104
4 Nitrogen and Phosphorous Recovery from Municipal Wastewater …
4.3 Nutrient Removal/Recovery from Wastewater
Wastewater contains high levels of nutrients such as nitrogen and phosphorous. As
already mentioned previously, release of high levels of nutrients to the environment
leads to eutrophication. Thus, the nutrients should be removed or recovered from the
water before it is discharged to the environment.
Nutrient recovery from wastewater is commonly a three-step process: (1)
nutrient accumulation by precipitation and biological nutrient removal, (2) nutrient
release and stabilization and (3) nutrient extraction [14]. Nutrient accumulation methods include chemical accumulation via precipitation, adsorption/ionexchange, algae accumulation, membrane filtration, etc. Nutrient release methods
include biological releases such as anaerobic digestion, thermochemical/chemical
release, such as hydrolysis, wet oxidation, incineration, gasification, and pyrolysis,
and bioleaching/extraction. Finally, the nutrient extraction and recovery technologies include chemical precipitation/crystallization, gas-permeable membrane and
absorption, liquid–gas stripping, electrodialysis, etc. [15].
Nitrogen is mostly removed through Biological Nutrient Removal (BNR), while
phosphorous can be removed by adsorption, chemical and biological methods.
Biological nutrient removal depends on the readily biodegradable chemical oxygen
demand (rbCOD) as the carbon source in the influent. In some plants, external carbon
sources such as methanol and sugar are added, but at an increased operational cost
[16].
4.3.1 Nitrogen Removal/Recovery from Wastewater
Total nitrogen in wastewater is present in the form of nitrate, ammonia, particulate
organic nitrogen and soluble organic nitrogen [17]. It should be noted that organic
nitrogen in wastewater is unable to be removed biologically and only the particulate
fraction can be removed through solids separation using filtration or sedimentation
[19]. Ammonia is toxic to the aquatic life as it results in increased oxygen demand
in receiving water when it is converted to nitrates, leading to the fast growth of
algae and eutrophication of the receiving water. The biological process for nitrates
removal involves nitrification–denitrification, which is a two-step process. In the
first step (nitrification) in an aerobic environment, ammonia nitrogen is converted to
nitrates (NO
−
3 ) and in the second step (denitrification) under anoxic conditions, the
nitrates are converted to NO 2 or nitrogen gas that escapes into the air.
4 Nitrogen and Phosphorous Recovery from Municipal Wastewater …
4.3 Nutrient Removal/Recovery from Wastewater
Wastewater contains high levels of nutrients such as nitrogen and phosphorous. As
already mentioned previously, release of high levels of nutrients to the environment
leads to eutrophication. Thus, the nutrients should be removed or recovered from the
water before it is discharged to the environment.
Nutrient recovery from wastewater is commonly a three-step process: (1)
nutrient accumulation by precipitation and biological nutrient removal, (2) nutrient
release and stabilization and (3) nutrient extraction [14]. Nutrient accumulation methods include chemical accumulation via precipitation, adsorption/ionexchange, algae accumulation, membrane filtration, etc. Nutrient release methods
include biological releases such as anaerobic digestion, thermochemical/chemical
release, such as hydrolysis, wet oxidation, incineration, gasification, and pyrolysis,
and bioleaching/extraction. Finally, the nutrient extraction and recovery technologies include chemical precipitation/crystallization, gas-permeable membrane and
absorption, liquid–gas stripping, electrodialysis, etc. [15].
Nitrogen is mostly removed through Biological Nutrient Removal (BNR), while
phosphorous can be removed by adsorption, chemical and biological methods.
Biological nutrient removal depends on the readily biodegradable chemical oxygen
demand (rbCOD) as the carbon source in the influent. In some plants, external carbon
sources such as methanol and sugar are added, but at an increased operational cost
[16].
4.3.1 Nitrogen Removal/Recovery from Wastewater
Total nitrogen in wastewater is present in the form of nitrate, ammonia, particulate
organic nitrogen and soluble organic nitrogen [17]. It should be noted that organic
nitrogen in wastewater is unable to be removed biologically and only the particulate
fraction can be removed through solids separation using filtration or sedimentation
[19]. Ammonia is toxic to the aquatic life as it results in increased oxygen demand
in receiving water when it is converted to nitrates, leading to the fast growth of
algae and eutrophication of the receiving water. The biological process for nitrates
removal involves nitrification–denitrification, which is a two-step process. In the
first step (nitrification) in an aerobic environment, ammonia nitrogen is converted to
nitrates (NO
−
3 ) and in the second step (denitrification) under anoxic conditions, the
nitrates are converted to NO 2 or nitrogen gas that escapes into the air.