Read the following text and speak on every stage of sewage (wastewater) treatment.
Text C. Sewage Treatment Process (continuation)
Primary treatment consists in temporarily holding the sewage in a quiescent basin called “primary clarifier” or “primary sedimentation/ settling tank”. The main purpose of primary treatment is the physical separation of solids and grease from the wastewater (heavy solids settle to the bottom while oil, grease and lighter solids float to the surface to be skimmed off). The settled and floating materials (sludge) are removed, separately treated or processed, and a homogeneous liquid is subjected to secondary (biological) treatment. As a result of primary treatment, 30- 40% of Biological Oxygen Demand (BOD) and 50% of Total Suspended Solids are removed. Primary clarifiers are equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank from where it can be pumped to further sludge treatment stages.
Secondary treatment is designed to degrade the biological content of the sewage (dissolved and suspended biological matter derived from human waste, food waste, soaps and detergent) using aerobic biological processes. The purpose of biological treatment is BOD reduction. The principle of the process is that simple bacteria (cells) eat the organic mat- ter which is transformed into cellular mass (floc) through their metabo- lism. The floc is precipitated at the bottom of a settling tank or retained as slime on solid surfaces. There are two broad types of biological treat- ment:
1. the treatment that includes mechanical means to create contact be- tween wastewater, cells and oxygen:
· activated sludge (aerated sewage containing aerobic microorgan- isms that help to break it down); such aerobic biological wastewater treatment is accomplished in activated sludge tanks;
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· 
trickling filters and rotating biological contactors where the bi- omass (biological films of bacteria, protozoa and fungi) grows on the media’s surface and eats or otherwise reduces the organic content.
2. the treatment without mechanical means:
· the sewage is made to flow by gravity through specially con- structed lagoons or wetlands where vegetation acts as a biological filter to the water.
Tertiary treatment provides a final treatment stage to remove disease- causing organisms and to increase the effluent quality (of 10 parts per million BOD and 10 parts per million Total Suspended Solids) before it is discharged back into the environment. Tertiary treatment processes can be physical, biological, or chemical including:
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· sand filtration (to remove residual suspended matter) or activated carbon filtration (to remove residual toxins);
· lagooning (to provide further biological improvement through storage in large artificial ponds or lagoons);
· nitrogen and phosphorus removal;
· disinfection (to reduce the number of microorganisms) using chlo- rine, ozone O3, or ultraviolet (UV) light;
· odour removal.
More than one tertiary treatment process may be used at any treat- ment plant.
Read the following text and say what the purposes of sludge treatment and disposal are.
Text D. Sludge Treatment and Disposal
Sludge is the residue that accumulates in the STP. It is solid matter that has settled out of suspension in sewage undergoing sedimentation in tanks or basins. Since a considerable quantity of sludge is produced dur- ing the sewage treatment process, treatment and disposal of sewage sludge are major factors in the design and operation of all water pollution control plants. Two basic goals of sludge treatment before final disposal are:
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· the reduction of sludge volume, which, in turn, reduces the costs of pumping and storage;
· the stabilization of the organic materials (stabilized sludge does not have an offensive odour and can be handled without causing a nui- sance or health hazard).
Treatment methods of sewage sludge may include a combination of the following processes:
· thickening,
· digestion,
· dewatering,
· disposal.
Thickening is usually the first step in sludge treatment, because it is impractical to handle thin sludge, slurry of solids suspended in water. Thickening is usually accomplished in a tank called a gravity thickener. An alternative to gravity thickening is dissolved-air flotation.
Digestion is a biological process in which organic solids are decom- posed into stable substances. Digestion reduces the total mass of solids, destroys pathogens, and makes it easier to dewater or dry the sludge. Most large STPs use a digestion system in which organics are metabo- lized by bacteria anaerobically (in the absence of oxygen), and in some STPs sludge digestion takes place aerobically (in the presence of oxy- gen). Both aerobic and anaerobic digestion converts about half of the organic sludge solids to liquids and gases.
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Dewatering is dehydration, or water removal. Digested sewage sludge is usually dewatered before disposal. Dewatered sludge still con- tains a significant amount of water (about 70%), but even at that mois- ture content, sludge no longer behaves as a liquid and can be handled as a solid material. Sludge drying beds provide the simplest method of de- watering. Drying is a combination of evaporation and gravity drainage through the sand. After about six weeks of drying, the sludge cake may have a solids content of about 40%. Alternatives to sludge drying beds include the rotary-drum vacuum filter and the centrifuge.
Disposal. The final destination of treated sewage sludge usually is the land. Dewatered sludge can be:
· buried underground in a sanitary landfill;
· spread on agricultural land as a soil conditioner and fertilizer;
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· incinerated if a suitable site for land disposal is not available, as in urban areas (in the case of incineration, air pollution control is a very important factor);
· dumped in the ocean (once an economical disposal method for many coastal communities, it is no longer considered a viable option);
· reutilized as an energy resource in many advanced countries.
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18. Read the texts of Unit 4 again and make notes under the follow- ing headings. Then use your notes to talk about Types of Sewage and Sewage Treatment.
1. Sewage. Types of sewage.
2. Wastewater composition.
3. Sewage treatment.
4. Sludge disposal.
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