Germany-based mine closure planning company GMB has developed the underwater nozzle pipelines (UNP) method, using natural water circulation to distribute lime and carbon dioxide (CO₂), as a more effective means of neutralising flooded opencast mines.

“The method is particularly suitable for expansive lakes with significant acid inflow, as large volumes can be treated in a short period,” comments Lausitzer und Mitteldeutsche Bergbau-Verwaltungsgesellschaft (LMBV) engineering division head Eckhard Scholz.

The LMBV is responsible for recultivation of the region’s lignite mining areas. Among other measures, it uses pilot and demonstration projects to develop new technical solutions for water treatment, from their technical inception to a scale suitable for actual remediation measures.

Having successfully demonstrated the UNP method in a pilot project at Lake Scheibe, in Lusatia, Germany, GMB is building another UNP on Lake Bernstein, as well as looking for additional areas of use for the solution. “As things stand, the system configuration and operation are specifically designed for Lake Scheibe. So we are currently reviewing what standardisation of the system components might look like”, explains GMB engineering office head Dr Michael Strzodka. Defining standard sizes for the floating pipes, pumps, the mixer and the nozzles would enable the method’s use to treat smaller ponds as well.

GMB points out that lignite can only be extracted from opencast mines if the ground water level is lowered to below the coal seam.

“Depending on the chemism from the overburden, the rising groundwater after mining in the pit lake may become acidic, owing to pyrite weathering. Thus, groundwater remediation projects seek to balance out the groundwater deficit and neutralise the lakes [of] former mines so that flora and fauna may be established,” says GMB.

Lime is usually used in this process, for example, using ships to introduce lime into the lake water with sprinkler systems. Large quantities of lime are needed to treat an entire water body, however, as there is only a low level of blending that takes place in the process, GMB developed an alternative. Lime mixed with lake water is pumped through the nozzles of floating pipes into the lake, where it uses the natural lake circulation to blend thoroughly, distributing the suspension evenly throughout the lake.

Germany’s Federal Mining Act stipulates that operators carry out rehabilitation measures for the surface areas of former mines. This also applies to the lakes they produce, highlights GMB.

“The lignite mining region of Lusatia alone has 36 pit lakes with an aggregate expanse of 146.8-million metres squared and a water volume of 2.287-billion metres cubed. Their pH values vary from 7.5 to an acidic 3, which precludes the emergence of a functioning ecosystem.”

This prompted GMB, a Senftenberg-based mining services specialists, to develop the UNP method using submerged floating pipes equipped with nozzles within a project group in 2010. Other partners in the group were Fels-Werke, Institut für Wasserwirtschaft, Ökologie und Siedlungsbau IWSÖ, the Kemmer/Harbauer Group and Linde Gas. They used the laboratories of the Brandenburg University of Technology Cottbus-Senftenberg, in Senftenberg.

“GMB applied to carry out both a pilot and a demonstration project – initial neutralisation and CO_{2 buffering,” states Scholz.}

The projects were carried out in Lake Scheibe, which holds about 110-million metres cubed of water. The lake had an acid concentration before primary neutralisation of 381-million moles.

A fixed plant was built on the lake shore for the first stage of the project, initial neutralisation took place between October 2011 and January 2012. Its purpose was to blend lake water with lime on site. Six pairs of nozzles were fitted at equal intervals of 20 m to a floating pipe located around 50 cm beneath the surface of the water. Afterwards, a 4% suspension consisting of 15 200 t of slaked lime and 440 000 m3 of lake water was pumped into the lake at a speed of about 7.6 m/s and a flow rate of 260 m3/h for six days a week over 16 weeks.

“The UNP method exploits the phases of lake circulation to ensure ideal blending with the lake water,” says Strzodka, adding that the lime is introduced to the epilimnion layer.

“But the water in the lake possesses different densities, which ensure that the lime is distributed throughout the lake during the phases of full circulation in spring or autumn. Distribution of the lime suspension is, therefore, entirely natural and does not require any external energy source. In this manner the pH value [of the lake] was increased from 2.9 to 7. The neutral range is between six and eight,” he notes.

Water circulation was used once more at Lake Scheibe in a secondary treatment from May to August 2014, which had become necessary as reacidification of the lake was caused by inflowing water with an acidic pH value. As a result, 3 000 t of lime in a 400 000 m3 suspension was introduced during this period, creating an alkali buffer of 0.18 mol/m3.

In a second stage, the UNP plant was developed further, in June 2015, which involved the introduction of CO₂ to the lake to induce hydrocarbonation. Here, the suspended CO₂ was fed into the hypolimnion using the lake bed, while the forced inflow of lime remained directly beneath the surface of the water. The process initially used the lake layering phase that occurs in summer and winter to add the two suspensions to the water in separate layers. In the subsequent full-circulation phase, an entirely natural blending, took place, thus allowing the lime to react with the CO₂.

“We would have needed a massive facility to produce hydrocarbonation outside the lake. Maintenance and operation alone would have been extremely costly. In contrast, our new method of using full circulation of the lake is practically free and very efficient. The lake is used as a fully- equipped reaction chamber,” explains Strzodka.

With the simultaneous treatment of the lake with lime and CO₂, a buffer is created and the water body requires far less additional treatment, leading to signifi- cantly lower costs. The treatment is complete as soon as the biologi- cal processes restore natural balance to the lake.

Scholz is satisfied with the planning and implementation: “The UNP system proved extremely effective during initial neutralisation of Lake Scheibe and is, therefore, outstanding value for money. Also, the project management team at GMB, the scientific advisers and the system engineers always managed to promptly find remedies to technical issues that arose during the operational phase.”