For over a century, anaerobic digestion has become a key technology in removing organic contents from waste streams, while at the same time enabling the recovery of energy and resources (Guo et al. 2017; Martins et al. 2018). In both industrial and scientific communities in this field, great efforts have been devoted to technical innovation, resulting in the development of many cutting-edge and emerging strategies for the management of waste streams, nutrients circulation, biogas production and removal of hazardous and persistent micro-pollutants, and insights into digestion microbiome (Arora et al. 2018; Gonzalez et al. 2018; Hassa et al. 2018). Therefore, the aim of this special issue is to bring together papers from multiple disciplines on the state-of-the-art emerging/innovative anaerobic processes and associated principles. This issue contains 11 selected research articles, covers research frontiers from the development, optimization and application of novel processes, as well as the functional microbiome in anaerobic digesters.

Engineering bioreactor/bioprocess design basing on advantages of established design science theory and knowledges on the targeting anaerobic microbiome could fundamentally and significantly improve digestion performance (Arora et al. 2018). In this issue, two novel bioreactors/processes were developed and applied for the wastewater treatment and resource recovery, respectively, i.e., an anaerobic filter membrane bioreactor (Diez et al. 2018) and anaerobic-based water resource recovery facility (WRRF) (Seco et al. 2018), providing interesting insights into the impacts of reactor configuration and operation condition on the digestion performance. For specific applications of bioreactors/processes, many puzzles await to be disentangled, including the effects of seeding microbiome (Kocamemi et al. 2018; Zhao et al. 2018), volatile fatty acids (VFAs) (Vargas-Morales et al. 2018), redox conditions (Pokorná-Krayzelová et al. 2018), temperature (Awad et al. 2018), organic loading rates and digestion substrate pre-treatments (Diamantis & Aivasidis 2018), which have been comprehensively tested and discussed in articles in this issue. These results genuinely provide invaluable to anyone involved in anaerobic digestion.

We would like to thank the organizing committee of the 15th IWA World Conference on Anaerobic Digestion (AD-15) for providing an excellent platform gathering international researchers, practitioners, designers, public officials and industry representatives and decision makers in the field of anaerobic digestion. This special issue is evidence of their great efforts and the great success of the conference.

Awad
M.
,
Tian
Z.
,
Gao
Y.
,
Yang
M.
&
Zhang
Y.
2018
Pretreatment of spiramycin fermentation residue using hyperthermophilic digestion: quick startup and performance
.
Water Science and Technology
78
(
9
),
1823
1832
.
Diez
V.
,
Iglesias
A.
,
Cámara
J. M.
,
Ruiz
M. O.
&
Ramos
C.
2018
A novel anaerobic filter membrane bioreactor: prototype start-up and filtration assays
.
Water Science and Technology
78
(
9
),
1833
1842
.
Gonzalez
A.
,
Hendriks
A. T. W. M.
,
van Lier
J. B.
&
de Kreuk
M.
2018
Pre-treatments to enhance the biodegradability of waste activated sludge: elucidating the rate limiting step
.
Biotechnol. Adv.
36
(
5
),
1434
1469
.
Hassa
J.
,
Maus
I.
,
Off
S.
,
Pühler
A.
,
Scherer
P.
,
Klocke
M.
&
Schlüter
A.
2018
Metagenome, metatranscriptome, and metaproteome approaches unraveled compositions and functional relationships of microbial communities residing in biogas plants
.
Appl. Microbiol. Biotechnol.
102
(
12
),
5045
5063
.
Kocamemi
B. A.
,
Dityapak
D.
,
Semerci
N.
,
Keklik
E.
,
Akarsubası
A.
,
Kumru
M.
&
Kurt
H.
2018
Anammox start-up strategies: the use of local mixed activated sludge seed versus Anammox seed
.
Water Science and Technology
78
(
9
),
1901
1915
.
Martins
G.
,
Salvador
A. F.
,
Pereira
L.
&
Alves
M. M.
2018
Methane production and conductive materials: a critical review
.
Environ. Sci. Technol.
52
(
18
),
10241
10253
.
Pokorna-Krayzelova
L.
,
Vejmelková
D.
,
Selan
L.
,
Jenicek
P.
,
Volcke
E. I. P.
&
Bartacek
J.
2018
Final products and kinetics of biochemical and chemical sulfide oxidation under microaerobic conditions
.
Water Science and Technology
78
(
9
),
1916
1924
.
Seco
A.
,
Aparicio
S.
,
González-Camejo
J.
,
Jiménez-Benítez
A.
,
Mateo
O.
,
Mora
J. F.
,
Noriega-Hevia
G.
,
Sanchis-Perucho
P.
,
Serna-García
R.
,
Zamorano-López
N.
,
Giménez
J. B.
,
Ruiz-Martínez
A.
,
Aguado
D.
,
Barat
R.
,
Borrás
L.
,
Bouzas
A.
,
Martí
N.
,
Pachés
M.
,
Ribes
J.
,
Robles
A.
,
Ruano
M. V.
,
Serralta
J.
&
Ferrer
J.
2018
Resource recovery from sulphate-rich sewage through an innovative anaerobic-based water resource recovery facility (WRRF)
.
Water Science and Technology
78
(
9
),
1925
1936
.
Vargas-Morales
G.
,
Chamy
R.
&
García-Gen
S.
2018
A VFA-based controller for anaerobic digestion of industrial winery wastewater
.
Water Science and Technology
78
(
9
),
1871
1878
.