On 23 January, more than 300 local and international consultants, contractors, subcontractors and suppliers attended a project briefing in Doha. The presentations were not for upcoming high-profile projects such as the football stadiums needed to stage Fifa’s 2022 World Cup, or even Doha’s metro network and new expressway programme. Instead, Public Works Authority (Ashghal) officials briefed companies on the Inner Doha Resewerage Implementation Strategy (Idris).

Tendering of the QR10bn ($2.72bn) scheme will start this year, and as one of the largest single infrastructure projects planned in Qatar, it has captured the interest of regional and international contractors.

Drainage overhaul in Doha

The scheme will radically overhaul the way sewage is managed in the capital. The city currently has three main drainage catchment areas: Doha South; Doha West; and Doha North. Additionally, there are two smaller sub-catchment areas covering Lusail, which is within the Doha North catchment area, and Doha Industrial Area, which is in the Doha South catchment area. There are also three operational main sewage treatment plants: Doha West; Doha South; and Industrial Area.

The Idris scheme covers the Doha South catchment area and will solve some of Qatar’s most pressing infrastructure problems. It will stop the existing foul sewerage network, which is overloaded and often results in street flooding, and allow for key components of the sewerage network reaching the end of their life to be replaced.

The scheme involves building a series of deep tunnels that will serve the Doha South catchment area for the next 50 years and will cater for the extensive planned redevelopment work. The concept is not a new one. Similar schemes have been built in Hong Kong and Singapore. Closer to home, the Strategic Tunnel Enhancement Programme is currently under construction in Abu Dhabi.

The main components of Doha’s Idris scheme include lateral interceptors and a main trunk sewer together with a terminal pumping station for transporting effluent beneath Doha to a New Doha South sewage treatment plant.

Once treated, the effluent will then be transported to any end-user via a treated sewage effluent return system. Once complete, up to 500,000 cubic metres a day (cm/d) of high-quality treated sewage effluent (TSE) will be produced that can be used for irrigation and other purposes.

The new gravity-based system means many of the existing pumping stations and sewage treatment plants in the Doha South catchment area can be decommissioned. More than 30 ageing pump stations are expected to be removed from service and the number rises to nearly 60 when combined with other Ashghal projects in Doha.

The project involves digging and building over 30 kilometres of trunk sewer and 76km of lateral interceptor sewers. The rest of the construction work involves building a terminal pump station, the New Doha South sewage treatment plant and 92km of TSE return facilities, together with the decommissioning of 37 pump stations in the Doha South area.

The main trunk lines will be about 40km long with internal diameters of 3-4.5 metres. Tunnel boring machines will be used to dig the lines at depths of 24-59m, with an average depth of 43m. They will have a structural lining with a corrosion protection coating, and will be divided into three main segments: north; central; and south.

The interceptors will have internal diameters ranging from 40 centimetres to 2.4m, and will be installed at depths of 4-40m below the surface. The average depth will be 23m. They will be dug using micro-tunnelling techniques. Geographically, the interceptors will be divided into three main areas: central Doha; Al-Wakrah; and Mesaieed Industrial City.

Divided pumping station

The terminal pumping station will be built 70m underground and will pump 12 cubic metres of effluent a second at full capacity. It will meet the mid-term requirements for the Doha South catchment area and will be designed to accommodate the installation of an adjacent station in the future.

The pumping station will be a fully divided facility to enhance reliability and allow for periodic redundancy for maintenance and repair. It will also include an odour extraction and treatment system, together with an emergency power generation system.

The New Doha South sewage treatment plant will have a capacity of 500,000 cm/d. The site has been designed for further expansions that will take the final capacity up to about 1.5 million cm/d. Effluent will undergo several treatment processes at the plant, including screening and grit removal, nutrient removal and high-level suspended solids reduction, disinfection and sludge processing.

The TSE return facilities will include a pump station with a 700,000-cm/d capacity that will move treated effluent along pipelines from the Doha West and existing Doha South sewage treatment plants to farms in the west of Qatar. Unlike the tunnels, the pipelines will be installed near the surface using cut and cover techniques.

US-based CH2M Hill was appointed in late 2011 to manage the overall scheme and prepare the concept and preliminary design.

Six packages

The actual construction work involves such a large scale that it will be completed in a series of six packages, some of which will be split further into individual contracts. They are:

Package 1: Three design and build contracts for the lateral interceptor sewers

Package 2: Three design and build contracts for the main trunk sewer

Package 3: One design, build, operate contract for the terminal pump station

Package 4: One design, build, operate contract for the New Doha South sewage treatment plant

Package 5: Three design and build contracts for the TSE return systems

Package 6: Two contracts for the decommissioning of the pumping stations

This year will be a busy 12 months for the project. The first contracts are expected be tendered and awarded in 2013, with construction on the project expected to start in mid-2014. Completion is planned for the end of 2018. By then, the construction sector should know all about the Idris scheme.