Dholera Industrial Effluent Equalization Tank: GPCB Codes

1. GPCB and DICDL CETP Inlet Discharge Thresholds

To protect the centralized CETP biological reactors and underground pipeline infrastructure, effluent leaving an industrial plot’s equalization and primary treatment plant must meet strict pre-treatment standards before discharge into the DICDL underground collection grid:

Note: High-COD or toxic chemical industries (e.g., semiconductor chemical suppliers, electroplating, dye intermediates) must deploy on-site Multiple Effect Evaporators (MEE) and Zero Liquid Discharge (ZLD) plants. Direct discharge into DICDL sewers without written GPCB CETP allotment is illegal.

2. Equalization Tank Sizing and Hydraulic Retention Time (HRT)

Under GPCB design guidelines and DSIRDA GDCR Section 12, an industrial effluent equalization tank must be sized based on total daily wastewater flow ($Q_{\text{day}}$) and discharge variability:

A. Minimum Hydraulic Retention Time Formula

The physical liquid volume of the equalization tank must satisfy:

$$V_{\text{eq}} \ge Q_{\text{avg}} \times \text{HRT}$$

Where: * $Q_{\text{avg}}$ = Average hourly wastewater generation rate ($\text{m}^3/\text{hr}$). * HRT Standards: * Continuous Manufacturing (3 Shifts): Minimum HRT of 8 to 12 Hours. * Batch Processing Industries (1 to 2 Shifts): Minimum HRT of 16 to 24 Hours to balance intermittent batch dump volumes. * Freeboard Allowance: Tanks must include a minimum 0.5-meter freeboard above the high-liquid level to prevent foam overflow.

3. Mixing and Aeration System Specifications

To prevent anaerobic septic conditions, sludge sedimentation, and volatile odor formation, equalization tanks in Dholera SIR must feature active homogenization:

1. Submerged Coarse Bubble Aeration: Compressed air grids using non-clog tubular EPDM diffusers must supply at least 1.2 to 1.5 m³ of air per hour per m³ of tank volume (or 0.02 to 0.03 m³/min per m² of tank floor area).
2. Mechanical Floating / Submersible Mixers: In chemical facilities where excessive aeration causes volatile organic compound (VOC) stripping, low-speed submersible mixers (propeller diameter ≥ 1.5 m) providing continuous velocity gradients greater than 0.3 m/sec across the entire floor must be deployed to keep solids suspended.
3. Automated pH Neutralization Skid: Dholera factories must install dual inline pH sensor transmitters inside the equalization basin that automatically trigger dosing metering pumps (injecting 10% Caustic Soda ($\text{NaOH}$) or 15% Sulfuric Acid ($\text{H}_2\text{SO}_4$)) to maintain neutral effluent between pH 6.8 and 7.8.

4. Civil Engineering and Anti-Corrosion Linings

Because raw industrial effluent contains aggressive acids, organic solvents, and high chlorides, equalization tank construction in Dholera SIR must comply with stringent structural codes:

• Concrete Grade & Waterproofing: The basin must be constructed from RCC Grade M30 or higher, utilizing sulfate-resisting cement with integral crystalline waterproofing compounds complying with IS 3370.
• Epoxy / FRP Lining: All internal wetted surfaces must be lined with a minimum 3 mm thick Fiberglass Reinforced Plastic (FRP) dual-layer coating or heavy-duty chemical-resistant vinyl ester resin to prevent sulfuric acid degradation of the structural concrete.
• Leak Detection Sump: Underground equalization basins must be built within a double-containment concrete pit or include sub-slab perforated drain tiles leading to an observation inspection sump. This ensures any tank leakage into Dholera’s marine clay water table is detected within 24 hours.

Frequently Asked Questions (FAQ)