Scene 8 · Inside the filter hall

Magnodol raises the pH. The nickel binds.*

Inside the treatment hall, the raw water from each Tiefbrunnen (deep production well) is dosed with O₂ in an Oxidator and enters the Vorfilter (pre-filter, also Druckfilter) of one of two parallel filter trains. The Vorfilter is a pressurised vessel packed with a bed of coarse-grained quartz gravel and sand (Kies grobkörnig); on this bed, iron precipitates first as Fe(OH)₃ and a partial fraction of the manganese follows as MnO₂ — the iron-then-manganese sequencing documented as a Lower-Rhine signature (Wisotzky, 2021, Ch. 17). The water then passes to the Fall-Verdüsungs-Filter (spray-aerated open filter), where it is atomised through a ring of nozzles onto a bed of half-burned dolomite — Magnodol, CaCO₃·MgO. CO₂ is stripped, the pH rises past the 6.5 mobilisation threshold, and the nickel is locked into freshly precipitated calcite and residual manganese oxide on the same grains (Kandemiroglu, 2011, Chapter 6.1; Wisotzky, Kandemiroglu & Plassmann, 2012).

Photograph of the filter hall interior at Wasserwerk Breyell, Niederrhein, Germany, showing four blue cylindrical pressure vessels arranged as two parallel filter trains across a central walkway with a doorway in the centre background. The left pair of vessels is annotated by the author with the numbers 3 and 4; the right pair with 2 and 1. Pressure gauges and electrical control panels are visible at floor level beside each vessel. Photograph by Osman Can Kandemiroglu (thesis Abb. 6.1; watermark @ock).

Train TB 2
vessels 3 → 4

Train TB 1
vessels 1 → 2

Iron(III) hydroxide precipitation · ¹

Fe²⁺ + ¼ O₂ + 5/2 H₂O → Fe(OH)₃↓ + 2 H⁺

orange flocs retained on the Druckfilter gravel bed

Partial manganese(IV) oxide precipitation · ²

Mn²⁺ + ½ O₂ + H₂O → MnO₂↓ + 2 H⁺

autocatalytic on existing MnO₂ grain coatings — iron must clear first (Wisotzky, 2021)

CO₂ stripping · ⁵

CO₂(aq) ↔ CO₂(g)↑

spray-aerated nozzle ring

Magnodol dissolution (chemical de-acidification) · ⁶

CaCO₃·MgO + H₂O + 3 H₂CO₃ ↔
Ca²⁺ + Mg²⁺ + 4 HCO₃⁻ + 2 H₂O

via Mg(OH)₂ intermediate — raises pH past 6.5

Calcite reprecipitation + Ni²⁺ capture · ⁷

2 HCO₃⁻ + 2 Ca²⁺ + Mg(OH)₂ ↔
2 CaCO₃↓ + 2 H₂O + Mg²⁺

Ni²⁺ co-incorporation site #1 — r² = 0.86 (Ni–Ca)

Residual MnO₂ + Ni²⁺ capture (Rest-Entmanganung) · ⁸

Mn²⁺ + ½ O₂ + H₂O ↔ MnO₂↓ + 2 H⁺

Ni²⁺ co-incorporation site #2 — r² = 0.66 (Ni–Mn)

Iron(II) oxidation (electron transfer) · ³

Fe²⁺ + ¼ O₂ + H⁺ → Fe³⁺ + ½ H₂O

half-reaction driving the Fe(OH)₃ precipitation in the Vorfilter

Manganese(II) oxidation (electron transfer) · ⁴

Mn²⁺ + ½ O₂ + 2 H⁺ → Mn⁴⁺ + H₂O

half-reaction driving partial MnO₂ in the Vorfilter and the residual MnO₂ capture in the F-V.-F.

Wasserwerk Breyell · filter hall interior

Four pressure vessels arranged as two parallel filter trains: Vorfilter (pre-filter) → Fall-Verdüsungs-Filter (spray-aerated open filter).

8 · inside the filter hall

stage 0 · photograph

Train TB 1 · vessels 1 → 2

→ raw water enters pH 5.8

① Vorfilter / Druckfilter iron removal

Iron(III) hydroxide precipitation · ¹

Fe²⁺ + ¼ O₂ + 5/2 H₂O → Fe(OH)₃↓ + 2 H⁺

orange flocs retained on the Druckfilter gravel bed

Partial manganese(IV) oxide precipitation · ²

Mn²⁺ + ½ O₂ + H₂O → MnO₂↓ + 2 H⁺

autocatalytic on existing MnO₂ grain coatings — iron must clear first (Wisotzky, 2021)

② Fall-Verdüsungs-Filter spray-aerated open filter · de-acidification

CO₂ stripping · ⁵

CO₂(aq) ↔ CO₂(g)↑

spray-aerated nozzle ring

Magnodol dissolution (chemical de-acidification) · ⁶

CaCO₃·MgO + H₂O + 3 H₂CO₃ ↔
Ca²⁺ + Mg²⁺ + 4 HCO₃⁻ + 2 H₂O

via Mg(OH)₂ intermediate — raises pH past 6.5

Calcite reprecipitation + Ni²⁺ capture · ⁷

2 HCO₃⁻ + 2 Ca²⁺ + Mg(OH)₂ ↔
2 CaCO₃↓ + 2 H₂O + Mg²⁺

Ni²⁺ co-incorporation site #1 — r² = 0.86 (Ni–Ca)

Train TB 2 · vessels 3 → 4

→ raw water enters pH 5.8

① Vorfilter / Druckfilter underlying half-reaction mechanisms

Iron(II) oxidation (electron transfer) · ³

Fe²⁺ + ¼ O₂ + H⁺ → Fe³⁺ + ½ H₂O

half-reaction driving the Fe(OH)₃ precipitation in the Vorfilter

Manganese(II) oxidation (electron transfer) · ⁴

Mn²⁺ + ½ O₂ + 2 H⁺ → Mn⁴⁺ + H₂O

half-reaction driving partial MnO₂ in the Vorfilter and the residual MnO₂ capture in the F-V.-F.

② Fall-Verdüsungs-Filter residual manganese removal

Residual MnO₂ + Ni²⁺ capture (Rest-Entmanganung) · ⁸

Mn²⁺ + ½ O₂ + H₂O ↔ MnO₂↓ + 2 H⁺

Ni²⁺ co-incorporation site #2 — r² = 0.66 (Ni–Mn)

→ Sammelbecken (clean-water reservoir)

pH 7.2

Treated water leaving the plant · nickel limit (20 µg/L) of the German Trinkwasserverordnung complied with.

Ni²⁺ through the treatment above 20 µg/L TrinkwV limit

raw water 20 µg/L drinking-water limit (TrinkwV) treated water

Figure (Scene 8). Cross-fade from photograph (thesis Abb. 6.1, © Osman Can Kandemiroglu) of the filter-hall interior at Wasserwerk Breyell, Lower Rhine, to a schematic of the two parallel filter trains. Each train consists of a pressurised pre-filter (Vorfilter, also Druckfilter) packed with a bed of coarse-grained quartz gravel and sand (German: Kies grobkörnig) under upstream O₂ dosing by an Oxidator, followed by a spray-aerated open filter (Fall-Verdüsungs-Filter, also Entsäuerungsfilter; the de-acidification filter) packed with a bed of half-burned dolomite (Magnodol, CaCO₃·MgO; semi-calcined dolomite of DIN EN 1017 Type A). The complete chemistry from the thesis (Kandemiroglu, 2011, Chapter 6.1) is annotated as six equations. In the Vorfilter: Eq. 15 (Fe²⁺ + ¼ O₂ + 5/2 H₂O ↔ Fe(OH)₃↓ + 2 H⁺) removes iron as orange flocs on the quartz gravel; Eq. 16 (Mn²⁺ + ½ O₂ + H₂O ↔ MnO₂↓ + 2 H⁺) removes manganese only partially, because manganese oxidation in such filters is microbial and autocatalytic on existing MnO₂ grain coatings and is inhibited while iron is still present (Wisotzky, 2021, Ch. 17; Rott & Meyerhoff, 1993) — the iron-then-manganese sequencing observed at Helenabrunn (Lower Rhine) is a documented signature of this process. Both Vorfilter reactions release 2 H⁺ per metal, depressing the pH further and consuming acid neutralisation capacity. At the top of the Fall-Verdüsungs-Filter, Eq. 17 (CO₂(aq) ↔ CO₂(g)↑) strips carbonic acid by spray atomisation through the ring of nozzles. On the falling sheet of water that lands on the dolomite bed, Eq. 18 (CaCO₃·MgO + H₂O + 3 H₂CO₃ ↔ Ca²⁺ + Mg²⁺ + 4 HCO₃⁻ + 2 H₂O) dissolves the Magnodol; mechanistically the MgO fraction first hydrates to Mg(OH)₂ (Eq. 19) which neutralises H₂CO₃* (Eq. 20), raising the pH past the 6.5 mobilisation threshold (Wisotzky, Kandemiroglu & Plassmann, 2012, Fig. 3). The pH rise drives two parallel nickel-capture pathways on the same dolomite grains. Eq. 21 (2 HCO₃⁻ + 2 Ca²⁺ + Mg(OH)₂ ↔ 2 CaCO₃↓ + 2 H₂O + Mg²⁺) reprecipitates calcite into which Ni²⁺ is co-incorporated — the macroscopic correlation r² = 0.86 (Ni–Ca) in the spent filter solids (Kandemiroglu, 2011, Tab. 8.7; Wisotzky, Kandemiroglu & Plassmann, 2012, Fig. 6) emerges from this site. Eq. 22 (Mn²⁺ + ½ O₂ + H₂O ↔ MnO₂↓ + 2 H⁺) is the residual manganese removal (Rest-Entmanganung) that completes the iron-then-manganese sequence under the now-alkaline conditions of the Magnodol bed; the freshly formed MnO₂ is the second Ni²⁺ co-incorporation site, yielding the parallel correlation r² = 0.66 (Ni–Mn) (Fig. 5). The two pathways are magnified at the grain scale in Scene 9. The treated water from both trains exits into the Sammelbecken (clean-water reservoir; Reinwasserbehälter) and from there into the drinking-water distribution network, where the nickel concentration complies with the 20 µg/L limit value of the German Trinkwasserverordnung. Schematic geometry is pedagogical, not to scale; vessel numbering follows the in-photograph labels but is not surfaced in the schematic because the chemistry is identical in the parallel trains.

Marker legend. Each chemistry card in the schematic carries a superscript marker (¹ – ⁸) instead of an inline equation number, so the equation in the box reads cleanly. The mapping below points each marker to its thesis equation number (Kandemiroglu, 2011, Ch. 6.1; German: Gleichung, abbreviated Gl.) and its peer-reviewed-paper equation number where one exists (Wisotzky, Kandemiroglu & Plassmann, 2012; also numbered on the project website at osmancankandemiroglu.com).

¹ Iron(III) hydroxide precipitation — thesis Eq. 15. TB 1 Vorfilter result. The Vorfilter Fe-precipitation step described in prose on the website (no inline Eq. number on the paper). Removes iron as orange flocs on the quartz gravel bed.
² Partial manganese(IV) oxide precipitation — thesis Eq. 16. TB 1 Vorfilter result, partial at the still-acidic raw-water pH (Wisotzky, 2021, Ch. 17). Same stoichiometry as ⁸ but proceeds only partially here, because the alkaline conditions needed for full Mn oxidation are not yet established.
³ Iron(II) oxidation half-reaction (electron transfer) — thesis Eq. 13. TB 2 Vorfilter, mechanism of marker ¹. Shows the underlying electron transfer: dissolved O₂ from the upstream Oxidator oxidises Fe(II) to insoluble Fe(III).
⁴ Manganese(II) oxidation half-reaction (electron transfer) — thesis Eq. 14. TB 2 Vorfilter, mechanism of markers ² and ⁸. Mn(II) is oxidised to Mn(IV) by dissolved O₂, microbially and autocatalytically on existing MnO₂ grain coatings; the reaction only proceeds once dissolved Fe has cleared (Wisotzky, 2021, Ch. 17).
⁵ CO₂ stripping (physical de-acidification) — thesis Eq. 17, paper Eq. 4. Fall-Verdüsungs-Filter spray nozzle ring; carbonic acid is stripped from the water by droplet flight through air.
⁶ Magnodol dissolution (chemical de-acidification) — thesis Eq. 18, paper Eq. 5. Fall-Verdüsungs-Filter dolomite bed. Half-burned dolomite (CaCO₃·MgO; semi-calcined to DIN EN 1017 Type A) dissolves; mechanistically the MgO fraction first hydrates to Mg(OH)₂ (thesis Eq. 19) which then neutralises H₂CO₃* (thesis Eq. 20), raising the pH past the 6.5 mobilisation threshold (Wisotzky, Kandemiroglu & Plassmann, 2012, Fig. 3).
⁷ Calcite reprecipitation + Ni²⁺ capture — thesis Eq. 21, paper Eq. 7. Fall-Verdüsungs-Filter bed, first Ni-capture site. Macroscopic correlation r² = 0.86 (Ni–Ca) in the carbonate-extractable fraction of the spent filter solids (Kandemiroglu, 2011, Tab. 8.7; paper Fig. 6).
⁸ Residual MnO₂ + Ni²⁺ capture (Rest-Entmanganung) — thesis Eq. 22, paper Eq. 6. Fall-Verdüsungs-Filter bed, second Ni-capture site, completes the Vorfilter's partial Mn removal under the now-alkaline conditions. Parallel correlation r² = 0.66 (Ni–Mn) in the aqua-regia-extractable fraction of the same solids (paper Fig. 5).

Display order in this scene: ¹ → ² → ³ → ⁴ → ⁵ → ⁶ → ⁷ → ⁸. TB 1 Vorfilter results (¹, ²) appear first; then the underlying mechanism (³, ⁴) is shown beneath TB 2 Vorfilter; then the Fall-Verdüsungs-Filter chemistry (⁵, ⁶, ⁷, ⁸) follows in the order in which the water meets each step.

* Wisotzky, F., Kandemiroglu, O. C., & Plassmann, C. (2012). Nickel release into groundwater and its fixation during drinking-water treatment (Nettetal/Lower Rhine) [original German title: Nickelfreisetzung in das Grundwasser und dessen Bindung bei der Wasseraufbereitung zu Trinkwasser (Nettetal/Niederrhein)]. gwf-Wasser | Abwasser, 153(7/8), 828–832. Peer-reviewed. This paper documents that the treated water at the Stadtwerke Nettetal facilities complies with the 20 µg/L limit value for nickel set by the German Drinking-Water Ordinance (Trinkwasserverordnung, TrinkwV); the limit is unchanged in the 2023 TrinkwV revision (Novelle) transposing EU Directive 2020/2184.

Treatment-chemistry context (pre-filter iron-then-manganese sequencing, microbial and autocatalytic manganese-oxide growth on existing grain coatings, the Helenabrunn three-stage Lower-Rhine treatment template, and half-burned dolomite as one of the canonical de-acidification options for restoring the calcium-carbonate–carbonic-acid equilibrium of corrosive groundwater): Wisotzky, F. (2021). Applied Groundwater Chemistry, Hydrogeology and Hydrogeochemical Modelling [original German title: Angewandte Grundwasserchemie, Hydrogeologie und hydrogeochemische Modellierung] (2nd, expanded ed.). Springer Spektrum, Berlin. ISBN 978-3-662-62755-6 (Ch. 3.7 calcium-carbonate–carbonic-acid equilibrium; Ch. 17 Lower-Rhine iron and manganese removal).

Plant-specific reactions Eq. 13–Eq. 22 (German: Gl. 13–Gl. 22) enumerated in this scene are taken verbatim from Kandemiroglu, O. C. (2011). Investigations into the origin of nickel in the raw water of the Breyell and Kaldenkirchen waterworks of Stadtwerke Nettetal GmbH and the hydrochemical behaviour of nickel during water treatment [original German title: Untersuchungen zur Herkunft des Nickels im Rohwasser der Wasserwerke Breyell und Kaldenkirchen der Stadtwerke Nettetal GmbH sowie zum hydrochemischen Verhalten des Nickels bei der Wasseraufbereitung] (Master's thesis, Chapter 6.1). Ruhr University Bochum, Chair of Applied Geology / Hydrogeology.