Micro -Nutrients play an important role in the functioning of microorganisms during anaerobic digestion. The anaerobic treatment of industrial wastewaters, requires the addition of Micronutrients suitable for micro-organisms since these wastewaters are devoid of essential metals. However, the dosing of nutrients is only effective if the metals are in a bio available from which in turn is dependent on the chemical speciation of the system.
Nutrients in anaerobic digestion may be divided into three categories, namely macronutrients, micronutrients and vitamins. Macronutrients refer to those elements that are required in large concentrations (above 1 mg/l). Micronutrients are those trace metals that are required in concentrations in the range of less than 1 mg/l.
However it is deficient in most of the nutrients such as N, P, K and S and other micronutrients such as Cu, Zn, Al, Co and Ni that are necessary for the functioning of the microorganisms. It is also unknown whether micronutrients that are present are in a form the microbes are able to assimilate. Anaerobic treatment of this effluent therefore requires nutrient supplementation.
Effluents produced by different types of industries will differ in their composition and concentration of the micro-metals present. The types of microorganisms within the digesters will also be different. Thus each effluent will require a micronutrient mix suitable for that effluent and process.
Anaerobic digestion of waste streams requires the presence of essential metals for the microorganisms to function. In chemical, petrochemical, sugar refining and paper making industries, wastewater streams undergoing anaerobic digestion are deficient in these Micronutrients required by the microorganisms for optimal biological functioning. Therefore these must be supplemented to the influent.
A high level of volatile fatty acids (VFA) is a good sign of anaerobic malfunction. For a balanced process, the acidogenesis, acetogenesis and methanogenesis degradation stages need to be equal.
Why the VFA is high..?
If the required micronutrients are not supplied, the functioning of the microorganisms is impaired. This leads to reactor acidification and subsequent loss in methanogenic activity, describes this phenomenon using methane and acid forming bacteria. There are many different methane forming bacteria as well as many different acid forming bacteria. When the system is in balance, the methane forming bacteria consume the acid intermediates as rapidly as they are formed. If the population of the methane bacteria is not high enough, or their functioning is impaired, the acid intermediates will not be consumed as swiftly as produced. This results in an increase in the volatile acid concentration in the digester.
Nutrient deficiency is signaled by a high volatile fatty acid (VFA) concentration. High VFA in the effluent is also the indication for toxicity. Once it has been determined that the nutrients are sufficient and bioavailable, the problem of toxicity should be investigated
An indication of micronutrient deficiency is the formation of sludge that bulks and foams and fails to settle in subsequent stages. Microorganisms that lack nutrients have difficulty growing into settleable flocs due to their unstable structures.