Editing Landfill (section)

===Acid formation (Phase III)===

Hydrolysis of the biodegradable fraction of the solid waste begins in the acid formation phase, which leads to rapid accumulation of [[volatile fatty acids]] (VFAs) in the leachate. The increased organic acid content decreases the leachate [[pH]] from approximately 7.5 to 5.6. During this phase, the decomposition intermediate compounds like the VFAs contribute much [[chemical oxygen demand]] (COD). Long-chain volatile organic acids (VOAs) are converted to acetic acid (C<sub>2</sub>H<sub>4</sub>O<sub>2</sub>), CO<sub>2</sub>, and hydrogen gas (H<sub>2</sub>). High concentrations of VFAs increase both the [[biochemical oxygen demand]] (BOD) and VOA concentrations, which initiates H<sub>2</sub> production by fermentative bacteria, which stimulates the growth of H<sub>2</sub>-oxidizing bacteria. The H<sub>2</sub> generation phase is relatively short because it is complete by the end of the acid formation phase. The increase in the biomass of [[acidogenic]] bacteria increases the amount of degradation of the waste material and consuming nutrients. Metals, which are generally more water-soluble at lower pH, may become more mobile during this phase, leading to increasing metal concentrations in the leachate.