Combining pH and electrical conductivity measurements to improve titrimetric methods to determine ammonia nitrogen, volatile fatty acids and inorganic carbon concentrations

Anaerobic digestion of waste Tunisian date (Phoenix dactylifera L.): effect of biochemical composition of pulp and seeds from six varieties

Large amounts of secondary date waste (pulp and seeds) are produced and discarded in Tunisia without proper valorisation methods. To study the possibility of valorising different varieties of Tunisian date waste (pulp and seeds) via anaerobic digestion, batch biochemical methane potential (BMP) tests were carried out under mesophilic temperature. The bio-methane production curves were fitted to the model of modified Gompertz in search of the kinetic parameters. The bio-chemical characterisation of the substrates from different varieties (total and volatile solids, COD and contents in carbohydrates, proteins, lipids, polyphenols) was realised. Principal component analysis (PCA) was used to investigate the correlations between the model parameters and biochemical variables. Results show that the biochemical compositions of date pulp and seeds strongly depend on the varieties. The BMP are in the range of 0.295-0.345 and 0.267-0.327 Nm³ CH₄·kg COD^-1 for pulp and seeds respectively, resulting from the significant biochemical variance among the varieties. The BMP of date seeds was significantly correlated with their VS/TS ratio, carbohydrate and protein contents (p < 0.05). For the pulp, significant correlation was found between BMP, carbohydrate and lipid contents. PCA shows that certain varieties (like pulp and seeds of Deglet Nour and seeds of Bejou) are most suitable for being valorised by anaerobic digestion. The most suitable date varieties for this innovative approach were revealed. This research provided useful knowledge for bioconversion of waste date pulp and seeds to biomass energy.

Accurate Estimation of Bicarbonate and Acetic Acid Concentrations with Wider Ranges in Anaerobic Media Using Classical FOS/TAC Titration Method

The determination of a volatile fatty acid content (FOS) and total alkalinity (TAC) can be carried out using Nordmann’s FOS/TAC titration method developed in the 1970s. This two-point titration (pH = 5 and 4.4) can be simply implemented and is widely employed by both the academic and industrial worlds. However, the present study proves that Nordmann’s method is only valid in limited ranges, since the titration of one FOS and TAC has an impact on the determination of the other, especially in extreme conditions. The present work develops a numerical tool with Scilab simulating the acid–base equilibria of titration. The program is efficient in predicting the experimental equivalent volumes obtained from Nordmann’s method with different combinations of sodium acetate and sodium bicarbonate contents. The mean absolute percentage errors (MAPE) between the simulation and experiment are below 7%. Two new formulas are developed, considering both equivalent volumes at pH = 5 and 4.4 to calibrate FOS and TAC values. The proposed formulas show their good performance in predicting various combinations of FOS and TAC contents in an anaerobic digestate at TAC ranging from 0 to 20,000 mg CaCO3·L−1 and FOS ranging from 0 to 31,000 mg HAc·L−1.

Recent developments in monitoring technology for anaerobic digesters: A focus on bio-electrochemical systems

With the increasing popularity of waste to energy conversion, demand for large-scale operation of anaerobic digestors has emerged in the market. However, the process instabilities in anaerobic digestors limit the expansion of facilities to high loading rates. The irregularities in the process can be addressed directly by altering the feedstock characteristics provided an on-hand, robust, and sensitive monitoring device is available. In this context, the bioelectrochemical system has emerged as an excellent tool for monitoring and optimizing the anaerobic process within the reactor. This article reviews the gradual evolution in techniques and approaches for monitoring of anaerobic digestion (AD) process. An analysis of the recently popular biosensing techniques has been done with a focus on the bioelectrochemical monitoring system and its operation mode. A brief attempt to highlight the current challenges in the field of bioelectrochemical process monitoring for AD has also been made, which can be supportive of future research.

A new electrochemical method for simultaneous removal of Mn2+and NH4+-N in wastewater with Cu plate as cathode

In this study, a new electrochemical method was used to simultaneously efficient removal of Mn²⁺ and NH₄⁺-N in wastewater with Cu plate as cathode. The effects of various reaction parameters on the concentrations of Mn²⁺, NH₄⁺-N and by-products (NO₃^--N and NO₂^--N, free chlorine and residual chlorine), as well as the removal mechanism were investigated. The results showed that the removal efficiencies of Mn²⁺ and NH₄⁺-N were 99.1% and 92.9%, and the concentrations of NO₃^--N, NO₂^--N, free chlorine and residue chlorine were 0.73 mg/L, 0.15 mg/L, 0.13 mg/L and 0.63 mg/L reacting for 3 h at room temperature, respectively, when the current density was 10 mA/cm², the mass ratio of ClO^- and Cl^- was 1:1, the initial pH was 9. The concentrations of Mn²⁺, NH₄⁺-N and by-products in wastewater met the integrated wastewater discharge standard (GB8978-1996). In addition, spherical manganese oxide was deposited on the anode plate, and spherical manganese oxide collapsed over electrolysis time. Manganese was mainly removed in the form of MnO, Mn(OH)₂ and MnO₂. NH₄⁺-N was mainly oxidized to N₂. Economic evalution revealed that the treatment cost was 2.93 $/m³.

Design, Development, and Performance Evaluation of a Fertigation Device for Distributing Solid Fertilizer

Solid fertilizers, which have a low operating cost, are widely applied in Chinese crop fields. In order to distribute solid fertilizer through fertigation, an innovative device with a simple structure was designed, which can feed, dissolve, and distribute fertilizer simultaneously. The parameters of the outlet pipe and fertilizer-feeding component were designed, and a preliminary equation for calculating the fertilizer-feeding flowrate was established. Then experiments were conducted to optimize the established equation. Obtained results showed that the deviation between the measured and the calculated feeding flowrate through the optimized equation was about 5%. This ensured that the fertilizer-feeding flowrate can be adjusted accurately. Experiments were also conducted to explore the effect of the working parameters on the fertilization uniformity of the designed device. It was found that as the fertilization time and inlet water flowrate increases, the fertilization uniformity increases but fertilizer concentration decreases. Based on the obtained results, it is concluded that the designed fertigation device outperforms the conventional pressure differential tank that is normally applied to distribute solid fertilizers.

A novel approach for rapidly measuring volatile fatty acids in anaerobic process

Volatile fatty acids (VFAs), the intermediate of the anaerobic process, are considered to be the critical, high-sensitive and reliable indicators of the process stability. Close monitoring and control of VFAs are paramount for the efficient operation of the anaerobic reactors. In this study, a buffer intensity-based mathematical model was developed, and the least square method was integrated into the model to solve the issue of non-linear fitting of the titration curve. An automatic analyzer embedded with the developed model was designed and implemented for measuring VFAs and alkalinity. Through model optimization, the pH range of 3.5-5.6 was found to be suitable for VFAs analysis. The developed approach was validated by different VFAs (up to 500 mg/L as acetic acid) and carbonate alkalinity concentrations (up to 1500 mg/L as CaCO₃) with high recovery rates (>0.9). Optimal ratios of carbonate alkalinity to VFAs are identified in the range of 2.4-7.5 for accuracy. Owing to the non-linear fitting of the titration curve, the impact of other weak acid subsystems (e.g., phosphate, ammonium and sulfide) can be negligible. The one-year real-time monitoring of environmental samples by using the automatic analyzer indicates a high consistency and stability compared with the 5 pH point titration. This approach proves to be rapid (<3 min/sample), accurate, reliable and can be applied for real-time automatic monitoring of the anaerobic process.

Electrical Conductivity for Monitoring the Expansion of the Support Material in an Anaerobic Biofilm Reactor

This article describes the use of the electrical conductivity for measuring bed expansion in a continuous anaerobic biofilm reactor in order to prevent the exit of support material from the reactor with the consequent loss of biomass. The substrate used for the tests is obtained from a two-stage anaerobic digestion (AD) process at the pilot scale that treats the liquid fraction of fruit and vegetable waste (FVW). Tests were performed with the raw substrate before anaerobic treatment (S1), the effluent from the hydrolysis reactor (S2), and the effluent from the methanogenic reactor (S3) to evaluate its effect on the electrical conductivity values and its interaction with colonized support material. The tests were carried out in a 32 L anaerobic inverse fluidized bed reactor (IFBR), which was inoculated with colonized support material and using two industrial electrodes at different column positions. The results with the previously digested samples (S2 and S3) were satisfactory to detect the presence of support material at the points where the electrodes were placed since the electrical conductivity values showed significant changes of up to 0.5 V, while with substrate S1 no significant voltage differences were appreciated. These results demonstrate that electrical conductivity can be used as an economic and simple mean for monitoring the support material expansion in order to avoid over expansion in the IFBR. It was also demonstrated that the conditions of the substrate in the methanogenic stage (pH and presence of volatile fatty acids) do not affect the operation of the electrical conductivity detection system.

Solid-state anaerobic digestion of wheat straw: Impact of S/I ratio and pilot-scale fungal pretreatment

Solid State Anaerobic Digestion (SSAD) of fungal pretreated wheat straw was evaluated in a leach bed reactor. During a first experiment, the effect of Substrate/Inoculum (S/I) ratios on the start-up phase was investigated. High S/I increased methane productivity but also raised the risk of reactor failure due to Volatile Fatty Acid (VFA) accumulation. With S/I ratios between 1.2 and 3.6 (Volatile Solid (VS) basis), the SSAD start-up using wheat straw was successful. Moreover, reactors were able to recover from acidification when the Total VFA/alkalinity ratio was lower than 2 gHAc_eq/gCaCO₃, with VFA concentrations lower than 10 g/L and a pH close to 5.5. The conventional threshold of 0.6 gHAc_eq/gCaCO₃ for stable wet AD is therefore not adapted to SSAD. During a second experiment, after the wheat straw was submitted to a fungal pretreatment in a non-sterile pilot-scale reactor, it was digested with an S/I ratio of 2.8-2.9. Under batch SSAD conditions, the biodegradability of pretreated wheat straw was slightly improved in comparison to the control (254 versus 215 NmL/g VS, respectively). Considering mass losses occurring during the pretreatment step, suboptimal pretreatment conditions caused a slightly lower methane production (161 versus 171 NmL/gTS_initial after 60-days anaerobic digestion). Nevertheless, pretreatment improved the start-up phase with lower acidification relative to controls. It would be particularly beneficial to improve the methane production in reactors with short reaction times.

Formation and characteristics of a ternary pH buffer system for in-situ biogas upgrading in two-phase anaerobic membrane bioreactor treating starch wastewater

Biochemical biogas upgrading retaining more CO₂ from biogas to form biomethane opens new avenues for sustainable biofuel production. For developing this technology, maintaining sustain pH for CO₂·H₂O is fundamental. This study proposes an innovative control strategy for in-situ biogas upgrading retaining and converting the CO₂ in the biogas into CH₄, via hydrogenotrophic methanogenesis without external agent. The Biogas-pH strategy limited pH drop over 7.4 by stop feeding and maintained the methanogenesis activity by biogas flow rate over 98 ml·h^-1. Low pH (7.37-7.80) decrease CO₂·H₂O as a substrate in stage-I, higher pH (7.40-8.41) enhances CO₂ to CO₂·H₂O transfer by 6.29 ± 2.20% in stage-II. Because of that 95% CO₂·H₂O converts to HCO₃^- and CO₃^2- when pH > 7.9, higher pH > 7.9 did not further upgrading the biogas. The carbonate buffer system shown open and close system characteristics in gas and liquid phase. The biogas CH₄ was upgraded from 67.27 ± 5.21% to 73.56 ± 5.01%.

A modified two-point titration method for the determination of volatile fatty acids in anaerobic systems

The volatile fatty acids (VFA) concentration plays important roles in the rapid start-up and stable operation of anaerobic reactors. It's essential to develop a simple and accurate method to monitor the VFA concentration in the anaerobic systems. In present work, a modified two-point titration method was developed to determine the VFA concentration. The results show that VFA concentration in standard solutions estimated by the titration method coincided well with that measured by gas chromatograph, where all relative errors were lower than 5.5%. Compared with the phosphate, ammonium and sulfide subsystems, the effect of bicarbonate on the accuracy of the developed method was relatively significant. When the bicarbonate concentration varied from 0 to 8 mmol/L, the relative errors increased from 1.2% to 30% for VFA concentration at 1 mmol/L, but were within 2.0% for that at 5 mmol/L. In addition, the VFA composition affected the accuracy of the titration method to some extent. This developed titration method was further proved to be effective with practical effluents from a lab-scale anaerobic reactor under organic shock loadings and an unstable full-scale anaerobic reactor.

A review on the applications of microbial electrolysis cells in anaerobic digestion

Anaerobic digestion (AD) has been widely used for biogas or biofuel generation from waste treatment. Because a low production rate and instability of AD occur frequently, various technologies have been applied to improvement of AD. Microbial electrolysis cells (MECs), an emerging technology, can convert organic matter into hydrogen, methane, and other value-added products. Recent studies showed that application of MEC to AD (MEC-AD) can accelerate degradation of a substrate (including recalcitrant compounds) and alter AD microbial community by enriching exoelectrogens and methanogens thus increasing biogas production. With stable microbial communities established, improvement of MEC-AD for methane production was achieved. MEC-AD process can be monitored in real-time by detecting electric signals, which linearly correlate with substrate concentrations. This review attempts to evaluate interactions among the decomposition of substrates, MEC-AD system, and the microbial community. This analysis should provide useful insights into the improvement of methane production and the performance of MEC-AD.

Fast characterization of solid organic waste content with near infrared spectroscopy in anaerobic digestion

The development of anaerobic digestion involves both co-digestion of solid wastes and optimization of the feeding recipe. Within this context, substrate characterisation is an essential issue. Although it is widely used, the biochemical methane potential is not sufficient to optimize the operation of anaerobic digestion plants. Indeed the biochemical composition in carbohydrates, lipids, proteins and the chemical oxygen demand of the inputs are key parameters for the optimisation of process performances. Here we used near infrared spectroscopy as a robust and less-time consuming tool to predict the solid waste content in carbohydrates, lipids and nitrogen, and the chemical oxygen demand. We built a Partial Least Square regression model with 295 samples and validated it with an independent set of 46 samples across a wide range of solid wastes found in anaerobic digestion units. The standard errors of cross-validation were 90mgO₂⋅gTS^-1 carbohydrates, 2.5∗10^-2g⋅gTS^-1 lipids, 7.2∗10^-3g⋅gTS^-1 nitrogen and 99mgO₂⋅gTS^-1 chemical oxygen demand. The standard errors of prediction were 53mgO₂⋅gTS^-1 carbohydrates, 3.2∗10^-2g⋅gTS^-1 lipids, 8.6∗10^-3g⋅gTS^-1 nitrogen and 83mgO₂⋅gTS^-1 chemical oxygen demand. These results show that near infrared spectroscopy is a new fast and cost-efficient way to characterize solid wastes content and improve their anaerobic digestion monitoring.