Activated Sludge

Large activated sludge aeration basin

This the largest, single-cell, activated sludge aeration basin I’ve ever seen, with a volume of nearly 12,000,000 gallons (45,425 cubic meters).


The Activated Sludge Process

The activated sludge process was “invented” around 1913 at the Lawrence Experiment Station in Lawrence, Massachusetts by Clark and Gage and in 1914 by Ardern and Lockett who were chemists with the Manchester Corporation in England.

The primary goal of the activated sludge process is to remove organic compounds (both soluble and insoluble) from wastewater in order to convert the organic material into a flocculant, microbial suspension (biomass) that will easily settle in a clarifier. In a municipal environment it is more likely that the organic compounds will be readily digestible which will result in a high rate of microorganism growth. In an industrial wastewater environment, the microorganisms must often degrade complicated organic compounds which will results in a much slower rate of bacterial growth. For either environment, this growth rate is important in both industrial and municipal wastewater systems because the organisms will reproduce at a rate greater than what will be needed to achieve the desired level of treatment. As a result of this excess growth, there is a need to remove or “waste” microorganisms from the activated sludge process. These waste activated sludge solids will then require further treatment including thickening and disposal and can make up a significant portion of the total cost of operation in a wastewater plant.

The typical, most basic layout of the activated sludge process is shown in Figure 1. Note that a primary clarifier is shown. This is a unit process often found in a municipal wastewater plant. In an industrial wastewater plant you would see an equalization basin instead of a primary clarifier.

Figure 1: Typical Activated Sludge Configuration

Activated sludge simple schematic

In Figure 2 a process flow diagram is shown for an industrial wastewater plant. The primary clarifier has been replaced by an equalization tank. CPI is a corrugated plate interceptor with the purpose of removing free oil before it can reach the complete-mix aeration basin which would add a significant load to the process. The dissolved air flotation (DAF) unit is actually a “Clari-DAF” designed as a circular tank with a bottom rake and surface skimmer.

There are several point to note in Figure 2. For example, you can see the use of a cationic flocculant polymer which is essential in helping to bring the mixed liquor suspended solids together to form a thick mass of solids that can be skimmed off the surface of the DAF. This plant was dealing with a major filamentous growth problem that required the use of calcium hypochlorite added directly to the biological reactor to destroy the sheath of the filamentous strands.

Figure 2: Actual Wastewater Plant Configuration in the Petrochemical Industry

More complicated activated sludge schematic

One more variation of an activated sludge wastewater system serving a petrochemical is shown in Figure 3. This plant is getting complicated. It has two major influent wastewater sources, one from a chemical plant and one from a petroleum plant. The flow from the chemical plant needs pH adjustment before it enters the primary clarifier. The flow from the petrochemical plant, coming in with the primary clarifier effluent, often results in the need to further adjust the pH. These waste streams are both deficient in sufficient quantities of ammonia and phosphate so nutrient addition is required. There’s a lot for the operators to keep track off, to monitor, to pay close attention to, before the wastewater enters the aeration tanks. An in organic coagulant is used to improve settling in the secondary clarifiers. pH adjustment is sometimes required on the effluent from this plant before the discharge enters the Mississippi River.

Waste activated sludge is thickened in a DAF which is then pumped to a sludge storage tank where it combines with solids removed from the primary clarifier. The sludge is dewatered on a belt filter press before being incinerated in a multiple-hearth incinerator. You don’t find too many multiple-hearth incinerators running in the United States these days. This particular unit process used to be one of my favorites to operate, many, many years ago when I was an operator at a municipal wastewater plant in the Northeast United States. All-in-all, this is a complicated wastewater system with many subsystems required to insure the activated sludge process can function properly.

Figure 3: A Complicated Activated Sludge Process

Complex activated sludge schematic

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