Purification of naturally polluted water from natural reservoirs, such as rivers, lakes, swamps, and puddles, in the absence of centralized water supply systems is a very urgent task. For these purposes, there are many large stationary or portable systems for water purification in large volumes [1
]. Such water purification systems are based on the use of various purification methods, are manufactured in series, and are capable of ensuring low cost of treated water; however, they are, overall, still relatively expensive [2
]. When it is necessary to clean small volumes of water in the field—a few liters or tens of liters—and the filter should be compact, easy to use, efficient, and cheap, problems can arise.
It should be noted that there are many compact devices for water purification in the field [3
]. Such devices that are designed for repeated use have some disadvantages. As a rule, antimicrobial cleaning with such filters is solved by pumping contaminated water through a 0.2 micron-hole nano-membrane using a portable pump (included in the filter equipment). The most compact and cheapest devices, such as a tube-straw, allow a person to drink decontaminated water from any puddle by sucking the water through the tube. The main types of compact water purifiers are (1) pump filters and purifiers, (2) gravity filters, (3) ultraviolet light purifiers, (4) bottle filters, (5) squeeze filters, and (6) straw-style filters. An analysis of the market has shown that such straw tubes cost at least 20 US dollars after a discount [4
]. But to pay $
20 for 10 L of drinking water even in the field is expensive. Moreover, such straw tubes provide only antimicrobial protection since they contain only a nano-membrane. In many cases, natural water must also be cleaned from odorous and dissolved toxic organic impurities. The problem of purification from the ions of heavy metals and radionuclides is also not solved. There are also reagent methods of water treatment, for example, coagulants and flocculants or disinfecting by chlorine-containing tablets [4
]. Such treatment of water with reagents is cheaper, but it is more complicated than simple filtration and does not always provide the necessary quality of water purification.
It is obvious that in many cases—for example, for tourists traveling in nature, military expeditions and hikes, and people working far from civilization—simple, cheap, efficient, and compact filters for high-quality cleaning of small quantities of polluted natural waters are need. Such filters may be intended for single use. Naturally polluted water may contain different types of impurities. Biological pollution includes bacteria, viruses, spores, pollen, parasites, etc. There may be relatively large mechanical particles in the water—clay, mineral particles, soil, and the remains of living organisms. Various organic substances and undesirable ionic impurities, for example, heavy metal ions and radionuclides, can be present in dissolved form in dirty water. The task of creating filters for complex purification of polluted natural waters can be solved by creating a filter containing several specialized membrane-elements, which are assembled in a bag, fastened, and sealed around the perimeter of the bag. Each membrane purifies contaminated water from a specific type of contamination. Four filled membranes are proposed, which will comprehensively purify water successively, one by one. Three of the four membranes are made with pulp and paper technology where the active sorbent is among the cellulose fibers as filler or fixed on the surface of the cellulose fibers. The first membrane in the course of the water cleans it from relatively large mechanical particles. This is the so-called deep membrane because the deposition of particles occurs throughout the entire volume of the membrane. The second membrane contains a sorbent that binds microbiological contaminations. The third membrane contains activated carbon and absorbs dissolved organic matter. The fourth membrane contains an ion exchanger, which absorbs unwanted ionic impurities.