Trace substances and gases

Trace substances are chemical compounds that can be detected in water at very low concentrations (in the micro- or nanogram range). Even though they occur only in trace amounts, some of these substances can have adverse effects on the environment and human health.

Since contaminated water resources cannot always be replaced by alternative, uncontaminated sources, appropriate strategies for reducing or removing trace substances must be considered today.

Proven and economically viable water treatment processes are already available for many of these challenges. Depending on the type and concentration of the contamination, the following methods, for example,

- adsorption processes
- degassing processes
- oxidation processes
- ion exchange
- nanofiltration
- reverse osmosis

are successfully in use. The removal of trace substances usually requires several process steps and depends on the water matrix as well as the intended use of the water — whether as process water or drinking water.

The radionuclides uranium and radon can be detected in low concentrations in natural drinking water in areas with elevated radioactivity. Radon is a radioactive noble gas that is invisible and has no smell or taste. It is produced during the radioactive decay of uranium.

Through contact with uranium-bearing rock layers, radon (reference dose 100 Bq/l) can dissolve into groundwater and enter water supply systems in this way. As a noble gas, radon does not form chemical compounds with other substances and can therefore be removed from the water relatively easily through aeration or degassing.

Intensive contact between water and air causes radon to be expelled and subsequently removed with the exhaust air. Adequate aeration of the systems is particularly important in this regard. Outdoors, the released radon dilutes rapidly due to constant air movement, so that concentrations in the ambient air generally remain very low.

While the noble gas radon easily outgases, uranium (limit value 0.010 mg/l) must be removed from the water by means of selective ion exchange.

When heavy metals are present in water, they often originate from geological formations (e.g., arsenic-bearing rocks), and less commonly from industrial discharges, contaminated sites, or mining activities.

The most important heavy metals in drinking water includes:

- Lead (Pb)
- Chromium (Cr)
- Nickel (Ni)
- Arsenic (As) (strictly speaking, a semimetal)

Even low concentrations can pose health risks. Therefore, strict legal limits apply to drinking water (e.g., the Drinking Water Ordinance). The following treatment methods are considered particularly suitable:

Precipitation and filtration
→ Conversion of dissolved metals into insoluble compounds and subsequent removal by filtration (high volume of heavy metal-containing sludge)

Adsorption on granulated iron hydroxide
→ Particularly effective for removing arsenic down to the detection limit

Adsorption on manganese dioxide
→ particularly effective for removing manganese and lead down to the limit of detection

Nanofiltration
→ partially removes dissolved metal ions, but also other substances

Reverse osmosis
→ very effective removal of nearly all dissolved heavy metals and substances

Pharmaceutical residues, pesticides, and other organic micropollutants are often difficult to degrade biologically, chemically stable, highly mobile in the water cycle, and detectable even at very low concentrations. 
Activated carbon is a key method for removing trace substances. Removal occurs through adsorption onto the large internal surface area of the carbon. The following are used:

Powdered activated carbon (PAC)
Granular activated carbon (GAC)

A disadvantage is that the activated carbon must be regularly replaced or regenerated.

Through oxidation with ozone (ozonation), numerous organic trace substances are partially degraded. This alters the molecular structure in such a way that biological degradation in a subsequent filtration stage is enabled or improved. An additional hygienic effect (disinfection) is also beneficial.

The combination of ozonation and biologically active activated carbon filtration (ozone biofiltration) is now considered a particularly effective process. In this process:

- organic residues are adsorbed,
- biodegradable oxidation products are further degraded (mineralized),
- and water quality is sustainably improved.

Advanced oxidation (AOP = Advanced Oxidation Process) is a combined process using ozone and hydrogen peroxide or ozone and UV. This combination produces highly reactive OH radicals, which can break down even substances that are difficult to oxidize.