The goal is to explain what pollution is, what kind of pollution a river can clean itself or not and to what degree, what the main polluters are, and what happens if there is too much pollution.
POLLUTION OF SURFACE WATERS WITH WASTEWATER
People use water for drinking and food preparation, for cleaning dishes and washing laundry, for personal hygiene, and flushing toilets. One large water consumer is industry, which uses water for cooling and as a part of other technological processes, as well as a raw material. It is also widely used in agriculture for irrigation and watering crops as well as for dispersing different substances. After the water is used, it becomes wastewater; most of it is treated in wastewater treatment plants and ends up in watercourses or lakes.
In the Danube region, the individual consumption of water ranges between 100 and 120 liters per capita per day in most countries, in line with the EU consumption rates (IAWD, 2015). This includes water used for drinking, cooking, personal hygiene, cleaning, and watering gardens – all the water used in households. If we add to this quantity of water all the drinking water from water supply networks used in industry and agriculture, the average use of water amounts to 300 l/day.
| Drinking | 2–3 l/day |
| Food preparation | 2–10 l/day |
| Washing dishes | 2–10 l/day |
| Minimal personal hygiene | 2–5 l/day |
| Short showering | 20–30 l/day |
| Bathing in a bathtub | 50–150 l/day |
| Washing laundry | 5–20 l/day |
| Toilet (8 l/per flush) | 25–50 l/day |
| TOTAL approximately | 60–250 l/day |
| + cleaning of households, washing cars, watering lawns and gardens, pools, etc. | |
The average consumption of water per inhabitant is related to its availability, i.e. dependent on the climate zone (arid, humid, rainfall, surface waters, etc). It also depends upon the economy in a specific area or state. The average consumption per person in the poorest areas of the world is six liters of water a day, which is barely enough for survival and certainly insufficient for minimal hygiene.
In the richest parts of the world, the average consumption can be up to 500 liters of water a day, which in addition to flushing toilets includes showering or bathing, watering lawns and gardens, changing the water in pools, running fountains, etc.
Point and diffuse pollution
We distinguish between point and diffuse pollution. Outflows of waste substances into sewage systems or spilled contents of tanks and discharges from dumps are examples of point pollution. Diffuse pollution is caused by fertilizing and intensive use of chemical substances in agriculture (pesticides, herbicides), polluted precipitation, pollution from roads and pollution caused by erosion with precipitation. Diffuse pollution is more problematic, because it is harder to observe and control; therefore, it is harder to take appropriate measures to reduce it.
Nature produces no waste nor pollution. The principle of nature is the recycling of materials, e.g. by the self-purification in rivers. Thereby, production and decomposition of materials are in an equilibrium. In contrast, humans have increased their production in such a way that ecosystems cannot handle the quantity, or the products are harmful (i.e. toxic or non degradable).
Organic / Chemical pollution
In wastewater, there are different substances that can be divided into three classes according to their decomposition rate: fast decomposing, slow decomposing, and non-decomposable. Considering their origin, they can be divided into natural and artificial (anthropogenic or man-made).
Natural substances in wastewater are mostly organic, originating from sewage (households, toilets) and animal farms. They are rapidly biodegradable, because decomposing organisms use them as food, recycling the organic substances into mineral forms of nutrients that can be absorbed by plants and used for their growth. The aquatic environment is adapted to such pollution and can handle it if the amount is not too high.
Organic substances contain mainly carbon, hydrogen, and oxygen. In addition to these elements, nitrogen, phosphorus, and sulfur occur in smaller amounts. Other elements are found only in trace amounts. The prevailing organic substances in sewage are proteins (40–60%), carbohydrates (25–50%), and fats (10%). Microorganisms use oxygen for biodegrading the organic matter. If oxygen runs out, processes of oxic decomposition change into anoxic decomposition.
Artificial substances in wastewater originate mainly from washing machines and industry. The composition and amounts of artificial substances in wastewater depends on the technological processes. Artificial substances are divided into biodegradable and biologically undegradable. Industrial wastewater can include hazardous substances; therefore, factories must clean up these substances before discharging their residual water into natural waters or piping them to a wastewater treatment plant. Further, hazardous substances are released into surface waters, such as heavy metals, PCBs, PAKs, hormone active substances, medicals, macro- and microplastic, etc. This type of pollution occurred only in recent centuries (after the Industrial Revolution), and therefore the aquatic environment is not yet adapted to it.
Main polluters: agriculture, industry, traffic, households
Pollution of industrial wastewater is expressed as the population equivalent (PE, one unit is equivalent to one inhabitant). Approximately 10,000 liters of water are needed, for example, to process 1 ton of sugar, which means the pollution of approximately 400 PE (bearing in mind an average consumption of 250 l of water per day per person), the production of 1 ton of cellulose equals the pollution from 4000 to 6000 PE; the production of 1 hectoliter of beer up to 2000 PE, etc.
Agriculture: Water is used for irrigation in agriculture, and only a small share of it returns to a watercourse. It is often loaded with pesticides, fertilizers (among which phosphates and nitrates are the most important chemicals), and with a low content of oxygen. High concentrations of nitrogen and phosphorus, originating from the excessive use of fertilizers and residual water from farms, accelerate the growth of algae in aquatic ecosystems, leading to eutrophication and the deterioration of water quality (i.e. loss of oxygen). Phosphorus in European surface waters is the growth limiting nutrient, and therefore special attention is given to remove phosphorus in waste water treatment plants.
Industry: Numerous chemical substances (pharmaceuticals, cosmetics, detergents, etc.) enter in different manners to natural waters.
Settlements: Many villages lack proper sewage systems to collect the households and direct them to wastewater treatment plants, generating diffuse pollution with fecal waste and other chemical substances in daily use.
Transportation: Pollutants (i.e. oil products) generated by car traffic, ships, aerial transport are washed away by precipitation from air and roads, ending up in natural watercourses.
References:
- [IAWD] International Association of Water Supply Companies in the Danube River Catchment Area, 2015: Water and Wastewater Services in the Danube Region. A state of the sector, Report, 132 pp.
- Kompare, B. In: Vahtar M., Zdešar M., Kompare B., Urbanc-Berčič O., 2005: Kako se reka očisti? – Priročnik za učitelje 3. Book collection Vodni detektiv. ICRO Domžale, Domžale, Slovenia. p. 18.