Role of Fermentation Technology in Sewage waste water

The mixture of domestic sewage, industrial effluent and ground water which has infiltrated into the sewers is present in Wastewater which is 99.99% water, with a small amount of dissolved or suspended solid matter. 

One of the applications of fermentation process is in the treatment of wastewater. In the process of sewage disposal, sewage is digested by enzymes secreted by bacteria. Solid organic matters are broken down into harmless, soluble substances and carbon dioxide. The digested solids, also known as sludge, accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. 

Biological treatment of waste can be carried out aerobically, process known as composting or anaerobically through anaerobic digestion. The degradable organic compounds e.g. carbohydrates, proteins, fats, etc. in the waste water are broken down by aerobic micro-organisms mainly bacteria and rarely fungi. The result is an effluent with drastically reduced organic matter content. The materials difficult to digest form a sludge is treated separately either by aerobic breakdown of raw waste-water or anaerobic breakdown of sludge. ^[3]

Aerobic Breakdown of Raw Waste Water

The most common method involve in aerobic breakdown of raw waste water is the activated sludge system. Activated sludge system involves the production of an activated mass of micro-organisms capable of stabilizing the organic content of a waste aerobically.

Waste water is introduced into an aerated tank of micro-organisms which are collectively referred to as activated sludge or mixed liquor. Aeration is achieved by the use  of submerged  diffused  or surface  mechanical  aeration  which  maintain the activated sludge in suspension.

A wide range of bacteria are involved, including Pseudomonas, Achromobacter, Flavobacterium to name a few. Microorganisms have to cope with an uncontrollably diverse range of organic and inorganic compounds some of which may be toxic to the organisms. The microorganisms occur in discreet aggregates known as flocs. Some of the settled biomass is recycled as ‘returned activated sludge’ inoculate the incoming raw sewage because it contains a community of organisms adapted to the incoming sewage. The solid undigested sludge may be further treated into economically valuable products. 

Use of Fermented Sludge in Nutrient Removal

Fermentation can be used to produce carbon source for both nitrogen and phosphorus removal.  It is a tool to reduce operational costs associated with biological nutrient removal. Removal of nitrogen and phosphorus from wastewater effluents is important to ensure environmental protection of surface waters. High concentrations of nutrients in rivers can cause eutrophication, oxygen depletion in the river waters and can stimulate algae growth. Thus it has been proposed as an efficient process to remove nutrients from wastewater. ^[6]

Biological Nitrogen Removal

The processes for biological nitrogen removal can be incorporated into both activated sludge and percolating filter plants.  The overall mechanism follows the route of nitrification (oxidation of ammonia to nitrite and nitrate) and denitrification  (reduction of nitrate sequentially to nitrite, nitric oxide, nitrous oxide and nitrogen). ^[3]

Biological denitrification processes generally recirculate nitrified effluent as a source of nitrate. This means that 100% removal can never be achieved. Only the nitrate contained in the recirculated stream will be removed. 

Biological Phosphorus Removal

Biological phosphorus removal is dependent mainly on the ability of the bacterium Acinerobacter spp. to release phosphate under anaerobic conditions and to  absorb  it  under aerobic conditions. 

The mechanism of Phosphorus removal by Acinetobacter spp. includes the following steps:

• Under anaerobic conditions, readily biodegradable organic matter becomes fermented to short chain fatty acids. These are stored in the cell as poly- hydroxyl butyrates (PHB).
• Under aerobic conditions the stored PHB is oxidized and energy is released allowing the assimilation of soluble ortho-phosphate.
• The ortho-phosphate is metabolised by the cell and excess quantities are stored in the cell as poly-phosphate.  The storage of excess phosphate is known as 'luxury uptake' of Phosphorus and it is this particular ability of the cell that is exploited in the nutrient removal process.