Wastewater contains solids and dissolved pollutants, including fecal matters, papers, sanitary items, food residues and a variety of other contaminants. The sewer network usually receives wastewater from commercial offices, residential buildings and industries, and the combined flow from these various sources is discharged into a sewage treatment plant, where it is treated.

The overall goal of the activated sludge is to reduce or remove organic matter, solids, nutrients, and other pollutants from wastewater. More specifically, the activated sludge process involves blending settled primary effluent wastewater with a culture of microorganisms into fluid called “mixed liquor”. The mixed liquor will be discharged in an activation tank, in which air is introduced into the system to create an aerobic environment that keeps the activated sludge properly mixed. The bacteria stabilize the substances that have a demand for oxygen (BOD5), before being discharged in a clarifier where suspended solids and liquid are separated.
Parameters of a wastewater treatment plant
Before describing the extended aeration process, it is important to define some parameters related to the functioning of a wastewater treatment plant:

Suspended SOLIDS
Some pollutants do not dissolve in water as their molecules are too big to mix between the water molecules. These materials are called particulate matters and can often be a cause of water pollution. The size of these particles is bigger than 1 micron (1 micron = 1 mm/1,000).

Suspended solids concentration of some effluents

Effluent Suspended SOLIDS (mg/l)
Municipal 150-300
Dairy industry 150-600
Brewery 1,000-2,000
Distillery 1,500-3,000
Sugar refinery 1,500-3,500

Biological oxygen demand (BOD5)
The amount of dissolved oxygen consumed in five days by biological processes breaking down organic matter.

Chemical oxygen demand (COD)
The COD test is used to indirectly measure the amount of organic compounds in water.

COD concentration of some effluents

Effluent COD (mg/l)
Municipal 700
Dairy industry 2,000-6,000
Brewery 2,000-3,000
Distillery 30,000-40,000
Sugar refinery 10,000-15,000

Ratio of biodegradability
Ratio of biodegradability= COD/BOD5

In case the ratio COD/BOD5 > 3 that means the pollution is not biodegradable. Thus, the treatment by sludge activation cannot be applied.

Mixed liquor volatile suspended solids
MLVSS is generally defined as the microbiological suspension in the aeration tank. The biomass solids in a biological wastewater reactor are usually indicated as total suspended solids (TSS) and volatile suspended solids (VSS). The mixture of solids resulting from combining recycled sludge with effluent wastewater in the bioreactor is termed mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS).

Food to microorganism ratio (f/m)
This is the relationship between the load of BOD5 entering the activation tank and the mass of bacteria available in the aeration tank. This parameter is very important, as it has a direct impact on the volume of the activation tank, the quality of sludge, and the process of nitrification/denitrification. A f/m ratio between 0.05 - 0.15 kg BOD5 /kg MLVSS is usually considered acceptable for an extended aeration process.

Advantages and drawbacks of a low f/m

Advantages Drawbacks
High degree of elimination of BOD5 Long retention time -> bigger activation tank
Good nitrification/denitrification Sludge more concentrated -> slower decantation
-> bigger area for the clarifier
Good settleability to sustain
shock and toxic loading
Higher consumption of oxygen

Sludge age
Sludge age is the average solids retention time in the biological treatment process. The solids retention time is equal to the mass of solids in the aeration tank divided by the mass of solids leaving the system.
A treatment plant with a high sludge age is characterized by a low f/m ratio and a long sludge retention time. When the sludge age increases, the phenomenon of defloculation occurs, and metazoans appear.

Level of BOD5 and total nitrogen after treatment
Sludge age (days) BOD5 (mg O2/l) Total Nitrogen(mg /l)
≈ 2 30 -
≈ 4 15 -
>13 - 20
>18 - 10
Sludge volume index
The SVI is the volume in ml occupied by 1 g of activated sludge after settling the aerated liquor for 30 minutes. SVI is a very important indicator of the quality of the sludge. In the clarifier, the value of the SVI should be between 100 and 180 ml/g of suspended matters. This parameter is an indicator of the quantity of sludge to be returned to the activation tank, and the quantity to be taken out of the system.

Extended aeration process
The extended aeration process is characterized by a low food to microorganism ratio, a long sludge retention time, and a concentrated sludge. The modification of one of these parameters will modify the entire process performance of the treatment plant. For example, by increasing the f/m ratio, the process of nitrification/denitrification will be less efficient, the sludge age will decrease, and the produced sludge will be less concentrate.

At the inception of a wastewater treatment plant, the quantity of pollution exceeds by far the quantity of food.

Evolution of the food to microorganism ratio
Sludge age (days) f/m ratio
2-4 Very high
7-8 Moderate
14-18 Low
>40 Very low

When the f/m ratio is high, microorganisms are in log growth phase, which is characterized by excess food and maximum rate of metabolism. As a result, microorganisms remain in a dispersed state and neither settle out of solution by gravity in the settling tank, nor can be separated easily from the effluent to be returned to the activation tank.

However, at low f/m ratio, the metabolic activity is in endogenous phase where the rate of metabolism by wastewater microorganism is low. The large mass of waste microorganisms present then competes for the relatively smaller amount of food available in the effluent, and under aerobic conditions rapidly flocculates to settle out of solution by gravity.
General process of a basic wastewater treatment plant
Screening is the first step in a wastewater treatment plant. The aim of the screening system is to retain, before the biological treatment plant, any solid having a dimension of 2 cm or more, which can disturb the good running of the plant. Once this stage is completed, the wastewater is discharged in an activation tank.

Activation tank
The screened wastewater is introduced inside the activation cell, where it is mixed with the activated sludge. The homogenization inside the bacterial environment and the oxygenation are obtained by means of positive displacement air blowers. The air is introduced into the tank through fine bubbles air diffusers, set in the best way to prevent dead spots in the aeration volume. In addition, for a proper denitrification process to occur in the activation tank, the aeration time should not exceed 13 hours per day, since the process takes place during an anoxic phase, when no free oxygen is available. The concentration of the sludge should be around 4 g/l; in case this parameter goes higher than this value, we should extract sludge from the clarifier.

The settling tank receives by gravity the bacterial suspensions (the settling rate should be around 0.6 m/h). Then progressively, the sludge leaves the liquid phase, due to a good retention period, and adapted surface load. Part of the sludge stuck in the bottom, the return activated sludge (R.A.S) is lifted, pumped, and taken back to the activation cells, in order to achieve the mineralization and avoid any anaerobic fermentation. The rest of the sludge (the wasted activated sludge (W.A.S)), is sent to the solid handling process. The suspended solids concentration of the recirculated and extracted sludge is about 8 g/l. The clarified water is taken by a crenellated weir, and evacuated by gravity towards the outlet. The retention time in the clarifier is approximately 3 hours, and the sludge volume index should be around 180 ml/g.

Nitrification- denitrification process

The nitrogen removal process consists of 3 steps:ammonification, nitrification and denitrification
Ammonification – Nitrification - Denitrification

The first step occurs in the sewer pipes before arriving to the station. The majority of nitrogen is converted from organic nitrogen to ammonium NH4+.

The conversion of ammonium to nitrate nitrogen is called nitrification; it is a two-step process. First, ammonium is converted to nitrite (NO2-). Then the nitrite is converted to nitrate (NO3-). In order to perform this step, the bacteria must have free oxygen to perform their works. The nitrification occurs only under aerobic conditions.

The conversion of nitrate (NO3-) to nitrogen gas (N2) is called denitrification. The process is performed under anoxic conditions.
Additional wastewater structures
Balancing tank
The aim of the balancing tank is to equalize concentration fluctuations of the incoming wastewater and to attenuate the effect of flow surges. Oxygen is injected into the balancing tank bottom, through air diffusers, in order to prevent anaerobic conditions. A balancing tank can also be useful in reducing the volume of the clarifier.

Grit and grease removal
The grit removal system is designed for the mechanical removal of grease, grit, and floating materials, and should take place before the biological treatment. The retention time varies between 15 and 20 minutes (with a minimum retention time of 5 minutes). In order to avoid any anaerobic fermentation, the tank should be aerated.

Sludge holding tank
The wasted activated sludge (WAS) can be stored in a sludge holding tank, which acts as a thickener allowing the sludge concentration to increase. In order to avoid anaerobic fermentation, the tank should be aerated.

Belt filter press
Belt filter presses are continuous-feed sludge-dewatering devices that involve the application of chemical conditioning, gravity drainage, and mechanically applied pressure to dewater sludge. In most types of belt filter presses, conditioned sludge is first introduced on a gravity drainage section where it is allowed to thicken. In this section, a majority of the free water is removed from the sludge by gravity. Following gravity drainage, pressure is applied in a low pressure section, where the sludge is squeezed between opposing porous cloth belts. The squeezing and shearing forces thus induce the release of additional quantities of water from the sludge. The final dewatered sludge cake is removed from the belts by scraper blades.