As the human population grows and industrial and agricultural activities increase, water quality is becoming one of the most pressing environmental and health issues of the 21st century. Efficient wastewater treatment is one of the main pillars of water safety; efficient water purification methods need to be applied whenever possible to protect biodiversity, ecosystem stability, and sanitary safety.
Although heavy metals, crude oil and other sporadic pollution sources from fallouts are often in the spotlight, the most common and persistent source of water pollution is organic matter – coming from sewage, domestic effluents and agricultural runoff. Many industrial sources such as slaughterhouse wastewater, wineries, and paper mill wastewater are also high in carbonaceous and nitrogenous compounds.
When the untreated wastewater enters natural water bodies – streams, rivers, lakes and seas, it can quickly deplete the water source of the dissolved oxygen (DO) through undesirable microbial activity. Hypoxia and anoxia are the leading causes of aquatic ecosystem disruptions worldwide, resulting in disastrous events – from fish die-offs in rivers to anoxic ‘dead zones’ in the seas, void of all life.
Also, excessive organic matter can present a problem in artificial ponds and lakes, compromising the water quality of these sensitive and often unbalanced aquatic systems.
How aquatic hypoxia and anoxia occur
Dissolved oxygen (DO) is a measure of molecular oxygen dissolved in water.
Naturally, oxygen enriches the water and dissolves in it through several mechanisms: production of oxygen by photosynthesizing plants and algae, wind action, natural water flow and interaction with the environment (e.g., crashing against rocks). This is important to remember for later – to understand how aerating systems work.
Oxygen depletion occurs when DO concentration falls below the level that the aquatic organisms require to survive, eventually leading to ecosystem collapse.
There are two stages of oxygen depletion in aquatic systems:
- Hypoxic or Dysoxic system is an aquatic system has 1-30% oxygen saturation; fish can’t stand less than 30% of DO.
- Anoxic system has 0% saturation – it is devoid of all oxygen and aerobic life.
In comparison, water from a healthy aquatic ecosystem has, on average, 80-120% of DO (6.5-8 mg/L).
Oxygen Depletion On The Example Of Eutrophication
The most common biological mechanism of oxygen depletion is called eutrophication and it occurs in several stages:
- Polluting effluent rich in organic matter enters the aquatic system.
- If temperature and salinity are favorable, microorganisms feeding on the effluent components begin to reproduce exponentially;
- Algae are usually the first to start taking advantage of the nutrient influx, which leads to the so-called algal blooms.
- Green algae that multiply can produce oxygen during the day; however, they consume oxygen during the night in the process of respiration.
- Aerobic oxygen-dependent organisms – including the algae – die from hypoxia, fall to the bottom of the aquatic system and decay, further consuming the dissolved oxygen.
- The whole aquatic system crashes and only anaerobic bacteria that do not require oxygen are able to survive.
- The degradation of organic matter by anaerobic organisms is slow, produces unwanted gas emissions and foul odors.
Eutrophication and other forms of oxygen depletion can also occur naturally in stagnant waters when conditions are favorable.
Importance of Dissolved Oxygen in Wastewater Ponds and Lagoons
Beyond natural ecosystems and life-supporting decorative ponds, it is highly important to point out that wastewater ponds and lagoons also require a sufficient level of oxygen to support aerobic microorganisms that remove pollution quickly and efficiently, without unpleasant smells and environmentally problematic side-products.
Aerobic water treatment ponds should maintain a minimum DO level of 2.0 mg/L at all times and within the entire pond.
Anaerobic water treatment ponds do exist, but are outdated and used only in specific circumstances – because the properties of anaerobic systems stated above.
Fortunately, there are effective technical solutions to tackle the hypoxia issue. Aeration is a process of actively increasing dissolved oxygen content in the water to improve its properties and purification capacities – by supporting aerobic organisms that clean the water.
Active aeration is performed by aeration systems – aerators.
Dissolved oxygen is measured by using a calibrated DO sensor. The sensor is put directly in the water and measures the amount of DO in terms of percentage (%DO), and metric units (milligrams of oxygen per litre of water, mg/L).
How Does Aeration Clean Water And Improve Water Quality?
By mimicking natural mechanical means of aeration such as wind action, mechanical aquatic aerators use engine power to agitate the water surface and insert atmospheric oxygen into the water, thereby increasing the water’s DO concentration.
In turn, aquatic microorganisms that feed on the carbonaceous and nitrogenous waste material start to reproduce and increase their numbers exponentially. However, now they cannot deplete the oxygen since it is artificially injected by an aerator at regular intervals, so the microbial activity continues until the organisms consume almost all of their food, cleaning the water in the process.
Aeration is used in artificial aquatic systems to support aquatic life (e.g., in aquariums, fish ponds), or to help and drive microbial wastewater treatment.
However, sometimes aeration alone is not enough. Since the needed microorganisms are usually not present in the wastewater tanks and lagoons or their numbers are not sufficient, they are added to the water through the activated sludge process.
The activated sludge process uses a combination of artificial aeration and a microbiological floc (bacteria and protozoa suspension) to treat wastewater. Aeration tank is the first component of the system and the place where most of the oxidizing action occurs. It is followed by the settling tank which serves to separate the biological floc by letting it settle to the bottom. From there, the floc can be separated and used again or further degraded, while the fully treated water can re-enter a natural system, or used otherwise.
Types of Aeration
There are several types of aeration, and each has its advantages and limitations.
Natural aeration includes:
- Plant and algae aeration. Chlorophyll-containing organisms produce oxygen in the presence of sunlight; however, they have to use the oxygen at night, so water systems that have too many algae or plants will be depleted of oxygen rather than enriched.
- Wind action. The wind breaks the surface of the water and allows oxygen to enter the aquatic system.
Artificial aeration includes:
- Mechanical aeration. Mechanical aerators provide surface aeration, so they are also called surface aerators. Similar to wind action, they agitate the water surface to cause splashes, waves and droplet sprays, allowing the oxygen into the water.
There are several main types of mechanical aerators:
- low-speed surface aerators,
- floating surface aerators,
- submersible motor aerators,
- paddle wheel aerators,
- and fountains.
Diffused aeration is subsurface aeration. Diffused aerators rely on a series of underwater blowers to insert air into the aquatic system.
- The main types of subsurface aerators include coarse bubble aerators, fine bubble aerators, and jet aerators.
These artificial aerators are utilized for handling municipal, industrial and agricultural wastewater, as well as to revive, chemically stabilize, or maintain stagnant small water bodies.
It is clear to see that aerators are the key components of wastewater treatment processes.
However, not all aerators are made equal – and not all types are appropriate for all purposes. The choice of aerator type will depend on the technical parameters, construction compatibility with the aquatic system in mind, and maintenance requirements.
Surface aerators are versatile and can be used for agriculture wastewater treatment, as well as in various industrial settings – treating wastewaters from the food processing industry, paper and pulp industry, and even the petrochemical industry.
Their other advantage is the low maintenance level. Besides periodic performance check-ups, surface aerators do not require regular diligent inspection and cleaning, as subsurface diffused aerators do.
Surface aeration increases evaporation rates, and the heat adds to this evaporation. Excessive evaporation can have an adverse effect on the process of oxygenation itself, and other water quality factors.
Oxygen Transfer Rate (OTR) is the mass of oxygen transferred by an aerator machine, per unit of time. It is an important technical parameter for aerator assessment – the key variable for aerator design, and one of the main indicators of aerator efficiency.
Specific kinds of aerators have specific OTR values. In general, surface aerators have a high OTR. However, design advancements can improve OTR even beyond this expected level.
For example, the larger intake and outtake openings of EEE Floating Aerators, as well as their larger diffusers and a high-velocity spray pattern, allow for a higher volume of oxygen transfer than the competing models in the same category.To Sum It Up
- Level of dissolved oxygen (DO) is perhaps the most important indicator of water quality and its ability to support aquatic life.
- Pollution from sewage, industrial and agricultural effluent negatively impacts the DO level by driving processes which deplete the aquatic system of oxygen and leads to its biological collapse.
- Active water aeration is the process of enriching the water with atmospheric oxygen via suitable aerator systems.
- Aerators can be used to maintain the quality of water in a man-made pond or to drive the purification of wastewater in ponds and lagoons.
- There are several main types and many sub types of aerators; the choice of an aerator depends on the type of aquatic system, aerator performance indicators such as Oxygen Transfer Rate (OTR), maintenance requirements, and overall product quality.
- Surface aerators are versatile, easy to maintain, and have a high OTR.
Although there are defined average values of OTR and other indicators for each aerator type, the fine-tuning of the design can enhance these properties; EEE Aerators have a number of design improvements that make their performance superior in comparison to competitors in the same category.