New Approaches for Phosphorus Approval

Current Phosphorus Removal Approaches in Conventional Treatment Involve Either Modifying Activated Sludge for Biological Nutrient Removal (BNR), by Precipitation with Metal Coagulants or a Combination of Both Processes

Both are successful at meeting current consents, but become less attractive when meeting lower target levels due to concerns over robustness of performance and excessive use of chemicals and energy. In this context, alternative approaches for phosphorus removal from wastewater effluents have to be explored.

However, it is important to look beyond just treatment performance and consider other factors as part of a broader examination of the technologies to understand the true value of emerging innovative approaches.
Such thinking sits within the broader discussion concerning the future of wastewater management and the potential paradigm shift from treatment plants to resource recovery factories. To illustrate, currently available tertiary treatment technologies for phosphorus removal to very low levels are generally based on coagulation: either ballasted coagulation, or coagulation followed by filtration in a sand filter, membrane or cloth filter. Consequently, these technologies are heavily dependent on the use of chemicals, their price and the associated costs for sludge treatment and disposal which does not make them economically attractive. In addition, these processes provide no easy route for resource recovery.

However, innovative approaches currently undergoing development at Cranfield University offer alternative ways to meet the very low phosphorus discharge consents as well as opportunities for resource recovery providing a more sustainable alternative.

Technology 1: Iron nanoparticles

The first technology is a physical process where phosphorus removal from wastewater is achieved by adsorption on iron nanoparticles embedded in an ion exchange resin (a resin currently used commercially for arsenic removal). The technology has been shown to remove phosphorus to extremely low levels (below 0.05 mgP/L) even at very short contact times (as low as 1 minute) allowing for small footprint units.

As with all contact processes, the media requires regeneration once its capacity to remove phosphorus has been exhausted. Regeneration is achieved through simple pH swing to electrostatically dislodge the phosphorus enabling selective recovery through generation of a phosphorus rich liquor. The regeneration liquor can be reused without impairment to the regeneration processes enabling highly concentrated phosphorus streams to be generated over multiple cycles. At preselected intervals the regeneration liquor is processed to remove the retained phosphorus through precipitation with calcium. This enables both recovery of the phosphorus and reuse of the regeneration liquid. The latter defines the overall total cost of the processes transforming the economic suitability of the approach and provides a route for nutrient recovery.

Technology 2: Algae systems

The second alternative is a biological process using algae for nutrients removal. The uptake of algae systems for nutrients removal from wastewater as either open ponds or photo bioreactors has been hindered by the costly downstream harvesting system and extended retention times (days) required (Ometto et al., 2014). Immobilised algae in alginate beads offer a more viable approach as harvesting is facilitated and high rate reactors with shorter contact times (6-20 hours) are used. Trials of the immobilised algae system demonstrated the full potential of the technology with near complete removal (below 0.1 mg/L) of not only phosphorus but also ammonium and nitrate (Whitton et al., 2014). In addition, similarly to BNR systems, algae have the added benefit of resource recovery with production of biogas from anaerobic digestion of the harvested biomass offering potential for energy neutral operation.

Technology 3: Reed beds with reactive media

These two novel technologies show great potential for integration in wastewater treatment trains; however, at this stage of development they require a level of infrastructure inappropriate for very small STWs where the low tech/low maintenance approach is prevalent. In this case other alternatives may be considered such as using reactive media in reed beds. Reed beds, known to be easy to implement and cheap to operate, are often used on small size STWs. These systems can then be optimised by replacing the bed media with reactive media, achieving phosphorus removal with minimal investment costs. For this, steel slag, a calcium rich waste by-product from the steel industry has been shown to be efficient at removing phosphorus.

Both are successful at meeting current consents, but become less attractive when meeting lower target levels due to concerns over robustness of performance and excessive use of chemicals and energy. In this context, alternative approaches for phosphorus removal from wastewater effluents have to be explored.

However, it is important to look beyond just treatment performance and consider other factors as part of a broader examination of the technologies to understand the true value of emerging innovative approaches.
Such thinking sits within the broader discussion concerning the future of wastewater management and the potential paradigm shift from treatment plants to resource recovery factories. To illustrate, currently available tertiary treatment technologies for phosphorus removal to very low levels are generally based on coagulation: either ballasted coagulation, or coagulation followed by filtration in a sand filter, membrane or cloth filter. Consequently, these technologies are heavily dependent on the use of chemicals, their price and the associated costs for sludge treatment and disposal which does not make them economically attractive. In addition, these processes provide no easy route for resource recovery.

However, innovative approaches currently undergoing development at Cranfield University offer alternative ways to meet the very low phosphorus discharge consents as well as opportunities for resource recovery providing a more sustainable alternative.

Technology 1: Iron nanoparticles

The first technology is a physical process where phosphorus removal from wastewater is achieved by adsorption on iron nanoparticles embedded in an ion exchange resin (a resin currently used commercially for arsenic removal). The technology has been shown to remove phosphorus to extremely low levels (below 0.05 mgP/L) even at very short contact times (as low as 1 minute) allowing for small footprint units.

As with all contact processes, the media requires regeneration once its capacity to remove phosphorus has been exhausted. Regeneration is achieved through simple pH swing to electrostatically dislodge the phosphorus enabling selective recovery through generation of a phosphorus rich liquor. The regeneration liquor can be reused without impairment to the regeneration processes enabling highly concentrated phosphorus streams to be generated over multiple cycles. At preselected intervals the regeneration liquor is processed to remove the retained phosphorus through precipitation with calcium. This enables both recovery of the phosphorus and reuse of the regeneration liquid. The latter defines the overall total cost of the processes transforming the economic suitability of the approach and provides a route for nutrient recovery.

Technology 2: Algae systems

The second alternative is a biological process using algae for nutrients removal. The uptake of algae systems for nutrients removal from wastewater as either open ponds or photo bioreactors has been hindered by the costly downstream harvesting system and extended retention times (days) required (Ometto et al., 2014). Immobilised algae in alginate beads offer a more viable approach as harvesting is facilitated and high rate reactors with shorter contact times (6-20 hours) are used. Trials of the immobilised algae system demonstrated the full potential of the technology with near complete removal (below 0.1 mg/L) of not only phosphorus but also ammonium and nitrate (Whitton et al., 2014). In addition, similarly to BNR systems, algae have the added benefit of resource recovery with production of biogas from anaerobic digestion of the harvested biomass offering potential for energy neutral operation.

Technology 3: Reed beds with reactive media

These two novel technologies show great potential for integration in wastewater treatment trains; however, at this stage of development they require a level of infrastructure inappropriate for very small STWs where the low tech/low maintenance approach is prevalent. In this case other alternatives may be considered such as using reactive media in reed beds. Reed beds, known to be easy to implement and cheap to operate, are often used on small size STWs. These systems can then be optimised by replacing the bed media with reactive media, achieving phosphorus removal with minimal investment costs. For this, steel slag, a calcium rich waste by-product from the steel industry has been shown to be efficient at removing phosphorus.

Source: WWT

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