(643c) Integrated Cyanobacteria-Methanotroph Processes for Dairy Waste Valorization

An alternative to synthetic fertilizers is the use of livestock manure as a biofertilizer. However, manure is difficult to handle in large animal farms due to its dilute nature [6]. The direct application of manure on soil can also lead to over accumulation of P in soil, as the manure N:P ratio is typical not optimal for most crops [7-8]. The management of manure is the second largest source of greenhouse gas emissions on dairy farms, after enteric methane. On average, manure handling contributes to 131 kg of CO₂-eq emissions per ton of manure [9].

Various solutions for manure management have been explored, including the use of anaerobic digestion systems [10]. This process generates a couple of valuable by-products: biogas and digestate. Digestate consists of both solid and liquid components, which are often separated and processed individually to maximize their value. With proper treatment, both forms of digestate can be utilized in a range of applications, such as animal bedding (solids) and nutrient-rich fertilizers (solids and liquids). However, the digestate has similar N:P rations as manure, and its application also introduces issues.

There has been recent interest in the use of advanced bioprocesses to recover nutrients from manure [11]. For instance, we have recently proposed the ReNuAl (Renewable Nutrients from Algae) process, which uses cyanobacteria (CB) strains that are engineered through synthetic biology to achieve flexible N:P ratios (tailored to meet the diverse nutrient needs of crops). The use of engineered CB strains offer multiple benefits as they can use solar energy (via photosynthesis) to capture waste nutrients and carbon dioxide produced from anaerobic digestion of manure [12]. Moreover, CB biomass can be used as a biofertilizer, contributing to the creation of a circular fertilizer economy. While the ReNuAl system effectively integrates bioprocesses for nutrient recovery, there remain untapped waste streams that could be further utilized to produce valuable products. A promising opportunity is the use of methane (which is a by-product of anaerobic digestion) to produce value-added products. Methanotrophs are microorganisms that can metabolize methane as their primary carbon source, converting it into biomass that is rich in proteins [13]. This valorization helps create new revenue streams and enables reduction of greenhouse gas emissions of the ReNuAl process. However, research in this area has been limited, particularly in optimizing methanotroph cultivation and integration within existing bioprocess systems.

In this work, we explore the potential of using methanotrophs to enhance the resource recovery efficiency of the ReNuAl system. We propose a bio-integrated system combining CB and methanotrophs to transform waste streams into value-added products such as fertilizers, biogas, and single cell protein. The proposed system integrates an anaerobic digester to produce biogas, which is then used by methanotrophs to generate single-cell protein. The process includes a filtration system followed by a thermal dryer to dehydrate the protein. Meanwhile, the digestate is sent to a solids-liquid separation (SLS) system for further processing, and the nutrient-rich extrudate is fed into a bioreactor to cultivate CB. Once the CB reaches the required growth stage, the contents of the bioreactor are sent through a dewatering system and then dried in a thermal dryer to produce a dry CB biomass. This closed-loop approach facilitates the conversion of waste into valuable bioproducts. Our approach targets three critical dimensions: (i) nutrient recovery through CB able to capture excess of P from manure runoff, (ii) emission mitigation via methanotrophic conversion of CH₄ to single cell protein, and (iii) circularity via closed-loop production of valuable bio-based products. We conducted a techno-economic analysis and evaluated environmental impacts across varying integration levels, identifying key process factors that can be optimized to guide the design of improved CB and microbial strains for enhanced system economics.

References

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