The purpose of preliminary treatment is to protect the operation of the wastewater treatment plant. This is achieved by removing from the wastewater any constituents which can clog or damage pumps, or interfere with subsequent treatment processes. Preliminary treatment devices are, therefore, designed to:
- Remove or to reduce in size the large, entrained, suspended or floating solids. These solids consist of pieces of wood, cloth, paper, plastics, garbage, etc. together with some fecal matter.
- Remove heavy inorganic solids such as sand and gravel as well as metal or glass. These objects are called grit.
- Remove excessive amounts of oils or greases.
Racks and Bar Screens
These consist of bars usually spaced three-quarter inches to six inches. Those most commonly used provide clear openings of one to two inches. Although large screens are sometimes set vertically, screens are usually set at an angle of 45 to 60 degrees with the vertical. The incoming wastewater is passed through the bars or screens and periodically the accumulated material is removed. The racks or screens may be cleaned either manually or by means of automatically operated rakes. The solids removed by these units can be disposed of by burial or incineration.
Grinders, cutters and shredders. These are devices to break or cut up solids to such size that they can be returned to the wastewater without danger of clogging pumps or piping or affecting subsequent treatment devices. They may be separate devices to grind solids removed by screens or a combination of screen and cutters installed within the wastewater flow channel in such a manner that the objective is accomplished without actually removing these larger solids from the wastewater. These latter devices are made by a number of manufacturers under various trade names and, in most cases, consist of fixed, rotating or oscillating teeth or blades, acting together to reduce the solids to a size which will pass through fixed or rotating screens or grids having openings of about one-fourth inch. Some of these devices are even designed to operate as a low-lift pump. Unfortunately, many plants with comminuting devices develop problems within subsequent treatment units due to a build up of the shredded solids. This is usually witnessed in the aeration system of activated sludge plants. These shredded solids tend to clog diffusers and cling to the impeller blades of mechanical aerators.
Wastewater usually contains a relatively large amount of inorganic solids such as sand, cinders and gravel which are collectively called grit. The amount present in a particular wastewater depends primarily on whether the collecting sewer system is of the sanitary or combined type. Grit will damage pumps by abrasion and cause serious operation difficulties in sedimentation tanks and sludge digesters by accumulation around and plugging of outlets and pump suctions. Consequently, it is common practice to remove this material by grit chambers. Grit chambers are usually located ahead of pumps or comminuting devices, and if mechanically cleaned, should be preceded by coarse bar rack screens. Grit chambers are generally designed as long channels. In these channels the velocity is reduced sufficiently to deposit heavy inorganic solids but to retain organic material in suspension. Channel type chambers should be designed to provide controlled velocities as close as possible to 1.0 foot per second. Velocities substantially greater than 1.0 foot per second cause excessive organic materials to settle out with the grit. The detention period is usually between 20 seconds to 1.0 minute. This is attained by providing several chambers to accommodate variation in flow or by proportional weirs at the end of the chamber or other flow control devices which permit regulation of flow velocity. There are also patented devices to remove grit. One development is the injection of air several feet above the floor of a tank type unit. The rolling action of the air keeps the lighter organic matter in suspension and allows the grit relatively free from organic matter to be deposited in the quiescent zone beneath the zone of air diffusion. Excessive quantities of air can cause the roll velocity to be too high resulting in poor grit removal. Insufficient quantities of air result in low roll velocities and excessive organic matter will settle with the grit. These grit chambers are usually called aerated grit chambers.
Grit chambers are designed to be cleaned manually or by mechanically operated devices. If cleaned manually, storage space for the deposited grit is usually provided. Grit chambers for plants treating wastes from combined sewers should have at least two hand-cleaned units or a mechanically cleaned unit with by-pass. Mechanically cleaned grit chambers are recommended. Single, hand-cleaned chambers with by-pass, are acceptable for small wastewater treatment plants serving sanitary sewer systems. Chambers other than channel type are acceptable, if provided with adequate and flexible controls for agitation and/or air supply devices and with grit removal equipment.
There are a number of mechanical cleaning units available which remove grit be scrapers or buckets while the grit chamber is in normal operation. These require much less grit storage space than manually operated units.
Grit always contains some organic matter which decomposes and creates odors. To facilitate economical disposal of grit without causing nuisance, the organic matter is sometimes washed from the grit and returned to the wastewater. Special equipment is available to wash grit. Mechanical cleaning equipment generally provides for washing grit with wastewater as it is removed from the chamber.
Quantity of Grit.
This depends on the type of sewer system, the condition of the sewer lines and other factors. Strictly domestic wastewater collected in well constructed sewers will contain little grit, while combined wastewater will carry large volumes of grit, reaching a peak at times of severe storms. In general, 1.0 to 4.0 cu.ft. of grit per million gallons of wastewater flow can be expected.
Manually cleaned grit chambers for combined wastewater should be cleaned after every large storm. Under ordinary conditions these grit chambers should be cleaned when the deposited grit has filled 50 to 60 percent of the grit storage space. This should be checked at least every ten days during dry weather.
When mechanically cleaned grit chambers are used, they must be cleaned at regular intervals to prevent undue load on the cleaning mechanism. Recommendations of the manufacturer should be rigidly observed. This plus experience, will determine the cleaning schedule.
A grit in which marked odors develop indicates that excessive organic matter is being removed in the grit chamber. Alternately, if sludge from a settling tank is excessively high in grit, or if there is excessive wear in pumps, comminutors, sludge collectors or other mechanical equipment, the reason is likely to be inefficient functioning of the grit removing process. In either case, a study of this unit should be made.
Disposal of Screenings and Grit.
Screenings decompose rapidly with foul odors. They should be kept covered in cans at the screens and removed at least daily for disposal by burial or incineration. The walls and platforms of the screen chamber and screen itself should be hosed down and kept clean. Grit containing much organic matter may have to be buried to prevent odor nuisances.
Pre-aeration of wastewater, that is aeration before primary treatment is sometimes provided for the following purposes:
- To obtain a greater removal of suspended solids in sedimentation tanks.
- To assist in the removal of grease and oil carried in the wastewater.
- To freshen up septic wastewater prior to further treatment.
- BOD reduction.
Pre-aeration is accomplished by introducing air into the wastewater for a period of 20 to 30 minutes at the design flow. This may be accomplished by forcing compressed air into the wastewater at a rate of about 0.10 cu.ft. per gallon of wastewater when 30 minutes of aeration is provided or by mechanical agitation whereby the wastewater is stirred or agitated so that new surfaces are continually brought into contact with the atmosphere for absorption of air. To insure proper agitation when compressed air is forced into the wastewater, air is usually supplied at the rate of 1.0 to 4.0 cubic feet per minute per linear foot of tank or channel. When air for mechanical agitation (either with or without the use of chemicals) is used for the additional purpose of obtaining increased reduction in BOD, the detention period should be at least 45 minutes at design flow. The agitation of wastewater in the presence of air tends to collect or flocculate lighter suspended solids into heavier masses which settle more readily in the sedimentation tanks. Pre-aeration also helps to separate grease and oil from the wastewater and wastewater solids and to carry them to the surface. By the addition of air, aerobic conditions are also restored in septic wastewater to improve subsequent treatment.
The devices and equipment for introducing the air into the wastewater are the same or similar to those used in the activated sludge process.
Pre-chlorination is the chlorination of a wastewater prior to primary treatment. In general, the objectives of pre-chlorination are not related to disinfection, and its use is related to either temporarily preventing further wastewater decomposition or reducing problems associated with wastewater decomposition. The objectives of pre-chlorination are:
- Odor control
- Protection of plant structures
- Aid in sedimentation, and
- Reduction or delay of Biochemical Oxygen Demand (BOD)
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 upsewer from the plant in forcemains, 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.
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.
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.
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.