Carbon sources play a critical role in biological nitrogen removal during wastewater treatment, where strict total nitrogen limits on effluent discharge apply. Organic carbon sources serve as electron donors in the denitrification for heterotrophic denitrifiers. The growing need for improved denitrification driven by increasing influent loads in a limited area and stricter nutrient discharge standards has increased the demand for external carbon sources. Conventional carbon sources such as methanol or ethanol, used in wastewater treatment, are often derived from fossil fuels, raising environmental and economic concerns. Therefore, this thesis explores an alternative solution for carbon source provision in denitrification, i.e., waste-derived volatile fatty acids (VFAs). Several VFAs, e.g., acetic acid, propionic acid, and butyric acid, are generated during the anaerobic digestion (AD) of various organic waste materials as intermediate metabolites, which are a sustainable alternative that holds great promise for optimizing denitrification processes while mitigating environmental impacts.

In this thesis membrane bioreactors (MBRs) were applied for efficient production and extraction of VFAs from organic waste. This novel membrane separation technique led to particle-free VFAs at a high yield of 0.65 g VFAs/g VSfed. However, this VFAs effluent contains impurities such as ammonium ions (NH4+) that can have adverse effects when applied in wastewater operations, especially in the denitrification process. Ammonium removal potential was explored using a low-cost natural zeolite, clinoptilolite. The VFAs effluent was subjected to an ion exchange process to remove ammonium. Under the determined optimum conditions, average removal efficiencies of 93 and 94% were found for NH4+ removal at 12 h equilibrium time for the synthetic and VFA effluents, respectively. Denitrification performance was investigated thoroughly, and denitrification rates were compared with those obtained using conventional carbon sources. Although methanol exhibits a faster nitrate (NO3−) removal capability than obtained using other carbon sources, there is a lack of synchronicity between the conversion of NO3− ions to nitrite (NO2−) and NO2− to N2. However, relatively few issues have been encountered with using VFAs as a carbon source. Although adding VFA as the sole carbon source exhibited a slower denitrification rate than obtained with methanol, 50% of methanol can be replaced by waste-derived VFAs, achieving performance comparable to that obtained with pure methanol. In addition, further upgrading of waste-derived VFAs was complemented with the nanofiltration process before their application in wastewater treatment to increase their carbon content. It was observed that the chemical oxygen demand of VFAs increased up to 4.3 times (from 26.5 to 113.7 g/L). The concentrated waste-derived VFAs (after nanofiltration) could be used in the denitrification process of wastewater treatment. In conclusion, the use of waste-derived VFAs emerged as a potential sustainable alternative replacement of the conventional carbon sources for wastewater denitrification.

Supplementation of alternative carbon sources is a technological bottleneck, particularly in post-denitrification processes due to stringent effluent nitrogen levels. This study focuses on enhancing the sustainability of wastewater treatment practices by partially replacing conventionally used fossil-derived methanol with organic waste-derived volatile fatty acids (VFAs) in moving bed biofilm reactors (MBBRs). In this regards, results of denitrification batch assays with sequential or simultaneous addition of VFA effluent from acidogenic fermentation of potato starch residue (AD-VFAPPL) and chicken manure (AD-VFACKM), simulated synthetic VFAs solutions (sVFAs), and methanol as carbon source were presented and discussed. Although methanol has proven superior in the conversion of nitrate to nitrite, VFAs are more effective when it comes to reducing nitrite. Although solely added AD-VFAPPL had a slower denitrification capability (0.56 ± 0.13 mgNOx-N removed/m2/day) than methanol (1.04 ± 0.46 mgNOx-N removed/m2/day), up to 50% of the methanol can be replaced by waste-derived AD-VFAPPL and achieve comparable performance (1.08 ± 0.07 mgNOx-N removed/m2/day) with the pure methanol. This proves that the co-addition of VFAs together with methanol can fully compete with pure methanol in performance, providing a promising opportunity for wastewater treatment plants to potentially reduce their carbon footprint and become more sustainable in practice while benefiting from recovered nutrients from waste.

The cornerstones of an efficient circular waste management strategy aiming for enhanced resource efficiency are maximizing organic waste valorization and improving residual conversion to biochemicals. In this regard, this study focuses on the production of volatile fatty acids (VFAs) from the effluent of fungi biomass cultivation on low-grade residues from the potato starch industry with batch and semi-continuous membrane bioreactors (MBRs) containing the effluent of already fermented potato protein liquor (FPPL) inoculated with chicken and cow manure. The effect of pH in the batch experiments on the production and yield of VFAs during acidogenic digestion was evaluated. Rapid generation of VFAs at a concentration of up to 11.8 g/L could be successfully achieved in the MBR. Under the optimal conditions, a high yield of 0.65 g VFAs/g VSfed was obtained for the organic loading rate (OLR) of 1 g VS/L/d using FPPL substrate and chicken manure as inoculum. The results show that the application of sequential multi-step bioconversion of potato starch industry residues has the potential to increase the variety of value-added products generated from a single organic residue while enhancing nutrient recovery capacity. 

Sustainable provision of chemicals and materials is undoubtedly a defining factor in guaranteeing economic, environmental, and social stability of future societies. Among the most sought-after chemical building blocks are volatile fatty acids (VFAs). VFAs such as acetic, propionic, and butyric acids have numerous industrial applications supporting from food and pharmaceuticals industries to wastewater treatment. The fact that VFAs can be produced synthetically from petrochemical derivatives and also through biological routes, for example, anaerobic digestion of organic mixed waste highlights their provision flexibility and sustainability. In this regard, this review presents a detailed overview of the applications associated with petrochemically and biologically generated VFAs, individually or in mixture, in industrial and laboratory scale, conventional and novel applications.

Various methods exist for the recovery of volatile fatty acids from organic mixed waste effluents, and among them, the membrane filtration process holds a great promise over other recovery methods due to their simplicity, sustainability and high efficiency. Hence, in this study, nanofiltration experiments were carried out using two commercial nanofiltration membranes of 200–300 Da and 300–500 Da under various pH (4, 5.4, 7 and 9) at constant pressure (15 bar) and temperature of 20-21 °C in order to achieve a higher amount of volatile fatty acids from the real mixed food waste-based effluent. Results showed that solution pH plays an important role in the physicochemical parameters such as total solids removal rate was above 80 % at pH 4, chemical oxygen demand, ammonia and phosphorus removed to some extent at pH 9. Subsequently, the concentration and recovery percentages of volatile fatty acids increased with solution pH 9; in particular, lower molecular weight cut-off membrane, i.e., 200–100 Da, appeared to be more effective with an increased concentration of total volatile fatty acids (16.94 g L−1) and recovery percentage above 90 % at pH 9. Membrane performance was also evaluated and correlated with recovery performance in terms of permeate flux reduction at lower pH. An important finding of this study was the concentration and recovery percentages of volatile fatty acids reached around 96 % after 3rd cycle by conducting a repeated sequencing nanofiltration process, which was identified as a promising option to enhance the recovery percentages of volatile fatty acids.

Fossil-based materials such as methanol are frequently used in the denitrification process of advanced biological wastewater treatment as external carbon source. Volatile fatty acids (VFAs) produced by anaerobic digestion of food waste, are sustainable compounds with the potential to act as carbon sources for denitrification, reducing carbon footprint and material costs. In this study, the effectiveness of food waste-derived VFAs (AD-VFA) was investigated in the post-denitrification process in comparison with synthetic VFA and methanol as carbon sources. Acetic acid had the highest rate of disappearance among single tested VFAs with a denitrification rate of 0.44 g NOx-N removed/m2/day, indicating a preferential utilization pattern. While AD-VFA had a denitrification rate of 0.61 mg NOx-N removed/m2/day, sVFA had a rate of 0.57 mg NOx-N removed/m2/day, indicating that impurities in AD-VFA did not play substantial role in denitrification. AD-VFA proved to be promising carbon source alternative for denitrification in wastewater treatment plants.

Acidogenic fermentation of chicken manure (CM) for production and recovery of volatile fatty acids (VFA) is an interesting biological waste-to-value approach compared to benchmark organic waste management strategies. Considering the wide range of high value applications of VFA, a semi-continuous immersed anaerobic membrane bioreactor (AnMBR) was applied to boost VFA productivity and yield, while reducing downstream processing stages assisting the recovery of VFA. In this regard, the effect of parameters such as pH and organic loading rates (OLR) on the overall bioconversion and filtration performance was investigated. Thermal-shocked CM was applied both as inoculum and substrate. A very high VFA yield (0.90 g-VFA/g-VS) was obtained in the treatment with no pH control (~8.2) at an OLR of 2 g-VS/(L·d), presenting 24% higher yield compared to that of the controlled pH. Batch assays further demonstrated the enhanced hydrolysis and acidogenesis activities at weak alkaline conditions. A long-term (78 days) fermentation and filtration was successfully performed, where stable membrane filtration performance was experienced for about 50 days under high-solid (suspended solid of 37–45 g/L) and high flux (20 L/(m2·h)) conditions. Results suggest that AnMBR of CM is a feasible and promising process for VFA production and recovery.

Volatile fatty acids (VFAs) produced during anaerobic digestion (AD) of organic waste are a promising alternative carbon source for various biological processes; however, their applications are limited due to the presence of impurities such as ammonium (NH4+). This study investigates the potential for removal of ammonium using a naturally occurring zeolite (clinoptilolite) from chicken manure (CKM) derived VFA effluent recovered from an anaerobic membrane bioreactor (MBR). Experiments were conducted for both synthetic and actual VFA (AD-VFA) solutions, and the effects of different parameters were investigated with batch and continuous studies. It was observed that the Langmuir-type isotherm provided the best fit to the equilibrium data in the isotherm investigations carried out with the AD-VFA solution. The maximum adsorption capacity (qm) was found as 15.7 mg NH4+/g clinoptilolite. The effect of some operational parameters on process performance such as pH, initial NH4+ loading and potassium ion (K+) concentration was investigated. The pH had a negligible effect on ammonium removal for a pH range of 3–7, while the removal efficiency of ammonium decreased with the increase of initial NH4+ loading and K+ concentration. At the optimum conditions determined in batch experiments, the ammonium removal from synthetic and AD-VFA solutions were compared and average ammonium removal efficiencies of 93 and 94% were found in 12 h equilibrium time for synthetic and AD-VFA solutions, respectively. Overall findings indicated that clinoptilolite has excellent potential for ion exchange when combined with biological processes such as acidogenic fermentation of VFAs to purify the solution from high-ammonium content.