Advanced anaerobic digestion of household food waste pretreated by in situ-produced mixed enzymes via solid-state fermentation: Feasibility and application perspectives.

Enzymatic pretreatment is an effective method which can improve the anaerobic digestion (AD) efficiency of household food waste (HFW). As an alternative to expensive commercial enzymes, mixed enzymes (MEs) produced in situ from HFW by solid-state fermentation (SSF) can greatly promote the hydrolysis rate of HFW and achieve advanced anaerobic digestion (AAD) economically sustainable. In this paper, strategies for improving the efficiency of the enzyme-production process and the abundance of MEs are briefly discussed, including SSF, fungal co-cultivation, and stepwise fermentation. The feasibility of using HFW as an applicable substrate for producing MEs (amylase, protease, and lignocellulose-degrading enzymes) and its potential advantages in HFW anaerobic digestion are comprehensively illustrated. Based on the findings, an integrated AAD process of HFW pretreated with MEs produced in situ was proposed to maximise bioenergy recovery. The mass balance results showed that the total volatile solids removal rate could reach 98.56%. Moreover, the net energy output could reach 2168.62 MJ/t HFW, which is 9.79% higher than that without in situ-produced MEs and pretreatment. Finally, perspectives for further study are presented.

Increasing the organic loading rate of household food waste anaerobic digestion by landfill leachate addition: Performance and mechanism.

A high amount of easily degradable organics and the absence of trace metals (TMs) in household food waste (HFW) lowered the stability and efficiency of anaerobic digestion (AD) of HFW. Leachate addition to the AD of HFW can provide ammonia nitrogen and TMs to address the accumulation of volatile fatty acids and the lack of TMs. To study the effect of leachate addition on increasing organic loading rate (OLR), both mono-digestion of HFW and AD of HFW with leachate addition were evaluated using two continuously stirred tank reactors. The OLR of the mono-digestion reactor only reached 2.5 g COD/L/d. However, with the addition of ammonia nitrogen and TMs, the OLR of the failed mono-digestion reactor increased by 2 and 3.5 g COD/L/d, respectively. The specific methanogenic activity increased by 94.4% and the hydrolysis efficiency increased by 135%. Finally, the OLR of mono-digestion of HFW reached 8 g COD/L/d, with a hydraulic retention time (HRT) of 8 days and methane production rate of 2.4 L/L/d. In the leachate addition reactor, the OLR reached 15 g COD/L/d, while the HRT and methane production were 7 days and 3.4 L/L/d, respectively. This study demonstrates that leachate addition substantially improves the AD efficiency of HFW. The two main mechanisms of increasing the OLR of an AD reactor are the buffer capacity of ammonia nitrogen and the stimulation of methanogen by TMs from leachate.

High-efficiency anaerobic co-digestion of food waste and mature leachate using expanded granular sludge blanket reactor.

Anaerobic digestion of food waste receives more and more attention for waste-to-energy conversion, while easy acidification and limited efficiency hinder its wide application. To improve anaerobic digestion of food waste, its anaerobic co-digestion with mature leachate was performed using an expanded granular sludge blanket reactor. With the chemical oxidation demand (COD) removal of around 80%, the methane production and organic loading rate of the reactor reached 5.87 ± 0.45 L/L/d and 23.6 g COD/L/d, respectively. The rate of COD converted to methane was ranging from 74% to 87%. The addition of mature leachate provided ammonium to avoid acidification and trace metals for microbial growth, and the efficiencies of four stages of anaerobic digestion were all enhanced. The predominant methanogenic genera were shifted to adapt the changing condition, thus stabilizing the system. These findings support high-efficiency bioenergy recovery from food waste and leachate in practice.

Differential Effect of Anaerobic Digestion on Gaseous Products from Sequential Pyrolysis of Three Organic Solid Wastes.

Studies have shown that anaerobic digestion (AD) has an effect on the liquid and solid product property of sequential pyrolysis, but its influence on the gaseous products is lacking. In this study, syngas produced by pyrolysis from three raw organic solid wastes and the corresponding digestates, i.e., food waste, vinasse, and cow manure were investigated. AD causes a decrease in the contents of volatile solid, fixed carbon, C, H, and N and an increase in the S content. The weight loss of the wastes mainly occurs at 200-550 °C during the pyrolysis and the loss of the food waste and vinasse is higher than that of cow manure. In the carbon (C)-containing gas, AD leads to a decrease in the CH4 content of the syngas, implying that the heat values of the digestates are lower than that of the raw substrates. After AD, the total amount of nitrogen (N)-containing gas from the vinasse increases by 40.1%, while that from cow manure decreases by 14.1%. On the contrary, the total amount of sulfur (S)-containing groups in the syngas from vinasse drop by 22.0%, while that from cow manure increases by 9.1%. In addition, slight changes in the C-, N-, and S-containing gases are found from food waste. The results indicate that AD has a different effect on the N- and S- containing gaseous groups from different organic solid wastes, and the mechanisms deserve further investigation. The findings supply a theoretical foundation for environmental-friendly application of syngas from the digestates.

Using an expended granular sludge bed reactor for advanced anaerobic digestion of food waste pretreated with enzyme: The feasibility and its performance.

The high content of solid organics in food waste (FW) results in a low and unstable anaerobic digestion (AD) efficiency. Improving methane production rate and process stability is attracting much attention towards advanced AD of FW. The feasibility of advanced AD of FW pretreated with enzyme was investigated by batch experiments and 164 days running of an expanded granular sludge bed (EGSB) reactor. Simulation study based on the results of batch experiments indicates it is possible to treat enzymatically pretreated FW using an EGSB reactor. During the running of an EGSB reactor, the organic loading rate went up to 20 g chemical oxygen demand (COD)/L.d, and the total COD removal rate reached 88%. The significance of this study is to achieve an advanced AD of enzymatically pretreated FW with a stable and efficient methane production with biogas residue being reduced greatly.

Carbon source and phosphorus recovery from iron-enhanced primary sludge via anaerobic fermentation and sulfate reduction: Performance and future application.

Anaerobic fermentation and sulfate reduction (AF-SR) was firstly used for recovery of carbon sources and phosphorus from Fe-enhanced primary sludge (Fe-sludge). With FeCl3 dosage of 30 mg Fe/L, 63.0% of the chemical oxygen demand (COD) and 97.3% of the phosphorus were concentrated from sewage into Fe-sludge. Batch anaerobic fermentation tests of Fe-sludge with and without sulfate addition (AF-SR and control) were performed. The results showed that volatile fatty acid concentrations of the control and AF-SR were 211.0 and 270.2 mg COD/g volatile suspended solids, respectively. Furthermore, 33.2% (control) and 56.2% (AF-SR) of the total phosphorus in Fe-sludge was released. The recovery performances of carbon source and phosphorus were calculated based on struvite precipitation. The available carbon source of the AF-SR system was 44.5% higher than that of the control. A novel integrated wastewater and sludge treatment process based on chemically enhanced primary sedimentation and AF-SR is proposed for future application.

Simultaneous release of polyphosphate and iron-phosphate from waste activated sludge by anaerobic fermentation combined with sulfate reduction.

Iron is widely used in sewage treatment systems and enriched into waste activated sludge (WAS), which is difficult and challenging to phosphorus (P) release and recovery. This study investigated simultaneous release performance of polyphosphate and iron-phosphate from iron-rich sludge via anaerobic fermentation combined with sulfate reduction (AF-SR) system. Batch tests were performed, with results showing that AF-SR system conducted a positive effect due to the relatively low solubility of ferrous sulfide in comparison with ferric phosphate precipitates. Simulation study was performed to investigate the total P release potential from actual waste activated sludge, finding that about 70% of the total P could release with the optimized pH of 7.0-8.0 and the theoretical S2-/Fe2+ molar ratio of 1.0. A potential new blueprint of a wastewater treatment plant based on AF-SR system, towards P, N recovery and Fe, S, C recycle, was finally proposed.

Pretreating anaerobic fermentation liquid with calcium addition to improve short chain fatty acids extraction via in situ synthesis of layered double hydroxides.

In situ synthesis of layered double hydroxides (LDHs) was proved to be an effective way to extract short chain fatty acids (SCFAs) from anaerobic fermentation liquid (AFL) as carbon source for biodenitrification, but the SCFAs content in SCFAs-LDH was unsatisfactory because of the existence of much carbonate in AFL. Pretreatment of AFL with calcium addition was investigated to remove carbonate and improve SCFAs extraction via LDHs synthesis. Results of batch tests showed that, the carbonate removal efficiency was as high as 76.6% when the calcium addition was 0.06 mol/L at pH 12. When using the optimal SCFAs/Al3+ ratio of 3.0, the total SCFAs content in SCFAs-LDH with pretreatment was improved to 46.5 mg COD/g LDH, which was 4.5 times of the control (10.4 mg COD/g LDH). These results suggest that adding calcium to AFL was an effective way to eliminate the negative effect of carbonates on SCFAs-LDH synthesis.

Nitrate removal from groundwater using negatively charged nanofiltration membrane.

A commercial nanofiltration (NF) membrane was modified using poly(sodium 4-styrenesulfonate) (PSS) to improve the nitrate rejection from groundwater. Fourier transform infrared spectroscopy, thermogravimetric analysis, zeta potential, and water contact angle analyses were performed, showing that PSS was successfully coated onto the membrane with the surface negative charge density being enhanced. The results of nitrate removal tests showed that the best PSS concentration was 1.5 mg/L, with the nitrate rejection rate of 88.8% and the permeate flux of 27.0 L/m2 h. The effect of initial nitrate concentration and solution pH on the nitrate removal performance of the modified NF membrane was investigated. The results indicate that the modified NF membrane can improve nitrate removal from actual groundwater, with little membrane permeate flux loss. Graphical abstract.

Pretreatment of food waste with high voltage pulse discharge towards methane production enhancement.

Anaerobic batch tests were performed to investigate the methane production enhancement and solid transformation rates from food waste (FW) by high voltage pulse discharge (HVPD) pretreatment. The total cumulative methane production with HVPD pretreatment was 134% higher than that of the control. The final volatile solids transformation rates of FW with and without HVPD pretreatment were 54.3% and 32.3%, respectively. Comparison study on HVPD pretreatment with acid, alkali and ultrasonic pretreatments showed that the methane production and COD removal rates of FW pretreated with HVPD were more than 100% higher than the control, but only about 50% higher can be obtained with other pretreatments. HVPD pretreatment could be a promising pretreatment method in the application of energy recovery from FW.