**1. Introduction**

Recently, energy has become a basic need for modern society. The need of using energy was increased due to population and consumption growth and because the community used various kinds of equipment in supporting convenience in life [1]. The current global energy supply is highly dependent on fossil sources (crude oil, lignite, hard coal, natural gas). These are fossilized remains of dead plants and animals, which have been exposed to heat and pressure in the Earth's crust over hundreds of millions of years. For this reason, fossil fuels are nonrenewable resources in which reserves are being depleted much faster than the new ones being formed [2].

Indonesia as a tropical country has abundant renewable energy sources as alternative energy to replace fossil energy. One alternative energy is biogas. Biogas is the final gas product of anaerobic digestion/degradation (in an environment without oxygen) by methanogenic bacteria [1]. Biogas is very potential as the latest energy source because its methane (CH4) content itself has a heating value of 50 MJ/kg. Methane (CH4) has one carbon in each chain, which can produce combustion more

environmentally friendly than that of the long carbon chain fuels. This matter is caused by the less amount of CO2 produced during short carbon chain fuel combustion [3]. One of the main advantages of biogas production is the ability to transform waste material into a valuable resource, by using it as a substrate for AD [2].

Anaerobic digestion (AD) has been extensively used to convert organic waste streams from various sources, such as agricultural, industrial, and municipal solid waste, to biogas. The AD process can operate in both liquid and solid states in terms of total solid (TS) content. In general, the TS content of liquid AD (L-AD) systems ranges from 0.5 to 15%, while solid-state AD (SS-AD) systems usually operate at TS contents of higher than 15% [4].

Anaerobic digestion (AD) relies on efficient conversion of organic matter into a valuable product known as biogas, with methane (CH4) as its main combustible constituent. The biogas can be used as energy for household cooking, lighting, heating, and other applications. The process is heavily dependent upon the mutual and syntrophic interaction of a consortium of microorganisms to break down the complex organic matter into soluble monomers such as amino acids, fatty acids, simple sugars, and glycerols. For AD process optimization, it is vital to understand these biological processes and their associated chemical reactions [5].

There are four basic stages involved in AD. These four basic stages make up the process of biogas production from various organic materials as it occurs in an anaerobic digester. These four stages are the hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The AD process is characterized by the decomposition of organic matter into methane, carbon dioxide, inorganic nutrients, and compost in an anaerobic environment [6].

Many different types of anaerobic digesters are available. These vary in configuration, retention time, pre- and posttreatment requirements, and operating temperature, among other things, depending upon the principal feedstocks being treated. During AD, the breakdown of organic compounds is achieved by a combination of many types of bacteria and archaea (microbes). The biomass added to the digester is broken down into sugars, amino acids, and fatty acids (hydrolysis), fermented to produce volatile fatty acids and alcohols (acidogenesis) followed by the conversion into hydrogen, carbon dioxide, and ammonia. In addition, methanogens produce biogas from acetic acid and hydrogen [7].

The addition of biostarter can maximize biogas production. The selection of a good starter is very important to speed up the process overhaul of organic matter. Rumen fluid can be used as a good biostarter because in it there are cellulolytic and methanogenic bacteria. Cellulolytic bacteria degrade an organic material to become a substrate of methanogenic bacteria [8]. The addition of rumen fluid can also shorten the time to reach peak production of methane gas compared to substrates that are not given rumen fluid [9].
