# Single-Tank SBR System with Holding Tank
## Air-Aspirating Wastewater Treatment System

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## 1. SYSTEM OVERVIEW

This system treats wastewater using a **Sequencing Batch Reactor (SBR)** with biological treatment in both aerobic and anoxic phases. An upstream **holding tank** buffers influent during the decant phase, allowing continuous feed while the main reactor is not accepting flow.

**Key Features:**
- Single SBR treatment vessel
- Air-aspirating aeration (no external blower)
- Two-stage pumping (aeration + non-aeration cycles)
- Separate holding tank for flow management
- Aerobic and anoxic treatment phases
- Mechanical decant mechanism

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## 2. EQUIPMENT LIST & FUNCTIONS

### 2.1 Holding Tank
**Function:** Pre-treatment buffer and flow equalization
- Receives incoming wastewater continuously
- Holds flow during SBR decant phase
- Typical capacity: 25-50% of SBR volume

### 2.2 SBR Reactor Tank (Main Treatment Vessel)
**Function:** Primary biological treatment through controlled batch cycles
- Aerobic phase: organic matter oxidation, nitrification
- Anoxic phase: denitrification, nutrient cycling
- Settling phase: solids separation
- Decant phase: clarified liquid removal

### 2.3 Fill Pump (Non-Aeration)
**Function:** Transfer wastewater from holding tank to SBR during fill phase
- Operates: Fill phase only
- Intake: Holding tank
- Discharge: SBR main vessel
- Control: Runs on level sensor trigger

### 2.4 Aeration Pump (Main Treatment Pump)
**Function:** Circulate wastewater through diffuser/air-aspirating system during aerobic phase
- Operates: Aerobic phase only
- Creates negative pressure that draws ambient air into wastewater
- Dissolved oxygen delivered without external blower

### 2.5 Decant Mechanism
**Function:** Selectively remove clarified supernatant after settling
- Options: Fixed or Floating Weir, Mechanical Arm, Telescoping Pipe
- Discharge: To further treatment train or discharge point

### 2.6 Air-Aspirating Diffuser System
**Function:** Transfer oxygen without mechanical blower; creates mixing
- Submerged diffuser tube/header
- Ejector/Venturi section (in aeration pump discharge line)
- Air intake line (with optional strainer)

### 2.7 Control Valves
- **Decant Isolation:** On decant line (solenoid-controlled)
- **Sludge Drain:** Bottom of SBR (manual or solenoid)
- **Air Intake:** Check valve + optional solenoid

### 2.8 Level Sensors
- **Holding Tank:** High/Low level sensors
- **SBR Tank:** High level + mid-level sensors
- **Optional:** additional analog level sensing

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## 3. OPERATIONAL SEQUENCE

### Phase Durations (Example)

| Phase | Duration | Fill Pump | Aeration Pump | Decant | Air | Process |
|-------|----------|-----------|---------------|--------|-----|---------|
| Fill | 15-30 min | ✓ ON | OFF | CLOSED | CLOSED | Wastewater load |
| Aerobic | 30-60 min | OFF | ✓ ON | CLOSED | ✓ ON | Oxidation + nitrification |
| Anoxic | 10-20 min | OFF | OFF | CLOSED | CLOSED | Denitrification |
| Settle | 10-20 min | OFF | OFF | CLOSED | CLOSED | Gravity settling |
| Decant | 10-15 min | OFF | OFF | ✓ OPEN | CLOSED | Effluent withdrawal |
| Idle | 1-5 min | OFF | OFF | CLOSED | CLOSED | Brief rest |

**Total Cycle Time:** ~90-160 minutes (1.5-2.7 hours)  
**Cycles per Day:** 9-16 batches (typical: 12-14 cycles/24hr)

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## 4. SYSTEM PARAMETERS & DESIGN CONSIDERATIONS

### Sizing (Small System Example)
- Design Flow: 20 m³/day
- Cycles per day: 12
- Flow per cycle: 1.67 m³/batch
- SBR Volume: ~2,000 L
- Holding Tank: 600 L minimum (25-50% of SBR)

### Performance Targets
- BOD: < 20 mg/L
- TSS: < 10 mg/L
- TN: < 10 mg/L (with aerobic + anoxic phases)
- MLSS: 2,000-4,000 mg/L
- DO Target: 2-4 mg/L during aerobic phase

### Air-Aspiration Ratio
- Typical: 1.5:1 to 2:1 (air volume : water volume)
- 100 GPM pump flow → ~150-200 CFM air induced
- No external blower required = lower energy, noise, maintenance

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## 5. ADVANTAGES & CONSIDERATIONS

### Advantages
✓ Compact single-tank design  
✓ Air-aspirating aeration (no blower)  
✓ Flexible operation for variable loads  
✓ Excellent nitrification & denitrification  
✓ Low energy consumption  
✓ Holding tank simplifies flow management  

### Considerations
⚠ Decant mechanism must be reliable  
⚠ Regular sludge wasting required  
⚠ Phase timings sensitive to influent strength  
⚠ No pump redundancy  
⚠ No direct phosphorus removal (needs chemical dosing)  

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**Document Status:** DRAFT  
**Created:** 2026-05-01  
**Version:** 1.0