Experimental and Modeling Assessment of the Main Bio-physical-chemical mechanisms and Kinetics in High-solids Anaerobic Digestion of Organic Waste
|Tila||Julkaistu - 5 joulukuuta 2018|
High-solids anaerobic digestion (HS-AD) is a well-suited strategy to enhance the overall AD efficiency for OFMSW treatment. HS-AD is operated at a total solid (TS) content ≥ 10 %, permitting to reduce the reactor size and overall operational costs. Nonetheless, the TS increase can result into biochemical instability, and even reactor failure by acidification. Both the high organic load and the buildup of inhibitors can be responsible for the HS-AD instability. The most notable inhibitor in HS-AD of OFMSW is the free ammonia nitrogen (NH3). Therefore, a balance is often required between enhancing the HS-AD economy and the ‘undesired’ instability for OFMSW treatment.
This PhD research investigated the main bio-physical-chemical mechanisms and kinetics in HS-AD of OFMSW, with the aim to optimize the industrial application and maximize the kinetic rates. Laboratory-scale batch and semi-continuous experiments highlighted the main strengths and weaknesses of HS-AD. Simultaneously, the development of a HS-AD model permitted to condense the experimental knowledge about the main bio-physical-chemical effects occurring when increasing the TS content in HS-AD.
HS-AD batch experiments required a tradeoff between the initial TS, the inoculum-to-substrate ratio (ISR), the alkalinity and the nitrogen content, to assess the effects of increasing the initial TS content upon the methane yield, TS removal and chemical oxygen demand conversion. Particularly, a low ISR led to acidification, whereas the NH3 buildup led to volatile fatty acid (VFA) accumulation, reducing the methane yield, whether or not co-digestion of OFMSW with beech sawdust was used.
In semi-continuous experiments, HS-AD of OFMSW required a reduced effluent compared to the influent to counterbalance the organic mass removal associated to the biogas production. Nonetheless, mono-digestion of readily-biodegradable OFMSW could not sustain a TS ≥ 10 % without exacerbating the risk of substrate overload. Overloading was associated to the high biodegradability of OFMSW and the NH3 buildup. Thus, adding sawdust to OFMSW permitted to operate the reactors up to 30 % TS, due to the lower biodegradability and nitrogen content of lignocellulosic substrates.
As the main novelty of this PhD research, a HS-AD model based on the Anaerobic Digestion Model No.1 (ADM1) was developed. This model simulates the reactor mass and TS in HS-AD, in contrast of models focusing on ‘wet’ AD simulations (TS < 10 %). Moreover, the HS-AD model considers also the TS concentration effect on soluble species. A ‘non-ideal’ bio-physical-chemical module, modifying predominantly the acid-base equilibrium constants, was subsequently coupled to the HS-AD model. Noteworthy, HS-AD is often characterized by a high ionic strength (I ≥ 0.2 M), affecting the pH, NH3 concentration and CO2 liquid-gas transfer, as the most important triggers for HS-AD inhibition.
The HS-AD model calibration required multiple experimental datasets to circumvent parameter non-identifiability. The model calibration showed that HS-AD of OFMSW might be operated at I up to 0.9 M and NH3 concentrations up to 2.3 g N/L, particularly at higher TS contents (25 - 30 %). Moreover, the model calibration suggested that a reversible non-competitive NH3 inhibition should be further tested. Further HS-AD model developments (e.g. precipitation) were also recommended. All these results might aid in the optimization of HS-AD for organic waste treatment, renewable energy and nutrient recovery.