UK Construction Online’s Matt Brown speaks with Dr Ana Lanham, Lecturer in Water Science and Engineering at the University of Bath, to discuss phosphorus removal technologies and the AMP6 programme.
Could you tell me about how phosphorus ends up in water and why its levels in needs to be controlled?
Phosphorus ends up in the water bodies a consequence of agriculture run-off (from the addition of fertilisers containing phosphorus) and from concentrated urban wastewater.
Phosphorus is a natural and essential element to all living forms as it is a major constituent of DNA and many other components in living cells. This is also why phosphorus is one of the key elements in fertiliser, along with nitrogen and potassium, as it promotes plant growth. In its geochemical natural cycle, phosphorus originates from phosphorite deposits, also known as phosphate rock, through weathering. It is then taken up by all forms of living organisms and released back into the environment through animal waste and plant decay. With the green revolution, this natural cycle has been severely enhanced with the man-made exploitation of these phosphate rock reserves to apply in agriculture and the chemical industries. This, together with a greater urbanisation, has resulted in increased amounts of phosphorus in soils and water bodies.
Phosphorus in itself, unlike other pollutants, is not toxic, on the contrary it is beneficial. However, if too much is available in water courses and marine environments then aquatic plants and organisms such as algae or specific bacteria called cyano-bacteria, will grow too much into what is known as algae blooms or eutrophication. This phenomena is utterly destructive to the ecosystems as this excessive growth depletes all the oxygen in the water body and prevents sunlight from penetrating into the water, reducing photosynthesis and literally suffocating all aquatic life.
One of the means of preventing eutrophication is to control the level of phosphorus in treated wastewater discharged into the environment. The recommended discharge levels depend on the recipient environment, e.g. if the treated wastewater is discharged into a small watercourse with low dilution, then phosphorus discharge levels might be lower than if the water is discharged into a high flow, high dilution stream. The other way to reduce eutrophication is to influence and inform farmers to adopt more informed or sophisticated practices for the use of fertilisers to minimise run-off.
Finally, it’s not just about controlling the levels of phosphorus in the water: by encouraging lower discharges from wastewater we are promoting the opportunity to recover an essential resource that is finite and rapidly being used up.
AMP6 has seen a focus on innovation in phosphorus removal, could you tell me about some of the new technologies out there?
Indeed, a majority of UK’s water utilities have joined efforts with the UK Water Industry Research (UKWIR) to look into the best options to meet the increasingly stricter requirements of the Water Framework Directive.
Unfortunately most of the P removal in the UK is achieved by chemical dosing using iron salts. This option is usually preferred because it’s safe and predictable, however it does result in a number of other issues for utilities such as continuous operational costs, increased sludge production and it will be difficult to continue to use it due to accrued costs if P discharge limits become stricter.
Therefore there are enormous opportunities for the increased adoption of already established technologies such as biological nutrient removal (BNR). The processes that govern these technologies, although still complex, are now quite mature. Therefore they can be used to efficiently remove most of the phosphorus in the wastewater. Not only that but the phosphorus is now concentrated in the biomass and the sludge can then be used as fertiliser or be processed to release and recover phosphorus in a purer form such as struvite crystals before undergoing anaerobic digestion. There are a number of established forms of this technology that can be easily retrofitted into the currently existing activated sludge plants, sometimes just by modifying the operational parameters, and iron dosing can still be kept as an SOS treatment. However there are also innovative variations such as the Nereda® system for aerobic granular sludge that is starting to be implemented in the UK.
Whilst chemical dosing and biological processes would remove 80-90% of P loads, there could be a need for polishing or tertiary treatment technologies around a combination of more or less established physical-chemical processes such as filtration, coagulation or adsorption such as the Blue PRO®, Comag® or Hydrok-Mecana® technologies currently under trial. Biological technologies including modified versions of bio-filters and algae ponds or passive and low carbon technologies with lagoons or constructed wetlands can also be used.
The principles for all of these technologies are mostly established, the innovation comes from the way they are combined, operated and monitored and their modularity or land footprint.
What are the advantages and drawbacks of these technologies?
The advantage of BNR processes is the reliance on the unique capacities of microorganisms to undertake the treatment of water. This is typically a cheaper, more efficient and more sustainable process than chemical processes. However, it is energy intensive, due to aeration and mixing costs and it does require some investment in skilled operators. The process, like activated sludge is also more sensitive to variations in wastewater composition and to environmental conditions and if run poorly might lead to greenhouse gas emissions.
The different variants of filtration, coagulation and adsorption technologies are currently emerging and offer very good prospects in terms of tertiary treatment. They would be able to offer a reliable P removal to low concentrations of P. However, they would imply an added cost to utilities to install them in addition to other treatments and in most cases they do not allow the recovery of phosphorus.
Finally the algae processes have shown promising results although there are still some limitations in terms of reactor design, growth and P removal yields and clarification. Low carbon or passive systems are quite well-known and some innovative features such as mechanical aeration or new design configurations have shown it is possible to remove nutrients whilst reducing some of the land footprint. They are also getting a new surge of interest due to low maintenance and low carbon.
Is chemical dosing a sustainable solution to lowering P levels?
The biggest criticism with chemical P dosing is that you are treating wastewater by adding another chemical compound. This of course results in added costs and in added environmental pressures from sourcing these materials to their fate in the treatment process. The iron or aluminium dosing is an unspecific process that precipitates with the phosphorus compounds but also to other compounds such as sulphides or even organic matter and then ends up in the sludge. These metal salts can cause a problem in themselves as they might interfere with further biological treatment of the sludge (i.e. anaerobic digestion) or with the legal limits in the application of sludge to land as fertiliser. On another note, once the iron and aluminium phosphates are formed it is much harder to recover the phosphorus.
In my view, chemical dosing is a fantastic option as an SOS strategy, to complement the potential fluctuations of biological processes. However in the long term it is a recurring cost, an unnecessary use of resources and a difficult to process to scale up if increasingly low discharge limits are to be met.
There have been calls for phosphorus to be recovered rather than just removed – is this a completely different technology?
Yes there is worldwide recognition that phosphorus is a depleting finite resource. To ensure the protection of our food production systems and chemical industries, recovering phosphorus from our wastewaters is an obvious solution where approximately 10-20% of the mined phosphorus could be recovered.
We are already recovering phosphorus through the addition of sludge to land. This is a practice that is still permitted in the UK and that allows the non-specific recovery of many essential minerals and nutrients. However this has also many drawbacks such as the potential spread of pathogens and concentration of heavy metals, as well as the fact that iron or aluminium bound phosphates may not be in a bioavailable form. Hence other countries in the EU have developed different regulations that ban the application of sludge to land, stimulating the need for other forms of recovery of phosphorus and nutrients in a more purified form. This is the case of struvite (magnesium ammonium phosphate) and hydroxyapatite (calcium phosphate).
Whilst the technologies for phosphorus removal are based on separating the phosphorus from the liquid phase into a solid phase (either biologically into the sludge or through chemical precipitation and adsorption mechanisms), the phosphorus recovery technologies imply that we then take the liquid or solid “stream” where phosphorus has been accumulated, release it into a liquid form and then precipitate it in a purified crystal. The most straightforward way to do this and less energy intensive is using the sludge of a BNR process, prior to anaerobic digestion, such as in Ostara’s WASSTRIP® and then PEARL® process, resolving many of the struvite issues in pipes and pumps. You can also recover phosphorus from incineration ash using thermochemical methods combining high pressures and temperature to convert the phosphorus in the ash to more bioavailable forms, leach the phosphorus and then recover it using precipitation or ion-exchange technologies.
Is the low P level consents wanted by water companies achievable within the AMP6 timeframe?
I would believe that it would be achievable for the most part. Many of these technologies or the knowledge we have over the processes is quite mature and the water utilities seem to be moving towards a greater interest and investment in this area. It might be that in the short term, technologies using chemical dosing will still prevail, but would be replaced or complemented with main-stream biological processes or with polishing biological/physicochemical processes. The industry does need to make some significant investments in making this possible and in my view should have a longer term perspective, where even if they are now not recovering phosphorus, align their investments with the possibility of doing this in the near future.
What can be done to speed up the development of these technologies?
I think the already existing collaboration between several water utilities and UKWIR are a great step forward in promoting potential technologies and in investigating and comparing potential options already available in the market and testing them at pilot scale.
The other effort should come from the regulatory agencies, to encourage a longer term planning for utilities – where they can act on the double front of removing the phosphorus to preserve the good ecological status, but also on promoting resource recovery of a potentially vital resource to ensure food security.
Finally, we are achieving significant progress in modelling full wastewater treatment plants and the interactions between utilities and academia by exchanging monitoring data to improve these models further will play a significant role in screening the best technologies for a particular situation.
What difference could incentivising farmers to reduce their discharges of nitrogen make to P levels?
This is a very important approach at catchment level management that the UK has been focussing on and that can indeed play a huge role in reducing nutrient levels overall in water bodies. Nitrogen loads are intrinsically tied to phosphorus loads and so a reduction of either one affects the other. Wastewater treatment can only go so far and it’s a great opportunity to recover phosphorus from a concentrated source, but it only contains maximum 20% of the phosphorus that we source, the rest is retained in the soil or incorporated into living organisms and ends up in agricultural run-off. Therefore finding strategies and working with farmers, as the example of Wessex’s Water EnTrade nutrient trading platform is a very good practice that should be encouraged.
Is the government taking enough of a lead on these issues?
There is a slow change across the world to move away from “wastewater treatment” to resource recovery. Wastewater treatment plants are a safeguard to the environment but they are also a mine of resources that can potentially be used. I am not aware at the moment of any particular incentives from the government other than ensuring the compliance to the Water Framework Directive in terms of its phosphorus discharge limits. I believe there is a recognition that this might be an issue for food security and the government should incentivise the adoption of longer term thinking for water utilities, including reviewing the regulations on waste materials to enable an easy commercial uptake of recycled phosphorus
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