Essential Nutrients

Aerobic Digestion

Biological treatment plants rely on a continuous and natural process of biological breakdown of effluent. Micro-organisms feed on pollutants and convert them into a sludge which is separated from the treated effluent discharge, and does not pass into the waterways and sewers. When biological oxygen demand (BOD) reduction is poor, less effluent can be treated and more is discharged, a situation which is both environmentally hazardous and illegal.

The removal of BOD in an aerobic treatment system is accomplished by the oxidation of organic matter (to carbon dioxide and water) to provide energy for the synthesis of the remainder of the organic material into micro-organisms. The extent of BOD removal is, therefore, critically dependent on the reduction in organic substrate brought about by continuous growth of micro-organisms. In order for this replication of micro-organisms to occur, the growth environment must contain a balanced provision of organic substrate, oxygen, growth factors and nutrients.

The nutrient requirements of micro-organisms can be conveniently sub-divided into macro and micro nutrients. Macro-nutrients are required in relatively large quantities to sustain microbial growth, whereas micro-nutrients, although fundamental to microbial metabolism, are required only in trace quantities.

Macro-nutrients

Carbon and oxygen constituents of the biomass are assimilated from the organic substrate and dissolved oxygen present in the waste-water. The objective of a biological treatment process is to maintain the carbon source as the limiting nutrient. Nitrogen is required for the synthesis of proteins and nucleic acids. Phosphorus is functional in both energy transfer and the synthesis of nucleic acids and phospholipids. If either nitrogen or phosphorus becomes a limiting nutrient due to insufficient concentration within the waste-water, the effective removal of organic matter will not be achieved, since the biomass will no longer be able to synthesize new cell material.

A number of industrial waste-waters are known to be deficient in nitrogen and phosphorus sources, including those from pulp and paper, textile, brewing and distillery, and certain food processing effluents. In order to maintain effective microbial ecologies within systems treating such waste-waters, and hence achieve maximum BOD removal, the waste-water must be supplemented by a nitrogen and phosphorus source at an appropriate ratio and addition rate. Sulphur is used in the synthesis of proteins. The majority of micro-organisms can derive sulphur from sulphate sources such as calcium or magnesium sulphates. Unless the waste-water is derived from a demineralized source, there should usually be no requirement for sulphur addition.

Micro-nutrients

Micro-nutrients perform the critical functions of enzyme activation, regulation of osmotic pressure and transfer of electrons in oxidation-reduction reactions. In addition, a number of other functions are recognized—cobalt is used in the formation of vitamin B 12, and calcium plays apart in the stability of the cell wall.

Necessary trace elements identified include potassium, magnesium, manganese, iron, cobalt, nickel, zinc, copper and molybdenum. The precise amount of these trace elements needed in the biological process depends on the nature of the micro-organisms. These quantities have never been satisfactorily determined, but we are funding research to quantify the nutritional requirements of the bacteria which typically occur in our biological treatment systems.

Natural Selection

The microbial population in an activated sludge plant can be seen as a constantly evolving eco-system. The nature and rate of the changes occurring within the population are in direct response to the environment around the micro-organisms. Thus a change in the organic composition of the waste-water will be followed by a gradual change. in the microbial strains which predominate in the sludge. In effect, the process of natural selection ensures the growth of the particular microbial population which is best adapted to survive. This adaptability of micro-organisms is fundamental to the operational flexibility of biological t. treatment systems. If, for example, an industrial waste-water contains a low concentration of iron, an additional constraint has been applied to the microbial population. The organic matter (represented by BOD) no longer represents the rate limiting component of the influent waste-water. The microbial population will evolve with the primary objective of survival and proliferation in an environment that has a low concentration of iron. As at result, the microbial strains which predominate will not be those most suited to removal of BOD from the waste-water, but will be those that either require less iron, or are the most effective in assimilating it. This has two effects. First, BOD removal in the treatment plant is adversely affected, since the optimum conditions for organic substrate removal by growth of micro-organisms do not exist. Second, there is a proliferation of filamentous bacteria within the microbial population. The living environment of the biological treatment system requires continuous monitoring by vigilant eyes, and tailored treatment to ensure optimum efficiency, compliance with the law and maximum return on investment. It is not only vital to keep the biomass alive but also to maintain a high level of activity, as nutrient deficiency can have a dramatic effect on effluent plant efficiency.

Engineered Selection and Bio-augmentation

Therefore, we at EversTech give Nature a little help by identifying potential problem areas in a particular environment or type of waste, and anticipate what Nature would do. We then formulate a combination of micro-organisms that are appropriate to that substrate and engineer the microbial shift. This is called "bio- augmentation.". This creates a more acceptable time-scale of acclimatisation in a commercial plant, even though there will be a degree of natural selection of appropriate strains, a dominance of the engineered biomass must attain, and, therefore, regular inoculation of the "formulation" must be made. The metabolism and metabolic pathways of the engineered biomass are very often different, so other advantages occur, such as odour control, less oxygen requirement, more resilience to adverse conditions etc. However, our vigilance, as far as nutrient requirements are concerned, must never waver, and signs that there may be a nutrient deficiency must never be ignored. Information, even of the. smallest significance, related to the substrate or waste water source, must always be communicated to the Process Engineer.

Author: D. Evers, Principal Consultant
©1996 EversTech LLC