Journal of Contaminant Hydrology 140-141 (2012) 24–33

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Journal of Contaminant Hydrology

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Wastewater treatment by soil infiltration: Long-term phosphorus removal

David Eveborn⁎, Deguo Kong 1, Jon Petter GustafssonDepartment of Land and Water Resources Engineering, Royal Institute of Technology, Teknikringen 76, SE-100 44 Stockholm, Sweden

a r t i c l e i n f o

⁎ Corresponding author. Tel.: +46 8 790 73 28.E-mail address: eveborn@kth.se (D. Eveborn).

1 Present address. Department of Applied EnvironmenUniversity, SE-106 91 Stockholm, Sweden.

0169-7722/$ – see front matter © 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jconhyd.2012.08.003

a b s t r a c t

Article history:Received 1 March 2012Received in revised form 13 August 2012Accepted 17 August 2012Available online 24 August 2012

Phosphorus (P) leaching from on-site wastewater treatment systems may contribute to eutro-phication. In developed countries the most common on-site treatment technique is septicsystems with soil infiltration. However, the current knowledge about long term P removal in soiltreatment systems is not well developed and the data used for estimation of P losses from suchsystems are unreliable.In this studywe sampled four filter beds fromcommunity-scale soil treatment systemswith an ageof between 14 and 22 years to determine the long-term P removal and to investigate the chemicalmechanisms behind the observed removal. For one site the long-term P removal was calculatedusing a mass balance approach. After analysis of the accumulated P, it was estimated that onaverage 12% of the long-term P load had been removed by the bed material. This indicates a lowoverall capacity of soil treatment systems to remove phosphorus. Batch experiments and chemicalspeciation modelling indicated that calcium phosphate precipitation was not an importantlong-term P removalmechanism,with the possible exception of one of the sites. More likely, the Premoval was induced by AlPO4 precipitation and/or sorption to poorly ordered aluminiumcompounds, as evidenced by strong relationships between oxalate-extractable Al and P.

© 2012 Elsevier B.V. All rights reserved.

Keywords:On-site wastewater treatmentSoil treatment systemPhosphorusRemoval mechanismsChemical speciation modellingBatch experiment

1. Introduction

Tominimize health risks and preserve goodwater quality inrural areas, various on-site techniques are used for treatment ofdomestic wastewater. In developed countries, most of thesetreatment systems are septic systems that involve a septic tankand a soil infiltration component. There are several terms in usethat refer to the soil infiltration component (e.g. soil absorptionsystem, septic infiltration system or soil treatment system);here, we use the term soil treatment system (STS). Septicsystems are the most widely used on-site technique inAustralia and North America (Beal et al., 2005; USEPA, 2002)as well as in several European countries, in e.g. the British Isles(Butler and Payne, 1995) and the Scandinavian countries(Ridderstolpe, 2009). More than 70% of the acceptable on-site

tal Science, Stockholm

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wastewater treatment applications in Sweden include a STS(Ryegård et al., 2006). A STS has several strengths. For example,it will usually provide acceptable organic carbon removal aswell as essential sanitary protection of groundwater sources(Beal et al., 2005). Moreover it is very efficient in terms ofenergy and resource use compared to more advanced treat-ment techniques (Weiss et al., 2008).

However, in recent years increased attention has been paidto phosphorus (P) removal. In Sweden the current legislationrequires at least 70% phosphorus removal in on-site wastewa-ter treatment systems (90% in so-called sensitive areas)(Swedish EPA, 2006). Since P removal was not considered akey issue during the early development of septic systems, thisaspect of STS has not been extensively studied. Consequently,the widespread use of STS has now been called into question.

Phosphorus emissions may contribute to eutrophication,especially in freshwater systems (Smith, 2003). Moreover,phosphorus is critical also for the environmental status of largeparts of the Baltic Sea, which suffers frommassive algal bloomsand anoxic bottom zones in deep basins (Boesch et al., 2006).Several studies have pointed out the importance of septic

25D. Eveborn et al. / Journal of Contaminant Hydrology 140-141 (2012) 24–33

systems for the phosphorus loads to surface waters (Jarvie etal., 2006; Withers et al., 2011). According to results fromwatershed modelling, phosphorus leaching from on-sitetreatment systems accounts for around 10% of the Swedishanthropogenic phosphorus load to the Baltic Sea (Brandt et al.,2009). However, the extent of P leaching from septic systems isunclear and in other studies, their contribution to the overall Pload has been found to be minimal (Geza et al., 2010). Todevelop tools for evaluating the environmental impact of Pfrom STS there is a need for improved knowledge on the Premoval capacity of such systems.

The most frequent use of STS is for treatment of domesticwastewater from single household (SH) systems. However,they are also used in small community-scale applications.Even though the exact design of STS varies (e.g. the soil, themechanism for spreading the wastewater, and the loadingrate), the fundamental treatment concepts are largely thesame.

The removal of P can be attributed to various biological,chemical and physical mechanisms. For P removal in STS tobe sustainable, the mechanisms involved need to be efficientenough to cause net accumulation of P during the life span ofthe system and then keep the P immobilized. It is reasonableto assume that biological P uptake (by plants as well asmicrobes) will reach steady state quickly at most sites. Thisprocess will lead to a certain extent of P removal. However,since P-enriched biota is generally not harvested, biological Puptake will be of minor importance in septic systems. On thecontrary, chemical phosphorus removal may be of majorinterest and could involve a number of mechanisms includingprecipitation of Al(III), Fe(III), and Ca phosphates as well assorption onto Fe(III) and Al(III) (hydr)oxide phases (Robertsonet al., 1998). At low redox potential, Fe(II) phosphatesmay alsobe formed (Zanini et al., 1998). However, sorption and precip-itation reactions can be difficult to distinguish from each othermacroscopically (Isenbeck-Schröter et al., 1993).

The knowledge about factors that determine the extent oflong-termphosphorus removal in soil treatment systems is notwell developed. The impact of STS on surrounding groundwa-ter has been studied to some extent (e.g. Robertson, 1995;Robertson and Harman, 1999; Robertson et al., 1998). Forexample, phosphorus migration may be comparably slow innon-calcareous soils (Robertson, 2003). Moreover, P removalis attributed mainly to processes in the unsaturated zone(Robertson, 2008; Zanini et al., 1998).

Typically, studies on removal capacity have been carried outas “black-box” investigations using an inflow/outflow (I/O)measurement approach. Both field and laboratory-scale studieshave used this approach (e.g. Aaltonen and Andersson, 1996;Nilsson, 1990). I/O field measurements face several methodo-logical problems that include: identification of representativelocations for sampling influents and effluents; consideration ofpossible interactions with groundwater and precipitation;retrieval of flow-proportional samples; sufficient monitoringtime to capture removal/release dynamics. It is possible to dealwith some of these aspects through a careful experimentalsetup (see Lowe and Siegrist, 2008), but the time aspect is still aproblem. I/O studies have reported very large variations onremoval efficiency between different filter beds (Aaltonen andAndersson, 1996). Phosphorus removal can also be studied bymeans of sorption experiments in the laboratory. However, the

results from such experiments are heavily method-dependent.For example, the initial phosphate concentrations, the liquid tosolid ratio, and the contact time will have a strong impact onthe result (Cucarella and Renman, 2009).

The aim of this study was to investigate the long-termphosphorus removal acquired in non-calcareous STS, and themechanisms behind the observed removal. Four community-scale STS with an age of between 14 and 22 years were chosenas case studies.

The specific objectives of this paper were to:

• apply a mass balance approach to determine P removalefficiency based on the amounts of P accumulated in the filterbeds

• obtain evidence for the chemical P removal mechanismsinvolved using laboratory-scale batch experiments andselective extraction data.

2. Material and methods

2.1. Investigated sites

Soil sampleswere collected from four community-scale STS(designed for between 100 and 350 persons) with constructedsand beds. Details about design, location, wastewater charac-teristics and maintenance of the four sites are summarized inTable 1. One site was located near Karlshamn, southernSweden (Halahult, Ha) and the other three near Östersund,central Sweden (Tullingsås, Tu; Alsen, Al; Rötviken, Rö)(Table 1). The most significant difference to conventional SHsystems was that the studied STS consisted of uncovered filterbeds (i.e. the wastewater was applied directly on the soilsurface) and the hydraulic loadwas comparably high, between12 and 40 cm d−1 (2 to 7 times higher than what is usuallyrecommended in Swedish guidelines for SH systems withsimilar soil texture). Thewastewaterwas pretreated by a septictank (Tullingsås and Halahult sites) or by ponds to removesuspended solids (Alsen and Rötviken sites). The pretreatedwastewater in Alsenwas lower in total P and in BOD comparedto the Tullingsås and Halahult sites (Table 1). After pretreat-ment, the wastewater was applied to the filter area intermit-tently by gravity (Tullingsås site) or by pumping (the othersites). The inflow was located at the center of each filter area(except for the Halahult site in which a mobile inflow devicewas used), and the flow direction of the wastewater throughthe filter bed was vertical. The filter sand originated from localgravel pits and had d10 grain diameters between 0.23 and0.51 mm (Table 1).

Beneath the approximately 1 m deep bedmaterial, a drain-age system collected the percolating water and piped it to therecipient (no impermeable liner existed at any of the sites).Each treatment plant contained two or more filter areas, whichallowed for regularmaintenance and hydraulic recovery. At theRötviken and Alsen sites, the STS were regularly maintained bysubstitution of surface material to suppress growth of vegeta-tion. This did not occur at the Tullingsås and Halahult sites.

2.2. Collection and basic characterization of samples

The STS were sampled at five different depths (where thedepth of the filter bed allowed to) by exposing the soil profile

Table 1General description of the filter beds.

Property/site Alsen (Al) Halahult (Ha) Rötviken (Rö) Tullingsås (Tu)

Locationa Krokom Karlshamn Krokom StrömsundPretreatment Ponds Septic tank Ponds Septic tankBed surface substitutionb Regularly Never Regularly NeverDesigned for (person equivalents) 350 100 200 225No. of filter beds 3 2 3 2Surface area (m2) 3×440 2×50 3×80 2×196Design hydraulic loading rate (cm d−1) 12 40 – 33Years in operation 17 22 14 16Grain size (d10) of bed material (mm) 0.40 0.51 0.28 0.23Daily P load (g d−1)c mean±SD – – – 420±230P in inflow (mg l−1)c mean±SD 2.1±1.6 6.0±4.0 – 6.7±3.6BOD7 in inflow (mg l−1)c mean±SD 30±19 76±56 – 110±67

a Municipality within which the site is located.b Scratching and substitution of bed surface material is performed on a regularly basis in some bed filter applications.c Mean value and standard deviation in inflow to operating infiltration bed. Statistics derived from datasets of 46–59 sample points per filter bed measured in

1992–1993 (Bylund, 2003).

26 D. Eveborn et al. / Journal of Contaminant Hydrology 140-141 (2012) 24–33

and collection of samples by hand in the 0–5, 5–15, 15 – 30,30–60 and 60–100 cm layers. A reference sample was alsocollected at each site, which represented filter bed materialthat had not been exposed to P-containing wastewater.Typically the reference samples were taken from the embank-ment that surrounded the filter area.

The STS were sampled at a single location near the waste-water distribution point. However, at the Tullingsås site,samples were collected from five different locations withinone of the filter areas, to consider spatial heterogeneity ofaccumulated P. Because the filter areas were subjected tonon-uniform hydraulic loads, they are likely to vary internallywith respect to the amounts of accumulated P. The selection ofsampling locations at the Tullingsås sitewasmade after a visualjudgment with respect to slope and vegetation of the bedsurface. After collection, all soil samples were placed in plasticbags and stored at +4 °C prior to further use. Field-moistsamples were analyzed for pH in 0.001 M CaCl2 (using a liquidto solid ratio of 2) and total C was determined for air-driedsamples using a LECOCNS-2000Analyzer. The bulk densitywasdetermined by collection of soil cores (by use of metalcylinders) in four replicates. The cores were dried at 105 °Cbefore weighing.

2.3. Mass balance calculations

The long-term P removal was determined by calculationof the ratio between the P accumulated in the bed materialand the estimated P load at the Tullingsås site. According tothe maintenance staff at the site, the two filter beds had beenequally utilized. From this it can be estimated that the annualP load to each filter bed was 76 kg. This estimation is basedprimarily on a large number of inflow measurements carriedout during 1992 and 1993 (Table 1).

The P accumulation estimate was based on measurementsin the five different sample locations (Fig. 1) and in five soillayers (in total 25 samples) to the bottom of the filter bed(1.15 m). For an individual sample, the accumulated P wasdefined and calculated as the difference between HNO3-digestible P in the sample and HNO3-digestible P in the refer-ence sample. The mean value and standard deviation (SD) for

the P accumulation in each sampled layer were computed. Thetotal amount of accumulated P (mP) was then determinedaccording to:

mp ¼Xn

l¼1

ρVcð Þl ð1Þ

wheremp is the sum of the mean accumulated P in layers l=1to n, ρ is the dry bulk density (kg m−3),V is the volume of layerl (m3), and c is the concentration of accumulated P (kg Pkg−1

soil). Eq. (1) was used repeatedly to calculate accumulated P(within one SD).

2.4. Analysis of HNO3-digestible P

HNO3-digestible P was used as a basis for the mass balancecalculations. Although this acid digestionmethodmay probablynot recover allmineral-bound P (see, e.g., Syers et al., 1967), it islikely to recover all reactive P, including all P that was sorbedas a result of wastewater infiltration. Samples were analyzedfor P by nitric acid digestion with 7 M HNO3 according to theSwedish standard method SS 028311 (Swedish StandardsInstitute, 1997). Briefly, 5 g soil was mixed with 20 cm3 7 MHNO3 and heated in an autoclave at 120 °C for 30 min.Subsequently, the P concentration in the extract was deter-mined with inductively coupled plasma emission spectrometry(ICP-OES) using a Perkin-Elmer Optima 3000 DV instrument.

2.5. Oxalate extraction

Reactive aluminium and iron (hydr)oxides, as well as phos-phorus associated with these fractions, were determined byextraction with ammonium oxalate (0.2 M oxalate buffer,pH 3) (van Reeuwijk, 1995). However, apart from P associatedwith aluminium and iron (hydr)oxides, other P species thatare unstable at low pH will also be dissolved (e.g. calciumphosphates). Field-moist samples (in duplicate) from the foursites were extracted using a liquid to solid ratio of 100:1,shaken for 4 h in the dark in an end-over-end shaker. Oxalate-extracted Fe, Al and P were analyzed by ICP-OES as above, and

Fig. 1. Illustration of filter bed and sample locations.

27D. Eveborn et al. / Journal of Contaminant Hydrology 140-141 (2012) 24–33

the results were reported on a dry weight basis. For the results,the coefficient of variation was in the range of 8±8% for metalconcentrations and 12±10% for phosphate concentrations.

2.6. Batch experiments

A series of batch experiments were undertaken to examinethe removal capacity and pH dependence of P desorption/dissolution. The batch experiments used samples from theuppermost layer (0–5 cm) of each soil and from the referencesoils, and theywere conducted at room temperature (21 °C). Ina pH-dependence experiment, field-moist samples (4 g) weresuspended in 35 cm3 solutions in polypropylene centrifugetubes (in duplicate) and shaken for 5 days in an end-over-endshaker. The solutions consisted of 0.01 MNaNO3with differentadditions of acid (from 0 up to 2.6 mM HNO3) or base (from0 up to 1.7 mM NaOH).

In a sorption experiment, field-moist samples (3 g) weresuspended in 30 cm3 solutions consisting of backgroundelectrolyte (0.01 M NaNO3) to which different PO4 additions(as NaH2PO4) had been made (3.1, 6.2 and 12.4 mg P/l). ThesePO4 additions did not alter the equilibrium pH to any consid-erable extent (b0.6 pH units). Conditions were otherwise thesame as for the pH-dependence experiment.

In both experiments samples were centrifuged for 20 minat 3000 rpmafter equilibration. The pH value and the alkalinitywere thenmeasured directly on subsamples by use of a PHM95pH meter (Radiometer Analytical, Lyon, France) and a titrator.After filtration through a 0.2 μm Acrodisc PF filter, the inor-ganic PO4\P concentration was determined colorimetricallyaccording to the acid molybdate method by use of flowinjection analysis (Aquatec-Tecator autoanalyser, Foss Analyti-cal, Copenhagen) whereas Fe, Al, Ca, Mg, K and Na weredetermined by ICP-OES using a Varian Vista Ax instrument(Agilent Technologies Inc., Santa Clara, CA), and F−, Cl- and SO4

2−

were determined by ion chromatography using a DionexDX-120 instrument (Dionex Corp., Sunnyvale, CA).

2.7. Chemical speciation modeling

To evaluate the conditions for precipitation of solid cal-cium and aluminium phosphate phases we used the chemicalequilibrium modeling software Visual MINTEQ (Gustafsson,2009). Activities of Ca2+, Al3+, and PO4

3− were calculated andused to investigate the conditions for precipitation. The follow-ing P phases were included in the investigation: amorphouscalcium phosphate, Ca3(PO4)2 (ACP), octacalcium phosphate,Ca4H(PO4)3 (OCP), dicalcium phosphate, CaHPO4 (DCP),dicalcium phosphate dihydrate, CaHPO4×2H2O (DCPD), hy-droxyapatite, Ca5(PO4)3OH (HAp), and variscite, AlPO4×2H2O,see Table 2 for constants and heats of reaction used. Equilib-rium constants and heats of reaction for soluble complexeswere taken from the default thermodynamic database of VisualMINTEQ, which mostly relies on the NIST compilation (Smithet al., 2003). The following parameters/concentrations wereused as input for the modeling: Temperature, alkalinity, pH,Ca2+, Mg2+, Al3+, Fe2+, K+, Na+, PO4

2−, SO42−, Cl−, F−, NO3

−.Concentrations were measured in the equilibrated soil extract,except for NO3

-, which was calculated based on the addition ofNO3 from NaNO3 and HNO3, as the original soil extract concen-trations were negligible compared to the additions. At pH>5(i.e., in all extracts except for those from the Tullingsås 0–5 cmsample), the calculated Al3+ activity was higher than the onecalculated from the solubility of “soil” Al(OH)3(s) (Gustafssonet al., 2001), and then the Al3+ activity was instead fixed by thesolubility of Al(OH)3(s), in agreementwith procedures used forlake water (Sjöstedt et al., 2010). For the Tullingsås 0–5 cmsample, the Al3+ activity was instead calculated using theStockholmHumicModel, as described by Sjöstedt et al. (2010).

3. Results

3.1. General properties

The response to wastewater application was different inthe four filter beds, as was evidenced by the results of pH and

Table 2Solubility constants and heats of reaction for Ca and Al phosphates used in the speciation calculations.

Reaction log Ks (25 °C)a ΔHr (kJ/mol)a

ACP1: Ca3(PO4)2(s)⇔3Ca2++2PO43- −25.5c −94c

ACP2: Ca3(PO4)2(s)⇔3Ca2++2PO43- −28.25c −87c

DCP: CaHPO4(s)⇔Ca2++PO43−+H+ −19.28 31

DCPD: CaHPO4×2H2O(s)⇔Ca2++PO43−+H++2H2O −19.00 23

HAp: Ca5(PO4)3OH(s)+H+⇔5Ca2++3PO43-+H2O −44.3b 0

OCP: Ca4H(PO4)3(s)⇔4Ca2++3PO43−+H+ −47.95c −105c

Variscite: AlPO4(s)×2H2O(s)⇔Al3++PO43−+2H2O −22.07d −9.4e

a Unless otherwise stated, the values are from Smith et al. (2003).b Solubility of HAp at 21 °C (McDowell et al., 1977).c Calculated from raw data given by Christoffersen et al. (1990).d Lindsay (1979).e Woods and Garrels (1987).

28 D. Eveborn et al. / Journal of Contaminant Hydrology 140-141 (2012) 24–33

total C. Acidification of the profile was seen in the Tullingsåsand Halahult sites, particularly in the surface layer (Table 3).In both cases a more than five-fold increase in total C wasobserved in comparison to the reference soil, which can atleast partly explain the decreased pH, as immobilized organicacids from the wastewater may have contributed to anincrease in organic matter. In the two other sites pH andorganic C appeared to have been less affected by the appliedwastewater. However, these sites were subject to surfaceregeneration on a regular basis (Table 1), which complicatesthe interpretation.

3.2. Phosphorus accumulation and mass balance

Generally, the concentration of oxalate-extractable P wasmuch lower than the concentration of HNO3-digestible P,

Table 3Properties of the filter bed materials. Data from oxalate extractions represent mean

Site Depth(cm)

pH Total C(%)

Alsen 0–5 7.07 1.25–15 7.28 0.7715–30 7.24 1.230–60 7.46 n.a.c

60–100 7.64 n.a.c

Ref. 7.26 0.59Halahult 0–5 6.56 3.2

5–15 7.44 0.3015–30 7.68 0.3930–60 8.16 n.a.b

60–100 8.12 n.a.b

Ref. 8.08 0.62Rötviken 0–5 7.28 0.35

5–15 7.38 0.2015–30 7.66 0.3130–60 7.50 n.a.c

Ref. 7.52 0.16Tullingsåsd 0–5 4.49 0.85

5–15 5.05 0.1615–30 5.00 0.1130–60 5.71 n.a.c

60–100 6.41 n.a.c

Ref. 6.85 0.13

a HNO3 extractable.b Oxalate-extractable.c n.a. = not available.d Values are from location 5 at the Tullingsås site.

particularly in the reference samples, in which the oxalate-extractable fraction constituted less than 22% of the HNO3-digestible P (Table 3). The vertical distribution of P in thedifferent filter beds followed slightly different patterns. AtHalahult and Tullingsås, the accumulation of P (P in thesample minus P in the reference) was significantly higher inthe uppermost layer of the soil (Fig. 2). The average Paccumulation at the Halahult and Alsen sites was 110 and320 mg P/kg soil respectively. At the Rötviken site, the Paccumulation was close to 0 mg P/kg soil. The P accumulationat different sample points in the filter bed at Tullingsås sitevaried between 95 and 880 mg P/kg. The total P load during16 years of operation at this site was 3300 mg P/kg whereasthe P accumulation in the soil within one SD varied between260 and 510 mg P/kg soil. The mass balance calculation forthe Tullingsås site thus indicated 12±4% P removal.

s of duplicate samples.

PHNO3a

(g kg−1)Feob

(g kg−1)Alob

(g kg−1)Pob

(g kg−1)

0.50 0.91 0.27 0.100.38 0.97 0.24 0.0480.62 1.1 0.24 0.0880.67 2.3 0.33 0.170.76 2.5 0.30 0.260.34 0.96 0.19 0.0101.1 0.87 1.3 1.30.44 0.23 0.19 0.120.37 0.23 0.11 0.0590.30 0.17 0.064 0.0420.29 0.18 0.080 0.0810.25 0.16 0.059 0.0430.48 0.52 0.20 0.0950.42 0.60 0.25 0.0840.38 0.57 0.28 0.0420.42 0.77 0.16 0.130.42 0.59 0.17 0.0940.98 1.6 0.86 0.700.68 1.2 0.50 0.330.62 1.1 0.39 0.280.65 1.3 0.41 0.320.69 1.3 0.38 0.270.42 1.2 0.41 0.070

Fig. 2. Depth distribution of HNO3‐digestible P (P-HNO3) in the four investigated sites. For the Tullingsås site, the depth distributions of all five sampling locationsare shown.

Fig. 3. Relationship between oxalate-extractable Al and P for filter bed samplesfrom all investigated sites.

29D. Eveborn et al. / Journal of Contaminant Hydrology 140-141 (2012) 24–33

3.3. Accumulation of iron and aluminium

At the Halahult and Tullingsås sites, oxalate-extractable Feand Al in the uppermost layer were much higher than in theother layers and in the reference samples, showing evidencefor the accumulation of Fe and Al (Table 3 and Fig. S1,Supplementary materials). At the Rötviken site there was littledifference in Fe and Al between the layers and at Alsen theconcentration of Fe and Al increased with increasing depth.Here, differences in maintenance should be considered a keyaspect for the differences between the Alsen and Rötviken siteson the one hand and the Halahult and Tullingsås sites on theother. For all sites, the depth distribution of oxalate-extractableP followed that of Al very well. For the filter bed samples therewas a very strong relationship between the two (r2=0.95), seeFig. 3, whereas oxalate-extractable Fe and P were poorlyinterrelated (r2=0.07). The molar ratio of oxalate-extractableP to Al ranged from 0.13 to 0.90 (median value 0.56).

3.4. pH-dependence experiment

In 3 of the 4 sites, dissolved PO4\P increased at higher pH(Fig. S2, Supplementary materials). If calcium phosphates hadbeen a major sink for phosphorus it would have been reason-able to expect increasing PO4\P concentration at low pH (as aresult of Ca\P mineral dissolution). However, only the resultsfor the Halahult site were consistent with such a pattern. Theresults for the 3 remaining sites, showing lowP solubility at low

pH, were consistent with sorption or precipitation involving Feand Al. According to the results from speciation modelling, theonly Ca\P mineral phase for which the saturation indexexceeded the solubility constant was hydroxyapatite (HAp)(this was true for the Halahult, Rötviken and Alsen sites,whereas the Tullingsås samples were always undersaturatedwith respect to HAp). However, there was nothing to indicateequilibrium with respect to HAp since its solubility line washighly exceeded (data not shown). A more relevant result may

Fig. 4. Modelled ion activity products and solubility lines for two amorphous calcium phosphates (left) and variscite (right) in equilibrated extracts from thepH-dependence experiment, see Table 2 for constants used.

30 D. Eveborn et al. / Journal of Contaminant Hydrology 140-141 (2012) 24–33

be that 3 of the 4 sites (Halahult, Alsen and Rötviken) werequite close to saturation with respect to ACP (amorphouscalcium phosphate) under neutral to alkaline conditions inthe pH-dependence experiment (Fig. 4). Moreover, the acidextracts from the Tullingsås and Halahult sites were close tosaturationwith respect to variscite,which indicates thepossiblerole of Al phosphate minerals in controlling P solubility underacidic conditions.

3.5. Sorption experiment

The sorption experiment indicated a low additional P re-moval capacity of the 0–5 cm samples of the used filter bedmaterials (Fig. 5). An equilibrium concentration that willkeep the removal rate at an acceptable level (“acceptable”means 70–90% P removal according to Swedish guidelines)would be around 1 mg/l P or below. However, this is lowerthan the concentration obtained in the control samples(i.e. without added P) for two of the used samples. Hencethese samples had already exhausted their P removalcapacity with respect to the guidelines. The sample with the

Fig. 5. Phosphorus sorption characteristics in the 0–5 cm top layer (closedsymbols) and in reference samples (open symbols) at the investigated sites

.

strongest additional removal capacity was the 0–5 cmsample at the Tullingsås site (80 mg/kg at an equilibriumconcentration of 1 mg/l). At this concentration, the referencesamples only sorbed 40 mg P/kg or lower.

As in the pH-dependence experiment, the saturationindices for the Halahult, Alsen and Rötviken sites were closeto the solubility constant of ACP when the activities of the freeions had been calculated using Visual MINTEQ (Fig. 6).However, in contrast to the pH-dependence experiment thedata points in this case tended to be located in the over-saturated region (compare with Fig. 4). Again, as for the pH-dependence experiments, samples from the Tullingsås andHalahult sites were close to the solubility of variscite.

4. Discussion

Interpretations and decisions about long-term P removal insoil infiltration beds have in the past been heavily dependenton I/O measurements. A scientifically based analysis of thelong-term P removal capacity and the P removal mechanismsinvolved has been largely lacking. To create a reliable know-ledge base for the estimation of long-term P removal in, and Pleaching from, STS it is necessary to use a variety of otherresearch techniques to determine the P removal capacity of thesystems that are in use today. There is also need for a theo-retical basis that will explain how and when the P removalcapacity can be reached.

In this study we determined the long-term P removal fromthe start of operation until the time of sampling by establishinga mass balance over a filter bed and then compare the esti-mated P load with the accumulated P (measured as HNO3-digestible P (P-HNO3) in sample minus P-HNO3 in a referencesample). In comparison to most I/O studies our result ofbetween 8 and 16% P removal is low. Aaltonen and Andersson(1996) reported between 38% and 60% P removal by 12monitored beds in Sweden. According to the Swedish EPA(Swedish EPA, 2008), 25–90% P removal is obtained in SwedishSTS. These figures are heavily based on the work by Nilsson(1990). USEPA (USEPA, 2002) claims that between 85 and 95%P removal is obtained (septic tank removal included), andrefers to Sikora and Corey (1976).

Fig. 6. Modelled ion activity products and solubility lines for two amorphous calcium phosphates and variscite in equilibrated extracts from the sorptionexperiment, see Table 2 for constants used. Closed symbols are for filter samples and open symbols are for reference samples.

31D. Eveborn et al. / Journal of Contaminant Hydrology 140-141 (2012) 24–33

Variations in chemical, physical and operational propertiesbetween the applications investigated here and those studiedearlier could not be completely ruled out as possible reasons tothe discrepancy between our study and earlier I/O measure-ment studies. The hydraulic load was larger (2–7 times) in theuncovered STS studied here, compared to in conventional SHsystems. There might also be differences in soil chemistry.However, it should be kept in mind that the Tullingsås site,which was the site selected for the mass balance calculations,had a strong mean P accumulation compared to the other sitesstudied. Our results are consistent with those of Robertson(2008), who investigated a septic system plume of phosphorusand concluded that it had migrated 16 m during 16 years ofoperation. Quick migration of phosphorus from several septicsystems was observed also in an earlier study by Robertsonet al. (1998).

From a soil chemistry perspective there are reasons to besceptical to the widely held view of the large P removalcapacity of STS since it needs ad-hochypotheses to explain howsand- and gravel-like materials can accumulate P well abovetheir P sorption capacity. For example, to reach a long-term Premoval of 90% at the Tullingsås site, it would have beennecessary to accumulate 3 150 mg P/kg, which is higher thanmost engineered materials that have been optimized for Premoval (Cucarella and Renman, 2009).

To ensure the reliability of the mass balance calculations,the most questionable background data were validatedagainst other information sources. For example, the P-HNO3

content in the reference samples (ranging from 248 to449 mg P kg−1, with an average of 362 mg kg−1) werefound to be within the range of the total P concentration inthe C horizon of sandy Swedish soils, in which the P contentrange between 87 and 1090 mg kg−1, with an average of306 mg kg−1 (SLU, 2012). Moreover, the estimated P loadswere in agreement with the P loads that could be calculatedbased on the number of persons connected to the treatmentplant. However, the data required for the P-load estimationwere taken from one of the first years during which thesystem was in use. As a consequence, there is a risk fortime-dependent changes (for example due to changes inpopulation density). Moreover, it should be kept in mind that

we needed to make some assumptions in the mass balanceapproach; these may have been reasonable for the studiedsites but not necessarily correct for all kinds of STS.

The accumulation of P was relatively high in the top layer(0–5 cm) of some of the filter beds (Halahult and Tullingsås)(Fig. 2). An interesting observation was that this increasecoincided with significantly increased amounts of oxalate-extractable Al and Fe (Fig. S1, Supplementary materials). Suchincreases have also been reported by other researchers (Zaniniet al., 1998). The reason for this increase is unclear butdeposition of Al and Fe compounds from the wastewater and/or biochemical degradation of the minerals in top of the filterbed are both possible hypotheses. Hence, metal accumulationin the surface horizon could be an important mechanism thatdetermines the phosphorus removal capacity of the STS. Thetrend described above was not as clear at the Alsen andRötviken sites (Fig. 2). However, in these two beds substitutionof surface materials has occurred regularly. This may obstructpurposeful interpretations of the long-term P accumulation.For the Rötviken site, the net P accumulation was even close tozero. The result for the Rötviken site may have been influencedalso by other factors (for example, the actual P load may havebeen low, or the reference sample usedmayhave been too highin P).

Our sorption experiment confirms the weak P removalcapacity of the studied sites (Fig. 5). The accumulation of Feand Al compounds in the filter bed surface provides a logicalreason to why the results from the sorption experimentswere sometimes better for the top layer samples than for thereference samples. The finding that the strongest P removalwas found in the top layer of the Tullingsås site could, however,also be related to the low pH value in this horizon (4.4), whichwould favour Al- and Fe-induced sorption or precipitation infilter beds (Robertson, 2003).

Aluminium compounds are the most important compo-nents governing P removal in most of the studied sites, asevidenced by the strong relationship between oxalate-extractable Al and P. The relevant process may involve surfacecomplexation of PO4\P to poorly ordered Al (hydr)oxide-type compounds such as allophane and imogolite. It seemslikely, however, that formation of AlPO4 precipitates (such as

32 D. Eveborn et al. / Journal of Contaminant Hydrology 140-141 (2012) 24–33

variscite) occur and may even be of considerable importancefor the observed P removal. This is indicated by the relativelyhigh ratio of oxalate-extractable P to Al in the filter beds(median value 0.56), which is larger than would be expected ifsorption to Al compounds was the only important process.Moreover, our results from Visual MINTEQ modelling stronglyindicate variscite to be an important P solubility control, at leastin the Tullingsås site.

Although our methods did not allow identification of AlPO4

precipitates, direct evidence of their formation in noncalcareousfilter beds have been obtained earlier (Robertson, 2003; Zaniniet al., 1998). Furthermore, Giesler et al. (2005) showed thatsuch compounds may be formed at high equilibrium PO4\Pconcentrations also in natural sandy forest soils. As indicated bythe pH-dependence experiment, there were indications ofaccumulation of calcium phosphate minerals only for theHalahult site. On the other hand geochemical modelling in-dicated that saturation with respect to ACP was reached for allbut the Tullingsås site when Pwas added (Fig. 6). This indicatesthat ACP can work as a temporary pool for P when the Pconcentrations in the influent water are high. Precipitatedcalcium phosphates may then dissolve again when the Pconcentration in the influent decrease. It is likely that similarmechanisms will take place in the equilibrium interfacebetween solid/dissolved Al\P and Fe\P compounds andadsorbed/free P (interactions with Fe and Al-(hydr)oxides). Inactual applications such inflow dynamics is likely to be verycommon due to leaching into pipe systems and dilution ofwastewater by rain and ground water interactions. In partic-ular, I/O measurements will not catch this kind of variationsunless they are running for a very long time. Net P leachingfrom STS can also be expected during periods when the bed isresting (as a consequence of P dissolution during percolation ofrain water and degradation of biological bound P). Negativevalues for P removal have also been observed in some data sets(Aaltonen and Andersson, 1996; Eveborn et al., 2009) and itseems likely that similar observations have been treated aserroneous data in many investigations.

This research suggests that the sand materials used inSwedish STS have poor conditions for long-term P removal,especially in community-scale systems with high hydraulicloads. This does not exclude the possibility that the situationmay be different in certain septic systems with much lowerloads. For example, Robertson (in press) recently reportedsatisfactory long-term P removal results for an SH system(with a much smaller hydraulic load) using a sand materialrich in aluminium and iron(III) (hydr)oxides. Thus for properassessment of the long-term P removal in STS, the hydraulicload and the filter bed properties are probably two importantfactors to consider.

5. Conclusions

• Our results, as obtained with a mass balance approach,showed much poorer P removal performance for soil treat-ment systems than previously reported. The method usedshould be well suited for estimation of long term P removal.Therefore current criteria for phosphorus losses from suchsystems can be questioned.

• A strong correlation between oxalate extractable P and Alindicated that the P removal mechanisms may involve both

surface complexation of PO4\P to poorly ordered Al (hydr)oxide-type compounds and formation of AlPO4 precipitates(such as variscite).

• In laboratory batch experiments, the pH dependence of theP solubility was consistent with Al\P association at threeof four sites. However, the remaining P removal capacitywas rather low.

• Based on chemical equilibrium modelling calcium phos-phates were not generally controlling the phosphate con-centration in the equilibrium solution except for at possiblyone site. In addition,modelling suggests that the formation ofvariscite (AlPO4×2H2O) is possible in sites with pH valueslower than 7.

• Future research is needed to provide reliable estimates ofthe P removal capacity in soil treatment systems and tofurther evaluate the identity and relative importance of theP removal mechanisms.

Acknowledgements

Many thanks to the master students Johanna Danielsson,Marie Erlandsson, Johanna Fredén, Hanna Hugosson andMalin Wahlqvist (Aquatic and Environmental EngineeringProgramme, Uppsala University) for laboratory assistance.Caroline Holm and Ola Palm, JTI (Swedish Institute of Agri-cultural and Environmental Engineering) are acknowledgedfor support during the study. We thank the Swedish Water &Wastewater Association and The Swedish Research CouncilFormas (project no. 2006‐632) for financial support of thisresearch.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttp://dx.doi.org/10.1016/j.jconhyd.2012.08.003.

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