11Th European Waste Water Conference Providing Phosphorus Removal for Rural Treatment Works

11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK

11TH EUROPEAN WASTE WATER CONFERENCE PROVIDING PHOSPHORUS REMOVAL FOR RURAL TREATMENT WORKS

Bowman, B.1 and Aboobakar, A.1 1United Utilities, UK Corresponding Author Email [email protected]

Abstract

There is an ongoing phosphorus challenge to achieve “good ecological status” in waterbodies across North West England. The size range of sites required to meet tight regulatory permits is increasing, in some cases with phosphorus targets below 0.5mg/l. In recent years, much of the focus has been on technological developments appropriate for large works, leaving a knowledge gap in how phosphorus removal can be achieved sustainably in small, rural works.

For works with population equivalents under 1000, there are added complications of limited workforce, power supply and access routes. Conventional approaches of chemical or biological nutrient removal may not be appropriate for these situations and alternatives are not readily available. Utilising the concepts of a circular economy we aim to deliver low maintenance, robust, sustainable phosphorus removal for small rural works. The role of reactive media for phosphorus removal and regeneration or reutilisation routes may be critical to delivering this goal.

Keywords

Phosphorus; Reactive media; Rural; Sustainable; WFD; WwTW

Introduction

The Water Framework Directive (WFD) has set an objective for wastewater treatment to achieve phosphorus (P) removal for the prevention of eutrophication in waterbodies. Modelling suggests that in order to meet ‘good’ ecological status in river catchments within the United Utilities region there will be a requirement to provide P removal at a large number of small treatment facilities (<5000PE). In addition, the permit requirements are becoming tighter, in some cases below 0.5mg/l.

Conventional P removal at small treatment facilities has typically been achieved through chemical processes. These processes contribute to significant operating costs and carbon emissions as well as increased customer impact due to chemical deliveries. This leads to a disproportional cost/benefit for the intervention. Chemical P removal operates through the generation of ferric or aluminium phosphate salts; these are largely unreactive precipitates from which P recovery is not viable. In the interest of future resilience this route does not meet our strategic vision.

Developments in technology to reach total P permit conditions less than 1mg/l have exploited economies of scale and intensive processes suitable for large, urbanised treatment facilities. These have limited suitability to rural catchments where a combination of catchment and end-of-pipe solutions has been shown to be the best approach to driving overall water quality improvements. A new type of approach is needed to suit the requirements for rural catchments which could also be an enabler to the recovery of phosphorus.

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK

The Integrated Catchment approach

The basis of the Integrated Catchment (IC) approach is to consider a river as a complete system; incorporating the water and wastewater catchments, full range of river users, point discharges and disperse pollution. This systems thinking model has been applied to the River Petteril in Cumbria to form a case study to roll out the approach across the region.

The River Petteril is a tributary of the much larger River Eden (which is a designated Site of Special Scientific Interest), located in Cumbria, in the Solway-Tweed River Basin area of the North West Region. The length of the river flows from Penrith in the south up to Carlisle in the north (confluence with the River Eden), parallel to the M6 motorway. For much of the length of the river, the Petteril catchment is rural, consisting mostly of agricultural activities, with Carlisle being the urban area of the catchment.

The current WFD ecological classifications of the Petteril and its tributaries range between “good” ecological status in the southern parts of the catchment and “moderate” for most of the river up to Carlisle with one of the tributaries, Blackrack Beck, currently classified as “poor” (Figure 1).

Figure 1: Map of the River Petteril catchment, with WwTWs, ecological classifications and designated drinking water nitrate vulnerable zones

Within the Petteril catchment United Utilities has 10 small WwTWs which serve a combined population equivalent of just over 2000. Four of these WwTWs were identified in the National Environment Programme (NEP5) as requiring a phosphorus permit in order to meet our “fair share” reduction towards achieving good ecological status in water quality by 2027. The cost to achieve these permits though

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK conventional means was disproportionate making this a good candidate for demonstrating the Integrated Catchment approach.

Figure 2: Applying the integrated catchment principles to develop an alternative plan for the Petteril catchment

The IC approach is summarised in Figure 2; in essence this is an innovative approach which aims to provide value for money for customers and targets interventions to the ‘right’ areas within a catchment. The approach involves:

• Collaboration and partnership working with customers and stakeholders within the catchment • Evidence gathering to build a better understanding of the inputs to the river allowing modelling analysis and scenario planning • Investigation and application of catchment interventions including those with additional benefits such as that mitigation of flooding • Asset solutions that are tailor made to small works.

Catchment sampling

Limited information was available regarding the sources of phosphorus within the catchment; particularly the contributions from diffuse pollution within the tributaries and discharges from our WwTWs. This drove us to develop a programme of investigations, sampling and monitoring key parameters such as: phosphorus, nitrates, ammonia, BOD, solids and flow across the catchment.

Our newly gathered evidence allowed us to:

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK

• Run and compare different scenarios (using the EA’s SIMCAT model) to achieve water quality improvements. • Identify key issues and risks at catchment level, and to target and prioritise solutions accordingly. • Engage with the EA to understand the differences between their cost benefit assessment and our required capital investment. • Influence the EA’s technical review of the Water Industry National Environment Programme (WINEP) and to agree the right permitting requirements for our WwWTs in the Petteril catchment.

For United Utilities this evidence-based approach set the precedent for using smart catchment strategic principles in decision-making, engaging with key stakeholders and driving the most affordable solutions for customers and the best outcomes for the environment.

The result for the Petteril catchment was a revision of permits to more accurately reflect the contributions throughout the river system and a flexible approach to integrate catchment level interventions with treatment works solutions to provide a greater overall benefit to river quality. To enable this approach a new way to approaching P removal for small, rural WwTW was required.

Reactive media for phosphorus removal Reactive media

In recent years a number of studies (Nilsson, et al., 2013) (Vohla, et al., 2011) (Molle, et al., 2005) have investigated the possibility of removing P from wastewater using materials with P-retentive characteristics; these materials are termed reactive media. There are a wide variety of reactive media products on the market, each of which appear to operate in subtlety different ways; however the main reaction pathway involves the generation of a phosphate precipitate utilising reaction sites on the media surface or through the dissolution of parts of the media to form new phosphorus-containing compounds (Molle, et al., 2005) (Nilsson, et al., 2013) (Vohla, et al., 2011).

These studies led to the theory that reactive media could be used as a tertiary treatment stage on small, rural treatment facilities in order to meet the required P targets. The technology aligns with existing treatment processes on these sites; it is not energy intensive, nor does it require frequent chemical deliveries or intervention to optimise performance.

As time passes, the media sorption capacity reduces to the point that treatment is not being achieved and the media needs to be replaced (Vohla, et al., 2011). The point at which this occurs is dependent on the reactive media in use and the volume of media installed for a given flow rate, the influence of phosphorus concentration is dependent on the reaction mechanism within the media. Spent media is rich in slow release, bioavailable phosphorus which is considered of value to agriculture in countries where this technology is already in use (Vohla, et al., 2011).

Pilot scale investigations

Design

Laboratory scale investigations were used to provide an indication of the possibilities of a range of reactive media materials. However, these tests are not able to mimic real wastewater complexity as well as flow and quality variations. For this reason, two commercial reactive media materials: Polonite and Phosclean (a formulation of apatite) were selected to take forward to pilot scale on a small, rural wastewater treatment facility. An additional media was selected to trial at pilot scale due to promising

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK laboratory results. This media is still in development and details are confidential, the media will be referred to as Media C for the rest of this paper.

The site chosen for the trials; Calthwaite WwTW has a population equivalent (PE) of 222, is situated in Cumbria within the River Petteril catchment and has been given an AMP6 phosphorus permit. Calthwaite WwTW has a pumped inlet with existing treatment as depicted in Figure 3. Flows passing through the pilot plant therefore returning the flows upstream of the take-off point was considered to have a negligible impact on concentrations.

Figure 3: Calthwaite WwTW existing treatment processes and pilot trial arrangement

Previous studies have typically seen reactive media used within a constructed wetland system. This has been considered to exacerbate potential issues when removing and replacing the media once it becomes saturated. One potential mitigation for this is to place reactive media into a distinct unit which can be more easily replaced (Vohla, et al., 2011). This is the principle used in the design of the pilot plant.

A sub-section of final effluent has been diverted from the final effluent chamber passes through one of three filter arrangements each containing a different reactive media, collected and returned to the main flow at the outlet of the tertiary SAF. The pilot trial arrangement is depicted in Figure 3. The media for each stream is held within two up-flow filters to ensure the media is continuously saturated with wastewater and improve plug-flow characteristics. Tertiary SAF effluent was sampled at the buffer tank and the outlet from each individual media stream, a spot sample was taken every day for the trial duration. All samples have been analysed on site using test kits with a sub-set of samples being sent to central labs for corroboration of on-site analysis.

Prior to commencement of the trial retention tests were carried out to determine the preferred hydraulic retention times. For the first phase of the trial a continuous flow was maintained across the filters to replicate the hydraulic retention time in average conditions. For the second phase of the trial the flow rate has been varied to more accurately mimic a diurnal profile as well as replicating dry and wet weather conditions to ascertain the impact of varying flow rates on the performance of the media.

Retention tests A batch test was carried out to identify the preferred hydraulic retention time in average conditions. The filter vessel was filled with media, then tertiary SAF effluent, a sample was taken every hour for 24 hours and analysed for total P. This was repeated twice for each media (Figure 4, Figure 5 and Figure 6)

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK

Figure 4: Phosclean retention test results Figure 5: Polonite retention test results

Figure 6: Media C retention test results

The retention tests showed that in all cases a target of <1mg/l total P was achieved in less than 24 hours. This was then used as the basis for the pilot trial flow rates.

Phosclean results

Effluent has been treated by Phosclean for 27 days: in this time the media has not performed as expected (Table 1: Phosclean performance under continuous fixed flow conditionsTable 1). Various changes have been made to improve performance, however none have been successful to date. Investigations are on-going to understand why the performance of this reactive media is not reflecting lab scale tests or batch test results which have also been carried out with tertiary SAF effluent from Calthwaite WwTW. The media is known to still be under development by the supplier and it is thought that this may be a cause of poor performance.

Table 1: Phosclean performance under continuous fixed flow conditions

Sample Total P (mg/l) Soluble reactive P (mg/l) average range average range Reactive media influent 4.95 1.94 – 6.83 4.72 1.97 – 6.63 Reactive media effluent 4.24 2.28 – 6.57 4.10 2.15 – 6.14

Polonite: Continuous fixed flow rate

A filter containing Polonite reactive media has been in operation for 39 days with a continuous fixed flow rate simulating average conditions. In this time the tertiary SAF effluent total phosphorus concentration has ranged from 2.15mg/l to 8.99mg/l, of this the soluble reactive phosphorus fraction

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK represents almost the entirety of total phosphorus present. Effluent following treatment showed a decrease in soluble reactive phosphorus (SRP) to 0.71mg/l (average) whilst the average total phosphorus concentration decreased to 1.93mg/l (Table 2 and Figure 7). The disparity between total P and SRP is believed to be due to the release of calcium phosphate precipitate which could be captured by a final solids capture process which also aligns with the vision of sustainable wastewater treatment. The efficacy of this process is currently under investigation, it is expected that with solids capture the treated effluent P concentration will be somewhere between the total P and SRP concentrations presented here.

Table 2: Polonite performance with continuous fixed flow conditions

Sample Total P (mg/l) Soluble reactive P (mg/l) average range average range Reactive media influent 5.75 2.15 – 8.99 5.62 1.78 – 8.86 Reactive media effluent 1.93 0.93 – 4.24 0.71 <0.5 – 2.88

Figure 7: Polonite performance under continuous fixed flow conditions

The tertiary SAF effluent data captured as part of the trial has emphasised how variable the concentration of phosphorus passing through the works has been. In mid-July an operational issue occurred with the tertiary SAF which we believe contributed to the very high concentrations observed and the sudden drop between the 21st July and the 23rd July. However, the variability is frequently seen and could be a feature of this particular influent flow. More will be understood on this as the trial continues.

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK

Polonite: Variable flow rate

The feed flow rate has now been modified to replicate a more representative flow pattern including wet and dry conditions. This phase of the trial is still in progress and will continue for a number of weeks. Initial results (Table 3 and Figure 8) have indicated that this has resulted in an improvement in performance, potentially due to the introduction of ‘rest periods’ which the supplier suggests will improve the reactivity of the media.

Table 3: Polonite performance under variable flow rate conditions

Sample Total P (mg/l) Soluble reactive P (mg/l) average range average range Reactive media influent 4.49 1.94 – 6.41 4.21 1.97 – 6.24 Reactive media effluent 1.12 0.53 – 1.64 0.5 <0.5 – 0.64

Figure 8: Polonite performance under variable flow rate conditions

A function of the reactions undertaken by Polonite is a significant increase in pH to alkaline conditions of up to pH11.5. This would require correction prior to release of effluent to the water course; investigations are ongoing into sustainable options for pH correction.

A wide range of parameters have been tested as part of the analysis. Initial indication suggest that concentrations following treatment by Polonite are less than or equal to the concentrations currently present in tertiary SAF effluent.

Media C: Continuous fixed flow rate

A filter containing Media C has been in operation for 36 days with a continuous fixed flow rate simulating average conditions. In this time the tertiary SAF effluent total phosphorus concentration has ranged

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK from 2.15mg/l to 8.99mg/l, of this the soluble reactive phosphorus fraction represents almost the entirety of total phosphorus present. Effluent following treatment showed a decrease in soluble reactive phosphorus (SRP) to 0.48mg/l (average) whilst the average total phosphorus concentration decreased to 0.53mg/l (Table 4 and Figure 9).

Table 4: Media C performance with continuous fixed flow conditions

Sample Total P (mg/l) Soluble reactive P (mg/l) average range average range Reactive media influent 5.85 2.15 – 8.99 5.57 1.78 – 8.86 Reactive media effluent 0.53 <0.50 – 0.97 0.50 <0.50 – 0.94

Figure 9: Media C performance under continuous fixed flow conditions

Media C: Variable flow rate

The feed flow rate has now been modified in the same way as the Polonite filter to replicate a more representative flow pattern including wet and dry conditions. This phase of the trial is still in progress and will continue for a number of weeks. Initial results (Table 5 and Figure 10) have indicated that this has not resulted in a detrimental change in performance.

Table 5: Media C performance under variable flow rate conditions

Sample Total P (mg/l) Soluble reactive P (mg/l) average range average range Reactive media influent 4.49 1.94 – 6.41 4.21 1.97 – 6.24 Reactive media effluent 0.69 0.32 – 1.87 0.58 <0.50 – 1.09

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK

Figure 10: Media C Performance under variable flow rate conditions

Initially using Media C with variable flows resulted in greater variability in the results, this has not continued as the trial has progressed. The cumulative effect of variable flow rates is yet to be determined; more information on this will be collected as the trial progresses.

Effluent treated by Media C has also been analysed for a wide range of parameters Initial indications suggest that concentrations of metals following treatment are less than or equal to the concentrations currently present in tertiary SAF effluent.

Discussion of pilot trial results to date The trial results so far indicate that reactive media could provide effective phosphorus treatment at small, rural treatment works. The laboratory and batch results using Phosclean do not appear to scale up to a continuous flow system. This may be due to the developmental status of the media being investigated or may be due to other reaction characteristics which have not been fully identified. Results using Polonite have been positive; however they also demonstrate the need for tertiary solids capture downstream and the increasing pH associated with this media. Media C has consistently produced an effluent with an average total phosphorus concentration less than 1mg/l. This product is in development and a number of steps need to be taken to ensure that it is fit for purpose prior to rolling out as a phosphorus treatment technology.

The trial is continuing over the next few months, in this time we aim to provide more confidence over treatment capability under variable flow rate conditions and complete the analysis into the likelihood of release of other compounds as a result of using reactive media.

Conclusion

Development of an integrated catchment approach for the River Petteril and investigation into sustainable P removal technologies has led to the following conclusions:

www.ewwmconference.com Organised by Aqua Enviro 11th European Waste Water Management Conference 3rd – 4th October 2017, Leeds, UK

1. The objective to reduce nutrient levels in rivers in order to meet the WFD has resulted in increasing numbers of P permits within rural catchments 2. A systems thinking approach has been applied to provide a greater benefit to the customer; applying a more relaxed permit at a treatment facility can enable catchment interventions leading to a greater overall benefit to river quality 3. An evidence-based approach has been used alongside collaboration with key stakeholders within the catchment to ensure that interventions are targeted where they are needed and all parties remain engaged in the overarching purpose 4. In order to provide P treatment at small WwTWs, typically technologies developed for large works have been scaled down. Consequently, the cost to provide treatment is disproportionate to the number of customers and the environmental benefit in many cases. There is a gap in the market for sustainable P treatment tailored to the needs of small WwTWs 5. A retention time of 24 hours was considered sufficient in batch tests. This is a reasonable volume to be suitable for small WwTWs 6. The reactive media Phosclean has so far not been found to be effective at removing phosphorus in this situation. Efforts are ongoing to understand why this is 7. The reactive media Polonite has shown consistent removal of phosphorus although there is a disparity between the total phosphorus in the treated effluent and the soluble reactive phosphorus implying that the media is reacting and releasing a phosphorus precipitate. If this can be captured in a final treatment stage then the potential of this product increases 8. Additional treatment stages may be required to provide solids capture and pH correction 9. Media C is providing very promising results with respect to the provision of sustainable phosphorus removal and has produced an effluent with an average total phosphorus concentration less than 1mg/l 10. There are more questions to be answered. Trial results to date have shown that there is potential in this approach to provide sustainable phosphorus removal which is tailored to small WwTWs and rural river catchments.

Acknowledgements

The work reported in this paper has been supported by the United Utilities Process Technology Team, Cumbria Wastewater Teams and Lancaster University. The reactive media has kindly been provided by Ecofiltration and ARM.

References

Molle, P. et al., 2005. Apatite as an interesting seed to remove phosphorus from wastewater in constructed wetlands. Water Science Technology, 51(9), pp. 193-203.

Nilsson, C. et al., 2013. Effect of organic load on phosphorus and bacteria removal from wastewater using alkaline filter materials. Water Research, Volume 47, pp. 6289-6297.

Vohla, C. et al., 2011. Filter materials for phosphorus removal from wastewater in treatment wetlands - A review. Ecological Engineering, Volume 37, pp. 70-89.