All Things Water Course I, Activated Sludge

Russ, the chief engineer at West Tech, introduces the activated sludge process, a crucial biological process in wastewater treatment plants worldwide. The session aims to provide an overview of the process, its parameters, and its role within a typical wastewater treatment plant. The company, West Tech, specializes in supplying and designing large-scale equipment for water and wastewater treatment plants. Russ has extensive experience in the field and holds a master's degree in civil engineering and business administration.

The video script details the typical flowsheet of a wastewater treatment plant, starting with screening to remove debris like rags and plastics. Following this is grit removal to prevent wear on pumps and settling issues. Primary treatment in the form of a clarifier is then described, which can remove about 50% of contaminants. The focus then shifts to the activated sludge process for treating the remaining dissolved and fine particulate contaminants. The script also touches on disinfection methods, solid handling, and the importance of various treatment parameters such as BOD and TSS.

The paragraph delves into the biochemistry behind the activated sludge process. It explains how microorganisms use oxygen to break down organic material in sewage, converting it into carbon dioxide, ammonia, and new microorganisms. The process is not balanced chemically due to the variety of organic chemicals involved, but it provides a basic understanding of how BOD is removed from wastewater. The paragraph also discusses the significance of BOD as a measure of organic matter that can be degraded by microorganisms.

Key operating parameters for activated sludge systems are discussed, including mixed liquor suspended solids concentration, which typically ranges from 2,000 to 4,000 milligrams per liter. The importance of maintaining an adequate dissolved oxygen concentration for the microorganisms to function is highlighted, with a target level of about two milligrams per liter. The potential issues of having too high or too low oxygen levels are also covered, such as the growth of filamentous bacteria and the creation of an anaerobic, smelly environment.

The concept of hydraulic retention time, which is the time the liquid stays in the system, is explained, typically ranging from four hours to 24 hours. The solids retention time (SRT) or mean cell residence time is also discussed, which is crucial for allowing microorganisms to reproduce and grow, typically lasting between 2 to 8 days. The importance of the food to mass ratio, which is the ratio of BOD to the mass of microorganisms, is emphasized, with a normal range of 0.2 to 0.5 to ensure effective treatment and prevent issues like viscous bulking.

Different types of activated sludge systems used in practical plant designs are outlined, including diffused aeration systems with coarse or fine bubble diffusers and mechanical surface aeration systems. The use of these systems in various tank shapes and the specific application of mechanical aerators in oxidation ditches is discussed. The advantages and considerations of each system type are highlighted, emphasizing their role in the aeration and mixing processes essential for effective wastewater treatment.

The video script concludes with a summary of the key treatment parameters and operating parameters discussed, such as BOD and TSS, as well as the importance of maintaining the right balance of microorganisms and their food supply. It reiterates the significance of the activated sludge process in wastewater treatment and briefly recaps the types of systems, including diffused air and mechanical surface aeration, that can be used to achieve effective treatment outcomes.