Sludge Accumulation and Characterization in Decentralized Community Wastewater Treatment Systems with Primary Clarifier Tanks at Each Residence

Abstract

Sludge accumulation, treatment and disposal can represent a high percentage of the operating cost for a wastewater system. This is especially important for small-scale and onsite wastewater treatment systems, where sludge removal can be one of the few operating costs of the system. In 2000, as a result of a large number of septic system failures, the community of Wardsville installed a Clearford Industries Inc. Small Bore Sewer™ (SBS™) system which included two-chamber 3600 L tanks located on the properties of individual homes. The tanks were collectively attached to a small bore piping system to deliver the effluent from the tanks to a small community wastewater treatment system. During the summer of 2007, a field study was initiated with a community survey, followed by a review of candidate sites, leading to the selection of 29 sites for site investigation and sampling. Sampling involved the collection of samples for sludge characterization along with the measurements of the height of solids (scum and sludge) within the tank. The data were analyzed to determine the factors having a statistically significant impact on solids accumulation rates within each of the two chambers of the tank. Household water usage was found to be the variable having the strongest association with sludge and scum accumulation, and models were estimated relating solids accumulation to water usage in order predict pump out frequency. A second field sampling program was conducted in Wardsville during April 2008, involving only the first chamber of 13 primary clarifier tanks. Overall contributions have been made in understanding and quantifying solids accumulation rates and sludge characterization in onsite primary clarifier tanks. As well, the information gained from the analysis of the data collected provides a meaningful insight into the factors influencing solids accumulation within individual residential primary clarifier tanks, and points to future research directions for understanding the factors influencing solids accumulation.