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).
Food materials contained in wastewater promote the growth of living organisms. Their respiratory activity then decreases the DO in the wastewater, unless this oxygen supply is replenished by natural or artificial means. The most common method of wastewater treatment today is the Activated Sludge System which requires oxygen to maintain the aerobic process.
A Floc develops when wastewater is aerated in the aeration tank or tanks. Maintained in suspension by aeration, this floc has a very high purification capacity. Floc accumulation in the aeration tank is aided by returning previously formed floc to the tank influent. This method is a type of activated sludge system.
After aeration, the activated sludge is allowed to settle in the secondary settling tank. Sludge produced in excess of the amount required for return to the primary settling tank is disposed of as water or processed for fertilizer. The effluent itself is further disinfected before disposal to destroy disease-producing organisms. The most common disinfectant is chlorine.
In this process, it is very important that the correct amount of air is maintained to provide adequate aeration. If the air supply is not maintained at a rate that DO balances the BOD, the organisms die due to lack of oxygen, and the process becomes septic because the sludge is no longer activated. Therefore, to balance the BOD and DO the ideal wastewater treatment blower should have a variable air volume and be selected to overcome the total pressure losses as indicated below, or changes in the inlet conditions of the fan:
1. Filter, inlet pipe and valves
2. Pipes and valves from the blower discharge flange to the air diffuser openings.
3. Pressure drop across air diffuser openings: minimum when clean and increases as the opening becomes fouled with deposits.
4. The static pressure due to the wastewater head (which represents 85% of the total loss).
5. Changes in blower inlet conditions.
Loss factors 4 and 5 are of utmost importance to the wastewater treatment system designer. This means you require a blower performance curve that has a pressure rise characteristic to compensate for variations in the resistance of the wastewater headers or air diffuser.
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.