- an environmental benefit from use of biogas as power supply
- a big saving in disposal cost due to waste volume reduction
- a return on investment of less than two years Introduction
Solid Waste Management in Greece has been remarkably upgraded during the last fifteen years, becoming a well organized and responsible activity, with specific goals and targets, as dictated by the EU directive on Municipal Solid Waste and landfill (99/31/EE). The new solid waste management plan comprises building of new landfill, increasing the investment for recycling and composting and modernizing the existing plants. The landfill of Ano Liossia is one of the biggest in Greece and serves the Athens area, with more than 3 million inhabitants with an average production rate of 1,1 Kg/day of solid waste (2004). The typical composition of the municipal solid waste in the recent years is the following: putrescibles 47%; paper and similar 20%; metals 4,5%; plastics 8,5%; glass 4,5%; others 15,5%. As part of the activity to upgrade the existing facilities, the community had to solve the problem of the landfill leachate.
Water percolating through landfills produces leachate, which may contain undesirable or toxic chemicals. Modern sanitary landfills are built to prevent leachate contamination of groundwater or surface waters. The bottom of the landfill is lined with impermeable layers, and the leachate is collected and treated before being released to the environment.
In order to reduce the ever increasing disposal costs and improve the environment quality in 2005 the Athens municipality implemented a project for treating the leachate by reverse osmosis followed by vacuum evaporation. The plant as a whole was designed to treat up to 400 cubic meter per day. The evaporation plant was designed for a maximum capacity of 200 cubic meters per day, being the RO yield around 50%. The plant was based around 4 identical evaporators, each of them connected to a cogeneration plant and an air cooler. The first one provided the necessary power to evaporate the leachate, the second one was used to cooling down and condensate the vapour. The solution offered the maximum combination of capacity and flexibility, and provided the most stable conditions for the evaporators (optimized settings).
Vacuum evaporation provides a thermal separation of the wastewater into two streams, a distillate and a concentrate. The distillate, representing around 80% of the inlet volume complied with the regulation in force and could be sent to municipal water treatment plant; the concentrate, representing around 20% of the initial volume, was disposed as before. The plant provided the enormous advantage of a volume and cost reduction of 95%, being the concentrate only 5% of the total capacity plant.
The leachate coming from Ano Liossia landfill is classified by the experts as an 'old' leachate (the landfill age is more than 5 years, indeed), being characterized by an alkaline pH, a low COD value a very low BOD/COD ratio (<0,1) and a high conductivity. The waste feeding the evaporators, is the leachate after the concentration by RO. This particular kind of wastewater, because of the high concentration of pollutants and the particular combination of metals, organics, and chlorides, reduces the effectiveness of traditional chemical-physical treatments and provided an excellent feed for the evaporation technique, especially when carried out under vacuum conditions.
The Vacuum Evaporation
Evaporation is a thermal treatment that allows separation of the water from the pollutants thanks to the different volatility of the compounds. It can be applied with little or no pre-treatment, and provides advantages over traditional water treatment techniques, such as full automation, very low chemicals consumption and high tolerance of variability to concentrations of pollutants. Typically, it provides complete separation of metals, ions, oils (more than 99% reduction) and a good separation of surfactants, COD and BOD (95% reduction). The separation of the latter is particularly effective at a low temperature. This condition is achievable by reducing the pressure inside the boiling chamber (vacuum evaporation). The distillate produced in this way is superior in quality to that produced evaporating at high temperature. In a typical vacuum evaporator, boiling takes place at around 35°C and 5 kPa (in the boiling chamber).
The Evaporation Equipment
The evaporator, heart of the waste water treatment system chosen by Ano Liossia municipality, is based on a series of 4 EW40000FF model, one of a standard range of skid mounted modular designs, produced by Led Italia. The unit is classified as a forced circulation, external shell and tube, vacuum evaporator and is able to produce up to 40.000 litres of distillate per day. This model is designed for low maintenance and continuous, unattended 24h, 7 day/week working. It is made of SAF 2507, a type of Super Duplex Stainless Steel, which resists the corrosive effects of chemical compounds concentrated by the continuous water evaporation. The hot water used by the evaporators is heated by the cogeneration group, that burns the biogas produced by the landfill, allowing big energy savings. The EW40000FF are equipped with modern PLC, digital recording system and data modem for remote controls.
The unit was designed around the principle of forced circulation that provides great advantages on treatment of industrial waste, as it reduces the fouling and scaling effects. This is achieved by continuously circulating the waste by means of a pump that keeps the liquid overheated in the heat exchanger. In this way, the evaporation takes place only in the boiling chamber (flash evaporation). This design provides benefits based on strong reduction of maintenance and a sensible increase on evaporation yield. Both benefits affect positively the return of investment.
The waste water is sucked via a filter into the boiling chamber by the vacuum. The inflow volume is controlled automatically by level sensors. The circulation circuit pumps the water through the primary heat exchanger where it is supplied with the heat necessary for boiling. Once heated, the water goes back to the boiling chamber, where it immediately boils (flash evaporation). The vapour rises through the demister to damp any droplets and is condensed against the U tube heat exchanger. From the top of the boiler, the distillate is sent to the storage tank where it is discharged automatically according to settings determined at the commissioning stage. The machine is powered by hot and cold water that provide evaporation and condensation respectively.
The yield of the installed equipments has shown a consistent performance: 80% distillate having low hardness and low conductivity, that can be sent to municipal water treatment plant; 20% concentrate to be disposed as before. The combination of evaporators and biogas cogeneration plant was ideal to provide the maximum energy efficiency, thus reducing the site demand for power and leading to further savings.
The application at Ano Liossia landfill has demonstrated the suitability of the evaporation process to treat landfill leachate. The Athens municipality has dramatically reduced the costs of waste disposal, has improved the quality of the environment and has been able to demonstrate to people and local authorities that they have a pro-active and responsible attitude to their environmental obligations.