Influence of trace elements on biogas production from mango processing waste in 1.5 m3 KVIC digesters

Cow manure stabilizes anaerobic digestion of cocoa waste

Anaerobic digestion of a mono-feedstock often causes low methane yields and process instability. An effective strategy to overcome these barriers is co-digestion with animal manure. The obtained process improvement is often attributed to buffer capacity, nutrients, vitamins and trace metals, and microorganisms present in manure, but it remains unknown which factor plays the key role in digester performance. Here, we investigated anaerobic digestion of cocoa waste in four different treatments: mono-digestion, addition of synthetic nutrients, co-digestion with sterile cow manure, and co-digestion with raw cow manure. Co-digestion with raw manure resulted in the highest methane yield of 181 ± 39 L kg^-1 VS (volatile solids), similar to the co-digestion with sterile manure, i.e., 162 ± 52 L kg^-1 VS. The supplementation of synthetic nutrients to the anaerobic digestion of cocoa waste only temporarily increased methane yield, indicating that this will tackle a lack of nutrients in the short term, but has a limited long-term contribution to the stabilization of the process. Hence, because of the inability of synthetic nutrients to stabilize the digestion process and the similarity between the digesters fed sterile and raw manure, both at the physico-chemical and microbial level, the key contribution of manure co-digestion with cocoa seems to be the provision of buffering capacity.

Multiple Effects of Different Nickel Concentrations on the Stability of Anaerobic Digestion of Molasses

Molasses is a highly thick by-product produced after sugarcane crystallization constitutes large amounts of biodegradable organics. These organic compounds can be converted to renewable products through anaerobic digestion. Nevertheless, its anaerobic digestion is limited due to its high chemical oxygen demand (COD) and ion concentration. The effects of nickel (Ni2+) on the stability of anaerobic digestion of molasses were established by studying the degradation of organic matter (COD removal rate), biogas yield, methane content in the biogas, pH, and alkalinity. The results showed that there were no significant effects on the stability of pH and alkalinity. Increased COD removal rate and higher methane content was observed by 2–3% in the digesters receiving 2 and 4 mg/L of Ni2+ in the first phase of the experiment. Ni2+ supplemented to reactors at concentration 2 mg/L enhanced biogas yield. Overall, it is suggested that the addition of Ni2+ has some effects on the enhancement of biogas yield and methane contents but has no obvious effects on the long-lasting stability of the molasses digestion.

Trace elements dosing and alkaline pretreatment in the anaerobic digestion of rice straw

The effect of trace elements (TEs) addition and NaOH pretreatment on the anaerobic digestion of rice straw was investigated in batch tests. Co, Ni and Se were added to the raw rice straw at different dosages. The NaOH pretreatment was applied to the rice straw both alone and in combination with the addition of TEs, in order to evaluate potential synergistic effects of the pretreatment and the TEs supplementation on the biogas production yields. The results obtained showed that the alkaline pretreatment was more effective than the TEs addition in increasing the cumulative biogas production, causing a 21.4% enhancement of the final biomethane yield, whereas the increase due to TEs dosing was not statistically significant. The analysis of volatile fatty acids (VFAs) confirmed that the NaOH pretreatment resulted in a higher production of VFAs, indicating an increased hydrolysis, while TEs addition did not cause significant changes in the VFA concentrations.

Enhanced biogas production from rice straw by selective micronutrients under solid state anaerobic digestion

Biomethanation of rice straw (RS) was studied in a batch mode at high total solid content (TSC) of 25% in outdoor pilot scale digesters. Performance was monitored for over six months by supplementing Nickel and Cobalt 15 and 10mgkg(-1) RS to each of mesophilic and thermophilic digesters for 35 and 21days retention time (RT), respectively. The average biogas production from mesophilic and thermophilic digesters were found varying 310 and 396Lkg(-1)TS, respectively. The corresponding figures for the control digesters were 225 and 270Lkg(-1)TS. Around 37 and 46% higher biogas production was recorded by supplementing the micronutrients in mesophilic and thermophilic digesters, respectively. Methane content in biogas was 57-59%. Matured compost had nitrogen, phosphorus and potassium contents of 1.0-1.2, 1.3-2.2, and 1.2-2.1%, respectively. The results demonstrated that the present process is faster, requires less than 85% water and produces green energy in addition to manure in less time compared to conventional process.

Modified batch anaerobic digestion assay for testing efficiencies of trace metal additives to enhance methane production of energy crops

Batch biochemical methane potential (BMP) assays to evaluate the methane yield of biogas substrates such as energy crops are usually carried out with undiluted inoculum. A BMP assay was performed on two energy crops (green cuttings and grass silage). Anaerobic digestion was performed both with and without supplementation of three commercial additives containing trace metals in liquid, solid or adsorbed form (on clay particles). In order to reveal positive effects of trace metal supplementation on the methane yield, besides undiluted inoculum, 3-fold and 10-fold dilutions of the inoculum were applied for substrate digestion. Diluted inoculum variants were supplemented with both mineral nutrients and pH-buffering substances to prevent a collapse of the digestion process. As expected, commercial additives had no effect on the digestion process performed with undiluted inoculum, while significant increases of methane production through trace element supplementation could be observed on the diluted variants. The effect of inoculum dilution may be twofold: (1) decrease in trace metal supplementation from the inoculum and (2) reduction in the initial number of bacterial cells. Bacteria require higher growth rates for substrate degradation and hence have higher trace element consumption. According to common knowledge of the biogas process, periods with volatile fatty acids accumulation and decreased pH may have occurred in the course ofanaerobic digestion. These effects may have led to inhibition, not only ofmethanogenes and acetogenes involved in the final phases of methane production, but also offibre-degrading bacterial strains involved in polymer hydrolysis. Further research is required to confirm this hypothesis.

Long-term anaerobic digestion of food waste stabilized by trace elements

The purpose of this study was to examine if long-term anaerobic digestion of food waste in a semi-continuous single-stage reactor could be stabilized by supplementing trace elements. Contrary to the failure of anaerobic digestion of food waste alone, stable anaerobic digestion of food waste was achieved for 368 days by supplementing trace elements. Under the conditions of OLR (organic loading rates) of 2.19-6.64 g VS (volatile solid)/L day and 20-30 days of HRT (hydraulic retention time), a high methane yield (352-450 mL CH(4)/g VS(added)) was obtained, and no significant accumulation of volatile fatty acids was observed. The subsequent investigation on effects of individual trace elements (Co, Fe, Mo and Ni) showed that iron was essential for maintaining stable methane production. These results proved that the food waste used in this study was deficient in trace elements.

Anaerobic digestion of Jatropha curcas L. press cake and effects of an iron-additive

Oil production from Jatropha curcas L. seeds generates large amounts of Jatropha press cake (JPC) which can be utilized as a substrate for biogas production. The objective of this work was to investigate anaerobic mono-digestion of JPC and the effects of an iron additive (IA) on gas quality and process stability during the increase of the organic loading rate (OLR). With the increase of the OLR from 1.3 to 3.2 g(VS) L(-1) day(-1), the biogas yield in the reference reactor (RR) without IA decreased from 512 to 194 L(N) kg(VS) (-1) and the CH₄ concentration decreased from 69.3 to 44.4%. In the iron additive reactor (IAR), the biogas yield decreased from 530 to 462 L(N) kg(VS) (-1) and the CH₄ concentration decreased from 69.4 to 61.1%. The H₂S concentration in the biogas was reduced by addition of the IA to values below 258 ppm in the IAR while H₂S concentration in the RR increased and exceeded the detection limit of 5000 ppm. The acid capacity (AC) in the RR increased to more than 20 g L(-1), indicating an accumulation of organic acids caused by process instability. AC values in the IAR remained stable at values below 5 g L(-1). The results demonstrate that JPC can be used as sole substrate for anaerobic digestion up to an OLR of 2.4 g(VS) l(-1) day(-1). The addition of IA has effectively decreased the H(2)S content in the biogas and has improved the stability of the anaerobic process and the biogas quality.