Zero Liquid Discharge (ZLD) in milk production is a wastewater treatment strategy where no liquid effluent leaves the plant. All wastewater generated during dairy processing is treated, recycled, and reused — leaving only solid waste for disposal.
Dairy plants (like milk processing units, cheese, butter, yogurt factories) generate wastewater with high organic load, making ZLD both challenging and valuable.
Why ZLD Is Important in Dairy Plants :
Milk processing wastewater contains:
- High BOD (Biological Oxygen Demand)
- High COD (Chemical Oxygen Demand)
- Fats, oils & grease (FOG)
- Lactose, proteins
- Cleaning chemicals (CIP systems)
Without treatment, it causes:
- Severe oxygen depletion in water bodies
- Odor problems
- Regulatory non-compliance
- Environmental penalties
Typical ZLD Process Flow for Milk Production
1. Pre-Treatment
Removes large solids and fats.
- Screening
- Oil & grease trap / Dissolved Air Flotation (DAF)
- Equalization tank
2️. Biological Treatment
Removes dissolved organic matter.
- Anaerobic reactor (UASB / Anaerobic digester)
→ Produces biogas (methane) usable for energy - Aerobic treatment (Activated Sludge / MBR)
This stage removes 90–95% of BOD/COD.
3. Tertiary Treatment
Polishing stage to prepare for reuse.
- Sand filtration
- Activated carbon filtration
- Ultrafiltration (UF)
- Reverse Osmosis (RO)
RO separates:
- Permeate (clean water) → reused in plant
- Reject (concentrate) → sent for evaporation
4️. Evaporation & Crystallization
This is what makes it “Zero Liquid Discharge”.
- Multiple Effect Evaporator (MEE)
- Mechanical Vapor Recompression (MVR)
- Crystallizer
Outputs:
- Recovered distilled water (reused)
- Solid salts/sludge (disposed or reused if possible)
Water Reuse Options in Dairy Plant
Recovered water can be reused for:
- Cooling towers
- Boiler feed (after polishing)
- Floor washing
- CIP pre-rinse
- Gardening
Benefits of ZLD in Milk Industry
✔ 95–99% water recovery
✔ Reduced freshwater consumption
✔ Energy recovery from biogas
✔ Regulatory compliance
✔ Sustainable branding
✔ Reduced environmental footprint
Challenges in Dairy ZLD
- High capital cost
- Energy-intensive evaporation
- Membrane fouling (due to fats & proteins)
- Sludge management
- Skilled operation required
Approximate Wastewater Characteristics (Dairy)
| Parameter | Typical Range |
|---|---|
| BOD | 800–3000 mg/L |
| COD | 1500–5000 mg/L |
| TSS | 200–1000 mg/L |
| pH | 6–10 |
Example Applications
Large dairy processors in water-scarce regions (e.g., India, Middle East) increasingly adopt ZLD systems to comply with strict environmental norms.
For example:
- Amul
- Nestlé
- Danone
have implemented advanced water recycling systems in various processing facilities.
Simplified ZLD Flow Diagram
Raw Effluent
↓
Screening → DAF → Equalization
↓
Anaerobic Reactor → Aerobic Treatment
↓
UF → RO
↓
RO Permeate → Reuse
RO Reject → MEE → Crystallizer
↓
Solid Waste + Recovered Water
