Malodour in treatment facility is one of the commonest problems that bug ETP plants. Most often it is related to disturbed microbial ecology in the plant. Malodour is a phenomenon that occurs frequently in anaerobic systems where conditions have become septic. Two important factors that trigger malodours are very low oxygen levels and absence of nitrates.
Microorganisms degrade cBOD by oxydising componds in presence of electron acceptors. Most microbes are able to use oxygen as a terminal electron acceptor while some can use Nitrates. When both these compounds are absent, microbes turn to alternative electron acceptors. These alternate electron acceptors and by-products of consequent degradation reactions produce compounds that give off malodours. Most of these compounds are sulphur based since sulphur compounds serve as electron acceptors in septic conditions. As a result hydrogen sulphide, dimethyl sulphide, mercaptans etc are formed and released. Along with these, organic acids and alcohols are also formed that add to foul smell.
If the sewage water contains high organic loads and has become septic, then bacteria break down organic molecules into Volatile Organic Compounds and Volatile Fatty Acids along with production of ammonia. When bacteria degarde cBOD in prsence of oxygen or nitrate no foul smelling compound is formed. Only when they switch to alternative electron sources that such foul smelling compounds are produced.
There are several ways to control the problem. One very direct and effective way is to increase oxygen levels. If that is not possible (as in anaerobic treatment plant) the next best solution is to add Nitrates in the wastewater. Nitrates can be added in the form of NaNO3 or Ca(NO3)2. Microbes cannot use sulphur or organic compounds as electron acceptors in presence of oxygen or nitrates.
Bioaggumentation with certain non-pathogenic Pseudomona is also a very good option as these organisms can facultative lower cBOD without production of excess hydrogen sulphide and ammonia.
Chemical methods like addition of alkali or metals salts are temporary. Such methods try reducing malodour by killing all bacteria and stopping anaerobic degradation. These methods are temporary as malodour returns after the effect of these chemicals has subsided and new growth has occurred.
Hydrogen sulphide, common malodour culprit, is also toxic to microbes that reduce ammoniacal nitrogen and it also serves as food source for unwanted filamentous microbes such as Beggiatoa and Thiothrix. Growth of such filamentous organisms leads to scum formation and foaming. Scum formation further complicates the treatment process. The sooner the problem is addressed the better it is.
