force crag mwts odour field trials 1. introduction - gov.uk · pdf filereport date: 2 february...

WRc Ref: UC11694 V0.2/16566-0 2 February 2017

1© WRc plc 2017

Client: Coal Authority Report Ref: UC11694 V0.2

Report Date: 2 February 2017 Contract No: 16566-0

Author: David Sivil, Richard Hooper

Force Crag MWTS Odour Field Trials

1. Introduction

1.1 Background

Following pilot scale testing the Coal Authority installed passive bio-bed reactors, referred to as vertical flow ponds (VFPs), at the Force Crag Mine Water Treatment Scheme (MWTS) in Cumbria to successfully remove metals from contaminated mine water draining from an abandoned mine prior to continued discharge to an adjacent beck. The Coal Authority is looking to establish a similar system at Nent Haggs. The natural biological treatment process generates hydrogen sulphide during a sulphate reduction process which converts metal sulphates to less soluble sulphides. The hydrogen sulphide is present as a dissolved phase in the treated water. Due to the proximity of the proposed site at Nent Haggs to local residents and businesses a chemical treatment system will be required to remove dissolved hydrogen sulphide from the treated water prior to discharge. Technical consultants AECOM and the University of Newcastle have developed an initial treatment design plan, which involves dosing the VFP effluent with hydrogen peroxide (H2O2) to remove dissolves sulphide.

The Coal Authority commissioned WRc to review the proposed odour mitigation and treatment plan and this is reported in detail in WRc Report UC11596 and summarised in WRc Report UC12123. Following the review WRc proposed a site based odour treatment trial at a surrogate site – Force Crag, to produce treatment specific test data to optimise the odour


2© WRc plc 2017

treatment plan proposed by AECOM and specifically the requirements for chemical dosing using hydrogen peroxide.

The influent mine water at Force Crag contains circa 10 times less sulphate than that predicted at Nent Haggs. The impact of higher sulphate concentrations on the concentration of dissolved sulphide in the VFPS has not been fully quantified. This information will be required to design the final dosing system for the Nent Haggs scheme.

Hydrogen peroxide (H2O2) is used in other comparable sectors to reduce dissolved hydrogen sulphide in water and thereby atmospheric odour emissions. The dosing requirements will be based on flow rates, residence times in the treatment zone and wide water quality.

WRc recommended that experimental tests should be carried out to determine the hydrogen peroxide dose, contact time, effectiveness for removing hydrogen sulphide from mine water effluent treated by a VFP and the residual concentration of hydrogen sulphide that could be released in a discharge scenario. Ozone, a strong oxidising agent, has also been used in odour abatement, by itself and in combination with hydrogen peroxide but this treatment scenario was not assessed in these trials.

WRc suggested that it would be beneficial to carry out the experimental tests on both an established VFP and, if it were possible, a newly commissioned VFP that contains new organic material which would produce VFP effluent with a higher chemical oxygen demand (COD) concentration. The existing Force Crag MWTS in the Lake District has VFPs similar to those proposed for Nent Haggs MWTS and was selected for experimental tests on an established VFP. A newly commissioned VFP at a MWTS is not currently available. VFP effluent from a recently commissioned VFP was, therefore, simulated by adding some soluble organic material, derived from a similar biological media to that used in a VFP, to samples of Force Crag MWTS VFP effluent.

WRc also recommended that additional test should be included in any field trials to compare the reduction in the sulphate concentration across the VFPs with the measured dissolved sulphide1 in the VFP effluent. Understanding the relationship between the sulphate reduction and sulphide generation within the VFP at Force Crag MWTS VFP would provide more information for estimating the generation of sulphide in the proposed VFP for the Nent Haggs MWTS.

1 Dissolved sulphide is present in water in two forms, molecular H2S and ionic sulphide ions, and the fraction of each form is dependent on the pH value of the water. For example at pH 7, 50% of the dissolved sulphide will be in the ionic sulphide form and 50% will be in the molecular H2S form. Molecular H2S is volatile but sulphide ions are not. However, the species are in equilibrium with each other, so if some of the molecular H2S volatilises some of the sulphide ions will change to the molecular form to try to maintain the equilibrium.


3© WRc plc 2017

1.2 Objectives

The objectives of WRc’s odour field trials at Force Crag MWTS were as follows:

Determine the hydrogen peroxide dose, contact time, effectiveness for removing hydrogen sulphide from effluent in mine water treated by a VFP (with and without the addition of soluble organic material) and the residual hydrogen sulphide concentration.

Measure and compare the reduction in the sulphate concentration across the VFPs with the measured dissolved sulphide in the VFP effluent.

1.3 Report contents

This report contains the results, conclusions and recommendations from the odour field trials that were carried out at Force Crag MWTS in September 2016.

1.4 Glossary

BOD: Biochemical oxygen demand COD: Chemical oxygen demand FeS: Iron suphide H2O2: Hydrogen peroxide MWTS: Mine Water Treatment System VFP: Vertical flow pond ZnS: Zinc sulphide

2. Supporting information from WRc’s technical review

2.1 Hydrogen peroxide dosing for removing hydrogen sulphide

AECOM has carried out an environmental assessment for the Nent Haggs MWTS. AECOM has assumed at the outline design stage that hydrogen peroxide dosing would remove all traces of hydrogen sulphide from the VFP at the design stage. They have consequently concluded there would be no odour emission from residual hydrogen sulphide. In addition AECOM has not specified the design contact time that hydrogen peroxide would need to be in contact with the VFP treated mine water to provide effective treatment. Additional parameters, such as effluent contact time would need to be considered in the detailed engineering design stage.

WRc has previously carried out experimental tests on behalf of a collaboration of water utilities to assess the removal efficiency of hydrogen sulphide in waste water treatment sewage by hydrogen peroxide dosing (WRc, 2006). WRc found that hydrogen peroxide dosing could achieve 99% removal after a contact time of 10 minutes. The UKWIR Technical


4© WRc plc 2017

Reference Document on odour control (Hobson, 2002) states that hydrogen peroxide is typically used up within 10 to 15 minutes of being dosed into wastewater. An article published by Alken Murray Corporation (2016) states that generally 90% of the reaction between hydrogen peroxide and hydrogen sulphide takes place within 10 to 15 minutes, with the balance reacting in an additional 20 to 30 minutes.

The Odour Control Options Appraisal by AECOM has assumed a hydrogen peroxide to hydrogen sulphide dosing ratio of 1.5:1 for treating the VFP effluent when it has become established and the COD has been estimated to be less than 10 mg/l. During the commissioning and establishment phases of the VFP the COD concentration has been estimated to be 100 and 3,000 mg/l, respectively, due to soluble organic material transferring from the biological media to the VFP effluent (AECOM 2016). It has been assumed that the hydrogen peroxide to hydrogen sulphide dosing ratios would be 4:1 and 3:1 respectively. The UKWIR Technical Reference Document on odour control (UKWIR, 2002) states that typical field applications of hydrogen peroxide require one to three kg of hydrogen peroxide per kg sulphide present in wastewater. This research was carried out on material which is directly relevant to the VFPs which will contain a high level of organic material during the commissioning phase.

WRc recommended that experimental field trials be carried out to determine the dose, contact and effectiveness of hydrogen peroxide dosing for removing hydrogen sulphide from VFP treated mine water. This work was suggested as a method to establish the proportion of the residual H2S remaining in hydrogen peroxide dosedVFP effluent compared to the H2S concentration in the undosed VFP effluent. .

2.2 Comparison of sulphate concentration reduction across VFP with hydrogen sulphide generation

WRc’s technical review noted that sulphate concentrations measured by Newcastle University in the effluent of the VFPs at Force Crag MWTS were approximately 10 to 15 mg/l lower than in the influent. WRc has calculated that if 10 to 15mg/l of sulphate was reduced and fully converted to hydrogen sulphide in the VFP effluent the hydrogen sulphide concentration would be 3.5 to 5.3 mg/l rather than 15 mg/l stated in the AECOM documents reviewed. This is based on sulphate concentrations in both influent and effluent.

Newcastle University had measured sulphide concentrations in the VFP effluent that did not exceed 2 mg/l. WRc recommended that further measurements were carried out at Force Crag MWTS to compare the reduction in the sulphate concentration across the VFPs with the measured dissolved sulphide in the VFP effluent. WRc also recommended that both COD and BOD data from Force Crag be reviewed to establish if there is a relationship between these variables and sulphide generation within the VFPs and transfer into the effluent.


5© WRc plc 2017

3. Method

3.1 Effectiveness of hydrogen peroxide for dissolved sulphide removal

WRc carried out the experimental work on 20/9/16 and 21/9/16 at Force Crag MWTS to minimise the storage time of test samples and the loss of dissolved sulphide prior to the tests which were carried out on site. There are two VFPs on the site but the inlet pipe to VFP2 was submerged, so the experimental work was only carried out on VFP1, see Figure 1.

Figure 1 Schematic of Force Crag MWTS and photographs of the VFP effluent discharge chamber and discharge pipe C

VFP1 treated mine water effluent was collected prior to the outlet cascade. The flow of the effluent over the cascade has the potential to release some, or all, of the dissolved sulphide from the effluent, which could result in a non-representative sample being tested The effluent samples were collected from the VFP1 discharge pipe ‘C’. A dissolved oxygen probe was used to measure the dissolved oxygen concentration in the samples to check that low dissolved oxygen conditions were maintained. The dissolved oxygen concentrations of the VFP effluent samples were generally between 0.9 and 1.4 mg/l, and are unlikely to have had

VFP1

Single Influent

pipe

Chamber containing all four effluent discharge

pipes (C, A, B and D)


6© WRc plc 2017

any significant effects on the results of the experimental trials. WRc consider that the dissolved oxygen levels observed in the effluent therefore have had limited effect on the dosing trial results.

It was thought possible that some hydrogen sulphide would de-gas from the VFP effluent over a period of time after it discharges into the effluent chamber. Consequently, initial tests were carried out on control samples (with and without additional dissolved organic material to mimic conditions in a ‘new’ biobed where there are leachable organics) stored for different periods of time prior to hydrogen sulphide measurements being made to establish when the equilibrium values of dissolved sulphide become stable. These preliminary tests found that the storing of the samples for 15 minutes prior to the tests allowed the dissolved oxygen concentrations to stabilise.

An experimental matrix of hydrogen peroxide dosing concentrations and contact times was carried out on samples of the VFP treated mine water with and without additions of soluble organic material. The experimental test parameter values (hydrogen peroxide dose and contact time) were adapted from the experimental results recorded.

Leachable organics from compost and bark currently used in the VFP biobed are likely to contain predominantly fulvic and humic acids. Attempts by WRc to generate the soluble organic material from a mixture of compost and bark produced an organic solution with a low COD concentration. As an alternative, an organic solution called ‘Ful-Power’ that contained fulvic acid, derived from freshwater humic sediments was purchased from Bioag Europe AG (http://bioag.eu/en/com-virtuemart-menu-products/humic-and-fulvic-acid-products/humic-fulvic-acid-products-for-agriculture-and-gardening). The COD concentration of the neat Ful-Power solution was measured as 209 mg/l.

The Ful-Power material was considered by WRc as an appropriate source of organic material for the tests to show the general effects of elevations in the COD concentrations in the VFP effluent on the removal of hydrogen sulphide by hydrogen peroxide. Whilst this may be an overestimate of the longer term COD concentration in a bio-bed, the experiment was designed to mimic the effect of additional organic material and the increase of peroxide dosing needed for effective sulphide removal.

WRc measured the dissolved sulphide concentration, pH value and temperature in the initial VFP effluent (with or without the additional soluble organic material) and after each set of experimental conditions. The chemical oxygen demand (COD) concentration was measured in the initial VFP effluent (with and without the additional soluble organic material).

For the field testing WRc used an established method for measuring the concentration of hydrogen sulphide in wastewater by measuring the concentration of hydrogen sulphide in the gas phase that was in equilibrium with the dissolved hydrogen sulphide in the liquid phase. The method involves filling a container two thirds with the wastewater sample, sealing the container and shaking it for 45 seconds and measuring the hydrogen sulphide concentration

http://bioag.eu/en/com-virtuemart-menu-products/humic-and-fulvic-acid-products/humic-


7© WRc plc 2017

in the headspace using either a Dräger hydrogen sulphide short-term gas detection tube or a Jerome hydrogen sulphide monitor.

WRc Technical Report TR142 (WRc, 1980) has information on the equilibrium relationship of volatile dissolved hydrogen sulphide in aqueous solutions with hydrogen sulphide concentrations in air. The report also has information on the proportions of dissolved hydrogen sulphide in aqueous solution that are in the volatile and non-volatile forms for a particular pH value (see note 1 on page 2 for an explanation of the different forms of hydrogen sulphide in aqueous solution). Using information from WRc Technical Report TR142, the concentrations of volatile and total hydrogen sulphide dissolved in each wastewater sample was calculated from the concentration of hydrogen sulphide in the ‘Shake’ test headspace. The ‘Shake’ test approach eliminated the need to add chemicals, such as zinc acetate, to preserve the dissolved sulphide in the form of zinc sulphide prior to laboratory analysis, which could have led to misleading dissolved sulphide measurements due to zinc sulphide that was already present in the VFP effluent.

3.2 Experimental overview

Each test in the hydrogen peroxide trials was carried out on an 8-litre sample of outlet effluent from VFP1 effluent when no additional organic solution was added. When organic solution was added the total size of the test samples was reduced to 0.66 litres to reduce the total volume of organic solution that was needed to be used on site. There are expected to be minimal differences between the results of tests carried out with 8 and 0.66-litre volume samples.

The hydrogen peroxide was mixed thoroughly with each sample of mine water (with or without the additional soluble organic solution) in a mixing container. After the specified contact time for a particular test the resultant sample was transferred to the ‘Shake’ test container for the dissolved sulphide measurement to be made. Twelve and one litre shake containers were used for the 8 and 0.66 litre samples, respectively, to maintain the same 2:1 liquid to headspace ratio.

A summary of the ranges of experimental conditions tested is shown in Table 1. The experimental results were used to calculate the initial sulphide concentration, the percentage removals and residual concentrations for each hydrogen peroxide dose, contact time and COD concentration tested.

3.3 Comparison of sulphate concentration change and dissolved sulphide generation across VFP

Samples of initial mine water influent collected prior to the VFP1 treatment system, and VFP1 effluent from two of the four effluent pipes (C and D) were analysed immediately on site for:

sulphate concentration;


8© WRc plc 2017

dissolved sulphide concentration;

dissolved oxygen;

pH value; and

temperature.

Table 1 The ranges of experimental conditions tested

Addition of soluble organic material

expressed as additional COD

(mg/l)

Range of VFP effluent storage time prior to tests (stabilisation time) tested (mins)

Range of contact times tested (mins)

Range of hydrogen peroxide dose to

hydrogen sulphide ratios tested (mg/mg)

0 0 to 20 0 0

15 5 to 30 4.7:1 to 20.4:1

50 15 0 0

15 15 12.0:1 to 25.8:1(a)

120 15 0 0 15 15 33.3:1(a)

Note: (a) Higher dose ratios were not tested on the grounds that they would likely to be uneconomical for a full scale process.

Sub-samples were sent for laboratory analysis for:

chemical oxygen demand (COD) (total and soluble);

biochemical oxygen demand (BOD) (total and soluble);

suspended solids;

zinc (total and soluble); and

iron (total and soluble).

A mass balance in terms of sulphur was calculated for the VFP system that covered the decrease in the sulphate (SO4) concentration across the VFP, the hydrogen sulphide (H2S) generated in the VFP and the estimated mass of zinc sulphide (ZnS) and iron sulphide (FeS) precipitated out within the VFP process.


9© WRc plc 2017

4. Results

4.1 Effectiveness of hydrogen peroxide for dissolved sulphide removal

A summary of the experimental results for the removal of hydrogen sulphide from VFP treated mine water using hydrogen peroxide are included in Table 2. Detailed results are contained in Table A1 in Appendix A.

Table 2 Summary of experimental results for the removal of hydrogen sulphide from VFP1 treated mine water by dosing hydrogen peroxide

COD (total) (mg/l) VFP1 effluent VFP1 effluent plus additional soluble organic material

<11 68 130 Optimum contact time (minutes) 15 15 15 Optimum hydrogen peroxide dose to hydrogen sulphide ratio (mg/mg) 10.2 25.8(b) 33.3(b)

% removal of hydrogen sulphide at optimum conditions 61.5 - 78.5%(a) 41.5% 13.9%

Notes: (a) A maximum percentage removal of hydrogen sulphide from VFP1 effluent of 82.1% was achieved with a higher H2O2 : H2S dose ratio of 18.7 mg/mg. (b) Higher dose ratios were not tested on the grounds that they would likely to be uneconomical for a full scale process.

The total COD concentration in the VFP1 effluent without any addition of soluble organic material was less than 11 mg/l, which was similar to that estimated for the proposed VFP for Nent Haggs during normal operation.

Hydrogen peroxide was found to remove a maximum of 82.1% of total dissolved sulphide present in the VFP1 effluent, using a contact time of 15 minutes and a hydrogen peroxide to hydrogen sulphide dose ratio of 18.7:1. Up to 78.5% removal of total dissolved sulphide was achieved by the same contact time, but approximately half the dose ratio (10.2:1). This was identified as the optimum dose when balancing the removal performance against the dose ratio needed. Therefore the indicative dosing estimated by AECOM (2016) appears to be an underestimate using the results from field trials.

The addition of the soluble organic material to the VFP effluent to increase the COD concentration by approximately 50 mg/l considerably reduced the effectiveness of hydrogen peroxide to remove total dissolved sulphide. For instance, a contact time of 15 minutes and hydrogen peroxide to hydrogen sulphide dose ratio of 25.8:1 achieved a removal of total dissolved sulphide of 41.5%.


10© WRc plc 2017

Increasing the VFP effluent COD concentration by approximately 120 mg/l (a similar concentration estimated for the proposed Nents Haggs VFP during the establishment phase) using a higher dose of soluble organic material reduced the removal of total dissolved sulphide to 13.9%, despite using a contact time of 15 minutes and hydrogen peroxide to hydrogen sulphide dose ratio of 33.3:1. This dose ratio was more than eleven times the dose ratio of 3:1 estimated by AECOM. The addition of this higher dose of soluble organic material was found to decrease the initial pH value of the VFP effluent from approximately pH 7 to 6. Changes in the pH of the VFP effluent during a commissioning phase and steady-state operational phase may have an influence on the volatile and non-volatile sulphide levels.

From the above results it can be predicted that the removal of hydrogen sulphide using hydrogen peroxide would be less than 13.9% for the commissioning phase of the proposed VFP at Nent Haggs when the COD concentration has been estimated to be 3,000mg/l.

4.2 Comparison of sulphate concentration change and dissolved sulphide generation across VFP

The comparison of the sulphate concentration reduction across VFP 1 to the generation of dissolved sulphide, zinc sulphide and iron sulphide within VFP1 are presented in detail in Table A2 (in Appendix A) and as a summary in Table 3 (in this section). The reduction of the sulphate concentration across VFP1 of 18 mg/l was equivalent to 6 mg/l expressed as the sulphur content. The total sulphide concentration generated was 0.59 mg/l, which was equivalent to 0.55 mg/l as sulphur, based on hydrogen sulphide. The zinc sulphide and iron sulphide generation estimates were 4.9 and 0.3 mg/l, respectively, which together corresponded to a sulphur content of 1.7 mg/l. A sulphur mass balance for VFP1 found that out of the 6 mg/l of sulphur from the reduction of sulphate, 2.3 mg/l of the sulphur (38%) could be accounted for by the generation of hydrogen sulphide, zinc sulphide and iron sulphide.

Additional analysis of the VFP influent and effluent streams, see Table A3 (in Appendix A), found that the COD concentrations were less than 11 mg/l and the BOD was less than 5 mg/l in both the VFP inlet and outlet. These concentrations were very similar to those measured previously by Newcastle University at Force Crag MWTS and used in the Nent Haggs MWTS Odour Control Options Appraisal Document (AECOM, 2016) as the estimated VFP effluent values for the Nent Haggs MWTS under established operation,

The sulphate concentration in the influent for VFP1 at Force Crag MWTS was measured as 24 mg/l and is only about 10% of the sulphate concentration of 250mg/l measured at Nent Haggs MWTS by Newcastle University. The generation of hydrogen sulphide by sulphate reducing bacteria, when they chemically reduce sulphate to sulphide in anaerobic conditions (such as sewer rising mains), has been found to be related to the COD and BOD concentrations in the wastewater. Therefore, it is possible that the sulphide generation in the VFP at Force Crag MWTS was limited by the low COD and BOD concentrations in the influent mine water rather than the influent sulphate concentration. Laboratory scale VFP plants operated by Newcastle University during the past ten years also found that the addition of


11© WRc plc 2017

carbon to VFPs increased the removal of sulphate in the VFPs even when the initial sulphate concentration was not changed, as reported in the Nent Haggs MWTS Odour Control Options Appraisal Document (AECOM 2016, Figure 2.1.1)

Further work is required to be carried out to strengthen confidence in the estimate of the hydrogen sulphide that would be expected to be generated in the VFP at Nent Haggs MWTS by re-examining previous experimental work carried out by Newcastle University on the full-scale VFPs at Force Crag MWTS, the pilot-scale VFP at Nenthead and laboratory scale VFPs.

Table 3 Comparison of the removal of sulphate in VFP1 with the generation of hydrogen sulphide, zinc sulphide and iron sulphide

Sample

Sulphate reduction Hydrogen sulphide

generation Zinc & iron sulphide

generation estimation

Total for H2S, ZnS &

FeS generation

Sulphate ions conc. (mg/l SO4)

Sulphur equiv conc. (mg/l)

H2S conc.(mg/l)


ZnS conc.(mg/l)

FeS conc. (mg/l)


Sulphur equiv conc.

(mg/l)

VFP 1 Influent 24.0 8.00 <0.01 <0.01 VFP1 Effluent (mean for effluent discharges C & D)

6.00 2.00 0.59 0.55

Mean change across VFP1 -18.0 -6.00 0.59 0.55 4.90 0.30 1.70 2.30


12© WRc plc 2017

5. Conclusions

The conclusions drawn from the September 2016 odour trials at Force Crag MWTS are as follows:

Effectiveness of removing hydrogen sulphide from VFP effluent by dosing with hydrogen peroxide: Hydrogen peroxide was found to remove a maximum of 82.1% of total dissolved sulphide present in the VFP1 effluent, using a contact time of 15 minutes and a hydrogen peroxide to hydrogen sulphide dose ratio of 18.7:1. Up to 78.5% removal of total dissolved sulphide was achieved by the same contact time but half the dose ratio (10.2:1). This was identified as the optimum dose when balancing the removal performance against the dose ratio needed but was still more than six times the dose ratio of 1.5:1 estimated by AECOM.

The effects of elevated COD concentrations in the VFP effluent on the effectiveness of hydrogen peroxide to remove hydrogen sulphide: The addition of organic material equivalent to the VFP effluent, to increase the COD concentration by the order of 50 mg/l, decreased the removal of total dissolved sulphide by hydrogen peroxide. For instance, a contact time of 15 minutes and hydrogen peroxide to hydrogen sulphide dose ratio of 25.8:1 the removal of total dissolved sulphide was 41.5%. A larger increase in the COD concentration of in the order of 100mg/l (a similar concentration estimated for the proposed Nents Haggs VFP during the establishment phase) reduced the removal of total dissolved sulphide to 13.9%, despite a hydrogen peroxide to hydrogen sulphide dose ratio of 33.3:1. This dose ratio was more than eleven times the dose ratio of 3:1 estimated by AECOM. From the above results it can be predicted that the removal of hydrogen sulphide using hydrogen peroxide would be less than 13.9% for the commissioning phase of the proposed VFP at Nent Haggs when the COD concentration has been estimated to be 3,000mg/l (Table 3).

Comparison of the reduction of sulphate across the VFP to the generation of hydrogen sulphide: The decrease in the sulphate concentration across VFP1 of 18 mg/l (equivalent to 6 mg/l expressed as the sulphur content) corresponded to the generation of hydrogen sulphide of 0.59 mg/l. The total concentration of sulphur ‘locked up’ in the VFP process as zinc sulphide and iron sulphide was estimated, from the analysis of the VFP influent and effluent for zinc and iron, to be 1.7 mg/l. A mass balance for sulphur across VFP1 found that 38% of the sulphur from the decrease in the sulphate concentration could be accounted by the sulphur in the hydrogen sulphide, zinc sulphide and iron sulphide.

Comparison of the sulphate concentrations in the VFP influent at Force Crag MWTS and the proposed Nent Haggs MWTS: The measured sulphate concentration in the influent for the VFP at Force Crag MWTS was only 10% (24 mg/l) of the sulphate


13© WRc plc 2017

concentration measured in the mine water influent that would be treated at the Nent Haggs MWTS (250 mg/l). However, the generation of hydrogen sulphide in VFPs may be limited by the COD/BOD concentrations in the VFP influent rather than the sulphate concentration.

The results from Force Crag, and other research, show that hydrogen peroxide dosing provides an effective treatment for removal of dissolved hydrogen sulphide (H2S): However, at realistic dosing rates there will still be a level of residual dissolved H2S in the water phase as the removal efficiency is never 100% and the dosing rates required to achieve even 90% removal are considerable. The impact of any subsequent release of hydrogen sulphide on identified nearby receptors would need to be further quantified. Factors, such as discharge turbulence, may help to dissipate this residual gas and additional/alternative dosing methods for odour control may be of benefit.


14© WRc plc 2017

6. Recommendations

WRc recommends that the following work is carried out:

Experimental trials with ozone and a combination of hydrogen peroxide and ozone: If the performance of hydrogen peroxide dosing for removing 78 to 82% of hydrogen sulphide, as measured during the Force Crag MWTS trials, is not deemed sufficient to enable atmospheric odour concentration limits around the proposed Nents Haggs MWTS to be met when the VFP has become established, WRc advises that additional experimental trials are carried out at the Force Crag MWTS to test alternative reagents to hydrogen peroxide. Furthermore, during the VFP commissioning and establishment phases the removal of hydrogen sulphide by hydrogen peroxide is not expected to be cost effective. Alternative reagents could include ozone and a combination of hydrogen peroxide and ozone. The trials would test whether ozone and/or a combination of hydrogen peroxide and ozone would be more effective than hydrogen peroxide for removing hydrogen sulphide.

Re-assess the range of potential estimates for the hydrogen sulphide that could be generated by the proposed VFP at Nent Haggs MWTS: WRc recommends that previous experimental data collected by Newcastle University from the Nenthead VFP pilot plant, Force Crag MWTS and laboratory tests are re-examined. The aim of the re-examination would be to determine how the hydrogen sulphide concentration in the VFP effluent corresponds to the influent sulphate, COD and BOD concentrations, COD and BOD concentrations leached from the bio-bed material during the commissioning and establishment phases and the removal of metal species within the VFP.

Estimate the percentage removal of hydrogen sulphide required to be achieved by odour control measures: WRc recommends that the Coal Authority commissions their environmental modelling contractor to adapt and re-run existing atmospheric dispersion models for Nent Haggs MWTS to estimate the maximum residual hydrogen sulphide concentration in the VFP treated mine water at the proposed MWTS that would meet the odour concentration limits chosen for the proposed MWTS site boundary. An estimate of the maximum residual hydrogen sulphide concentration would allow the minimum performance level of the odour control measures to be determined for the re-assessed estimate for the sulphide concentration in the VFP effluent.


15© WRc plc 2017

7. References

AECOM, 2016. “Haggs Mine Water Treatment Scheme – Odour Control Options Appraisal” – March 2016, Ref: 47072599_MARP014, AECOM (Rev 0 LIB_20160407)

Alken Murray Corp. Pollution Control, 2016. “Solving the Hydrogen Sulphide Problem.” Alken Murray Corp. Pollution Control, Virginia, USA (http://www.alken-murray.com/H2SREM5.HTM)

UKWIR, 2002. UKWIR Report “Odour Control in Wastewater Treatment – A Technical Reference Document.” Ref No.01/WW/13/3.

WRc, 1980. “The determination of safe limits for the discharge of volatile materials to sewers”. WRc Technical Report TR142.

WRc, 2006. “Use of Additives to Control Odour in Wastewater Treatment Processes.” WRc Portfolio Report P6830.

WRc, 2016. “Nent Haggs Minewater Treatment Scheme – Independent Review of Proposed Odour Abatement.” WRc Draft Report UC11596.01.

© WRc plc 2017 The contents of this document are subject to copyright and all rights are reserved. No part of this document may be reproduced, stored in a retrieval system or transmitted, in any form or by any means electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of WRc plc.

Any enquiries relating to this report should be referred to the Author at the following address:

David Sivil, Richard Hooper WRc plc, Frankland Road, Blagrove, Swindon, Wiltshire, SN5 8YF

Telephone: + 44 (0) 1793 865134 Fax: + 44 (0) 1793 865001 Email: [email protected] Website: www.wrcplc.co.uk

http://www.alken-murray.com/H2SREM5.HTM)

mailto:[email protected]

http://www.wrcplc.co.uk


16© WRc plc 2017

Appendix A Detailed results for odour trials at Force Crag MWTS A1 Detailed results of hydrogen sulphide removal trials from VFP treated mine water using hydrogen peroxide

Table A1 Detailed results for the hydrogen peroxide dosing trials

Date

Test condition Analysis of initial VFP1 sample Analysis of resultant VFP1 sample dosed with H2O2

Stabil-isation

time prior to

tests (mins)

Organic material added

COD (mg/l)

Total test

volume (litres)

Contact time

(mins)

H2O2 dose (ml)

Sample Ref.

Volatile sulphide

(mg/l) pH

Total sulphide

(mg/l)

H2O2 to H2S

dose ratio

(mg/mg)

Sample Ref.

Volatile sulphide

(mg/l) pH

Total sulphide

(mg/l)

Total sulphide removal

(%)

20/09/16 15 No <11 8.0 10 5 Mean of FC12 & FC13

0.172 7.15 0.400 4.7 FC14 0.039 7.42 0.148 63.1

15 10 9.4 FC18 0.028 7.36 0.086 78.5 15 20 18.7 FC19 0.023 7.34 0.072 82.1 30 5 4.7 FC22 0.028 7.38 0.089 77.8 30 20 18.7 FC23 0.028 7.36 0.086 78.5

21/09/16 15 No <11 8.0 5 20 FC26 0.183 6.96 0.367 20.4 FC29 0.055 7.35 0.172 53.1 15 5 5.1 FC30 0.041 7.39 0.138 62.5 15 10 10.2 FC31 0.032 7.38 0.104 71.8 15 10 10.2 FC35 0.041 7.49 0.141 61.5

21/09/16 15 Yes 68 0.660 15 1 FC32 0.206 6.88 0.352 12.9 FC34 0.115 6.98 0.229 34.9 15 2 25.8 FC36 0.133 6.80 0.206 41.5 Yes 130 0.660 15 2 FC38 0.252 5.95 0.273 33.3 FC37 0.206 6.18 0.235 13.9


17© WRc plc 2017

A2 Detailed results for the analysis of VFP1 influent and effluent

Detailed results for the analysis of VFP1 influent and effluent are contained in Table A2 and Table A3. The results in the former table are used to carry out a sulphur mass balance across VFP1.

Table A2 Analysis of VFP1 influent and effluent and sulphur mass balance

Sample

Sulphate reduction across VFP1

Hydrogen sulphide generation in VFP1 Zinc sulphide generation in VFP 1 Iron sulphide generation in VFP 1

Total for H2S, ZnS & FeS

Sulphate conc (SO4

mg/l)

Sulphur equiv (mg/l)

Total dissolved hydrogen sulphide

(H2S mg/l)


Zinc (total) (mg/l)

Zinc (soluble)

(mg/l)

Zinc sulphide

generation estimate

(mg/l)


Iron (total) (mg/l)

Iron (soluble)

(mg/l)

Iron sulphide

generation estimate

(mg/l)



VFP 1 Influent 24.0 8.0 0.00 0.00 3.51 3.25 0.61 <0.23

VFP 1 Effluent - Quadrant C 5.0 1.7 0.62 0.59 0.13 <0.018 0.28 <0.23

VFP 1 Effluent - Quadrant D 7.0 2.3 0.55 0.52 0.26 0.04 0.56 <0.23

Mean change across VFP 1 -18.0 -6.0 0.59 0.55 3.31 3.22 4.9 1.6 0.19 <0.23 0.3 0.1 2.3

Table A3 Additional analysis of VFP1 influent and effluent

Sample Dissolved

oxygen (mg/l)

Temperature (oC) pH COD (total)

(mg/l) COD

(soluble) (mg/l)

BOD (total) (mg/l)

BOD (soluble)

(mg/l)

Suspended solids (mg/l)

VFP 1 Influent 5.9 11.9 6.34 <11.0 <11.0 1 <1 4

VFP 1 Effluent - Quadrant C

1.4 13.5 7.04 <11.0 <11.0 2 3 4

VFP 1 Effluent - Quadrant D

1.3 13.3 7.02 <11.0 <11.0 2 4 4

force crag mwts odour field trials 1. introduction - gov.uk · pdf filereport date: 2 february...

Documents