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+# settler
+
+> **Reflects code as of `7bf464b` · regenerated `2026-05-11` via `npm run wiki:all`**
+> If this banner is stale, the page may be out of date. Treat as informative, not authoritative.
+
+## 1. What this node is
+
+**settler** is an S88 Unit that models a secondary clarifier. It takes the upstream reactor's effluent stream, performs a 13-species TSS mass balance, and splits it into three Fluent envelopes: clarified effluent, surplus sludge, and return sludge. A downstream return pump (rotatingMachine child) draws the return-sludge flow.
+
+## 2. Position in the platform
+
+```mermaid
+flowchart LR
+    upstream[reactor<br/>upstream<br/>Unit]:::unit
+    settler[settler<br/>Unit]:::unit
+    downstream[reactor<br/>downstream<br/>Unit]:::unit
+    return[rotatingMachine<br/>return pump<br/>Equipment]:::equip
+    tss[measurement<br/>type=quantity (tss)<br/>position=atequipment]:::ctrl
+
+    upstream -.stateChange.-> settler
+    settler -->|Fluent inlet=0,1,2| downstream
+    return -->|child.register downstream| settler
+    settler -.F_sr.-> return
+    tss -->|quantity (tss).measured.atequipment| settler
+    classDef unit fill:#50a8d9,color:#000
+    classDef equip fill:#86bbdd,color:#000
+    classDef ctrl fill:#a9daee,color:#000
+```
+
+S88 colours: Unit `#50a8d9`, Equipment `#86bbdd`, Control Module `#a9daee`. Source of truth: `.claude/rules/node-red-flow-layout.md`.
+
+## 3. Capability matrix
+
+| Capability | Status | Notes |
+|---|---|---|
+| TSS mass-balance split (3 streams) | ✅ | Effluent / surplus / return derived from `F_in * Cs[12] / C_TS`. |
+| Particulate zeroing in effluent | ✅ | Species 7–12 set to 0 in effluent when `F_s > 0`. |
+| Particulate concentration in sludge | ✅ | Species 7–12 scaled by `F_in / F_s` in surplus + return. |
+| Return-pump flow draw | ✅ | `F_sr` = min(pump flow, F_s). Surplus = F_s − F_sr. |
+| F_s clamp to F_in | ✅ | Prevents negative effluent when X_TS_in > C_TS. |
+| Manual influent override | ✅ | `data.influent` lets ops supply `{ F, C }` directly. |
+| Multiple reactor upstreams | ❌ | Only one `upstreamReactor` slot; last registration wins. |
+| Stateful FSM | ❌ | Stateless transform — recomputes on every push. |
+
+## 4. Code map
+
+```mermaid
+flowchart TB
+    subgraph nodeRED["nodeClass.js — adapter (BaseNodeAdapter)"]
+        nc["buildDomainConfig()<br/>static DomainClass = Settler<br/>static commands"]
+    end
+    subgraph domain["specificClass.js — orchestrator (BaseDomain)"]
+        sc["Settler.configure()<br/>ChildRouter rules<br/>getEffluent — TSS split<br/>_connectReactor (manual listener)"]
+    end
+    subgraph commands["src/commands/"]
+        cmds["index.js + handlers.js<br/>data.influent + aliases"]
+    end
+    nc --> sc
+    nc --> cmds
+```
+
+| Module | Owns | Read first if you're changing… |
+|---|---|---|
+| `specificClass.js` | All domain logic: getEffluent split, reactor + machine + measurement wiring, getOutput, getStatusBadge. | Mass-balance math, child wiring, telemetry shape. |
+| `commands/` | Single command (`data.influent`) + aliases + payload validation. | Manual-influent topic, new aliases. |
+
+Settler is small enough (~140 LOC) that no concern-split was needed (per P6.6).
+
+## 5. Topic contract
+
+> **Auto-generated** from `src/commands/index.js`. Do NOT hand-edit between the markers. Re-run `npm run wiki:contract`.
+
+<!-- BEGIN AUTOGEN: topic-contract -->
+
+| Canonical topic | Aliases | Payload | Unit | Effect |
+|---|---|---|---|---|
+| `data.influent` | `influent`, `setInfluent` | `any` | — | Push the influent stream (payload: {F: flow m3/h, C: [concentrations mg/L]}). |
+| `child.register` | `registerChild` | `string` | — | Register a child node (typically a measurement) with this settler. |
+
+<!-- END AUTOGEN: topic-contract -->
+
+## 6. Child registration
+
+```mermaid
+flowchart LR
+    subgraph kids["accepted children (softwareType)"]
+        m["measurement"]:::ctrl
+        r["reactor<br/>upstream"]:::unit
+        mach["machine<br/>downstream"]:::equip
+    end
+    m -->|"&lt;type&gt;.measured.&lt;position&gt;"| h_m[_connectMeasurement]
+    r -.stateChange.-> h_r[_connectReactor<br/>manual listener]
+    mach -->|registered| h_mach[_connectMachine<br/>sets returnPump]
+    h_r --> pull[upstreamReactor.getEffluent]
+    pull --> emit[notifyOutputChanged]
+    classDef ctrl fill:#a9daee,color:#000
+    classDef unit fill:#50a8d9,color:#000
+    classDef equip fill:#86bbdd,color:#000
+```
+
+| softwareType | filter | wired to | side-effect |
+|---|---|---|---|
+| `measurement` | any | `_connectMeasurement` | Re-emits on settler's measurements; `quantity (tss)` updates `C_TS`. |
+| `reactor` | `positionVsParent=upstream` (warns otherwise) | `_connectReactor` | Stores as `upstreamReactor`; subscribes to its **own** `emitter` (NOT `measurements.emitter`) for `'stateChange'`. |
+| `machine` | `positionVsParent=downstream` | `_connectMachine` | Stores as `returnPump`; sets `machine.upstreamSource = settler`. |
+
+### 6.1 Reactor ↔ settler wiring (the load-bearing bit)
+
+The reactor pushes its `stateChange` event on `reactor.emitter`, not `reactor.measurements.emitter`. The standard `router.onMeasurement` path can't subscribe — so settler attaches the listener manually inside `_connectReactor`. On each fire, settler **pulls** the upstream effluent via `reactor.getEffluent` and copies it into `this.F_in` + `this.Cs_in`.
+
+`reactor.getEffluent` historically returned either an array (3-stream) or a single envelope — the 2026-03-02 `_connectReactor` fix preserves both shapes:
+
+```js
+const raw = this.upstreamReactor.getEffluent;
+const effluent = Array.isArray(raw) ? raw[0] : raw;
+this.F_in = effluent.payload.F;
+this.Cs_in = effluent.payload.C;
+this.notifyOutputChanged();
+```
+
+If you change the reactor's effluent shape, this is the line to update.
+
+## 7. Lifecycle — what one stateChange does
+
+```mermaid
+sequenceDiagram
+    participant reactor as upstream reactor
+    participant settler as settler
+    participant pump as return pump child
+    participant downstream as downstream consumer
+    participant out as Port-0 output
+
+    reactor->>settler: emitter.emit('stateChange')
+    settler->>reactor: pull getEffluent
+    reactor-->>settler: { F, C[13] }
+    settler->>settler: F_in = F, Cs_in = C
+    settler->>pump: read measurements.flow.measured.atequipment
+    pump-->>settler: returnFlow
+    settler->>settler: getEffluent — split into 3 inlets
+    settler->>out: [Fluent inlet=0, Fluent inlet=1, Fluent inlet=2]
+    out->>downstream: 3 msgs on Port 0
+```
+
+The split runs lazily inside `getEffluent`: each call recomputes from current `F_in`, `Cs_in`, `C_TS`, and the pump's reported `flow.measured.atequipment`.
+
+## 8. Data model — `getOutput()`
+
+Port 0 carries the 3-envelope Fluent stream directly; Port 1 (this snapshot) is the scalar dashboard view.
+
+<!-- BEGIN AUTOGEN: data-model -->
+
+| Key | Type | Unit | Sample |
+|---|---|---|---|
+| `C_TS` | number | — | `2500` |
+| `F_eff` | number | — | `0` |
+| `F_in` | number | — | `0` |
+| `F_return` | number | — | `0` |
+| `F_surplus` | number | — | `0` |
+
+<!-- END AUTOGEN: data-model -->
+
+**Concrete sample** (typical operating point):
+
+```json
+{
+  "F_in": 1000,
+  "C_TS": 2500,
+  "F_eff": 850.0,
+  "F_surplus": 50.0,
+  "F_return": 100.0
+}
+```
+
+`F_eff + F_surplus + F_return = F_in` always holds (modulo float). Particulates concentrate by `F_in / F_s` in the surplus + return streams.
+
+## 9. Configuration — editor form ↔ config keys
+
+```mermaid
+flowchart TB
+    subgraph editor["Node-RED editor form"]
+        f1[Name]
+        f2[Position vs parent]
+        f3[Logging level]
+    end
+    subgraph config["Domain config slice"]
+        c1[general.name]
+        c2[functionality.positionVsParent]
+        c3[general.logging.logLevel]
+    end
+    f1 --> c1
+    f2 --> c2
+    f3 --> c3
+```
+
+| Form field | Config key | Default | Range | Where used |
+|---|---|---|---|---|
+| Name | `general.name` | `Settler` | string | display + Port-1 topic |
+| ID | `general.id` | `null` | nullable string | child registration key |
+| Software type | `functionality.softwareType` | `settler` | string | parent-side router filter |
+| Position vs parent | `functionality.positionVsParent` | `downstream` | enum: `upstream` / `atEquipment` / `downstream` | parent-side routing |
+| Logging level | `general.logging.logLevel` | `info` | enum | logger threshold |
+
+Settler has no operational config of its own — all behaviour is driven by the runtime state (`F_in`, `Cs_in`, `C_TS`). Tune behaviour by feeding it different reactor effluents or `C_TS` measurements.
+
+## 10. State chart
+
+Skipped — settler is stateless. Each `stateChange` (or `data.influent`, or `quantity (tss)` update) recomputes the 3 Fluent streams from scratch.
+
+## 11. Examples
+
+| Tier | File | What it shows | Status |
+|---|---|---|---|
+| Basic | `examples/basic.flow.json` | Inject `data.influent`, watch 3-stream split | ✅ in repo |
+| Integration | `examples/integration.flow.json` | reactor (upstream) + settler + return pump | ✅ in repo |
+| Edge | `examples/edge.flow.json` | F_s clamp + zero-influent fallback | ✅ in repo |
+
+One screenshot per tier where helpful. PNG ≤ 200 KB under `wiki/_partial-screenshots/settler/`.
+
+## 12. Debug recipes
+
+| Symptom | First thing to check | Where to look |
+|---|---|---|
+| `F_eff` negative or NaN | `C_TS` zero or `Cs_in[12]` huge. F_s clamp should prevent — confirm clamp present. | `specificClass.js → getEffluent` |
+| Settler never updates after reactor changes | Reactor child not on `'upstream'` position, or listener attached to wrong emitter. | `_connectReactor` — listens on `reactor.emitter`, NOT `measurements.emitter`. |
+| Return-sludge flow = 0 | `returnPump.measurements.type('flow').variant('measured').position('atEquipment')` empty. Wire a flow measurement on the pump. | `_connectMachine`, pump measurement chain. |
+| 3 Fluent envelopes not arriving downstream | `payload.inlet` selector on the downstream reactor mismatches (0=eff, 1=surplus, 2=return). | downstream reactor's `data.fluent` handler. |
+| `quantity (tss)` updates don't change `C_TS` | Measurement child's `asset.type` not `quantity (tss)` exactly. | `_updateMeasurement` switch. |
+
+## 13. When you would NOT use this node
+
+- Use settler for **secondary clarification** downstream of a biological reactor. For primary sedimentation (raw sewage), the species-7-12 zeroing is wrong — model that as a separate process.
+- Don't use settler as a generic mass-balance node — the 13-species ASM3 vector is hard-coded.
+- Skip settler when the downstream reactor doesn't need a 3-stream split (e.g. single-tank SBR). A direct reactor → reactor wire is lighter.
+
+## 14. Known limitations / current issues
+
+| # | Issue | Tracked in |
+|---|---|---|
+| 1 | Only one `upstreamReactor` slot — multi-reactor settlers not supported (last registration wins). | `_connectReactor` |
+| 2 | TSS mass balance uses index 12 (`X_TS`) hard-coded — coupled tightly to ASM3 species ordering. | `getEffluent`, `_updateMeasurement` |
+| 3 | Settler depends on `mathjs` (~14 MB install) but only uses it transitively via reactor; no direct mathjs call in settler code. | `package.json` |
+| 4 | No flow-balance check at runtime — if particulate concentration drives F_s above F_in, the clamp masks an upstream bug rather than warning. | `getEffluent` |