Aerobic landfill (European Approach)

Results from lab scale as well as field scale investigations shows that there is still

significant emissions might occur in some traditional anaerobic landfill long time periods after landfill closure. While landfill gas emissions are expected for 3 decades, the production of leachate is predicted for decades to centuries (Ritzkowski et al., 2007).

These emissions will cause significant negative environment since most of the protection barriers of the old landfill are missing depending on its type and quality.

Although a complete sealing of landfill may temporarily reduce emission as result of dry-conservation of the biodegradable waste fraction, the emission potential remain.

The interrupted emission will restart once surface capping system is damaged resulting in the infiltration of water.

Although lysimeter studies tend to investigate the decomposition behavior of fresh waste under aerobic conditions, the conclusions drawn from these studies cannot be absolute until full-scale studies are under taken using new waste (Rich et al., 2008).

Currently, the aerobic in situ-stabilization by means of air injection (European approach) mainly attempts to stabilize and modify the inventory of organic matter inside aged landfill body in order to reduce the emission potential in an accelerated, controlled and sustainable way, unlike the semi-aerobic landfill. When used as remediation technique, the aerobic landfill technique will interrupt the five-stage degradation process of anaerobic landfill. The organic matter (e.g. hydrocarbons, partly non or only hardly degradable under anaerobic conditions) are significantly faster degraded due to the enabling faster and more extensive aerobic degradation process under this landfill scenario. It results in an increased carbon discharged via the gas phase as well as reduced contaminants concentration in leachate. As the carbon dioxide, instead of methane in anaerobic landfill, is the main compound in the extracted off-gas, the negative impact of landfill gas emissions towards the increased global warming effect can be significantly reduced. Besides, with respect to the leachate quality, a reduction of organic compounds as well as ammonium can be expected. Comparison of the specific parameters of three different landfill scenarios (i.e. anaerobic, semi-aerobic and aerobic landfill) can be found in Table 1-4. Apart from the positive effects to the environment, the aerobic in situ stabilization is associated with a significant cost saving potential due to a quantitative and qualitative reduction of the aftercare period (Rich et al., 2008).

The aerobic landfill method can be grouped into several categories base on the purpose, structure and power source:

1) High pressure aeration (shock pressure concept)

High pressure aeration is mostly associated with the Bio-Puster concept (Ritzkowski et al., 2012). The aeration is realized by shock pressure releases (up to 6 bars) from lances, using air which might be enriched by additional oxygen and potentially by

Table 1-4. Comparison of specific parameters of three different landfill scenarios (modified from Rich et al., 2008).

Anaerobic landfill Semi-aerobic landfill Aerobic landfill

Process

Five stages: aerobic, fermentation, acetogenesis, methanogenesis, oxidation

Passive drawing of air into waste mass due to temperature gradient

Aerobic conditions achieved by forcing air

into waste mass

Temperature range

30–65 ^oC (optimum range for LFG generation 30–

45^oC); Most UK landfill sites operate at 30–35 ^oC

40–50 ^oC (1) 40–70 ^oC (2); ideally 54–66 ^oC (3)^a

pH range

5–9 (7–8 during methanogenesis stage; ideally 6.8–7.5)

Ideally above 8 (4)

7.5–8.5 (5): fewer acids are produced than in anaerobic landfill, as fermentation reactions

are limited

Timescale Estimates vary from

decades to centuries 30 years (6)

2–3 years (7); 4–5 years (8); other attempts at estimates less successful

Emissions CO2, CH4, H2O, trace

pollutants CO2, H2O, trace pollutants CO2, H2O, trace pollutants

a These two reading were obtained from sites with different operating characteristics.

Source: (1) Yoshida, 2005; (2) Ritzkowski et al., 2003; (3) Read et al., 2001; (4) Lee et al., 1994; (5) Stessel and Murphy, 1992; (6) Chong et al., 2005; (7) Cossu, 2005; (8) Prantl et al., 2005.

nutrients. Supplied by a compressed air distribution net work, each lance features a quick release valve which is intermittently opened once a specified positive pressure has been built-up. The release aeration gas is capable of penetrating both, highly and weakly compacted waste materials.

The systems normally operated in parallel to the aeration and with an increased extraction capacity of 30% of the gas volumes used for aeration, as shown in Fig.

1-6(A). The extracted gas will be treated by biofilters or activated carbon. This aeration concept is mainly associated with the implementation of landfill mining projects since it is a costly technique.

2) Low pressure aeration

This technology is applied in the most of the aeration cases for accelerating solid waste biostabilization in situ. Different from the high pressure concept, the positive

pressure that used for aeration does not exceed 0.3 bars (normally ranges from 20-80 mbars). Moreover, because water addition is often not necessary when applying low pressure in situ aeration to landfills without final top cover, the aeration method is very advantageous to use to biologically stabilization waste in old landfill or open dumping

Waste

Drainage layer Drainage

pipe Mineral layer

Geological barrier

Drainage layer Mineral layer

A B

Surf ace cover

Surf ace cover Compressed

air in

Off-gas Compressed air in

Air in Off-gas

Waste

Waste

Drainage layer Drainage

pipe Mineral layer

Geological barrier

Drainage layer Mineral layer

C D

Biocover

Surf ace cover

Air in Air in Off-gas Air in (optional)

Waste

Fig. 1-6. Schematic of different aeration concepts. A: high pressure aeration; B: Low pressure aeration with parallel off-gas extraction; C: Low pressure aeration without off-gas extraction; D:

Low pressure aeration/air venting.

site without a base liner (Ritzkowski et al., 2007). The low pressure aeration concepts can be further sorted into four categories.

The first type is active aeration with off-gas extraction. Currently most of applications of this landfill concept are based on the AEROflott, AIRFLOW or Smell-Well concept (Ritzkowski et al., 2012). The air distributes inside the waste layer through convection and diffusion and it can be further directed by the simultaneously operated off-gas extraction system. This method is flexible due to the air flow inside the waste can be controlled by means of the selection of the located aeration and gas extraction wells, as shown in Fig. 1-6(B). The difference among the applications of this aeration concepts mainly exist with the selection of the off-gas treatment manners.

The second type is active aeration without off-gas extraction. In this scenario, air is supplied into waste layer without simultaneously operating with extraction system for off-gases. In this case, the landfill cover will function as a biological filter layer. This landfill scenario may easier to maintain compared with the type one, although it can result in a significantly lowered emission reduction. Aeration in this concept can be realized by a system of vertical gas wells in the waste body or through air injection into the unsaturated soil zone beneath the waste. In the later case, the soil will function as air distribution layer for evenly distributes the air from bottom to top, as shown in Fig.

1-6(C).

The third type is passive aeration (air venting). In this scenario, air is introduced into the waste layer through surface or open gas well by negative pressure induced inside the landfill body, as shown in Fig. 1-6(D). The gas well of this system are only perforated in the deeper waste layer in order to increase the aeration volume of waste as well as avoid short circuits near landfill surface. In order to ensure a gradual aeration, the passive aeration starts from the surface then shifts into the deeper layer. The extracted gas volumes under the aeration concept are considerably higher than the gas production rate of the landfill waste. The extracted off-gas can be treated by means of biofilter.

The fourth type is energy self-sufficient long term aeration. The systems consist of wind driven aspirators which is mounted on some of the existing gas wells, and pneumatic air pumps powered by wind wheels. The compressed air is directed into existing gas wells, which results in a continually oxygen supply of the already widely stabilized landfill (Ritzkowski et al., 2009). This aeration may be applied for long term aeration compared with the aforementioned active and passive aeration concepts. The systems can be established during transitional period between the end of active forced aeration and the subsequent final surface cover installation. Through the on-going low air supply, the restart of landfill gas generation can be widely avoided in the long term.

Apart from the technical and fanatical factors, local climatic conditions (temperature and precipitation levels) should be taken into account when selects an appropriate aeration technology for the specific landfill. As summarized by Ritzkowski and Stegmann (2012), landfills located in a temperate climate might be actively aerated by means of the low or high pressure concept since the moderate temperature and precipitation levels operational problems occur infrequently. On the other hand, a humid/tropical climate might result in operational problem due to the accumulation of water in landfill and the off-gas extraction system, thus significantly reducing the operation efficiency. Landfills under the later climate conditions might be suitable for the semi-aerobic concept, since the temperature differences between the ambient and the waste is sufficient. A location with extreme seasonal temperature variations, however, faces the most challenging situation. In particularly, the low temperatures problems which lead to an increased formation of condensate in the off-gas extraction system may occur. The active aeration concepts without off-gas extraction may be suitable for these conditions but it might fall short as to an inefficient biological methane oxidation in the landfill surface.

The landfill aeration technology has been widely used among the world in achieving different objectives (for example reduction of emission potential and aftercare period (Heyer et al., 2005), remediation of abandoned site (Cossu et al., 2003), and odour reduction (Jacobs et al., 2003)), and have found the results to be successful. However, there are still some uncertain issues or questions need to be satisfactorily addressed. The first one is related to financial cost. Although some investigation (Rich et al., 2008) shows that landfill aeration will benefit from multiple aspects such as shorten the long-term environmental costs in anaerobic landfill and landfill recover, finding a more suitable aeration concept with high oxygen utilization rate is still necessary; the second issue is about the behavior of heavy metal in leachate phase and release of non-methane organic compounds in gas phase after introducing air into a landfill. In general, the mobility of heavy metals in solid waste is high in the initial aerobic phase (Phase I) of the anaerobic landfill, but decrease as the oxygen levels decline since heavy metals tend to be retained within the waste under the reducing conditions of the anaerobic stages (Kjeldsen et al., 2002). According the research made by Kim, 2005, heavy metals, such as Al, Cu, Cr and Pb, are not immobilized in aerobic landfill as they are in the anaerobic landfill, which will lead to problems with the leaching of these toxic elements; the third issue is the rising temperature after air injection. The rising temperature as result of exothermic processes is commonly observed after air is introduced into the landfill.

Temperature profile at some areas in the landfill was increased up to 70 ^oC (Laux et al.,

2010). In order to avoid the combustion risk, landfill aeration has to stop and waits for the reduction of temperature in waste layer, or switch to a discontinuous aeration mode (Oncu et al., 2012). In this case, anaerobic degradation may be restart as indicated by the gas composition (Oncu et al., 2012). The switch of aerobic condition into anaerobic condition under high temperature condition might cause serious environment impact as described in the Chapter 4 of this dissertation.