Sunday, September 18, 2022

Data Centers will Increase Salinity in the Occoquan

Salt contamination to drinking water sources is an area of emerging concern. There are currently no restrictions on the salt content in water sent to UOSA nor are there hard limits on salt content for drinking water. As usual research is just being undertaken to identify all the sources of the problem as salt contamination to the Occoquan Reservoir has reached the taste level.

The salinity in the reservoir has been rising over time and several times a year has reached the level that can be tasted.  The rising salt in the reservoir is primarily from watershed runoff during wet weather and reclaimed water from UOSA during dry weather. Sodium concentration in the UOSA water  is higher than in outflow from the watersheds and will continue to rise with the increase in blowdown water from data center cooling. However, increasing paved areas will also increases the salt content in the storm runoff in the watershed.

Increased development in the Bull Run and Occoquan watershed as outlined in the PW Digital Gateway CPA just approved last week by our PW Planning Commission  will increase paved surfaces and runoff and decrease forested and agricultural land. The result will be to increase runoff and increase salinity and chemical and sediment contamination flowing into the runoff. When generally open rural area is developed stormwater runoff increases in quantity and velocity washing away stream banks, flooding roads and buildings carrying fertilizers, oil and grease, and road salt to the Occoquan Reservoir.

Data centers will also increase the salinity in the wastewater discharged to the Occoquan Reservoir. Data centers consume water directly for cooling.  A lot of water. According to a report from Dr. Venkatesh Uddameri of the Water Resource Center at Texas Tech, the typical data center uses about 3-5 million gallons of water per day. That is about as much water as 30,000-50,000 people.

To keep operating data centers need to keep their equipment cool and most commonly use cooling towers to reject heat from the computer equipment. Cooling towers reject heat by transferring heat from the water source to the atmosphere through evaporation and sensible heat transfer. As coolant water evaporates, total dissolved solids (TDS), salts and scaling ions accumulate in the cooling system, resulting in the need to ‘blow down’ the remaining water and replace it with freshwater. Water sources that are used in cooling towers often contains salts (such as chlorine, sulphates and carbonates and metal ions (such as iron and manganese) and these are concentrated through evaporation. In addition, water softeners using brine (salt water) solutions are used to prevent mineral scale from building up.

The only way to remove salt from the drinking water supply is to invest billions of dollars (from your water rates) in building and installing desalination equipment in the region’s water treatment plants which are not currently capable of removing salt from the source water. There is no other source of water to supply our area. The costs to add treatment lines at Fairfax Water to keep the Occoquan Potable is estimated to cost between $1 and $2 billion. This is a cost that will be borne by the water rate payers including the 350,000 in Prince William County.

In Summary, there are two ways that data center will increase the salt in the Occoquan:

  1. Pavement.  In winter all that pavement in parking lots around backup generators and roads will be sprayed with brine solution or salted and increase the salt content in the runoff.
  2. Cooling. Cooling evaporates some of the water concentrating the salt that is already in the water. In addition to increase the efficiency and life of cooling equipment they soften it (which means they add brine, salt water). The minerals and salt build up in the cooling tower and are blown out. The cooling tower blow down contains high salt levels and is sent to the wastewater treatment plant which has no ability to remove salt.


To read more about data center and water and rising salinity in the Occoquan Reservoir:

Will data centers imperil drinking water in Northern Virginia? | Pollution & Solutions |

Salt in a Data Centre (

Salt in fresh water sources becoming worrisome in D.C. region, experts say - The Washington Post

Data center water usage remains hidden - DCD (

(13) Water Considerations for Data Center Site Development | LinkedIn

Data Center Cooling | Saltworks Technologies

(Saltworks Technologies is the name of the company that sells water treatments for the cooling towers. Obvious right?)

The staggering ecological impacts of computation and the cloud - MIT Schwarzman College of Computing

The Problem of Salt in Wastewater - The Salt Miner

Continuous Monitoring and Partial Water Softening for Cooling Tower Water Treatment (

Freshwater salt pollution threatens ecosystem and human water security (


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