Long-term solutions for water treatment

International experience with operating wetlands suggests that wetlands could reduce construction, monitoring and annual operating costs for water treatment by one order of magnitude, however, there are no reliable costs available on long-term maintenance. Even less hard data are available regarding handling and disposal of the loaded organic matter from the wetlands.

To decrease the long-term costs of water treatment, the feasibility of alternative technologies at the Wismut sites was evaluated. The potential uses of innovative cost efficient technologies were identified in:

— the replacement of conventional treatment plants which become inefficient due to the decrease of the contaminant load in the feed water, such as the mine waters;

— the treatment of low volume streams exhibiting an unchanged level of contamination over long periods of time such as in case of seepage from waste rock dumps and tailings ponds which would require a separate conventional treatment leading to high specific costs dominated by labour costs, as well as

— the in situ remediation of ground water contaminated by seepage from the waste rock dumps, tailings ponds or mine waters.

Following the success of alternative water treatment methods at numerous sites, it was decided to test the alternative methods for removal of radioactive and metallic contaminants from the discharges typical for the Wismut remediation programme.

The passive water treatment systems are presently in the stage of technology development and pilot scale field testing. The required developmental effort in this area was well represented by the large number of contributions at the WISMUT’97 Workshop on “Water Treatment and Residues Management - Conventional and Innovative Solutions” (1).

Using the international experience, Wismut in co-operation with external partners is testing the feasibility of the following passive-biological technologies for treatment of mine, ground and surface water:

— Constructed wetland,

— Reactive permeable walls,

— In-situ removal of contaminants by microorganisms.

The completed and ongoing lab scale and pilot scale tests are described below.

7.1. Pilot scale test of a passive/biological treatment of the Poehla-Tellerhäuser mine water in a constructed wetland

The Poehla site comprises the relatively small Poehla-Tellerhäuser mine which has a flooding capacity of 1 million m³. In the course of 1995, contaminated flood water reached the level of natural overflow to the surface.

On average, flood water is in the order of 17 m³/h.

Therefore, a conventional water treatment plant was commissioned in 1995 to separate U, Ra, As, Fe, and Mn from flood waters. This plant uses the same technology that is used for the treatment of flood waters from the Schlema-Alberoda mine (cf. Section 6.2).

Table 4 shows the development of relevant contaminant concentrations in the flood water from 1995 through 2000.

The requirement to treat the overflowing mine water (removal of radium and arsenic) is ongoing, and based on geochemical modelling, it is predicted to last approx. 20 years.

A closer look at the development of the water quality at the Poehla site indicates the need for a long-term optimisation strategy.

Table IV. Contaminant loading of the Poehla-Tellerhäuser mine water (main components, average values in the 2^nd half of 1995 and in the 1^st half of 2000) and the permitted discharge concentrations of the water treatment plant

Component Unit Concentration Discharge value 2^nd half of 1995 1^st half of 2000

U_nat mg/l 1.6 0.1 0.2

Ra-226 mBq/l 1 400 4 650 300

As mg/l 0.9 2.3 0.1

Fe mg/l 17 9.3 2.0

Mn mg/l 3.7 0.7 2.0

SO₄ mg/l 140 5 200

In Summer 1998, the first constructed wetland of Wismut was put into experimental operation, treating a partial stream of the Poehla-Tellerhäuser mine water overflow. The lay-out of the constructed wetland is in Figure 4.

The constructed wetland was placed into a former retention basin. The basin was subdivided by concrete walls into five compartments creating the environment for the various reactions and various chemical/physical and biological efficiencies required at each stage. Since early 2000, an aeration cascade is in operation at the front end of the system.

The water movement is achieved by an overall gradient across the system realised by the arrangement of overflows and base drainage between the compartments.

In the first compartment the oxidation of Fe(II) and the subsequent precipitation of iron hydroxide takes place followed by sedimentation of the precipitate. The iron precipitation is accompanied by adsorption of arsenic and radium.

Then the mine water passes through two compartments with gravel filters having different grading. The material serves both as filter and provides a surface for establishment of micro-organism populations. To promote the growth of autochthonous micro-micro-organisms, nutrients were built into the compartments. The biomass can act either as a sorption agent for radium and uranium or as a catalyst for initiation of precipitation. Depending on the environmental conditions in the compartments, whether aerobic or anaerobic, other chemical reactions of contaminant separation become effective.

The last process step of the constructed wetland system is a finishing compartment. The compartment is filled with compost like matter and gravel on which helophytes are planted.

The prime aim is to raise the oxygen content in the compartment. In addition, the plants and the microorganisms in the root zone of the helophytes remove the remaining contaminants.

The present throughput through the constructed wetland is 3.5 m³/h.

Wasserbehandlung an WISMUT SanierungsstandortenWasserbehandlung an WISMUT Sanierungsstandorten

WIS M UT 1 4

3

G1 G2 partition walls drainage

sampling tube

Gravel basin with marsh plants anaerobic/aerobic

Gravel basin aerobic/anaerobic overflows

2

water channel flow direction

Sedimentation basin

Cascade

5

Gravel basin with marsh plants for oxigenation

Intake structure

Sludge tra p