Exercise 3. Read and translate the text. Choose an abstract of ten lines and get ready to read it aloud.
Stages of sewage treatment
Pre-treatment removes materials such as trash, tree limbs, leaves, that can be easily collected from the raw wastewater before they damage or clog the pumps and skimmers of primary treatment clarifiers.
In the primary sedimentation stage, sewage flows through large tanks, commonly called primary clarifiers or primary sedimentation tanks.
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.
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 60% to 65% of suspended solids, and from 30% to 35% of BOD from the sewage.
Secondary treatment is designed to substantially degrade the biological content of the sewage which is derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. The bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc.
Secondary treatment systems are classified as fixed-film or suspended-growth systems.
Fixed-film or attached growth systems include trickling filters and rotating biological contactors, where the biomass grows on media and the sewage passes over its surface.
Suspended-growth systems include activated sludge, where the biomass is mixed with the sewage and can be operated in a smaller space than fixed-film systems. However, fixed-film systems can provide higher removal rates for organic material and suspended solids.
The purpose of tertiary treatment is to provide a final treatment stage to raise the effluent quality before it is discharged to the environment (sea, river, lake, ground, etc.).
More than one tertiary treatment process may be used at any treatment plant. The main processes are removal of nutrients, nitrogen, phosphorus, sand filtration and sometimes disinfection.
The purpose of disinfection in the treatment of wastewater is to substantially reduce the number of microorganisms in the water to be discharged back into the environment. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Generally, short contact times, low doses and high flows all militate against effective disinfection. Common methods of disinfection include ozone, chlorine and ultraviolet light.
Chlorination remains the most common form of wastewater disinfection due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment.
Ozone is considered to be safer than chlorine because, it is generated onsite as needed and shouldn’t be stored. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators.
Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement.