References
  -  L. Lu, D. Xing, B. Liu, N. Ren, Enhanced hydrogen production
    from waste activated sludge by cascade utilization of organic
    matter in microbial electrolysis cells, Water Res., 46 (2012)
    1015–1026. 
-  L. Lu, D. Xing, N. Ren, Pyrosequencing reveals highly diverse
    microbial communities in microbial electrolysis cells involved
    in enhanced H2 production from waste activated sludge, Water
    Res., 46 (2012) 2425–2434. 
-  D. Wang, Y. Duan, Q. Yang, Y. Liu, B. Ni, Q. Wang, G. Zeng,
    X. Li, Z. Yuan, Free ammonia enhances dark fermentative
    hydrogen production from waste activated sludge, Water Res.,
    133 (2018) 272–281. 
-  R. Sun, D. Xing, J. Jia, A. Zhou, L. Zhang, N. Ren, Methane
    production and microbial community structure for alkaline
    pretreated waste activated sludge, Bioresour. Technol., 169 (2014)
    496–501. 
-  A.K. Wahidunnabi, C. Eskicioglu, High pressure homogenization
    and two-phased anaerobic digestion for enhanced
    biogas conversion from municipal waste sludge, Water Res.,
    66 (2014) 430–446. 
-  Y. Wang, J. Zhao, D. Wang, Y. Liu, Q. Wang, B. Ni, F. Chen,
    Q. Yang, X. Li, G. Zeng, Z. Yuan, Free nitrous acid promotes
    hydrogen production from dark fermentation of waste activated
    sludge, Water Res., 145 (2018) 113–124. 
-  X. Dai, X. Li, D. Zhang, Y. Chen, L. Dai, Simultaneous
    enhancement of methane production and methane content
    in biogas from waste activated sludge and perennial ryegrass
    anaerobic co-digestion: the effects of pH and C/N ratio,
    Bioresour. Technol., 216 (2016) 323–330. 
-  X. Wang, G. Yang, Y. Feng, G. Ren, X. Han, Optimizing feeding
    composition and carbon-nitrogen ratios for improved methane
    yield during anaerobic co-digestion of dairy, chicken manure
    and wheat straw, Bioresour. Technol., 120 (2012) 78–83. 
-  S. Astals, M. Esteban-Gutierrez, T. Fernandez-Arevalo, E. Aymerich,
    J.L. Garcia-Heras, J. Mata-Ahiarez, Anaerobic digestion
    of seven different sewage sludges: a biodegradability and
    modelling study, Water Res., 47 (2013) 6033–6043. 
-  M. Pohl, K. Heeg, J. Mumme, Anaerobic digestion of wheat
    straw - performance of continuous solid-state digestion,
    Bioresour. Technol., 146 (2013) 408–415. 
-  E. Baker, J.M. Keisler, Cellulosic biofuels: expert views on
    prospects for advancement, Energy, 36 (2011) 595–605. 
-  Y. Xi, Z. Chang, X. Ye, R. Xu, J. Du, G. Chen, Methane
    production from wheat straw with anaerobic sludge by heme
    supplementation, Bioresour. Technol., 172 (2014) 91–96. 
-  M. Kim, C. Liu, J.W. Noh, Y. Yang, S. Oh, K. Shimizu, D.Y. Lee,
    Z. Zhang, Hydrogen and methane production from untreated
    rice straw and raw sewage sludge under thermophilic anaerobic
    conditions, Int. J. Hydrog. Energy, 38 (2013) 8648–8656. 
-  M. Kim, Y. Yang, M.S. Morikawa-Sakura, Q. Wang, M.V. Lee,
    D.Y. Lee, C. Feng, Y. Zhou, Z. Zhang, Hydrogen production by
    anaerobic co-digestion of rice straw and sewage sludge, Int. J.
    Hydrog. Energy, 37 (2012) 3142–3149. 
-  N. Alemahdi, H.C. Man, N. Abd Rahman, N. Nasirian, Y. Yang,
    Enhanced mesophilic bio-hydrogen production of raw rice
    straw and activated sewage sludge by co-digestion, Int. J.
    Hydrog. Energy, 40 (2015) 16033–16044. 
-  Z.N. Abudi, Z. Hu, N. Sun, B. Xiao, N. Rajaa, C. Liu, D. Guo,
    Batch anaerobic co-digestion of OFMSW (organic fraction of
    municipal solid waste), TWAS (thickened waste activated sludge)
    and RS (rice straw): influence of TWAS and RS pretreatment
    and mixing ratio, Energy, 107 (2016) 131–140. 
-  X. Dai, C. Hu, D. Zhang, L. Dai, N. Duan, Impact of a high
    ammonia-ammonium-pH system on methane-producing
    archaea and sulfate-reducing bacteria in mesophilic anaerobic
    digestion, Bioresour. Technol., 245 (2017) 598–605. 
-  G. Wang, X. Dai, D. Zhang, Q. He, B. Dong, N. Li, N. Ye, Twophase
    high solid anaerobic digestion with dewatered sludge:
    improved volatile solid degradation and specific methane generation
    by temperature and pH regulation, Bioresour. Technol.,
    259 (2018) 253–258. 
-  H.K. Goering, P.J. Van soest, Forage Fiber Analysis, in
    Agriculture Handbook, Agricultural Research Services, Department
    of Agriculture, USA, 1970. 
-  L. Wei, X. Li, J. Yi, Z. Yang, Q. Wang, W. Ma, A simple approach
    for the efficient production of hydrogen from Taihu Lake
    Microcystis spp. blooms, Bioresour. Technol., 139 (2013) 136–140. 
-  A.F. Yakunin, P.C. Hallenbeck, Purification and characterization
    of pyruvate oxidoreductase from the photosynthetic bacterium
    Rhodobacter capsulatus, Biochim. Biophys. Acta-Bioenerg., 1409
    (1998) 39–49. 
-  H.G. Wood, B. Jacobson, B.I. Gerwin, D.B. Northrop, Oxaloacetate
    transcarboxylase from Propionibacterium, Method
    Enzymol., 13 (1969) 215–230. 
-  Y. Miron, G. Zeeman, J.B. Van Lier, G. Lettinga, The role of
    sludge retention time in the hydrolysis and acidification of
    lipids, carbohydrates and proteins during digestion of primary
    sludge in CSTR systems, Water Res., 34 (2000) 1705–1713. 
-  T. Ghose, Measurement of cellulase activities, Pure Appl.
    Chem., 59 (1987) 257–268. 
-  X. Wang, Y. Zhang, J. Zhou, T. Zhang, M. Chen, Regeneration of
    elemental sulfur in a simultaneous sulfide and nitrate removal
    reactor under different dissolved oxygen conditions, Bioresour.
    Technol., 182 (2015) 75–81. 
-  M.M. Estevez, R. Linjordet, J. Morken, Effects of steam
    explosion and co-digestion in the methane production from
    Salix by mesophilic batch assays, Bioresour. Technol., 104 (2012)
    749–756. 
-  Yadvika, Santosh, T.R. Sreekrishnan, S. Kohli, V. Rana,
    Enhancement of biogas production from solid substrates using
    different techniques - a review, Bioresour. Technol., 95 (2004)
    1–10. 
-  A. Wilkie, E. Colleran, Pilot-scale digestion of pig slurry
    supernatant using an upflow anaerobic filter, Environ. Technol.
    Lett., 7 (1986) 65–76. 
-  F. Di Maria, M. Barratta, Boosting methane generation by
    co-digestion of sludge with fruit and vegetable waste: internal
    environment of digester and methanogenic pathway, Waste
    Manage., 43 (2015) 130–136. 
-  Y. Maspolim, Y. Zhou, C. Guo, K. Xiao, W.J. Ng, Determination
    of the archaeal and bacterial communities in two-phase
    and single-stage anaerobic systems by 454 pyrosequencing, J.
    Environ. Sci., 36 (2015) 121–129. 
-  T. Kindaichi, T. Ito, S. Okabe, Ecophysiological interaction
    between nitrifying bacteria and heterotrophic bacteria in
    autotrophic nitrifying biofilms as determined by microautoradiography-fluorescence in situ hybridization, Appl. Environ.
    Microbiol., 70 (2004) 1641–1650.