Managing Nutrient Release in Polishing Ponds: Preventing Summer Algae Blooms & Effluent Issues

Polishing ponds are a common and cost-effective way to further treat wastewater, reducing nutrients, recalcitrant organics, suspended solids and pathogens. However, these ponds can sometimes become sources of water quality problems themselves, particularly in summer. This blog post explores how nutrient release from sludge in polishing ponds can contribute to algae blooms and effluent quality issues, especially during warmer months.
The Role of Polishing Ponds
Polishing ponds, also known as maturation or stabilization ponds, are secondary or tertiary treatment steps that utilize natural processes to remove pollutants. Wastewater flows into large, ponds where sunlight, bacteria, and algae work together to break down organic matter. While effective at removing pathogens and some nutrients, these systems are not without their challenges.
The Summer Sludge Problem
Over time, solids settle at the bottom of polishing ponds, forming a layer of nutrient-rich sludge. This sludge is composed of dead algae, microbial biomass, and inorganic particles. While stable under aerobic conditions, the warmer temperatures of summer can lead to significant problems:

• Increased Microbial Activity: Higher temperatures accelerate the metabolic rates of anaerobic bacteria within the sludge. These bacteria decompose organic matter, releasing dissolved nutrients like ammonia, phosphates, and organic nitrogen compounds back into the water column.
• Reduced Oxygen Levels: Warm water holds less dissolved oxygen than cold water. Additionally, the decomposition processes in the sludge consume oxygen, leading to anaerobic or anoxic conditions at the pond bottom. This lack of oxygen further promotes the release of nutrients, especially phosphorus, which is often bound to iron under aerobic conditions but released when oxygen is scarce.
• Thermal Stratification: In summer, the pond water can stratify into layers with different temperatures. The warmer, less dense water at the surface can prevent mixing with the cooler, denser water at the bottom. This stratification can trap released nutrients in the lower layers, allowing them to accumulate to high concentrations before eventually mixing into the upper layers.

Algae Blooms: A Feast for Nutrients
The release of these accumulated nutrients into the water column acts as a potent fertilizer for algae.

• Rapid Algal Growth: With abundant nitrogen and phosphorus, algae populations can explode, leading to what are commonly known as "algae blooms." These blooms often manifest as a visible green, blue-green, or even reddish discoloration of the pond water.
• Cyanobacteria (Blue-Green Algae): Of particular concern are blooms of cyanobacteria, often referred to as blue-green algae. While technically bacteria, they photosynthesize like algae. Many species of cyanobacteria can produce toxins (cyanotoxins) that are harmful to humans, animals, and aquatic life.
• Reduced Light Penetration: Dense algae blooms block sunlight from reaching the deeper parts of the pond, inhibiting the growth of beneficial submerged aquatic vegetation that could otherwise compete with algae for nutrients.
• Diurnal Oxygen Swings: During the day, algae produce oxygen through photosynthesis, often leading to supersaturated conditions. However, at night, they respire, consuming oxygen. In dense blooms, this respiration can drastically deplete oxygen levels, leading to anoxic conditions that stress and kill fish and other aquatic organisms.

Effluent Quality Issues
The consequences of summer sludge nutrient release and subsequent algae blooms extend beyond the polishing pond itself, directly impacting effluent quality:

• Increased Total Suspended Solids (TSS): Algae themselves contribute to TSS, making it difficult for the WWTP to meet effluent limits for suspended solids.
• Elevated Biochemical Oxygen Demand (BOD): While algae produce oxygen during the day, their eventual death and decomposition contribute to BOD in the effluent. This means the effluent requires more oxygen from the receiving water body to break down organic matter, potentially harming aquatic life.
• High Nutrient Concentrations: Despite the pond's intended purpose, the release of nutrients from sludge can lead to effluent with elevated levels of nitrogen and phosphorus, contributing to eutrophication in downstream rivers, lakes, and coastal waters. This can fuel further algae blooms and dead zones.
• Disinfection Challenges: High TSS from algae can interfere with disinfection processes (e.g., UV or chlorination), making it harder to effectively remove pathogens.
• Taste and Odor Compounds: Some algae, particularly cyanobacteria, produce compounds that cause unpleasant tastes and odors in the water, which can be a concern if the receiving water body is used for drinking water or recreation.

Mitigation Strategies
Addressing nutrient release from sludge in polishing ponds during summer requires proactive management:

• Regular Sludge Removal: Periodically dredging or dewatering the ponds to remove accumulated sludge is one of the most effective ways to reduce the internal nutrient load.
• Aeration & Mixing: Introducing aeration mixing systems can help maintain aerobic conditions throughout the water column, preventing anaerobic nutrient release from the sludge and promoting beneficial microbial activity.
• Chemical Precipitation: In some cases, chemicals like alum or ferric chloride can be added to precipitate phosphorus, binding it into insoluble forms that settle out. Difficulty in dosing makes frequent use of precipitation an inefficient solution.​
• Vegetated Treatment Systems: Integrating constructed wetlands or other vegetated treatment systems can provide an additional polishing step, utilizing plants to uptake excess nutrients before discharge.
• Flow Management: Optimizing hydraulic retention times can help prevent excessive sludge accumulation and improve overall pond performance.
• Adding Biological Growth Media: Adding MBBR or Fixed Film media to support biofilm in the ponds can help polish the effluent nitrogen and phosphorus prior to discharge. 
  
• Bacteria tablets & Spikes: Solid time release bacteria designed to sink to the sludge layer are a newer option. They have worked in zones with high organic solids but should be applied before summer temperatures increase anaerobic degradation and fermentative respiration. Think of the tablets/spikes are more of maintenance product between dredging.