IMPACTS OF THE WATER FRAMEWORK DIRECTIVE ON THE … · Organised by Aqua Enviro Technology Transfer...

5th European Water & Wastewater Management Conference

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Organised by Aqua Enviro Technology Transfer

IMPACTS OF THE WATER FRAMEWORK DIRECTIVE ON THE WATER INDUSTRY: IRON

CONSENTING

Comber S, Mistry R, Gardner M, and Georges K

Atkins

Abstract

To meet the targets of the Water Framework Directive, existing Environmental Quality

Standards (EQS) for many substances, including iron, are under review. The main purpose of

iron dosing at wastewater treatment works is to reduce effluent phosphorus concentrations in

order to meet other requirements of European legislation. If the iron EQS value is reduced, a

subsequent tightening of surface water discharge permits may result. An UKWIR-funded project

looked to assess the current impact of iron dosing on downstream water, sediment and

ecological quality as well as the likely impact of tightening iron discharge permits on the Water

Industry. Analysis of pre- and post iron dosing water quality and ecology downstream of several

treatment works shows that tightening of permits would provide no obvious environmental

benefit at significant costs to the Water Industry and subsequently the consumer. A series of

permitting options were provided to ensure EQS compliance without incurring disproportionate

costs to the water industry.

Keywords

Iron discharge consenting, WFD, phosphorus reduction

Introduction

Iron salts are currently used at approximately 700 Wastewater Treatment Works (WwTW)

serving over 20 million customers (UKWIR, 2011) as a coagulant within wastewater treatment to

precipitate phosphorus into sludge. This process is used in order to meet European Directives

for the control of phosphorus discharged to the aquatic environment, with the majority of works

dosing to achieve phosphorus permits of either 1 or 2 mg/l. Where iron is added during the

treatment process a permit to discharge is generally applied by the Environment Agency to

ensure that the receiving water Environmental Quality Standard (EQS) is not exceeded, thus

ensuring there is no negative impacts on downstream ecology. The current EQS set under the

Dangerous Substances Directive (DSD) is 1 mg/l as dissolved iron.

The Water Framework Directive (WFD) is a relatively new European Directive which sets very

stringent targets for water quality and ecology. As part of its implementation the DSD is being

repealed and new EQS are being derived for a large range of potentially toxic substances. Iron is

not a particularly toxic metal owing to its instability in water; however, excessively high

concentrations can cause toxicity and so has meant an EQS was set under the DSD. Iron may

now be included as a WFD Annex VIII Specific Pollutant and a revised EQS set by the

Environment Agency, is likely to be published for consultation in 2011. Given that the WFD EQS

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setting methodology invariably leads to tighter EQS than those set under the DSD, there is risk

to the Water Industry of a downward pressure on new and existing iron permits in the future.

Furthermore, in many cases phosphorus permits are also being tightened to meet WFD

objectives and are likely to become more extensive in the future. This in turn puts pressure on

water companies to use more iron. Recent estimates (UKWIR, 2011) also suggest that to achieve

the phosphorus EQS, many more wastewater treatment works (WwTW) may require some form

of phosphorus removal (potentially at least an additional 1,000 WwTW) in an attempt to meet

in-river phosphorus EQS.

Any decrease in iron permit values would put significant pressure on the Industry to install

extensive tertiary treatment (sand filters) to reduce effluent concentrations or seek alternatives

to its use. Concerns regarding cost and increasing carbon emissions have lead to the Water

Industry via UK Water Industry Research (UKWIR) to review the current permitting policy, the

fate and behaviour of iron in receiving waters; impacts on downstream ecology and water

quality; costs of additional treatment and to offer options for possible future permitting regimes

(UKWIR, 2011). Decisions regarding the regulation of iron in the UK have yet to be made; this

paper summarises the key outcomes of the industry’s research.

Iron Chemistry

Forms of iron in water

It is important to note that iron is one of the Earth’s most abundant elements and is present in

the crust at around 5% by mass. In effluents and surface waters iron is chemically unstable in

solution and should be present as solid iron hydroxides. Concentrations of the toxic forms of

iron, namely the dissolved free metal ion, are therefore very low and in most cases, not

measurable. Only in acidic waters in areas rich in iron minerals would appreciable

concentrations of dissolved iron be detected, prior to precipitation downstream as an orangey,

‘ochrous’ deposit. Low concentrations of filterable iron may be detected in the aqueous phase

owing to inadequacies in the determination of dissolved iron.

An operationally defined particle size cut off of 0.45 µm is used for filtering samples and

classifying a metal as ‘particulate’ or ‘dissolved’. Colloidal iron (oxidised iron hydroxides in an

intermediate phase between dissolved and precipitation as a solid) passes through the filter and

is therefore included in the ‘dissolved’ fraction; ‘filterable iron’ would be a more accurate

description. Colloidal species are relatively unreactive, their toxicity is much less than the free

metal ion. In some cases its slow oxidation or binding to substances which maintain its solubility

in rivers and effluents, such as dissolved organic matter or in the case of effluents, synthetic

chelating agents used in detergents, can also mean iron is detectable in the water column.

Figure 1 provides a diagrammatic summary of iron states in typical surface waters, illustrating

the complexation of iron as well as the different forms in which it may exist.





Figure 1: Summary of iron states in typical surface water conditions

Aquatic toxicity of iron

Owing to its lack of solubility, iron is not an especially toxic metal and has always been viewed in

this light by the regulator. It has not been classified as a Priority Substance or Priority Hazardous

Substance by the EU, who have left individual Member States to decide whether to include it as

a Specific Pollutant or not. The UK is considering iron as a specific pollutant.

The existing DSD EQS is 1 mg/l dissolved metal and exceeedances would only be expected in

iron-mineral rich areas. Furthermore, its instability in water complicates toxicity testing, as there

are often uncertainties over the form of iron to which an organism is being exposed, as well as

its concentration.

Consequently, the derivation of EQS and permit conditions are complex and generally imprecise.

Recent standard setting methodologies have in some cases set EQS based on firstly identifying

waterbodies of ‘good’ ecological status then setting a chemical standard based on observed

concentrations. However, because iron is unstable in water and will tend towards the solid

phase, water concentrations will be low in most types of water (pristine, polluted or otherwise)

of any ecological quality and significantly less than any toxicity threshold, which would lead to a

very low EQS being set based on observed concentrations. Table 1 provides a summary of how

different forms of iron vary in solubility and bioavailability and therefore toxicity.





Table 1: Summary of solubility and bioavailability of Fe2+and Fe3+

Reduced Toxicity Risk Increased Toxicity Risk

Filterable iron background levels are low in

river environment (10-30 µg/l)

Fe2+ is more toxic that Fe3+

Complexation with organic ligands reduces

toxicity

Complexation with organic ligands prolongs

presence in water

Precipitated iron offers little in way of

chemical toxicity

Iron precipitation onto sensitive

membranes (gills) has negative impacts

Despite its low toxicity there is some evidence to suggest that iron precipitates can exert a

smothering effect and may affect gill function (Gerhardt, 1992). The data supporting these

concerns are, however, limited and uncertainties associated with the forms of iron to which

organisms are exposed and the subsequent effects are substantial.

Iron Permitting

Over the previous water industry Asset Management Planning (AMP) cycles the derivation of

iron consents (now permits) have been broadly based on the Environment Agency’s ‘river needs’

no deterioration policy, which was developed for the implementation of the DSD. The method

utilises the Environment Agency’s River Quality Planning (RQP) tool to generate an effluent

permit based on a downstream target concentration, which is itself based on the EQS. However,

the RQP outputs can have a tendency to generate too strict a permit for low dilution scenarios

and too generous a permit at high dilutions. As a result, previous permits have generally been

negotiated at a local level based on technical feasibility, monitoring data, or in some previous

cases set as 1/7th of observed average effluent suspended solids concentration.

WwTW iron permits are set as total iron and are typically 4 mg/l (Figure 2), which owing to the

complex chemistry of iron rarely reflects available dilution (Figure 2). This de facto emission limit

value has been generated via negotiation and knowledge of effluent quality achievable, fate in

receiving water rather than strict adherence to modelling outputs and Environment Agency

policy. For compliance purposes, under the DSD all metal permits are in theory set as maximum

values, which now is accepted as a statistically flawed approach, with percentile values being

preferred for permitting under the WFD.





05

101520253035404550

1 2 3 4 5 6 7 8 9 10 >10

Fre

qu

en

cy (

%)

Fe permit (mg/l total Fe)

Figure 2: Percentage of permits applied to iron for WwTW based on available

data (453 WwTW).

0.01

0.1

1

10

100

1000

10000

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Ap

pro

xim

ate

dilu

tio

n

Permit (mg-Fe/l)

Figure 3: Estimated dilution (River: WwTW flow) for WwTW with an iron permit

based on available data





Where dosing iron, WwTW effluents exhibit concentrations significantly less than the permitted

discharge limit. This is not because of the use of extensive tertiary treatment, to meet a permit

expressed as a maximum allowable concentration (MAC), but because the physico-chemical

characteristics of iron will result in its precipitation and accumulation in the sludge rather than

discharge in the effluent (Figure 4).

Dissolved concentrations of iron in dosed effluents are typically 25% of the total values.

Exceedances of the permitted MAC in WwTW effluents are ephemeral and associated with the

discharge of high concentrations of suspended solids (Figure 5). The instability of iron in solution

would make setting permits on ‘dissolved’ or ‘filterable’ iron difficult owing to likely changes in

form between sampling and receipt by the laboratory, thereby necessitating filtration on-site.

This will need to be considered on the basis of how any new EQS is expressed.

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6 7 8 9 10

Me

asu

red

co

nce

ntr

atio

ns

(mg-

Fe/l

)

Permit level (Total Fe - mg/l)

mean concentration of WwTW effluent monitoring data

95%ile concentration of effluent monitoring data

Actual permit concentration

Figure 4: Measured iron levels against permit conditions for one company





1

10

100

1000

0.001 0.01 0.1 1 10 100

Tota

l su

spe

nd

ed

so

lids

(mg/

l)

Total Fe (mg/l)

Figure 5: Relationship between total iron & suspended solids. Transparent bar

denotes typical WwTW permit range of 3 to 5 mg/l

Downstream impacts of iron dosing

Chemistry

Examination of river monitoring data provided by the Environment Agency reflects the lack of

stability of iron in the water column (UKTAG, 2011). Measured concentrations of total or

dissolved iron downstream of WwTW pre- and post iron dosing showed no significant change

(Figures 6 and 7). A case study on the Warwickshire Avon has been used to illustrate this.

SIMCAT water quality modelling was used to confirm this conclusion and illustrated that even

dosing up to the permitted concentration still had little impact on downstream concentrations.

00.10.20.30.40.50.60.70.80.9

Tota

l iro

n (m

g/l

)

Dark bars denote post iron dosing, light bars pre dosing

Figure 6: Dissolved iron concentrations in rivers immediately downstream of a

WwTW pre- and post iron dosing for phosphorus reduction.





0.00

0.04

0.08

0.12

0.16

0.20

0.24D

isso

lve

d ir

on

(mg

/l)

Dark bars denote post iron dosing, light bars pre dosing

Figure 7: Dissolved iron concentrations in rivers immediately downstream of a

WwTW pre- and post iron dosing for phosphorus reduction.

Sediment

Particulate iron concentrations in 10 WwTW effluent discharges dosing iron were calculated for

1,973 sampling occasions. Average iron content was 6% (Figure 8) which falls in the middle of

that observed in UK stream sediments. Available data therefore suggest no impact of WwTW

dosing iron on downstream sediment iron concentrations, largely owing to the abundance of

iron in UK soils and sediments.

0102030405060708090

100

0 5 10 15 20 25 30 35

Pe

rce

nti

le (%

)

Particulate Fe in WwTW effluent dosing iron (%)

Figure 8: Cumulative distribution of particulate Fe percentages in WwTW

effluent (shaded area represents typical composition range of iron in UK sediments)





Ecology

Extensive ecological quality monitoring has been undertaken in the UK over the past 30 years or so. A ranked system, RIVPACS (Wright, 1994)) has been developed to compare observed and expected biodiversity of invertebrates (bottom-dwelling organisms) sampled by disturbing the river bed and capturing and counting the species present then ranking their ‘quality’ described as BMWP score (e.g. may flies and caddis flies rank highly as they are associated with clean water and chironimid worms low as they are associated with low oxygen, sediment rich poor quality water), the number of taxa present (NTAXA) and the average score per taxa (ASPT).

Examination of ecological data downstream of WwTW pre- and post-dosing failed to show any evidence of systematic degradation in ecological status. Indeed, in more cases than not, improvements were observed (Table 2). Given that the ecological measurements were based on invertebrate surveys on organisms which would most likely to be impacted by ‘smothering’ caused by precipitation of iron onto gills, and included data sets likely to be used to set any revised EQS for iron, the significance of this conclusion should not be under stated.

Table 2: Summary of differences in biological scores following

commencement of iron dosing at six WwTW

WwTW BMWP

Statistically

significant?1 NTAXA

Statistically

significant? ASPT

Statistically

significant?

1

✓ ✓ ✓

2 ✓ ✓ ✓

3 x ✓ ✓

4 x x ✓

5 x x ✓

6 ✓ ✓ X

Note: up arrows show increased score, down arrows the opposite, heavier shading shows statistical significance

1 t-Test: Two-Sample Assuming Equal Variances (95% confidence interval)





Further investigation into those sites with a significant decrease in biology scores shows that

corresponding upstream sites have also demonstrated a decline in scores (although not

significantly) (Figure 9). It is not possible therefore to conclude that iron dosing has caused a

negative impact on ecology at these sites.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Av

era

ge

AS

PT

sco

re

Upstream to Downstream along River Gipping

pre dosing post dosing

Sto

wm

arke

t Ww

TW

Ne

ed

ham

Mar

ket

Ww

TW

Figure 9: Cumulative impact on ASPT score from iron dosing at Stowmarket &

Needham Market WwTWs

WwTW 6 WwTW 4





Possible impacts of EQS changes

The data suggests that a modest reduction in the value of the current EQS (1 mg/l dissolved iron

as an annual average) may be accommodated based on existing treatment technologies

(potentially down to an EQS of 0.5 mg/l dissolved iron depending on permitting policies).

Any significant reduction in the EQS or strict adherence to a no deterioration policy, would lead

to the requirement for additional treatment (with commensurate costs and carbon emissions),

which may not be achievable using existing tertiary technologies. Figure 6 shows two scenarios

of possible additional costs should in-river iron targets change and show that depending on the

assumed current effluent quality, downstream targets of less than 0.5 mg/l dissolved iron lead

to an increasing risk of the necessity of tertiary treatment with commensurate rises in costs and

carbon emission.

The setting of downstream target concentrations of iron of less than 0.5 mg/l (dissolved) would

lead to an increasing requirement to install tertiary sand filters with commensurate increases in

costs and carbon emissions. In terms of additional costs to customers, assuming the retrofitting

of sand filters to works currently without them, figures for a single company (Anglian Water,

pers comms) suggests that the Capex would be £38.3 million with an Opex of £2.9 million,

leading to an increase in customers’ bills of £6 per household over 5 years (UKWIR, 2011)

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

0.0

0.4

0.8

1.2

1.6

2.0

0 0.2 0.4 0.6 0.8 1 1.2

Ton

ne

s C

O2/y

ear

Wh

ole

life

co

st (

£b

illi

on

)

(20

07

, 20

yr,

6%

dis

cou

nt)

Downstream target (mg-Fe diss/l)

(A) Addional whole life cost vs d/s Fe target

Whole life cost (£ billion)

Carbon dioxide (tonnes/yr)

Figure 8: Estimate cost curves where WwTW assumed to be discharging 1 mg-Fe

diss/l (roughly equivalent to a 4 mg-Fe tot/l permit).





0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

0.0

0.4

0.8

1.2

1.6

2.0

0 0.2 0.4 0.6 0.8 1 1.2

Ton

ne

s C

O2/

year

Wh

ole

life

co

st (£

bill

ion

)

(200

7, 2

0 yr

, 6%

dis

cou

nt)

Downstream target (mg-Fe diss/l)

(B) Addional whole life cost vs d/s Fe target

Whole life cost (£ billion)

Carbon dioxide (tonnes/yr)

Figure 8: Estimate cost curves where WwTW assumed to be discharging 0.2 mg-

Fe diss/l – typically achieved at moment.

Summary and recommendations for future permitting

The outcome from the review suggests that iron dosing at WwTW for phosphorus removal has

no significant impact on downstream sediment, water chemistry or ecology. The data questions

the need for iron permitting at all, unless it can be justified.

If deemed necessary, the UKWIR research (UKWIR,2011) concluded that a 4 mg/l total iron limit

would be protective of the existing EQS and should be at the vast majority of WwTW without

the need for sand filters. Alternatively, application of the EA’s river needs consenting policy

taking account of dilution and setting a permit as a 95th percentile appears also to be effectively

a cost-neutral option. However, this option would lead to significant relaxation of permits at

WwTW with high dilutions with some tightening of permits at WwTW with very low dilutions.

Setting a 95%ile permit based on existing effluent quality would represent a significant

tightening compared with current permit values and as it does not take dilution into account,

does not reflect any of the impacts (or lack of them) that the WwTW has on the receiving water.





Acknowledgements

The project team would like to thank the following for supporting this project:

• Gordon Wheale, UKWIR Project Manager for allowing us to present this project and paper.

• Environment Agency for allowing access to environmental monitoring data, ecological data and a view of the draft iron EQS.

• Dave Trigg of Staffordshire University for supplying the ecological data.

• The Project Steering Group for their support in providing valuable data, without which the project could not have been completed. .

• Manali Chalke (Brunel University) for undertaking the MSc and providing valuable synthesis of the ecological data available downstream of WwTW dosing iron.

• Mark Scrimshaw (Brunel University) for guidance and suggestions regarding the MSc.

References • Gehardt A. (1992) Effects of sub-acute doses of iron on Letophlebia marginata. Freshwater

Biology, (27), 79-84.

• UKWIR (2011) A review of the setting of iron limits for wastewater treatment works effluents (11/WW/20/4): . http://www.ukwir.org/ukwirlibrary/93693

• Wright, FW (1994) Development of RIVPACS in the UK and the value of the underlying data-base. Limnetica 10(1), 15-31


http://www.ukwir.org/ukwirlibrary/93693

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