Page review information
Edition covered
Water/wastewater controls practice — IEC 61511 · IEC 62443 · ISA-18.2 · AWWA/EPA context — exact governing revisions not yet recorded — verify on the linked standards pages
Content status
Reviewed
Coverage
Control patterns for filtration and clarification (run/backwash state machine, turbidity control); illustrative, site design governs.
Last reviewed
August 2026
Primary reference
The official published standard — required for final decisions

Confirm the governing edition, jurisdiction, and project-specific requirements before applying anything on this page.

Water/Wastewater — System Reference

Filtration and Clarification

Scope: Rapid gravity filters, pressure filters, and lamella clarifiers. Filter run/backwash state machine, turbidity-driven backwash initiation, filter-to-waste logic, and coagulant dosing integration.

Standards Applicability

Standard Role in this system
EPA SWTR Turbidity < 0.3 NTU in 95% of measurements; < 1 NTU at all times — continuous logging required
ISA-18.2 Alarm priority for turbidity spike, head loss alarm, backwash tank low
IEC 61511 SIF: Filter effluent isolation if turbidity > 1.0 NTU (protects clearwell from contamination)

Filter Run / Backwash State Machine

stateDiagram-v2
    [*] --> Standby
    Standby --> Filtering : Start command received
    Filtering --> BackwashInitiate : dP > 2.5m H₂O\nOR runtime > 24h\nOR turbidity spike
    BackwashInitiate --> AirScour : Close effluent valve\nOpen BW supply
    AirScour --> Backwash : Air scour complete (3–5 min)
    Backwash --> Rinse : Backwash complete (8–12 min)
    Rinse --> Filtering : Turbidity < 0.2 NTU for 5 min
    Rinse --> Alarm_FailToReturn : Turbidity still > 0.5 NTU\nafter 2 backwash cycles
    Filtering --> Standby : Stop command
    Alarm_FailToReturn --> Standby : Operator resets

Turbidity-Driven Filter Bypass Logic

flowchart TD
    A[AT-201\nFilter Effluent Turbidity] --> B{Turbidity > 0.3 NTU?}
    B -->|No| C[Normal Operation\nLog to SCADA historian]
    B -->|Yes| D{Duration > 15 min?}
    D -->|No| E[Alarm: High Turbidity\nCheck filter and coagulant dose]
    D -->|Yes| F[Initiate Backwash Sequence]
    F --> G{Post-backwash turbidity\n< 0.2 NTU?}
    G -->|Yes| C
    G -->|No — 2nd cycle| H[Close Filter Effluent Valve XV-201\nAlarm: Filter Offline — Operator Action Required]
    H --> I[Route raw water to adjacent filter\nif available]

Key Engineering Decisions

Filter-to-waste is non-negotiable after backwash. Backwash water contains the TSS and organisms removed from the media. Returning this to the clearwell would spike turbidity and could compromise disinfection efficacy. Route to backwash recovery basin or drain — not clearwell.

Multiple filter sequencing: Do not backwash more than one filter simultaneously (reduces plant capacity). Stagger backwash triggers by 2-hour minimum offset between filters.

Turbidity analyzer location: Sample point must be downstream of the filter bed, upstream of the clearwell. Dead time between the filter and the analyzer determines PID response time.

Trust Boundary — Engineering Judgment Required

This site is an independent educational reference for industrial automation standards. It paraphrases and organizes guidance in original language; it is not a substitute for the official published standards, professional engineering judgment, or legal review, and content has not been independently verified against current published editions.

Items marked Review pending await technical review against the identified edition. Items marked Planned have no reviewed source material behind them. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.