Text B. Sewage (wastewater) treatment process

Sewage (wastewater) treatment is the process of removing contaminants from wastewater, both industrial and domestic. It includes physical, chemical, and biological processes to remove physical, chemical, and biological contaminants. Its objective is to produce an environmentally safe sewage water (treated effluent) and a solid waste (sludge or biosolids) suitable for discharge or reuse back into the environment. Reuse is often for agricultural purposes, but more recently, sludge is being used as a fuel source.

It used to be said that " the solution to pollution is dilution". Nature has an amazing ability to cope with small amounts of water wastes and pollution discharged into a body of water. A natural process of stream self-purification occurs. Densely populated communities generate such large quantities of sewage, however, that dilution alone does not prevent pollution. This makes it necessary to treat wastewater to some degree before disposal. Sewage treatment plants (STPs) reduce pollutants in sewage to a level which nature can handle. The sewage treatment plant plays vital role in the process of removing the contaminants from sewage to produce liquid and solid (sludge) suitable for discharge to the environment or for reuse.

Sewage can be treated close to where it is created (in septic tanks, biofilters, aerobic wastewater treatment systems), or collected and transported via a network of pipes and pump stations to a municipal wastewater treatment plant. Industrial wastewater often requires specialized treatment processes.

Stages of conventional sewage treatment at the wastewater treatment plant involve:

1. pretreatment (preliminary treatment);

2. primary treatment;

3. secondary treatment;

4. tertiary treatment.

Here is step-by-step guide describing what happens at each stage of the treatment process.

Preliminary treatment removes materials that can be easily collected from the raw wastewater before they damage or clog the pumps and skimmers of primary treatment clarifiers. Pretreatment includes: screening, grinding, sand and grit removal, dissolved air flotation, wastewater flocculation, prechlorination or preaeration, equalization.

Screening (straining) is the removal of all large objects (wood, stones, dead animals) in the influent sewage water using a screen (a trash rake, a mechanically cleaned bar screen, a manually cleaned screen in smaller or less modern plants); the materials are then sent to a landfill. Sand and grit settlement is accomplished in a sand trap or a grit chamber where the velocity of the incoming wastewater is carefully controlled to allow the materials to settle. Screens, grinders, and sand and grit traps are provided for the protection of other equipment in the STP. Dissolved air flotation and wastewater flocculation aid in the removal of suspended solids and oil in the primary clarifier and reduce the biological loading on secondary treatment processes. Prechlorination or preaeration may be required to prevent odour problems and to eliminate septic conditions where wastewater has abnormally long runs to the plant. Equalization structures are used to regulate diurnal flow variations and to equalize flows to treatment facilities.

***aerobic waste water treatment – аэробная (биологическая) очистка сточных вод (очистка сточных вод от органических загрязнений в специальных водоотстойниках с помощью аэробных микроорганизмов)

biofilter – биологический фильтр, биофильтр

biosolids – твёрдые вещества биологического происхождения; " биосолидс" (органический материал, получаемый в результате обработки сточных вод)

dilution – разжижение, разбавление; растворение; разведение; раствор; растворённое состояние

dissolved air – растворённый (в жидкости) воздух

grit chamber [trap] – песколовка; гравиеловка

preaeration – предварительная аэрация

prechlorination – предварительное хлорирование

pretreatment – предочистка, предварительная очистка (сточных вод)

primary treatment – первичная обработка

primary treatment clarifier – установка (отстойник) первичной очистки сточных вод

raw sewage (wastewater) – неочищенные сточные воды, необработанные сточные воды

sand trap – песколовка

secondary treatment – доочистка (сточных вод); вторичная очистка (сточных вод)

septic tank – септик(тенк), отстойник, септический резервуар, септик (сооружение для предварительной очистки сточных вод)

sludge - густая грязь; ил, тина; осадок; отстой; шлам; осадок сточных вод; осадок в резервуаре со сточными водами; очищать от грязи; превращать в осадок, отстой; давать осадок, образовывать осадок

tertiary treatment – доочистка, третичная очистка (сточных вод)