When Safe Water Tastes Bad: How Cities Strip Out Earthy, Sulfur, and “Pool” Flavors

Indonesia’s health rules say potable water must be clear, colorless, tasteless and odorless (“tidak berasa, tidak berbau”) (text-id.123dok.com). Yet a national survey (Riskesdas 2013) found only about 94.1% of households had “good” water (no turbidity, color, taste or odor), while 2.6% reported a noticeable taste and 1.4% an odor in their drinking water (text-id.123dok.com).

That matters because consumers reject water well below health-based limits when it tastes or smells off, the WHO notes (www.ncbi.nlm.nih.gov). Chlorine taste, for instance, is detectable far under toxic thresholds (www.ncbi.nlm.nih.gov). Utilities understand the stakes: “Unpleasant tastes or odors undermine consumer confidence” even when water is microbiologically safe (www.ncbi.nlm.nih.gov) (www.americancityandcounty.com).

Trace compounds that trigger complaints

Many taste-and-odor drivers register at parts per trillion (ppt) or low parts per billion (ppb), well below safety benchmarks but above human detection thresholds (www.gmcnetwork.com) (www.ncbi.nlm.nih.gov). Operators monitor the odor threshold number (TON, a sensory dilution rating) or use trained panels to pinpoint nuisance levels.

• Geosmin and 2‑Methylisoborneol (MIB): earthy/musty, typically from cyanobacteria or Actinobacteria; odor thresholds ≈4–10 ng/L (www.gmcnetwork.com).
• Hydrogen sulfide (H₂S): “rotten egg” smell; human odor threshold ~0.05–0.5 mg/L H₂S.
• Chlorine/chloramine: “swimming‑pool” notes; detectable well below regulatory limits (www.ncbi.nlm.nih.gov).
• Metals (Fe, Mn, Cu): metallic tastes, red/brown staining; iron often objectionable above ~0.3 mg/L (www.ncbi.nlm.nih.gov).
• Organics and disinfection byproducts (DBPs): phenolic or solvent-like tastes; many VOCs (volatile organic compounds) have low µg/L thresholds (e.g., toluene) (www.ncbi.nlm.nih.gov).

Other culprits include decaying vegetation (humic acids), algal pigments, and distribution issues like biofilms or nitrification. The upshot: utilities must diagnose the molecule before choosing the remedy.

Aeration and air stripping (volatile removal)

Aeration exposes water to air to strip volatile gases. It is very effective for H₂S, methane, radon and many VOCs, with 70–90% H₂S removal typical; performance is most effective below ~2 mg/L H₂S and often followed by a granular activated carbon polish (extension.uga.edu). Forced‑draft or packed towers routinely cut high radon or H₂S by 90–99% before filtration or disinfectant addition; radon can drop >99% via cascade aerators. Aeration also oxidizes iron, manganese and CO₂ (raising pH) at moderate efficiencies. It is not effective for non‑volatile tastes like geosmin/MIB or for inorganic salts.

Post‑stripping polishing with granular activated carbon can remove trace H₂S that remains (extension.uga.edu).

Activated carbon adsorption (PAC/GAC)

Activated carbon—powdered (PAC) or granular (GAC)—adsorbs a wide range of organics. Utilities report >90% geosmin/MIB removal with PAC in the low mg/L range, while GAC can remove >95% across many thousands of bed volumes (dependent on water quality) (www.ncbi.nlm.nih.gov). PAC doses of 1–10 mg/L are common during algal episodes; GAC filters are often sized for 10–20 minutes of empty‑bed contact time (EBCT, a filter sizing parameter). Activated carbon also removes many VOCs, DBP precursors, pesticides and phenolic compounds (www.americancityandcounty.com).

Limitations include eventual saturation and competition from natural organic matter. Even so, powdered activated carbon additions are a fast, targeted response to algal events, especially when paired with accurate feed from a chemical dosing pump, while GAC filters provide continuous control.

Advanced oxidation processes (ozone and UV‑peroxide)

Advanced oxidation processes (AOPs, radical‑based oxidation methods) such as ozone, O₃/H₂O₂ (peroxone), UV/H₂O₂ and UV‑chlorine generate hydroxyl radicals that rapidly degrade odorants. Ozone alone is a strong oxidant; AOPs intensify radical formation. In one full‑scale reservoir study with MIB, geosmin and chlorinated ethers, a pre‑ozone dose of 1.0 mg/L removed <30% of MIB (and <20% of certain halogenated compounds). Increasing to 1.5 mg/L downstream ozone pushed all odorous compounds below human thresholds; 1.0 mg/L ozone followed by GAC completely eliminated the odor problem (enveurope.springeropen.com).

AOPs also reduce bromide‑based DBPs (disinfection byproducts) because they consume bromide, though operators must manage byproducts like bromate from ozonation and control residual H₂O₂ or aldehydes. For UV‑based schemes, pairing process design with UV systems is a common route within the AOP toolbox referenced above.

Operational levers in distribution systems

Beyond treatment trains, operators use biological filtration (biologically‑active carbon) to biodegrade some odorants, and alternative oxidants (chlorine dioxide, permanganate) for specific organics. Disinfectant strategy matters: switching from free chlorine to chloramine or chlorine dioxide can alleviate chlorine taste/odor, and finished‑water aeration can strip residual chlorine or chloramines. In distribution, regular flushing and maintaining disinfectant residuals help control biofilm‑related tastes and nitrification (www.ncbi.nlm.nih.gov).

Treatment matchups by contaminant

• Geosmin/MIB (earthy/musty, ppt levels): PAC/GAC adsorption; ozone/AOP; enhanced coagulation plus filtration; algal source control. Effective use cases include PAC pulses and ozone‑GAC trains (www.ncbi.nlm.nih.gov). Deploy PAC during bloom events or route flows through GAC media.
• Hydrogen sulfide (rotten egg, ≈0.1–1 mg/L): Aeration/air stripping; strong oxidants (chlorine, permanganate, ozone or H₂O₂); GAC polishing; source aeration (extension.uga.edu).
• Iron/manganese (metallic/rusty): Aeration and filtration; green‑sand filters; oxidation with chlorine/ozone plus sedimentation. Where appropriate, media such as green‑sand support Mn/Fe removal.
• Chlorine/chloramine (chlorine flavor): Adjust disinfectant dosing and contact time; convert to chloramine; activated carbon to remove residual (www.ncbi.nlm.nih.gov). A final pass through GAC can polish residuals.
• Phenols/industrial organics (medicinal/chemical): GAC/PAC adsorption; ozone/AOP; source protection. Dosing PAC provides fast response; pairing with AOPs helps degrade recalcitrants (www.americancityandcounty.com).
• Hardness/sulfate (bitter, salty): Blending or dilution; softening (ion exchange) above taste thresholds (typically >200–300 mg/L hardness). Options include ion‑exchange softeners or, for salts, nanofiltration.

WHO guidance specifically notes H₂S tastes can be removed by aeration, activated carbon or oxidation (www.ncbi.nlm.nih.gov). The strongest programs combine methods—pre‑ozonation plus GAC, or PAC with oxidation. An engineering review lists PAC, GAC, ozone and AOP among remedies for “the most difficult T&O problems” (www.americancityandcounty.com).

Troubleshooting guide for customer complaints

• Earthy/musty: Typically geosmin/MIB after warm weather or rain. Check taste panels or GC analysis and algal counts. Solutions include adding PAC (several mg/L) or routing through GAC filters; ozone or peroxone ahead of filtration also works. O₃/GAC trains often reduce 2‑MIB/geosmin below human thresholds (enveurope.springeropen.com). Utilities sometimes issue brief notices clarifying these algal compounds are harmless while treatment proceeds.
• Chlorine taste/odor: Indicates high free chlorine residual. Check free and total chlorine; if free Cl₂ >0.5–1.0 mg/L, review dose and contact time. Consider conversion to chloramine or chlorine dioxide; small GAC polish or extended clearwell aeration can reduce residual. WHO frames disinfectant odor control as an operational fix (dose/precursor management) (www.ncbi.nlm.nih.gov).
• Rotten egg/sulfur: Classic H₂S. Check sulfide/sulfate and anaerobic conditions. Aeration is first‑line—design to strip ~90% H₂S at <2 mg/L; follow with GAC to remove traces (extension.uga.edu). Chemical oxidation (chlorine or permanganate) converts H₂S to sulfate; well shock chlorination can suppress sulfur bacteria. In networks, hyperchlorinate and flush if H₂S appears from decaying chloramine.
• Metallic taste/red water: Check Fe, Mn, Cu at taps; inspect for corrosion. Use aeration plus filtration or green‑sand filters; sequester iron with polyphosphate; lower pH or add orthophosphate to curb copper leaching. For high‑metal sources, softening or ion exchange may be required.
• Chemical or fuel‑like odor: Possible industrial/petroleum contamination. Sample VOCs (benzene, toluene, xylene). If confirmed, apply GAC and locate the leak; consider distribution flush and public notices (boil or do‑not‑drink) until resolved.
• Bitter or salty: Often sulfate or chloride (e.g., seawater intrusion). Check conductivity/chloride/sulfate. If exceeding taste guidelines (e.g., >250 mg/L chloride or sulfate), blend sources or remove salts via nanofiltration or use a softening step for high hardness (>300–400 mg/L as CaCO₃).
• Musty/plastic/medicinal (phenolic): Potential phenols or chlorophenols. Confirm by lab tests. Use PAC or GAC; ozonation oxidizes many phenols. Reduce chlorophenols by removing organic precursors via coagulation or carbon.

Proactive planning is essential. As one engineering review put it: “Nothing elicits consumer complaints faster than bad‑tasting or foul‑smelling water.” Utilities should monitor raw water, train operators, and maintain spare PAC/GAC stocks or ozone capacity—before the calls start (www.americancityandcounty.com) (www.americancityandcounty.com). That approach sustains compliance with Indonesian standards (“tidak berasa, tidak berbau,” text-id.123dok.com) and international guidance.