Municipal wastewater is typically generated from domestic and industrial sources and may also include urban run-off. Domestic wastewater is generated from residential and commercial areas, including institutional and recreational facilities. In the rural setting, industrial effluents and storm water collection systems are less common. Industrial wastewater commonly originates in designated development zones or, as in many developing countries, from numerous small-scale industries within residential areas.
Within the household, tap water is used for a variety of purposes, such as washing, bathing, cooking and the transport/flushing of wastes. Wastewater from the toilet is termed "black" and the wastewater from the kitchen and bathroom is termed "grey". They can be disposed of separately or they can be combined. Generally, the wealthier a community, the more waste is disposed by water-flushing off-site.
Domestic wastewater generation is commonly expressed in litres per capita per day or as a percentage of the specific water consumption rate. Domestic water consumption, and hence wastewater production, typically depends on water supply service level, climate and water availability. In moderate climates and in industrialising countries, 75 per cent of consumed tap water typically ends up as sewage. In more arid regions this proportion may be less than 50 per cent due to high evaporation and seepage losses and typical domestic water-use practices.
Industrial water demand and wastewater production are sector-specific. Industries may require large volumes of water for cooling (power plants, steel mills, distillation industries), processing (breweries, pulp and paper mills), cleaning (textile mills, abattoirs), transporting products (beet and sugar mills) and flushing wastes. Depending on the industrial process, the concentration and composition of the waste flows can vary significantly. In particular, industrial wastewater may have a wide variety of micro contaminants, which add to the complexity of wastewater treatment (WHO, 1997).
The typical composition of untreated domestic wastewater is presented in Table 1.
Industrial wastewater differs from domestic wastewater as it usually contains relatively high levels of toxic elements and compounds. The composition and flowrates of industrial waste streams vary significantly and depend on the nature and the size of the industry (Table 2). Rigorous pre-treatment is required in water reclamation facilities that receive industrial wastewater to ensure the reliability of the biological treatment processes by excluding potentially toxic compounds. Treatment of industrial wastewater needs to be customized for each industrial sector (WHO, 1997; US EPA 2004).
Table 2: Typical combination of pollutants in waste streams of different industrial sectors (Source: Shi, 2009)
Iron and steel
BOD, COD, oils, metals, acids, phenols and cyanide
Textiles and leather
BOD, solids, sulphate and chromium
Pulp and paper
BOD, COD, solids, chlorinated organic compounds
Petrochemicals and refineries
BOD, COD, mineral oils, phenols and chromium
COD, organic chemicals, heavy metals, SS and cyanide
Fluorine and SS
COD and organic chemicals
SS, metals, acids and salts
Stormwater is rainwater that has fallen on impervious surfaces (such as roofs, sidewalks, driveways, parking lots, streets) and is allowed to run freely, picking up pollutants and toxins, until it falls into a stormwater drain system (https://whollyh2o.org/rainwater-stormwater/item/49-rainwater-and-stormwater-two-sides-of-the-same-story?.html).
In separate sewer systems, stormwater and wastewater are discharged in two separate systems. Collected municipal and industrial wastewater is treated in wastewater treatment plants, and after treatment it is either discharged to receiving water bodies or reused for beneficial purposes. Collected stormwater is discharged directly into receiving water bodies, possibly treated in a stormwater treatment tanks. For hydraulic relief stormwater tanks it can be integrated in the stormwater sewer.
The composition of stormwater is considerably different; it depends on the contamination of precipitation, the surface structure and its contamination, the intensity and storm duration, the precipitation water amount etc. Stormwater in private areas or roofs is usually of low contamination, while stormwater of industrial areas, streets and highways is usually of high contamination. The average figures of the outflow composition of precipitation water detected in Germany by Boller and Höflinger (1996) are presented in Table 3 (Rozkošný et al., 2014).
Table 3: Concentration of heavy metals in stormwater, roof and road runoff (Source: Rozkošný et al., 2014)
Note: 1) roof covering is formed by copper sheet 2) roof covering is formed by galvanized or more precisely zinc sheet
Requirements for precipitation drainage in the separate sewer systems in Germany [NRW, 2004 Circular of the Ministry of the Environment and Nature Conservation, Agriculture and Consumer Protection North Rhine-Westphalia, - IV-9 031 001 2104 - 26.05.2004] describe the expected pollution of the precipitation water depending on the area of origin. Based on the expected level of pollution, stormwater is classified into three categories
Rainwater refers only to the rain that falls on the roof, which can be harvested into a storage tank prior to contact with the ground (see for example Figure 1). Rainwater quality is much higher that storm water, since groundwater generally contains various contaminants including soil, organic matter, fertilisers from gardens, oil residues from driveways etc (http://www.nationalpolyindustries.com.au/knowledge-base/what-is-the-difference-between-stormwater-and-rainwater/).
Rainwater is relatively free from impurities except those picked up by rain from the atmosphere, but the quality of rainwater may deteriorate during harvesting, storage and household use. Wind-blown dirt, leaves, faecal droppings from birds and animals, insects and contaminated litter on the catchment areas can be sources of contamination of rainwater, leading to health risks from the consumption of contaminated water from storage tanks.
Microbial contamination of collected rainwater indicated by E. coli (or, alternatively, thermotolerants coliforms) is quite common, particularly in samples collected shortly after rainfall. Pathogens such as Cryptosporidium, Giardia, Campylobacter, Vibrio, Salmonella, Shigella and Pseudomonas have also been detected in rainwater. However, the occurrence of pathogens is generally lower in rainwater than in unprotected surface water, and the presence of non-bacterial pathogens, in particular, can be minimized. Higher microbial concentrations are generally found in the first flush of rainwater, and the level of contamination reduces as the rain continues. A significant reduction of microbial contamination can be found in rainy seasons when catchments are frequently washed with fresh rainwater.
Rainwater is slightly acidic and very low in dissolved minerals; as such, it is relatively aggressive. Rainwater can dissolve heavy metals and other impurities from materials of the catchment and storage tank. In most cases, chemical concentrations in rainwater are within acceptable limits; however, elevated levels of zinc and lead have sometimes been reported. This could be from leaching from metallic roofs and storage tanks or from atmospheric pollution. Rainwater lacks minerals (http://www.who.int/water_sanitation_health/gdwqrevision/rainwater.pdf).
Precipitation runoff from roads (Figure 2) can cause pollution on surface water, groundwater and soil, due to its hydrological and hydraulic characteristics, as well as its chemical and physical contents. Measures like infiltration, retention and treatment can reduce the hydraulic and pollutant loads of road runoff.
Heavily polluted precipitation runoff from road drainage can derive from:
Combined sewer systems are designed to collect storm water runoff, municipal and industrial wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to water bodies (Figure 3).
During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason combined storm water tanks are built at suitable sites to store the excess wastewater. However, the storage capacity of the storm water tanks is also limited, and in the event of particularly heavy rainfall, they are designed to overflow and discharge excess wastewater directly to nearby streams, rivers, or other water bodies (see Figure 4). These overflows, called combined sewer overflows (CSOs), contain not only storm water but also untreated human and industrial waste, toxic materials, and debris (US EPA).
CSOs can cause considerable water pollution. The ammonium nitrogen content in the receiving water bodies can increase (Figure 5). This may lead to fish die-off, as ammonium can be partly present in the aqueous phase as ammonia, which is highly toxic to fish. In addition, the oxygen content of water can be significantly reduced. Simultaneously, organic and mineral solids can be discharged into water bodies via CSOs leading to formation of digested sludge in the receiving bodies, oxygen consumption and clogging of the sand-gap systems (habitat for aquatic organisms).
Natural or man-made water bodies that appears on the surface, such as a streams and rivers, lakes, ponds, or reservoirs. Surface water, unlike groundwater is generally of bad quality and need treatment before human consumption because it accumulates pathogens, metals, nutrients and chemicals as it flows across contaminated surfaces.
River water is an important surface water resource for households, agriculture (e.g. irrigation, animal husbandry) and industry (e.g. processing water and energy production). Moreover, rivers also provide many ecosystem services (e.g. tourism) (Figure 6). River water quantity and quality strongly depend on its runoff system, the seasonal changes and the general soil and vegetation it flows through on its way to the sea. There are several methods to extract river water for use; some of them need skilled labour. To protect rivers, wastewater has to be treated before it is discharged into it.
Lakes are surface water sources, with water levels changing depending on the seasons. Lakes usually play an important role in the supply of water for the regional population (household use), industry and agriculture. Lake management has to consider that the amount of water extracted is not higher than water entering the lake; pollution and eutrophication; as well as sedimentation to guarantee an ecological balance and a constant quality of water for use. There are natural lakes and man-made lakes, also called reservoirs.
Man-made reservoirs, sometimes called artificial lakes, are important water sources in many countries around the world. In contrast to natural processes of lake formation, reservoirs are artificial, usually formed by constructing a dam across a river or by diverting a part of the river flow and storing the water in a reservoir. Upon completion of the dam, the river pools behind the dam and fills the artificially created basin (UNEP 2000). Seasonal changes of runoff and precipitation feed the reservoir. There are big differences in the size of man-made reservoirs such as big artificial lakes or small pond-like waterbodies. The stored water can be used for irrigation, drinking water after purification or to produce energy (http://www.sswm.info/category/concept/concept-introduction).
Groundwater is water found underground in the cracks and spaces in soil, sand and rock. It is stored in and moves slowly through geological formations called aquifers (The Groundwater Foundation, 2018). It is also called subsurface water, to distinguish it from surface water found in water bodies such as rivers, lakes or oceans.
Groundwater bodies are formed/replenished when precipitation infiltrates below the ground surface into the soil zone. When the soil zone becomes saturated, water percolates downward. A zone of saturation occurs where all the interstices are filled with water. There is also a zone of aeration where the interstices are occupied partially by water and partially by air. Groundwater continues to descend until, at some depth, it merges into a zone of dense rock. (https://www.britannica.com/science/groundwater).
The major ion constituents in groundwater are: Na+, Mg2+, Ca2+, Cl-, HCO3-, SO4 2- (Table 4). The total concentration of these six major ions normally comprises more than 90% of the total dissolved solids (TDS) in the water, regardless of the water’s salinity. The most abundant dissolved gases are N2, O2, CO2, CH4, H2S and N2O, and they can significantly influence the subsurface environment. Their concentration in groundwater is generally lower compared to the inorganic constituents (Candella & Morell).
Table 4. Dissolved inorganic constituents in groundwater
(greater than 5mg/L)
(0.001 - 5.0 mg/L)
*trace constituents are those present in a concentration lower than 0.001 mg/L
The Groundwater Foundation (2018) https://www.groundwater.org/get-informed/basics/groundwater.html
The salinity of sea water ranges from 30,000 to 44,000 mg/L (http://www.iwa-network.org/desalination-past-present-future/). Saltwater can be desalinated to provide water suitable for human consumption or irrigation (https://en.wikipedia.org/wiki/Desalination). Desalinated water is produced through the process of desalination, which removes mineral components from the feed water.
Desalinated water is stored prior to or during distribution, and the issues that may arise during storage are related to the potential introduction of microbial or chemical contaminants and the corrosion of materials affecting water quality. Desalinated water is more corrosive (low minerals concentration) than other drinking-water sources, so the stability of water is crucial for the elimination of its corrosive effect on pipes in the distribution network.
The reliability and flexibility of water supply can be increased by blending desalinated sea water with groundwater or water from other sources (WHO, 2011).
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