Multivariable advanced nonlinear controller for bioethanol production in a non-isothermal fermentation bioreactor

Design for fermentation bioreactor controllers is challenged by the nonlinear process kinetics and the lack of online measurements for key variables. This work developed a multi-input, multi-output advanced nonlinear control structure for a continuous, non-isothermal, constant volume fermentation bioreactor. Utilizing feedback linearization control for the bioreactor feed to regulate glucose concentration, and backstepping control for the cooling jacket feed to regulate reactor temperature. A developed novel estimator for biomass concentration was incorporated to provide online estimates for the unmeasurable state variable. Simulation results showed the control structure ability in efficiently establishing a combination of dynamic and fixed set points, despite disturbances in the bioreactor feed temperature and glucose concentration. Expanded bioreactor control authority increased operational flexibility and enhanced the potential for performance improvements. This work illustrated the effectiveness of feedback linearization and backstepping control in designing controllers for biological systems with nonlinear dynamics, complex interactions, and input disturbances.

Transition of municipal sludge anaerobic digestion from mesophilic to thermophilic and long-term performance evaluation

Strategies for the transition of municipal sludge anaerobic digestion from mesophilic to thermophilic were assessed and the long-term stability and performance of thermophilic digesters operated at a solids retention time of 30days were evaluated. Transition from 36°C to 53.3°C at a rate of 3°C/day resulted in fluctuation of the daily gas and volatile fatty acids (VFAs) production. Steady-state was reached within 35days from the onset of temperature increase. Transitions from either 36 or 53.3°C to 60°C resulted in relatively stable daily gas production, but VFAs remained at very high levels (in excess of 5000mg COD/L) and methane production was lower than that of the mesophilic reactor. It was concluded that in order to achieve high VS and COD destruction and methane production, the temperature of continuous-flow, suspended growth digesters fed with mixed municipal sludge should be kept below 60°C.

Modeling the fate and effect of benzalkonium chlorides in a continuous-flow biological nitrogen removal system treating poultry processing wastewater

The fate and effect of the antimicrobial compounds benzalkonium chlorides (BACs) on the biological nitrogen removal (BNR) processes for a continuous-flow, three-stage laboratory-scale BNR system were modeled. Three kinetic sub-models, corresponding to each reactor, were developed and then combined in a comprehensive ASM1-based model. Kinetic parameters for the three sub-models were evaluated using experimental data obtained from independent batch assays. The biodegradation of BACs was modeled with a mixed-substrate Monod equation. The inhibitory effect of BACs on the utilization of degradable COD and denitrification was modeled as competitive inhibition, whereas non-competitive inhibition was used to model the effect of BACs on nitrification and inhibition coefficients were evaluated. The model simulated well the long-term performance of the BNR system treating a poultry processing wastewater with and without BACs. Enhanced BAC degradation by heterotrophs and increased resistance of nitrifiers to BACs, reflecting acclimation/enrichment over time, is a salient feature of the model.

Fate and effect of benzalkonium chlorides in a continuous-flow biological nitrogen removal system treating poultry processing wastewater

Quaternary ammonium compounds (QACs) are used for sanitation in many poultry processing facilities. This work investigated the fate and effect of a mixture of benzalkonium chlorides (BACs), a class of QACs widely used in commercial antimicrobial formulations, on the biological nitrogen removal (BNR) processes. A laboratory-scale BNR system was operated continuously for 670 days, fed with poultry processing wastewater amended with a mixture of BACs. Initially, the nitrogen removal efficiency deteriorated at a BAC feed concentration of 5 mg/L due to the complete inhibition of nitrification. However, after 27 days of operation, the system recovered and achieved 100% ammonia removal. High nitrogen removal efficiency was achieved even after the feed BAC concentration was stepwise increased up to 120 mg/L. Batch nitrification assays performed before, during, and after BAC exposure, showed that rapid microbial acclimation and BAC biodegradation contributed to the recovery of nitrification achieving efficient and stable long-term BNR system operation.

Nitrate reduction in a simulated free-water surface wetland system

The feasibility of using a constructed wetland for treatment of nitrate-contaminated groundwater resulting from the land application of biosolids was investigated for a site in the southeastern United States. Biosolids degradation led to the release of ammonia, which upon oxidation resulted in nitrate concentrations in the upper aquifer in the range of 65-400 mg N/L. A laboratory-scale system was constructed in support of a pilot-scale project to investigate the effect of temperature, hydraulic retention time (HRT) and nitrate and carbon loading on denitrification using soil and groundwater from the biosolids application site. The maximum specific reduction rates (MSRR), measured in batch assays conducted with an open to the atmosphere reactor at four initial nitrate concentrations from 70 to 400 mg N/L, showed that the nitrate reduction rate was not affected by the initial nitrate concentration. The MSRR values at 22 °C for nitrate and nitrite were 1.2 ± 0.2 and 0.7 ± 0.1 mg N/mg VSS(COD)-day, respectively. MSRR values were also measured at 5, 10, 15 and 22 °C and the temperature coefficient for nitrate reduction was estimated at 1.13. Based on the performance of laboratory-scale continuous-flow reactors and model simulations, wetland performance can be maintained at high nitrogen removal efficiency (>90%) with an HRT of 3 days or higher and at temperature values as low as 5 °C, as long as there is sufficient biodegradable carbon available to achieve complete denitrification. The results of this study show that based on the climate in the southeastern United States, a constructed wetland can be used for the treatment of nitrate-contaminated groundwater to low, acceptable nitrate levels.

Effect of temperature and benzalkonium chloride on nitrate reduction

The effect of temperature and benzalkonium chloride (BAC) on nitrate reduction was investigated in batch assays using a mixed nitrate reducing culture. Nitrate was transformed completely, mainly through denitrification, to dinitrogen at 5, 10, 15 and 22 °C. In the absence of BAC, reduction of individual nitrogen oxides had different susceptibility to temperature and transient nitrite accumulation was observed at low temperatures. When the effect of BAC was tested up to 100 mg/L from 5 to 22 °C, denitrification was inhibited at and above 50mg BAC/L with transient nitrite accumulation at all temperatures. The effect of BAC was described by a competitive inhibition model. Nitrite reduction was the denitrification step most susceptible to BAC, especially at low temperatures. BAC was not degraded during the batch incubation and was mostly biomass-adsorbed. Overall, this study shows that low temperatures exacerbate the BAC inhibitory effect, which in turn is controlled by adsorption to biomass.