Mathematical model for predicting sludge accumulation in septic tanks based on operational parameters: effect of hydraulic mixing

Abstract

Reporting sludge accumulation in septic tanks as a constant rate is no longer accepted, and there is a need for a model that predicts sludge accumulation in septic tanks taking into consideration the continuous consumption of sewage solids by microorganisms. This study presents the development of mathematical model that predicts sludge accumulation in septic tanks and to investigate the effect of mixing on solids digestion. The mathematical model takes into consideration the effect of different operational parameters (influent and effluent) as well as the bacterial kinetics in predicting the sludge accumulation in septic tanks. The model predicts with R 2 > 0.88 the sludge accumulation rate in standard septic tank calculated by empirical models developed by Weibel in 1955, and by Bounds in 1995. The model was also used to estimate the typical pump-out interval for different tank volumes and the sludge accumulation inside septic tanks at different operational parameters.

Challenges to Accurate Estimation of Methane Emission from Septic Tanks with Long Emptying Intervals

“…It is comparable to 0.04 L/(cap·d) in a previous study in Hanoi and 0.07 L/(cap·d) in six cities in Indonesia, while it was high in some other countries: 0.5 L/(cap·d) in Thailand, 0.1 L/(cap·d) in France, and 0.1 L/(cap·d) in Canada . Potential factors for the difference in accumulation rate include the type of wastewater entering the septic tanks (blackwater or both black and graywater), solid and organic strength of the influent, diets, flushing water quantity, hydraulic retention time, and temperature. ,, …”

“…36 Potential factors for the difference in accumulation rate include the type of wastewater entering the septic tanks (blackwater or both black and graywater), solid and organic strength of the influent, diets, flushing water quantity, hydraulic retention time, and temperature. 17,37,38 3.2.2. Septage Composition.…”

“…The basic function of a septic tank is to remove solids by separating settleable solids and scum of wastewater. A proportion of the settled digestible matter is stabilized in a septic tank; the untreated effluent is discharged, and a mix of stabilized and undigested solids accumulates at the bottom of the tank. , The solids retained in septic tanks must be emptied at a proper time to maintain the functionality of the septic tank. − The recommended emptying interval is 1–5 years. − In this study, we use the term emptying interval to refer to the time from the latest emptying or the time after construction if there was no emptying practice. In low- and middle-income countries, long emptying intervals appear to be common; for instance, the average emptying intervals of septic tanks in Southeast Asia are 8.1 (Hanoi), 12.7 (Mandalay), and 16 (six cities in Indonesia) years, indicating that the septic tanks are not operated under recommended conditions.…”

“…A proportion of the settled digestible matter is stabilized in a septic tank; the untreated effluent is discharged, and a mix of stabilized and undigested solids accumulates at the bottom of the tank. 17 , 18 The solids retained in septic tanks must be emptied at a proper time to maintain the functionality of the septic tank. 19 − 21 The recommended emptying interval is 1–5 years.…”