Choosing the Proper Equipment for Extended Aeration Lagoons
Extended aeration lagoons can be an excellent solution to the wastewater treatment needs of a small municipal or industrial wastewater treatment plant. Where land is inexpensive and available, the extended aeration lagoon is simply the most cost-effective and easiest design approach. Because the extended aeration lagoon naturally results in sludge settling, decomposition and clarification, the need for and expense of separate clarifiers and sludge removal is eliminated.
For the extended aeration lagoon system to have long-term operating success, the equipment chosen to dispense and distribute the air must be durable, rugged, maintenance free, economical, and have good mixing and oxygen transfer.
Mechanical surface aerators are used because of their low cost and simplistic design. In an extended aeration lagoon, these aerators are generally located a significant distance from the shoreline, where they are most effective.
This location, however, can present an accessibility problem when maintenance is required. As mechanical surface aerators employ mechanical moving parts, they are prone to problems resulting from the demanding lagoon environment.
In an extended aeration lagoon, suspended solids naturally settle to the bottom as an inherent function of the process. In such an environment, course and fine bubble diffusers will clog, requiring ongoing maintenance.
Static tube aerators have long been utilized in extended aeration lagoons with an impressive record. These devices have many inherent characteristics that make them well suited for this application. Static tube aerators are known to require essentially no maintenance.
The internal diffusers in the static tube act to create a turbulent environment for the air and liquid mediums passing through them, thus, shearing the rising bubbles into smaller diameter bubbles and increasing the oxygen transfer efficiency. The vertical, tubular nature of the static tube acts as an airlift pump. Depending on the suspended solids content of the wastewater in an extended aeration lagoon, mixing can be as important, if not more important, than the actual oxygen delivered.
As a result, the water passing through the static tube aerator produces a circular mixing effect. The static tube aerator is the ideal aeration/mixing device for the extended aeration lagoon environment. As a course bubble diffuser, the oxygen transfer is sufficient for the lagoon process.
Static tube aerators offer the lowest installation cost when compared to any of the alternative aeration devices available on the market today. Because of their superior mixing ability, fewer aerators are required, they can be spaced at larger intervals, and they require less aeration piping. There are no moving parts or any expendables. If maintenance cost is included in the evaluation procedure, static tube aerators truly stand alone as a solution for extended aeration lagoons, as they historically have required little to no maintenance.
Aeration system control strategies
The modern small wastewater treatment plant has unique demands that the responsible design engineer should heed in developing a controls strategy. Capital cost must be kept to a minimum.
However, the “cost” to be considered is not solely composed of the initial capital investment. Operational and maintenance costs over the life of the equipment greatly exceed the initial cost of the equipment. Control policies to be implemented in a plant must reduce the operational investment of an end user. The most direct way to reduce operational costs in the small wastewater plant is to simplify and streamline the work of the human element.
Regardless of the approach, a small wastewater plant’s instrumentation and control package must possess a number of qualities to be justified and practical. In general terms, the system must be fail-safe, rugged, easy to understand and troubleshoot, flexible and, of course, economical.
Today, it is quite common in the small municipal wastewater treatment industry for one operator to care for a number of plants. Thus, remote and autodialer systems that will notify an operator when a particular event transpires (e.g. blower shutdown) are fundamental. The operational philosophy of many small plants is to tie in a few simple alarms to the autodialer system that will notify the operator of an issue. Additional items that can be introduced into the system control panel (SCP) include mechanical alternators that can start a stand-by blower in the event of a main blower trip, and/or alternate blowers based upon a mechanical timer for equal “wear and tear.”
In recent years, programmable logic controller (PLC) technology has become both reliable and affordable. Numerous models are available, depending on the specific application. For a small wastewater plant, in addition to matching all the options of a mechanical alternator, a PLC-based SCP can provide for more efficient, trouble-free operation by allowing the aeration system to respond to ever-changing process variables (e.g. dissolved oxygen, diurnal flow, etc.). The net result is a system that is capable of running efficiently, without continual human intervention.
In the pursuit to design the most cost-effective, efficient and trouble-free system possible, the aeration equipment supplier should provide the control system. Because the blowers, underwater aerators and system controls encompass the aeration system, a single-source supplier ensures compatibility and responsibility. Single-source responsibility equates to quick, effective solutions, and simplified overall design and installation.