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What's happening and our landfill bog

Leachate Measurement problems

While we were researching our automated system, we looked at a couple of other types of measuring systems and encountered a few problems. The obvious choice initially was a pressure transducer system, which is a similar idea to those used on submarines. You locate the transducer itself in the bottom of the well or leachate chamber, and the depth of leachate is given by how much pressure the transducer is receiving at the depth of liquid compared to atmospheric pressure - it's a simple but often effective idea, and in normal circumstances very accurate. The deeper the liquid, the higher the pressure.

The problem is in this application is the difference in pressure in wells and leachate towers caused by landfill gas. The pressure in a leachate tower or well is unlikely to be atmospheric, which is usually the reference point that these type of sensors rely on. If the well is sealed, without gas extraction, pressure can rise up to two bar! This means that the sensor thinks there is much more liquid in the well than there is - due to the gas pressure in the well also exerting an influence on the liquid. Hence, false low reading!

 On the other hand, if active gas extraction is taking place in the site and gas is being taken from the well being measured, the pressure inside the well may be lower than atmospheric pressure, thus giving a false high reading.

These systems are normally calibrated initially by mechanical dip meters, which of course means that the well is open to atmosphere when the calibration is taking place, hence giving an initial accurate level reading and a false sense of security in the integrity of the data.

So, if you have a pressure based level system on your site or are thinking of buying one, you might want to get the vendor to prove that the readings are accurate in all circumstances, or get caught out when the Environment Agency does random checks on your site.

Landfill restoration

We recently completed a contract for Norfolk County Council which involved the final restoration of one of their landfill sites - we seeded the ground using a special landfill mix of seed. We completed the contract at a fraction of the cost of our competitors and in a much shorter timescale - just another area of landfill systems that we can offer. Contact us for a quote, we think you will pleasantly surprised!

Surface laid pipe versus buried

There has always been a debate on which is better - surface laid pipe or buried. In all truth, each system has it's advantages and disadvantages. Buried pipe means that grass cutting and aftercare activities are much easier, it is visually less intrusive and means that temperature variances are reduced.

Changes in temperature can affect MOPE pipe significantly, polyethylene has a high co-efficient of expansion and a surface laid system is subject to extreme changes in temperature, especially in the UK with our famous changeable weather! With a surface laid pipe system, care must be taken to include designed expansion points in the system, and pipe may be required to be pinned to the surface to prevent slippage over the site - we have often been called out to relocate pipe onto retaining bunds after expansion has caused it to slip off. Power sources for pumping systems can also be a problem on the surface of the landfill - electric cables for pumps, air feeds for pneumatic pumps, they can all be damaged by plant moving across they site.  Breakages in joints can also occur. However, surface laid systems are MUCH easier to install and maintain - leaks, low points and other problems can be visually identified.

Buried systems do not normally suffer from thermal expansion problems, especially if they are buried 500mm or more underground. However, burying pipe work on a landfill site has it's own problems. Landfills are not solid ground, they settle and decompose - often at differing rates from one area to another. This can cause lows in buried gas pipe, which can fill up with condensate blocking gas flow, or even worse - breakages in joints underground. Finding these problems when a system has been buried can be a time consuming and expensive process, causing downtime for the flare / power generation system on site and potential environmental problems with odours. Buried pipe work systems can also be damaged by excavation required - for example to install a drainage system or re-level the site contours. Installation of a buried system can also be in the region of 2-3 times the cost of a surface laid system.

So what is the better system? There is no real defined answer to that question, some people prefer buried and some prefer surface laid, and a lot depends on the circumstances of the site, i.e. planning considerations, post closure usage such as farming etc and over-tip plans. Whatever the system that is chosen though, the designer must ensure that all of the above factors are taken into consideration. 

Sufficient expansions points should be designed into surface laid systems - road crossing points across the site should also be taken into consideration, and safety is certainly an issue with power sources for pumps. Surveying the system is a MINIMUM requirement after installation, full CIA  is preferable.

Sufficient falls and test points should be designed into buried systems to ensure that problems, when they do occur, can be found. It is also a very good idea to survey in all gas systems just after installation, with accurate CIA records.

Landfill Litter Fencing

We recently won a contract to install landfill litter fencing for a large waste operator in the South East - just another service that we can provide to the landfill industry.

Leachate recirculation

This is the big subject at the moment in landfill gas generation. Improving the output of gas from sites is the holy grail of all landfill gas power generation companies - and is related to our current water crisis in the UK!!

We are not getting as much rainfall as we used to, especially in the south of the UK, and one result from that is that landfill sites are experiencing less leachate problems. The Environmental Managers and EVA officers will think that that is a blessing, less leachate, less costs in disposing of the problem, but as I am sure that all the landfill power generation companies will agree with me on this one, it also means less gas and slower degradation rates of the landfill itself.

Landfill gas is only generated when the conditions are right, when there is an absence of oxygen, when the exothermic reaction of degradation produces enough temperature for anaerobic bacteria to reproduce and consume the degradable carbon fraction within the waste mass  when there is enough moisture. Anaerobic bacteria, like us, need water to live, and also to move around the waste. Leachate flows within the waste mass are the transport system that the bacteria use.  Bacteria is usually found in soils used for cover (not the only source, but a big one) and the transport mechanism for the bacteria is from the rainwater running down through the soils and into the deeper waste mass. Thermal reactions within the mass also produce leachate currents running laterally and (through vapour generated by heat) upwards as well. Hence, less rain, less bugs, less gas.

There has been some great research done in Florida on this and other waste related subjects - see http://www.floridacenter.org/ . There has also been a paper written by Debra Reinhart and A. Basel Al-Yousfi on the subject which can be accessed here.

So, how do you increase the waste degradation rates in 'dry' waste? There are several possibilities:

1. Introduce steam into the waste mass - in theory, this has two benefits, to increase moisture content locally and to increase the waste temperature - sounds ideal, but it has to be done carefully otherwise the high temperatures involved will boil the bacteria!
2. Introduce 'clean' water into the waste mass - fine and dandy but it does have the downside of increasing the overall leachate content within the site (which the Environment Agency are not too keen on).
3. Introduce leachate pumped from the 'wet' bottom of the site (assuming you have any) to the 'dry' waste nearer the surface. This has several advantages, you are introducing moisture and you are also 'seeding' the fresher waste with anaerobic bacteria taken from the bottom of the site. If you can design the system so that waste is pumped directly from the leachate extraction wells into the recirculation points then you will also be transporting heat into the waste mass, as well as having increased quantities of live bacteria. Putting the leachate into a tank prior to recirculation inevitably means that some bacteria will die as the leachate cools in the tank.

However, a word of warning. Introducing leachate at the top of any cell has it's risks, notably possible leachate outbreak along the side flanks of the cell - which may lead down onto a nice green field or lovely uncontaminated stream, so sufficient preparation in the form of a study should take place before it is attempted. The EA will also probably want to see that you have done your homework, and that the system has sufficient safeguards and controls - this may involve some Civil Engineering works as well as safety cut-offs in the recirc system itself. The gas extraction system will have to be able to cope with the increased production rates (sufficient wells, extraction pipe work etc),  may involve a change in your PPC so expect some legal / consultancy costs.

Having said that, the results are often spectacular for gas production, and the system should pay for itself in a fairly short timescale.  If used on active tip faces or 'temporary capped' areas waiting for an over-tip, settlement rates will increase creating additional void. It also has the advantage from the Environment Agencies point of view that the landfill itself will be less of a risk in the longer term as the site will not gas for as long a period.

We have several systems for recirculation leachate - and can provide technical assistance and feasibility studies on the subject so if this idea is of interest to you, send us an email or give us a call.

SDR rating and pipe specification - leachate and gas systems

I was asked recently what SDR meant in terms of pipework: here is the answer (courtessy T-Mex's website - good chaps!)

The wall thickness of PE (HDPE, MDPE and HPPE) pipe is referred to as the SDR (Standard Dimensional Ratio). The SDR is the pipe's wall thickness in relation to it's outside diameter. For most landfill applications either an SDR 17.6 or SDR 11 material is normally specified.

Pressure Rating

For PE80 (MDPE and HDPE) material an SDR rating of 17.6 provides a pressure rating of PN6 (6 bar) and an SDR rating of 11 provides a pressure rating of PN10 (10 bar). For PE100 material SDR ratings of 17.6 and 11 provide pressure ratings of PN10 and PN16 respectively.

The operational pressure of most landfill gas management systems is between 50 mbar and 150 mbar suction and accordingly an SDR17.6 PE 80 material, giving a pressure rating of PN6, is generally deemed acceptable. Where the installation may be subjected to additional stresses (e.g. due to depth of cover or excessive lateral movement) the use of the thicker walled SDR 11 material to provide a greater tensile strength is sometimes considered.

PN10 rated pipe is normally used in leachate systems - for both air feed (if applicable) and leachate return. For sizing pipework for leachate volumes, you will need to calculate the total possible maximum flow in the system - this can be determined by simply adding the flow rating of your pumps and using a flow calculator - Mears make excellent gas and water flow calculators!

Electric pumps normally have a manufacturers stated flow rating and pressure output (head), and reciprocal air pump maximum output can be calculated by multiplying the cylinder displacement (usually in litres) by the maximum number of strokes per minute. For air feed systems, it is important that you size the pipework to give sufficient air flow to the farthest reaches of the site, and be warned, air diaphragm pumps, whilst excellent performers, are notoriously air hungry!!

Gas pipe rating

How do you size a gas pipe - by this I mean what diameter of pipe do you use when designing a landfill gas extraction system? For the gas main, a good place to start is to estimate the area's production rate from the wells that the main will serve, or even better, to measure the actual flow. The calculated flow should be no more than 10 metres per second (use a mears calculator) to calculate velocity - careful when you calculate the flow rate, use the ID of the pipe for the calculation, so the SDR rating of the pipe makes a difference.  Take into account further wells that may be introduced into the system in the future. Pressure loss in gas systems from the start of the pipe to the end should also be taken into consideration, as a rule of thumb I don't like a system to have more than 10mb from one end to another. A good way to achieve a lower pressure drop and velocity with smaller diameter pipe is to introduce a ring main system - a continuous pipe that runs around the site (or cell) joining to itself. This system also has the advantage that if it becomes blocked (with condensate due to a pump failure, or due to pipe settlement) at any point the gas can run both ways around the blockage. Sometimes you can only tell if a main is blocked in a ring main by seeing suction pressure swing and then you still get reasonable gas flow! I prefer ring main systems as a basic design, they are more robust.

There are several schools of thought on well connection pipe sizing - one is to make is as big as possible, even to the extent of running fairly large diameter pipe (160mm and above) directly by the well and tapping into it with a short feed of flexible pipe - this system normally collects gas from 5-10 wells. The other is to make and individual connection to a well in fairly small (say 63mm)  pipe and to run it down to the main, creating a higher velocity and thus clearing the pipe of condensate due to high gas flow. Other use 90mm, or 110mm pipe as individual well connections. I cannot really advise the best system out of these, as a lot depends on the site itself - whether tipping operations are still going on, whether the landfill is fairly flat or has good falls, whether the pipe work system is to be buried or not. In general I prefer larger pipe to smaller pipe but we have used small diameter pipe on many occasions where the situation demands it. Another factor to take into consideration is the gas production of the well itself - some gas wells, and more relevantly Leachate extraction points, can produce HUGE amounts of gas - I once upgraded the connection on ONE leachate tower on a site from 90mm to 160mm, which gave a 300kW improvement in the power generation on the site! Unfortunately, you don't always know the production of any individual well until it's connected - check the suction pressure on the take off point and compare it to the suction on the well - there should be more than about 2mb difference, otherwise either the pipe is undersized or is blocked with condensate.

Camera survey

We recently did some camera survey work on wells using our new all singing survey camera - have a look at the results!

Move the mouse over the picture to start the video!

Web news

We have fixed some broken links to various sites - hope this helps!

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Last modified: 06/08/06