Landfilling of waste leads to the production of leachate which usually has a high BOD and COD together with nitrogen, inorganics and organic halides. Many factors determine the quantity and composition of leachate – waste types, disposal methods, construction and age of the landfill, and climatic/seasonal effects being amongst the most important.
Leachate is now required to be treated prior to discharge into surface waters. The most suitable treatment method varies with leachate composition and required discharge standards. PCI Membraness, part of ITT Industries Inc., offers state-of-the-art membrane technology for leachate treatment, which is often used as part of the overall treatment system to meet the requirements of individual landfill sites.
The first stage in the treatment of many leachates is biological pre-treatment which breaks down organic compounds, and removes nitrogen and some inorganics through flocculation. For dilute or methanogenic phase leachate, biological pre-treatment may not be necessary. PCI-Membranes ultrafiltration (UF) plant, as part of a membrane bioreactor, can be used as part of the activated sludge process. UF separates the pre-treated leachate from the activated sludge, the latter being recycled. UF replaces the drum filter, biological contactor and settlement tank in the traditional biological pre-treatment process, and has a number of advantages:
Reverse osmosis (RO) is a filtration process which dewaters aqueous solutions such as landfill leachate by the application of an elevated pressure which causes water to diffuse through the polymeric membrane. The membrane is impervious to large molecules (the contaminant compounds), but allows very small molecules (primarily water) to pass through. RO is a non-thermal process and since no phase change occurs, it is energy-efficient.
Liquid flow within the system is tangential to the filtration surface thus inhibiting the formation of deposits, ensuring the maintenance of a high processing capacity. Residual inorganic and non-degradable organic compounds are removed by RO. A high ammonia rejection rate is ensured through acidification of the RO feed. Typically, 75~80% of the RO feed is recovered as water fit for discharge to surface waters. The RO concentrated leachate can be recycled to the landfill or, further treated by evaporation and drying.
Typically, 75~80% of the RO feed is recovered as water fit for discharge to surface waters. The RO concentrated leachate can be recycled to the landfill or, further treated by evaporation and drying. The condensate from evaporation and drying stages can also be treated in the RO process. Depending upon the composition of the leachate and discharge standards, the RO process may comprise 1, 2 or 3 purification stages in order to generate permeate fit for discharge to surface waters. The first stage uses open tubular membranes which can handle large quantities of suspended material without blockage or damage; later stages use spiral-wound membranes which economically treat the solids-free leachate (permeate). Thin-film com- posite membranes are used throughout with the benefits of high pollutant re retention, long membrane lifetime, and ease of cleaning. The first stage usually uses RO membranes but in the treatment of some leachates with high salt levels, nanofiltration (NF) membranes may be used. NF membranes allow salt to pass into the permeate. Since salt content limits the achievable concentration factor, NF allows a higher concentration factor to be realised in the first stage. However, the NF permeate may need more post-treatment than RO permeate.
The energy requirements of a leachate treatment process comprising biological pre-treatment, reverse osmosis and evaporation, can usually be adequately satisfied by the energy recoverable from landfill gas.
Typical treatment of landfill leachate (shows the use of PCI’s membrane systems)
|Raw Leachate||Pretreated Leachate||Permeate RO||Permeate RO2|