Automatic Wasting & Activated Sludge management for sewage treatment works - Roger Powell

There are three key controllable processes in an activated sludge process:

1) Dissolved Oxygen in the aeration basins 2) Return Activated Solids flow rate 3) Waste Activated Solids flow rate After discussing basic activated sludge treatment plant operation, this paper will present a proven method of reliably controlling the flow rate of the Waste Activated Solids thus effectively controlling the bacteria population to a value that gives the most efficient treatment of wastewater for minimal cost to treat.

Activated Sludge Treatment Plant Purpose and Operation

The purpose of a wastewater treatment plant is to remove biological oxygen demand (BOD) and reduce nitrates, nitrites, phosphorus, and total suspended solids prior to releasing the water back into the environment. If the wastewater is properly treated, then its release will not have a negative effect on the environment that it is released into.

An activated sludge treatment plant uses bacteria in suspension to treat the waste through consumption of the waste. Please refer to the diagram below. Raw wastewater enters the plant through mechanical bar screens to remove the large debris that would damage plant equipment. After that, easily settleable materials are removed from the wastewater by the primary clarifier so that they don’t have to be biologically treated later in the aeration tanks, reducing treatment costs.

The aeration basin is where the food and oxygen are provided to the bacteria for them to consume the wastes in the water and not suffocate and die due to lack of oxygen in the water. Because raw wastewater containing food is always entering the aeration tanks, the bacteria that have finished eating get hydraulically pushed into the secondary clarifiers. In these clarifiers, the bacteria settle to the bottom and clean water is reintroduced into the environment. Most of the bacteria on the bottom of the clarifier are returned back into the aeration basin (Return Activated Sludge) so that they can continue to eat again. This forms a closed loop system of bacteria eating in the aeration basin, being separated from clean water in the secondary clarifiers, and returned back to the aeration basin to eat again. Also keep in mind that the bacteria, like any living organism that is provided a steady supply of food and a good environment to live in, will procreate and produce more bacteria.