Contribution to nutrient recovery from anaerobically digested liquid effluent using engineered biochar

Sohrab Haghighi Mood

doi:10.7273/000005006

Phosphorus (P) and Nitrogen (N) are necessary nutrients for plant growth. However, the high contents of these nutrients cause eutrophication, a global environmental problem. Large, confined animal feeding operations (CAFOs) produce large quantities of manure responsible for releasing nutrients (N and P) and pathogens into water bodies. Although anaerobic digestion (AD) is a promising technology to convert manure carbon into methane, it does not solve the problem associated with P and N management. AD liquid effluent still contains large quantities of nutrients. An integrated concept for nutrient recovery that includes Dissolved Air Flotation (DAF), pyrolysis, advanced oxidation, and adsorption is proposed as a complementary technology to recover P and N from AD effluent. Engineered chars were synthesized to adsorb PO43- and NO3-. AD fiber was the first feedstock to produce biochar for PO43- adsorption. It was found that by treating AD fiber with ammonia from the pyrolysis step up to 750o C, it was possible to obtain biochar with a capacity to adsorb up to 63.1 mg/g. However, the method used with AD fiber failed to produce engineered biochars with PO43- adsorptive properties when using Douglas Fir and Wheat Straw. It was found that the low content of Mg or Ca was the main reason for the low adsorption capacity of these lignocellulosic materials. The understanding of the need to dope with the biochar with N and Mg led to the development of a generalized method for producing engineered biochars with high PO43- adsorption capacity from any lignocellulosic material. The results show a synergistic effect of nitrogen functionalities and Mg on biochar surface/structure, resulting in improved phosphate adsorption. Biochars doped with Mg and N showed phosphate adsorption of 216 (a 37-fold increase compared with raw biochars). A new adsorption mechanism was proposed to explain the obtained results. A similar method was used to synthesize an adsorbent for NO3-. Fe and N were introduced to the biochar structure. The experimental optimization strategies with a central composite experimental design for the biochar were conducted. The optimum pyrolysis temperature, metal loading(FeCl3), and pyrolysis time(under ammonia gas) were 800 °C, 6 g, and 30 min, respectively. The nitrate adsorption capacity of biochar at pH 3 and 7 are 20.67 and 9.4, respectively, showing that the adsorption capacity of biochar increased by around 96% at neutral pH compared to pristine biochar. Finally, the biochars with high nitrate and phosphate adsorption capacity were applied to recover P and N from AD effluent. An integrated operation system was used to recover P and N from AD effluent. O3 oxidation degraded organic matter and converted different forms of P and N into PO43- and NO3-. The rate of PO43- release was much faster than the rate of NO3-. The higher P adsorption efficiency was achieved when organic P was degraded and converted into PO43- by ozone oxidation. It was possible to achieve PO43- removal of 121 mg/g from ozonated organic P model compound (Fosfomycin sodium) (a 51-fold increase compared with the adsorption capacity of biochar in fosfomycin solutions without ozone treatment). A 2.3-fold increase in P adsorption capacity was also achieved when DAF effluent was treated with ozone.

Contribution to nutrient recovery from anaerobically digested liquid effluent using engineered biochar

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