Water treatment is the science of improving any waste water supply and then treating it it a manner that can be safely used for all domestic purposes including drinking and irrigation. In short, wastewaster treatment is the science of handling ingluent (raw sewage) in such a manner that effluent (sewage plant output) is in no way dangerous or offensive to life or property.
How the treatment system works
The strength of sewage and its ability to pollute water is described by two basic conditions:
- B.O.D. (Biological Oxygen Demand)
- Suspended solids in mg/liter
As an example, when a body of water, like a river is heavily overloaded with sewage which is trying to oxideize or decay, the action of sewage demands oxygen to proceed. This B.O.D. is supplied by D.O (dissolved oxygen) in the water. Therefore, if the B.O.D. exceeds the D.O. then there is no oxygen left for the river life. All sewage treatment is aimed at satisfying oxygen demand and reducing suspended solids.
The primary step in this process is accomplished by allowing the sewage to settle in a large tank. While the effluent from the settling tank contains less suspended solids and it is cleaner, nothing has been done to satisfy the B.O.D. in this simple settling tank. Let us approach the controlled decomposition of sewage with a process, such that the D.O. (disolved oxygen) present in the water-sewage mixture will always satisfy the Biological Oxygen Demand. By definition this is essentially an aerobic process (micro-organisms depend upon Oxygen for survival).
All living things are dependen upon Oxygen for respiration. Gaseous Oxygen furnishes their normal needs. Terrestrial oranisms utilize the Oxygen of the atmosphere and aquatic ornganisms depend on Oxygen dissolved in water. The atmosphere contains 21% Oxygen by volume, while water holds only 0,8% in solution at normal temperature of 50ºF. The aquatic environment is then inherently more sensitive to the Oxygen Demand of the ornganisms that populate it.
The food materials contained in sewage encourage the growth of living organisms. Their repiratory activity then diminishes the D.O. in the sewage, unless this Oxygen supply is replenished by natural or artificial means. The most common method of waste water treatment today is the Activated Sludge System which requires Oxygen to maintain the aerobic process.
A Floc develops when the sewage is aerated in the aeration tank or tanks. Kept in suspension by aeration, this floc has a very high purifying capacity. The accumulation of the floc in the aeration tank is helped by returning previously formed floc to the tank influent. This method is one type of activade sludge system.
Following aeration, the activated sludge is permitted to settle in the secondary settling tank. Sludge prodeced in excess of the amount required for return to the primary settling tank is disposed of as a wate or processed for fertilizer. The effluent itself is further disinfected prior to disposal in order to destroy disease producing ornganisms. The most common disinfectant is Chlorine.
In this process it is very important that the correct amount of air is maintained to provide proper aeration. If the air supply is not maintained at a rate such that de D.O balances the B.O.D, the organisms then perish due to lack of Oxygen, and the process becomes septic because the sludge is no longer activated. Therefore to balance the B.O.D. and the D.O. the ideal wastewater treatment blower should be of varying air volume and selected to overcome the total of pressure losses as indicated below, or changes to the blower inlet conditions:
1. Filter, inlet piping and valves
2. The piping and valves from the blower discharge flange to the actual air diffuser openings.
3. The pressure drop across air diffuser openings - minimum when clean and increases as the opening become encrusted with deposits.
4. The static pressure due to sewage head (representing 85% of the total loss).
5. Changes in inlet conditions to the blower.
The loss factors 4 and 5 are of paramount importance to the sewage treatment system designer. This means that he requires a blower performance curve that has a rising characteristic in pressure to compensate for variations in sewage head or air diffuser resistance.
Advantages of Multistage Centrifugal Blowers in Wastewater Treatment
1. Multistage Centrifugal Blowers offer total flexibility in their operaing range since they can be throttled on either inlet or outlet to reduce air volume while maintaining a stable pressure curve. Reduction of air volume reduces the power consumption proportionally while no such saving can be reached with positive displacement blowers.
2. Multistage Centrifugal Blower's simple drive arrangement allows operation via direct coupling, V-belt or gear increaser.
3. No elaborate inlet silencer is needed as blowers are quiet.
4. No discharge silencer is needed as the process piping can be connected via rubber sleeve or expansion joint to the blower outlet flange to further reduce noise.
5. No emergency pressure relief valve is ever needed since the curve does not permit it to reach excessive or destructive pressure.
6. No elaborated vent valves (bleeding to atmosphere) are needed to control the volume. The Multistage Centrifugal Blower can be throttled on either inlet or outlet to adjust the volume and save HP.
7. No high temperature safeguards are required since the design pressure ratio of Centrifugal Blowers cannot produce excessive or unsafe temperatures.
8. No elabore pressure lubrication system is required since Multistage Centrifugal Blowers are equipped with recirculating oil-lubricated anti-friction bearings.
9. Since there is a ample space between the impeller circumference and the housing, there are no wearing parts, against the tight contact between the lobes and the housing of P.D. Blowers.
