Microbes in Sewage Treatment

The Problem Sewage Treatment Solves

Every day, cities and towns generate vast volumes of wastewater -- sewage -- carrying human excreta, food waste, detergents, and other organic and inorganic material washed down drains and toilets. This sewage is overwhelmingly water (over 99 per cent by some estimates) but the small remaining fraction of solid and dissolved organic matter is enough to cause serious environmental harm if released untreated: dumped directly into a river or lake, this organic load would trigger explosive, uncontrolled growth of naturally occurring decomposer microbes in the water body, which in consuming the organic matter would also consume the water's dissolved oxygen, suffocating fish and other aquatic organisms and devastating the ecosystem. Municipal Sewage Treatment Plants (STPs) exist to prevent this, and they work by deliberately harnessing -- under controlled conditions -- exactly the same microbial decomposition process that would otherwise happen destructively in the open environment.

Primary Treatment: Physical Removal

The first stage sewage passes through at a treatment plant is primary treatment, which is purely physical and involves no microbial action. Sewage is passed through a sequence of filters to remove large floating debris, and then allowed to stand in settling tanks, where sand, grit and other dense suspended solids settle out under gravity as a primary sludge, while lighter materials are skimmed off the surface. What remains is a still-cloudy effluent, now free of large solids but still carrying substantial dissolved and suspended organic matter.

Secondary (Biological) Treatment: Where the Microbes Take Over

The effluent from primary treatment is passed into large aeration tanks for secondary treatment, and it is here that microorganisms do the essential work. The effluent is continuously agitated and mechanically aerated -- air is pumped through it -- which allows populations of aerobic bacteria and filamentous fungi naturally present in sewage to grow vigorously, forming clumps called flocs. These microbial flocs consume the dissolved and suspended organic matter in the effluent as their food source, breaking it down and, in the process, dramatically reducing what is called the sewage's Biochemical Oxygen Demand, or BOD.

Biochemical Oxygen Demand (BOD) as a Pollution Measure

BOD is defined as the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria, and it is the standard measure used to assess how polluted a water sample is with biodegradable organic material. A high BOD value means the water contains a large amount of organic matter that will demand -- and deplete -- a correspondingly large amount of dissolved oxygen as it decomposes; this is precisely the situation that suffocates aquatic life if untreated sewage reaches a natural water body. Sewage treatment aims to lower BOD substantially -- essentially transferring the oxygen-demanding decomposition process into the controlled, aerated tanks of the treatment plant, where it can be managed, rather than letting it happen uncontrolled in a river.

What Happens to the Sludge …