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2.4 Factors affecting the stability of the biogas production by dark fermentation

Apparently, there are several factors affecting the stability of the biogas production by
dark fermentation are mainly comprehensive the operating conditions such as pH, temperature,
buffer capacity, and mixing intensity, and feedstock including waste composition, waste
concentration, toxic and inhibitory compounds; all of these factors are influence directly on the
microorganisms are discussed below (Boe, 2006).

2.4.1 Substrate

Various types of substrate that can be used to produce biogas pass through dark
fermentation are comprehensive pure sugars, such as sucrose, glucose, and lactose, and organic
wastes from food factories such as apple processing wastewater, potato processing wastewater,
starch wastewater, food waste, and palm oil mill effluent. However, the uses of pure sugars are
only for trying to understand the microbial physiology of hydrogen production, it is not provided
for hydrogen production in the industrial scale since it is too expensive. The selection of organic
substrates to be used in dark fermentation for hydrogen production must consider including cost,
availability, carbohydrate content, and also biodegradability (Kongjan, 2010). Moreover, the
concentration of substrate also must consider because several researches reported about at high
concentration of substrate could cause the substrate inhibition which could potentially inhibit or
overload the process and lead to decrease in biodegradability.

2.4.2 Nutrients

The sufficient amounts of macro- and micro-nutrients are playing a vital role on
microbial cell growth. Carbon, hydrogen, oxygen, and nitrogen are the major components of
macro-nutrients in biomass cells. Moreover, sulphur, phosphorus, potassium, calcium,
magnesium, and iron are required for specific proteins. Meanwhile, the micro-nutrients including
nickel, cobalt, copper, manganese, molybdenum, zinc, selenium are also required in smaller
amount. However, the present of these macro- and micro-nutrients in high concentration could
be inhibitory.

Skim latex serum (SLS) is deficient in the macronutrients in carbon, while palm oil mill
effluent (POME) is also deficient in nitrogen which is crucial for microbial growing. Thus, in