Valorization of Byproducts from Hydrothermal Liquefaction of Sewage Sludge and Manure: the Development of a Struvite-Producing Unit for Nutrient Recovery

Abstract

Hydrothermal liquefaction (HTL) presents a promising technology to recover energy from sewage sludge and manure as bio-crude oil. The liquid and solid phases after HTL are considered as byproducts; however, the latter is rich in nutrients, which offers a possibility for nutrient recovery and manufacturing of valuable products. This study focuses on the simultaneous recovery of phosphate and ammoniacal nitrogen as struvite by combining the liquid HTL byproduct and leachate from the solid HTL byproduct. To implement a flexible struvite-producing unit, the effect of an inlet-mixed solution composition on the struvite precipitation was investigated on a laboratory scale and preferable operating conditions were identified. These operating conditions were used afterward to evaluate and adapt an air-agitated system for struvite precipitation subsequent to the HTL production chain. Moreover, the precipitation process in the air-agitated system was evaluated using synthetic model solutions compared to real HTL byproducts to offer insight into the effect of impurities on the product quality. The product, which has a struvite content of more than 75 wt %, was manufactured from the HTL byproducts of manure and sewage sludge as well. It has potential for use as a fertilizer based on its elemental composition but the use of citric acid was found to be effective to improve the quality of the product, especially of sewage sludge. The results showed that the air-agitated system could be effectively implemented in the HTL production chain. It has the potential to enhance features of the product, leading to a ready-to-use fertilizer in shape (particle size) and quality (struvite content).

Circular Economy Model Through Phosphorus Recovery to Obtain Struvite from Sewage Sludge

“…In addition, micronutrients such as Fe, Cu, Zn or Co were detected. However, small amounts of toxic or dangerous agricultural compounds, such as Cr, As or Hg, were also present [28,41].…”

“…Both treatments require the addition of Mg and, through aeration, achieve CO2 stripping to reach a slightly basic pH (8-8.5) which causes the spontaneous precipitation of struvite in the presence of ammonium [24][25][26]. At the laboratory scale, phosphorus recovery as struvite was studied in a liquid effluent after WO treatment of municipal sewage sludge [27,28], process similar to the applied in this work.…”

Disposition of Solids from Hydrothermal Liquefaction of Biomass: Current Understanding, Research Gaps, and Opportunities

“…As for the land application of biosolid residuals from anaerobic digestion biosolids for plant fertilizers, bioavailability of the nutrients in HTL solids was found to be lower than that of the sewage sludge feedstock and simultaneously demonstrated that heavy metal content in the solids exceeded the threshold values for use as a fertilizer . However, recovery of low bioavailable phosphorus has also been demonstrated using HTL solids and other residuals to recover between 42 and 92% as struvite. , Struvite, a crystalline mineral of ammonia, magnesium, and phosphate (NH 4 MgPO 4 ·6H 2 O), can be directly used as a slow-release inorganic fertilizer, helping to reduce fertilizer runoff and the consumption of fertilizers. Several companies offer struvite recovery from digester effuents with full-scale systems operational throughout the US.…”

“…39 However, recovery of low bioavailable phosphorus has also been demonstrated using HTL solids and other residuals to recover between 42 and 92% as struvite. 39,40 Struvite, a crystalline mineral of ammonia, magnesium, and phosphate (NH 4 MgPO 4 •6H 2 O), can be directly used as a slow-release inorganic fertilizer, helping to reduce fertilizer runoff and the consumption of fertilizers. Several companies offer struvite recovery from digester effuents with full-scale systems operational throughout the US.…”

Upgrading of Hydrothermal Liquefaction Biocrudes from Mono- and Co-Liquefaction of Cow Manure and Wheat Straw Through Hydrotreating and Distillation

“…Solids from HTL carry around 100 kg of carbon from single feedstock processing, however this number was reduced to 56 kg in the combined approach. The solids from HTL of agricultural waste streams are rich in phosphorus [45], and further processing is needed to retrieve fertilizergrade forms of this element [46]. Thus, the reduction of carbon yielded as solids in the HTL process can also be beneficial for the handling and valorization of this fraction, greatly benefiting agricultural management of land and soil nutrient circularity.…”