Mine drainage and landfill leachate treatment

This article presents an outline of the chemistry involved for aerobic systems and presents some recent case studies in the US where mine drainage is successfully being treated. Additional research has also been undertaken in the use of algal mats for the treatment of ammonia and other compounds in landfill leachate.

Mine drainage treatment

Successful treatment of mine drainage from coal mines usually requires the use of aerobic wetland processes. Drainage within deep abandoned mine workings is commonly oxygen deficient with reducing conditions often being prevalent.

As a consequence, the iron within the minewater is predominantly in the ferrous (iron II) state, although some ferric (iron III) iron will also be present. On exposure to the surface the ferrous form will oxidise to ferric form:

Fe2+ + 1/4O2 + H+ —› Fe3+ + 1/2H₂O

In addition, the pH of the minewater (due predominantly to the presence of sulphate from oxidised sulphide minerals) can be acidic. The rate of iron oxidation is controlled by the pH of the minewater. Below ground and at acid pH levels (pH 2-3) oxidation of the minewater is controlled by bacteria and is relatively slow -the oxidation taking a number of days. At more alkaline pH levels (>pH 5), and in the presence of air, the oxidation is not bacterially controlled and can take a matter of minutes.

Aerobic wetland systems are designed to encourage the oxidation process and are consequently relatively shallow (about 0.3m deep), vegetated and with surface flow predominating.

As ferrous iron is converted to ferric iron in the wetland, a hydrolysis reaction takes place which causes the precipitation of ferric hydroxide or oxyhydroxide:

Fe3+ + 3H₂O —› Fe(OH)3 + 3H+

or

Fe3+ + 2H₂O —› FeOOH + 3H+

Therefore, by encouraging oxidation processes, iron will be removed in the aerobic wetland by ferric hydroxide precipitation causing the build-up of the characteristic red ochre often observed at coal mine drainage sites.

A consequence of this reaction is the production of acidity (hydrogen ions), which lowers the pH of the minewater. This can reduce the oxidation rate (as outlined above) and cause distress to plants growing in the aerobic cell. Plants such as reeds (i.e. Typha latifolia and Phragmites australis) are encouraged to grow as they pass oxygen through their root system causing aeration of the substrate.

Blue Ridge landfill

The Blue Ridge Refuse Disposal Facility is operated by Waste Management International and is located in Irvine, Kentucky, US. The landfill is situated within a shale containing elevated pyrite, and as a consequence the underdrainage at the site contains elevated levels of iron and sulphate.

The underdrainage is directed into the local river system and has caused the deposition of iron hydroxide through sections of the system.

The US EPA served a pollution notice on the landfill operators requiring immediate action. Sampling revealed trends in the dissolution of iron from the underlying pyritic shales.

Following analysis, the iron concentrations were then used to calculate the load requiring treatment. Once the sizing had been established and treatability studies undertaken, construction could begin.

Construction had to be undertaken in such a way that the wetlands produced an aesthetically pleasing treatment system capable of treating winter storm flows. The system is currently removing iron to below discharge consent level and has received US EPA approval.

Landfill leachate treatment

Wetlands and algae have been shown to be effective in treating a variety of other parameters which can impact water quality. These include SS, BOD, nitrogen compounds, phosphate and faecal coliforms. One of the more toxic contaminants is ammonia (ionised and un-ionised) and the removal of this parameter has been shown to be successfully achieved in constructed wetland systems – although the kinetics of removal are slow in a surface flow system.

Algae, on the other hand, produce oxygen when they photosynthesise and a treatment system using certain types of algae has the potential to be more effective at removing ammonia than a surface flow wetland.

In an attempt to optimise the treatment of ammonia contaminated leachate, a treatability study using blue green algae is being investigated. This has culminated in the building and operation of a pilot microbial mat system (Biomat) at the Waste Management Outer Loop Facility in Louisville, Kentucky, US.

Microbial mats are natural systems dominated by Oscillatoria sp. – a blue green algae (cyanobacteria). Such systems can be developed relatively rapidly by enriching the water surface with silage (grass clippings). This, coupled with the incorporation of nitrifying bacteria and purple autotrophic bacteria, is used in the pilot system. Nutrients in the form of phosphate were supplied by the leachate, although trials were also undertaken to establish the effectiveness of adding rock phosphate.

A number of processes operate to remove ammonia from the leachate. These support the photosynthesis which produces oxygen during the photoperiod and, because it is entrapped in the algal structure, also during the night. These processes are enhanced by others, detailed below:

Chemotaxis. The ammonium ion is bound to the negative surface charge of the algae, where it is assimilated into the algal cell or transformed by nitrification processes.

Direct Assimilation. Cyanobacteria can assimilate the ammonium ion. This, coupled with chemotaxis, operates independently from photosynthesis reactions, providing a more efficient mechanism for rapid ammonia removal.

Nitrification. Mats produce a slimy matrix which supports nitrifying bacteria.

It becomes apparent, therefore, that the mats provide an integrated treatment method which can remove ammonia from landfill leachate. The attachment media used comprised coconut matting. The pH of the algal system produces free ammonia (the only type nitrifiers can use), although the levels of ammonia can be relatively high (3-5% at pH 7-8); however, these are also nitrified. Studies at the site have shown that the incorporation of limestone berms support nitrifiers which further enhances ammonia removal.

The two-year study has culminated in the scaling up of the pilot cells to enable treatment of all the leachate from the landfill.