Waste Water Treatment Overview

Any activities, whether domestic or industrial produce wastewaters or effluents containing undesirable contaminants that must be removed before the water is discharged to a natural receptor.

In general, conventional wastewater treatment consists of a combination of physical, chemical and biological processes and operations to remove insoluble particles and soluble contaminants from effluents. There are various sources of water contamination. For categories of water are generally distinguished:

• Rainwater (runoff from impermeable surfaces)
• Domestic wastewater
• Agricultural water
• Industrial wastewaters

A wastewater treatment process generally consists of five successive steps as follows. General processes involved in each step are referred in Figure 1.

1. Preliminary treatment or pre-treatment (physical and mechanical)
2. Primary treatment (physicochemical and chemical)
3. Secondary treatment (chemical and biological)
4. Tertiary of final treatment (physical and chemical)
5. Treatment of the sludge formed (recycling, incineration, etc)

Figure 1: Waste water treatment processes

The objective of preliminary treatment is the removal of coarse solids and other large materials often found in raw wastewater. Preliminary treatment operations typically include coarse screening, grit removal and, in some cases, comminution of large objects.

During the primary treatment, the removal of settleable organic and inorganic solids by sedimentation is achieved, and materials that will float (scum) by skimming. Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD5), 50 to 70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment in primary clarifiers.

The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids. In most cases, secondary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes. Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-products (principally CO2, NH3, and H2O).

The microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary effluent. The sedimentation tanks used in secondary treatment, are often referred to as secondary clarifiers. The biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing.

An adaptation of the activated sludge process is often used to remove nitrogen compounds, in a nitrification-denitrification process.

Tertiary and/or advanced wastewater treatment is employed when specific wastewater constituents which cannot be removed by secondary treatment must be removed, or when the water wants to be reused in other processes. Often tertiary treatments include disinfection, which normally involves the injection of a chlorine solution. Ozone and ultraviolet (uv) irradiation can also be used for disinfection. The bactericidal effects of chlorine and other disinfectants are dependent upon pH, contact time, organic content, and effluent temperature.

The residue that accumulates in sewage treatment plants is called sludge (or biosolids). Treatment of sludge may include a combination of thickening, digestion (where organics are metabolized by bacteria anaerobically), and dewatering processes. The final destination of treated sludge usually is the land. Dewatered sludge can be buried underground in a sanitary landfill. It also may be spread on agricultural land in order to make use of its value as a soil conditioner and fertilizer. Since sludge may contain toxic industrial chemicals, it is not spread on land where crops are grown for human consumption.

It is important to remark that the problems encountered during wastewater treatment are generally complex as the effluent contains pollutants of various types depending on its origin. Since, there are different types of effluent to treat, each wastewater treatment plant shall be designed with its own characteristics requiring specific treatment processes.

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