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Effluent quality from 200 on-site sewage systems: design values for guidelines
K J Charles1, N J Ashbolt, D J Roser
1Cooperative Research Centre for Water Quality and Treatment, School of Civil and Environmental Engineering, University of New South Wales, Sydney NSW 2052, Australia. kcharles@unsw.edu.au
Summary
On-site sewage system effluent quality often exceeds guideline limits, increasing disposal area failure risk. Adopting the 80th percentile for system design, particularly for septic tanks, is proposed to improve wastewater treatment performance.
Area of Science:
- Environmental Engineering
- Wastewater Treatment
- On-site Sewage Systems
Background:
- Effluent quality from on-site sewage treatment systems is crucial for disposal area design and system longevity.
- Current regulatory guidelines may not accurately reflect actual effluent quality due to poor septic tank performance and high disposal system failure rates.
Purpose of the Study:
- To evaluate the accuracy of current effluent quality guidelines for on-site sewage systems.
- To propose an improved design parameter for effluent disposal areas to minimize system failures.
Main Methods:
- Analysis of septic tank effluent quality data.
- Comparison of observed effluent quality with existing regulatory guidelines.
- Assessment of aerated wastewater treatment system performance against accreditation guidelines.
Main Results:
- Septic tank effluent quality is poorer than currently suggested by guidelines.
- Aerated wastewater treatment systems generally meet accreditation guidelines, but nutrient data is insufficient.
- Proposed design values for septic tank effluent: 660 mg L(-1) Suspended Solids (SS), 330 mg L(-1) Biochemical Oxygen Demand (BOD), 250 mg L(-1) Total Nitrogen (TN), and 36 mg L(-1) Total Phosphorus (TP).
Conclusions:
- Current guidelines for septic tank effluent quality are inadequate.
- The 80th percentile of system performance should be adopted as the design value for sizing effluent disposal areas.
- Implementing the 80th percentile design value can help minimize failures associated with overloading of disposal areas.