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Proceedings Sardinia 2007, Eleventh International Waste Management and Landfill Symposium S. Margherita di Pula, Cagliari, Italy; 1 - 5 October 2007 © 2007 by CISA, Environmental Sanitary Engineering Centre, Italy

WOODLAWN BIOREACTOR - PROGRESS REPORT ON THE WORLD'S LARGEST BIOREACTOR PROJECT

S. HANCOCK*, S. RAINFORD**, C. ALEXANDER**, D. IFE* AND M. JORGENSEN*

*URS Australia Pty Ltd, Level 6, 1 Southbank Boulevard, Southbank 3006, Melbourne, Australia **VEOLIA Environmental Services, Level 4, 65 Pirrama Road, Pyrmont, NSW 2009, Australia

SUMMARY: The Woodlawn Bioreactor has now (April 2007) been in operation 2.5 years and has gone through a variety of operational issues which have generated management responses to ensure that the objectives of the facility, namely the orderly disposal of 480,000 t of waste per annum; the expected generation of energy and, finally, the avoidance of environmental impacts is achieved. This paper describes the issues encountered which include initial waste saturation with acid sulfate rich runoff and groundwater inflows; H2S generated OH&S issues, the competition of sulfur reducing bacteria with methanogens and the maintenance of effective gas harvesting, gas generation and monitoring. During this process a great deal has been learned about the processes within the waste mass and how these can be controlled and utilized. An essential element of the learning process has been and will continue to be, the focus of monitoring on parameters which reflect the processes which are sought to be managed to achieve optimal benefit.

1. GENERAL

The development and studies surrounding the Woodlawn Bioreactor have been presented in several papers at Sardinia. These include Hancock, S. et al (2001), Moreau-Le Golvan, Y. et al (2003 and 2005). Many other technical contributions have assisted the authors and these are referenced herein or in the above listed papers.

This paper outlines the various technical challenges which have emerged in the operation of the Woodlawn Bioreactor and reviews aspects of the previous 18 months of operation.

The technical issues of concern covered in this paper are ▪ Occupational Health and Safety (OH&S) issues arising from the potential release of hydrogen

sulfide (H2S) gas; ▪ Implications of the presence of H2S in landfill gas as a source of energy for power generation

(trends of H2S and LFG production); ▪ Indications arising from leachate chemistry in respect to gas production and the opportunities

for increased gas production (description of leachate chemistry and trends);

Sardinia 2007, Eleventh International Waste Management and Landfill Symposium

▪ Ongoing monitoring requirements. The Woodlawn Bioreactor is being developed in a former open pit base metal mine located to the south of Sydney in the Southern Tablelands of New South Wales (Figure 1).

Figure 1. Woodlawn Bioreactor

Plate 1. Woodlawn Bioreactor Landfill


This base metal (Cu, Zn, Pb, Ag) mine operated from 1974 and closed as an underground mine in 1998. The pit is located on the Great Dividing Range and has its upper rim at approximately 820m AHD. The open pit was approximately 200m deep and has a rectangular conical shape which extended from a small sump at the base to have an area of approximately 38ha within the upper rim. The rocks which abut the pit dip steeply to the south west and are only permeable for groundwater to the extent that interconnecting fractures are exposed. Inflows to the open pit were <1 – 2 L/s on mine closure and inflows to the underground working were also very small. During mining operations, horizontal drainage bores had to be drilled into the rock to relieve groundwater pressure buildup behind the rock faces. Over the years, some of these bores have clogged and they are now at various stages of operability.

The mineralization included sulfides of copper, zinc and lead along with significant pyritic material (FeS2, Fe2S3). These sulfides become oxidized when they are exposed to the atmosphere and water. This has given rise to any water (groundwater or runoff) accumulating in the pit being extremely acid (pH <2.5) and enriched in sulfate (commonly >10,000mg/L). Bore MB16, located adjacent to the waste rock dump, encountered groundwater with a sulfate concentration of 36,500 mg/L and a pH of 3.5.

Underground workings which extend to elevations some 650m below the open pit were developed from benches within the open pit. These remain largely unflooded. Waste rock from the underground workings was tipped across the east face and to a lesser extent across the northern batter. These are a source of acid drainage in the pit.

It was recognized that the chemistry of leachate generated from putrescible municipal waste (in particular the negative redox potential and high bicarbonate concentrations) would act to neutralize and, with acid drainage controlled, reverse the potential for acid drainage issues to persist within the mine open pit to affect the external environment via springs. Further, with the site located 80km NE of the national capital Canberra and 240km south of Sydney on the major rail line, the site could serve Sydney and possibly Canberra as a valuable waste management facility (Figure 1).

2. DESIGN

Woodlawn was permitted by the New South Wales government as a Bioreactor facility in 2001 and was commissioned by engineering works including a haul road, stormwater runoff detention ponds and a basal liner including leachate and groundwater underdrain systems over the period 2003 to 2004. The first wastes were received by rail in September 2004 and waste receival operations have continued uninterrupted since then.

During the planning for landfill development of the site, it was decided that installation of a liner system on the walls of pit was not necessary because the bulk permeability of the rock mass approximated the regulatory barrier requirement of 1 x 10-9 m/sec. A basal leachate collection system was placed at the base of the pit to facilitate leachate collection. The basal liner was placed on top of waste rock fill in the basal sump, and a 300mm thick leachate collection layer of crushed rock was placed on top of the compacted clay liner for leachate collection .This is provided with herringbone network of 150mm perforated HDPE pipes and the sump is overthickened with over 2 metres for retrospective drilling of vertical leachate extraction points if required.


3. INITIATION OF OPERATIONS

Extreme high intensity rainfall events in late 2004 and in early 2005 caused premature saturation of the waste placed to that time with acid sulfate-rich water deriving from runoff overtopping control structures and infiltrating into waste rock accumulations within the pit.

Following premature saturation, and based on previous work carried out prior to start of operations, it was recognized that in time the combination of the acid water with the biodegradable waste would generate pH levels at which H2S generation would commence. This issue had the potential to make leachate management difficult due to odour and represented a potential O H & S issue for workers at the active face in the pit.

An immediate program of works, including installation of further stormwater control infrastructure plus comprehensive site and workplace monitoring was thus implemented. Additional infrastructure included selective lining and barrier systems across permeable waste rock surfaces and along drainage lines; installation of a slurry wall and additional leachate monitoring bores and gas extraction systems. The additional monitoring included shallow and deep gas concentrations, leachate and influent groundwater chemistry and, as soon as LFG extraction systems were in place, extracted gas chemistry. Groundwater and leachate levels were also monitored along with extracted volumes.

To alleviate surfacewater inputs to the waste it was necessary to prevent as much acid mine drainage as possible from reaching the waste. This was largely achieved by creating a vertical barrier wall in the form of a slurry-filled trench, around the perimeter of the waste on the eastern and northern margins where most of the infiltrated water inflow occurred. This was backed up by a French Drain (gravel filled trench) to allow separate removal of the groundwater seepage prior to its entry into the landfilled waste. The latter has had some success but is limited in its performance probably due to poor penetration to the fractured rock at the base of the wall. Additional leachate pumpage has been provided for by a series of vertical leachate pumping wells which tap leachate both from saturated waste and from flooded former gas drainage gravel blankets.

The progressive diversion of groundwater, wherever possible, plus the installation of leachate wells and pumping systems has returned control to these systems and should finally allow full control of leachate levels within the waste mass.

Final storage (removal and disposal) of acid mine drainage / leachate mixture has been a major challenge to the site. A considerable amount of work is being carried out by Veolia and the University of Queensland to look at treatment options of leachate affected acid mine drainage and acid mine drainage affected leachates in various proportions and salinities. Evaporation of leachate from the site, as long as no odour issues are released, is consistent with water management for the site and numerous ponds already exist for this purpose. Over 70Ml of acid mine drainage has been evaporated from the site over the past 6 months alone so, as can be seen, climatic conditions are very beneficial. Treatment trials are well underway and results for anaerobic treatment supplemented by chemical treatment appear promising in removing the odour constraints resultant from thiol generation.

4. MONITORING SYSTEM

The monitoring system for the Woodlawn Bioreactor includes elements which address: ▪ Landfill gas both in the waste and in the gas harvesting system; ▪ Leachate in situ and at discharge; ▪ Groundwater and infiltrated stormwater;


▪ Void waters including influent spring water, stormwater in the retention dams, evaporation dam; and,

▪ Geotechnical loading and temperature within the waste. The purpose of the monitoring is to ensure that operations maintain a safe workplace and that the operating systems applied are beneficial in sustaining efficient waste placement stabilization and and effective gas generation and collection environment.

By having a multifaceted monitoring system, some anomalies have been detected. One anomaly which will be of interest to operators of bioreactor landfills is the apparent presence of massive gas accumulations under pressure within the waste mass. These “bubbles” can significantly inhibit waste wet up and can distort leachate levels in adjacent monitoring wells which reflect the gas pressure rather than genuine saturation levels.

This anomaly was discovered when drilling vertical gas extraction wells into the waste. These wells commonly recorded dry waste while drilling at depths apparently below saturation level. As the gas pressure were relieved by outflow, it was noted that the leachate levels in surrounding wells could decline by as much as 4m.

5. GAS SYSTEM

5.1 Gas Extraction System

The gas extraction system comprises the following components: First, a gas extraction layer comprising a blanket of crushed rock layer with horizontal slotted

pipe connected to a perimeter gas collector around the margin of the waste mass. Second, a gas extraction layer comprised of horizontal slotted pipes placed in trenches excavated into the waste and connected to a ring manifold. Currently, vertical “pinwells” are also punched into the waste mass along lines that are connected with and transfer gas into the horizontal manifold system.

A condensate collection system collects entrained condensate from the gas lines at a rate of about 200 L/day. This condensate is then applied back over the waste. The gas pipeline transfers gas from the collection manifolds along the haul road out of the open cut to a gas flare at the surface. This was located initially near the rim of the open pit, but was subsequently moved to the power station outside the pit. The power station and flare system complete the whole system.

5.2 H2S Issues

H2S gas is generated by the reduction of sulfate present in infiltrated mine runoff and from sulfur containing materials within the waste. H2S upon release, will then migrate along paths of least resistance throughout the waste mass becoming gradually dissolved into the leachate and/or be attenuated by reaction with metal ions within the leachate to be precipitated as sulfides such as pyrite (Fe, Fe2S3) and chalcopyrite (Cu, FeS) and other minerals, dependent upon the pH/Eh regime. H2S may however generate and persist in zones of the waste subject to irrigation with sulfate rich leachate and where acetogenic and acidogenic biodegradation processes are predominant. H2S is an odorous and toxic gas and represents an OH & S risk to workers if it is permitted /able to release at high concentrations in the working environment.

Since January 2006, the maximum value for H2S monitored weekly at 20cm below waste surface at 20 separated locations (using steel probe and dedicated gas monitor) have fallen from over 160 ppmv to less than 20 ppmv. Average H2S values for the same data set have fluctuated between 2 and 50 ppmv associated with the intermittent operation of the gas extraction systems. The maximum H2S concentrations appear to represent “hot spots”; since H2S maxima can


correlate to a low average H2S concentration. The averages however, are seldom greater than 20% of the maximum. No correlation of H2S concentrations appears to be evident with rainfall but it is possible that H2S maxima correlate to irrigation events when all LFG generation rates are increased until H2S is swept from the system.

The “hot spot” concept is confirmed by Draeger Tube sampling results which shows that, at depth in bores H2S accumulates to high concentrations. The concentrations generally decline rapidly on venting and in some cases reduces to levels similar to the samples monitored 20cm below surface of waste. It is noted that when the gas extraction system was closed down between end of March and mid-May 2006 that the average H2S concentration trend in shallow gas monitoring was irregular, towards increasing values, with a marginal increased detection percentage in the monitoring performed. Results obtained when the gas extraction system was reactivated (in May 2006) have confirmed the rapid reduction of H2S concentrations across the active subsurface when gas movement is initiated. Initially high H2S concentrations were evident at the Haase Flare (>1000 H2S ppmv) mixed with high proportions of methane and carbon dioxide. However, the H2S concentrations were reported to have rapidly declined after pumping only 450 m3 of gas over 3.5 hours. The gas extraction system was then shut down and restarted again after 9.5 hours and monitoring shows the H2S concentration had risen again by accumulation in the waste mass above the leachate saturation level, but again it declined to below 200 ppmv as a consequence of the pumping.

Since October 2006, the active gas extraction system has been working continuously with extraction effective across the whole waste mass. The H2S levels now (December 2006) recorded in the gas are < 80 ppmv H2S

The leachate saturation levels measured in early Jan 2007, when coupled with sulfate and pH determinations, indicate that some weakly acid sulfatic waters are still entering the waste at least from the waste rock pile. This would have maintained H2S “hot spots” in this area where and when a positive hydrostatic head persists from the waste rock into the waste. This H2S may be supplemented by some elevated sulfate levels in leachate used for waste irrigation. The fact that H2S does not release to the surface however is likely to be a consequence of biotic and abiotic inhibition of H2S in the significantly wet up zone of the waste being irrigated. Notably, the sulfate levels in the leachate are markedly lower away from the original leachate sump and, while elevated (>2000 mg/L) where the pH is mildly acidic, they fall very substantially as the leachate pH rises into the methanogenic range of 6.8 to 7.4 (Reinhart & Townsend, 1998).

Other sampling points show the similar characteristics when sampled over time, with the readings for H2S concentrations being highly erratic. Considerable variation in the measured H2S concentrations is attributed to measurement error. Much work has been done to resolve this issue but it remains a challenge.

Based upon all the H2S gas monitoring data to hand it would appear that pockets of H2S gas do generate and accumulate within the semi saturated waste, but these are subject to attenuation by abiotic and biotic reactions within the waste mass and by dilution when gas extraction is practised. The magnitude of the O H & S risk these pockets represent is possibly exaggerated by Draeger Tube derived results, but even these values have reduced markedly since the early period of waste accumulation and the inundation of the waste by sulfate acid mine drainage. In particular, the harvested gas H2S concentrations are mostly very low indicating that H2S “hot spots” represent a small and diminishing volume within the waste mass

The monitoring results demonstrate that while, as expected, H2S is generated by reduction of acid sulfates at depth in waters saturating the wastes, the operation of the gas extraction systems and the operational use of crushed haematite (Fe2O3) as periodic thin reactive layers in the waste can be observed to have acted in a manner which has reduced the risk of significant releases of H2S at the surface. Leachate irrigation, while potentially introducing some sulfate, from the monitoring data also seems to inhibit H2S release to the surface. This is presumably due to the


irrigation stimulating biological attenuation of H2S in the upper waste layers. Quite apart from the use of hematite and the gas extraction system, monitoring of the gas

flows is showing that despite the very high initial sulfate concentrations (>20,000 mg/L) and the low pH (<3.0) in some influent groundwater (infiltrated mine runoff), these chemical characteristics are reacted out by contact with the waste mass. This process progressively reduces the free sulfur content in LFG and normalises the pH to neutral.

5.3 LFG Gas Composition Trends

Monitoring of LFG at 20 points, at 20 cm beneath the waste surface, each week shows that, at least since May 2005, significant methane has been generated in, presumably, the saturated waste at depth. This is evident from the methane concentration commonly exceeding 5% in air diluted gas. Five percent is the maximum methane concentration one would expect to find in acetogenic decaying waste where oxygen and oxide levels are still high. The presence of any methane at concentrations above 5% is an indication of active methanogenesis at depth.

Flare gas monitoring records show carbon dioxide to methane ratios in excess of 0.5 by June 2005 and some records show the ratio near 1.00. Thereafter there is a clear indication in the results of the produced gas of methanogenic biodegradation being established at depth. This ratio has persisted through 2006.

The very rapid onset of methanogenesis is interpreted to reflect three phenomena in our opinion. These are: i. Abiotic hydrolysis of the organic waste mass by the acidic water inflow; ii. Rapid neutralisation of the acid waters in the centre of the site at least; and iii. Rapid onset of methanogenic conditions to generate the methane (less than 7 months). The work of Farquar and Rovers (1973), Pacey (1992), Augenstein (1991), Christensen et al (1996) and others has shown that hydrolysis, fermentation and acetogenic phases must proceed before methanogenesis can commence. This is because these phases of degradation produce the volatile fatty and acetic acids which are the most easily metabolized energy substrate for the methanogenic bacterial populations.

Rudolph V., and Krol, A. (1994) showed that under ideal laboratory conditions these processes could be stimulated and methanogenic degradation started in less that one month. However, in large moisture constrained landfill masses, the onset of methanogenisis at any significant scale has been noted from experience to take generally longer than 18 months (Cossu et al 1996).

While many wet landfills exist, few if any have been as closely or as early monitored for LFG production as the Woodlawn Bioreactor. Certainly, the combination of acid drainage induced waste hydrolysis and the availability of moisture appear to have combined to give rise to a very early onset of methanogenisis despite the potential for competition from sulfur reducing bacteria.

One other factor is also clear from the surface sampled gas monitoring points and this is that the operation of the gas harvesting system is efficient in maintaining aerobic conditions in the upper layers of the waste. It is rare for the oxygen concentration, in the upper layers, to fall below 10% when the gas extraction system is operating. This means that the upper layers of the waste mass are effectively composting aerobically, which in turn preconditions them for rapid acetogenic and fermentative decomposition. This activity also expedites the on-set of methanogenesis. These results indicate that the level of compaction being achieved at the facility is appropriate for bioreactor optimisation.

Since the reinitiation of the gas extraction system in mid May 2006, the CO2:CH4 ratio has fluctuated between 0.6 and 1.0 with the range increasing to mainly between 0.8 and 1.0 since November 2006. Some oxygen leakage is evident at all times with the lowest levels being


mostly greater than 4%. This seems likely to reflect bore construction and is declining over time as the CO2 and CH4 percentages increase.

5.4 VolumetricGas Production Trends

Gas production monitoring is carried out at the flare and the results are summarised in Table 1.

Table 1 Gas production data - flare

Average

content of CH4

Average operating hours

per day

Average landfill gas flow measured

Average CH4 monthly flow

Month % h m3/h m3 CH4

January 3

1 48.0% 2

133 3,958 Februar

y 2

8

March 3

1

April 3

0

May 3

1 32.0% 1

90 893

June 3

0 32.0% 1

90 864

July 3

1 58.0% 2

102 3,668

August 3

1 39.0% 1

95 1,149 Septem

ber 3

0 41.0% 4

115 5,658

October 3

1 36.0% 24

160 42,854 Novemb

er 3

0 51.0% 24

164 60,221 Decemb

er 3

1 54.0% 24

184 73,924

January 3

1 55.0% 24 300 122,760

Volume of CH4

flared 315,948 m3 CH4


6. LEACHATE

6.1 Chemistry

While analyses of leachate have been undertaken since early 2005, the early waste penetrating wells and the leachate and drainage pump sumps were affected by the initial acid sulfate rich water invasion. These wells have now been replaced by new (LE2 to LE17) and more representative analyses are summarised in Table 2.

The pH values in all the leachate bores mostly reflect acetogenic conditions. Notably to the north and west these are closer to and extend into the methanogenic range. This reflects the integration of leachate generating within the waste mass with methanogenic degradation at depth and acetogenic degradation shallow. This is to be expected.

The leachate Redox potential, where recorded, is negative, but only strongly negative (near -300mV) in LE2 where the pH is well in the methanogenic range. The leachate sump recorded positive redox potential indicating the continuing influx of oxidizing waters into the leachate system this point where the slurry wall could not be completed. The negative redox data are consistent with reducing environments encountered in a waste mass undergoing active biodegradation.

Bicarbonate values are all very high indicating that methanogenic conditions must exist. VFA where recorded are high, mostly in excess of 5,000mg/L and commonly greater than

50,000mg/L. Over the period of analysis they appear to be increasing which is consistent with increased degradation being stimulated by the leachate irrigation.

The BOD:COD ratios vary from <0.04 to above 0.8. These reflect the high COD and inhibited biodegradation in the deep saturated waste compared with the less restricted biological activity in LE6 in the northern clay lined enlargement. LE7 and LE2 show rising BOD overtime before a slow decline which is also reflected in VFA diminution. This is considered to be indicative of rapid waste stabilisation and will be accompanied by a reduction in gas generation locally from this section of waste.

Sulfate values are higher than common for MSW landfills. Concentrations lie mostly in the range 2,000–3,000mg/L. Some lower values occur which correlate to higher pH. This is to be expected but the data is insufficient for defining the proportionality of different biochemical processes. Reduction of sulfate in the leachate used for irrigation would reduce the potential for sulfur reducing bacteria competition to inhibit methane generation. This will be achievable by calcium treatment which would reduce the SO4

2- to <1,500mg/L for irrigation. Studies into this are proceeding.

Sulfide levels are very variable but are mostly quite low at <3mg/L and sulfite concentrations, when tested, are in the range 150 to 300 mg/L. These conditions indicate variable conditions of oxidation exist within the waste.

Ammonium concentrations are higher than normal for most MSW landfills and commonly exceed 2,000mg/L. This seems to reflect the acid hydrolysis which the basal waste has undergone. At these high concentrations some nitromonas bacterial competition with methanogenic bacteria may occur but insufficient data is yet available to assess this aspect.

TDS as determined by laboratory methods exaggerate the level of TDS due to the sludges which precipitate from the solution as gases are driven off during drying. TDS by summation of major ions show values which are dominated by SO4

2-, NH4+, NA+, K+ and Cl-. These values are

typical of acetogenic leachates and decline where higher pH values are found . TOC values are all high with about 60% represented by VFA. This indicates that substantial

substrate remains for degradation. The TKN values are all well in excess of the ammonia concentrations which indicate that

substantial organic nitrogen remains to sustain both acetogenic and methanogenic metabolism.


6.2 Discussion

With the leachate wells having varying depths and screen intervals, the samples collected for analysis are inevitably integrations of fluids deriving from deep, long saturated wastes and shallower saturated and semi saturated waste affected by leachate generated in and drained from overlying waste. That methanogenesis is strongly established at depth is clear from the gas generation even though the leachate analyses indicate that acetogenic and/or acid hydrolysed waste leachate is dominant. It is significant that the leachates are pregnant with readily biodegradable organic compounds. These will feed methanogens as the pH adjusts around acetogenic population clusters to give rise to increasing methane generation.

The analyses of the new leachate wells show the pH leachate to vary from neutral pH (7.2) to mildly acidic pH (5.2). With sulfate concentrations one to two orders of magnitude lower than those from the leachate sump. They have substantial iron in solution more than sufficient to react out the SO4

2- concentrations after reduction to S2-. That the SO42- ion is still present is an

indication of Fe complexation which may be as jarosite (K2Fe6(OH)2(SO4)4) or pisanite ((FeCu)SO4.7H2O. The Fe will precipitate at higher pH or be reduced to pyrite or chalcopyrite.

Irrespective of the iron situation, the leachate is in a pH range which is not exceptional for MSW leachates where gypsum as plasterboard is present. It seems that normal methanogenic bacteria are operating but that they must be competing with sulfur reducing bacteria (Townsend et al 2002). This will increase CO2 production relative to methane which drives the pH down. This may delay methanogenesis but is likely to represent only a marginal decrease in methane generation and one which will diminish with time as the S2- ion is precipitated with heavy metals especially where the bulk waste mass is away from the perimeter of the pit.

Details of leachate analyses will be evaluated to determine how sulfate ion is distributed around the site. Sulfate ion concentrations are variable as a consequence of time, irrigation and gas or leachate extraction activities. The present data is insufficient to determine which of these influences are dominant.

The sulfate concentration and pH will initially be the best basis for determining the leachate extraction areas and regimes where the best water chemistry for bio-stimulation of methanogenesis exists, while at the same time reducing methane inhibition by sulfur reducing bacteria.

7. INFLUENT WATER

7.1 Influent Chemistry

.Table 3 Water chemistry outside of the slurry wall Parameter Surface Water Groundwater

North Wall South Wall OSW02 LMB5

pH 7.2 2.8 4.0 3.8

Conductivity (uS/cm)

3,300 16,000 16,000 10,900

SO42- (mg/L) 1,400 20,000 13,000 10,900


The influent water comprises a combination of groundwater from the Palaeozoic bedrock mass, interflow (shallow seepages of surface water deriving from infiltrations into the waste rock on the walls of the pit) and rainfall. The water chemistry from bores in the waste rock outside of the slurry wall are summarised in Table 3. The analyses indicate the sulfur loading originating from the waste rock is quite variable, nonetheless the concentrations are very high and it is undesirable that this water is permitted to enter the waste

7.2 Discussion

The issue of acid sulfate drainage and groundwater ingress into, and saturation of a portion of the waste, has been found to have a limited effect in terms of H2S hazard potential. In a saturated landfill, hydrogen sulfide gas generation, as opposed to sulfide ion, is likely to be limited to zones where gas solubility and H2S generation occur (e.g. zones of high temperature (>35oC), low pH (<4.8), and high TDS concentration). Whilst these may be optimal conditions, H2S gas will still be emitted from the waste, as different horizons of the waste pass through to maturity. This will be reflected by changing environmental conditions of the leachate.

The influent water captured within the runoff system and the water captured by the French Drain are pumped out of the pit for disposal by evaporation.

Issues of localised levels of H2S remain which need careful management over time.

8. OBSERVATIONS

8.1 Leachate Management and Use

The initial waste saturation appears to have given rise to a predigestion of the saturated waste consistent with acetogenic and acidogenic decomposition. This has expedited the development of hydrogenic and methanogenic bacterial degradation and the early generation and release of methane.

It is to be expected that sulfur reducing bacteria will, however, compete with methanogens for the carbon substrate and consequently the methane generation capacity per unit dry weight of biodegradable waste may have been reduced. Gurijala & Sufitia (1993) and Fairweather & Barlaz (1998) support this - whereas Yoda et al (1978) contradicts it. Inhibition effects are, however, offset by the fact that saturation causes the total waste mass to be included in some methanogenic degradation and, since only a relatively small proportion of the waste mass was completely inundated with acid sulfate-rich water, the long term gas reduction potential should be only marginally affected.

Indeed, over time and with leachate pumping and recycling, it is to be expected that the sulfate concentration in the leachate will be depleted significantly, especially as new intrusions will be minimised by more efficient interception measures for run-off and removal off-site. Further, oscillation of the leachate level will lead to periodic desaturation and revitalisation of methanogenesis is expected with markedly increased CH4 generation.

Energy-rich gas generation optimisation is important. This requires evaluation of leachate chemistry from the leachate recovery wells, across the site, so that pumping can concentrate on those wells which will represent the least potential for methanogenic competition.

As leachate extraction and recycling continues, the volume of information regarding chemistry, flow rates, saturation and gas production will increase. This will enable future management decisions to have more impact on bioreactor optimisation.


8.2 Methane Gas Inhibition

Suppression of methane generation can occur as a consequence of: i. Sulfur reducing and ammonia denitrifying bacteria competing for organic substrates to an

extent which is currently not defined; ii. Hydrogen sulfide concentrations (H2S > 550mg/L) inhibiting either acetogenic or

methanogenic activity; iii. Exhaustion of VFA created initially by the acidogens and acetogens under semi-saturated

conditions before the exhaustion of the main mass of the biodegradable waste component. Thereafter the substrate for methanogenic metabolism becomes concentrated upon less biodegradable compounds (lignins) to generate methane more slowly;

iv. Hydrogen gas partial pressure inhibiting acetogenic activity; v. Free ammonia gas (NH3 > 80mg/L) inhibiting methanogenic activity as pH rises above 7.0 The combination of leachate and gas chemistry against gas production statistics should allow evaluation of waste energy exhaustion or suppression.

Sulfur compounds can inhibit methane production in several ways. These include the toxicity of undissociated H2S to acetogenic (AB), sulfur reducing (SRB) and methanogenic bacteria (MB) and the competition that sulfur and sulfate reducing bacteria represent to acetogenic and methanogenic bacteria. The degree of SRB competition is then variable dependent upon temperature (solubility of H2S decreases as T increased), saturation levels, waste density and COD : SO4 ratio.

Notably, sulfate reducing bacteria can produce acetate as well as H2S. The H2S is then reacted out with metals or is dissociated to become a pH buffer along with bicarbonate released by degradation of the acetate and the hydrolysis of CO2. Sulfur reducing bacteria will then further reduce the H+ and HS- in solution using some of the acetate.

Ammonium ions are released in the hydrolysis and biodegradation of organic matter including especially the proteinic amino acids which form the cell walls and from the electrolytes in the cells. Ammonia (NH3) gas always exists in a pH dependent equilibrium with aqueous ammonium ion (NH4

+). Concentrations at the same temperature and pressure will vary from being relatively very low at pH (<6) to high at pH above 10. As a consequence, where the NH4

+ concentrations are high, the equilibrium level of associated NH3 can be severely inhibitory due to its toxicity to both AB and MB.

From laboratory studies it is known that free NH3 at concentrations above 80mg/L can be toxic. At pH 7.0, all NH3 would be present as NH4

+, and from empirically derived equilibrium constant at 25 ºC, a NH4

+ concentration in excess of 14 000 mg/L in the leachate would be required to produce NH3 at toxic levels. Note, however, that a single unit increase in pH to 8.0 will induce a 10 fold increase in the relative concentration of NH3, or conversely, a 10 fold decrease in NH4

+ (1400 mg/L) to produce the same toxic concentration of NH3.

8. CONCLUSIONS TO DATE

The Woodlawn Bioreactor is just over 2 years old and the initiation period appears to be largely complete with the major site development issues now either largely addressed by engineering work or understood and scheduled for attention.

The experience being obtained from monitoring at the site is being used to optimise management to maximise the returns available from the landfill gas, both for energy generation and other purposes.

The events which gave rise to acid sulfate saturation of the wastes shortly after placement


appear to have lead to the rapid onset of methanogenesis, albeit somewhat inhibited by competitions from sulfur reducing bacteria. The magnitude of this inhibition is yet to be evaluated, but given the mass of the waste now contributing CH4 it would appear likely to be small in the long time frame of gas generation (50 years) available for this site. Equally, the early concerns over H2S derived OH&S issues now seem to have been abated by reactions in the mass plus the operation of the gas extraction system from very early in the initiation period. The fact that the gas system and the landfill operated without any use of daily cover also calls into question the need for daily cover, at least in so far as the inhibition of odour and oxygen ingress is concerned. Indeed, the ingress of oxygen has probably contributed to the very high values of VFA now recorded.

The combination of available moisture, increased rates of oxidizing hydrolysis plus the capacity now in place to oscillate the leachate levels seems likely to be very effective in stimulating the maximum CH4 gas productions the waste mass is capable of producing.

These challenges have required the site management techniques to be well informed by an understanding of the complex interactions. This is a learning process which will need to continue into the future.

REFERENCES

Augustine D and J. Pacey, 1991, Modelling of Landfill Methane Generation, in Proc of Sandmia 91, Third International Landfill Symposium CESa Caglian, Italy pp115-48.

Berge, N.D. and D.R. Reinhart, 2003, In-Situ Nitrification of Leachate in Bioreactor Landfills, in Proc of Sardinia 2003, Session D15, P429, CISA, Cagliari, Italy, October.

Cossu R, Christiansen T and R Stegmann (eds), 1996, Landfilling of Waste: Biogas, E & F.N. Spon, London. UK

Farquar, G.J and S. A Rovers 1973, Gas Production from Landfill decomposition, Water, Soil and Air Pollution 2, 493.

Fairweather R. & Barlaz M.A., 1998, Hydrogen Sulfide Production during decomposition of Landfill inputs, in Journal of Environment Engineering April.

Gurijala, Rao, K and Joneth, M, Suflta, 1993, Environmental Factors Influencing Methanogenesis from Refuse in Landfill Samples, in Environmental Sci, Tednol, V27, pp1176-1181.

Hancock, S., Ife, D., Rainford, S., and Smith, L., 2001, Woodlawn Mine Rehabilitation: Bioreactor Issues for a Deep Pit, Proc. Of Sardenia 2001, Eighth Int. Waste Mgt. and Landfill Symp., CISA, Env, San, Eng, Centre, Cagliari Italy, Vol IV pp87-97

Moreau-Le Golvan, Y., Triquenaux, C., Bogner, J., Smith, L., and Munary, M.-L, 2003, Methanogenesis Optimisation by Controlling pH and Sulfate in a Bioreactor – Applied Research for Woodlawn Bioreactor (Australia), Proc. Of Sardinia 2003, Ninth Int. Waste Mgt and Landfill Symp, CISA, Env, San, Eng, Centre, Cagliari Italy, Abstracts pp456-457

Moreau-Le Golvan, Y., Lagier, T., Smith, L., and Lane, M., 2005, A Strategy to Achieve Optimal Performance at Full Scale Operational Bioreactor Landfill, Proc. Of Sardinia 2005, Tenth Int. Waste Mgt and Landfill Symp, CISA, Env, San, Eng, Centre, Cagliari Italy, CD

Pacey J.G., 1981, Prediction of Landfill Gas Production and Recovery in Gas Research, Proc of International Conference, Los Angeles, September – October. CA, USA, Government Institutes Inc, Rockeville, MD, pp896-915.

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