There are two sources that provide Riyadh city with potable water. These include desalinated sea water from AI Jubail plants which accounted for about 60%.
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Al-Mutaz,I.S., and Sami O.A. Al-Yousef, "Lime Recovery from Riyadh Water Treatment Plants", The Fourth Gulf Water Conference, Bahrain, Feb. 13-18, 1999 Lime Recovery from Riyadh Water Treatment Plants Ibrahim S. Al-Mutaz 1 and Sarni O.A. Al-Yousef
1) Chemical Eng. Dept., College of Engineering, King Saud University, P 0 Box 800, Riyadh 11421 2) Riyadh Water & Sewage Authority, Riyadh, Saudi Arabia
Abstract There are two sources that provide Riyadh city with potable water. These include desalinated sea water from AI Jubail plants which accounted for about 60%. The remaining 40% comes from the treated brackish water from deep and shallow wells. These plants are operating at close to design capacity and presently operated and maintained by Riyadh Water and Sewage Authority. Massive amount of chemicals are used in these plants. The type and amounts of chemicals often used in water treatment plants basically depend on the ground water quality and chemical characteristics. However, large quantities of lime, Ca(OH)2, are consumed for softening. The objective of the softening process is to prevent scaling of membranes in the RO units. The produced sludge from the softening is drained to the sludge thickener on a daily basis where it should be disposed. Proper sludge disposal is required for safe environment. So sludge must be dewatered before discarding. The dewatered softening sludge contains about 85% calcium carbonate (CaC03) and 12% magnesium hydroxide, (Mg(OH) 2 . The insoluble magnesium hydroxide can be converted into soluble magnesium bicarbonate by carbonation of the sludge with carbon dioxide, C02. The remaining solid will be rich in calcium carbonate which can be easily separated. After recalcining of the carbonated sludge calcium oxide is produced which can be hydrated readily into lime_ This paper will focus on the feasibility of the recovery of lime from Riyadh water treatment plants. Brief description of softening process in Riyadh water treatment plants will be summarized as well as the characteristics of the softening sludge produced from these plants. Processes used for lime production from softening sludge will be reviewed including sludge conditioning and dewatering. The technical description of these processes will be given.
Keywords: Lime, lime recovery, sludge, water treatment.
Introduction Riyadh has five large water treatment plants. These are Shemessy Water Treatment Plant, Malaz Water Treatment Plant, Sulbukh Water Treatment Plant, Manfouha Water Treatment Plant and Buwayb Water Treatment Plant. These plants were built by Degremont of France and use reverse osmosis system with hollow fine fiber and polyamid membrane of B-9 (0840) permeators manufactured by DuPont except Buwayb plant which has a spiral wound configuration and cellulose acetate membrane of CAROGA 81SOS manufactured by UOP[l]. Shallow wells in Wadi Nisah, Wadi Namar and Wadi Hayir and the deep wells ofMinjur formation are the principal sources of raw water in Riyadh. Table 1 shows the typical analysis of raw water after cooling and net water produced by the Riyadh Water Treatment Plants. Clearly large amount of hardness and alkalinity had to be removed as well as the disolved soilds[2]. Table 1: Water analysis Item Temp. pH T. Hardness Ca. Hardness Mg. Hardness Total alk. Cond. TDS Ammonia Nitrite Nitrate Chloride Sulfate Iron Alu. Chlorine Silica
oc
oom as CaC03 oom as CaC03 ppm as CaC03 ppm as CaC03 uS/cm mg/l. as ion mg/Las ion mg/I.as ion mWJ.as ion mg/i.as ion mWJ.as ion mg/Las ion mWJ.as ion mg/I.as ion mg/I.as ion
Raw Water
Net
33 8.0 670 470 200 160 2000 1350
