Algae are a diverse group of aquatic, photosynthetic organisms. They manifest in water systems in two primary forms, each requiring different removal strategies. Planktonic algae are microscopic, free-floating cells that cause the water to appear uniformly green or cloudy, often called “green water.” Filamentous algae are macroscopic, forming long, stringy strands or dense mats that cling to surfaces or float on the water’s surface.
Filtration is the most effective physical removal method for achieving long-term water clarity. Chemical treatments, such as algaecides, kill the algae but leave dead organic matter suspended in the water. This decaying material consumes dissolved oxygen and releases nutrients, which can trigger subsequent algae blooms. Therefore, filtration is necessary to physically remove both living and dead algae biomass before it pollutes the aquatic environment.
Physical Removal and Manual Techniques
Before engaging a mechanical system, manual removal techniques offer an immediate solution for controlling large accumulations of algae. Specialized nets and skimmers physically lift floating filamentous algae mats and debris from the water’s surface. This rapid action immediately reduces the overall organic load, which is important during a heavy algae bloom.
For algae adhering to the sides of pools, ponds, or submerged surfaces, brushing or scraping is required to dislodge the growth. Once scrubbed, the algae become suspended in the water column, making them accessible for subsequent removal by a filter system or water vacuum.
A pond or pool vacuum sucks up the dislodged algae and the layer of sludge or detritus that settles at the bottom. This detritus is a nutrient-rich food source for new growth. Partial water changes are also an indirect form of physical removal, as they dilute dissolved nutrients like phosphates and nitrates, making the environment less hospitable for future algae growth.
Dedicated Mechanical Filtration Systems and Media
Mechanical filtration systems physically trap suspended particles, including microscopic algae, as water is forced through a porous medium. Pressure filters, such as sand or bead filters, house the media in a sealed vessel and are highly effective for maintaining clear water. These systems require backwashing, which reverses the water flow to flush trapped solids out of the media and away from the system, preventing excessive clogging.
A filter’s effectiveness is determined by the size of the particles it can reliably capture, measured in microns. Coarse media, such as certain sponges or Japanese mats, are placed first to remove large debris and string algae, typically capturing particles above 50 microns. Finer filter media are necessary to capture the single-celled planktonic algae that cause green water, which can be as small as 5 to 30 microns.
Silica sand, a common filter medium, typically removes particles in the 20 to 100-micron range. Newer glass media can improve this performance to 5 to 10 microns. The highest clarity is often achieved using diatomaceous earth (D.E.) filters, which utilize the microscopic skeletons of diatoms to trap particles as minute as 2 to 5 microns. To maximize the filter’s effect, the pump system must be correctly sized to ensure the water body’s volume is circulated frequently. For decorative ponds, a turnover rate of at least 1.5 to 2.5 times per hour is often recommended to maintain high water clarity.
Pre-Filtration Clarification Methods
For water systems struggling with persistent green water, specialized clarification methods prepare microscopic algae for capture by the mechanical filter. One effective method is ultraviolet (UV) sterilization, which uses a specific wavelength of UV-C light to disrupt the algae’s DNA. This damage prevents reproduction and causes the individual cells to perish and clump together, a process known as natural flocculation.
The newly formed clumps are large enough to be easily caught by the mechanical filter media, which previously could not trap the minute cells. The efficiency of a UV unit depends on the clarity of the water entering it. Therefore, a pre-filter is necessary to remove larger sediments that could block the UV light’s penetration.
Chemical clarifiers are another pre-filtration treatment that uses binding agents to aggregate particles. Chemicals like aluminum sulfate or chitosan act as flocculants by neutralizing the slight negative electrical charge on the surface of algae cells. This charge neutralization, known as coagulation, allows the microscopic cells to overcome their natural repulsion and bind together into larger masses called flocs. These flocs can then be easily skimmed from the surface or routed through the mechanical filter for final physical removal.