Tracing morphological characteristics of activated sludge flocs by using a digital microscope and their effects on sludge dewatering and settling

ABSTRACT

In wastewater treatment by the activated sludge (AS) process, settleability and dewaterability of AS are key issues that are directly related to the treated water quality and sludge treatment costs. Several studies investigated the relationship between the shape of AS flocs and their settling/dewatering property. To quantify the floc morphology, it is imperative to attach a camera to a microscope or move the stage manually. Hence, labour and equipment costs may increase. In this study, by combining a digital microscope and an automatic stage, more than 100 magnified floc images were rapidly obtained from one AS sample dropped on a slide glass, and shape parameters were collectively calculated using an analysis software. During 1-year monitoring of four wastewater treatment plants in Sapporo City (Hokkaido, Japan), the morphological parameters and extracellular polymeric substances (EPS) quantity/quality of AS were analyzed on the basis of their correlation to the time to filtration (TTF) and sludge volume index (SVI), which are indicators for describing the dewatering and settling properties of AS, respectively. In one plant, larger, denser, and smoother flocs tended to contain less EPS and exhibited better sludge dewaterability. In another plant, larger, denser, and smoother flocs were considered to contribute to better settlement. Especially, an equivalent high-density floc diameter and the ratio of mixed liquor suspended solids (MLSS) concentration to the total floc area were commonly suggested to explain AS dewaterability and settleability.

GRAPHICAL ABSTRACT

KEYWORDS:

• Activated sludge
• extracellular polymeric substances
• microscopic image
• sludge volume index
• time to filtration

Acknowledgements

We are grateful to the staff of Sapporo City for providing samples and to Mr. Sho Taniuchi of Hokkaido University for technical support with respect to sampling and equipment maintenance. We would like to thank Uni-edit (https://uni-edit.net/) for editing and proofreading this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available on request from the corresponding author, YN. The data are not publicly available because some information on sewage samples requires permission from the city.

Additional information

Funding

This research was supported financially by the JST-Mirai Program [grant number JPMJMI18DB], JSPS KAKENHI [grant numbers 21H04568, 20KK0090, 19K21979, and 17K18894], a Gesuido Academic Incubation to Advanced (GAIA) Project awarded by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) [grant number 2016-4], the Northern Advancement Center for Science & Technology (NOASTEC) [grant number 2016-Startup-7], and the Toda Scholarship Foundation [grant number 2016-1].