CO-TREATMENT AT EXISTING STPs
This approach leverages existing wastewater treatment infrastructure, allowing cost-effective treatment of faecal sludge through integration into under-utilised Sewage Treatment Plants (STPs). This is particularly effective where STPs have spare capacity.
Implementation Steps
Step 1: Identification of Underutilised STPs
STPs operating below their designed capacity should be shortlisted for potential co-treatment. A technical checklist helps ensure suitability.
Key Criteria:
- Current utilization below designed capacity
- Technology compatible with FS treatment
- Adequate receiving infrastructure
- Accessible location for desludging vehicles
- Willingness of operating authority
Step 2: Clustering of Villages
Villages within 15 km road distance from the identified STP form a cluster for centralized faecal sludge collection and treatment.
Cluster Planning Considerations:
- Maximum 15 km road distance to STP
- Population and household count
- Predominant containment types
- Road accessibility and conditions
- Avoid overlap with other cluster boundaries
Step 3: Quantification of Faecal Sludge
Accurate estimation of FS volume is critical for planning. Use the triangulation approach with three complementary methods:
Population Method
Projections using Arithmetic Progression (AP), Geometric Progression (GP), or Incremental Increase methods.
Volume of OSS Systems
Based on septic tank design specifications, number of users, and desludging frequency.
Transportation Method
Operational data from existing desludging vehicles and service records.
Step 4: Co-treatment Technology Selection
Two primary approaches for integrating FS into existing STPs:
A. Dilution Method
Direct addition of faecal sludge into sewage influent. This is the simplest method requiring minimal infrastructure changes. However, it requires continuous monitoring to prevent shock-loading of the treatment process.
Schematic of dilution method for FS co-treatment
Key Requirements:
- Controlled addition rates to maintain dilution ratios
- Monitoring of BOD/COD levels
- Adequate spare capacity in STP
- Receiving bay for desludging vehicles
B. Solid-Liquid Separation
This approach extracts solids before introducing the liquid fraction to the STP. It minimizes shock-loading and allows separate treatment of solids. Two common technologies:
Planted Drying Beds (PDB)
Natural treatment system using wetland plants. Solids are retained and dewatered while liquid percolates through. Low maintenance and suitable for rural/peri-urban areas.
Screw Press
Mechanical dewatering system that separates solids efficiently. Higher capital cost but smaller footprint. Suitable where land is limited.