Continuous sand filters used in major Saudi irrigation project

The Advanced Wastewater Treatment Plant, north of Dhahran, in Saudi Arabia, will be able to recycle 10 mmgd of high quality tertiary water for irrigation purposes, saving approximately 3 billion gallons of precious groundwater every year.

By Chris Taylor

June 5, 2003 -- The Advanced Wastewater Treatment Plant, north of Dhahran, in Saudi Arabia, will be able to recycle 10 mmgd of high quality tertiary water for irrigation purposes, saving approximately 3 billion gallons of precious groundwater every year. Twenty-four DynaSand filters, from Nordic Water Products, were used in this project.

The Kingdom of Saudi Arabia is located in one of the world's most arid regions, yet beneath the surface lie reserves of fresh water; a resource which worldwide is only now being seen as finite, and in need of conservation and protection. In a country where desert temperatures can approach 45 degrees Centigrade (113 degrees Fahrenheit), this concern is even more immediate.

The protection of this life-giving element falls in Dhahran to DUD - Dhahran Utilities Department - which performs a critical function in the distribution of water, the collection of wastewater, and water well maintenance - and in a constant programme of education and support for the principles of water conservation, and to seek ever more effective ways to convince people of the precious value of water.

As part of DUD's continuing campaign to conserve water through reclamation and treatment, last summer the Department celebrated the completion of the expansions of Dhahran North Sewage Treatment Plant (NSTP) and the Advanced Wastewater Treatment Plant (AWTP). Combined, the projects will add 40 per cent to the plants' treatment volume capacity, and will contribute to the annual saving of billions of gallons of groundwater.

The work was undertaken by Saudi Aramco which owns and operates many water reclamation facilities. The NSTP is located in Doha, and the AWTP is located inside the Dhahran community. These two plants serve the Saudi Aramco community as well as Doha, Dana, and King Fahd University of Petroleum and Minerals (KFUPM).

The Dhahran NSTP has been expanded from 8 to 10 million gallons per day (mmgd) to meet anticipated demand by the year 2010; and in line with original plans to increase the recycling of tertiary water, the AWT plant has been expanded from 3 to 10 mmgd. Work at the NST plant included the installation of two additional settling tanks and a bypass overflow line to divert effluent to the emergency pond during heavy rains. About 3 mmgd of the NSTP secondary effluent is further treated at the tertiary level for use in irrigation throughout the community's planted areas.

The expansion activities at the AWT plant included a new 6 mmgd continuous backwash filter, a 1.5 million gallon storage tank, a new irrigation pump station and tertiary distribution piping. The project also replaced the existing hazardous chlorine gas used in both plants with sodium hypochlorite.

With the project completed, the AWT plant will be able to recycle 10 mmgd of high quality tertiary water for irrigation purposes, saving approximately 3 billion gallons of precious groundwater every year. In Dhahran now, 100 per cent of wastewater is treated to a tertiary level and is suitable for irrigation; the effluent pumped out of the NSTP and treated at the secondary level is pumped to the AWTP and treated at the tertiary level.

The greatest use for reclaimed water irrigation is for community landscaping and sports fields. Community landscaping includes parks for family activities, common green areas in housing developments, and roadside strips. In Dhahran, some reclaimed water is also used to water resident's front gardens, and to grow grass turf for use within the community.

Abqaig, al-Hasa and Tanijib communities all have tertiary wastewater treatment facilities, and DUD is currently conducting a study to upgrade Rahima and 'Udhailiyah STPs to a tertiary level. According to Munir M. Rafie, Aramco's executive director, Community, Buildings and Office Services, this is in line with the company's interest in the treatment of sanitary drainage water, because of the company's belief in protecting the health and sanitation of the environment and community. He added that the treatment plants are the most developed in the region, and are operated according to the latest technology
available worldwide.

Throughout the wastewater treatment and effluent re-use processes, the fitration requirements were met by Nordic Water Products (formerly Waterlink) and that company's DynaSand continuous sand filter. Its proven reliability with more than 8,000 installations over 10 years, and no moving parts for maintenance, made it the sound choice for this application. A further key advantage is that the filter does not have to be taken out of operation for backwashing or cleaning: simultaneous with the filtration process, fouled sand is cleaned in a dedicated wash process and the suspended solids discharged with the wash water.

In the DynaSand filter fouled sand is continuously removed from the filter bed, washed and recycled without interruption to the filtration process. The DynaSand filter is based on the counterflow principle.

The water to be treated is admitted through the inlet distributor in the lower section of the unit and is cleaned as it flows upward through the sand bed, prior to discharge through the filtrate outlet at the top.

The sand containing the entrapped impurities is conveyed from the tapered bottom section of the unit, by means of an air-lift pump, to the sand washer at the top. Cleaning of the sand commences in the pump itself, in which particles of dirt are separated from the sand grains by the turbulent mixing action. The contaminated sand spills from the pump outlet into the washer labyrinth, in which it is washed by a small flow of clean water.

The impurities are discharged through the wash water outlet, while the grains of clean sand (which are heavier) are retained to the sand bed. As a result, the bed is in constant downward motion through the unit. Thus, water purification and sand washing both take place continuously, enabling the filter to remain in service without interruption.

Twenty-four individual filter modules were installed, each having six square metres of horizontal surface area, to be installed in six concrete cells. The filters' designed flow capacity is 3,950 gallons per minute, equivalent to 897 cubic metres per hour, with maximum capacity some 17 per cent beyond that.

One of the factors that made this project particularly challenging was that it entailed connecting and installing expansion pipes and equipment inside the plants whilst they were operating at full capacity.

David Barrie is Aramco's Project Engineer for Community Projects Division in Dhahran. He claimed the project's timescale - starting in the fourth quarter of 1999 - had also been very tight, but that the team had pulled together to meet it. "The project's construction period was 19 months, which set a new standard for such projects in Aramco. It was very challenging, but I was fortunate to have an excellent team and a cooperative contractor. After completion, the project was turned over to Dhahran Utilities for operations."


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