WASTE OR RESOURCE: ’S BENEFICIAL USE OF WASTE STREAMS

Paul Weber1, Joe Wildy1, Fiona Crombie1, William Olds1, Phil Rossiter1, Mark Pizey1, Nathan Thompson2, Paul Comeskey2, Dave Stone2, Andrew Simcock3, Andy Matheson1, Karen Adair4, Mark Christison5, Hayden Mason6, Mark Milke7

1-Solid Energy Ltd; 2-Stockton Alliance New Zealand Ltd; 3-Biodiesel New Zealand Ltd; 4-Bio-Protection Research Centre, Lincoln University; 5- City Council; 6-Holcim New Zealand Ltd; 7-Natural and Civil Engineering School, University of Canterbury Correspondence to: [email protected] Abstract

In the last decade Solid Energy New Zealand Ltd - in collaboration with its research, applied technology and business partners - has made considerable progress in identifying waste resource streams which can be beneficially reused to support the state owned enterprise’s business objectives. Within the company’s business, Christchurch City’s high-quality biosolids are being used as a topsoil supplement in mine site rehabilitation. Coal ash from boilers is being used to create a capping material which reduces the formation of acid mine drainage. Cement kiln dust, a by-product of cement-manufacturing, is being used for waste rock capping to reduce acid mine drainage, and as a binding agent to fill and make safe underground voids prior to . Waste mussel shell is being used to create sulphate- reducing bioreactors to lower acidity and metals loads in mine water.

Within Solid Energy’s renewable energy companies (Nature’s Flame and Biodiesel New Zealand) - approximately 150,000 tonnes of wood pellet fuel has been manufactured from untreated plantation-grown pinewood offcuts, shavings, and sawdust; 6,000 tonnes of used cooking oil has been collected for conversion into biodiesel. Used engine oils from mining fleet operations and other suppliers are recycled and sold for boiler fuel. Biodiesel glycerol, a by-product of biodiesel production, is used to boost the creation of methane by anaerobic digestion at Christchurch City Council’s wastewater treatment plant. The gas is then converted into electricity and used for commercial heating purposes. Finally, municipal biosolids, in part dried by methane derived from anaerobic digestion boosted by biodiesel glycerol, has been successfully trialled as added-value fertiliser for biofuel crops.

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Introduction

Solid Energy extracts, processes, markets and distributes coking, thermal and specialist coal from underground and opencast mines at Huntly in Waikato, , and Westport on the West Coast, and in Southland. More than half of annual output is sold for export to major international steel industry customers. In New Zealand, Solid Energy’s customers include New Zealand Steel’s Glenbrook Mill and Huntly Power Station and industries such as dairy and meat processing, cement making, and the timber industry.

Solid Energy businesses Nature’s Flame and Biodiesel New Zealand (BDNZ) have been developing bioenergy opportunities. BDNZ produces its high-quality Biogold™ fuel from used vegetable oil and oilseed rape grown locally as a break crop. Nature’s Flame is New Zealand’s leading producer of clean-burning wood pellet fuel.

In the last decade Solid Energy New Zealand -- in collaboration with its research, applied technology and business partners -- has made considerable progress in identifying waste resource streams which can be beneficially reused to support the state owned enterprise’s business objectives, including the objective of having a net positive effect on the New Zealand environment. One component of this is the diversion for beneficial use at mine sites of bulk waste streams that might otherwise go to landfill. This results in a string of benefits. Scarce and increasingly expensive landfill space is preserved, extending the life of these facilities; the waste resource often substitutes for a raw, primary resource; the waste generator reduces their disposal costs; and the system reduces Solid Energy’s environmental impact. This paper discusses these projects.

Biosolids for mine site rehabilitation Field trials using biosolids were begun in collaboration with local authorities at (Christchurch City Council) and Mine (Watercare, Hamilton City Council) with the aim of developing successful, cost-effective systems for rehabilitating disturbed sites.

From 2007 - 2011 field trials were undertaken to evaluate one-off dose rates of biosolids incorporated into the soil matrix prior to traditional rehabilitation (where the final landform is shaped, overlain with a topsoil medium, and then planted). The outcome demonstrated that a one-off dose was the most suitable approach at Stockton mine site and Solid Energy 2 and Christchurch City Council (CCC) are now actively engaged in a project which, when fully operational, will result in the beneficial reuse of up to 6,000 tonnes of dry biosolids each year. This involved significant collaboration between the parties and CCC has commissioned a $30M thermal drying system for the biosolids, which reduces transport rates and produces a more manageable product for Stockton. At Rotowaro trials proved that full-scale application was unsuitable due to operational issues.

From 2009 to 2011 field trials were undertaken at Rotowaro mine to evaluate low-dose rate repeat surface applications of biosolids at application rates equivalent to the (textbook) agronomic requirement for nitrogen (200 kg N/ha.yr) and at application rates equivalent to twice the agronomic requirement for nitrogen (400 kg N/ha.yr). In August 2012 Solid Energy applied for resource consents for operational use of biosolids at Rotowaro mine and plan to start applying biosolids operationally from summer 2012/2013.

Solid Energy has worked from a premise that any use of biosolids (at Rotowaro and Stockton) should not compromise future land use options and thus the soil contaminant ceiling limits for metals and organic wastewater contaminants are below agricultural limits set by the Waikato District Council and have been taken in part from the accepted NZWWA Guidelines (2003); Canadian Environmental Quality Guidelines (CCME); USEPA Region 9 PRGs (Residential soils); and the Petroleum Guidelines (1999).

Nitrogen leaching is a significant issue in Waikato and thus biosolids application rates at Rotowaro are based on nitrogen loading and subsequent nitrogen loss (per Ha) rather than potential contaminants. Results from the 2009 - 2010 trials (200 kg N/ha.yr) demonstrated that the rate of nitrogen loss per unit area amounted to 5.6 kg N/ha.yr. Results from the 2010 - 2011 trials (400 kg N/ha.yr) demonstrated that the rate of nitrogen loss per unit area amounted to 5.8 kg N/ha.yr. These rates are well below the 30 kg N/ha.yr ceiling level applied to the site (this ceiling limit being typical of a dry stock farming unit in the area). Figure 1 shows nitrogen loss from Waipuna area at Rotowaro mine site into Te Whā stream and shows that immediately following the application periods there were minor spikes in the nitrogen discharge, although not significantly above the background levels.

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Figure 1. Nitrate nitrogen concentrations in site discharge to Te Wha Stream 2009 to 2012. Pink and red lines indicate 200 kg N/ha.yr application rates; purple and blue lines indicate 400 kg N/ha.yr.

Figure 2: Stockton Mine biosolids plots (month 10). Right ~300 dt/ha of biosolids. Left is the control plot which received a standard rate of inorganic fertiliser but no biosolids.

To date ~2,000 dry tonnes of biosolids have been used for rehabilitation at Stockton and 2,600 wet tonnes have been used at Rotowaro for rehabilitation. Significant beneficial outcomes for all parties involved in this project are expected over the next few years.

Coal ash for preventing acid mine drainage

Coal combustion produces ash (clinker and fly ash) is derived from minerals present in the coal such as clay and alumina-silicates, and un-burnt coal. Typically coal ash is 5-8 wt% of the coal burnt and this material is often disposed to landfill. Solid Energy has resource consents to beneficially reuse coal ash at Stockton Mine to manage acid mine drainage

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(AMD). AMD is generated by the oxidation of pyrite and the subsequent release of acidity and metals (Equation 1). Further details are available (e.g., Elder et al., 2011; Weber et al., 2008). Essentially, however, if oxygen is excluded, the production of AMD stops.

7 15 FeS2 + /2H2O + /4O2  Fe(OH)3 + 2H2SO4 (1)

Management, monitoring, and regulatory compliance standards for the coal ash resource consent were based on the Pennsylvania Department of Environmental Protection, Residual Waste Management Regulations (25 PA Code Chapter 287) 1992, and the West Virginia Department of Environmental Protection (DEP) guidelines1.

Typically, compacted fly ash has low permeabilities (10-7 m/s or lower), which means that it is difficult for water and oxygen to penetrate, thus making it an ideal covering material for potentially acid-forming material. Research has demonstrated that oxygen can be excluded by placing fly ash in 300mm layers over acid-forming rock. An additional benefit of using ash is that it has a moderate acid neutralisation capacity (ANC) ranging from 24 - 350 kg H2SO4/t (Table 1), which can also neutralise acid generated in the underlying rock. This is also reflected in the alkaline paste pH of the material (Table 1).

Table 1. Paste pH and ANC results for different products used on site for AMD control.

Sample Paste pH ANC (kg H2SO4/t) Various aglime products (<2mm) 8.4 - 10.8 747 - 966 CKD 11 - 13.9 479 - 788 Coal ash 5.96 - 11.96 24.5 – 354.2 Acid forming sandstone control 3.3 <1

The standard method for capping potentially acid-forming (PAF) waste rock overburden areas at Stockton to exclude oxygen and water is to use 400mm layers of compacted granite to achieve permeability of ~10-6 m/s. This material is then covered by 400mm of topsoil. Oxygen monitoring results (using 2m long oxygen probes) have indicated that coal ash significantly retards oxygen ingress (Figure 3).

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Figure 3. Oxygen concentration beneath capping materials. Atmospheric oxygen content is 20.9%

Because there can be issues with elevated metals in some coal ash, Solid Energy ensures that any ash it employs in rock capping does not interact with AMD. The least-risk option, and the best AMD management approach, is to place ash as an oxygen-excluding layer on top of acid-forming rock. To date, 9,100 tonnes of coal ash have been used at Stockton for the construction of oxygen-excluding covers for AMD management.

Cement Kiln Dust (CKD) covers for preventing acid mine drainage Holcim’s cement plant produces approximately 30,000+ tpa of cement kiln dust (CKD), a by- product of cement manufacture. CKD is highly alkaline with a paste pH ranging from 11-14 and an ANC ranging from 479 – 788 kg H2SO4/t and is a great source of neutralisation. Solid Energy has demonstrated that this material can also act as an oxygen-excluding cover (Figure 3) and has significant alkalinity (Table 1) that then percolates down through the underlying acid-forming rock, neutralising any acidity generated.

A 5,000 tonne CKD trial was undertaken at Stockton mine in 2006 to confirm the environmental benefits and operational logistics of using CKD. Oxygen probes and lysimeters to capture drainage were installed 2 metres under the CKD layer and control site (PAF waste rock only). Results demonstrated that this was an effective cap to exclude oxygen. Leachate collected from the lysimeters indicated that both the control and the CKD- capped rock produced pH 2.8 for the first year. Thereafter the pH of the CKD-treated rock was slightly higher than the control until the final sample, which had a pH of 5.7 compared to pH 2.5 in the control. Acidity results (Figure 4) also agree with decreased acid loads.

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35 Control 30 CKD Treatment 25 20 15 10

5 Acid Acid Load (g CaCO3) 0 0 20 40 60 80 100 Weeks

Figure 4. Control and CKD leachate acidity (derived from lysimeters).

In 2007 Solid Energy was granted resource consent by the West Coast Regional Council to use CKD for engineered caps to control AMD (40,000 tpa). Today, CKD is also mixed with granite to produce a cohesive material that resists erosion and has a greater ANC than 100% granite, thus providing neutralization to the underlying PAF rock. To date, ~90,000 tonnes have been delivered to Stockton for the construction of oxygen-excluding covers for AMD management.

Cement Kiln Dust (CKD) for the stowage and filling of underground voids

Solid Energy has obtained a second resource consent for the use of CKD, for the purpose of manufacturing a controlled, low-strength fill (CLSF) for stowage to fill voids created by historic underground mining (Figure 5a). The CLSF is an engineered fill comprised of overburden rock obtained from site, small proportions of general purpose cement (GPC) as required, and CKD from Holcim’s cement plant. Once a void has been filled with CLSF and the material has “set”, it prevents heavy mobile plant (excavators and trucks) from falling into the workings (safety issue) and also ensures that overburden does not contaminate the coal resource following blasting of the overlying rocks (waste minimisation and mining efficiency). Stowage using CLSF also helps in the suppression and control of underground fires in the old workings.

The collaborative research between Solid Energy and Holcim (lab trials, 100kg field trials, and operational trials at Stockton’s MG15 block) proved that the CKD provided pozzolonic properties to the CLSF, enabling improved cementitious behavior and the need for less

7 cement. Further research has demonstrated that once the CLSF is excavated, the material can be re-used a second time, blended with granite, and then applied as an engineered cap with similar permeabilities to other capping processes that reduce AMD.

Fig 5a. CLSF as placed within the MG15 void Fig 5b. Exposed CLSF during mining (MG15)

Waste mussel shell for AMD treatment

Mussel shells are ~92% by weight CaCO3 and thus an excellent source of neutralisation. Significant waste shell is available in New Zealand. An early trial using 10 tonnes of mussel shell beneath acid-forming rock demonstrated its capacity to neutralise acidity (Weber et al., 2008). A second trial was established at Stockton to build a mussel shell sulphate-reducing bioreactor (SRB) to treat AMD (e.g., Crombie et al., 2011). This work builds on from collaborative research with the University of Canterbury Natural and Civil Engineering School where SRB systems were investigated as part of a PhD project by Dr Craig McCauley.

Approximately 160 tonnes of fresh mussel shell, were placed in a 450 m3 sedimentation pond to treat influent AMD (< 1L/s). Significant improvements to water quality were observed (Table 2). Further details are available (Crombie et al., 2011). The results demonstrated that waste mussel shell from the seafood industry can be beneficially reused to treat AMD. To date ~800 tonnes of shell has been deployed to Stockton for beneficial purposes such as this. This technology could be rolled out to other sites in New Zealand and internationally, with significant waste-reduction benefits. Worldwide biofouling, due to mussels and other biofoulers, is a multi-billion dollar problem and this work could provide a solution for the waste created.

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Table 2. Influent and effluent water analysis for the mussel shell bioreactor. Dissolved Metals (mg/L) Ammonia EC Acidity DO pH Nitrogen Al Fe Ni Zn (µS/cm) (mg/L CaCO ) (mg/L) 3 (mg/L) Influent Min <0.003 0.02 0.001 0.5 2.1 332 240 2.1 0.001 Mean 47 31 2.47 1.2 2.8 1,229 422 8.5 0.11 Max 80 140 079 2.2 4.0 1,621 790 10.2 0.32 Effluent Min <0.003 <0.02 <0.0005 <0.001 6.2 943 0 0.5 0.01 Mean 0.013 0.17 0.028 0.008 6.9 1,377 0.3 2.8 4.6 Max 0.21 0.92 0.038 0.045 8.6 2,110 9.9 6.1 46

Renewable Energy businesses

Natures Flame was acquired in 2003 to produce premium wood pellets. To date, approximately 150,000 tonnes of quality wood pellet fuel has been manufactured from untreated pine wood offcuts, shavings and sawdust. In October 2009 Nature’s Flame commissioned New Zealand’s largest wood pellet fuel plant at Rakanui Road, Taupo. The $34M plant produces pellets that are valued nationally and internationally as a clean burning, low carbon emitting renewable fuel. Current production volumes for the Taupo plant are 30Ktpa now (using 6MW of drying heat) at 4 t/hr with scale up to 90Ktpa planned when a new geothermal or biomass heat plant is connected (18MW of drying heat) at 12t/hr. The site is consented up to 150Ktpa. This will be a significant renewable energy supplier in the future.

BDNZ produces its high-quality Biogold™ fuel from used vegetable oil and oilseed rape grown locally as a break crop. Approximately 6,000 tonnes of used cooking oil has been collected from food industry sources for conversion into biodiesel. BDNZ produces approximately 2 million litres of biodiesel per annum. Used engine oils from mining fleet operations and other suppliers is recycled and sold for boiler fuel via BDNZ. 10% of biodiesel production is a biodiesel glycerol byproduct.

Biodiesel glycerol for energy via anaerobic digestion

Worldwide the increase in biodiesel production has created an oversupply of biodiesel glycerol and depressed its commodity value. BDNZ produces up to 400,000 L pa (2011 figues) of biodiesel gylcerol and a solution was required. In 2008 research began to develop 9 a re-use alternative that would benefit the environment, reduce waste management costs, and provide a source of revenue. Research proved that the biodiesel glycerol was a good candidate for anaerobic digestion to produce methane (energy).

Laboratory trials were started at the University of Canterbury simulating the conditions typical of existing full-scale digesters currently used at Christchurch City Council’s (CCC) Bromley Wastewater Treatment Plant (Bromley). Each digester was fed with primary wastewater sludge and it was found that the addition of 3% to 6% biodiesel glycerol resulted in 200% to 300% increases in biogas production volumes (Figure 6a)

Figure 6a. Laboratory biogas production for Figure 6b. Bromley biogas production and significant raw sewerage co-digested with glycerol increases in gas following the addition of glycerol.

The CCC has a requirement for biogas to provide renewable energy. Thus a collaborative research programme between BDNZ, Solid Energy, and CCC commenced in 2009 and a full- scale field trial was undertaken at Bromley to determine the benefits of glycerol introduced to the Bromley anaerobic digesters’ influent feed. Results demonstrated that a significant increase in biogas output during periods of glycerol dosing (Fig 6b).

On the basis of the successful trials the biodiesel glycerol anaerobic digestion programme became fully operational in 2011 and continues to operate today. As of August 2012 Christchurch City Council is successfully co-digesting 100% of the biodiesel glycerol produced by BDNZ and there is capacity to take all expected future glycerol production. Additional lab work has been undertaken on using glycerol to dose anaerobic ponds used to manage dairy effluent, with similar significant increased biogas production observed.

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Biosolids for biofuel growth on marginal land One attractive goal of producing biofuels is achieving carbon-neutrality. Some biofuel crops, however, require significant inputs of agrichemicals, which can be energetically demanding to produce. Based on the significant growth responses where biosolids were applied at Solid Energy’s mine sites, a collaborative research project was initiated between Solid Energy and the Bio-Protection Research Centre at Lincoln University. The objective was to compare the use of biosolids to that of urea fertiliser on (1) the seed yield of biodiesel crop plants, and (2) soil chemistry. The work will soon be published and further details are available from the authors (Adair et al., in prep).

Results demonstrated that yield, determined by the weight of seed produced per plant, increased with the addition of biosolids compared to the control soil for both plant species investigated (B. napus and C. sativa). Seed yield from plants growing in soil amended with the low biosolids treatment was similar to that of plants grown in soil amended with urea fertiliser; however seed yield from the high biosolids treatment was greater than the fertiliser treatment (Figure 7).

Figure 7 – Mean seed yield per C. sativa (left) and B. napus (right) plant when grown in control soil, with urea fertiliser and two levels of biosolids (low: 200 kg ha-1 N equivalent and high: 400 kg ha-1 N equivalent applied prior to sowing

These results suggest that the use of biosolids to grow biofuel crops has significant advantages for growth rates. The use of biosolids may thus enable poor marginal land to be used productively for growing biofuel crops, which could have national importance.

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Conclusions

Solid Energy in collaboration with its research, applied technology and business partners, has made considerable progress in identifying waste resource streams which can be beneficially reused. Significant tonnes of waste have been diverted from landfill (Table 3), which contributes to significant beneficial resource reuse within the country. Further work is planned to look at other opportunities in the future and maximising the value within the current projects.

Table 3. Waste products beneficially reused by Solid Energy (since 2006). All values are total tonnes to date except where indicated by tpa (tonnes per annum). Waste Product (diverted for beneficial reuse to date) Tonnes (approx.) Biosolids 4,600 Coal Ash 9,100 Cement Kiln Dust 90,000 Mussel Shells 800 Waste Cooking Oil 6,000 tpa Glycerol 400 tpa Wood offcuts (sawdust, shavings etc) 150,000 Compost (derived from composting organic waste from freezing works) 3,000 tpa

References Adair, K., Wratten, S., Boyer, S., Barnes, A.M., Waterhouse, B., Smith, M., Weber, P. (in prep) Belowground impacts of enhanced biofuel crop production with biosolids. Soil Biology and Biochemistry.

Crombie, F.M., Weber, P.A., Lindsay, P., Thomas, D.G., Rutter, G.A., Shi, P., Rossiter, P., Pizey, M.H. (2011). Passive treatment of acid mine drainage using waste mussel shell, Stockton coal mine, New Zealand. In Proc. 7th Australian Acid and Metalliferous Drainage Workshop, 21- 24 June 2011, Darwin, . Ed L.C. Bell and B. Braddock (JK Tech Pty Ltd)

Elder, D.M., Pizey, M.H., Weber, P.A., Lindsay, P., Rossiter, P.J., Rutter, G.A., Thomas, D.G., Crombie, F.M., Cooper, T., Wildy, J.J., 2011. Addressing the environmental effects of mining on the Ngakawau River. Water New Zealand Annual Conference, Rotorua 9-10th November, 2011.

Weber, P.A., Lindsay, P., Hughes, J.B., Thomas, D.G., Rutter, G.A., Weisener, C.G., Pizey, M.H., 2008. ARD minimisation and treatment strategies at Stockton Coal Mine, New Zealand. In "Proceedings of the Sixth Australian Workshop on Acid and Metalliferous Drainage", Burnie, Tasmania. 15-18 April 2008. (Eds LC Bell, BMD Barrie, B Baddock, and RW MacLean) pp. 00-00 (ACMER: Brisbane).

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