14

1 Anaerobic Co-digestion as a Smart Approach for Enhanced Biogas Production

Digesters with constant mixing can take up higher OLR, and it has been reported

that OLR increased up to 300 kg COD/m³/d using super high rate anaerobic

bioreactor (SAB) that works on a principle of spiraling baffle running through the

middle of the digester body [15].

Mixing helps in uniform distribution of feedstocks during AcD and provides access

of metabolic intermediates, microbial interaction; prevents stratification and release

of trapped methane that has been observed with completely stirred/mixed tank reac-

tors (CSTRs) [4]. Mixing of digester content can occur naturally to some extent by

rise of methane bubbles, which is by itself not sufficient for optimum biogas pro-

duction, hence auxiliary mixing is essential. It has been reported that intermittent

mixing leads to better biogas production in comparison to continuous mixing [4].

As we know that four groups of microbes are responsible for biogas production, an

attempt has been made to build two-stage digesters basically dividing microbial role

of hydrolysis/acidogenesis and acetogenesis/methanogenesis [18]. The first-stage

hydrogenic reactor (HR) and the second-stage methanogenic reactor (MR) are linked

but operated at different pH [19] and only recommended for digesting sugar-rich

feedstocks [20]

1.4

Digestate/Spent Slurry

The effectiveness of AcD can be evaluated based on the quality of the digestate/spent

slurry of the digester. The composition of the digestate will naturally differ from ini-

tial feedstock, there should have been a drastic reduction in total solids content and

COD. With richness in nitrogen and potassium and low on carbon content, the diges-

tate can be an excellent source for organic manure for crop production, could support

by minimizing usage of chemical fertilizers, and bedding can prevent soil erosion

and help to retain soil fertility [21]. There have been few concerns on long-term

impact on usage of manure as fertilizer:

●chances of altering preexisting and natural soil microflora,

●impact of excessive nitrogen emissions from manure applied farm lands,

●presence of recalcitrant compounds, and

●slow degrading remnant organic matter contributed by manure.

There has been considerable research over the aforesaid drawback, and we have

conclusive results with reports stating minimal or of minor relevance with no major

impaction on overall soil fertility [22]. Manure can be packed and stored over of

period of few months without much loss in nitrogen content and has been evaluated

for storage during different seasons for their efficacy [23]. The grade of the manure

would vary and generally rely on the nature of feedstocks digested, for example, AD

of agricultural feedstocks may yield manure with less nitrogen content while live-

stocks waste or dairy waste manure may be nitrogen-rich, especially liquid compost;

accordingly soil management plan is essential to determine the quality and quantity

of manure and its influence on appropriate soil type before any large-scale applica-

tion of manure over farm land [24].