Emerging Water Pollutants and Wastewater Treatments
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(d) Tertiary and/or advanced treatment (TWWT)
This is the last stage of wastewater treatment. The TWWT is employed specifically
to remove pollutants or contaminants which escaped during secondary treatment.
Dissolved solids, heavy metals, refractory organics, suspended solids, phosphorus
and nitrogen are necessary to be removed by specific treatment procedures. Advanced
treatments are also called tertiary treatments as they are often preceded by high-rate
secondary treatment. Tertiary treatment processes, however, are at times coupled
with primary or secondary treatments (e.g., the addition of chemicals to primary
clarifiers or aeration tanks for phosphorus removal) or used to substitute secondary
treatment (e.g., primary effluent overland flow treatment).
Nitrogen content present in influents that is usually in their ammonia form is unaffected as they pass through the first two treatment phases. Near-complete nitrification
of the nitrogenous component occurs in the third aerobic zone, converting them to
nitrites and nitrates sequentially. The mixed liquor rich in nitrate generated from the
aerobic zone is then recycled back to the initial anoxic zone. Nitrates recycled are
denitrified anaerobically and nitrogen gas is generated (which completely escapes
to the atmosphere) by facultative bacteria, which use influent organics hydrogen
donors. Unrecycled nitrates are reduced by endogenous bacteria respiration in the
second anoxic zone. Dissolved oxygen levels are raised again in the final re-aeration
zone to avert subsequent denitrification that may affect settling in the secondary clarifier. To further prevent denitrification, the level of dissolved oxygen is raised in the
final re-aeration zone; this would impair settling in the secondary clarifiers.
(e) Disinfection
The disinfectant usually employed for wastewater disinfection is chlorine. It is
injected in form of a solution into the wastewater at the chlorine dosage tank. The
tank is automated and discharges chlorine at a specific time interval. The dosage
of chlorine to be used is dependent on wastewater strength and other factors, but
common dosage ranges are between 5 and 20 mg/l. Other disinfectants, though not
commonly used are ultraviolet (UV) irradiation and ozone. The chlorine dosage tanks
are often automated rectangular channels with designs providing a contact time of
about 25–30 min. However, the standard chlorine contact time of 1 h or more may
be needed for target irrigation purposes of reclaimed effluents. Other factors which
depend on chlorine disinfectants include acidity and alkalinity (pH), impact time,
effluent temperature and lastly, organic contents.
8 Other Wastewater Treatment Techniques
Commonly available high rate processes include rotating biological contactors
(RBC), activated sludge processes, oxidation ditches and trickling filters otherwise
referred to as biofilters. Combining two processes in the serial order for instance; activated sludge subsequent to biofilters is usually utilized to treat community wastewater
containing highly concentrated organic matters, which are of industrial origin.
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(d) Tertiary and/or advanced treatment (TWWT)
This is the last stage of wastewater treatment. The TWWT is employed specifically
to remove pollutants or contaminants which escaped during secondary treatment.
Dissolved solids, heavy metals, refractory organics, suspended solids, phosphorus
and nitrogen are necessary to be removed by specific treatment procedures. Advanced
treatments are also called tertiary treatments as they are often preceded by high-rate
secondary treatment. Tertiary treatment processes, however, are at times coupled
with primary or secondary treatments (e.g., the addition of chemicals to primary
clarifiers or aeration tanks for phosphorus removal) or used to substitute secondary
treatment (e.g., primary effluent overland flow treatment).
Nitrogen content present in influents that is usually in their ammonia form is unaffected as they pass through the first two treatment phases. Near-complete nitrification
of the nitrogenous component occurs in the third aerobic zone, converting them to
nitrites and nitrates sequentially. The mixed liquor rich in nitrate generated from the
aerobic zone is then recycled back to the initial anoxic zone. Nitrates recycled are
denitrified anaerobically and nitrogen gas is generated (which completely escapes
to the atmosphere) by facultative bacteria, which use influent organics hydrogen
donors. Unrecycled nitrates are reduced by endogenous bacteria respiration in the
second anoxic zone. Dissolved oxygen levels are raised again in the final re-aeration
zone to avert subsequent denitrification that may affect settling in the secondary clarifier. To further prevent denitrification, the level of dissolved oxygen is raised in the
final re-aeration zone; this would impair settling in the secondary clarifiers.
(e) Disinfection
The disinfectant usually employed for wastewater disinfection is chlorine. It is
injected in form of a solution into the wastewater at the chlorine dosage tank. The
tank is automated and discharges chlorine at a specific time interval. The dosage
of chlorine to be used is dependent on wastewater strength and other factors, but
common dosage ranges are between 5 and 20 mg/l. Other disinfectants, though not
commonly used are ultraviolet (UV) irradiation and ozone. The chlorine dosage tanks
are often automated rectangular channels with designs providing a contact time of
about 25–30 min. However, the standard chlorine contact time of 1 h or more may
be needed for target irrigation purposes of reclaimed effluents. Other factors which
depend on chlorine disinfectants include acidity and alkalinity (pH), impact time,
effluent temperature and lastly, organic contents.
8 Other Wastewater Treatment Techniques
Commonly available high rate processes include rotating biological contactors
(RBC), activated sludge processes, oxidation ditches and trickling filters otherwise
referred to as biofilters. Combining two processes in the serial order for instance; activated sludge subsequent to biofilters is usually utilized to treat community wastewater
containing highly concentrated organic matters, which are of industrial origin.