Some of the energy released by the substrates is captured by the bacterial cellsand stored in high-energy phosphate bonds for use in cellular activity. Energy thatis not captured by the bacterial cells is lost to the environment as heat. New bacterialcells and carbon dioxide are products of biochemical reactions that involveorganic compounds (Equation 8.3).substrates (organic compounds) A bacterial cells + carbon dioxide (8.3)Catalysts are involved in biochemical reactions. These catalysts are known asenzymes. Enzymes are large proteinaceous molecules that greatly accelerate the

The net results of anaerobic digestion of solids are significant decreases inpercent solids and percent volatile solids in digester sludge. The first and secondstages of anaerobic digestion are achieved through the activities of facultativeanaerobes and anaerobes, whereas the third stage is achieved through the activityof only anaerobes, the methane-forming bacteria.Within the digester all carbohydrates are degraded inside the cell of facultativeanaerobes and anaerobes. Carbohydrates too large to enter the cell, that is, in aninsoluble or complex soluble form, must be hydrolyzed into smaller, soluble sugars outside the cell through the use of exoenzymes (Figure 8.2). Once hydrolyzed, thesmaller, soluble sugars enter the cell, where they are degraded by endoenzymes.

CHEMICAL OXYGEN DEMAND:It measures the equivalent amount of oxygen required to chemically oxidize organic compounds in water. thechemical oxygen demand(COD) test is commonly used to indirectly measure the amount oforganic compoundsinwater. It is expressed in milligrams per liter (mg/L) also referred to as ppm (parts per million), which indicates themassof oxygen consumed per liter ofsolution. The basis for the COD test is that nearly all organic compounds can be fully oxidized tocarbon dioxidewith a strongoxidizing agentunderacidicconditions.BIOLOGICAL OXYGEN DEMAND:Biochemical oxygen demand(BOD) is the amount ofdissolved oxygenneeded by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period.