ANS:A - acid fermentation

The first stage of the neutral process of sludge digestion is typically referred to as acid fermentation. During acid fermentation, organic matter present in the sludge is broken down by acid-forming bacteria into volatile fatty acids (VFAs), carbon dioxide (CO2), and hydrogen gas (H2). This stage is characterized by the production of acidic compounds such as acetic acid, propionic acid, and butyric acid. The acidic environment created during acid fermentation helps to solubilize and partially degrade complex organic compounds present in the sludge, making them more accessible to subsequent stages of digestion. This initial breakdown of organic matter sets the stage for further digestion processes to occur. Therefore, acid fermentation is recognized as the first stage in the neutral process of sludge digestion, preceding subsequent stages such as methane fermentation.