Anaerobic Digestion Return to Process Page

What are Anaerobic Digesters and what do they do?    After all the solids from the various settling tanks are collected, they are pumped into Primary digester #1.  The reason that the process is called anaerobic is because the entire process takes place in the absence of oxygen.  The La Porte Wastewater Treatment Facility has three digesters.  Two Primary digesters and a Secondary digester.  The two Primary digesters are heated to 95 degrees Fahrenheit and mixed, where the Secondary digester is unheated and is not mixed.   In Primary digester #1, untreated solids (also called sludge) are deposited into the digester and mixed with methane gas.  This form of mixing looks very similar to the way bubbles of air in an aquarium rise to the surface.  As the bubbles of methane rise to the surface in the digester, they carry the nearby solids upwards.  Once they reach the surface, the solids settle back towards the bottom of  the digester.  The methane gas is then captured by a large floating cover and pumps inject the gas back into the bottom of the digester where it continues the mixing process.  While the digester is being mixed, it is also being heated.  In the center building of the digesters there is a large hot water heat exchanger system.  Pumps remove solids from the digester and run it through a series of tubes that are heated by hot water.  This heats the solids which are then returned to the digester.  By carefully controlling the temperature of the water, the temperature of the solids and therefore the temperature of the digester can be maintained at 95 degrees.    The reason that the temperature is maintained at 95 degrees, is that temperature provides the best environment for the bacteria in the digester to perform their job.  There are two main types of  bacteria in the digestion process, acetogens and methogens. The acetogenic bacteria take the solids material and break the complex organics (sugars, carbohydrates, proteins, fats, etc) in the solids down into volatile fatty acids.  During this step the bacteria produce carbon dioxide, hydrogen, acetic acid, and lactic acid.  For this reason, the acetogenic bacteria are commonly referred to as "acid formers".  One the acetogenic bacteria have done their work, the methogenic bacteria start.  The methogenic bacteria use the volatile fatty acids to form methane gas, while some additional bacteria will use the hydrogen and carbon dioxide to also form methane gas.  As long as the acetogenic bacteria are breaking the complex organics down, there is food for the methogenic bacteria and the end result is the production of biogas which is about 60% methane gas, 40% carbon dioxide gas with trace amounts of other gases.  The methane that is produced by the methogens is captured and is the gas that is used to heat the digester and also provide for mixing.  As the solids are being processed through Primary digester #1, the same volume of solids are being transferred to Primary digester #2 where the same process is taking place.  Between the two Primary digesters, the solids are treated for approximately 60 days. Once the solids have been through treatment, they are called Biosolids and they are transferred into the Secondary digester where the solids are allowed to settle.  The clear water that gathers at the top (called supernatant) is removed and placed back at the start of the Treatment Facility where is goes through the entire treatment process.  The heavier solids on the bottom of the Secondary digester are pumped out to the Storage Tank where they accumulate until they can be finally disposed of.