What Is Wastewater Lagoon Aeration?
Lagoon Aeration is the process by which both oxygen and mixing are utilized in a wastewater treatment lagoon system in order to create an aerobic treatment environment. Through proper aeration and retention time of typically 20-40 days, aerated lagoons are capable of producing effluents of below 10 mg/L BOD, TSS and, in warmer climates, Ammonia/nitrogen below 10 mg/L. The purpose of this article is to briefly describe and evaluate different lagoon aeration technologies in terms of aeration, installation and operation & maintenance.
According to an EPA factsheet, the purpose of aeration in wastewater treatment is twofold: 1) to supply the required oxygen to the metabolizing microorganisms; and 2) to provide mixing so that the microorganisms come into contact with the dissolved and suspended organic matter (EPA 1999). This definition is perhaps the most important consideration when evaluating lagoon aeration technologies because the effectiveness of a wastewater aeration system depends directly on its ability to both aerate and mix. Without proper aeration, an aerated lagoon will fail to meet its effluent requirements-at that point, installation, operation and maintenance costs will become of lesser importance in comparison.
There are two general categories that lagoon aeration systems fall into. One is surface aeration and is characterized by mechanical equipment floating on the surface of the water; the other is diffused aeration that releases bubbles from a point typically located at or near the bottom of a lagoon. This article will consider mechanical surface aeration, coarse and fine bubble diffused aeration, and a aeration diffuser that combines both coarse and fine bubble in one portable unit.
Mechanical surface aerators are tethered to shore and situated on the surface of the water by floats. There they mix and aerate liquid by use of a motor-mounted propeller. This propeller rapidly moves surrounding wastewater, facilitating the introduction of ambient air. Motor sizes for surface aerators run from 2.5 horsepower to 100+ horsepower.
The advantage of surface aerators is that they are portable and thereby easy to install by simply floating the aerator into its location. Moreover, they are relatively inexpensive in terms of up front capital costs. However, mechanical surface aeration, at 1.25-2.25 pounds of oxygen per horsepower hour, is regarded as less energy efficient than diffused aeration (EPA 2002). Considering that aeration accounts for 60-80% of consumption at most facilities, outdated and inefficient aeration systems are a primary candidate for replacement (EPA 2006). Today, mechanical aerators are still utilized at thousands of wastewater facilities across the nation, but are losing ground to competitors offering increased efficiency and decreased maintenance requirements.
In addition to the relatively low oxygen transfer efficiency of surface aerators, their location on the surface of the water has two distinct disadvantages. First, surface aerators have a limited ability to mix lower than 6′ below where they are located. Sludge accumulation inevitably occurs on the bottom as a result, which can, among other things, take away from the volume of the lagoon, thereby lowering the retention time and circumventing the treatment process. Second, surface aerators have become known to have high maintenance requirements due to having a motor close to the surface of the water. This is especially true in colder climates and is part of the reason that the State of Illinois has banned surface aerators in new lagoon systems.
Coarse Bubble Aeration
Also known as large bubble diffusers, coarse bubble diffusers get their name from the type of bubble they produce. Coarse bubble diffused systems are composed of dozens to hundreds of coarse bubble diffusers, each attached to a PVC or stainless steel air supply line fixed to the bottom of the lagoon. The most common coarse bubble aerator for lagoons is what is known as a static tube. When large bubbles are released at the bottom of the tube, they create an effect whereby water is pulled from below the static tube and pushed towards the surface.
The primary advantage of this technology is its robust design and ability to mix. First, the circulation and turbulence created by the static tube ensures that the entire water column is mixed adequately and sludge accumulation is limited. Second, the fact that there are no moving parts in the water and the orifices are less prone to clogging makes this a very robust solution.
However, while more efficient than surface aerators, with aeration efficiencies between approximately 2-3 pounds of oxygen per horsepower hour, coarse bubble diffusers are 40-50% less efficient than their fine bubble diffuser cousins. Moreover, a fixed aeration header grid makes them quite labor intensive to install and difficult to access for maintenance should anything go wrong. Indeed, due to these factors it is quite rare to see coarse bubble diffusers, new or retrofits, being installed in aeration lagoons today.
Fine Bubble Aeration
Fine bubble technology is a form of subsurface aeration that introduces air into water via very small (or fine) bubbles (<2mm in size). Thousands of fine bubbles promote the transfer of oxygen to water due to their collectively large surface area and low buoyancy, maximizing air-water contact. Typical fine bubble diffusers systems for lagoons can be dropped in from the surface and are self weighted and supplied air from flexible weighted tubing connected to a air lateral pipe floating on the surface. The primary advantage of fine bubble technology is its high efficiency, capable of providing 6-7 lb/O2 hp-h, making it 40-60% more efficient than any other lagoon aeration solution. Moreover, there are many fine bubble technologies specifically designed to retrofit lagoon systems relatively easily without dewatering or installing fixed piping-this makes installation relatively cost effective.
There are two primary disadvantages of fine bubble systems for lagoons. First, because the fine bubbles are so small, they are unable to create a significant amount of turbulence and mixing, which can lead to sludge accumulation and potential treatment problems over time. Second, all fine bubble diffusers require regular maintenance as they are subject to fouling/clogging. Fouling is the process whereby bacteria build up around and impinge upon the orifices where air is diffused. Depending on the type of fine bubble diffuser used, whether that be perforated tubing, EPDM disks or tubes, cleaning is needed on a regular basis.
MARS Aeration Alternative
The MARS Lagoon Aeration diffuser utilizes patented Double Bubble™ technology, which combines two components integral to proper wastewater treatment. The first component is the fine bubble membrane, which harnesses the benefits of efficient oxygenation. The second component works in collaboration with the fine bubble membrane: the coarse bubble static tube aerator. This provides substantial turbulence and mixing capacities that help the MARS treat the entire water column. Together, these components allow the MARS to oxygenate and treat water effectively while still using energy efficiently. The Triplepoint MARS system also features a portable design–each MARS has its own weighted legs and is fed air via flexible weighted tubing. With its flexibility in placement, the MARS can be utilized to manage practically any wastewater treatment cell by simply lowering it in from the surface.
Maintenance is equally as simple–every unit can be individually raised for quick inspection, without dewatering the lagoon. The system’s blowers, the only components requiring some regular maintenance, are located onshore where they can be easily accessed. This also allows for hassle free, onshore treatment control. Designed with no moving parts, the MARS System minimizes maintenance requirements–making it the most sustainable lagoon treatment option available.