A design brief for treating cooling tower blowdown in palm-oil mills lays out how to neutralize pH and strip hardness, silica, metals and salts so the stream can enter POME (palm oil mill effluent) treatment or be reused — and why that matters under Indonesia’s discharge rules.
Industry: Palm_Oil | Process: Cooling_Systems
Cooling tower blowdown may be just 10–20% of a cooling circuit’s flow, but it carries a concentrated load of salts and treatment chemicals that rules out direct discharge (researchgate.net). One EPA-cited “Water A” case logged about 1320 mg/L Na⁺, 400 mg/L Ca²⁺, 400 mg/L Mg²⁺, 208 mg/L Cl⁻ and 5000 mg/L sulfate, pushing total dissolved solids (TDS, the sum of dissolved salts) into the 7–8 g/L range (nepis.epa.gov). Blowdown pH often sits around ~8–9 due to corrosion inhibitors or alkalis, and it accumulates scale/corrosion inhibitors and other pollutants (researchgate.net).
In Indonesia, mills commonly route blowdown to the POME system, but combined effluent must meet strict standards: BOD₅ (biochemical oxygen demand, 5 days) ≤ 250 mg/L, COD (chemical oxygen demand) ≤ 500 mg/L, TSS (total suspended solids) ≤ 300 mg/L, oil & grease ≤ 30 mg/L, ammonia ≤ 20 mg/L, and pH 6–9 (mail.saka.co.id). Since blowdown is mostly inorganic with low BOD/COD, the practical objectives are clear: neutralize pH and remove hardness (Ca/Mg), silica, heavy metals and any residual treatment chemicals before sending it to POME or discharge.
Priority removals and design targets
Hardness (Ca²⁺, Mg²⁺) drives scale and disrupts downstream processes. Lime–soda or caustic softening routinely precipitates over 95% of divalent ions (researchgate.net). Where ion exchange fits better, a sodium-cycle softener using softening resins offers a compact alternative.
Silica is a notorious fouler. Electrocoagulation (EC, an electrochemical process using sacrificial Al or Fe electrodes) can remove about ~99.5% of dissolved silica; one review reports 0.4–0.5 moles SiO₂ removed per mole of Fe/Al dosed (researchgate.net; researchgate.net). Conventional coagulation with aluminum or iron salts (e.g., alum, ferric chloride) is also effective, and many plants standardize on liquid coagulants for operational simplicity.
Heavy metals (Fe, Cu, Zn, Ni) often arrive via corrosion. Raising pH to ~8–9 precipitates insoluble hydroxides; adding coagulant and filtering polishes to discharge levels often below 0.5 mg/L. One bench case cut copper from ~250 ppb (0.25 mg/L) to <15 ppb with proper pretreatment and an ultrafiltration (UF) barrier (scribd.com). As a membrane pretreatment and a physical barrier to fines, UF is a common safeguard.
Suspended solids and colloids are removed by clarification and media filtration. Clarifiers routinely knock out >90% of TSS, and ballasted sand systems (e.g., Actiflo) can achieve <30 mg/L TSS quickly. A gravity unit such as a clarifier, aided by flocculants, is typical before filters.
Organic residues from inhibitors are low but not zero. Granular activated carbon (GAC) or biologically active carbon can remove >90% of TOC (total organic carbon) when required (researchgate.net). A downstream activated carbon filter is an inexpensive polish.
Salinity (TDS/chlorides/sulfates) is the pivot for reuse. Reverse osmosis (RO, a pressure-driven membrane process) or electrodialysis (ED, an ion-selective electrical separation) can strip bulk salts; literature reports ED removing >90–95% of major ions, including ~96% sulfate and 93–95% Ca/Na (researchgate.net). For lower-pressure hardness control, nanofiltration is often paired with RO, while brackish streams align with brackish-water RO.
Example process configuration and controls
Screening and pre-filtration come first. Mills that draw from rivers can carry debris into cooling circuits, so a coarse screen or bag filter prevents nuisance solids from hitting chemistry steps; packaged lines for waste‑water physical separation are common.
pH adjustment follows. Where blowdown runs alkaline (typical), dosing acid such as sulfuric or injecting CO₂ to pH ~8.5 sets up precipitation. If an acidic slug arrives after a cleaning event, caustic or lime raises pH back toward neutral. Inline control favors metered addition via a dosing pump to maintain a tight setpoint.
Chemical softening and precipitation do the heavy lifting. Lime (Ca(OH)₂) and soda ash (Na₂CO₃), or simply NaOH, precipitate CaCO₃ and Mg(OH)₂ while also dropping Fe, Al, Zn, Cu as hydroxides; the target is >95% hardness removal and >90% metal precipitation (researchgate.net). If inhibitor carryover is resilient (e.g., phosphonates), periodic acid dosing or ozone pre‑oxidation helps. As an alternative or polish, sodium-cycle ion exchange with softener units can trim residual hardness.
Clarification and flocculation settle the precipitate. A gravity basin or lamella pack settles CaCO₃ and metal hydroxides; ballasted floc (sand plus coagulant) accelerates to <30 mg/L TSS. Compact modules such as a lamella settler fit tight mill footprints, while polymer aids from flocculant lines sharpen settling.
Neutralization returns the train to biological- and discharge-safe pH. After softening, effluent often sits at pH ~10–11 and is re‑acidified to ~7 by CO₂ absorption or dilute acid, meeting the pH 6–9 corridor mandated in Indonesia (mail.saka.co.id).
Filtration and adsorption polish the effluent. Dual‑media beds — a silica layer such as sand under anthracite — sweep remaining fines, while a downstream GAC filter adsorbs residual organics and color. Where membranes follow, ultrafiltration (UF) is standard pretreatment to protect RO from colloids.
Advanced desalting is optional and goal-driven. A single‑stage RO typically recovers 50–60% of water (ide-tech.com), so two‑pass designs or RO+ED stacks are used when higher recovery is needed. In systems modeled after IDE’s MAX H₂O Desalter, the RO concentrate is looped to an integrated salt precipitation unit to crystallize sparingly soluble salts, achieving >95% recovery in practice (ide-tech.com). When simplicity is paramount, limiting RO recovery to ~70% still slashes blowdown volume. For hardness trimming at lower pressures, NF often precedes RO. If disinfection is specified, a final UV stage such as an ultraviolet unit inactivates microbes without chemicals.
For completeness, enhanced make-up and blowdown reuse trains have also piloted biologically active carbon followed by UF and RO, delivering 80–120 μS/cm conductivity water (scribd.com).
Measured performance and effluent quality
Silica: EC with Al or Fe electrodes removes ~99.5% (researchgate.net). Hardness: acid/base softening removes >95% Ca/Mg (researchgate.net). Heavy metals: precipitation typically leaves <0.1–0.5 mg/L as soluble metal; bench tests cut Cu from 0.25 mg/L (~250 ppb) to <15 ppb (scribd.com).
Solids and oil: flocculation plus filtration reduce TSS below 30 mg/L and oil/grease to <10 mg/L, meeting POME-facility expectations (often <50 mg/L TSS). TDS: ED can strip 90–96% of major ions (e.g., 96% sulfate, ~94% Ca/Na) (researchgate.net). In a pilot, BAC+UF+RO produced ~80 μS/cm conductivity and ~70 μg/L TOC (scribd.com). Even without full membrane polishing, pairing softening with UF/activated carbon yields near drinking-water quality (~120 μS/cm, TOC <0.5 mg/L). Final neutralization ensures pH 6–9 as required in Indonesia (mail.saka.co.id).
Water recovery, compliance and footprint
Recycling treated blowdown back into the cooling system reduces fresh intake and disposal — IDE cites >93% water recovery in practice and describes how “recycling the treated blowdown back into the cooling tower…dramatically reducing the consumption of both blowdown and makeup water” (ide-tech.com; ide-tech.com). In one configuration, integrated salt precipitation in the RO concentrate achieved >95% recovery (ide-tech.com), and overall programs report up to 94% water recovery (ide-tech.com).
Beyond recovery percentages, there’s a measurable footprint effect: Müller et al. found that recycling cooling blowdown (instead of fresh makeup) cut the cooling system’s water footprint by ~13% (sciencedirect.com). For a mill processing hundreds of tonnes of fruit daily, even a 13% water savings translates to millions of liters conserved per day. Related discussions on palm oil’s water footprint appear in Kospa et al. (2017) (researchgate.net).
All of this sits atop a simple compliance anchor: deliver neutral pH and low minerals so combined effluent meets BOD₅ ≤ 250 mg/L, COD ≤ 500 mg/L, TSS ≤ 300 mg/L, oil & grease ≤ 30 mg/L, ammonia ≤ 20 mg/L, and pH 6–9 (mail.saka.co.id). With a front end built around screens, dosage control, precipitation and clarification — supported by membrane-ready pretreatment and the right desalting step — the concentrated 10–20% of flow becomes a compliant stream or even a new make‑up source (physical separation; chemical dosing; UF pretreatment; RO/NF/UF systems).
Source data and examples referenced: composition and pH ranges from US EPA and peer‑reviewed reviews (nepis.epa.gov; researchgate.net), silica removal by EC (researchgate.net), metals and UF polishing (scribd.com), membrane pilot performance (scribd.com), and IDE’s blowdown management cases and recovery figures (ide-tech.com; ide-tech.com).
