Sewage treatment covers any artificial process to which sewage is subjected in order to remove or alter its objectionable constituents so as to render it less dangerous or offensive. Sewage before being disposed of either in river, land or other places has generally to be treated to make it safe both for environment and living beings. Following points can direct us to the need for sewage treatment:
- Sewage is both dangerous to health and a potential nuisance
- Raw or untreated sewage discharged into the stream forms objectionable deposits on the bed
- Grease and soap contained in the sewage will rise to the surface of the stream and from a scum
- Fish may be killed and stream may be spoiled from any recreational use
- Disease bearing bacteria may be carried in the bodies of the cows that wade in contaminated stream, those bacteria may reach the milk during the process of milking
- Sewage may contain bacteria that cause typhoid, fever, various forms of dysentery and cholera.
|Flow Diagram for Sewage Treatment Stages|
Sewage can be treated close to where it is created, a decentralised system or be collected and transported via a network of pipes and pump stations to a municipal treatment plant, a centralised system. Industrial sources of wastewater often require specialized treatment processes. Sewage treatment generally involves few stages, i.e. Preparatory, Primary, Secondary treatment.
Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment.
Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment.
Sewage treatment is done in order to remove or alter its objectionable properties so as to render it less dangerous or offensive. Following reasons may be taken into account:
- To stabilize the sewage without causing odor and nuisance and without endangering public health
- To prevent damage done to receiving waters, whether they are rivers, canals, coastal waters and tidal estuaries.
Primary treatment is the first process in the wastewater treatment plant to remove a significant fraction of organic particulate matter (suspended solids). These suspended solids contribute to biochemical oxygen demand (BOD5) of the wastewater. Thus, removing suspended solids also reduces BOD5. The process is important because the reduction of suspended solids and BOD5 lowers the oxygen demand, decreases the rate of energy consumption, and reduces operational problems with downstream biological treatment processes. Primary treatment also serves the important function of removing scum and inert particulate matter that was not removed in the grit chamber. The scum consists of grease, oil, plastic, leaves, rags, hair, and other floatable material. The treatment reduces the Biochemical Oxygen Demand (BOD) of the waste by about 15 to 30% which is a measure of the strength or pollution potential of the waste matter.
|Disc type of Fine screen|
Screening is the very first operation carried out in a sewage treatment plant and consists in passing the sewage through the different types of screens, so as to trap and remove the floating materials.
The specific objectives are:
- To prevent formation of unsightly scum in settling and aeration tanks.
- To prevent clogging sprinklers nozzles or the surface of trickling filters.
- To prevent pumping machinery and siphons etc. from damage.
- To prevent formation of sludge banks or unsightly floating matter in the receiving bodies of water.
Cutting Screens or Comminutors are usually provided in plants that do not have primary clarifiers, fine screens or mechanically cleaned bar screens. This kind of device breaks the larger sewage solids to about 6mm in size, when the sewage is screened through them. They consist of revolving slotted drum through which the sewage is screened. Cutters mounted on the drum shear the collected screen against a comb until they are small enough to pass through 5mm to10mm wide slots of the drum. These are usually arranged in pairs to facilitated repairs and maintenance. Cutting Screens should always be preceded by grit chambers to prevent excessive wear.
A grit chamber is an enlarged channel or a long basin, in which the cross section is increased so as to reduce the flow velocity of sewage to such an extent that the heavy inorganic material settle down by the force of gravity and lighter inorganic materials remain in suspension. They are nothing but kind of sedimentation tanks, design to separate the intended heavier inorganic materials by the process of sedimentation due to gravitational force.
They remove the inorganic particles having specific gravity of about 2.65. Such as sand, gravel grit, eggshells, bones etc of size 0.2mm larger to prevent damage to pumps or to prevent their accumulation in sludge digesters.
Sewage contains lot of grease and fatty oils, which forms a scum in sedimentation tanks and interferes with oxidation process in aeration tanks. Skimming tanks or primary settling basins are about 1 m deep and the scum accumulations are removed manually or buried or burnt with detention time of about 10 minutes. This tank generally includes an aerating device which blows air through the sewage at a rate about 0.1 ft3 of air per gallon of sewage. The rising air tends to coagulate the grease and oil and cause them to rise to the surface where they can be removed easily.
In the primary sedimentation stage, sewage flows through large tanks commonly called primary sedimentation tanks. The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. Grease and oil from the floating material can sometimes be recovered for saponification.
The dimensions of the tank should be designed to effect removal of a high percentage of the floatables and sludge. A typical sedimentation tank may remove from 50 to 70 percent of suspended solids, and from 30 to 35 percent of biochemical oxygen demand (BOD) from the sewage.
The goal of Sedimentation Tank has been framed in the context of an arbitrary percentage removal of total suspended particles without a justification for the selection of the percentage removal or a means of assessing whether or not the goal has been achieved. A currently evolving philosophy is that the Sedimentation Tanks should be designed on the basis of the oxidative capacity of the downstream biological processes. Sedimentation Tanks can remove more BOD and solids for less operational cost than any other treatment process. Thus, it makes both economic sense and design sense to remove, to the maximum extent possible, the settleable solids and settleable BOD5 by primary settling. From an operational perspective, under most situations the design should minimize the conditions that promote biological activity in the Sedimentation Tanks. An exception to this approach occurs when biological phosphorus removal is to occur downstream. In this case, the Sedimentation Tanks may be used to generate volatile fatty acids to promote biological phosphorus removal.
Circular and rectangular tanks are the most common configurations. Square tanks with circular sludge collection mechanisms have been used. These have generally proven unsatisfactory because of sludge build up in the corners, and fouling of the more complex sludge collection mechanism. Stacked rectangular tanks have been used where space is highly restricted. They have a much higher construction cost and require more complex structural design. Plate settlers have become an important design alternative in primary sedimentation. Of the alternatives, rectangular and circular tanks with and without plate settlers are favored for primary sedimentation.
Septic Tank refers to the anaerobic bacterial environment that develops in the tank and which decomposes or mineralizes the waste discharged into the tank. Septic tanks can be coupled with other on-site wastewater treatment units such as bio-filters or aerobic systems involving artificial forced aeration.
A septic tank generally consists of a tank of between 4000 and 7500 liters (1,000 and 2,000 gallons) in size connected to an inlet wastewater pipe at one end and a septic drain field at the other. These pipe connections are generally made via a T pipe which allows liquid entry and exit without disturbing any crust on the surface. Today, the design of the tank usually incorporates two chambers (each of which is equipped with a manhole cover) which are separated by means of a dividing wall which has openings located about midway between the floor and roof of the tank.
Wastewater enters the first chamber of the tank, allowing solids to settle and scum to float. The settled solids are anaerobically digested, reducing the volume of solids. The liquid component flows through the dividing wall into the second chamber where further settlement takes place, with the excess liquid then draining in a relatively clear condition from the outlet into the leach field, also referred to as a drain field or seepage field, depending upon locality.
The Imhoff tank 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. An Imhoff tank is a two-stage septic system where the sludge is digested in a separate tank. This avoids mixing digested sludge with incoming sewage. Also, some septic tank designs have a second stage where the effluent from the anaerobic first stage is aerated before it drains into the seepage field. The 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 are being superseded in sewage treatment by plain sedimentation tanks using mechanical methods for continuously collecting the sludge, which is moved to separate digestion tanks. This arrangement permits both improved sedimentation results and better temperature control in the digestion process, leading to a more rapid and complete digestion of the sludge.
Preparatory and Primary treatments are the first processes in the wastewater treatment plant to remove a significant fraction of organic particulate matter (suspended solids). These suspended solids contribute to biochemical oxygen demand (BOD5) of the wastewater. Thus, removing suspended solids also reduces BOD5. The process is important because the reduction of suspended solids and BOD5 lowers the oxygen demand, decreases the rate of energy consumption, and reduces operational problems with downstream biological treatment processes. Primary treatment also serves the important function of removing scum and inert particulate matter that was not removed in the grit chamber. The scum consists of grease, oil, plastic, leaves, rags, hair, and other floatable material. Thus Preparatory and Primary Treatment serves the entire treatment process well and without these processes, total system may not work properly.
1. Aziz M.A. 1972, Sewage Engineering, Dacca
2. Kundu D., Preperatory and Primary Treatment: A Literature Review
3. Davis M.L., Water and Wastewater Engineering: Design Principle and Practice