## 2 Application of Membranes in Contamination Situations
### Groundwater Contamination
Among the factors influencing redox reactions, the following are important:oxygen content in the water feeding a given aquifer,
* distribution and reactivity of organic matter and other potential reducers in the groundwater reservoir,
* distribution of potential redox buffers in the groundwater reservoir, and
* intensity of groundwater exchange [77]. However, it should be remembered that the presence of contaminants in groundwater in the form of organic, inorganic, radioactive, or microbiological compounds is in most cases a consequence of human activity on the soil surface, the migration of pollutants in the process of infiltration, diffusion, together with rainwater, into groundwater. On the other hand, higher and undesirable concentrations of contamination may be the result of environmental conditions. High concentrations of heavy metals such as Fe and Mn are often the result of a geological basis. In such a situation, actions are taken outside the system to subject water purification processes for drinking or other purposes, e.g., for the brewing industry. Therefore, they do not require taking corrective actions at the place of contamination, but only adaptation to the recipient's requirements. Thus, membrane systems are subject to different requirements depending on the purpose of remediation. Many different factors must be taken into account when selecting a membrane in the event of contamination. A given solution often has to be dedicated to both the type of pollution, the characteristics of the system to be cleaned, environmental conditions, including the presence of microorganisms, and the purpose of the activities carried out. Remediation typically involves extracting the groundwater from the well, treating it at various stages of the separation and purification process, and putting the water back underground. Processing can include various steps, including but not limited to, solids filtration, ozonation, air stripping, adsorption, or pH control. Strict water quality standards must be met before water is reintroduced. The rehabilitation process sequence is site-specific and the design of the process must depend on the condition of the aquifers and local regulations. An example is a reclamation carried out by 3M factory [78], the task of which was not only to remove contamination from the water, but also to bring the concentration of dissolved oxygen (DO) in the purified water to the DO concentration of the water in the receiving aquifer, i.e., to a concentration <1-2 mg/L. Membrane degassing (MDG) was used, using 3M\({}^{\text{TM}}\) Liqui-Cell\({}^{\text{TM}}\) 4 \(\times\) 28 Membrane Contact (MC) membrane contacts and an ultrafiltration system consisting of 3M\({}^{\text{TM}}\) Liqui-Flux\({}^{\text{TM}}\) UF modules as pre-treatment for MCR, with the goal of ultrafiltration membranes were to remove solid particles and colloidal substances, as well as reduce organic carbon and biological pollutants [78]. Metals present in the environment, such as As, Cd, Cu, and Hg, are very dangerous to human health and environmental safety. However, their removal from groundwater is very difficult due to the presence of metals in various oxidation states and the formation of various compounds, depending on the physicochemical properties of the environment. Unfortunately, the development of industry, including plant protection products, has resulted in more and more of these pollutants in groundwater. Many membrane techniques are used to remove them, including nanofiltration, reverse osmosis, the effectiveness of which depends on, among others, the pH of the solution (Table 3). One of the increasingly used techniques for removing metals, including arsenic, is nanofiltration. However, it should be noted that performance, water permeability, and resistance to contamination may vary depending on the type of membrane. In the case of NF/RO membranes used to remove arsenic and depending on the pH of the environment, it may be from 5% to 99% [92; 93]. [PERSON] et al.