Aerated Lagoon Design: Partial Mix vs. Complete Mix

According to an EPA wastewater technology fact sheet, the purpose of wastewater aeration is twofold: to supply the required oxygen to the metabolizing microorganisms; and to provide mixing so that the microorganisms come into contact with the dissolved and suspended organic matter.

aerated lagoon

Robust mixing accelerates aerated lagoon treatment.

Thus, mixing is a critical component of wastewater lagoon aeration. You can design an extremely efficient aeration system, but if it fails to adequately mix and keep solids in suspension, eventually sludge will build up and lead to diffuser clogging, short circuiting, and insufficient treatment. (For more information on what can happen when there is inadequate mixing, see our blog Causes and Effects of Wastewater Lagoon Sludge Explained.) This article will define and compare partial-mix and complete-mix aerated lagoons, and outline some design considerations to optimize treatment and energy efficiency.

Partial-mix aerated lagoon

In a partial-mix aerated lagoon, the aeration is designed to provide the oxygen needed for biological treatment, and to provide only partial-mix conditions. As a result of this being a partial-mix environment, there is not sufficient energy/mixing to keep 100% of the biological solids from settling. A small percentage of the settled solids will lurk at the bottom in an anaerobic zone, or sludge storage area, where they decompose anaerobically and slowly. Depths generally range from 6–20 feet, with the shallower lagoons likely to have been facilitative lagoons upgraded with aeration to improve treatment. The vast majority of aerated lagoons in the United States are partial mix and use either surface or diffused aeration.

Complete-mix aerated lagoon

aerated lagoon

The complete mix cell is followed by a quiescent cell (foreground) for the settling of solids.

By definition, complete-mix lagoon aeration is designed to keep 100% of the solids in suspension. When the biological solids are kept in suspension at all times, they digest up to 10X faster as compared to a partial-mix environment. There are a number of reasons that a complete-mix aeration system would be selected, including needing to achieve more treatment in the same footprint; the EPA compares a complete-mix aerated lagoon system to an activated sludge plant without the sludge recycling. Another reason would be if you are returning sludge from a clarifier or DAF system and need to maintain a suspended solid level to further enhance treatment. For further information on this approach see our LRAS Advanced Lagoon Treatment page.  Finally, complete-mix lagoons can be utilized as flow equalization basins, where plant influent is stored during peak flow conditions so that a continuous flow can be pulled into the primary treatment system. By completely mixing the basin influent, solids are not allowed to settle so they do not have to be removed later.

Design considerations

Area of influence: In a partial-mix aerated lagoon design, there should be sufficient aerators positioned closely enough together to avoid large dead zones, where sludge can form in drifts, causing short circuiting and clogging diffusers.

As a rule, the maximum areas of influence of each aerator should touch on the influent end of a lagoon and be no more than 25-50′ apart on the effluent end of a lagoon. Different types of diffusers have different areas of influence. The MARS Lagoon Aerator, for example, can have areas of influence up to 175’ in diameter around the diffuser depending on depth. Purely fine-bubble diffusers can have areas of influence between 20–25’ in diameter because they are unable to move as much water per assembly. This generally means that more fine-bubble diffusers are needed for mixing, increasing capital and long-run maintenance costs.

Long run life-cycle costs: Long run life-cycle costs are a balance between capital and operational costs. A complete-mix system typically requires 0.75 hp per 1,000 cubic feet of capacity; a partial mix system will use 0.5 hp per 1,000 cu ft of capacity. As a result, complete-mix lagoons require more horsepower and aeration equipment as compared to partial-mix systems. Purchasing less aeration equipment means lower initial capital costs; however, in the long run, there will still be a cost of sludge removal. Purchasing enough aeration equipment for complete mix will translate to higher capital and long run operational costs.

While a partial-mix aerated lagoon system requires fewer aerator units and less power to run, retention times are longer, so it lacks the treatment capacity of a complete-mix system. Adapted from the EPA’s Principles of Design and Operation of Wastewater Treatment Pond Systems, the following chart compares how much dedicated overall acreage (including dikes and roadways) is required for each lagoon type to treat a municipal wastewater flow of 1 mgd.

aerated lagoon

A complete-mix aerated lagoon system needs a fraction of the land requirement of a facultative or partial-mix system because it packs a lot more treatment into the same space.

Lagoon expansion with MARS aeration: a case study

Converting a facultative or partial-mix aerated lagoon to complete mix is in effect a lagoon expansion: The system can handle more influent and provide more treatment in the same footprint.

Triplepoint’s MARS aeration can be used to create a complete-mix environment, combining the energy-efficient oxygenation of fine-bubble diffusers with the robust mixing capability provided by the coarse-bubble static tube.

aerated lagoon mixing

The MARS aeration complete-mix environment turns over Lakewood’s lagoon every four minutes.

The Lakewood Wastewater Authority in Lake Odessa, MI, rehabilitated their 0.65 acre, 1.4 million gallon flow equalization lagoon with MARS aerators to create a complete-mix environment. Fed air via three onshore 50hp blowers (one redundant), 48 MARS lagoon aerators now transfer 3,840 pounds of oxygen into the water per day. Furthermore, lagoon mixing improved greatly, with a complete turnover once every four minutes, keeping all sludge solids in suspension. Most importantly, the MARS aerators have kept the pond aerobic, with a strong dissolved oxygen content to maintain healthy BOD breakdown and keep odors at bay.

Adding MARS aeration and mixing to a wastewater lagoon increases capacity and improves treatment. Download our MARS Lagoon Aeration brochure.

Aerated lagoons, of course, have higher energy demands than facultative lagoons, and complete-mix lagoons require more energy than partial-mix lagoons. This added cost, however, is offset by better quality treatment, increased capacity, and reduced sludge handling.

Contact Triplepoint for Assistance Designing an Efficient Lagoon Aeration System—we can provide you with design calculations, budgetary costs, preliminary layouts, and lifecycle cost analysis.

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