Continuous Sand Filters Used in Major Saudi Irrigation Project

In Dhahran now, 100 percent of wastewater is treated to a tertiary level and is suitable for irrigation.

By Chris Taylor

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

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 percent 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 (mgd) 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 mgd. 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 mgd 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 mgd 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 chlorine gas used in both plants with sodium hypochlorite.

In Dhahran now, 100 percent 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 filtration requirements were met by Nordic Water Products (formerly Waterlink) and that company's DynaSand continuous sand filter. The filters have been used in more than 8,000 installations over 10 years. They have no moving parts, which simplifies maintenance, and they do 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.

The DynaSand filter is based on the counterflow principle. The water to be treated is admitted through the inlet distributor (1) 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 (2) at the top. The sand containing the entrapped impurities is conveyed from the tapered bottom section of the unit (3), by means of an air-lift pump (4), to the sand washer (5) 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 (6), in which it is washed by a small flow of clean water. The impurities are discharged through the wash water outlet (7), while the grains of clean sand (which are heavier) are retained to the sand bed (8). 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 at the Advanced Wastewater Treatment Plant, each having six square meters 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 meters per hour, with maximum capacity some 17 percent beyond that.

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