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Phosphorus fate and dynamics in greywater biofiltration systems Harsha S. Fowdar, Belinda E. Hatt, Tom Cresswell, Jennifer J. Harrison, Perran Louis Miall Cook, and Ana Deletic Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04181 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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Environmental Science & Technology

Phosphorus fate and dynamics in greywater biofiltration systems

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Harsha S. Fowdar,*,a,b , Belinda E. Hatta,b, Tom Cresswellc, Jennifer J. Harrisonc, Perran

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L.M Cookb,d, Ana Deletica,b

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a

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University, VIC 3800, Australia

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b

Cooperative Research Centre for Water Sensitive Cities, Melbourne, Australia

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c

Environmental Research, Australian Nuclear Science and Technology Organisation

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(ANSTO), New Illawarra Road, Lucas Heights, NSW 2234, Australia

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d

Monash Infrastructure Research Institute, Department of Civil Engineering, Monash

Water Studies Centre, School of Chemistry, Monash University, VIC 3800, Australia

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*Corresponding author: E-mail addresses: [email protected]; Tel.: +61 3 9905

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5581; fax: +61 3 9905 4944

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ABSTRACT

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Phosphorus, a critical environmental pollutant, is effectively removed from stormwater by

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biofiltration systems, mainly via sedimentation and straining. However, the fate of dissolved

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inflow phosphorus concentrations in these systems is unknown. Given the growing interest in

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using biofiltration systems to treat other polluted waters, for example greywater, such an

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understanding is imperative to optimise designs for successful long-term performance. A

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mass balance method and a radiotracer,

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partitioning of phosphorus (concentrations of 2.5 – 3.5 mg/L, >80% was in dissolved

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inorganic form) between the various biofilter components at the laboratory scale. Planted

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columns maintained a phosphorus removal efficiency of >95% over the 15-week study

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P (as H3PO4), were used to investigate the

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period. Plant storage was found to be the dominant phosphorus sink (64% on average).

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Approximately 60% of the phosphorus retained in the filter media was recovered in the top 0-

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6 cm. The

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dissolved phosphorus in the system (up to 57% of input P).Plant assimilation occurs at other

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times, potentially liberating sorption sites for processing of subsequent incoming phosphorus.

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Plants with high nutrient uptake capacities and the ability to efficiently extract soil

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phosphorus, for example Carex appressa, are, thus, recommended for use in greywater

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biofilters.

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P tracer results indicate that adsorption is the immediate primary fate of

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1. INTRODUCTION

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Phosphorus is an essential element of life; however, when present in excess, it becomes an

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environmental pollutant of concern, triggering eutrophication, oxygen depletion, and

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biodiversity loss in surface waters.1,2 In particular, discharge or unintended percolation of

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nutrient-rich waters through irrigation and agricultural drainage, non-point sources, certain

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mining industries and leaking sewers into water bodies lead to an increase in phosphorus

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levels in surface waters.1,3-7 Resulting algal blooms in Australian and U.S freshwaters have

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been reported to cost the community between AUD180 and 240 million8 and USD2.2 billion9

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respectively every year in terms of value losses in recreational water usage, drinking water

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and food production.

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Biofiltration systems (also known as biofilters and bioretention systems) are a promising

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technology developed to treat nutrient rich waters before discharge into receiving waters.

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They have been traditionally employed to treat stormwater runoff from urban areas and have

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grown in popularity in recent years due to their efficient pollutant removal, flexible design,

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microclimate benefits and amenity value.10 In addition, there is now increasing interest in 2 ACS Paragon Plus Environment

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their use for treating polluted waters other than stormwater; for example greywater,

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groundwater and partially treated wastewaters within urban areas.

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Biofilters are essentially planted soil- or sand-based filtration systems, often with a

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permanently saturated zone at the bottom.10 They mainly operate as gravity filters and differ

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in design and operation to vertical (sub-surface) treatment wetlands in that they are

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ephemeral systems, that is, they are wet for only a few hours after water application and dry

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for the majority of time. Biofilters therefore employ different soil media and plant species

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than vertical stormwater or wastewater wetlands.

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Biofilters have been demonstrated to effectively remove nutrients from stormwater,

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particularly phosphorus (P); a number of black-box studies reported total phosphorus (TP)

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removal from stormwater to be typically >75%.10-12

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governing removal mechanisms are sedimentation (because over 50% of P in stormwater is in

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particulate form)13 and adsorption,14,15 while plant uptake is speculated to be negligible.16,17 A

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recent laboratory-scale biofilter column study from our research group found P removal from

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light greywater (washing basin and shower/bath discharges) to be highly variable

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(unpublished data). The greywater mix used contained TP concentrations of 2.5 – 3.5 mg/L,

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an order of magnitude higher than typical stormwater P concentrations (approximately 0.35

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mg/L)18 and most of the P was in dissolved inorganic form. Plants appeared to influence P

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retention, with planted columns retaining from 7 - 85% of P, depending on the plant species,

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compared to unplanted columns retaining only 7% after one year of greywater application

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(unpublished data).

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According to a survey of open literature, there have not been any detailed process studies

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conducted to determine the fate of P in biofilters (either greywater or stormwater) even

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though this knowledge is imperative for optimisation of system design and prediction of

It has been hypothesised that the



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long-term performance and maintenance requirements. Studies of similar treatment systems,

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for example sub-surface flow wastewater wetlands, indicate that microbial P assimilation

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accounts for only a minor percentage of retained P.19 Adsorption onto the filter media has

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been found to be a more important P removal pathway than plant P assimilation in systems

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treating wastewater under high loading rates.20-22 However, for low loaded subsurface flow

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wetlands systems, plant P uptake can be a more significant pathway. For example, 86% of the

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P removed in a sub-surface flow horizontal wetland treating nursery runoff (influent P = 0.58

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mg/L of which 88% was in dissolved inorganic form) was found to be due to storage in plant

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biomass.23 In another study involving a 15 year old wetland, while P removal efficiency was

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low (24%), plant uptake and accumulation in soil accounted for similar storages (10% and

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14%, respectively).24

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While the fate of P in greywater biofilters can be postulated based on observations of

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wastewater wetland systems, these two systems are inherently different in design and

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operation, as discussed previously. Further to this, wastewater contains far more P than

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greywater (e.g. 5-30 mg P/L)25 thus preventing the direct translation of those observations to

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the greywater biofilter context. In a similar way, the findings from stormwater biofilter

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studies could not be directly transferred due to low inflow P concentrations and differences in

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loading dynamics.

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The objective of this work was to investigate the fate of P (largely present in dissolved

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inorganic form) in greywater biofilters. Specifically, this paper attempts to quantify the

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retention of P in different biofilter components using a mass balance method. It examines the

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distribution, form and availability of P retained in biofilter media. A novel technique

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employing a

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system. The tracer results were compared with the mass balance results for an assessment of

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how biofilter P storages vary across time. This paper provides critical information that

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P radiotracer was also used to trace the immediate fate of incoming P in the


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reinforces our understanding of P retention mechanisms in biofilters and hence enables the

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design of biofilters to be optimised for treatment of higher levels of incoming P. This is also

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a rare study on the mechanisms of P retention in soil-plant based systems, and as such could

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have wider implications for the design of other plant-soil based water treatment technologies.

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2. MATERIALS AND METHODS

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2.1 Experimental Set-up and Procedure

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Biofilter columns were constructed using 10 cm diameter polyvinyl chloride (PVC) pipe and

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filled with 20 cm of fine sand (0.5 mm) and gravel drainage layer (Figure

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1). The outlet pipe was raised to a height of 20 cm from the bottom to create a permanently

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saturated zone. Six columns were planted with one Carex appressa plant each (obtained from

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a local nursery as young seedlings) while four columns were left unplanted. Carex appressa

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is an Australian native sedge that is commonly used in stormwater biofiltration systems as it

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was found to be very effective at removing nitrogen.11,16,26 The planted columns were placed

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under grow lights in the hydraulics laboratory at Monash University, Australia. They received

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synthetic greywater at a TP concentration ranging from 2.5 – 3.5 mg/L; Filterable Reactive

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Phosphorus (FRP, considered as phosphate) accounted for about 80 - 85% of the TP

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concentration. The synthetic greywater was prepared using a modified version of the recipe

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developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO;

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Australia),27 corresponding to bathroom wastewater discharges. Details about the preparation

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method are given in the supporting information (Table S1). Typical quality parameters are:

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Total Suspended Solids (TSS), 88 mg/L; Total Nitrogen (TN), 5.9 mg/L; Biological Oxygen



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Demand (BOD), 110 mg/L and Total Organic Carbon (TOC), 36 - 50 mg/L (see Table S2 in

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supporting information for full synthetic greywater quality).

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Figure 1. Laboratory biofilter column profile

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Synthetic greywater was applied at a loading rate of 38 mm/day 3 - 5 days per week for a

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period of 14 weeks; this loading rate corresponds to 0.3 L/day and a theoretical saturated

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zone retention time of 48 hours. The columns received a TP mass load of 6.7 g/m2 during this

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period. At the end of the 14th week, the columns were transferred to the radioecology

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greenhouse at the Australian Nuclear Science and Technology Organisation (ANSTO) in

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Lucas Heights, Australia for

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influent was enriched with 0.65 MBq 32P (as carrier-free H3PO4 in dilute HCl; the addition of

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which did not affect the pH of the greywater) before dosing following the same regime as that

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used at Monash on three consecutive days. One Carex column was not spiked with the tracer

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and served as a control.

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P radiotracer addition and sampling. The synthetic greywater

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2.2 Water, Plant and Media Sampling and P analyses

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During the repetitive dosing events (first 14 weeks), influent and effluent water samples from

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each column were collected on each dosing day and composited at the end of each week. The

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composite influent/effluent samples were stored below 40C for analysis of TP. During one

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dosing event in week 13, the influent and effluent water were also sampled and analysed for

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FRP, total dissolved phosphorus (TDP), TN, oxidised nitrogen (NOx), ammonium (NH4+),

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total dissolved nitrogen (TDN) and TOC. Water quality analyses were conducted in a NATA

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(National Association of Testing Authorities; Australia) accredited laboratory using standard

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methods.

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During

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occasion for analysis of TP and

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acidified with 0.1 mL of concentrated HCl and left to equilibrate for five minutes, after which

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the sample was filtered using a 0.45 µm syringe filter and analysed for 32P activity.

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At the completion of the tracer experiment, the saturated zone was drained and the entire

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water volume collected before harvesting the plants from the biofilter columns. Above-

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(shoots) and below-ground (roots) biomass were separated. The shoots were clipped into

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small pieces while the total root biomass was washed several times in deionised water to

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remove any attached filter media. Two sub-samples of the entire plant (shoot and roots) of

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one planted column were sent for auto radiographic imaging for qualitative information on

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11, 19-21, 29-31 cm using a 3 cm diameter corer.

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Plant samples and a filter media sub-sample from each depth were dried at 60OC to constant

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mass for dry weight determination. Sub-samples of each of the dried shoot and root samples

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were ashed in a muffle furnace at 500°C for two hours. After cooling, 5 mL of concentrated

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P spiking events, influent and effluent water samples were collected on each 32

P activity. The 20 mL aliquot taken for

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P analysis was

P uptake. Filter media samples were collected at the following depth intervals: 0-2, 4-6, 9-



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HCl was added and the solution evaporated on a hotplate at 150°C until approximately 1-2

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mL remained.28 The solution was filtered using a 0.45 µm syringe filter into a liquid

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scintillation vial and made up to 20 mL with deionised water.

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To measure inorganic and organic pools of P in the filter media, the filter media samples and

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the raw media (that is, filter media before greywater application) were analysed for TP, Bray

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P-1 extractable phosphorus and acid extractable phosphorus. TP was determined by

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performing an aggressive leach using 6.25 g of media and 25 mL of aqua regia (concentrated

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HNO3 and HCl (1:3)) at 100°C for two hours. Bray P-1 extractable phosphorus is an

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agronomic soil test, measuring available P in soil for plant uptake.29 This fraction essentially

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represents a percentage of the total inorganic P present in the soil. Bray P-1 extraction

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comprises adsorbed and sparingly soluble mineral phrases which can be made available to

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plants and microbes through changes in soil condition. It was determined by mixing 6.25 g of

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media with 25 mL of 0.03 M NH4F – 0.025 M HCl for 30 minutes at 21°C.28 Acid extractable

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phosphorus using 1 M HCl measures the soil inorganic phosphorus,30 present in either

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adsorbed form or in mineral phase. It was determined by shaking 6.25 g of media with 25 mL

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of 1 M HCl for two hours. Filtered extractants were analysed for

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concentration. 32P activity in the water samples and plant and media extracts was analysed by

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Cerenkov counting using a Liquid Scintillation Analyser (Perkin Elmer Tri-Carb 2900TR

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LSC, 0-30 keV window, total 10 minute count time per sample (5 minutes, 2 cycles)).

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Measured

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and correcting for counting efficiency as well as radioactive decay according to the 32P half-

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life (T½=14.268 days). P concentration in the water samples was analysed using flow

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injection analysis methods31 and in the plant and media extracts by Inductively-Coupled

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Plasma Atomic Emission Spectrometry (ICP AES; Thermo Scientific iCAP 7600) against

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matrix-matched standards. The difference between TP and inorganic phosphorus (as

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P activity and P

P count rates were converted to activities by subtracting instrument background


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extracted by the 1 M HCl) largely represents the organic phosphorus fraction but may also

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include inorganic phosphorus that was not extracted by the acid (for instance, more stable

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inorganic phosphorus).32

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2.3 Data Analysis

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Phosphorus load reduction by the planted and unplanted systems was calculated as a

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percentage of the influent load and plotted over time to assess the performance of the system

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and observe the evolution in system phosphorus retention capacity.

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A phosphorus mass budget was calculated as follows:

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Pin = Peff + Pplant root + Pplant shoot + Pmedia + Punaccounted for

(1)

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where Pin is the cumulative influent mass of phosphorus over 15 weeks of greywater loading,

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Peff is the cumulative effluent mass, Pplant root and Pplant shoot represent the P accumulated in the

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root and shoot biomass respectively, Pmedia is the accumulated mass in the sand media and

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Punaccounted for is the amount unaccounted for or overall error. The initial plant P content was

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not measured before greywater dosing since the process is a destructive one; Pplant root and

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Pplant shoot were taken as the total plant P content in the respective plant part at time of harvest.

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P measured on a separate set of 10 Carex appressa plants of approximately similar age and

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size as those at the beginning of the study amounted to an average of 3.8 mg (±0.86 standard

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deviation). This was found to be approximately 10% of total P measured after the 15 weeks

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study period. Hence the error in estimating P recovered in plant biomass as a result of

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greywater dosing was fairly small (95%) but

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thereafter declined over time, likely as a consequence of a decrease in the phosphorus

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sorption capacity of the sand media. Del Bubba, et al. 33 studied the P-adsorption capacities of

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13 sands for use in wetland systems and noted that the sorption capacities of sand to be

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quickly exhausted within 2 months to approximately one year of operation, depending on

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type of sand. As such, after 14 weeks of greywater application, removal efficiency in the

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unplanted columns was low at 30% (±1.4 SD). The significantly lower removal efficiency in

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week 13 (unplanted columns) is likely an artefact of small variations in influent concentration

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since no significant difference in effluent P concentration was observed between week 13 and

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14 (p>0.05).

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Figure 2. Variation in phosphorus load reduction over time, expressed as a percentage of total influent load per week.

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P radiotracer was added in week 15. Data shown are means ± SD (n=4).

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The comparison of P concentrations in the saturated zone pore water (“old” water retained

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from previous dosing) and the effluent from the previous dosing (Figure S2) indicate that

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most of the removal is taking place in the upper filter layers as incoming water percolates

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through the media. P in the influent was mainly in dissolved form with FRP making up >80%

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of the TP. In the effluent of the Carex columns, FRP, particulate phosphorus (PP), and

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dissolved organic phosphorus (DOP) were

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