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Prevent Harmful Algal Blooms with Nutrient Inactivation

Anna Chase and Matt Ladewig | July 13, 2023

Harmful algal blooms (HABs), which typically consist of cyanobacteria, have become an increasingly widespread issue in water sources across the United States.

HABs can release bio-toxins, which negatively impact aquatic life and threaten the health of animals and people who live near or use the water. Due to these impacts, HABs can adversely affect various water uses, including drinking water, animal watering and recreational use. These blooms can also cause or contribute to numerous other water quality issues, including:

  • Deoxygenation of water
  • Accelerated eutrophication
  • Emission of compounds that cause taste and odor
  • Increased turbidity
  • Discoloration
  • Increased volatility in pH
  • Mobilization of toxic metals from the sediment

Data indicate that HABs are increasing in frequency and intensity around much of the world and there is a growing need to address these blooms in a proactive manner, particularly in areas where HABs are coupled with water scarcity or contamination challenges.

Nutrient inactivation treatment has proved to be a successful mitigation approach . Following best practices in planning, implementing and monitoring can minimize or prevent HABs both now and for the future.

Cause of HABs – The Role of Internal Loading of Phosphorus from Sediments

  • HABs occur in waterbodies with high levels of phosphorus, which allows for rapid growth of algae and cyanobacteria.
  • In many cases, bottom sediments contain high levels of phosphorus due to long-term accumulation from watershed sources. Under certain environmental conditions, phosphorus may be released from the sediments into the water column. This is known as internal loading.
  • In these situations, reducing external inputs of phosphorus (by implementing stormwater improvements, reducing fertilizer use in the watershed, etc.) may not sufficiently decrease phosphorus levels to prevent HABs from developing.

Nutrient Inactivation is a Proven Solution

Although there are a variety of established and emerging lake management approaches to address internal loading, sediment phosphorus inactivation, also known as nutrient inactivation, is one of the most widely used.
This approach involves the use of flocculants or precipitants to capture and bind reactive phosphorus in lake bottom sediments, rendering it unavailable for use by algae and cyanobacteria. A variety of materials have been used for nutrient inactivation, including:

  • Alum (aluminum sulfate)
  • Aluminum chlorohydrate (ACH)
  • Ferric chloride
  • Ground limestone (GCC)
  • Polyaluminum chloride (PAC)
  • Precipitated calcium carbonate (PCC)
  • Proprietary technology (e.g., Phoslock®, EutroSORB® G)
  • Sodium aluminate
  • Zeolite

The most appropriate nutrient inactivation option depends on several factors and should be selected in consultation with a qualified lake management professional. However, the option with the most extensive track record in North American lakes, ponds and reservoirs is alum.

Alum Treatment Key Considerations


  • The suitability and feasibility of alum treatment as a HAB management measure is something that should first be evaluated as part of a comprehensive lake management plan.
  • Once selected as a preferred course of management, the dose of alum will need to be determined using sediment and water quality, hydrology, bathymetry and other supporting data.
  • In waterbodies with low alkalinity, sodium aluminate may be applied concurrently with alum to balance the water pH during treatment.


  • Alum treatment is a precision process that requires specialized equipment and professional training.
  • When alum is injected into a waterbody it quickly forms a whitish flocculent solid (aluminum hydroxide) which is very effective at attracting and binding phosphorus.
  • This flocculant settles, initially forming a thin blanket over the bottom and becoming incorporated into sediments over a few weeks to months.
  • The aluminum hydroxide flocculant intercepts and binds with phosphorus released from the sediment, forming aluminum phosphate, a stable compound that will persist indefinitely under most in-lake environmental conditions. Phosphorus bound in this way is kept out of the water column and is inaccessible to algae and cyanobacteria.

  • The effective duration provided by an alum treatment depends on several factors. However, a properly dosed and applied alum treatment can be expected to reduce internal loading of phosphorus and therefore decrease the likelihood of HABs, for multiple years to decades.


  • Improper application of alum can result in pH fluctuations that could impact aquatic life or their habitats. Therefore, a thorough understanding of baseline conditions, informed treatment design and implementation of a monitoring program are recommended as a part of any alum treatment program. These elements are often also required as part of the permitting process.
  • Post-treatment monitoring is also recommended to both document the initial effectiveness of the treatment and measure trends in water quality over time.

Next Steps: TRC Can Help

HAB events can severely degrade water quality and affect beneficial uses of lakes, ponds and reservoirs. In many cases, phosphorus release from the sediments is the primary driver of HABs in these waterbodies. Nutrient inactivation treatments, such as alum, reduce the amount of phosphorus released which can minimize or eliminate HABs for years. However, nutrient inactivation is not appropriate for every waterbody with recurring HABs.

TRC’s lake management experts can help you diagnose the drivers behind the HABs in your waterbody, determine if nutrient inactivation makes sense and, if so, lead the way in helping you design, permit and implement an effective treatment. We also have the expertise to assist you in HAB issue assessment, community outreach and risk communications and the implementation of other treatment and management approaches.

Contact our Experts Below to Learn More

Anna Chase

Anna is a water resources scientist with more than eight years of experience in the monitoring and management of freshwater lakes and ponds. She has completed projects for numerous state and municipal governments, water suppliers, lake associations, and federal agencies. Anna specializes in surface water quality monitoring, aquatic invasive species management, data analysis and research, and macroinvertebrate sampling and taxonomic identification. She holds a bachelor’s degree in Biology and Environmental Studies from Bowdoin College and a master’s degree in Zoology from the University of New Hampshire. Anna can be reached at

Matt Ladewig

Matt Ladewig is a limnologist and Certified Lake Manager with more than 20 years of experience in the monitoring, modeling, and management of aquatic ecosystems. He has completed studies on more than 100 water bodies for a diversity of clients, including water suppliers, state and municipal governments, lake associations, and private landowners. Mr. Ladewig has developed comprehensive lake management plans as part of many of these studies, often with a focus on the management of nutrients, algae/cyanobacteria, or aquatic nuisance vegetation. He also has practical experience permitting and implementing biological, chemical, and mechanical/physical components of lake management programs, as well as the monitoring programs needed to evaluate success. Contact Matt at

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