Why Cali's Water Profile Is Its Own Design Basis
A data center in Cali, Colombia in 2026 needs a treatment train sized for the Cauca valley's warm wet-bulb (~22–24 °C) and harder makeup water, not a generic arid-site design. Cooling-tower blowdown runs 25–30% of makeup at 4 cycles of concentration and must clear Resolución 631/2015 TDS and suspended-solids limits before discharge to sewer under a CVC permiso de vertimientos. The defensible train is side-stream multi-media filtration → hollow-fiber UF → brackish-water RO at 50–70% local recovery → UV or chlorine dioxide on the reuse stream, with a 1–3 month on-site pilot to confirm silica-tolerant antiscalant selection. CAPEX lands at $0.4–0.9M for 5 MW and $0.8–1.6M for 10 MW, with $1.50–3.00/kgal OPEX (Genesis Water Technologies, 2026).
Most published 2026 data-center water guidance defaults to Bogotá, Phoenix, or a generic arid-site brief, and three Cali-specific facts make that guidance non-transferable. First, Cali sits near 1,000 m elevation, so atmospheric pressure is close to sea level — RO specific flux and MVC specific energy from vendor curves are usable with only a small altitude correction, unlike the 10–20% flux derate and 5–15% MVC energy uplift that the Bogotá sibling case requires at 2,640 m / 0.74 atm (HydropureWater, 2026). Second, the annual mean wet-bulb sits in the ~22–24 °C band — warmer than Bogotá's 10–14 °C — so the cooling tower cannot push cycles of concentration as high before silica and CaCO₃ scaling force blowdown. Third, Cauca valley makeup is influenced by Rio Cauca tributary chemistry, which carries a hardness- and silica-bearing profile typical of Andean piedmont sources, although the supplied research does not give Cali-specific TDS, hardness, or silica numbers.
Because that local chemistry is qualitative in the source set, the buyer must run influent tests on Emcali potable supply, any private well, or any planned reclaimed-water makeup before locking in the train. Minimum parameters to characterize are TDS, hardness as CaCO₃, silica, alkalinity, chloride, sulfate, and temperature, plus a 30-day composite to capture dry-season swings. On the supply side, Emcali potable supply tightens during El Niño dry periods, so a 5–20 MW Cali site is an industrial water user on the same network as the city — supply-side risk must be modeled alongside discharge-side risk when the EIA chapter is written, not treated as an operational afterthought.
Cooling-Blowdown Chemistry: What Has to Be Removed Before Discharge or Reuse
At 4 cycles of concentration, blowdown is 25–30% of makeup water volume — the volumetric and concentration problems have to be solved in the same unit operation (Genesis Water Technologies, 2026). For a Cali site on 1,000–1,500 m³/day of makeup, that translates to 250–450 m³/day of blowdown to manage every day the cooling tower is in service, and the dissolved load it carries is what determines the train.
Blowdown TDS runs 1,200–6,000 mg/L, which is 4–8× the makeup value depending on COC and source water; suspended solids sit in the 10–50 mg/L range from corrosion products, biofilm fragments, and airborne particulates; and the dissolved load is dominated by Ca²⁺, Mg²⁺, silica, and bicarbonate alkalinity plus whatever treatment chemicals are being fed to the loop: biocides, scale inhibitors, corrosion inhibitors, dispersants (Genesis Water Technologies, 2026). The cooling-side quality targets the train has to hit are TDS 500–1,500 mg/L, hardness <200–400 mg/L as CaCO₃, suspended solids 10–25 mg/L, bacterial counts <10,000 CFU/mL, pH 6.5–8.5, and alkalinity 50–200 mg/L as CaCO₃ (Genesis Water Technologies, 2026). Municipal secondary effluent is typically 600–1,200 mg/L TDS, which already falls inside the cooling target band and is useful framing if the operator wants to blend with potable makeup (Genesis Water Technologies, 2026).
Legacy chromate and high-phosphate inhibitor programs are a specific compatibility risk: both chemistries poison downstream membranes and trigger Resolución 631 caps on hexavalent chromium and total phosphorus, so the chemical-program audit has to happen before the RO pilot, not after. The table below summarizes the parameter envelope the engineer is designing against.
| Parameter | Blowdown at 4 COC (Cali envelope) | Cooling-side reuse target |
|---|---|---|
| TDS | 1,200–6,000 mg/L | 500–1,500 mg/L |
| Hardness (as CaCO₃) | Driven by makeup + COC | <200–400 mg/L |
| Suspended solids | 10–50 mg/L | 10–25 mg/L |
| Silica | Concentrates with COC | <150 mg/L (RO permeate basis) |
| Bacterial count | Variable | <10,000 CFU/mL |
| pH | 7.0–8.5 typical | 6.5–8.5 |
| Alkalinity (as CaCO₃) | Concentrates with COC | 50–200 mg/L |
Sources: Genesis Water Technologies, 2026; Res. 631/2015 caps confirmed against the local permit envelope.
The 2026 Regulatory Stack in Cali: Resolución 631, CVC Permits, and the Concesión de Aguas
Resolución 631/2015 (Ministerio de Ambiente y Desarrollo Sostenible) is the binding industrial discharge instrument in Colombia, and it sets caps on TDS, total suspended solids, and treatment-chemical residuals that any data-center blowdown train has to clear before the first liter reaches the sewer (HydropureWater, 2026). Decreto 3930/2010 and Decreto 4728/2010 frame the discharge regime to surface water and to sanitary sewer, and they require characterization of the non-domestic load before a permiso de vertimientos is granted — not a self-declared Ficha Ambiental (HydropureWater, 2026).
CVC — Corporación Autónoma Regional del Valle del Cauca — is the regional ambient authority for Cali and the surrounding Valle del Cauca, and the permiso de vertimientos it issues is a binding permit, not a regulatory formality (HydropureWater, 2026). Law 99/1993 and Decreto 2811/1974 cover the concesión de aguas for makeup intake from Emcali, a private well, or a non-conventional source; concession timelines are jurisdiction-specific and the supplied research does not give Valle del Cauca numbers, so the buyer should request the current CVC cronograma before the EIA chapter is locked. If the operator wants to call recovered blowdown "reuse" rather than "internal recycling" for GRI, CDP, and ISSB disclosure, Resolución 1207/2014 and the Viceministerio de Agua Potable guidelines are the right citations to put in the EIA chapter (HydropureWater, 2026).
Approval timelines for water-reuse permits in the cited framework run 6–18 months depending on jurisdiction (Genesis Water Technologies, 2026), and that is the planning buffer the Cali project has to absorb before mechanical completion. The permit sequence in practice runs: (1) influent and effluent characterization, (2) EIA chapter including the permiso de vertimientos application to CVC, (3) concesión de aguas application for makeup, (4) Resolución 1207/2014 alignment if the recovered stream is to be classified as reuse, and (5) operational commissioning under CVC oversight. None of these steps is interchangeable with the CAR Cundinamarca process used in the Bogotá sibling case, so the EIA author needs a Cali-specific cronograma.
The Defensible Four-Stage Train for a 5–20 MW Cali Site
The defensible train for a 5–20 MW Cali site is a four-stage membrane chain with on-site disinfection, sized so each unit operation does one job and the next stage is not punished for the previous one's shortfall — the same design principle the Bogotá article applies for the same chemistry at higher altitude (HydropureWater, 2026). Because Cali is close to sea level, RO and UF vendor curves can be used with only a small altitude correction, which is the design simplification relative to the Andean sibling case.
Stage 1 — a self-cleaning 10–25 µm multi-media filter for RO pretreatment treats 1–5% of circulation flow continuously, drops suspended solids to a level the UF can accept, and lands at $50,000–$200,000 CAPEX for typical data-center flow rates (Genesis Water Technologies, 2026). A PLC-controlled chemical dosing skid upstream keeps coagulant feed steady on variable Cauca-valley raw water.
Stage 2 — a hollow-fiber UF skid at 0.01–0.1 µm pore size delivers 90–95% recovery with no chemical coagulant, removes bacteria, biofilm fragments, and colloidal silica, and protects the RO from biofouling — the dominant failure mode on silica-rich feed. UF systems operate at 10–30 psi and tolerate the high-TDS blowdown without extensive pretreatment; backwash with permeate keeps membranes productive, with chemical cleans every 1–3 months depending on feed (Genesis Water Technologies, 2026).
Stage 3 — an industrial brackish-water RO system at 50–70% local recovery (lower than the 75–80% sea-level norm because silica and CaCO₃ scaling dominate on Cauca-valley water) produces permeate at 10–50 mg/L TDS, suitable for direct return to the cooling-tower basin. Antiscalant must be selected for silica tolerance, and a 1–3 month on-site pilot is the only defensible way to lock in the recovery number (Genesis Water Technologies, 2026).
Stage 4 — disinfection on the reuse stream. A UV sterilizer on the RO permeate line at 40 mJ/cm² controls Legionella and biofilm fragments without producing DBPs, and a chlorine dioxide generator for biofilm control handles biological fouling where biofilm control is the priority. The unit-operation table below is what the RFQ to a HydropureWater-class supplier should be built around.
| Stage | Unit operation | Key spec | Function in the train |
|---|---|---|---|
| 1 | Multi-media filter (10–25 µm) | 1–5% of circulation flow | Drops SS to UF-acceptable level |
| 2 | Hollow-fiber UF | 0.01–0.1 µm, 90–95% recovery, 10–30 psi | Removes bacteria, colloids, biofilm fragments |
| 3 | Brackish-water RO | 50–70% local recovery, 10–50 mg/L permeate TDS | Dissolved solids cut to cooling-tower spec |
| 4 | UV / ClO₂ | UV 40 mJ/cm²; ClO₂ residual per program | Disinfection without DBPs |
Source: Genesis Water Technologies, 2026. For phased buildout over an 18–24 month data-hall fill, see the 2026 commissioning-duration guide and the 2026 UPW loop metals and silica spec guide for tighter permeate targets on adjacent loops.
Cycles of Concentration, Antiscalant Selection, and the Silica-to-Hardness Ratio
Four cycles of concentration is the safe 2026 default on silica-bearing Cauca-valley feed, producing 25–30% blowdown at 1,200–6,000 mg/L TDS (Genesis Water Technologies, 2026). Sites with side-stream filtration holding suspended solids below 15 µm can push to 4–5 COC, but only if a silica-tolerant antiscalant program is in place and the Langelier Saturation Index is monitored on the concentrate, because the saturation index climbs faster at warmer wet-bulb than it does in cooler Andean sites.
The silica-to-hardness ratio — not TDS alone — drives antiscalant selection, the recovery ceiling, and whether the concentrate has to be sent to a thermal stage (Genesis Water Technologies, 2026). On Cauca-valley water, hardness and silica typically rise together because the Rio Cauca tributary carries both, so the operator has to characterize both parameters and feed them into the antiscalant vendor's projection software before the recovery number is locked. The supplied research does not give specific pilot duration or feed-chemistry numbers for a Cauca-valley source, so the on-site pilot is the buyer's checklist: recovery sweep, clean-in-place frequency, antiscalant dose response, and concentrate LSI trend across dry-season and wet-season feed.
Decision Framework: Partial Reuse, High-Recovery CTBD, or Full ZLD
The decision framework is driven by three Cali-specific risk axes: El Niño-driven freshwater availability, capacity at the Emcali potable network, and the discharge limits CVC attaches to the permiso de vertimientos. Default to partial reuse first, escalate only when the permit or the water budget forces it.
Partial reuse at 60–85% overall recovery is the 2026 default for a 5–20 MW Cali site: it cuts freshwater demand and discharge volume at the same time, and it is the configuration Resolución 631/2015 and CVC expect when an industrial operator asks for a multi-year discharge permit (HydropureWater, 2026). High-recovery closed-circuit thermal brine concentration (controlled salt precipitation, ~95% recovery, ~1 mg/L silica permeate) is the right upgrade for sites near the Cauca river recharge zone or the Farallones foothills where concentrate disposal is restricted — framing adapted from the Bogotá sibling case (HydropureWater, 2026). Full ZLD (RO + MVC + crystallizer) at 95–99% overall recovery reaches $3–8M CAPEX and $5–15/kgal OPEX and only pencils at 20 MW when CVC refuses concentrate discharge or El Niño-driven freshwater rationing is the binding constraint (Genesis Water Technologies, 2026). Discharge-only is a stopgap: Resolución 631 TDS caps plus $5–15/kgal direct discharge fees erode the savings within a year (Genesis Water Technologies, 2026).
| Tier | Configuration | Trigger condition | Where it pencils |
|---|---|---|---|
| Discharge-only | Sewer under Res. 631 | Stopgap; permit pending | TDS <1,500 mg/L achievable |
| Partial reuse | Side-stream + UF + RO | Default for 5–20 MW | 60–85% overall recovery |
| High-recovery CTBD | Controlled salt precipitation | Near Cauca recharge, Farallones | ~95% recovery, ~1 mg/L silica permeate |
| Full ZLD | RO + MVC + crystallizer | 20 MW, concentrate refused, freshwater curtailed | 95–99% overall recovery |
The partial-reuse and high-recovery CTBD rows are both anchored on the industrial brackish-water RO system as the workhorse stage; ZLD adds MVC and crystallizer downstream.
2026 CAPEX and OPEX Bands for a Cali Data Center
A 5 MW Cali site on 1,000–1,500 m³/day makeup (250–450 m³/day blowdown) lands at $0.4–0.9M CAPEX and $1.50–3.00/kgal OPEX for a side-stream + UF + RO train at 50–70% local recovery (Genesis Water Technologies, 2026). A 10 MW site roughly doubles the train to $0.8–1.6M CAPEX, with OPEX in the same per-kgal band because fixed costs dilute across more volume. A 20 MW site is where ZLD becomes a real option — full ZLD at $3–8M CAPEX and $5–15/kgal OPEX is only justified when concentrate discharge is refused and freshwater is curtailed during the dry season. Modular UF and RO skids let the operator phase capacity in 1–2 MW increments, which matters when the data hall is built out over 18–24 months. For an Andean-altitude comparison and a more conservative derate, see the Bogotá sibling case; for a Mediterranean-climate reference, see the Ankara 2026 engineering guide.
| Site size | Train | CAPEX (USD) | OPEX (USD/kgal) |
|---|---|---|---|
| 5 MW | Side-stream + UF + RO (50–70% local recovery) | $0.4–0.9M | $1.50–3.00 |
| 10 MW | Side-stream + UF + RO, phased skids | $0.8–1.6M | $1.50–3.00 |
| 20 MW | Side-stream + UF + RO + MVC + crystallizer (ZLD) | $3–8M | $5–15 |
These are screening-grade bands, not EPC tender numbers — adjust for Colombian import duties, IVA, and the Valle del Cauca seismic / wind derating that local buildings require. The partial-reuse bands are anchored on the industrial brackish-water RO system and the hollow-fiber UF skid as the two workhorse skids.
Frequently Asked Questions
What CAPEX and OPEX should a 5–20 MW Cali data center budget for cooling-blowdown treatment in 2026?
A 5 MW Cali site on 1,000–1,500 m³/day makeup lands at $0.4–0.9M CAPEX and $1.50–3.00/kgal OPEX for a side-stream + UF + RO train at 50–70% local recovery; a 10 MW site is $0.8–1.6M CAPEX at the same per-kgal OPEX; a 20 MW ZLD case is $3–8M CAPEX and $5–15/kgal OPEX (Genesis Water Technologies, 2026). These are screening-grade bands, so the buyer must add Colombian import duties, IVA, and Valle del Cauca seismic / wind derating before the steering-committee number.
How do I pick a treatment supplier that can clear both Resolución 631/2015 and the CVC permiso de vertimientos?
Shortlist suppliers who can show prior CVC permiso de vertimientos approvals, not just generic Colombian experience, and who will commit to a 1–3 month on-site pilot to confirm the 50–70% local RO recovery on Cauca-valley feed (Genesis Water Technologies, 2026). Ask for a written compliance matrix mapping each unit operation to the Resolución 631/2015 caps, plus a draft EIA chapter section the operator can hand to the CVC reviewer.
What influent tests are required before the train is locked?
The minimum is TDS, hardness as CaCO₃, silica, alkalinity, chloride, sulfate, and temperature, plus a 30-day composite to capture dry-season swings — the supplied research does not give Cali-specific numbers, so these tests are the buyer's input, not a vendor assumption. Without them, the silica-to-hardness ratio that drives antiscalant selection and the recovery ceiling cannot be confirmed (Genesis Water Technologies, 2026).
Is MVC necessary on a Cali site, or is partial reuse enough?
For most sub-20 MW Cali sites, partial reuse at 60–85% overall recovery is the defensible default and is the configuration Resolución 631/2015 and CVC expect on a multi-year permiso de vertimientos (HydropureWater, 2026). MVC on the RO concentrate at 95–98% recovery should be reserved for sites where CVC refuses concentrate discharge or where El Niño-driven freshwater rationing is the binding constraint, not specified as a default. Because Cali is close to sea level, vendor MVC curves are usable with only a small altitude correction.
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