Guangxue Wu

Effects of thermo-chemical pre-treatment of grass silage on methane production by anaerobic digestion.

Dried grass silage (GS) was pre-treated at different NaOH loading rates (1%, 2.5%, 5% and 7.5% by volatile solids (VS) mass in grass silage) and temperatures (20 °C, 60 °C, 100 °C and 150 °C) to determine effects on its bio-degradability in terms of the hydrolysis yield and degradation of ligno-cellulosic materials for biogas production. At 100 °C and the four NaOH loadings, up to 45% of the total COD was solubilised and up to 65.6%, 36.1% and 21.2% of lignin, hemicellulose and cellulose were removed, respectively; biological methane production potentials obtained were 359.5, 401.8, 449.5 and 452.5 ml CH₄/g VS added, respectively, being improved by 10-38.9% in comparison with untreated GS. VS removals following anaerobic digestion were 67.6%, 76.9%, 85.3%, 95.2% and 96.7% for untreated GS and GS treated at the four NaOH loadings, respectively. 100 °C and the NaOH loading rate of 5% is recommended as a proper GS pre-treatment condition.

Methane production from anaerobic co-digestion of the separated solid fraction of pig manure with dried grass silage

Anaerobic co-digestion of the solid fraction of separated pig manure (SPM) with dried grass silage (DGS) was evaluated in three identical continuously stirred tank reactors (CSTRs) at 35±1 °C. The feedstock contained 20% DGS in CSTR1, 30% DGS in CSTR2 and 40% DGS in CSTR3 on a volatile solids (VS) basis. Organic loading rates (OLR) of 1.0, 1.5, 2.0 and 3.0 kg VS/m(3)/d were studied and it was found that the OLR affected the digester performance more than the DGS proportion in the feedstock. Tripling the OLR increased volumetric methane yields by 88% and decreased specific methane yields by 38%. At the OLR of 3 kg VS/m(3)/d, post-methane production potentials of digestates ranged from 38% to 41% of total methane production potentials of the feedstock. An energy yield estimation on a 654-sow pig unit showed that 268-371 MWh/a electricity and 383-530 MWh/a heat would be generated.

Production of polyhydroxybutyrate by activated sludge performing enhanced biological phosphorus removal

In this study, polyhydroxybutyrate (PHB)--a biodegradable plastics material--was produced by activated sludge performing enhanced biological phosphorus removal (EBPR) in batch experiments under anaerobic, aerobic and anaerobic/aerobic conditions. Under anaerobic conditions, the maximum PHB content of the dry biomass was 28.8% by weight, while under aerobic or anaerobic/aerobic conditions, the maximum PHB content was about 50%. The PHB production rate with respect to the volatile suspended solids (VSS) was: (i) 70 mg/(g VSS)h under aerobic conditions that followed anaerobic conditions, (ii) 156 mg/(g VSS)h under anaerobic condition, and (iii) 200mg/(g VSS)h under aerobic conditions with energy also supplied from polyphosphate. A side stream, with initially anaerobic conditions for PHB accumulation and phosphorus release, and then aerobic conditions for PHB accumulation, was proposed. In this side stream, biomass with a high PHB content and a high PHB production rate could be both achieved.

Effect of the solid content on anaerobic digestion of meat and bone meal

The effect of the solid content on anaerobic digestion of meat and bone meal (MBM) was investigated in batch reactors at MBM solid contents of 1%, 2%, 5% and 10%. There was no significant difference in the specific methane (CH(4)) production potential with respect to the total volatile MBM solids (TVS) applied at these solid contents, which ranged from 351 to 381 ml CH(4)/g TVS. However, the highest CH(4) yield with respect to the removed volatile MBM solids (RVS) was 482 ml CH(4)/g RVS at the MBM solid content of 5%; the CH(4) yields were 384-448 ml CH(4)/g RVS at the other MBM solid contents. The lag time of CH(4) production rose with the increase in the solid content. The longer lag time at MBM solid contents of 5% and 10% was due to inhibition caused by high concentrations of volatile fatty acids (VFAs) and free ammonia in the reactors, but the inhibition was reversible. The production of VFAs during the digestion varied with solid contents: at the solid content of 1%, only acetic acid was detected; at 2%, both acetic and propionic acids were detected; and at 5% and 10%, acetic, propionic, butyric and valeric acids were detected. After 93-day digestion, the volatile MBM solid reduction was 92%, 91%, 79% and 80% at MBM solid contents of 1%, 2%, 5% and 10%, respectively.

Effect of salinity on the activity, settling and microbial community of activated sludge in sequencing batch reactors treating synthetic saline wastewater

The effects of salinity on the activity in nutrient removal, settling and microbial community of activated sludge in sequencing batch reactors (SBRs) treating synthetic saline wastewater were investigated. Two SBRs, one treating synthetic saline wastewater (the N-Reactor, with NaCl addition) and the other treating fresh synthetic wastewater (the C-Reactor, without NaCl addition), were operated for 68 days. Three salinities (in terms of concentrations of NaCl)--10, 20 and 40 g NaCl/l--were examined. The microbial activity described with the specific glucose utilization rate, specific nitritation and nitratation rates, and specific phosphorus release and uptake rates, was inhibited in the N-Reactor, in comparison with that in the C-Reactor, except that the specific nitritation and nitratation rates were improved at the salinity of 10 g NaCl/l. The sludge yield coefficient decreased at salinities of 10 and 20 g NaCl/l but it rose at the salinity of 40 g NaCl/l. The settling of activated sludge flocs, in terms of the sludge volume index (SVI), was improved by adding NaCl. Particularly in the first 5 minutes during the SVI measurement, activated sludge flocs in the N-Reactor settled much faster than those in the C-Reactor. However, the effluent from the N-Reactor contained higher suspended solids than the effluent from the C-Reactor. The microbial diversity decreased with increasing the salinity, and the microbial community structure was greatly influenced by the salinity. Bacteriodetes and Actinobacteria were the dominant phylums detected with molecular fingerprinting techniques.

Hydrolysis and acidification of grass silage in leaching bed reactors

Hydrolysis and acidification of grass silage (GS) was examined in leaching bed reactors (LBRs) under organic loading rates (OLRs) of 0.5, 0.8 and 1.0 kg volatile solids (VS)/m(3)/day. The LBRs were run in duplicate over five consecutive batch tests (Batch tests 1-5) to examine the effects of pH, leachate dilution and addition of inoculum on the process of hydrolysis and acidification. The highest GS hydrolysis yields of 52-58%, acidification yields of 57-60% and VS removals of 62-66% were obtained in Batch test 4. Increasing OLRs affected the hydrolysis yield negatively. In Batch test 4, the reduction of lignocellulosic materials was up to 74.4% of hemicellulose, 30.1% of cellulose and 9.3% of lignin within 32 days. Cellulase activity can be used as an indicator for the hydrolysis process. Methane production from the LBRs only accounted for 10.0-13.8% of the biological methane potential of GS.

Effect of bacterial communities on the formation of cast iron corrosion tubercles in reclaimed water.

To understand the role bacterial communities play in corrosion scale development, the morphological and physicochemical characteristics of corrosion scales in raw and disinfected reclaimed water were systematically investigated. Corrosion tubercles were found in raw reclaimed water while thin corrosion layers formed in disinfected reclaimed water. The corrosion tubercles, composed mainly of α-FeOOH, γ-FeOOH, and CaCO3, consisted of an top surface; a shell containing more magnetite than other layers; a core in association with stalks produced by bacteria; and a corroded layer. The thin corrosion layers also had layered structures. These had a smooth top, a dense middle, and a corroded layer. They mostly consisted of the same main components as the tubercles in raw reclaimed water, but with different proportions. The profiles of the dissolved oxygen (DO) concentration, redox potential, and pH in the tubercles were different to those in the corrosion layers, which demonstrated that these parameters changed with a shift in the microbial processes in the tubercles. The bacterial communities in the tubercles were found to be dominated by Proteobacteria (56.7%), Bacteroidetes (10.0%), and Nitrospira (6.9%). The abundance of sequences affiliated to iron-reducing bacteria (IRB, mainly Geothrix) and iron-oxidizing bacteria (mainly Aquabacterium) was relatively high. The layered characteristics of the corrosion layers was due to the blocking of DO transfer by the development of the scales themselves. Bacterial communities could at least promote the layering process and formation of corrosion tubercles. Possible mechanisms might include: (1) bacterial communities mediated the pH and redox potential in the tubercles (which helped to form shell-like and core layers), (2) the metabolism of IRB and magnetic bacteria (Magnetospirillum) might contribute to the presence of Fe3O4 in the shell-like layer, while IRB contributed to green rust in the core layer, and (3) the diversity of the bacterial community resulted in the complex composition of the core layer, and gas producing bacteria (sulfate-reducing bacteria and methanogenic bacteria) played a role in the formation of the porous core layer.

Effect of extracellular polymeric substances on corrosion of cast iron in the reclaimed wastewater

Microorganisms were cultured in the R2A medium with inoculum from biofilm in a reclaimed wastewater distribution system and then extracellular polymeric substances (EPS) were extracted from the culture. Characterization of EPS and their effects on the corrosion of cast iron were examined. EPS extracted from different culturing stages contained different proportions of protein and polysaccharide but with similar functional groups. All types of EPS could inhibit cast iron corrosion and the EPS from the stationary stage had the highest inhibition efficiency. The inhibition efficiency was increased with addition of a small amount of EPS while decreased with excessive amount of EPS. EPS formed a protective film on the metal surface, which retarded the cathodic reduction of oxygen. Excessive amount of EPS promoted anodic dissolution through EPS-Fe binding. The CO and C(O, N) in EPS could be the anodic electrochemical sites with possible products of C(C, H).

Distributions and activities of ammonia oxidizing bacteria and polyphosphate accumulating organisms in a pumped-flow biofilm reactor

The spatial distributions and activities of ammonia oxidizing bacteria (AOB) and polyphosphate accumulating organisms (PAOs) were investigated for a novel laboratory-scale sequencing batch pumped-flow biofilm reactor (PFBR) system that was operated for carbon, nitrogen and phosphorus removal. The PFBR comprised of two 16.5l tanks (Reactors 1 and 2), each with a biofilm module of 2m(2) surface area. To facilitate the growth of AOB and PAOs in the reactor biofilms, the influent wastewater was held in Reactor 1 under stagnant un-aerated conditions for 6 h after feeding, and was then pumped over and back between Reactors 1 and 2 for 12 h, creating aerobic conditions in the two reactors during this period; as a consequence, the biofilm in Reactor 2 was in an aerobic environment for almost all the 18.2 h operating cycle. A combination of micro-sensor measurements, molecular techniques, batch experiments and reactor studies were carried out to analyse the performance of the PFBR system. After 100 days operation at a filtered chemical oxygen demand (COD(f)) loading rate of 3.46 g/m(2) per day, the removal efficiencies were 95% COD(f), 87% TN(f) and 74% TP(f). While the PFBR microbial community structure and function were found to be highly diversified with substantial AOB and PAO populations, about 70% of the phosphorus release potential and almost 100% of the nitrification potential were located in Reactors 1 and 2, respectively. Co-enrichment of AOB and PAOs was realized in the Reactor 2 biofilm, where molecular analyses revealed unexpected microbial distributions at micro-scale, with population peaks of AOB in a 100-250 microm deep sub-surface zone and of PAOs in the 0-150 microm surface zone. The micro-distribution of AOB coincided with the position of the nitrification peak identified during micro-sensor analyses. The study demonstrates that enrichment of PAOs can be realized in a constant or near constant aerobic biofilm environment. Furthermore, the findings suggest that when successful co-enrichment of AOB and PAOs occur in biofilm environments, such as in the PFBR system, they do so at different zone depths in the biofilm.

Inhibitory effect of copper on enhanced biological phosphorus removal.

Copper inhibition of enhanced biological phosphorus removal (EBPR) was examined in batch experiments under anaerobic and aerobic conditions. Inhibition was represented by both acetate uptake and phosphorus release coefficients under anaerobic conditions, and by a phosphorus uptake coefficient under aerobic conditions. The results showed that copper inhibition of EBPR occurred mainly during aerobic phosphorus uptake and a first-order phosphorus uptake coefficient can be better used to describe the inhibition effect. For the aerobic phosphorus uptake using the EBPR activated sludge, (i) copper inhibition started at 0.07 mg/l, (ii) 50% and 100% inhibition occurred at 0.30 mg/l and 0.53 mg/l, respectively, and (iii) the inhibition constant was 0.48 mg/l.