Algae-based method could help treat waste water in India, say researchers

Indian researchers have proposed a theory for a sustainable wastewater treatment method using a microalgae-bacterial consortium to remove toxic ammonium. The method, proposed in a recently published study, leverages algae’s photosynthesis mechanism to produce oxygen, cutting energy costs by up to 90% compared to traditional aeration methods.

This is pertinent given that India’s wastewater situation is critical. According to a 2022 Niti Aayog report, India generates approximately 39,604 million litres per day of wastewater in rural regions, while in the urban centres, the wastewater generation has been estimated as 72,368 MLD for the year 2020-’21.

Most of the wastewater in India is generated from agricultural runoff, industrial effluents, and sewage, all of which contain various toxic chemicals. One of the most concerning pollutants in significant quantities is ammonium. Formed from gaseous ammonia, ammonium is a highly toxic nitrogen compound that can exist as nitrate, nitrite, or ammonia. It poses serious risks to living organisms, being lethal to animal cells and harmful to aquatic life. Ammonium can also cause water acidification and contribute to eutrophication, where excessive nutrients in the water lead to harmful algal blooms.

“It is crucial to remove ammonium from wastewater to prevent damage to aquatic life and maintain the overall quality of surface and groundwater,” says Kannan Pakshirajan, a professor in the Department of Biosciences and Bioengineering at the Indian Institute of Technology Guwahati. He further explains, “Ammonia can harm mammalian cells and human health due to its potential to alter water pH. Drinking water is typically slightly alkaline, and if its pH becomes acidic, it becomes unfit for consumption.”

Cost-effective way

Traditional ammonium removal methods involve oxygenation, which is energy-intensive, accounting for up to 90% of a treatment plant’s energy consumption.

Pakshirajan and his team have developed an approach for wastewater treatment using a microalgae-bacterial consortium, specifically targeting ammonium removal. “Biological methods, such as those involving microorganisms in anaerobic digestion, are preferred over chemical methods for ammonium removal due to their environmental sustainability and lower energy requirements,” Pakshirajan explains.

The study authors note that traditional ammonium removal methods often come with significant drawbacks, including high energy costs and the generation of waste-activated sludge.

“The first step is converting ammonia into another, less harmful nitrogen species like nitrite or nitrate through oxidation. For this, we need to supply oxygen. What is often done is, we aerate the system by means of any pumping mechanism or mechanical operations or even supply pure air into the system by compressed air and so on. This is energy consuming, and the major cost involved in the wastewater treatment,” according to Pakshirajan.

In contrast, the new system is self-activated and leverages the microalgae’s ability to produce oxygen through photosynthesis, reducing the need for external aeration. This can potentially save 50%-90% in energy costs.

While chemical methods are also commonly used to remove ammonium, Pakshirajan emphasises that the new approach is a green technology that aligns with the country’s sustainability goals. “Using chemicals may be faster, but it often requires more energy, which is not favourable for the environment,” he says.

Using light and algae

The team designed a photo-sequencing batch reactor where ammonia is first converted into a less toxic form, such as nitrate, and eventually into nitrogen, the least toxic form. This is a biological process involving both bacteria and microalgae.

Ammonia oxidation to nitrogen requires oxygen, while nitrate conversion to nitrogen occurs under anoxic conditions. The microalgae-bacterial consortium facilitates this process. The algae produce oxygen through photosynthesis, which supports ammonia oxidation, and in the absence of light, they not only stop producing oxygen but also consume the remaining oxygen.

“During the conversion of ammonia to nitrate and nitrate to nitrite, a significant amount of oxygen is required. We can regulate this oxygen demand by exposing the algae to light and blocking it – what we call the light and dark cycle. This process is what we refer to as the photo-activated sludge process,” Pakshirajan explains.

In this system, the “sludge” is a microbial flock composed of algae, ammonia-oxidising bacteria, nitrate-oxidizing bacteria, and denitrifying bacteria. The system can operate in batches, continuously, or sequencing batch modes, depending on the wastewater treatment needs, the paper notes.

The absorption of ammonia by the bacteria manifests as increased biomass which is advantageous, says Pakshirajan. When more biomass is present and there is more light, more oxygen will be produced through photosynthesis. “So, this way, both the algae and the bacteria and so are we,” he points out.

Pakshirajan describes this approach as a cost-effective, nature-based solution for wastewater treatment. “On one hand, we use entirely natural substances, mimicking the natural processes of ammonium absorption, and on the other, we reduce the costs associated with aeration, oxygen supply, and pumping that are typically involved in traditional systems,” he explains.

TV Ramachandra, a scientist at the Centre for Ecological Science, Indian Institute of Science, Bengaluru, says he believes the concept is well-known. “A microalgae-bacterial consortium is often used at the inlet of lakes,” says Ramachandra, who has led lake rejuvenation efforts in Bengaluru.

While he acknowledges that the concept itself is not novel, he commends the modelling approach developed in the study. However, he suggests that incorporating more field samples would have likely yielded better results.

Pakshirajan also points out some limitations of the study. “Since ammonia is toxic, higher concentrations can hinder the efficiency of ammonia-oxidising bacteria,” he notes. However, he adds that the ammonium concentration in wastewater typically falls within the tolerable range for these bacteria, so this is not a major concern.

This article was first published on Mongabay.