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This post explores lagoon wastewater treatment replacement and retrofit options for small communities faced with stringent NPDES treatment requirements. It also includes an overview of disinfection options for lagoon and mechanical wastewater treatment systems.

Stringent NPDES Treatment Requirements

Small communities with aerated lagoon and facultative controlled discharge lagoon wastewater treatment systems are facing increasingly stringent treatment requirements for ammonia, pH, dissolved oxygen, and bacteria with new National Pollution Discharge Elimination System (NPDES) Permits. Required by the EPA, DNR, and/or other state regulatory agencies, the permit is authorized under the Clean Water Act for the regulation of point sources that discharge pollutants into U.S. waters.

“Meeting these requirements is essential to protect the environment and our water supply, which is the most valuable resource we have, for future generations,” states Lindsay Beaman, PE, Business Unit Leader of Snyder & Associates’ Cedar Rapids office.

Protecting Aquatic Life

Ammonia is a natural by-product of the decomposition of all nitrogen-containing organic matter. This means it’s something we all contribute to because the human body produces ammonia. Industrial users, such as food processing, fertilizer, and dog food plants, also contribute to ammonia levels, but in higher concentrations.

Ammonia promotes algae growth in bodies of water. It’s converted to nitrite and nitrate through aerobic biological activity, which depletes dissolved oxygen in water. At elevated concentrations, it’s toxic to aquatic life. Some types of algae are toxic to humans as well.

The success of aerated and facultative lagoons in removing ammonia is limited to moderate-temperature weather. Once the water temperature dips below 40 degrees Fahrenheit, the bacteria needed to treat ammonia become inactive. As a result, many small, Midwestern communities need alternative solutions for ammonia reduction that can withstand cold winter temperatures.

The goal of new NPDES permit limits for pH is to ensure discharged water has a similar pH to the receiving stream.

“PH keeps everything in balance, creating a healthy environment where aquatic life can flourish,” explains Beaman. “If the pH becomes too acidic or basic, aquatic plants and organisms are negatively impacted.”

Disinfection, the third component of new NPDES permit limits, is essential because bacteria, protozoa, and viruses present in water can lead to disease and illness. Even just a single exposure can have hazardous health effects.

“Bodily contact with polluted water can be detrimental to human health,” explains Beaman. “Children and other stream users may inadvertently ingest water from receiving streams or come in contact through a wound, cut, or scrape, so disinfection is crucial.”

4 Lagoon Alternatives for Ammonia Reduction

For small livable communities seeking alternative methods of ammonia reduction, the solution is lagoon enhancement or replacement. Mechanical plant options exist for lagoon replacement, while Lemna systems, NitrOx® Reactor systems, and Submerged Attached Growth Reactor (SAGR) systems are common ways to enhance an existing lagoon.

Mechanical Plants

AeroMod and Sequencing Batch Reactors (SBR) are two examples of mechanical plant options for lagoon replacement. Both systems utilize the activated biosolids process to treat wastewater.

Currently under construction, the Pleasantville Wastewater Treatment Plant is an example of Sequencing Batch Reactor use to achieve NPDES permit effluent limits while expanding plant capacity to accommodate increased influent flow.


A Lemna system utilizes modular, insulated covers to help retain heat. They also keep sunlight out, hindering algae growth, which can cause problems with suspended solids in the effluent. Nitrifying and denitrifying reactors are plug-in components added to the back of the lagoon to finish the process the lagoon starts. Overall, Lemna systems are relatively easy to install with few moving parts and minimal human intervention needed for operation.

NitrOx Reactor

A NitrOx Reactor is a compact, easy to install system that enhances lagoon systems by heating the water to maintain the temperature needed to keep bacteria active. Situated next to a lagoon, these low-cost systems are relatively easy to maintain and operate. However, because the system helps regulate water temperature, utility costs will likely increase.

Norway’s Wastewater Treatment Plant improvement project utilizes a NitrOx Reactor.

Submerged Attached Growth Reactor (SAGR)

Typically built as an addition to an existing lagoon, SAGR systems are an effective solution for meeting stringent EPA water quality standards in cold to moderate climates. They’re also fairly easy to operate and maintain. Constructed below grade, SAGR systems take advantage of natural, geothermal heat and the earth’s natural insulating properties to maintain water temperature during average winter conditions.

Wastewater treatment plant improvements for the City of Colesburg and the City of Lenox both utilize SAGR systems.

3 Disinfection Options for Lagoon & Mechanical Wastewater Treatment

Regardless of which solution you choose, supplemental disinfection will be necessary to meet NPDES requirements.

Natural Disinfection

Natural disinfection allows wastewater to sit exposed to the elements for approximately 180 days as bacteria dies off. Without the need for chemicals, maintenance, and utilities, it’s affordable and easy to use. However, natural disinfection can’t be guaranteed, and it requires more space than other options.

The City of Colesburg is utilizing a SAGR system and UV disinfection to achieve NPDES compliance.


This method of disinfection utilizes chemicals to disinfect wastewater. While effective, it requires space, labor, chemical use and storage, which are factors communities should take into account. Another downside is the pass-through of chemicals into the receiving stream.

Ultraviolet (UV)

UV disinfection utilizes light to neutralize harmful microorganisms. UV systems come in many shapes and sizes, allowing them to be tailored to specific community needs. However, low light transmittance impairs the effectiveness of UV disinfection, so it’s not ideal for all lagoon systems.

The Right Solution for Your Community

The optimal wastewater treatment solution for any community involves a number of factors including current system conditions, available space, and budget.

It’s also essential to take operator licensing into account. Whether you’re looking to upgrade or replace your wastewater treatment facility, you’ll need licensed staff to operate it. Depending on how sophisticated yours will be, higher certification for operators may be required.

Many communities in need of lagoon enhancement or replacement are experiencing population decline with much of the remaining population on fixed incomes. The burden of expensive wastewater treatment system initiatives is borne by the residents, often in situations where city rates haven’t increased for a long time.

“We can assist with grant and loan applications and help appeal to the DNR for time extensions while affordable alternatives are explored,” says Beaman.

Once the appropriate solution is determined, project planning, permitting, and construction typically occurs over a five-year period.

“We encourage any community facing the challenges associated with lagoon wastewater treatment facilities and stringent NPDES treatment requirements to proactively access their options,” shares Beaman. “There’s no such thing as a one-size fits all solution. Ensuring your community is positioned to meet wastewater treatment requirements long-term demands careful thought, planning, and implementation.”

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