Aerobic treatment of wastewater

Definition

It is any treatment process that uses aerobes to biodegrade or oxidise or remove the unwanted organic or inorganic compounds.  In the treatment of wastewaters, the processes can be divided into suspended growth processes (e.g. activated sludge process, Figure 1) or attached growth processes such as rotating biological contactors (Figure 2) or trickling filters (Figure 3). [1]

Aerobe is a micro-organism that needs free or dissolved oxygen to develop. [1]

Treatment Processes

Activated sludge process

A continuous, aerobic biological treatment for wastewater dating from 1913, that uses a culture of bacteria suspended in the wastewater in an aeration tank to adsorb, absorb and biodegrade the organic pollutants. Flocs are formed that may reach 0.1 mm in diameter in the aeration tank. These are kept in suspension either by air blown into the bottom of the tank (diffused air system) or by mechanical aeration (see Figure 6). The mixture of the activated sludge and the wastewater in the aeration tank is known as the mixed liquor. The concentration of mixed liquor is known as mixed liquor suspended solids (MLSS), but is also measured as mixed liquor volatile suspended solids (MLVSS). The mixed liquor flows from the aeration tank to a sedimentation tank, where the activated sludge flocs combine together into larger particles that settle as a sludge. Most of the sludge from the sedimentation tank returns to the aeration tank. The dissolved oxygen in the aeration tank should be at least 0.5 mg/l, preferably 1 to 2 mg/l (see oxgen activated sludge). The aeration tank may be designed on aeration periodpreferably on F:M ratio or mean cell residence time. If the ammonia present is to be oxidised to nitrate, the plant must be designed for nitrification. In municipal wastewater, the activated sludge process is usually preceded by primary sedimentation. In the start up of an activated sludge plant, the time needed for establishing the appropriate bacteria and protozoa can be greatly reduced by seeding the new aeration tank with sludge from one that is working well. Activated sludge treatment demands only about one seventh of the land occupied by trickling filters. It also does not suffer from filter flies and has little smell. The operating cost of the aeration tank can be high and skilled attention is essential. Many varieties of activated sludge treatment exist, including contact stabilisation, extended aeration, modified aeration, oxygen activated sludge. [1]

Figure 1: Activated Sludge process, flow diagram. [1]

Rotating Biological Contactor, RBC

It is an aerobic wastewater treatment in which a horizontal shaft just above the surface of the wastewater carries closely spaced discs up to 3 m diameter or random plastics media in circular wire cages, that revolve with the shaft (see Figure 2). Some 25 to 45% of the discs or other media are submerged. The slow rotation develops a biofilm which oxidises the wastewater. When the slime layer becomes too thick, it sloughs off and is settled in a separate tank or compartment. In some plants, especially those for small communities, the discs are contained in the same tank as the zones for primary and secondary sedimentation and sludge storage. Some anaerobic sludge digestion will occur and the plant is desludged two to four times yearly. Little maintenance is needed.

Figure 2: Rotating biological contactor [1]

Trickling Filter

It is an aerobic biological treatment for wastewater that was developed in Salford, England in 1893. In a typical standard rate trickling filter (Figure 3), the wastewater trickles down through a 2 m deep bed of coarse stones, 24 to 100 mm diameter. Many other types exist, possibly using plastic media in a packed tower, as in high rate trickling filters. It is an attached growth process with a biofilm developing on the media. The attractions of the standard rate trickling filter are its low power cost and simplicity. Its disadvantages are the capital cost for large works, the large land area compared with activated sludge and the absence of versatility in operation. The trickling filter is not wholly aerobic. After the biofilm has formed, there is an anaerobic layer at the surface of the stones which little or no oxygen can reach. The main micro-organisms are aerobes, anaerobes and facultative anaerobic bacteria, but fungi are present also on aerobic surfaces, where they compete with bacteria for food. Algae occur at the top of the filter where there is sunlight. Trickling filters have been used to nitrify activated sludge effluent, and anaerobic filters can be used for denitrification. [1]

Figure 3: Trickling filter [1]

Advantages of Aerobic Treatment Processes

The advantages of aerobic treatment relative to anaerobic treatment are as follows:

1. Good process stability
2. High effluent quality
3. Smaller reactor sizes [4]

Disadvantages of Aerobic Treatment Processes

The disadvantages of aerobic treatment relative to anaerobic treatment are as follows:

1. High energy consumption
2. Higher nutrient requirements
3. High sludge yield
4. High operating costs [4]

Relative Advantages and Disadvantages of Aerobic and Anaerobic Treatment Processes [4]

Process	Anaerobic	Aerobic
Advantages	High energy consumption Higher nutrient requirements High sludge yield High operating costs	Good process stability High effluent quality Smaller reactor sizes
Disadvantages	Low sludge yield Low energy consumption Generation of biogas Low nutrient requirements	Sensitivity to toxicity and influent fluctuation Requires more monitoring Higher capital costs Usually requires downstream aerobic polishing prior to discharge

Figure 5: Fate of carbon and energy in Aerobic and Anaerobic wastewater treatment. [5]

References

1. Dictionary of Water and Waste Management, Elsevier
2. Environmental Engineers’ Handbook
3. Wastewater treatment: biological and chemical processes By M. Henze
4. Environmental Bioengineering By Lawrence K. Wang, Joo-Hwa Tay, Stephen Tiong Lee Tay
5. Biological wastewater treatment: principles, modelling and design By M. Henze