By: Shi-Chun(Timothy) Jou
Sewage treatment is a process in which the pollutants are removed. The ultimate goal of sewage treatment is to produce an effluent that will not impact the environment  . In the absence of sewage treatment, the results can be devastating as sewage can disrupt the environment.
The general processes of sewage treatment are primary, secondary and tertiary treatment. Primary treatment involves physical separation of sewage into solids and liquid by using a settling basin. The liquid sewage is then transferred to secondary treatment which focuses on removing the dissolved biological compound by the use of micro-organisms. The micro-organisms usually use aerobic metabolism to degrade the biological matter in the liquid sludge. Then tertiary treatment is required to disinfect the sewage so that it can be released into the environment. The solid sewage separated from primary treatment is transferred to a tank for sludge digestion which involves anaerobic degradation using micro-organisms .
The environment of the sewage treatment plant has to be controlled precisely because bacteria are sensitive to the oxygen level, pH level, temperature, and level of nutrient. In order for efficient degradation of biological matter to occur, these factors are controlled manually.
Sewage is composed of organic matter such as carbohydrates, fats, oil, grease and proteins mainly from domestic waste. It also contains dissolved inorganic matter such as nitrogen species and phosphorous species mainly from agricultural use . It is essential to remove the nutrients before they are released to the environment because it interferes natural habitats by altering the chemical composition such as pH or oxygen level both directly and indirectly.
Oxygen level is an important factor to secondary and tertiary treatment processes. Secondary treatment, oxygen is required as a terminal electron acceptor in organic matter degradation. For example, nitrification by Nitrosomonas and Nitrobacter species requires dissolved oxygen to occur . Oxygen in secondary treatment is provided manually by pumping oxygen into the sewage continuously which occurs in an aeration tank . In tertiary treatment, the removal of excess organic matter is enhanced by settling the sewage in a lagoon. This process is also aerobic, but it depends on the diffusion of oxygen because most organic matter has been degraded by secondary treatment .
Acidity plays a crucial role in the breakdown of organic matter because pH affects the solubility of compounds which indirectly affect the accessibility by bacteria . Also, bacteria responsible for organic matter degradation are sensitive to the pH of the environment. Extremely high or low pH levels are able to kill bacteria, deposition of organic matter occurs due to lack of degradation . Hence, the pH of sewage treatment is controlled to be around 7. A nitrifier in secondary treatment, Nitrosomnas requires a pH between 6~9 in order to be viable .
The effect of temperature is influential for secondary treatment, but it is not important in primary treatment. Bacterial growth is sensitive to temperature because high temperature can increase the fluidity of the phospholipid bilayer which leads to cell lysis. However, bacteria are known to have higher enzymatic activity at higher temperature because of increased thermal energy. For example, when thermophilic sludge treatment is compared to mesophilic treatment, the sludge biodegradability is higher with thermophilic degradation . Hence the temperature has to be controlled precisely to maximize the efficiency of degradation but also allow the cell to remain viable.
There are a lot of nutrients available in the sewage because of human waste and agricultural runoff . Bacteria can harvest the electron from organic matter and transfer it to a terminal electron acceptor which results in the break down of organic matter and energy conservation .
There are several microbial processes, and the microbial processes can be catergorized into aerobic and anaerobic.
After primary treatment, liquid and solid phases are physically separated. The liquid phase is treated with aeration to allow aerobic degradation of the nutrients. The two important microbial processes at this stage are nitrification and phosphorous removal. Nitrification occurs in two discrete steps. First of all, ammonium is oxidized to nitrite by Nitrosomonas.spp, and nitrite is further oxidized to nitrate by Nitrobacter.spp. Phosphorous removal can occur biologically by the process of “enhanced biological phosphorous removal.” The process is demonstrated by the cell taking up phosphorous within their cell, and the biomass is filtered .
In the liquid component of sewage, denitrifying bacteria reduce nitrate into dinitrogen gas which liberates nitrate from the sewage . The solid component of the sewage separated in primary treatment is fermented by bacteria anaerobically .
Microorganiasms can also be categorized by its metabolism.
Microorganisms with aerobic microbial process
Members of the Nitrosomonas genus is a gram negative bacterium responsible for the first stage of nitrification in sewage. They oxidize ammonium into nitrite. This bacterium prefers a pH around 6-9 and nitrify optimally at 20-30°C .
Members of the Nitrobacter genus is a gram negative bacterium responsible for the second stage of nitrification in the sewage. It oxidizes nitrite to nitrate using oxygen as a terminal electron acceptor. The bacteria has an optimum pH of 6~8, and an optimum temperature of 0~40°C .
Microorganism with anaerobic microbial process
Members of Pseudomonas genus is a gram negative denitrifying bacteria that use the chemical energy in organic matter to reduce nitrate into dinitrogen gas . Also, members of the bacteroidetes phylum are the gram negative bacteria responsible for the anaerobic fermentation of the solid sludge .
A research has shown the correlation between nutrient removal efficiency, light wavelength and light intensity. Xu et al. discovered that red and high intensity light maximizes the nutrient removal efficiency . Also, the use of pre-treated sludge is found to generate electricity in a microbial fuel cell. This can potentially lead to production of renewable energy.
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2 Microbes in sewage treatment
All living things, including ourselves and microbes, need food to grow, maintain and repair their cells, and to provide a source of energy for life. However, we cannot digest all of the food we eat and what remains undigested, ends up in the sewage system. About 10 billion litres of sewage are produced every day in England and Wales and this has to be treated to remove harmful substances and pathogenic microbes before the waste can be safely released into the environment. The main component of sewage is organic matter (undigested food) but there are other substances such as oil, heavy metals, nitrogen and phosphorous compounds (from artificial fertilisers and detergents) which also have to be removed. Here you will consider the important role of microbes in the sewage treatment process.
Sewage is actually a mixture of all types of waste water, including rain water and domestic water from toilets, baths and sinks. When sewage arrives at a treatment works (shown schematically in Figure 5), it is first filtered to remove large objects (e.g. condoms, tampons and cigarette ends) which have got into the system. These usually go to a landfill site or incinerator. The remaining material is then allowed to ‘settle’so that much of the solid material drops to the bottom of a tank. This solid material is then removed and usually buried in landfill, burnt or, after further treatment, used as fertiliser on agricultural land.
What then remains is the liquid portion, or effluent, which is rich in suspended organic matter and some pathogenic microbes. This liquid portion will ultimately be released into rivers or the sea but it is vital to first reduce the organic matter content and eliminate harmful microbes. To do this the liquid is fed into an aeration tank containing a complex community of microbes. The contents of the tank are mixed mechanically with air or air is bubbled through the tank. The microbes then use the organic material in the sewage as their source of carbohydrate for respiration.
The oxygen in the air allows the microbes to respire aerobically.What are the products of the aerobic respiration of carbohydrates like glucose?
Aerobic respiration can break down carbohydrates completely, producing just carbon dioxide and water (together with energy needed for the microbes to stay alive).
Aerobic respiration is the most efficient way of breaking down organic matter although some compounds in the effluent are not broken down completely. The tanks often contain porous solid materials, on which biofilms can develop, increasing the numbers of microbes and so the efficiency of the breakdown process. During this process, a fairly solid material known as activated sludge is formed. This contains a mix of microbes and undigested material. Since it contains all of the essential microbes to break down incoming waste, some of it is added to batches of new sewage (Figure 5). After this aerobic digestion, and a variety of other purification procedures, the liquid portion of the sewage is usually safe to discharge into rivers or the sea. The remaining activated sludge material is subjected to various other types of biological processes to reduce further the amount of organic matter it contains. Anaerobic bacteria are often used in this subsequent stage, since, although they grow more slowly, they can break down more complex materials that are difficult to degrade using microbes that respire aerobically. The gases produced in this anaerobic process are carbon dioxide and methane, a mixture called biogas, which can be collected and subsequently burned for energy production.
Sewage processing reduces the concentration of potentially harmful bacteria such as E. coli and Salmonella in the original sewage as many of them die during the processing because the conditions are not appropriate for them. It is also important to reduce the amount of organic compounds in the effluents released into rivers from sewage works. If this is not done, then microbes naturally present in the river use the organic compounds as a source of energy and reproduce in huge numbers. Since they respire aerobically, they use up much of the oxygen dissolved in the water, leaving little for other organisms such as invertebrates or fish, many of which will die. Sewage must therefore be treated to reduce the amount of organic matter, and thus reduce the Biological Oxygen Demand or BOD, defined as the amount of oxygen required by the aerobic microbes to decompose the organic compounds in a sample of water.
The process of sewage treatment can be thought of as a complex form of composting. The compost heap which you may have in your garden is like a miniature sewage treatment works. The centre usually becomes anaerobic as existing oxygen is used up. Closer to the top of the heap aerobic processes take place. Apart from the raw material, the other big difference between a sewage treatment works and a compost heap is that inside a compost heap, temperatures become high – well above 60 °C – which is detrimental to most species of microbes, but in which some can flourish.
Sewage works rely on the diverse capabilities of microbes for the complete process to be effective. Name three types of microbial activity that are used in the process and one that is not.
Aerobic and anaerobic respiration and fermentation play an important role in sewage treatment. Photosynthesis is generally not used in the sewage works because it is a process that builds organic compounds from carbon dioxide, whereas the objective of sewage works is to break down organic compounds.
Many of the microbes used in food and in industry are anaerobic so that they must be cultured in conditions without oxygen. In principle, anaerobic culturing is very similar to aerobic culturing. However, it does involve some specialist equipment and procedures which are described in Box 1.
Box 1 Growing anaerobic microbes and large-scale culturing
Why do you think anaerobic culturing would be more challenging than aerobic culturing?
Although some anaerobic microbes can tolerate small amounts of oxygen, many are obligate anaerobes, which means they are killed by oxygen. When culturing them, special efforts must be made to ensure that at no time during their manipulation using aseptic techniques are they exposed to oxygen.
Microbiologists have devised a range of methods for culturing anaerobic microbes. One of the most important pieces of equipment is a cabinet flushed with a gas such as nitrogen which allows manipulations to be carried out in an oxygen-free environment. The operator carries out the work through rubber gloves connected to the inside which prevents oxygen from entering the cabinet. When anaerobic microbes are cultured on Petri dishes, the dishes are also stored in airtight containers that are constantly flushed with nitrogen or carbon dioxide to keep them anaerobic. Many anaerobic organisms also need some unusual elements to grow, and these are added to the culture media.
Growing anaerobic microbes on a large scale is essential for the industrial processes of brewing beer, making wine and carrying out other types of fermentation and for producing large quantities of microbes, for example yeast for bakeries. This is often done in giant metal vats called fermentation reactors which can hold many hundreds or thousands of litres of microbial culture (Figure 6c). The vats are provided with sensors to measure temperature and nutrient levels that allow computers to control the conditions within the reactors to ensure maximum growth of the microbes. One of the difficulties with such cultures is preventing the microbes from settling to the bottom of the vats.
Why is this likely to be a problem?
Those microbes which settled first would be covered by those settling later and therefore would not be able to access the nutrients in the culture medium. They would probably die and decompose, and their breakdown products could then contaminate the culture, as well as resulting in a lower yield of microbes from the whole vat.
To prevent settling, the vats can be stirred mechanically by large paddles (Figure 6a) or oxygen-free air (mostly nitrogen) can be bubbled through to circulate the contents of the vat (Figure 6b). If aerobic microbes are being cultured, then similar vats are used but air or oxygen is bubbled through.
The growth of populations of microbes in the fermentation reactors (and in all other situations too), follows the basic pattern shown in the growth curve in Figure 7. When microbes are first added, they begin growing and dividing slowly as their enzyme systems adjust to the presence of new nutrients. This is referred to as the lag phase. After a period of time the microbes begin to grow and divide very rapidly to take advantage of the favourable growth conditions – the exponential phase. There may then come a point at which the microbes have used up some vital nutrient in the growth medium and they are no longer able to divide – the stationary phase. Eventually the microbes become old and start to die – the death phase. These growth stages are important for industrial processes. To produce the maximum amount of microbes and microbial products, they must be constantly provided with new nutrients to prevent them entering the stationary phase, hence the valves and tubes entering the fermentation reactors in Figure 6c. These are then harvested by being extracted from the medium in which the microbes are growing.