This chapter explains what self-purification ability of a water system is, what happens when we add pollution to the biological cycle in a river, what physical, chemical, and biological processes are important, and what kind of pollution a water system can clean by itself.
SELF-PURIFICATION ABILITY OF WATER SYSTEMS
When the water in the physical environment becomes polluted, it is purified after a certain period. We call this the self-purification ability of water. A long time ago, this process was summarized in the Slovenian proverb saying that, “the water is cleansed, when it runs over seven stones”. It seems to be very simple; however, it is actually a complex interweaving of different, interdependent natural processes that involve physical, chemical, and biological processes working simultaneously. Only when the amount of wastewater does not exceed the natural capacity of a river to clean itself does the self-purification result in a substantial decrease in pollutant concentration. However, due to the new technologies and materials we use nowadays, the pollutants are not just “natural” anymore, but they include different sorts of technologically synthesized substances that are new to the natural environment and can be harmful to the preservation of its quality and self-purification ability. Furthermore, the increase in human population and activities has led to an increased amount of wastewater, which far exceeds the self-purification capacities of natural systems.
There are several types of physical processes involved in the self-purification of a river, and the most important are:
Dilution – the mixing of discharged wastewater with the stream water. Dilution is more efficient if the watercourse is fast flowing and turbulent. Further, dilution increases with an increasing ratio of stream discharge and pollution discharge.
Sedimentation – the settling down of solid particles onto a river bottom and their removal from the watercourse. In still or slow-flowing waters, large particles will settle out quite quickly, while small particles will stay in a suspension for prolonged periods, eventually settling down. During floods or heavy runoff, increased turbulence may lead to the resuspension of solids, causing increased turbidity of the water.
Straining or filtering – mainly the interception of floating particles by the movement of water through gravel and sand in a river bottom where small bits of organic matter or inorganic material may be filtered. Filtering also occurs along a stream, where large bits of debris are filtered through vegetation and deposited, often to be remobilized during storms and high waters.
Aeration – the process of transferring gases, especially oxygen, from the air into the water. Dissolved oxygen in water is necessary for the respiration of water organisms and facilitates many chemical and microbiological processes in water responsible for the reduction of pollutant concentrations. Aeration is most intense at the contact of water with the atmosphere, in quickly flowing waters, and at rapids and waterfalls. The temperature, atmospheric pressure (altitude), and amount of dissolved substances in water all affect the rate of aeration.
Grinding and dissolution – other physical processes important in self-purification ability of rivers that help transform organic matter into smaller particles that are easily included in the biological cycle.
Adsorption is the adhesion of molecules of gas, liquid, or dissolved solids to a surface of suspended particles or fixed substratum such as stones, plant stems, etc. Adsorption of pollutants by organic matter leads to decreased toxicity.
The chemical processes in a watercourse, especially oxidation and reduction, significantly contribute to the decomposition of dead organic substances. For organic pollutants with groups that are easily hydrolyzed (chemically broken down due to reaction with water), hydrolysis is one of the most important ways for their transformation and disintegration. Photolysis is decomposition under the influence of light. In contrast to adsorption, absorption means the uptake or dissolution of atoms, molecules or ions, e.g. a gas is absorbed by liquids. Absorption or assimilation is also used for biological uptake. Many chemical processes are closely connected to physical processes; therefore, the term “physical-chemical processes” is often used.
Self-purification of water is based, above all, on biological processes. These are based on the physical-chemical processes that are occurring inside living organisms. In this process, decomposers act directly, while all other organisms in a river that form different life communities cooperate indirectly. Their composition depends upon many factors (amounts of water, the water flow velocity, temperature, the morphology of the river bed, etc.). To understand the biological processes and their role in the self-purification abilities of a river, one must understand the cycling of substances in a river ecosystem. Organisms that have the decisive role in procedures of self-purification in watercourses are those that decompose dead organic substances into basic elements and mineral compounds (mineralization) that plants and animals can use as food and build into their bodies (assimilation).
Most organisms that are decomposing in watercourses live on or in the river bottom. Therefore, it is important that the river bottom is heterogeneous. In most aquatic ecosystems, we can find a complex mixture of algae and mosses together with cyanobacteria, heterotrophic microbes, and detritus (the breakdown products of dying organisms) that is attached to all submerged surfaces, including bottom sediment, rocks, aquatic plants and submerged leaves and branches of other plants. This layer is known as a biofilm or periphyton, and it is an important food source for larger invertebrates such as snails and insects as well as fish.
Periphyton plays a vital role in aquatic ecosystems. With phytoplankton and macrophytic vegetation, it contributes to primary production, nutrient fixation, and the circulation of matter in the ecosystem. Periphyton is a good bioindicator because it reacts very quickly to changes in the hydrological regime and water quality. Algae in the periphyton can absorb excess nutrients, heavy metals, and various toxic substances from water, thus improving water quality.
Complete and incomplete self-purification
We distinguish between complete and incomplete self-purification. Both forms are usually present in the water environment.
Complete self-purification – aerobic decomposition:
Complete self-purification occurs only in an aerobic environment, meaning in the environment in which oxygen is present and where there is a sufficient quantity of light and nutrients. This leads to perfect decomposition, meaning to inorganic matter (i.e. mineralization). Mineralizing substances have no smell.
Aerobic decomposers in water need oxygen to survive and do their work. Lapping waves and babbling brooks help increase the level of dissolved oxygen that is crucial to so many creatures in river ecosystems. In natural waters, aerobic microorganisms decompose dead animal and plant organic matter to simple organic salts, which are used again as nutrients for the growth of plants and animals. During this process, oxygen from water is converted to carbon dioxide, which is used by vegetation under the presence of light. Carbon that is necessary for plant growth is fixed, and oxygen is released into the water. Thus, the cycle of self-purification requires a sufficient amount of dissolved oxygen at first. If there is not enough oxygen, many parts of the system suffer: the aerobic decomposers cannot digest dead matter, insects cannot develop and mature, fish kills may occur, etc. Eventually, the river changes, starting at the microbial level.
Incomplete self-purification – anaerobic decomposition:
If the aerobic group of organisms is absent because there is not enough dissolved oxygen, which is usually deep in the interior of sediments or in some heavily polluted waters, complete mineralization is not possible. In addition to CO2 and H2O, organic substances decompose to methane and some other highly reduced organic compounds (for example, hydrogen sulfide (H2S)) that have an unpleasant odour. In such anaerobic conditions, natural waters become an unfavourable environment for life, with unpleasant odours.
In nature, decomposition occurs at various rates, depending on temperature, chemical composition, and other factors. Easily decomposable substances can decompose within approximately two days, while heavily decomposable ones can decompose for a month or even longer; this can even take years in an anaerobic environment in sediments. Carbon’s organic substances decompose faster than nitrogen’s organic substances.
INCLUDING ORGANIC (WASTE) MATERIAL INTO BIOLOGICAL CYCLE
Some organisms (decomposers) use organic substances in wastewater as food for their growth and development. In this way, organic carbon is included in a biological cycle, pollution is radically reduced, and watercourses are purified. The processes are copied in biological water treatment plants.
A problem occurs if there is too much organic pollution, or if the substances are of artificial origin so that natural decomposers are unable to decompose them.
If there are too many nutrients (phosphates and nitrates) that are food for plants, the plants flourish. Phosphorus is the growth-limiting nutrient in our regions. Some algae grow so fast that by night, when they are not producing oxygen, but they are only using it for breathing, the oxygen runs out. This can cause the death of all life in the water. When this happens, decomposers have to decompose the remains of plants and animals, which means that the oxygen is needed again. If there is no oxygen, the dead organic material rots. We call this phenomenon eutrophication, which in our region is mainly triggered by phosphorus as a growth-limiting factor.
Furthermore, a greater problem occurs if substances of anthropogenic origin are in the wastewater (products of chemical, petrochemical or pharmaceutical industries), for which adequate decomposers are not found in nature. These substances are often poisonous for the environment and, moreover, the processes of mineralization and breakup in the environment to harmless substances needs a lot of time.
The self-purification capacity of a river has served as an inspiration for constructing wastewater treatment plants (WWTPs). Years ago, sewage was dumped into waterways, and its degradation was trusted to the natural processes, such as dilution and bacterial decomposition. Increased human population resulted in a greater volume of domestic and industrial wastewater, requiring that communities give nature a helping hand.
Wastewater treatment represents an important form of pollution control. Wastewater is collected in sewers and delivered to plants for treatment. In treatment plants, wastewater is cleaned so it can be discharged into streams or reused for certain purposes.
Wastewater treatment plants (WWTPs) copy natural processes and are constructed in various steps, each of them gradually increasing purification efficiency:
- 1st step: mechanical removal of solid waste particles.
- 2nd step: biological step, activated sludge (bioreactor) with bacteria, reducing mainly organic carbon.
- 3rd step: chemical step, phosphorus removal by >90% through (co-)precipitation with Iron Chloride or Iron Sulfate.
- 4th step: filtration or ultrafiltration to further eliminate phosphorus, efficiency >95%, up to 98%.
- 5th Recently, more sophisticated technologies (similar to those for drinking water treatment) are used to remove toxic and hazardous substances such as charcoal, UV-radiation and ozonation.
- 6th Newest research focuses on separating substances (e.g. urine from faeces) and recycling these sources for further use, e.g. urine is rich in phosphorus and can be turned into and used as fertilizer, thus substituting chemical fertilizers.
- Schwoerbel, J. (1994): Methoden der Hydrobiologie. Süsswasserbiologie. 4. neubearbeitete Auflage, UTB 979, Gustav Fischer, Stuttgart, 368 pp.
- Kompare. B. (2005). In: Vahtar, M., Zdešar, M., Kompare, B., Urbanc-Berčič, O.,: Kako se reka očisti? – Priročnik za učitelje 3. Book collection Vodni detektiv. ICRO Domžale, Domžale, Slovenia, p.40.
More information on self-purification processes can be found here: