspot provided

This presentation will share some key lessons learned from (a) the design methodology during the detail design phase, (b) the commissioning during start-up phase, and (c) the first few years of operation.