Aerobic biological treatment systems

The main aerobic biological wastewater treatment process includes:

High rate processes:

(i) Activated sludge

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(ii) Oxidation ditch

(iii) Trickling filter

(iv) Bio filter (Biotower)

(v) Rotating biological contactor

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Low rate processes:

(i) Facultative stabilization ponds

(ii) Aerated lagoons

High rate processes

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High rate processes are characterized by relatively small reactor volumes and high concentration of microorganisms when compared with low rate processes. Consequently, the growth rate of new organisms is much greater in high rate systems because of the well- controlled environment.

Activated sludge process. This process originated in England in the early 1900s and earned its name because sludge (mass of microbes) is produced which aerobically degrades and stabilizes the organic matter of a wastewater. The typical layout of an activated sludge process

Activated sludge is, at present, the most widely used biological treatment process for both domestic and industrial wastewaters. The activated sludge process refers to a continuous aerobic method for biological wastewater treatment, including carbonaceous oxidation and partial nitrification. The expression ‘activated sludge’ alludes to slurry of microorganisms that remove organic compounds from wastewater, these microorganisms are themselves removed by sedimentation under aerobic conditions. In an activated sludge system, soluble and unsettle able biodegradable organic compounds are degraded by bacteria in an aerated basin, and biomass is carried over with the influent into a secondary settling tank where solids are allowed to settle and concentrate; they are then removed. Part of the activated sludge (settled solids) is drawn off as waste; the rest (30-40%) is recycled to the aeration basin to maintain a constant population of microorganisms.

The process relies on a dense microbial population being mixed in suspension with the wastewater under aerobic conditions. In the presence of adequate nutrients and oxygen, a high rate of microbial growth and respiration is achieved. This results in the utilization of organic matter present, to produce end products such as carbon dioxide, ammonia, phosphate and sulphate and the biosynthesis of more microorganisms. In activated sludge systems, organic load removals of 85-95% can be achieved.

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Trickling filters

A trickling filter or bio filter consists of a circular basin or tower filled with support media such as broken stones, slag, plastic rings, modular plastic fills, etc. Wastewater is distributed over the media continuously. Microorganisms become attached to the media to form a biological slime layer (bio film). Organic matter in the wastewater diffuses into the bio film, where it is stabilized. Oxygen is supplied to the film by the natural flow of air either up or down through the media, depending on the relative wastewater temperature and ambient air temperature. The thickness of the bio film increases as new microorganisms grow. Frequently, portions of the bio film slough off the media. The sloughed biomass is separated from the liquid in a secondary settling tank and discharged to sludge processing. Clear liquid from the secondary settling tank is called as secondary effluent and a portion is often recirculated to the filter to maintain constant hydraulic distribution of the wastewater over the filter.

Rotating biological contactors

Rotating biological contactors (RBCs) are fixed-film reactors similar to trickling filters in that microorganisms responsible for biodegradation are attached to support media. In the case of the RBC, the support media are slow rotating discs that are partially submerged in a semi circular tank receiving primary effluent.

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A biological contactor consists of a series of closely spaced circular plastic discs of PVC. The discs are partially submerged in waste water. By gentle rotation of the discs, the bio films are alternately exposed to the contaminants in the wastewater and oxygen in the atmospheric air. The disc rotation affects oxygen mass transfer and maintains the biomass in aerobic condition. Oxygen is supplied to the attached bio film from the air when the bio film is out of the water and from the liquid when submerged. Oxygen is transferred to the wastewater by surface turbulence created by disc rotation. Sloughed pieces of bio film from the disc surface are removed and segregated by providing a secondary settling tank

High-rate biological treatment processes remove not less than 85% of the BODs and suspended solids originally present in raw municipal sewage. Activated sludge process generally produces an effluent of slightly higher quality than bio filter or RBCs. However, they remove very little phosphorus, nitrogen and non-biodegradable organics.

Low-rate biological treatment systems

Natural low-rate biological treatment systems are available for the treatment of municipal sewage and tend to be lower in cost and less sophisticated in operation and maintenance.

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Stabilization pond

Stabilization ponds are shallow ponds, typically 1-2 m deep, where raw sewage or partially treated sewage can be decomposed by symbiotic action of algae and bacteria. These ponds are designed to maintain aerobic conditions throughout, but more often the decom­position taking place near the top layer is aerobic, while the bottom (benthic) layer is anaerobic.

In stabilization ponds, algae utilize C02, sulphates, nitrates, phosphates, water, and sunlight to synthesize their own organic cellular material and give off free oxygen as a waste product. This oxygen, dissolved in pond water, is available to bacteria and other microbes for their metabolic process, which include respiration and degradation of organic material in the pond. The settle able organic matter deposits at the bottom to undergo anaerobic decay this is the algal-bacterial symbiosis, in which

1. Microorganisms use oxygen dissolved in the water and

2. Break-down organic waste materials, to produce

3. End products such as C02, H20 and plant nutrients such as nitrates, phosphates and sulphates,

4. Algae use as raw material in photosynthesis, to give out oxygen, thus

5. Replenishing the depleted oxygen supply and maintaining an aerobic environment.