Clogging layer development in septic tank effluent soil absorption systems

On-site wastewater treatment systems throughout North America most commonly use soil absorption beds as the final disposal of wastewater. With proper design and use, these systems function well. However, failure rates as high as 30% are reported with excessive clogging as one of the main causes. Laboratory and field-scale research indicate the rate of clogging layer development is dependent on the hydraulic loading rate and effluent strength. However, current designs typically consider only hydraulic loading and little research has been performed on actual systems.; In this study 15 actual absorption beds were assessed to determine the proportion of the bed that had failed due to clogging layer development. Absorption bed input conditions of flow and effluent quality were also measured. A mathematical relationship was developed to allow prediction of a time to absorption bed failure based on the input wastewater conditions. The term that best predicted the rate of clogging was the daily loading of BOD per unit area of bed. This term is descriptive of water quantity and quality and was an improvement over the typical method of considering water loading alone.