The following points highlight the four main objectives of pre-chlorination. The objectives are:- 1. Odor Control 2. Protection of Plant Structures 3. Aid in Sedimentation 4. Reduction or Delay of Biochemical Oxygen Demand.
Objective # 1. Odor Control:
The decomposition of wastewater starts in sewers and becomes objectionable only after anaerobic decomposition has taken over. The degree of putrefaction that occurs is related to the time the wastewater is in the sewers which, in turn, depends on the length and grades of the sewers.
Odor problems, therefore, develop where the sewers are long or where it is necessary to collect sewage in pump sumps and subsequently pump the wastewater to a treatment plant. There are few places in this state where the sewers are so long that putrefaction occurs to such a degree that offensive odors rise from the sewers before the wastewater reaches the wastewater treatment plant.
If such a condition occurs, it may be possible to chlorinate the wastewater at a manhole on a trunk sewer. The amount of chlorine required varies depending on how long the decomposition of the wastewater must be delayed. It is not necessary to add sufficient chlorine to satisfy the chlorine demand, but merely sufficient to destroy odors and slow bacterial decomposition. Thus, no residual chlorine is produced. Doses of four to six mg/L are generally sufficient to control odors.
Chlorine may be applied up sewer from the plant in force mains, pump suction wells, screen chambers, grit chambers, trickling filter influent, settling tanks or wherever there is an odor problem. Normally, the practice is to start with a fairly high dose of chlorine (10 mg/L) to quickly control the odors, and gradually reduce the dose over a period of time to determine the minimum that will satisfy the local condition.
The production of offensive odors at pumping stations is a fairly common occurrence. Chlorination of the wastewater as it enters the pump sump or in the pump sump is effective as a preventative measure. The amount of chlorine required varies with the different situations but is less than that required to produce a residual. Generally, it is about the same as the chlorine demand or 25 to 50 lbs. per million gallons, but the minimum effective dose must be found by trial and error for each installation.
Another common occurrence is for wastewater to be septic, or a source of odor, as it is received at the wastewater plant. To prevent disagreeable odors during treatment, chlorination of the influent of the primary sedimentation tank is practiced which also aids in the settling properties of the sludge solids. If the purpose is only odor control and not disinfection, the chlorination need not be sufficient to produce a residual.
Generally, a dose that will destroy all the reducing substances and thus slow the rate of decomposition is used. How great this dose must be depends to a large extent on how far putrefaction proceeded before the wastewater reached the plant. When putrefaction is far advanced, the chlorine dose may be equal to or greater than the dose which would produce a residual if the wastewater were fresh.
A similar situation may develop when the wastewater is received fresh but becomes septic during the treatment process. This often occurs in a new plant where the initial wastewater flow is far less than the design flow and the detention period in the primary tanks is greatly prolonged.
Again pre-chlorination of the tank influent is used to delay putrefaction and resulting odors. In this case, the chlorine dose will be much less than that required if the wastewater were septic. The amount of reducing substances in the wastewater will be low and a dose of two to five mg/L of chlorine may be sufficient to prevent odors.
Objective # 2. Protection of Plant Structures:
Decomposition of wastewater can proceed to the point of hydrogen sulfide production, but, owing to location or low concentration, odors are not a problem. If this occurs in a pumping station, intercepting sewers or treatment plant, there may be serious corrosion.
The remedy is similar to that for odor control — chlorination sufficient to prevent hydrogen sulfide formation or to destroy hydrogen sulfide if it has been produced. The points of application are similar to those used for odor control but the quantity of chlorine may be less because only hydrogen sulfide has to be controlled. Minimum chlorine dose cannot be found without laboratory tests.
In general though, this is a specific problem and the dose of chlorine can be found by trial and error. It may not be necessary to destroy all the hydrogen sulfide but only to reduce the concentration to one or two mg/L so that the amount evolved will be a minimum.
Hydrogen sulfide causes structures to be damaged and weakened due to corrosion and can result in shutdown of the plant for repair. Generally, it is an economic problem, but factors other than cost must be considered. One such factor is the toxic nature of hydrogen sulfide.
Objective # 3. Aid in Sedimentation:
Pre-chlorination at the influent of a settling tank is sometimes practical for the benefit of improved settling. Generally, such benefits are incidental to the use of pre-chlorination for some other purpose.
However, when there is a choice of the point of chlorine application, it is well to bear in mind that improved sedimentation, heavier sludge, and improved grease and oil separation are obtainable when chlorination of the primary influent is practiced.
Objective # 4. Reduction or Delay of Biochemical Oxygen Demand:
Chlorination of raw wastewater to produce a residual of 0.2 to 0.5 mg/L after 15 minutes contact may cause a reduction of 15 to 35 percent in the BOD of the wastewater. Generally, a reduction of at least 2 mg/L of 5 day BOD is obtained for each mg/L of chlorine applied up to the point at which a residual is produced.
When units of a plant become overloaded, use can be made of chlorination to reduce the load until additional treatment facilities can be provided as the use of chlorine for BOD reduction is usually not economical. Chlorine is also used when the additional load is only temporary, such as when supernatant is returned from sludge digesters or when a plant receives intermittent discharges of industrial wastes.
Occasionally, chlorination of the plant effluent to a relatively high residual is practiced to delay or reduce the BOD load on receiving waters during short periods of extremely low stream flow. This is only an emergency procedure but does offer some aid under such conditions. Generally, the higher the residual carried the more the load is reduced, but care must be taken to prevent fish kills by chlorine.