Patrick Reynolds, PhD

Environmental Consultant & Website Designer
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Portfolio of Landfill Management Projects

Stabilisation

Landfill 2000 - A Field Trial of Accelerated Waste Stabilisation

Project Objectives
This study investigated alternative landfill management procedures designed to accelerate the stabilisation processes occurring in municipal solid wastes (MSW). The Landfill 2000 test cells were located at Dewsbury Landfill, UK. The work was carried in two purpose built prototype waste processing cells each containing 1000 tonnes of MSW. The research objectives were to:
  • Establish optimum cell conditions that would give a three year waste degradation period
  • Quantify the methane generating potential of the waste/sewage sludge mix
  • Determine the biological condition of the waste digestate after a three year period and assess its suitability as an environmentally friendly soil conditioning medium
Technical Background to the Project
The project investigated a concept proposed by West Yorkshire Waste Management termed ‘Landfill 2000’. The concept was based upon anaerobic digestion of MSW in purpose built waste processing cells. For such a concept, the prime objective was to recover the usable gas potential of the waste under controlled but accelerated anaerobic conditions. The resulting digestate was either removed to landfill or further processed to produce a soil conditioning medium. In any subsequent landfill operation, inorganic contaminants are flushed from the material using a lower level of environmental protection than is commonly expected for un-processed municipal solid waste. In order to achieve more rapid waste stabilisation, effort was required in two areas: the first involving the development of control procedures over the stabilisation processes in a landfill environment, and the second being more focused attention on the quality and characteristics of waste going to landfill. Overall, an alternative operational strategy for landfill was sought to treat the landfill as a process and not simply a ‘black box’ repository for waste. The work was undertaken over the period April 1991 to March 1995.

Overview of the Work Programme
The study examined the operational features of the Landfill 2000 concept, which made the ambitious assumption that the usable gas extraction period can be contracted into a three year waste incubation period. Two key design features were adopted in the design of the trials to reflect this objective. These included the use of:
  • Digested sewage sludge in a mixture with municipal solid waste (to provide a source of essential moisture and microbial innocula)
  • Additional liquid to enable leachate recirculation (to promote microbial movement in the wastes including the mobilisation of degradable substrates and essential nutrients)
Plans for constructing two experimental reactors at the Lower Spen Valley Landfill at Ravensthorpe, West Yorkshire were agreed in 1990 and a high-density polyethylene (HDPE) flexible membrane liner was chosen as the waste containment medium. Two cells, each 36-metre-long by 23-metre-wide with a maximum depth of 5 metres were excavated, and the sides and base lined with a 1.5 millimetre sheet. A 1 millimetre sheet was used as the closure cover. The liner material was manufactured and installed by Agru of Austria, with quality assurance and quality control aspects of the installation overseen by Golder Associates (UK) Ltd.

Approximately 1000 tonnes of MSW and Sewage sludge mix (10:1) overlaid leachate drainage media and a collection pipework system in the base of each reactor. Additional pipework was laid in a protective cover layer over the waste, allowing gas collection and leachate recirculation. One reactor (Cell 2) was set up as a control to compare standard landfill practice and operational procedures with a more manipulative approach in a second reactor (Cell 1), based on the ability to recirculate leachate. Imported liquor was introduced and recirculated in Cell 1 to provide the required moisture content and nutrient mobility for the process to thrive. Gas quality and temperature readings within each reactor were monitored from a number of discrete probes along with the ability to measure gas pressure and gas flow rate (device patented, Bartington and Reynolds). Atmospheric pressure and ambient temperature measurements were also included. Data was recorded from all installed sensors using data-logging equipment housed in a site cabin positioned between the two reactor cells. The physical chemical and biological condition of the waste was determined at the end of a four year incubation period, by carrying out a sample recovery exercise using a percussion drilling technique. Waste characterisation work included category and distribution assays, interstitial liquid analyses, other physical and chemical analyses, including determination of biochemical methane potential (BMP).

Key Issues Relevant to the Development of Sustainable Landfill Practices
  • Examination of the static water balance for the recirculation cell showed that substantial movement of moisture took place through the body of the waste, rather than being confined to specific ‘by-pass’ routes. Lithium tracer tests indicated a mean hydraulic retention time of about six months. The water balance data suggest a total bed volume of about 640m3 , of which nearly 380m3 was assumed to be mobile water within the wastes. At an effective recirculation rate of close to 3 m-3d-1, we estimated that the system was operating at a mean hydraulic retention time of between 130 and 210 days.
  • Biological stabilisation processes within the experimental cells generally progressed in a bi-modal fashion. Examination of leachate composition within the drainage blanket and interstitial liquid within the wastes provided the evidence to support this conclusion. Volatile acid rich leachates, typical of acidogenic conditions within landfilled MSW, predominated within the wastes of both cells, although leachate recirculation led to a more even distribution of these dissolved organics throughout the body of the waste. By comparison, the sump leachates were typically methanogenic.
  • Further evidence of bi-modal behaviour was obtained through the analysis of low level hydrogen in the gas phase immediately above the saturated zone (2.5-3.0 metres depth) and at 1.5 metres in the unsaturated waste. In the recycle cell, hydrogen levels remained unperturbed by induced organic flow between the waste and the saturated zone, indicating that the methanogenic potential of the drainage media was well developed by a steady flow of substrate, induced by leachate recirculation. On the other hand, hydrogen levels in the non-recycle cell remained high, following the removal of standing leachate in the cell. We concluded that the induced flow of organic rich liquid into the saturated zone of this cell provided a shock loading of substrate far in excess of the capability of methanogens to remove it efficiently. Consequently, the methanogenic potential in the base of the non-recycle cell was not as well developed as in the recycle cell.
  • The quality of landfill gas, in terms of the methane content, was higher in the leachate recycle cell (50-60% CH4 v/v compared with 45-50% CH4 v/v in the non-recycle cell). Specific landfill gas (LFG) yields from the two cells were estimated from measured flow data and range from around 20 to 9 m3 LFG per tonne ‘as received’ MSW (wet weight) per year, for the leachate recycle and non-recycle cells, respectively.
  • Assuming a maximum LFG yield of around 200 m3. dry tonne-1 MSW, calculations show that around 60% of the readily available gas had been generated in the leachate recycle cell in comparison with 23% in the non-recycle cell. Data in our report showed that the readily degradable fraction of MSW accounted for around 18% w/w of the total mass, with 12% classified as ‘moderately’ degradable, 31% classified as ‘slowly’ degradable and 40% biologically inert. Category and distribution assays of recovered samples from both cells supported these claims and showed the wastes to be largely unaltered by the three year incubation in the experimental cells.
  • Laboratory tests carried out to determine the residual biochemical methane potential (BMP) of the recovered waste samples from the cells yielded results which, when equated with the measured gas production figures, compared favourably with more recent estimates of MSW methane yield cited in the literature.
Conclusions
In view of the comparatively high gas yields obtained in the trials, we concluded that the stabilisation processes within the waste of both experimental cells had been advanced significantly by the addition of water (in the form of digested sewage sludge) at the time of waste emplacement. Further improvements had been achieved by leachate recycling activity.

The gas yields recorded were particularly remarkable in view of the low temperature conditions maintained in the shallow waste cells (6-17oC) when compared with temperatures normally achieved in deeper more insulated operational landfills (20-40oC). By assessing the Biochemical Methane Potential (BMP) of recovered waste samples and extrapolating data for volumetric gas flow under passive venting from the cells, we were able to make broad predictions about MSW stabilisation periods. For the leachate recycle cell, a predicted stabilisation period of around 7 years compared with approximately 17 years in the non-recycle cell. These stabilisation periods were estimated on the basis of removal of readily degradable organics and did not take account of the removal of more recalcitrant carbon in the waste.

Hydraulic behaviour within the recirculation cell was difficult to interpret. However, retrospective examination of water balance data indicated that, at an effective recirculation rate of close to 3m3.d-1, a mean hydraulic retention time (HRT) of between 130 and 210 days has been successfully achieved.

We concluded that leachate recirculation techniques could be developed at full scale operational landfills where mean HRTs of between 1 and 5 years are required to meet current guidance for flushing bioreactor conditions. However, changes in effective porosity of saturated basal waste layers should not be discounted. Our calculations suggested that following an initial waste porosity of around 25% at the start of recirculation, this rapidly diminished in the saturated layers of waste to around 3%, within 30 months. Our assumption had been that stabilisation activity within this saturated layer led to accelerated settlement and compaction in this region. The implication we derive is that the saturated basal layer of a flushing bioreactor landfill should be constructed to minimise this effect in the short to medium term. We recommended that investigations be carried out on potential methods for retaining waste porosity characteristics in saturated waste layers at the base of landfills.

Although leachate recirculation had been responsible for a more uniform distribution of soluble organics and the development of more neutral pH conditions within the wastes, bimodal activity predominated in both cells, with the saturated zone presenting significant opportunities for optimising methanogenic removal of dissolved organic carbon. We recommended that further work be undertaken to investigate the use of low level hydrogen analysis as a means of monitoring, and hence managing, the performance of this zone in bioreactor landfills.

We also concluded that there was little scope for recovering the usable gas potential of landfilled wastes within a three year period, as proposed in the original ‘Landfill 2000’ concept. Although stabilisation of the wastes had been accelerated through leachate recirculation activity, other waste characteristics changed little over the four year period of the trial and therefore the waste material would be unsuitable as a soil conditioning medium, without further pre-treatment.

The trials demonstrated successfully that there is substantial scope for accelerating the removal of the readily degradable fraction of MSW in the landfill environment. The trials also provided some insight into the development of control procedures and drew attention to the changes in structural characteristics of waste under conditions of significant leachate recirculation.
Codisposal

Landfill Codisposal Studies

During the early 1990's. the guidance in the UK's Waste Management Paper 26, for the practice of co-disposing industrial waste with domestic refuse was re-assessed by the Department of the Environment. At this time, the draft EC Landfill Directive (91/C190/01) proposed controls and loading limits for co-disposal as a means of sorbing and storing non-degradable components of liquid waste, wherein the solid components of domestic refuse present a wide variety of physical and chemical surfaces which may interact with fluids passing over them. To determine the ability of refuse to alter the nature of the incoming waste, it was deemed essential to consider both the physico-chemical absorptive properties of the refuse together with the initial and future characteristics of the landfill.

Studies initiated by the Department of the Environment were designed to evaluate the effects of acid and organic ligand on both the release and retention of heavy metals by aged and recently emplaced refuse; to determine the chemical stability and partitioning of the retained metal within refuse; and to provide additional data on metal loadings which may be safely co-disposed.

Refuse/metal equilibration studies were carried out using two loadings of heavy metal, the first as proposed for UK and EC guidelines, the second tenfold higher than the first, resembling conditions in the vicinity of a co-disposal trench. Two acid loadings, one at a half, the other at five times the UK and EC guidelines, were simultaneously applied to the refuse. In addition, the retention and partitioning of metals at each loading was determined in the presence and absence of a EDTA, which represents the strongest ligand found in industrial process waste. All experiments were carried out for a period of one week, after which sequential extractions were carried out on the dried ground waste samples to determine the metal partitioning within the refuse material.

Based on the general level of metal retention by the refuse, the results of this study suggested that the UK guidelines and draft EC limits for the co-disposal of individual metals were satisfactory. The results provided no justification for the EC proposal of a total heavy metal loading limit of 100 g/tonne. The results also confirmed that research into the anaerobic biodegradability of ligands was essential, since the presence of EDTA was shown to reduce metal stability and, at low metal loadings, caused the quantities of individual metals removed to fall below EC limits.

In this study, the direct interactions between components of refuse revealed that paper, soil and plastic were capable of retaining similar quantities of metals. Although textiles were present in refuse to a lesser extent, their relative metal uptake capacity exceeded all other components. These findings highlighted the importance of refuse composition in relation to metal removal by the process of co-disposal. The recycling of various components of the refuse would undoubtedly affect the capacity of the refuse to retain metals. Also, the fact that most metals are contained within biodegradable components of the refuse was a cause for concern since the eventual biodegradation of these components would result in the release of metals.

In summary, the following requirements must be satisfied with respect to understanding and controlling co-disposal of heavy metals to landfill:
  • Determination of binding intensities and capacities for refuse components
  • Determination of the relative abundance of these components
  • Consideration of the effect of major competitors (Ca2+, Mg2+, Na+, H+, Cl-)
  • Evaluation of the kinetics and complexation of metal redistribution among refuse components during the active life of a landfill
Microbiology

Assessment of UK Funded Projects on Landfill Processes

The Programme funded by the UK Department of Trade and Industry was not operated in isolation. It has leant heavily on landfill research conducted in the UK (notably funded through the Dept of Environment) and elsewhere (particularly the USA). Although the DTI-funded programme has been innovative in some respects, in others it has developed an existing line of research. Regardless of this, there was a need to assess whether any of the findings made in the programme have already found applications or are likely to be applied in the control of landfill gas production with or without more research.

Recommendations
  • A feasibility study be initiated which has the following objectives: establish a reliable procedure for extraction of DNA and RNA (rRNA and mRNA) from landfill and leachate samples, show that the extracted nucleic acids can be used as templates for qualitative and quantitative assessment of a target species (probably a methanogen), define strategies for assessment of other species/functional groups using nucleic acid probe and PCR technologies. This study will lay the foundation for all detailed studies of the microbial species in landfills, their prevalence, source and distribution (solid surfaces or leachate). The results will have an impact, together with recommendation 2 on (a) deciding whether to design management strategies primarily for solids or leachate and (b) in defining an appropriate microbial innoculum source.
  • The activities of the landfill microbial populations in leachate or on solid surfaces should be estimated. The microbial populations in the two environments will be investigated as part of recommendation 1 but a full picture of the landfill microbiologies also requires knowledge of the relative activities in leachate and on surfaces and on the interactions between the two. Methods for achieving the aims should be assessed with priority given to methods capable of measuring waste degradation/methanogenesis in situ.
  • In situ methods of metabolic analysis can be exploited also in prediction of the onset of the methanogenic stage of landfills, i.e. the transition from acidogenesis to methanogenesis. A separate study is required to focus on this area. The use gaseous and dissolved hydrogen measurement warrants investigation as a method of predicting the onset of methanogenesis in normal landfill practice, and as an early warning of stress in co-disposal sites.
  • The results that emanate from the studies recommended in 1 and 2 (above) will determine the balance of landfill management towards solids degradation and leachate control. Management options can be considered prior to the above studies and an expert study is recommended.
  • Further studies to determine the extent of anaerobic methane oxidation are recommended. This work should, however, be focused to study the impact of landfilling sulphate-rich wastes. Such wastes can arise through co-disposal or by infiltration of saline water. This study will provide a rational basis for deciding whether to attempt gas abstraction at sites containing sulphate-rich wastes.
  • Further analysis of waste recycling schemes is required. An analysis of the impact of recycling schemes should be made in relation to the waste management options available and, specifically, whether the wastes that remain following removal of recyclables are more suited to one disposal strategy than another. The options that should be considered are continuation of present landfilling, more widespread adoption of cell-type strategies or aerobic composting (with consequences for energy recovery). Anaerobic treatment strategies should be considered in relation to the possible need for codisposal of the waste (enhanced in putrescibles) with a buffering medium.
  • Related to recommendation 6 is the need for a detailed assessment of the cost benefit analysis of waste sorting (irrespective of recycling) and controlled emplacement in landfills or landfill cells.
  • Improved methods of predicting the methane potential are needed both prior to waste emplacement and following emplacement. The recommendations made in for laboratory BMP assays should be tested. Also, the feasibility of measuring methane production from a cored sample that remains in situ in a landfill should be investigated.
  • The role of alternate electron acceptors on the activity of the microbial community has not been investigated. It is recommended that the influence and availability of sulphate and iron on the extent and rate of anaerobic biodegradation to methane be determined. This would allow clearer determination of the BMP of waste samples, whilst also providing a realistic prediction of the economic value of different landfilling strategies.
  • Further identification of the cellulolytic microorganisms and determination of the chemical, biological and physical factors affecting their activity. Identification of cellulolytic bacteria and determination of the cellulolytic activity would be performed in pure and mixed culture studies to determine the influence of other trophic groups on the rate of cellulose degradation and the pattern of carbon flow within the landfill environment. This work would include the effect of the current codisposal and future recycling strategies on the breakdown of cellulosic material in landfill.
  • The recycling of the various components of refuse is predicted to affect the capacity of the refuse to retain metals arising from the practice of co-disposal. Also, since at least two of the main adsorbing components (paper and textiles) are biodegradable, the need for research on the long-term chemical form of metal retention in refuse is evident. Since a previous report by WRc (Reynolds et al.) showed that the presence of EDTA reduced metal waste binding capacity, research into the anaerobic biodegradability of ligands, which are present at high concentrations within industrial waste, is essential.
  • The use of an assay to determine acute toxicity of the water-soluble fraction of industrially derived waste is proposed which will provide an effective method of determining acceptable loading criteria in landfill. The Microtox system based on measurement of bacterial bioluminescence is suggested due to the small sample volume required, the rapidity of testing, the ease of defining the end-point, and its reported effectiveness in determining acute toxicity of wastes containing toxic organic constituents and heavy metals.
  • To operate landfill cells as bioreactors, effective methods to encourage the movement of leachate through the waste are required. If recirculation strategies are to be effective, it is essential to understand the factors limiting the movement of liquid through the waste mass. Also, to enhance the rate of waste degradation by the process of leachate recirculation, it is important to determine the methanogenic potential of the biomass within the waste mass and drainage material.
Groundwater

Long-Term Monitoring of Non-Contained Landfills

Main Objectives of the Study
  • To produce long-term monitoring data for landfill gas and leachate for noncontained landfill sites, so that the effects of modern and historic landfill practices may be compared and further information gathered on the long-term impact of waste disposal activities, in order to provide the UK Department of Environment with the technical background needed to establish and improve waste management policy and to conduct EC negotiations
  • To provide practical experience of recommended landfill monitoring protocols, and to identify improvements
  • To identify and assess leachate attenuating mechanisms, based on both field and laboratory studies
Main Findings
At the time of this study (1996), the annual volume of leachate production in selected non-contained landfill sites was shown to be related directly to the efficiency of the capping materials in preventing ingress of rain, so that greater volumes of leachate would be expected formed from the older waste deposits at sites than from the more recent deposits which are capped with low permeability soils.

The strength of leachate and rate of landfill gas generation was shown to decrease some 15 to 20 years after deposit of older wastes. It was probable that the onset of decline in leachate strength would be delayed in the case of the more recent waste deposits, because of the more efficient capping and less efficient flushing from the wastes.

Deep (>50 m) unsaturated zones encouraged effective attenuation of the organic components of leachate, by methanogenesis, and of ammonia by cation exchange. In this case, the rate of progress of the pollution front towards the water table would be slow and any impact on groundwater quality would be expected to be further reduced by continued attenuation in the unsaturated zone.

Laboratory studies, in the form of flow-through column tests carried out to supplement the understanding of the attenuating processes derived from the field investigations and monitoring surveys, suggested that such tests may be developed to provide a reliable method for assessing the attenuating characteristics of the formation in advance of establishment of a landfill.
Leachate Treatment

Leachate Treatment Field Trials

This study, which was carried out in 1994, was intended to demonstrate the operational features of a pilot-scale anaerobic digester linked to a secondary stage nitrification plant for the on-site treatment of landfill leachate.

During the first year of operation, excessive inorganic scale accumulation on the interior of a 4m3 Upflow Anaerobic Sludge-Bed/Filter reactor and pipework surfaces resulted in operational problems, and forced the eventual shutdown of the pilot-plant. These studies highlighted the requirement for iron removal prior to anaerobic treatment, as evidenced by the low percentage of volatile matter (17.5%) and the high proportion of iron (10.5% w/w) in the sludge after operation for one year. Pre-treatment of the metals within the leachate was initiated by the addition of alkali, for metal precipitation, followed by neutralisation with acid. The maximum efficiency of the pre-treatment system was between 0.1 and 0.15 kgFe.m-3.d-1, which was approximately 60% of the maximum loading rate. Pre-treatment with alkali also removed calcium, zinc and phosphate to varying amounts. Results presented in this study suggested that a COD:phosphate ratio of 1000:1 was sufficient to support anaerobic decomposition within the digester, under the conditions used.

Extended operation of the pilot-scale digester at a HRT of 2 days resulted in a maximum treatment of 4 kgCOD.m-3.d-1, equating to 55% removal of COD. The relatively low organic treatment efficiency observed in comparison to previous laboratory studies, where treatment efficiencies in excess of 10 kgCOD.m-3.d-1 have been recorded, was shown to be caused by: (i) the variable leachate strength; (ii) heavy metal concentration leading to blockages; (iii) leachate feed arrangement to the reactor; and, (iv) poor hydraulic contact with the sludge.

The results of this study showed that the process of nitrification was inhibited at COD and BOD loadings above 0.5 and 0.25 kg.m-3.d-1, respectively. The study proved the necessity for a secondary stage aerobic treatment to metabolise residual organic material from the digester effluent. Treatment of residual organics in the digester effluent by the activated sludge unit provided an effluent of nutrient quality sufficient for nitrification. Subsequent studies showed that operation of the anaerobic digester, activated sludge unit and a Rotating Bological Contactor (RBC) in series proved successful as a treatment process for the removal of organics and the nitrification of ammonia from leachate.

This study further demonstrated the possibility of treating high concentrations of nitrate, resulting from the nitrification of leachate. The use of an external carbon source for denitrification was unnecessary. In terms of organic and nitrogen removal, high efficiencies were achieved. A four stage, organic-nitrification-denitrification system used in this study was capable of reducing COD, BOD and ammoniacal-nitrogen concentrations of 15,000, 9,240 and 1,100 mg.l-1 to 1,300, 35 and 45 mg.l-1, respectively.

The information supplied drew attention to those process features of the trial which have positively and negatively influenced the process performance. The use of an anaerobic upflow sludge bed/filter and an attached-growth system for denitrification, such as a fluidised-bed reactor, were proposed as improvements to system used in this study.