Biological Wastewater Treatment: Aerobic Versus Anaerobic

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What Is Biological Wastewater Treatment?

Biological wastewater treatment is a process that removes organic impurities from wastewater. It’s part of the secondary treatment process that occurs after mechanical or chemical processes remove substances, grit and suspended solids from the water.

There are two primary types of biological wastewater treatment processes:

  1. Aerobic
  2. Anaerobic

Aerobic wastewater treatment requires the use of oxygen, while anaerobic does not .  Both processes utilise natural organisms, such as bacteria, to further break down the organic waste that remains in the water.

Read more about wastewater treatment, aerobic, anaerobic systems and more below:

 

How Municipal Wastewater Treatment Process Works

Municipal Wastewater treatment typically involves a three-step process to rid it of contaminants so it can be recycled, reused or released into bodies of water.

The three key wastewater treatment steps are:

  1. Primary treatment
    The primary step of the wastewater treatment process handles the removal of solids. This typically happens in a large holding tank where heavier solids sink and lighter solids float. Suspended solids are collected mechanically and the liquid is sent along to the next step in the process.
  2. Secondary treatment
    This step is sometimes referred to as biological wastewater treatment, as it involves the use of bacteria, microorganisms and other biological agents to break down biodegradable contaminants to safe levels, which enables the water to be released or reused.
  3. Tertiary treatment
    This third step encompasses any additional treatment that might be necessary to remove pollutants such as heavy metals and chemical compounds that biological treatment methods cannot remove.
Plant automation made practical by on-line analytical capabilities and real-time control (RTC) helps wastewater treatment operators at every skill level be more effective at proactive management of process efficiency.

Factors to Consider When Choosing a Secondary Wastewater Treatment Method

When it comes to secondary wastewater treatment, there are several things to consider when deciding which method to employ. As mentioned above, there are two different kinds of secondary methods - aerobic and anaerobic.

The method you choose depends on a variety of factors, including:

Aerobic Treatment

Generally, aerobic treatment, which requires oxygen, is the better choice for wastewater streams with lower BOD or a lower concentration of contaminants.

Anaerobic Treatment

Anaerobic treatment is used for streams with a higher concentration of impurities and for warm wastewater streams.

 

How Aerobic Wastewater Treatment Systems Work

In aerobic wastewater treatment, wastewater flows from the primary treatment source into tanks or ponds, where it encounters biological substances, such as bacteria, that need oxygen in order to break down waste.

In a tank setup, air is pumped into the tank to add oxygen to the water and assist the bacteria's biological processes. However, in an open-air pond setup, the pond's surface area is large enough that pumping in extra oxygen isn't necessary.

This aerobic wastewater treatment results in the production of activated sludge—aerated sewage containing aerobic microorganisms developed under controlled conditions.

When added to raw sewage, activated sludge oxidizes the organic solids and separates any solids that have dissolved into the sewage mixture, allowing for easier filtration.

Aerobic processing of waste water in a treatment plant Aeration Basin

 

Oxygen in Aerobic and Anaerobic Wastewater Treatment

Aerobic and anaerobic wastewater treatment are similar processes for eliminating contaminants in wastewater but there are some key differences in the systems that enable these two different types of biological wastewater treatment.

Due to the need for oxygen in the aerobic digestion process, the initial investment in building such a system can be higher. An aerobic wastewater treatment system must have either large shallow ponds that allow microbes in the water to come into contact with the air or tanks that introduce oxygen into the water with aeration pumps.

In contrast, the anaerobic digestion process requires no introduction of oxygen, which means it can be done in closed tanks. Thus, it can be more cost-effective to set up than an aerobic system but it’s also a slower process. Additionally, anaerobic treatment can produce biogas—usually methane—which can be used in a variety of ways, including for vehicle fuel or the generation of electricity.

 

How Anaerobic Wastewater Treatment Systems Work

Anaerobic wastewater treatment uses a different biological process to break down contaminants — one that does not require the addition of oxygen. In this process, wastewater flows from the primary treatment process directly into closed tanks or lagoons, where biological organisms that don't require oxygen break down the impurities.

This process also results in the creation of activated sludge, but at a much lower quantity than aerobic treatment produces.

 

Key Differences: Aerobic Versus Anaerobic Wastewater Treatment

Aerobic and anaerobic wastewater treatment are similar processes for eliminating contaminants in wastewater but there are some key differences in the systems that enable these two different types of biological wastewater treatment.

Due to the need for oxygen in the aerobic digestion process, the initial investment in building such a system can be higher. An aerobic wastewater treatment system has to have either large shallow ponds that allow microbes in the water to come into contact with the air or tanks that introduce oxygen into the water with aeration pumps.

In contrast, the anaerobic digestion process requires no introduction of oxygen, which means it can be done in closed tanks. Thus, it can be more cost-effective to set up than an aerobic system, but it’s also a slower process. Additionally, anaerobic treatment can produce biogas (usually methane), which can be used in a variety of ways, including for vehicle fuel or the generation of electricity.

Aerobic Treatment

  • Requirements:
    Requires oxygen for the biological process to work.
  • Energy Efficiency: 
    Requires energy to introduce oxygen into the process.
  • Appropriate for:
    Low-to medium-strength wastewater (fewer than 1,000 ppm), e.g., municipal sewage and refinery wastewater.
  • Production:
    Relatively large amounts of activated sludge; no biogas.
  • Equipment and Technologies Used:
    Trickling filters, rotating biological reactors, oxidation ditches.

Anaerobic Treatment

  • Requirements:
    Uses a biological process that does not require oxygen.
  • Energy Efficiency:
    Does not require oxygen to be added to holding tanks, resulting in more energy efficiency than aerobic treatment.
  • Appropriate for:
    Medium-to high-strength wastewater (greater than 4,000 ppm), e.g., food and beverage industry wastewater.
  • Production:
    Biogas (methane and carbon dioxide); relatively low amounts of activated sludge.
  • Equipment and Technologies Used:
    Anaerobic lagoons, septic tanks, anaerobic digesters, continuous stirred tank reactors, upflow anaerobic sludge blankets.

 

 

Why Exact Measurements are Critical in Biological Wastewater Treatment

When you’re managing a biological wastewater treatment system, having exact measurements is the key to maintaining an efficient system that works the way it should.

There are a number of details that must be considered to keep a treatment system running properly and none can be overlooked if the goal is to run an efficient system without wasting money or energy.

Dissolved Oxygen and Wastewater

One of those details is dissolved oxygen (DO) and the amount and frequency of replenishment that’s needed as water leaves a treatment facility. DO levels must be precisely monitored so the treatment facility stays in compliance with regulatory discharge standards and to protect the environment.

The energy it takes to maintain an aerobic treatment system is another area where precise monitoring is important. Powering the aeration system in an aerobic treatment process can account for as much as 70% of the energy costs in a wastewater treatment plant. Plants that aerate constantly might also be maintaining a higher DO level than they need, which wastes money and energy.

Monitoring of Dissolved Oxygen (DO)

Careful monitoring of DO increases the efficiency of aeration basins and taking online DO measurements can help a facility adjust aeration to match the DO required by the organic load.

Continuous measurement and adjustment of DO ensures that the biological materials have enough oxygen to consume organics, while allowing the facility to conserve energy by reducing aeration when possible. It also prevents over-aeration, which impacts downstream processes.

 

 

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