System

Water Treatment Plant — 50 ML/d conventional surface water treatment facility. Session 207 begins the first component-level decomposition of a scaffolded system with 10 subsystems, 10 stakeholder requirements, and 15 system requirements. No subsystem requirements, interfaces, or verification entries existed prior to this session. Decomposition status advanced from scaffolded to in-progress.

Decomposition

Prioritised the {{entity:Chemical Storage and Dosing Subsystem}} and {{entity:Disinfection Subsystem}} as the first two subsystems for decomposition. Chemical dosing was selected for its safety criticality (chlorine gas handling drives {{sys:SYS-REQS-007}}) and its high interface count — it connects to coagulation, disinfection, distribution, and SCADA. Disinfection followed as the critical public health barrier, tightly coupled to the chlorine feed system.

Chemical Storage and Dosing was decomposed into 8 components: {{entity:Alum Bulk Storage and Metering System}} {{hex:D6A53018}}, {{entity:Polymer Preparation and Feed System}} {{hex:56D53218}}, {{entity:Chlorine Gas Storage and Feed System}} {{hex:47D73059}}, {{entity:Caustic Soda Storage and Feed System}} {{hex:D6B73018}}, {{entity:Fluorosilicic Acid Storage and Feed System}} {{hex:D7973059}}, {{entity:Powdered Activated Carbon Feed System}} {{hex:D6D51218}}, {{entity:Chemical Containment and Emergency Safety System}} {{hex:55F77859}}, and {{entity:Chemical Dosing Control System}} {{hex:55F77A18}}. The control system serves as the central hub, connecting to all six feed systems via 4-20 mA/Modbus, while the safety system independently monitors the chlorine room and can override the control system via hardwired interlocks.

Disinfection was decomposed into 4 components: {{entity:UV Disinfection Reactor}} {{hex:D7F73058}}, {{entity:Chlorine Contact Tank}} {{hex:CE851218}}, {{entity:CT Compliance Monitoring System}} {{hex:51F77B58}}, and {{entity:Disinfection Residual Analyser Network}} {{hex:54E77018}}. The UV-first, chlorine-second sequence provides independent barriers — UV for Cryptosporidium (4-log at 40 mJ/cm2), chlorine CT for bacteria and viruses plus distribution residual.

flowchart TB
    DCS[Dosing Control System] --> ALM[Alum Storage and Metering]
    DCS --> POL[Polymer Prep and Feed]
    DCS --> CL2[Chlorine Gas Storage and Feed]
    DCS --> NAO[Caustic Soda Storage and Feed]
    DCS --> HFS[Fluorosilicic Acid Feed]
    DCS --> PAC[PAC Feed System]
    CSS[Containment and Safety] -->|gas detection| CL2
    CSS -->|safety interlock| DCS
    ALM -->|alum injection| COAG[Coagulation Subsystem]
    POL -->|polymer injection| COAG
    CL2 -->|Cl2 solution| DIS[Disinfection Subsystem]
    NAO -->|caustic| DIST[Distribution Pumping]
    HFS -->|fluoride| DIST

```mermaid
flowchart TB
    FILT[Filtration Subsystem] -->|filtered water| UV[UV Disinfection Reactors]
    UV -->|UV-treated water| CCT[Chlorine Contact Tank]
    RAN[Residual Analyser Network] -->|residual readings| CTM[CT Compliance Monitor]
    CTM -->|CT calculation| CCT
    RAN -->|4-20mA signals| SCADA[SCADA Subsystem]

Analysis

Lint returned 2 findings: one medium-severity note on {{stk:STK-NEEDS-002}} lacking statistical parameters for capacity testing (inherited from scaffold session), and one low-severity note that VER/ARC entries lack “SHALL” keyword (expected for non-normative entries). One orphan: {{arc:ARC-DECISIONS-001}}, which is structurally expected since architecture decisions do not participate in derives/verifies linksets. Cross-domain similarity search on {{entity:Chlorine Contact Tank}} returned structural analogs (propellant tanks, bridges, structural beams) at 94-97% Jaccard — confirming the classification captured its physical containment nature but not yielding process-engineering insights for missing requirements.

Requirements

Created 18 subsystem requirements ({{sub:SUB-REQS-001}} through {{sub:SUB-REQS-018}}), 10 interface requirements ({{ifc:IFC-DEFS-001}} through {{ifc:IFC-DEFS-010}}), 13 verification entries ({{ver:VER-METHODS-001}} through {{ver:VER-METHODS-013}}), and 2 architecture decisions. All IFC requirements have corresponding VER entries (100% IFC coverage). Three critical SUB requirements also have VER entries: chlorine gas detection 5-second response ({{sub:SUB-REQS-004}}), ventilation 20 ACH verification ({{sub:SUB-REQS-005}}), and duty/standby pump switchover ({{sub:SUB-REQS-011}}). VER coverage for SUB+IFC combined: 13/28 = 46%. All SUB and IFC requirements are traced to parent SYS requirements via derives linksets. Key safety chain: {{sys:SYS-REQS-007}} derives {{sub:SUB-REQS-003}}, {{sub:SUB-REQS-004}}, {{sub:SUB-REQS-005}}, {{sub:SUB-REQS-009}}, and {{ifc:IFC-DEFS-006}}.

Next

Eight subsystems remain undecomposed: Raw Water Intake, Coagulation and Flocculation, Sedimentation, Filtration, Sludge Handling, Treated Water Storage and Distribution Pumping, SCADA and Instrumentation, and Electrical Power and Emergency Generation. Next session should prioritise SCADA and Instrumentation — it has the highest interface count across all subsystems and its decomposition will define the control architecture that constrains all other subsystems. The medium-severity lint finding on STK-NEEDS-002 should be addressed during QC review, not now, as it belongs to the scaffold-session requirements.