Anaerobic filter is very popular among biofilm reactors and
HRT is effective on the performance when treating low-strength
wastewater while OLR is limiting parameter when AF is fed by
high-strength wastewater [67]. AF can be operated by single-fed
as up-flow, down-flow [24,57] or horizontal direction [67] and by
multi-fed. Up-flow AF has been widely used for the treatment of
whey and it can be operated efficiently for the treatment of low
and high strength dairy wastewater at short HRT and high OLR [14].
Reported operational conditions and treatment performances of
anaerobic filters for the treatment of dairy wastewater are summarized
in Table 2. Rajinikanth et al. [67] concluded that low-density
polyethylene was very effective packing media for the growth of
high quantity biomass in up-flow AF. Biomass concentration within
AF was 21 g L−1 while solid wash-out and production of excess
quantity of viscous exopolymeric matters were observed due to
the rapid acidifying of wastewater at higher OLR. For the treatment
of dairy wastewater with 30 g COD L−1, maximum COD removal
efficiency was 80% at OLR of 17 kg COD m−3 d−1. Researchers recommended
intermittent recirculation in order to improve the
liquid mixing inside the AF and prevent the clogging. They also
reported that a post-treatment stage after AF is necessary to comply
with the discharge limits. Similarly, Rajinikanth et al. [69] used
low-density polyethylene as packing media in an up-flow AF for the
treatment of whey with 30 g COD L−1 at 33 ◦C. They obtained 80%