4 Nutrient Recovery: Phosphorous and Nitrogen
One of the several challenges faced during wastewater treatment is to comply with
the discharge limits of phosphorus and nitrogen containing species, namely phosphate, ammonia, nitrites, and nitrates. If from one side discharging effluents with
high amount of nitrogen and phosphorous to natural water courses will cause
detrimental environmental problems, as eutrophication, from the other side, phosphorous sources are becoming depleted [58]. The increasing need of phosphorous
fertilizers in agriculture to support the food demand of a growing population makes
phosphorous a vital element and its recovery from waste streams, imperative
[58, 59].
Nutrient removal and recovery from wastewater treatment can be achieved using
combinations of physical, chemical, and/or biological processes, as extensively
reviewed by [36, 60]. The process choice is naturally dependent on the characteristics of the wastewater, but typically, nutrient recovery is performed during
secondary and tertiary treatment, with this latest stage, being used to polish nutrient
concentrations down to the regulatory standards [61]. Chemical processes can occur
through the addition of, e.g., iron to promote nitrates denitrification to ammonia, or
magnesium, which co-precipitates with phosphates and ammonia producing struvite, a mineral fertilizer. It must be considered however the cost of the added
chemicals and possible water contamination by side-reaction products or the
chemical itself, if added in excess [60, 62]. Physical processes, such as adsorption
or membrane filtration, do not require chemicals addition and a wide range of
commercial products is available for effective nutrient recovery. However, rapid
material saturation and degradation due to poor mechanical stability can lead to
high investments and maintenance costs [60, 62].
Biological processes have long been studied and widely applied for wastewater
treatment due to their simple and economical operation with efficient removal of
biodegradable mater and nutrients [37, 60]. The biological nutrient removal occurs
in a combination of dissipative and/or assimilative processes. In conventional
biological treatment, ammonia removal is accomplished through assimilation into
the biomass, and through dissipation in nitrification/denitrification processes (which
occur in conventional activated sludge and microalgae-bacterial systems) or
deammonification (in anammox systems) [30, 36]. In the dissipative cases, reactive
nitrogen is removed from the liquid through conversion into gaseous nitrogen
species, such as N 2 O and N 2 [36]. Hence, the only recovered nitrogen is the fraction
that becomes assimilated into the biomass, which can then be directed to further
downstream applications [30, 36, 37, 61]. Increasing nitrogen recovery is one of the
current challenges in WWTP, being required further technological developments to
minimize nitrogen losses to the atmosphere.
In the case of phosphorous, its biological removal from wastewaters occurs
mainly through assimilation into the biomass and internal accumulation as
polyphosphate [58]. When polyphosphate (Poly-P) storage occurs without a previous phosphorous starvation, it is called as luxury uptake [47] and this feature is
New Phototrophic Factories for Resource Recovery
423
One of the several challenges faced during wastewater treatment is to comply with
the discharge limits of phosphorus and nitrogen containing species, namely phosphate, ammonia, nitrites, and nitrates. If from one side discharging effluents with
high amount of nitrogen and phosphorous to natural water courses will cause
detrimental environmental problems, as eutrophication, from the other side, phosphorous sources are becoming depleted [58]. The increasing need of phosphorous
fertilizers in agriculture to support the food demand of a growing population makes
phosphorous a vital element and its recovery from waste streams, imperative
[58, 59].
Nutrient removal and recovery from wastewater treatment can be achieved using
combinations of physical, chemical, and/or biological processes, as extensively
reviewed by [36, 60]. The process choice is naturally dependent on the characteristics of the wastewater, but typically, nutrient recovery is performed during
secondary and tertiary treatment, with this latest stage, being used to polish nutrient
concentrations down to the regulatory standards [61]. Chemical processes can occur
through the addition of, e.g., iron to promote nitrates denitrification to ammonia, or
magnesium, which co-precipitates with phosphates and ammonia producing struvite, a mineral fertilizer. It must be considered however the cost of the added
chemicals and possible water contamination by side-reaction products or the
chemical itself, if added in excess [60, 62]. Physical processes, such as adsorption
or membrane filtration, do not require chemicals addition and a wide range of
commercial products is available for effective nutrient recovery. However, rapid
material saturation and degradation due to poor mechanical stability can lead to
high investments and maintenance costs [60, 62].
Biological processes have long been studied and widely applied for wastewater
treatment due to their simple and economical operation with efficient removal of
biodegradable mater and nutrients [37, 60]. The biological nutrient removal occurs
in a combination of dissipative and/or assimilative processes. In conventional
biological treatment, ammonia removal is accomplished through assimilation into
the biomass, and through dissipation in nitrification/denitrification processes (which
occur in conventional activated sludge and microalgae-bacterial systems) or
deammonification (in anammox systems) [30, 36]. In the dissipative cases, reactive
nitrogen is removed from the liquid through conversion into gaseous nitrogen
species, such as N 2 O and N 2 [36]. Hence, the only recovered nitrogen is the fraction
that becomes assimilated into the biomass, which can then be directed to further
downstream applications [30, 36, 37, 61]. Increasing nitrogen recovery is one of the
current challenges in WWTP, being required further technological developments to
minimize nitrogen losses to the atmosphere.
In the case of phosphorous, its biological removal from wastewaters occurs
mainly through assimilation into the biomass and internal accumulation as
polyphosphate [58]. When polyphosphate (Poly-P) storage occurs without a previous phosphorous starvation, it is called as luxury uptake [47] and this feature is
New Phototrophic Factories for Resource Recovery
423