Steam Injection

In the past few years several Landfill Bioreactors have been constructed with varying results but basically with similar conclusions. Bioreactors increase the biodegradation rate of organic material; increases gas generation and increase the rate of settlement of the refuse. Regulators seem to favor the idea that a bottom liner should be more substantial for a Landfill Bioreactor than a standard Subtitle D landfill due to the addition of liquids. However, in the past five years it has been realized in the industry that most groundwater impacts around standard landfills are from migrating gas not leachate. Landfill Bioreactors equipped with adequate LFG collection systems reduce the long-term risk of groundwater impacts by accelerating biodegradation and removing the gases, which contain toxic compounds much faster than conventional landfills. However, the introduction of liquid to a landfill must be carefully controlled.

The main purpose of introducing liquids into a Landfill Bioreactor should be to increase the humidity in order to accelerate biodegradation, not to saturate the refuse. The liquid is currently introduced through various distribution methods, (i.e. surface flooding, sprinkling, or injection wells). However, a key problem with using liquid to raise the humidity is being able to achieve uniform distribution without saturating portions of the refuse. Saturation of the refuse results in several concerns:

1. Increased leachate generation.
2. Regulators require additional liner protection.
3. Uneven loading on slope liners requiring over-designing.
4. Gas is bound up in liquids making extraction difficult.
5. Liquids carry suspended solids and calcium carbonate, which can block bottom drains.
6. It adds material to a landfill instead of reducing it.

Currently, the amount of liquid to be introduced to a Bioreactor is typically based on the field capacity (i.e. the moisture content above which the liquid will begin to drop out under gravity forces) of the refuse. This can be as high as 40% by volume and is not well known or easily defined for MSW. Unfortunately, as the organic material biodegrades the overall capacity of the refuse to hold liquid reduces, releasing the liquid and allowing it to flow.

Even with all these challenges the Landfill Bioreactor is still the best landfill system for disposing of the organic threat to the environment that we are creating in our lifetime and not leave it for our next generation.

Also we should be taking advantage of the electric generating potential now when we need it the most. Therefore, a better method to raise the humidity in the refuse is needed.
A method, which would clearly and significantly improve the "moisture distribution" while minimizing the concerns listed above, would be to change the water into steam prior to injecting it into the landfill.

Liquid flow is mainly controlled by gravity, which is downward and then laterally under a head pressure. Steam flow is controlled by expansion pressure and temperature differential and moves in all directions. Therefore, the injected steam will expand throughout a much larger area within the waste and raise the humidity in the prism. Liquid cools the refuse and inhibits biodegradation until the temperature recovers. Steam will preheat the refuse and enhance biodegradation immediately. Decomposition is most active when the temperature is maintained around 100 degrees F.

When organic material decomposes it creates voids in the landfill. If liquid is pumped into these voids it could inhibit settlement because liquid is not compressible. Steam will not fill these voids; it will increase the rate of settlement and recover more air space. Also, steam will not carry suspended solids and calcium carbonate, which can plug up bottom drains.

Steam expands up to 1,600 to 16,000 times its original volume, therefore requiring only a fraction of the water to achieve widespread coverage within the waste prism compared to application of water at the surface. For ever 1,600 gallons of water used in a conventional bioreactor only 1 gallon of water converted to steam would be used and with better distribution. This will minimize the potential of the liquids migrating to the bottom of the landfill and to the groundwater.

The steam can be produced in a gas-fired boiler, heat exchanger on a flare or from the exhaust steam from a power plant and injected into the waste prism through a series of strategically located injection wells.
In Situ Landfill Profiling

Prior to installing a steam injection system or a liquid system, it is important to know where to place the injection wells and the gas collectors. The best way to obtain this information is to perform a Piezo-Penetrometer Test (PPT) Profile of the landfill.(Please review the web page "Landfill Profiling") This will identify dense layers that can act as an aquitard and can locate low-density layers that will transmit the steam further into the landfill. The PPT Profile will provide information as to the current in-situ conditions of the landfill prior to steam injection. Follow-up PPT investigations will indicate the effects of the steam injection has made on the refuse and whether any adjustments should be made to the process. It can also monitor the settlement by measuring the distance between the dense layers.

The 20-ton PPT rig is also used to install the 2" diameter steel injection and gas collector wells following the PPT Profiling by the hydraulic push-in method producing no cuttings. The depth control of the PPT with the aid of inclinometers in the cone is very exact so there is little danger of perforating a slope or bottom liner.

It is important that the steam injection system be monitored to meet regulatory requirements. The PPT rig can also install moisture and temperature sensors in the landfill to monitor the migration of the steam through the refuse. Another advantage of steam over liquid is that temperature sensors are much more responsive in a landfill, than moisture sensors.

The PPT Profiling method can provide comprehensive in-situ data; STI believes that the PPT method will be instrumental in the success and advancement in Landfill Bioreactors. Not only will it be used in lined landfills but as the acceptance of PPT and steam injection grows within the industry, it will also be instrumental in converting old closed unlined landfills into Steam Injected Bioreactors as well.

Steam Injection is also useful in Aerobic Bioreactors. When air is injected into a landfill it will remove moisture from the refuse. If too much moisture is removed spontanious combustion will ocurr. To prevent this, liquid is usually introduced into the refuse by applying liquid from the surface. However, the liquid will flow downward by the path of least resistance and not moisturize all the refuse. The air will travel to places in the refuse that the liquid will not migrate to. If steam is injected into the air stream then the moisture in the refuse will be maintained. Evaporation and injected liquid will cool the refuse, slowing biodegradation steam will maintain the temperature of the refuse even in the winter months.

The above steam process is considered a low temperature and low pressure system (245 to 300 degrees F), which is useful in biodegradation of organic waste in landfills. Laboratory studies have indicated that high temperature (300 to 600 degrees F at 95 psi) and high pressure steam is effective in melting/shrinking plastics in landfills. The steam can induce about 25% to 30% shrinkage and does not produce any off-gassing. It is recommended that a low temp. phase be conducted first to biodegrade organics. Then a high temp. phase be performed to shrink and open plastic garbage bags and allow the steam into this refuse. This would be followed by another low temp. phase to biodegrade the refuse in the garbage bag.

This combination could recover as much as 50% of the airspace depending on the amount of plastic in the landfill.

High temperature steam can also be injected into an above ground containers containing refuse, compacted, prior to placement into the landfill, thereby reducing its volume and saving airspace.

STI is looking forward to assisting you with your Landfill Bioreactor projects. This process has been awarded a U.S. patent.