Geosmin, 2-MIB and taste and odour management in drinking-water reservoirs — monitoring thresholds, UK DWS standards, destratification and GAC treatment.
Schmidt stability, thermocline timing, bubble-plume sizing and seasonal operating protocols for drinking-water reservoir destratification.
Reducing trihalomethane and haloacetic acid precursors through reservoir aeration, destratification and enhanced coagulation — UK DWS and WHO compliance.
Aeration and destratification for drinking-water reservoirs — taste and odour control, thermal mixing and DBP precursor control.
Aeration Types & Comparison Guide
Taste and odour events are the leading cause of consumer complaints to UK water companies. Two volatile metabolites account for the majority of incidents: geosmin (trans-1,10-dimethyl-trans-9-decalol), which imparts an earthy/musty smell at concentrations as low as 4 ng/L; and 2-methylisoborneol (2-MIB), responsible for a camphor-like musty note detectable at 5–10 ng/L. Both are produced by cyanobacteria (Anabaena, Oscillatoria, Microcystis, Planktothrix) and by actinomycete bacteria (Streptomyces, Nocardia) in reservoir sediments.
Thermal stratification creates the conditions for both organism groups: a warm, nutrient-enriched epilimnion where cyanobacteria out-compete green algae from June onwards; and an anoxic hypolimnion where actinomycetes in organic sediments produce geosmin independently of any phytoplankton bloom. This means that eliminating a visible cyanobacterial bloom does not guarantee the end of a taste event — sediment-derived geosmin may continue for weeks after bloom collapse.
Key regulatory context: The UK Drinking Water Standards 2018 (SI 2000/3184 as amended) contain no numerical limit for geosmin or 2-MIB. However, Regulation 4 states that water must contain no substances at concentrations that could cause taste or odour preventing its normal use. The DWI enforces this through undertakings and enforcement notices when TON > 3 is sustained. The WHO TGV is 10 ng/L for geosmin and 10 ng/L for 2-MIB (WHO 2022).
Key producers, detection thresholds, and applicable standards
| Compound | Source Organisms | Odour Threshold | Taste Threshold | Treatment Priority |
|---|---|---|---|---|
| Geosmin | Anabaena, Oscillatoria, Planktothrix; Streptomyces (sediment) | 4–6 ng/L (earthy/musty) | 10 ng/L | Destratification; KMnO₄ 0.5–1.0 mg/L; GAC EBCT 7.5 min |
| 2-Methylisoborneol (2-MIB) | Oscillatoria, Pseudanabaena; Streptomyces | 5 ng/L (camphor/musty) | 10 ng/L | Destratification; PAC 10–20 mg/L; O₃/UV-AOP for polishing |
| 2-Isopropyl-3-methoxypyrazine (IPMP) | Anabaena; Phormidium | 2 ng/L (bell pepper) | 1 ng/L | Bloom prevention; GAC; UV-AOP |
| Sulphide (H₂S) | Anaerobic sediment bacteria; SRB | 0.1 µg/L (rotten egg) | 0.05 mg/L | Hypolimnetic aeration; oxidation with Cl₂/O₃ |
| Dimethyl sulphide (DMS) | Microcystis, Phaeocystis | 10 ng/L (sweetcorn) | 25 ng/L | Destratification; break-point chlorination |
Regulatory treatment of geosmin, 2-MIB and cyanotoxins differs between jurisdictions. Our assessments apply the framework of the country of operation — UK, EU, Australian, US, Canadian or WHO guidance as instructed.
| Compound | UK (DWS 2018 / DWI) | EU (DWD 2020/2184) | Australia (ADWG 2022) | USA (EPA SDWA) | WHO (GDWQ 2022) | Canada (GCDWQ) |
|---|---|---|---|---|---|---|
| Geosmin | No PCV. "No taste or odour affecting normal use" (Schedule 1, Part 3). DWI uses TON testing and WHO TGV in practice. | No specific PCV. "Acceptable to consumers and no abnormal change" (DWD Art. 4). | 10 ng/L aesthetic guideline (NHMRC ADWG 2022, Chapter 10). | No federal MCL under SDWA. EPA SDWA Section 1401: "no taste or odour". State drinking water programs may set alert levels. WHO TGV applied in practice. | 10 ng/L TGV (taste/odour threshold of general population). | 10 ng/L aesthetic objective (GCDWQ, Health Canada). |
| 2-Methylisoborneol (2-MIB) | No PCV. TON test threshold. Detectable at 5–10 ng/L by most consumers. | No PCV; organoleptic requirement. | 10 ng/L aesthetic guideline (ADWG 2022). | No federal MCL. WHO TGV applied. Some utilities use internal alerts at 5 ng/L. | 9 ng/L TGV (sensory panel threshold). | 10 ng/L aesthetic objective. |
| Microcystin-LR | 1 µg/L (WHO TGV applied; DWI monitoring requirement; EA/DEFRA blue-green algae guidance). | 1 µg/L PCV (DWD 2020/2184/EU, Annex I Part B; mandatory from Jan 2023). | 1.3 µg/L (ADWG 2022, health guideline). Cylindrospermopsin 1 µg/L. Saxitoxin 3 µg STX eq/L. | 0.3 µg/L HA (infants, children ≤6 yrs, 2019); 1.6 µg/L HA (older children to adults). No federal MCL; several states (e.g. Ohio, New Hampshire) have formal standards. | 1 µg/L TGV (GDWQ 2022). Cylindrospermopsin 0.7 µg/L TGV. Saxitoxin 6 µg STX eq/L TGV. | 1.5 µg/L MAC (Health Canada 2022). Cylindrospermopsin 1 µg/L MAC proposed. |
| Cylindrospermopsin | Monitoring under EA guidance; WHO TGV applied. | Watch-list monitoring under DWD Annex II; TGV applied. | 1 µg/L (ADWG 2022). Produced by Cylindrospermopsis raciborskii — highly significant in subtropical/tropical Australian reservoirs. | 0.7 µg/L HA (children); 3 µg/L HA (adults); EPA 2019. | 0.7 µg/L TGV (GDWQ 2022). | 1 µg/L MAC proposed (Health Canada). |
The ADWG 2022 uses a four-tier alert system based on cell counts and toxin screening. Aeration is the primary preventive intervention; it is most effective when deployed before the reservoir enters Alert Level 1.
Cyanobacteria detected; cell counts generally < 500 cells/mL for most toxic genera. Initiate monitoring programme; review aeration operating regime. No immediate supply restriction.
≥ 2,000 cells/mL Microcystis equivalent (or ≥ 15,000 cells/mL other toxic genera). Consumer communication protocol initiated; increased toxin screening; consider full aeration deployment.
≥ 100,000 cells/mL Microcystis eq. (or visible scum). Consider restricting abstraction; consult regulator (state EPA / DH); maximum aeration deployment; increased treatment monitoring.
≥ 500,000 cells/mL Microcystis eq. (or dense, persistent scum). Do not abstract for potable supply without specialist toxin treatment; mandatory regulator notification. Aeration effectiveness limited at this stage — prevention is critical.
Warm-Climate Note: In subtropical and tropical Australian reservoirs (Queensland, Northern Territory, northern Western Australia), Cylindrospermopsis raciborskii is a dominant bloom-forming cyanobacterium producing cylindrospermopsin — a hepatotoxin not commonly encountered in temperate UK or European reservoirs. Southern hemisphere stratification peaks December–March (opposite to UK). Destratification programmes must be timed accordingly, typically commencing in September–October before summer stratification consolidates.
Deploy online TON monitor or fortnightly geosmin/2-MIB GC-MS sampling at abstraction intake and within the water column (epilimnion, metalimnion, hypolimnion). Alert threshold: geosmin or 2-MIB > 10 ng/L, or TON > 2 at treatment works inlet.
Distinguish cyanobacterial bloom production (phytoplankton net samples, fluorescence probe) from actinomycete sediment source (geosmin at all depths with no bloom present). Different sources require different primary controls.
Deploy thermistor string (1 m intervals). Calculate Schmidt stability. If stability > 50 J/m², thermocline is consolidated. Destratification still.but must be staged to avoid drawing cold anoxic water to the surface rapidly.
Start diffused-air system at 30% capacity and increase over 7 days. Monitor temperature profiles twice weekly. Target: temperature differential surface-to-bottom < 1°C. Bloom biomass will decrease as epilimnion cools and nutrient mixing dilutes P concentrations.
Dose KMnO₄ (0.5–1.5 mg/L as Mn) at the intake works if geosmin > 15 ng/L. KMnO₄ oxidises geosmin but is not effective for 2-MIB. For 2-MIB, use powdered activated carbon (PAC, 10–20 mg/L) dosed ahead of coagulation, or increase GAC EBCT at the treatment works.
After the event, audit destratification startup timing vs the thermocline formation date. If the bloom established before the system was activated, advance spring startup by 3–4 weeks the following year. Commission sediment oxygen demand (SOD) testing if actinomycete source confirmed.
Primary control — eliminates bloom conditions before geosmin production begins. Bubble plumes from floor-mounted diffuser manifolds. No moving parts in water. Typical air demand: 1–3 Nm³/h per 1,000 m³ reservoir volume. Lower lifecycle cost than any chemical treatment option.
Reactive dose to treatment works during a taste event. Effective contact 20–30 min before coagulation. Dose 10–20 mg/L for geosmin; 15–25 mg/L for 2-MIB. Spent PAC increases sludge volume. Used as event response, not prevention.
DWTP post-filtration barrier. EBCT 7.5–15 min removes geosmin to < 5 ng/L. 2-MIB requires 12+ min EBCT. Effective for several years before exhaustion but must be reactivated or replaced. High capital cost; most economic where T&O events are annual.
Our Reservoir Assessment — Algae & Taste and Odour Dimension: We assess cyanobacterial bloom risk, geosmin and 2-MIB production potential and phytoplankton functional-group succession as one of six scored health dimensions, delivered at 12-month strategic, monthly and hour-by-hour diurnal resolution. Applicable to drinking-water reservoirs and raw-water lakes worldwide. Results-only output available before any NDA is signed. Request a pre-NDA assessment preview →
Schmidt stability, thermocline timing, and bubble-plume air-flow sizing for UK reservoirs.
Read MoreHow NOM from algal lysate and anoxic mobilisation feeds THM and HAA formation at the treatment works.
Read MorePhosphorus control and cyanobacterial bloom management for nutrient-enriched waterbodies.
Read MoreOverview of all aeration strategies for drinking-water reservoir management.
Read MoreSend us your site parameters and water quality targets — we will recommend the most appropriate aeration strategy and equipment.
Our expertise spans multiple industries with sector-specific water treatment solutions.