Farming the algae Spirulina
Credit: PWRDF/ Flickr
By Gary Hartley

Extracting wastewater nutrients could make microalgae a more financially feasible feed

Safely reusing nutrients from wastewater could help produce microalgae for animal feed more cheaply, research from South Korea suggests.

Algae has considerable promise as a highly nutritious source of sustainable protein for livestock, with potential to replace up to 19% of conventional proteins by 2050. But producing algae at scale remains more costly than traditional feedstocks such as soybean.

One way to reduce these costs is by applying ‘circular economy’ principles, using the nutrients from wastewater to culture viable algae species. Previous studies had done this using raw wastewater, but the Korean team set out to do things differently, using a three-step process of nutrient extraction, priming the nutrients for reuse and algae cultivation. 

They extracted nitrate and phosphate from a livestock wastewater using what are known as drinking water treatment residuals, solids left from drinking water cleaning processes which are known to bind with phosphorus in manures. They then chemically encased the residuals in chitosan, a tough fibrous sugar, as gel beads.

Following these initial steps, they cultured the microalgae Spirulina platensis in flasks, comparing the use of the extracted nutrients to a control treatment, a typically used culture medium. They measured several metrics including dry weight and amino acid composition, as well as carrying out toxicity testing.

Cost-cutting, nutritious and safe

They found that using the experimental approach had no detrimental impact on algae being produced. It could also reduce culture medium costs by 18% and overall production costs by 4-9% when compared to the typical growth medium. They did note, though, that costs for the direct use of wastewater are virtually nil, meaning that to make their approach economically stack up in this comparison, new substances to ensnare nutrients and reuse the culture medium will need to be developed.

The team’s nutritional analysis of the S. platensis produced in the work revealed that protein content was lower when using the experimental medium than the control. Despite that, the essential amino acid content of the end product was similar to that found in eggs, milk and soy proteins.

The nutrient extraction approach is a more controlled and safe way of culturing S. platensis than using raw wastewater,the scientists wrote in the journal Heliyon. Their toxicity tests validated this both in lab analysis and after feeding the algae to rats, which exhibited no changes. 

Quality shows through

“A key highlight of our work is the quality of the S. platensis biomass derived from these reclaimed nutrients,” they wrote. “Our results indicate this biomass is not only free from toxins but also protein-rich, boasting an essential amino acid profile. This makes it a strong contender for animal feed, suitable for diverse livestock and fish species.

“Our study goes beyond merely addressing concerns over using wastewater in biomass production. It underscores the broader potential of repurposing waste materials.”

Although the results were promising, the technique still needs further refinement, including refining the nutrient extraction methods. Once this is done, the approach “should be cost-effective and scalable,” they added.

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