Biological ProcessesBiological treatment methods for controlling water pollution
Water treatment• Water treatment describes those processes used to make water more acceptable for a desired end-use.• These can include use as drinking water, industrial processes, medical and many other uses.• The goal of all water treatment process is to remove existing contaminants in the water, or reduce the concentration of such contaminants so the water becomes fit for its desired end-use.• One such use is returning water that has been used back into the natural environment without adverse ecological impact.• The processes involved in treating water for drinking purpose may be solids separation using physical processes such as settling and filtration, and chemical processes such as disinfection and coagulation.
Activated sludge• This is the most common type. It consists in a set of two basins.• In the first, air is pumped through perforated pipes at the bottom of the basin, air rises through the water in the form of many small bubbles.• These bubbles accomplish two things. – provide oxygen form the air to the water – create highly turbulent conditions that favor intimate contact between cells, the organic material in the water and oxygen.
• The second basin is a settling tank, where water flow is made to be very quiet so that the cellular material may be removed by gravitational settling.• Some of the cell material collected at the bottom is captured and fed back into the first basin to seed the process.• The rest is treated anaerobically (= without oxygen) until it is transformed into a compost- type material (like soil).
• The cost of an activated-sludge system is chiefly due to the energy required to pump air at high pressure at the bottom of the aerator tank (to overcome the hydrostatic pressure of the water).• Another disadvantage is that the operation is accomplished in two separate basins, thereby occupying a substantial amount of real estate.
Trickling filter• A trickling filter consists in a bed of fist-size rocks over which the wastewater is gently sprayed by a rotating arm.• Slime (fungi, algae) develops on the rock surface, growing by intercepting organic material from the water as it trickles down.• Since the water layer passing over the rocks makes thin sheets, there is good contact with air and cells are effectively oxygenated.
• Worms and insects living in this “ecosystem” also contribute to removal of organic material from the water.• The slime periodically slides off the rocks and is collected at the bottom of the system, where it is removed.• Water needs to be trickled several times over the rocks before it is sufficiently cleaned.
• Multiple spraying also provides a way to keep the biological slimes from drying out in hours of low-flow conditions (ex. at night).• Plastic nets are gradually replacing rocks in newer versions of this system, providing more surface area per volume, thereby reducing the size of the equipment.
Biological Contactor• This is essentially a variation on the trickling filter, with the difference being that solid material on which slime grows is brought to the water rather than water being brought to it.• Rotating disks alternate exposure between air and water.
Imhoff tanks• The Imhoff tank, named for German engineer Karl Imhoff (1876–1965), is a chamber suitable for the reception and processing of sewage.• It may be used for the clarification of sewage by simple settling and sedimentation, along with anaerobic digestion of the extracted sludge.• It consists of an upper chamber in which sedimentation takes place, from which collected solids slide down inclined bottom slopes to an entrance into a lower chamber in which the sludge is collected and digested.• The two chambers are otherwise unconnected, with sewage flowing only through the upper sedimentation chamber and no flow of sewage in the lower digestion chamber.
Imhoff tanks in the Bochum-Langendreer sewage treatment in Germany
Anaerobic Lagoon• Anaerobic Lagoon or Manure Lagoon is a man-made outdoor earthen basin filled with animal waste that undergoes anaerobic respiration as part of a system designed to manage and treat refuse created by Concentrated Animal Feeding Operations (CAFOs).• Anaerobic lagoons are created from a manure slurry, which is washed out from underneath the animal pens and then piped into the lagoon.• Sometimes the slurry is placed in an intermediary holding tank under or next to the barns before it is deposited in a lagoon.
• Once in the lagoon, the manure settles into two layers: solid or sludge layer and the liquid layer.• The manure then undergoes the process of anaerobic respiration, whereby the volatile organic compounds are converted into carbon dioxide and methane.• Anaerobic lagoons have been shown to harbor and emit substances which can cause adverse environmental and health affects.• These substances are emitted through two main pathways: gas emissions and lagoon overflow.
• Gas emissions are continuous (though the amount may vary based on the season) and are a product of the manure slurry itself.• The most prevalent toxic gasses emitted by the lagoon are – ammonia, hydrogen sulfide, methane, and carbon dioxide.• Lagoon overflow is caused by faulty lagoons, such as cracks or improper construction, or adverse weather conditions, such as increased rainfall or strong winds.