Treatment of wastewater using aquatic plants

Treating Wastewater with Algae

Aphanizomenon Algae

Algae are a group of microorganisms capable of photosynthesis. They can exist as unicellular (some species are smaller than certain bacteria) or multicellular forms (such as seaweeds, which can grow several meters long). Botanists classify algae based on the types of products they synthesize and store in their cells, as well as their pigments.

Algae grow rapidly, can endure environmental changes, thrive in wastewater, and are rich in nutrients and protein. Thus, these characteristics are leveraged for:

1. Wastewater Treatment and Nutrient Recycling. Biological activity in algae ponds removes organic matter and nutrients from wastewater, converting them into nutrients within the algae cells through photosynthesis. Most urban, agricultural, and livestock wastewater can be treated using algae pond systems.

2. Converting Solar Energy into Biological Energy. Algae utilize solar energy to perform photosynthesis, producing sugars, starches, etc. Therefore, using algae for wastewater treatment is considered an effective method to convert solar energy into the energy of living organisms.

Asterionella Algae

3. Eliminating Pathogens. Through the wastewater treatment process involving algae, pathogens present in wastewater are eliminated due to the following factors:

• Daily pH fluctuations in the algae pond caused by photosynthesis
• Toxins secreted from algae cells
• And the exposure of pathogens to sunlight (UV)

Typically, wastewater treatment is combined with algae production and harvesting to remove organic matter. However, harvesting algae is challenging (due to their small size), and most algae have thick cell walls, making them difficult for animals to digest. They are often contaminated with heavy metals, pesticides, and residual pathogens from wastewater.

The reactions occurring in algae ponds are primarily “symbiotic activities between algae and bacteria.”

Essential Factors for Algae-Based Wastewater Treatment

Ceratium Algae

Nutrients: Ammonia is the primary nitrogen source for algae to synthesize proteins through photosynthesis. Phosphorus, Magnesium, and Potassium are also nutrients that affect algae growth. The ratio of P, Mg, and K in algae cells is 1.5:1:0.5.

Depth of the Algae Pond: The depth of the algae pond is determined based on optimizing light availability during the algae’s synthesis process. Theoretically, the maximum depth of an algae pond is about 4.5 to 5 inches (12.5 cm). However, experimental models suggest that the optimal depth lies between 8 to 10 inches (20 to 25 cm). In practical production scenarios, the depth of the algae pond should exceed 20 cm (ideally between 40 to 50 cm) to ensure adequate retention time for wastewater within the pond and account for volume lost due to sedimentation.

Hydraulic Retention Time (HRT): The optimal hydraulic retention time is the duration required for the nutrients in wastewater to be converted into nutrients within algae cells. Generally, it is recommended to select a retention time greater than 1.8 days and less than 8 days for wastewater in ponds.

Chlamydomonas Algae

Loading BOD into the Algae Pond: The amount of BOD loaded into the algae pond affects algae productivity. If the BOD load is too high, the environment in the algae pond can become anaerobic, negatively impacting the symbiotic interactions between algae and bacteria. Some experiments conducted in Thailand indicate that under tropical conditions, a pond depth of 0.35 m, an HRT of 1.5 days, and a BOD loading of 336 kg/(ha/day) are optimal for algae ponds, achieving a productivity of 390 kg/(ha/day).

Mixing and Recirculation: The mixing process in algae ponds is essential to prevent algae cells from settling to the bottom and to facilitate nutrient exposure to algae, promoting photosynthesis. In large algae ponds, mixing also prevents temperature stratification and anaerobic conditions at the bottom. However, mixing can create disadvantages by resuspending settled solids and hindering light diffusion into the pond. Moraine and colleagues (1979) suggest that the flow rate in algae ponds should be around 5 cm/s. Recirculation helps retain active bacterial and algae cells; it also promotes aeration and accelerates reactions within the algae pond.

Harvesting Algae: Algae can be harvested using nets or sieves, by creating flocs or flotation, or through biological harvesting using herbivorous fish and invertebrates that consume algae.

Treating Wastewater with Larger Aquatic Plants

Dinobryon Algae

Aquatic plants are species that grow in water environments. They can cause some disadvantages for humans due to their rapid growth and wide distribution. However, utilizing them for wastewater treatment, composting, and as food for humans and livestock can mitigate these disadvantages and also generate profit.

Main Types of Aquatic Plants

Submerged Aquatic Plants: These plants grow beneath the water surface and can only thrive in water sources with adequate light. They can cause issues such as increasing water turbidity and blocking light diffusion into the water. Thus, these types of aquatic plants are ineffective in cleaning waste.

Floating Aquatic Plants: The roots of these plants do not anchor to the soil but float on the water’s surface, with their stems and leaves developing above the water. They drift on the surface according to wind and currents. Their roots provide a substrate for bacteria to attach to for waste degradation.

Emergent Aquatic Plants: These plants have roots anchored in the soil but have stems and leaves that grow above the water. They typically thrive in areas with stable tidal regimes.

Some Exemplary Aquatic Plants

The Role of Aquatic Plants in Treatment Systems

Some Reference Values for Designing Water Hyacinth Ponds for Wastewater Treatment

Euglena Algae	Pediastrum Algae	Synura Algae

Le Hoang Viet – Cited by Chongrak Polprasert (1989)