4.3.2.1 Activated Sludge Process

Categories: 4.3.2 Suspended growth Process
Time of issue: 2022-04-28 18:29:43
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Activated Sludge Process

An activated sludge process refers to a multi-chamber/tank reactor that makes use of highly concentrated and suspended microorganisms in aerobic conditions to degrade organic matters and remove nutrients from wastewater to produce a high-quality effluent.

Treatment Process:

The activated sludge process usually includes an aeration tank following by a clarifier. In the aeration tank with air or oxygen bubbled through, the suspended microorganisms will degrade soluble, colloidal, and particulate organic substances in the wastewater. After biodegradation process, the physical liquid-solid separation takes place in the follow-up clarifier. The two tanks are connected at the bottom, and the settled microorganisms in the clarifier are pumped back into the aeration tank for recycling. The activated sludge process can also achieve biological nitrification and denitrification as well as biological phosphorus removal.

Design Criteria:

Selection of reactor type. Important factors shall be considered when selecting a reactor type, including: (i) wastewater characteristics (e.g., pollutant concentration, pH); (ii) local environmental conditions (e.g., temperature, which effects microbial reaction rates); (iii) effluent discharge requirements; (iv) the effects of reaction kinetics; (v) oxygen transfer requirement; (vi) construction and operation costs; and (vii) expansion to meet future demand.

There are two typical types of activated sludge processes, which are plug-flow process and complete-mix process as illustrated in Table 4.3. The plug-flow process, also known as conventional activated sludge process, requires more space than the complete-mix process, but have less risks at short circuiting. Designers can make proper modifications to these processes to design a system that is more suitable for the available site conditions. The energy and labor requirements are main factors influencing the selection of modifications.

Table 4.3 Two typical types of activated sludge reactor

Plug-flow reactor	Complete-mix reactor

Both influent and return sludge enter at the head of the aeration tank, and travels through the tank at a constant rate to the point of discharge. The concentration of soluble substances and suspended solids varies along reactor length. Oxygen demand varies along the length of the reactor – decreases gradually from the inlet to the outlet of the aeration tank. Aeration capacity is uniform along the length of the aeration tank. Suitable for low-strength wastewater. with minimal peak load considerations.	Soluble substances and suspended solids distribute uniformly within the aeration tank. Oxygen demand is uniform in the aeration tank. The complete-mix process has greater hydraulic loading capacity and is able to handle surges in loading.

Key parameters

Mixed liquor suspended solids (MLSS): typically 2,000 to 4,000 mg/L, which represents the concentration of microorganisms in the wastewater. The setting of MLSS should take into consideration the air requirement and clarification capacity of the system.
Dissolved oxygen (DO): usually set at around 2 mg/L. Too high DO will consume too much energy and favor the growth of filamentous organisms that not easy to settle down. Too low DO will lead to anoxic or anaerobic conditions.
Hydraulic retention time (HRT): typically 4 to 24 hours. It depends on how many pollutants the system is designed to remove. Too short HRT will lead to insufficient time for microorganisms to break down the organic matters in wastewater.
Solid retention time (SRT): typically 3 to 5 days for BOD removal, and 3 to 18 days for nitrification. SRT is normally longer than HRT as the sludge is returned to the tank from clarifier. Too short SRT will lead to over waste of microorganisms and reduce the MLSS.
Food to mass ratio (F/M): typically 0.2 to 0.5. Too high F/M will lead to insufficient BOD removal and favor the viscous bulking with poor settling capacity. Too low F/M will favor the growth of filamentous organisms.
Solid recycle rate: 15%-100%.
Tank volume: the aeration tank volume should be equivalent to the daily design flowrate.

Aeration. In the activated sludge system, oxygen is transferred into wastewater by either a diffused aeration system or a mechanical aeration system. Oxygen transfer efficiency depends on the contact time between the bubbles and the wastewater, the size of the bubbles, and the turbulence of the wastewater. Longer contact time, smaller bubbles and more turbulence of the wastewater creates greater transfer efficiencies. As a diffused aeration system will consume approximately 50% of the total energy consumption of the entire wastewater treatment system, designers need to take the energy consumption into consideration when designing the aeration system. A combination of fine bubble diffusers, full floor coverage diffuser placement, and an annual cleaning requirement may be the most efficient system.

Operation and Maintenance:

General

Require a highly skilled and technical operator to manage the system.
Regular monitoring on the quality of influent (BOD, pH, flow rate), the dissolved oxygen level, MLSS, the sludge age, the balance of loading to organism mass (F/M ratio) in the reactor to ensure proper operation of the reactor.
Daily observations should be performed for the clarifier, such as checking for unusual vibrations or noises, scum collection, weirs, and floating solids.
Biomass will be removed when the clarifier is more than one-third full.
Require regular maintenance of the mechanical equipment, including blowers, diffusers, and pumps for return sludge and waste sludge.

Aeration system

To control the dissolved oxygen level in diffused air systems, the operator can: (i) adjust the air valves; (ii) control the blower output, such as using variable frequency drives (VFD); (iii) increase or decrease the number of blowers in operation; (iv) clean or replace diffusers; (v) change the number of diffusers; and (vi) control the treatment process, such as control the MLSS levels.
To control the dissolved oxygen level in mechanical aeration systems, the operator can: (i) increase or decrease the aerator speed by using VFDs; (ii) increase or decrease the aerator submergence by adjusting the water level in the aeration tank; (iii) increase or decrease the number of aerators in operation; and (iv) control the treatment process, such as control the MLSS levels.
During operation, when unusual noise or vibration occur in the aeration system, the operator needs to check: (i) the lubrication degree of blowers and motors; (ii) discharge pressure and temperature; (iii) filters and obstructions; and (iv) blower seals.
aeration is not maintained at a sufficiently high level (DO>2mg/L), or is too high (DO>4mg/L) which can cause foaming.

Sludge management

The color of healthy activated sludge is tan to brown.
If young sludge is observed (such as billowing whitish foam), it is necessary to decrease wasting rates to increase the amount of solids under aeration, reduce the F/M ratio, and increase the sludge age.
If old sludge is observed (such as dark brown foam with greasy or scummy appearance), it is necessary to increase wasting rates to reduce the amount of solids under aeration, increase the F/M ratio, and decrease the sludge age.
If there is no sludge blanket in the clarifier and a thin return activated sludge, the operator shall decrease the sludge return rates.
If the MLSS level is low and there is a sludge blanket build-up of solids, the operator shall increase the sludge return rates.
If filamentous bulking sludge is observed, under which circumstance the sludge blanket in the clarifier is near the surface and large quantities of MLSS are carried into the system effluent, the operator shall increase the F/M ratio.

Capital Cost and O&M Cost

High capital costs
High operating costs due to high energy consumption needed for aeration. Electricity requirement depends on the daily treatment volume, the wastewater strength (BOD, NH₃-N), and the configuration of aeration system.

Applicability