Engineering and modification of biochar is providing many possible ways to use bio-derived waste materials. In another example biochar derived from corn silage, cow manure and pig slurry fermentation waste as feedstock is used as a sorbent; modified with ferric iron and calcium by chemisorption, the biochar recovered phosphorus from sludge wastewater. The dewatered fermentation wastes are pyrolyzed to make biochar using waste heat from biogas combustion and pyrolysis gas. The authors compare the costs of the calcium-biochar sorbent vs. ferric-biochar sorbent, struvite, or sludge water at an application rate of 1 kg CaP /ha. The biochar sorbents are less expensive than struvite or direct sludge water applications and the phosphorus from the biochar is more bioavailable to plants. From these studies and more we can see the promise of biochar for recycling waste feedstocks into useful adsorbent carriers of nutrients or other products, as alternative crystallizing agents for struvite potentially replacing more expensive chemical precipitants, or as sorbents for the removal of pollutants from wastewaters. In the following study on the hydrothermal carbonization of digestate, an anaerobic digestate may be used to recover nutrients. Hydrothermal carbonization of anaerobically digested biomass is a way to make another biochar, referred to by the authors as "hydrochar" due to the process used to make it. The authors modeled recovery of ammonium, nitrate, phosphorus, and potassium from the hydrothermal carbonization process. The hydrothermal carbonization of biogas digestate of cattle manure was simulated using a model nutrient solution of glucose (as the organic carbon source) plus known amounts of potassium dihydrogen phosphate, ammonium chloride, potassium chloride, sodium nitrate and sodium nitrite. Modeling was used to determine how carbon, nitrogen, and phosphorus are incorporated into biochar or lost to process waters or gases during the formation of biochar. The gases measured included CO2, CO, methane, N2, nitrous oxide, and ammonia. Ammonia and nitrate were found in both the hydrochar and the process water; the relative amounts of ammonia and nitrates depended on the feedstock composition and the conditions for anaerobic digestion of the cattle manure. The carbon from the manure ended up in the biochar, while phosphorus was mainly lost to the wastewater. Thus, processes for anaerobic digestion of biomass feedstock to produce biogas can be integrated with use of the digestate for nutrient recovery as biochar, although processes must be carefully managed to be efficient sources for fertilizer.