Wastewater Treatment Types

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Rural unsewered areas, for the most part, use septic systems. In these, a large tank, known as the septic tank, settles out and stores solids, which are partially decomposed by naturally occurring anaerobic bacteria. The solids have to be pumped out and hauled by tank truck to be disposed of separately. They often go to municipal wastewater treatment plants, or are reused as fertilizer in closely regulated land-application programs. Liquid wastes are dispersed through perforated pipes into soil fields around the septic tank.

Most urban areas with sewers first used a process called primary treatment, which was later upgraded to secondary treatment. Some areas, where needed, employ advanced or tertiary treatment. Common treatment schemes are presented in the following paragraphs.

Primary Treatment. In primary treatment, floating and suspended solids are settled and removed from sewage.

Flow from the sewers enters a screen/bar rack to remove large, floating material such as rags and sticks.

The liquid and solid material removed from domestic septic tanks is called septage. Most septage is hauled to municipal sewage treatment facilities and most septage haulers must be licensed.

clarifier a tank in which solids settle to the bottom and are subsequently removed as sludge outfall the place where effluent is discharged into receiving waters

It then flows through a grit chamber where heavier inorganics such as sand and small stones are removed.

Grit removal is usually followed by a sedimentation tank/clarifiers where inorganic and organic suspended solids are settled out.

To kill pathogenic bacteria, the final effluent from the treatment process is disinfected prior to discharge to a receiving water. Chlorine, in the form of a sodium hypochlorite solution, is normally used for disinfection. Since more chlorine is needed to provide adequate bacteria kills than would be safe for aquatic life in the stream, excess chlorine is removed by dechlorination. Alternate disinfection methods, such as ozone or ultraviolet light, are utilized by some treatment plants.

Sludge that settles to the bottom of the clarifier is pumped out and dewa-tered for use as fertilizer, disposed of in a landfill, or incinerated. Sludge that is free of heavy metals and other toxic contaminants is called Biosolids and can be safely and beneficially recycled as fertilizer, for example.

Secondary Treatment. Primary treatment provided a good start, but, with the exception of some ocean outfalls, it is inadequate to protect water quality as required by the Environmental Protection Agency (EPA).

With secondary treatment, the bacteria in sewage is used to further purify the sewage. Secondary treatment, a biological process, removes 85 percent or more of the organic matter in sewage compared with primary treatment, which removes about 50 percent.

The basic processes are variations of what is called the "activated sludge" process or "trickling filters," which provide a mechanism for bacteria, with air added for oxygen, to come in contact with the wastewater to purify it.

In the activated sludge process, flow from the sewer or primary clarifiers goes into an aeration tank, where compressed air is mixed with sludge that is recycled from secondary clarifiers which follow the aeration tanks. The recycled, or activated, sludge provides bacteria to consume the "food" provided by the new wastewater in the aeration tank, thus purifying it.

In a trickling filter the flow trickles over a bed of stones or synthetic media on which the purifying organisms grow and contact the wastewater, removing contaminants in the process. The flow, along with excess organisms that build up on the stones or media during the purification, then goes to a secondary clarifier. Air flows up through the media in the filters, to provide necessary oxygen for the bacteria organisms. Clarified effluent flows to the receiving water, typically a river or bog, after disinfection. Excess sludge is produced by the process and after collection from the bottom of the secondary clarifiers it is dewatered, sometimes after mixing with primary sludge, for use as fertilizer, disposed of in a landfill, or incinerated.

Advanced or Tertiary Treatment. As science advanced the knowledge of aquatic life mechanisms and human health effects, and the need for purer water was identified, technology developed to provide better treatment. Heavy metals, toxic chemicals and other pollutants can be removed from domestic and industrial wastewater to an increasing degree. Methods of advanced treatment include microfiltration, carbon adsorption, evaporation /distillation, and chemical precipitation.

Industrial Waste Treatment. Depending on the type of industry and the nature of its wastes, industries must utilize methods such as those used for advanced treatment of sewage to purify wastewater containing pollutants such as heavy metals and toxic chemicals before it can be discharged. Industries are permitted to discharge directly to receiving waters under the National Pollution Discharge Elimination System (NPDES) permit system or to municipal sewers under the Industrial Pretreatment Program. Pollution prevention programs are very effective in helping industries reduce discharged pollutants, by eliminating them at the source through recycling or through the substitution of safer materials. More and more industries are approaching or attaining zero discharge by cleaning and reusing their water over and over and over.

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