Valorization of mineral N from animal housing as organic fertilizer or animal feed

General introduction

Making animal feed components via microbial biomass culture (Single Cell Protein or SCP) in a fermenter that one "feeds" with the nitrogen source ammonia from livestock buildings is, in theory, a feasible scenario when a number of factors are favorable. That is the conclusion of the Nitrogen Capture Feasibility Study that ILVO delivered in 2024. The objective was to find a useful and technically-economically realistic reuse of ammonia captured by chemical or biological air scrubbers in livestock houses. Air scrubbers then prevent the ammonia (or volatile nitrogen) from entering the atmosphere as vapor, where it adds to the nitrogen problem for nature. You need a nitrogen source to produce microbial biomass (so-called Single Cell Protein or SCP). in fermentation processes. Two possible valorization strategies for the end-production SCP were considered: Is it possible to use the SCPs as slow-release fertilizer or as an edible protein-rich component in animal feed?  

Research approach

A critical desk study was used to work through the entire technical process for the two scenarios, from barn to valorization end use. It involved these three steps: (1) nitrogen capture with air scrubbers on barns, (2) microbial biomass production in a digester and (3) conversion to end products. Then the legislative and regulatory aspects related to the use of SCP as slow release fertilizer or animal feed were also summarized. Assessments of economic feasibility and environmental sustainability were also made. 
Scenario 1 is: Ammonia emissions are captured using a chemical air scrubber, resulting in ammonium sulfate as a result of scrubbing. Enriched wash water is used as a nitrogen source in microbial fermentation (food source for bacterial growth in the fermenter). Fermentation occurs in place with a widely available carbon source, namely a potato processing plant. The wastewater there, as a plus point, contains a lot of organic matter (with thus a high chemical oxygen demand or COD), which makes it appropriate as an energy source in Single Cell Production. The necessary post-processing of the produced SCP, such as the pasteurization, centrifugation and drying steps are performed and the protein powder is used as part of animal feed.
Scenario 2 ran like this: SCP is produced on-farm after capturing ammonia emissions there with a biological scrubber. The wash water from the biological scrubber is used as a nitrogen source for fermentation. The carbon source is methane obtained during a manure fermentation process. 

Relevance/Valorisation

In both scenarios, the captured nitrogen is valorized as an animal feed component. Valorization as an organic fertilizer was not considered further because market prices for animal feed components are much higher, so as an animal feed component, economic feasibility is evaluated as being more likely. After consulting literature and obtaining insights from experts, the two scenarios were weighed for their economic feasibility and environmental impact. For scenario 2, the verdict is that production of SCP is currently not an economically interesting option for the cattle farm.
For scenario 1, the researchers conclude that only in the most favorable circumstances, i.e., with lower energy prices (as in January 2021) and with delivery of the produced energy to the grid, the price of the Single Cell Proteins (€ 1.05 /kg protein) will come out between trot (€ 0.93 /kg protein) and soy (€ 1.12 /kg protein) - under those circumstances it is competitive. The nuance is that the cost of storage, higher workload, additional contacts, uncertain market, etc. have not yet been factored in. Finally, the valorization of secondary raw materials such as wash water from chemical and biological air scrubbers via fermentation appears to be challenging from a regulatory perspective in 2024.