The place where every drop of water counts

In the middle of the desert in Riyadh - Saudi Arabia, where water is as rare as diamond, a mile deep, and very salty, our client wanted to produce 5,650 m3/d irrigation and potable water from his 5,750 m3/d limited well water source. This sounded like mission impossible and I remember watching a couple of the engineers listening to the conversation where puzzled and started looking at each other mystified. To add to the confusion, the client wanted to recover all of the filter backwash water, floor drains, and spent regeneration chemicals so the discharge from the plant is put to an absolute minimum. The client presented a water analysis that on its own would scare the most daring adventurer. The water has it all, hardness, silica, iron, suspended solids, and above all boiling hot!

Image. Reverse Osmosis

This sounded crazy enough for our design engineers to relax, recline the chair and have a cup of coffee. There should be some way to make this possible; after all, if a client wants to pay for something, then it can be done.

Looking around within the client facility, we found an existing RO plant producing 1,550 m3/d out of 2,200 m3/d, 650 m3/d from that plant are lost as brine and filter backwash, but not anymore. We wanted back these 650 m3/d, we can always make a use out of it.

So with this 650 m3/d, we had 6,400 m3/d to work with.

First, the well water was directed to a set of cooling towers, the cooling tower will serve dual purposes, it will cool the water down and saturate the water with oxygen, so it will be easily oxidized. Now we can see a smile on the face of our client, since he wont need to worry about throwing expensive chemical down the drain to oxidize the iron.

Two-day retention storage and equalization tanks where constructed to collect the well water allow amble time for all iron to oxidize and the water to cool. Water is then brought a flash-mixing chamber followed by a slow mixing chamber. Accurate amounts of Flocculant and coagulant are metered into the water, so they help form just big enough flocs to be captured by subsequent media filtration. The media filters to capture these solids and iron particles were a piece of art on their own, the filters were designed to minimize the backwash flow requirements, sub surface air scouring header and laterals were designed to reduce the backwash interval and lost b/w water. Special wedge-wire nozzles were used with airflow equalization and distribution metering orifice. The filters yielded an exceptional water quality, where the use of downstream micron filtration was not absolutely necessary. The water is then passed to a set of low-pressure RO plants recovering 75% of the feed water.

The brine form the LP- RO units is sent to the real work horse of the plant. A 3,000 m3 equalization tank was constructed to receive along with the brine, other sources of wastewater from filters backwash drain, chemicals cleaning and regeneration, and floor drains.

Two Ultra High Lime and Soda Ash (UHL-SA) clarifiers in series were constructed to treat all of this waste soup. In the first clarifier, pH is raised to 11.2 by the addition of lime, this converts all original water bicarbonates to carbonate where it combine with original water calcium carbonate hardness which settles out of solution, the excess hydroxide group from the lime is combined with free magnesium ions forming a net of magnesium hydroxide, that also precipitates out of solution. The magnesium hydroxide has a gelatinous structure that adsorbs silica ions and lowers the effluent silica levels.

Between the two clarifiers CO2 gas is added, the CO2 combines with the excess lime present in the effluent of the 1st clarifier, lowering the pH and precipitating further amounts of calcium carbonate. The 2nd clarifier acts as a polishing stage settling more of the suspended solids and lower the load on the subsequent multimedia filtration. The multimedia filters downstream of the UHL-SA were also designed with the same state of art technology as of the main stream with the additional high solids retention capacity and deeper media depth.

The filtered water downstream of the media filters contained low level of calcium and magnesium hardness, low silica content and virtually composed of Sodium, chloride, and sulfate salts. Alkalinity was carefully controlled so no scaling will form in the high pressure RO (HPRO).

The HPRO was designed with special high rejection membranes running at high pressure. The chemical composition of the feed water allowed for 75% recovery of the waste stream.

Two waste streams coming from the plant were sent to different locations, the brine was sent to an evaporation pond, circulation pumps and vaporizing nozzles were used to assist in evaporation rates keeping the area requirements for evaporation to a minimum. The lime slurry was sent to sludge drying beds. Decanted water from the lime slurry is reclaimed back and put into the system for further recovery.

The plant was successfully commissioned and put in service. Fine-tuning was done to reduce the lime slurry by up to 20% through reducing the lime and adding caustic instead. Part of the slurry was recycled back to the inlet of the clarifiers providing seeds for precipitate particles to form on improving the solids capacity and minimizing solids carryover.

By the end of the project, the client was able to yield 7,200 m3/d from total inlet of 7,950 m3/d.

The scarcity of water has pushed the envelop for the water engineering to the limits and in countries as dry as Saudi Arabia this pose the maximum challenge for engineers to keep coming with innovative and effective solutions.