xiii.Organic loading rate (OLR)

The OLR is the rate at which organic matter is added to the digester volume in relation to either time or the substrate's biological conversion capacity. The organic loading rate is directly promotional to the quantity of volatile solids loaded in the digester and hence it influences the biogas yield, with more methane production for lesser the OLR. If the reactor has higher concentration VFA's, then it implies that it is overloaded [101]. The loading rate of a digester is the daily volume of volatile solids given to the digester, and methane production are favorable with high loading rates [33].

xiv.Biological oxygen demand (BOD) and chemical oxygen demand (COD):

*The BOD and the COD affect biogas production too. The biochemical* oxygen demand determines the amount of oxygen needed by the microbes to decompose the substrate. If the BOD is high, then a more rapid organic degradation is achieved which leads to more biogas production. The COD measures all organic and inorganic biodegradable matter in feedstock or substrate. The value of COD of an organic waste helps predict the theoretical methane yield from the substrate. From experiments, if 0.5 l/gm of COD is removed, the approximate methane production is about 0.35 l/g [101, 111].

#### **2.6 Energy requirements for biogas process**

The production of biogas necessitates the addition of heat to keep the substrate at an appropriate temperature and the constant stirring of the substrate, either by hand or with an electric motor-driven stirrer [99].

Therefore, process heat energy requirement is determined by the relation in Eq. (3)

$$Q\_t = M \ast C \ast \left(T\_2 - T\_1\right) \tag{3}$$

where

Qt = Overall heat required to heat the slurry in Kilojoule (KJ);

M = Mass of the slurry in (Kg);

C = The specific heat capacity of slurry and is expressed in KJ/Kg°C.;

T2 = Desired digester slurry temperature in °C;

T1 = Temperature of the digester charge/slurry in °C,

M = Digester size (V) in m3 × density of slurry (ρ) Kg/m3 ; Density of slurry (ρ) = (Density of water + substrate density dung)/2; Specific heat of slurry = (specific heat of water (4.2KJ/Kg°C) + specific heat of the substrate e.g., cow dung (2.8KJ/ Kg°C)/2 = 3.5 Kg/Kg°C.

The digestor effluent contains waste heat that can be recovered and used to produce the necessary heat energy. The slurry's input temperature may rise by a few degrees Celsius as a result. This results in a potential 50% reduction in the total heat required by thermophilic digesters [31, 112]. External heating is most effective for digesters of a large or medium size and requires approximately twice as energy intensive as central heating. A total of 850–1000 W/m<sup>2</sup> K−1 is the typical range for space heating, and 300–400 W/m<sup>2</sup> K−1 is the typical range for space cooling [112].

Parameters such as microbe species, feedstock pretreatment, and biogas purification processes, substrate composition, substrate properties, and ideal reactor conditions must be managed to produce biogas profitably. Further investigation is required and have been actively a high interest area toward the enhancement of biogas production and biogas utilization through the application of engineering, biology/biotechnology, and technical innovations in order to achieve cost-effective biogas production [10, 113].
