Wastewater treatment

Wastewater contains a large fraction of organic matter which needs to be treated before discharge. Aerobic respiratory bacteria are capable of degrading organic matter into carbon dioxide and water in the presence of the oxygen.  It leaves environment clean without any residue. But the action of aerobic respiratory bacteria is inhibited when there is no adequate oxygen.  

                              Then anaerobic respiratory bacteria become active. They are also able to degrade organic matter into carbon dioxide. But they use non oxygen terminal electron acceptors such as sulfate, ferric and nitrate ions. They reduce sulfate ions into odiferous toxic gas hydrogen sulfide, water insoluble ferric into ferrous and nitrate into nitrogen gas which cause infertility in aquatic ecosystems. So although anaerobic respiratory bacteria degrade organic matter completely into carbon dioxide, they cause environmental pollution.

                                       Under anaerobic conditions, fermentation also takes place. Fermentative bacteria such as Clostridium use organic matter as their substrate and degrade them partially into acids, alcohols and aldehydes. Acids cause acidification of aquatic water bodies. Acids dissolve minerals and release toxic metal ions to the water. Ex: Al (3+). Alcohols and aldehydes may be volatile which cause atmospheric pollution. They cause producing unpleasant odor. If enough oxygen is available, these small organic compounds can be degraded into carbon dioxide completely.  But in most cases oxygen is still not available, so anaerobic respiratory microorganisms degrade these small organic compounds into carbon dioxide and cause pollution.

                             Therefore anaerobic respiration and fermentation are not the proper solutions for wastewater treatment. Aerobic respiration is the best solution. Wastewater is collected from the all over the premises and directed into an aeration tank. Here water is continuously aerated for 4-8 hours. Mechanical aerators are used to oxygenate the water. The oxygen concentration should be maintained in-between 0.5-2mg/l. it is impossible to retain wastewater until the all organic matter degrade.  After retention time, wastewater is transferred to the settling tank or primary clarifier. Here unoxidized or partially oxidized organic matter is allowed to settle down at the bottom while clear water is directed for further treatment.  A fraction of sediments is returned to the aeration tank as the activated sludge to enhance the treatment process.

Sludge digestion: The sediment that is collected from the bottom of settling or sedimentation tank is transferred into an anaerobic digester. Sludge is completely degraded into carbon dioxide. The sludge mostly contains the high strength organic matter which is needed to degreed into small organic compounds by fermentative bacteria such as Clostridium. They degrade the organic matter partially into acids, alcohol and aldehydes. Then small organic compounds are degraded completely into carbon dioxide by anaerobic bacteria. Most of that carbon dioxide is reduced into Methane by methanogens such as Methanomonas and Methanococcus. Methane is a combustible gas which can be used as alternative fuel for fossil fuel. The reaming in the digester is transferred into another setting tank for settling down for sediments. A portion of this sediment is returned to the digester for seedling the incoming unoxidized sediments. Rest is disposed as a valuable fertilizer for agriculture and land escaping.