Wasteserv has very recently taken over the operations of the Regenerative Thermal Oxidizer (RTO) gas plant at Maghtab. This news brought about a satisfactory level of appreciation in terms of Wasteserv’s commitment to control gaseous emissions, however this was also accompanied by criticism on the energy recovery potential of the gases.

This article attempts to explain the energy recovery systems for both the disused Maghtab and Ta’ Zwejra Landfills, essentially conveying the message that not all energy recovery systems are meant to produce electricity.

First of all, there needs to be made a clear distinction between energy recovery systems for uncontrolled dumps (such as the former dump at Maghtab) and for engineered landfills, as is the case for the Ta’ Zwejra and Ghallis facilities.

Both the dump at Maghtab and the Ta’ Zwejra engineered landfills are located within the Maghtab Environmental Complex. However, this is basically where the similarities end. The differences in land filling patterns have resulted in emissions with very different characteristics.

Energy recovery from the Closed Maghtab Dump

Energy recovery from gas emissions is carried out by extracting the gases that form within the landfill and use them to produce any form of energy. Waste produces methane if allowed to degrade in an environment where air is not present. If air or water (oxygen) is provided, the waste will have self-combustion and internal fires are sustained within the mass of the landfill.

At the old Maghtab site, we have no control over the limit of air intake to the system. This means that we have some methane gas where air is not present and furthermore, fires where air is finding its way to the waste. WasteServ thus needs to capture the gases before they can reach the surface of the landfill and escape into the atmosphere. To do this, WasteServ installed some 400 steel wells that are connected to two main gas pipes. One pipe is dedicated for methane (the good gas), and the other is dedicated to toxic gases, referred to as the bad gas.

This network of gas pipes are linked to a group of powerful booster pumps which provide for all the flow requirements within the network. This guarantees a constant supply of gas through the Regenerative Thermal Oxidizer (RTO) treatment facility, which is has been commissioned recently at Maghtab. These boosters, which are operating at 75 per cent of their capacity, ensure an extraction of 6000 cubic meters of landfill gases every hour.

The new project to Rehabilitate the Closed Landfills (and eventually convert it into a recreational park) shall produce the necessary capping to make the surface of the landfill less permeable to air and rainwater. The situation within these old landfills is expected to have a degree of change from aerobic to anaerobic, where the air presence is restricted and the generation of methane gas will be encouraged. Only when the inclusion of air is restricted may the capacity of the boosters be increased further.

The operation of the plant is rather straightforward and in straightforward terms, this facility may be considered as a very complicated furnace. Delving into the chemistry of the treatment of harmful gases goes beyond the scope of this article, however what is relevant is that it does so at an elevated temperature exceeding 900 degrees Celsius.

The RTO is preheated electrically until it reaches the correct operating temperature, thus allowing for the initial circulation of the gas within the system. Although the vast majority of the landfill gas generated from the Maghtab dump is not combustible, that tiny fraction of flammable gas (two pre cent methane) is sufficient to maintain the high temperatures of the system.

Indeed, following preheating, electrical supply is cut off, and the RTO is practically self-sustaining. The energy available in this gas flow is actually being recovered to treat the toxic gases. Although not directly involved in the generation of electricity, the energy recovered from the landfill gas of the Maghtab dump substantially substitutes the electrical loads, which would have been required to maintain the heat.

In so doing, the plant has so far treated 37,830 tonnes of CO2 equivalent.

Energy recovery from the Ta’ Zwejra engineered landfill

Electrical generation from landfill gas is associated with the engineered landfills within the Maghtab Environmental Complex, namely the Ghallis and Ta’ Zwejra Landfills, in clear contrast with that of the closed Maghtab dump.

The energy recovery system for the Ta’ Zwejra landfill is somewhat easier to understand. The percentage of combustible (flammable) fuel present in the gaseous emissions averages at about 50 per cent (a substantial improvement from the Maghtab’s dump two per cent), which allows for direct combustion of the gas. The gas extraction system for Zwejra will be physically very similar to what has been installed in the Maghtab dump, however in this case the gas will be fed to electrical power generators that do not differ considerably from normal combustion engines.

It is estimated that energy recovered from the Ta’ Zwejra and Ghallis landfill gas will be in the order of 1000 kW. This will contribute to the power produced from the whole Maghtab Environmental Complex, which is equivalent to the consumption of 3000 average households.

WasteServ has also started demonstrated the potential of using landfill gas for electrical generation. The first experiment of this sort was the lighting of a 50ft tall Christmas tree, which was fixed on the apex of the Maghtab dump for all to see during the festive season.

Treatment of leachate and leachables

The degradation of waste is a chemical process that changes the organic fractions of the waste from the solid state to the liquid and gaseous states. This reaction is easily noticed and may be experienced in the domestic environment, when a rubbish bag is left unattended for a couple of days. A twoday-old bin will often have an unbearable stench (gaseous state) and leave a viscous liquid (leachate) at the bottom.

It is the responsibility of every landfill operator to control both. Besides systems to control the gaseous emissions, an engineered landfill is designed in such a way to collect the leachate in a sump-like structure. The bottom liner of the Ta’ Zwejra and Ghallis landfills are constructed in a number of cells, which allows sectional collection and storage of this leachate.

The former Maghtab dump on the other hand has no such liner installed however since its core has a highly elevated temperature, the leachate evaporates and travels alongside the gas emissions, towards the vacuum created by the Booster pumps. When the hot evaporated gases get in contact with the cold pipe structure, the leachate condenses back, and is collected at a low spot in the gas main.

As part of this same project, Wasteserv has also installed a Reverse Osmosis (RO) system, which treats both condensate and leachate from either landfills. The second-class water that results from the treatment process is used for irrigation and cleansing purposes within the Maghtab Complex itself. The RO has the possibility to produce a maximum of 5000ltrs of water per hour. This is however limited by the amount of condensate and leachate feed it is provided.

Some history on operating engineered landfills locally

In 2004, WasteServ developed the Ta’ Zwejra Engineered Landfill, which was the first engineered facility locally. Since this site was developed with the necessary engineering to be compliant, it had to be distinguished from the closed dump. The area occupied by this cell was known as Habel Ta’ Zwejra, hence the name Ta’ Zwejra Engineered Landfill at the limits of Maghtab, was adopted for this facility.

WasteServ had submitted a development application to have this facility permitted to be operated and managed as an engineered landfill. On 1 December 2005, MEPA approved the first national Integrated Pollution, Prevention and Control (IPPC) permit to authorize WasteServ to operate and manage the non-hazardous, non-inert Landfills at Ta’ Zwejra, which permitted the change of use from the storage of waste to that of land filling.

Since the period taken to process development and operational permits was of a longer duration than expected, WasteServ continued to extend the utilization of Ta’ Zwejra Engineered Landfill to provide the necessary void space until the first cell of the Ghallis Engineered Landfill was developed. Between April 2004 and December 2006, four cells were constructed and engineered at Ta’ Zwejra facility.

Currently, WasteServ is initiating the process of capping and extracting gas from the Ta’ Zwejra Landfill. Tests are also being carried out to find the optimum use of the gases generated.

More information about WasteServ can be obtained by visiting website www.wasteservmalta.com or by calling the WasteServ Freephone 8007 2200.

Ing. Nicholas Vella is the Project Leader for the Rehabilitation of Disused Landfills Project within WasteServ Malta Ltd.