Softening as a Pretreatment Step
One documented route is to route tower blowdown into a softening plant so it can be blended with raw water and then treated. In the OSTI report, all the tower blowdown from two plants was recycled to a makeup water softening plant where it was blended with raw water and the mixture lime-soda ash softened [S1]. The same report describes a separate installation of a 1000-Gpm softening plant for hot tower blowdown water, indicating that the blowdown stream is sent through a dedicated softening train rather than a combined softener [S1].
The intent of softening blowdown before RO is to reduce the calcium load that would otherwise scale the membranes. The OSTI report records that hot, 100% tower blowdown water (450 ppm Ca as CaCO3) was softened to 50 ppm at room temperature in a laboratory evaluation [S1]. The previous maximum of 300 ppm calcium as CaCO3 in the tower water was the only firm maximum value established during that test program [S1]. Corrosion test coupons were installed in the largest cooling tower water system to evaluate any change in corrosion rates when the softened blowdown was returned to the loop [S1]. These figures are reported as project observations, not as a universal specification for every site [S1].
Membrane-Based Blowdown Recovery
A second documented approach combines partial softening with a membrane-based blowdown recovery (BDR) array. NREL describes a BDR system provided by Aqualogix as a customized retrofit of a BDR onto an existing partial water softening (PWS) system, with a membrane array installed on the tower loop [S2]. The BDR configuration included a break tank, a secondary blowdown valve, and an existing blowdown valve taken out of service, so the recovery side of the system could intercept the concentrated stream before it went to drain [S2].
The evaluation was designed to test the manufacturer's claim that this BDR system technology will reduce blowdown (the flushing of cooling tower water with high concentrations of minerals) by more than 45%, will reduce water consumption by more than 15%, and will deliver a payback in under 5 years [S2]. The excerpt notes that makeup water is introduced to dilute the remaining solids and chemicals and to replace water lost through blowdown and evaporation, which describes the role of makeup water in the system context [S2]. The excerpt does not give the full permeate or recovery figures, so a universal percent-recovery number for RO on cooling tower blowdown is not established by this evidence [S2].
Adjacent Planning Context
The Indonesian planning study frames the challenge as treating blowdown cooling tower water and reject water from reverse osmosis systems so the combined stream can be returned as cooling tower makeup water, aiming to design a water recycle system that processes blowdown and RO reject water at a non-woven facility in Sidoarjo [S3].
Cascading Within the Plant
The OSTI report describes cascading tower blowdown to the K-31 system to be utilized as firewater or to be blown down to the K-29 system, which is a layout decision rather than a chemistry decision [S1]. The report also notes that the tower blowdown requirement was cascaded to the K-31 system to be utilized as firewater or to be blown down to the K-29 system [S1].
Engineering Decision Factors Supported by the Excerpts
- Hardness control upstream of the membranes. Softening the blowdown before RO is documented, with 450 ppm Ca as CaCO3 softened to 50 ppm at room temperature in a laboratory test [S1].
- Blending with raw water. The blowdown was blended with raw water and the mixture lime-soda ash softened, suggesting a combined train rather than a standalone blowdown softener [S1].
- Integration with partial softening. The BDR array was installed as a customized retrofit onto an existing partial water softening system, with the existing blowdown valve not in service [S2].
- Manufacturer-claimed performance targets under test. The BDR evaluation targeted manufacturer claims of more than 45% blowdown reduction, more than 15% water consumption reduction, and a payback in under 5 years [S2].
- Cascading within the plant. The OSTI report describes cascading tower blowdown to another system to be used as firewater or to be blown down further, which is a layout decision rather than a chemistry decision [S1].
Key Design Questions Left Open by This Evidence
- What is the site-specific calcium, silica, and alkalinity profile of the blowdown that would feed the RO unit?
- Is the softener sized for the blended raw-water-plus-blowdown flow, or for a hot 100% blowdown sidestream [S1]?
- Which BDR or RO array configuration has been demonstrated for the actual data center cooling-water chemistry, given that the referenced BDR evaluation was testing manufacturer claims rather than reporting verified field performance [S2]?