The landfill leachate treatment plant has the purpose of purifying such liquid for final discharge in surface waters up to concentration limits defined by the environmental regulation 3 of D. Lgs. 152/2008 . The basic technology used is that of cross-flow membrane filtration, which may also be supported by a biological treatment.
Cross-flow filtration membranes, discovered in 1959 and initially used in the field of desalination, have now reached a degree of sophistication suitable to multiple uses: treating waste water, process fluids, intermediates, etc.
The degree of filtering will depend on the combination between the size of the pores of the membrane and chemical characteristics of the fluid to be treated. Membrane filtration can be divided into four categories depending on the size of separated substances:
- REVERSE OSMOSIS (RO) OR HYPERFILTRATION
- NANOFILTRATION (NF)
- ULTRAFILTRATION (UF)
- MICROFILTRATION (MF)
The differences between the different types of membranes are related to pore sizes and to the characteristics of the material; in reverse osmosis membrane pores are the smallest.
In any case, landfill leachate is separated into two fractions: one which crosses the membrane (permeate) and one that is retained (concentrate).The latter is purged continuously to ensure a "self-cleaning" effect on the surface of the membrane.
Purpose and short description of the plant:
The leachate treatment plant is built according following the multiple barrier treatment principle, in order to ensure greater safety in the achievement of the quality objective for purified water. It is composed of four main sections:
1. Storage and equalization tank
This tank has the function of ensuring adequate storage capacity to cope with temporary stops (e.g. ordinary and extraordinary maintenance) and to ensure the equalization of waste water. This makes concentrations of pollutants more consistent over time, in the following sections.
2. Aeration section and biological oxidation – optional in case of higher BOD/COD ratio.
The leachate is prefiltered on a coarse filter and fed to a tank for biological oxidation; air is injected by means of a system based on venturi injector and mixer nozzles. This allows a partial stabilization of wastewater and the partial transformation of the ammonia to nitrate, which is more easily rejected by the reverse osmosis membranes that follow. Also, part of more easily biodegradable organic pollutants is degraded to non polluting compounds (CO2 and water). Excess biomass can be recirculated to the landfill where natural stabilisation processes (predominantly anaerobic digestion) are completed. This section of the treatment can be by-passed in the case of solutions containing a negligible amount of biodegradable substances. In such case the tank has the sole role of hold-up and homogenization.
3. Ultrafiltration (UF) section
In this section, a very tight filtration is performed (membrane pores have a diameter in the order of 0.01 micron), thus guaranteeing a good clarification of wastewater, which is necessary for effective functioning of the reverse osmosis stage that follows. Ultrafiltration also constitutes a total barrier against large organic molecules, colloids and other macromolecules, which remain in the oxidation tank and are purged with excess sludge. Ultrafiltration is performed with tubular membranes (polymeric or ceramic) in a cross flow filtration regime.
4. Reverse osmosis (RO) concentration on polymeric spiral wound membranes.
This reverse osmosis section will be designed on two stages and two passes to ensure the highest possible concentration ratio and at the same time achieve a high quality permeate (for discharge in surface waters in accordance with Italian table 3 of Legislative Decree No. 152/2008). The filtrate from the first RO passage is refiltered on membranes with a very high rejection capacity. The reduced amount of concentrate to be returned to the landfill, in accordance with regulations (see e.g. Italian Decree of 13 January 2003, implementation of Directive 1999/31/EC on the landfill of waste), optimizes the recovery process. To be pinpointed is how the current legislation encourages on-site treatment of leachate and stresses that the resulting concentrates, similar to the waste already present in the landfill, must be destined mainly to recycle in the landfill itself, with obvious advantages:
- Accelerated stabilization of the landfill
- Production of biogas that is richer in methane, which is especially useful when such biogas is recovered for energy purposes
- Retainment of pollutants within the landfill
We can also say, according to several field experiences, that the recycling of concentrated leachate into the landfill is sustainable in the long term, because we are recycling components that come from the same landfill and we are not adding any extra pollutants. Many of the salts will be deposited in the landfill as low water-soluble salts (e.g. calcium sulfate) or will be incorporated into the life cycle of the landfill (e.g. alkalinity, nitrate salts, etc.).
The quantity of leachate treated daily must be determined through the evaluation of historical data series, possibly related to different seasons and different periods of rainfall. This will allow to estabilish treatment capacity that is in line with the maintenance of the correct level of leachate in landfill. Too low level would result in reduced pollutants digestions and biogas production. Too high level would result into inconsistent biogas production and potentially environmental pollution.
Regarding the characteristics of raw leachate, reference must also be made to recent data and historical data, for which detailed analysis is available.
In any case, the quali-quantitative variability of leachate is limited by installing upstream of the plant a suitable volume homogenization tank, with a capacity of around 20 days. It allows to establish an adequate storage and to deal with unforeseen situations (e.g. rainfall, extraordinary treatment plant maintenance, etc.).
Treatment on multiple sections optimizes each step in the removal of specific contaminants, at the same time making each step easier to manage. The final treatment with reverse osmosis membranes, which are the tightest and therefore most delicate components, gives warranty on discharged water quality.
We once again stress how the multiple barrier approach optimizes removal in each section of the different types of contaminants:
- aeration and preatreament phase eliminates the more easily biodegradable compounds and some ammonia;
- ultrafiltration membranes will retain colloids and macromolecules;
- reverse osmosis membranes will retain dissolved salts, metals and organic compounds with low molecular weight.
Integrity of reverse osmosis membranes, which represent the most delicate part of the system, is controlled continuously by means of conductivity measurments at various points of the plant. A sudden increase in conductivity signals a problem generally linked to mechanical failure.
The technical plants are pre-assembled on skid and installed within 20' containers, in order to facilitate the installation and ensure protection from unauthorized access. The container and UF module are placed on a cement platform near the storage and oxidation tanks.
To give an idea, the typical characteristics of Landfill Leachate as it is treated with these systems:
- high BOD5 values (> 15,000 mg / l);
- high BOD5 / COD ratios (> 0.7);
- pH in acid range (5-6);
- NH3 (500-1000 mg / l) resulting from the hydrolysis and fermentation of protein compounds;
- unpleasant smell.
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