Importance of water
Water, one of the world's most important but unfortunately finite source, which is being endlessly used and reused. Wastewater treatment is the process in which wastewater as well as the sewage, is filled with bacteria, chemicals and other contaminants is cleaned so that it can be recycled back safe for use. Once the treatment is complete, all forms of solids called sludge, regardless of the shape and size that was present in the wastewater will be removed. Besides that, oxygen gets restored into the water, which then eventually ends up in the lakes and rivers which require oxygen rich water to support the lifestyle of the aquatic organisms.
Wastewater includes a combination of domestic sewage (toilets, kitchen, and laundry) on a smaller scale and on a larger scale this consists of industrial effluent, schools as well as businesses (chemical and wastes, hospitals, shopping centres). Wastewater is also obtained from storm water infiltration and ground water which enters the sewer through the cracks present.
Generally the waste can be broken down naturally with bacteria and other biological organisms especially when it is just household or business waste. However, wastes obtained from industries are generally toxic and require a physical/chemical treatment plant, which uses both chemical reactions and physical processes to process the wastewater.
Overview of the treatment:
As an overview, the wastewater treatment occurs in three stages which will be discussed thoroughly. It begins with the preliminary and primary treatment where 40-60 % of the solids are removed (City of Columbia). Followed by the secondary treatment where 90% of the pollutants are removed, hence completing the liquid portion process (City of Columbia). The next step would be the treatment and removal of the sludge (bio-solids). The number of stages of the treatment varies but generally go up to four depending on the quality of water being treated.
Below is a picture of an aerial view of a general wastewater treatment plant.
Below, is the overall process diagram of the wastewater treatment.
(Source: Wastewater treatment and principals and regulations brochure)
The preliminary treatment is the first stage in wastewater treatment with its main purpose removing coarse solids and large materials found in raw water to make the water suitable for the main treatment process. It also ensures that the pumping equipment does not get damaged. This initial stage involves various different processes which include screening, grit removal and odour control.
Initially, the sewage is screened to remove large objects which include plastics and paper. This step is crucial to make sure that there is no blockage in the pipe system as well as no damages to the equipment. This is done generally by passing the sewage through mechanically raked bar screens (consisting of vertical bars spaced close together) which are used to capture the large objects and remove them from the wastewater stream. The screenings (material which have been cleared in this step) is disposed safely at a landfill site. Below is an example of a raked screen bar image used in industries.
(Source : Huber Technology, 2009)
The next step is the grit removal which includes grit, stones and dirt. However, before the sewage enters the grit tanks, Ferrous Chloride (FeCl2) and lime are added to improve the subsequent chemical treatment. Ferrous Chloride precipitates phosphorus thus reducing the growth of toxic algae in the water. Lime on the other hand is added to increase the pH level which aids the Ferrous Chloride in removing phosphorus and other material from the sewage.
Various different types of detritus tanks, grinders and cyclonic inertial separation are used including a comminutor and grit chamber to remove the coarse solids. A comminutor is actually a grinding pump which houses a rotating cutting screen that makes shreds large organic matter, therefore making it easier for microorganisms to decompose the organic matter. This step also further prevents any damage to the machines and pumps in the process.
The effectiveness of the chemicals added prior in the grit chamber is improved by adding and mixing compressed air into the wastewater. In order to allow the heavier inorganic materials settle out of the waste stream, the velocity of the incoming sewage is controlled. The air flow is also adjusted to create velocity near the bottom part of the chamber to catch the grit in a current hence, allowing it to settle.
Chlorination is another step that could be used in the preliminary treatment. However, as chlorination can be used for all the different stages in the treatment, the equipment has to be design specially and carefully for the same operations.
All the disposals collected from the preliminary stages are disposed of safely in a landfill.
The next step, following the preliminary treatment is the primary treatment. The purpose of this step is to remove the particles which are able to settle by sedimentation which includes organic nitrogen, organic phosphorous and heavy metals (New York Water operations 2007). This is done by passing the wastewater through the primary sedimentation tanks or primary clarifiers where solid particles are removed by physical settling due to its density, buoyancy and the force of gravity. Coagulants and flocculants such as solid and liquid Aluminium Sulphate and Aluminium Hydroxide Chloride (Accepta Water Treatment, 2010) are often added to expedite this process by encouraging the aggregation of particles. However, the pH level has to be constantly adjusted as they tend to reduce the pH levels of the wastewater.
(Source : City of Camarillo, 2010)
Above is an image of a sedimentation tank. They are designed to hold wastewater for numerous hours by then which most of the heavy solids would settle at the bottom of the tank. They would then form thick slurry known as sludge and also floating material such as fats, oil and grease to rise to the surface which would then be skimmed off. There are mechanical scrapers that have been designed for the tanks with the purpose of collecting the sludge at the bottom and the scum floating on the top. Both the sludge and skimmed material are generally pumped to a solid treatment process.
The sedimentation process is basically mainly to produce a liquid which is able to be treated biologically by reducing the biological oxygen demand, also known as BOD of water. BOD is the quantity of oxygen that is needed by aerobic microorganisms to decompose organic matter in a sample of water. The degree of water pollution can also be measured by the BOD. (The American Heritage® Dictionary of the English Language) When the solids are removed at these early stages, BOD can be reduced by 30-40 percent hence, increasing the efficiency of microbial digestion at a later stage. (Food and Agricultural Organisation of the United Nations, 2010)
The next stage, called the secondary treatment focuses on removing the remaining suspended and dissolved organic matter in the sewage. It is also known as the biological stage as the biodegradable organic contaminants that are dissolved would be broken down by microorganisms cultivated and added to the wastewater such as bacteria and protozoa. These microorganisms feed on the suspended and dissolved organic matter that remained from the primary clarifier. These bacteria can be categorised into aerobic or anaerobic bacteria, which is actually their need to oxygen. Generally, aerobes can degrade pollutants at a higher rate as opposed to anaerobes. (Waste Management, 2004)
Numerically, the anaerobic treatment produces 0.1-0.2 kg biomass or sludge per kg BOD as opposed to the aerobic treatment which produces 0.5-1.5 kg biomass or sludge per kg BOD. (V. Jegatheesan, C. Visvanathan and R. Ben Aim, 2008) Also, some factors that increases their rate of degradation is the quantity of their food source and the temperature of the sewer. This secondary treatment can actually be carried out in numerous different methods.
Anaerobic is defined as ‘does not require oxygen'. (MedicineNet.com) Firstly, the sewage is flown into anaerobic large tanks or ponds, therefore allowing anaerobic digestion to take place. Anaerobic digestion is when biodegradable material is broken down by the organisms without the presence of oxygen. The product of this digestion includes methane, carbon dioxide and sludge where to our advantage; methane can be used as an energy source categorising the anaerobic digestion as a renewable energy source. There is a membrane cover at the surface of the tanks/ponds which captures the methane and it is then used to generate electricity by combustion in a gas engine as well as reduce greenhouse gas emissions and odour. Besides that, mixing in the anaerobic process requires less energy compared to the aeration step carried out in the aerobic process. The emission of landfill gases into the atmosphere is also reduced with this anaerobic digestion. However, larger treatment plants are needed for the anaerobic process as they have slower reaction rates. (Guerrero F. Omil, R. Méndez and J. M. Lema , 1998)
The three main steps of this process are:
1. Hydrolysis and Acidogenesis
C 6 H 12 O 6 à 2C 2 H 5 OH + 2CO 2
(Organic compound) (Ethanol) (Carbon Dioxide)
Hydrolysis is a process where the covalent bonds are broken with the use of water. Therefore the complex organic compounds are broken down into their constituent part by enzymes. Subsequently, acidogenesis is where acidogenic bacteria produces short-chain product by converting the hydrolysis products through fermentation and other metabolic processes.
2C 2 H 5 OH + CO 2 à CH 4 + 2CH 3 COOH
( E thanol) ( Carbon Dioxide) (Methane) (Acetic A cid)
2CO 2 + 4H 2 à CH 3 COOH + 2H 2 O
(Carbon Dioxide) (H ydrogen) (Acetic Acid) (W ater)
Acetogenesis is the process where acid and alcohol are converted into acetate, hydrogen and carbonic gas by acetogens categorised into homoacetogens, syntrophes and suphoreductors. The acetic acid production process may be carried out by Clostridium acetium, Actobacterwoodiiand Clostridium termoautotrophicum. The products of this process vary with the type of bacteria, temperature and pH levels.
CO 2 + 4H 2 à CH 4 + 2H 2 O
(Carbon Dioxide) (H ydrogen) (Methane) (W ater)
CH 3 COOH à CH 4 + CO 2
(Acetic Acid) (Methane) (Carbon D ioxide)
The third step of this process is methanogenesis, a form of anaerobic respiration in which methanogens (microbes) that exist in deep sediments convert soluble matter into methane. The majority of the methane production is from the conversion of acetic acid and the rest comes from the reduction of Carbon Dioxide by hydrogen.
In addition to above, Sulphur, sulphite and nitrate under anaerobic conditions are reduced. To produce sulphides by sulphur reduction, Sulphur Reducing Bacteria (SRB) use sulphate or sulphite as electron acceptors and organic compounds such as acetate as electron donors. This is the main reason behind the rotten egg smell that exists from wastewater as they are kept for long periods of time under these anaerobic conditions. As for Denitrification however, nitrogen gas is produced by the reduction of nitrates using the organic compounds in the wastewater by denitrifying bacteria (DB). The bacteria generally require a carbon food source as energy for the conversion of nitrogen.
6NO 3 - + 5CH 3 OH 3N 2 + 5CO 2 + 7H 2 0 + 6OH -
( Nitrate) (Methanol) (Nitrogen G as) (Carbon Dioxide) (Water) (H ydroxide)
A ero bic Treatment
The next step in this secondary treatment of wastewater is the activated sludge process where atmospheric air or pure oxygen is bubbled through the sewage, combined with microorganisms to create biological flocculants which reduces the organic content of the wastewater quite significantly. This occurs because of the bacteria and protozoa feed on the remaining organic materials in the wastewater. (Guerrero F. Omil, R. Méndez and J. M. Lema , 1998)
Generally, the sewage is transferred into large ponds or tanks which are called surface-aerated basins that have floating surface aerators to promote the biological oxidation of wastewaters. These floating aerators create an oxygen rich aerobic environment in the sewage by removing most of the BOD therefore encouraging the growth of the aerobic microorganisms. Furthermore, the aerators provide mixing required for dispersing the air as well as contacting the reactants namely oxygen, microbes and wastewater. In the final clarifiers, the mixing process improves the settling of the biological solids. As the biological oxidation processes are highly dependent on the temperature changes, increasing the temperature to a certain threshold increases the rate of microbial decomposition. Surface aerated vessels mainly operate at temperatures ranging from 4 °C to 32 °C. (Beychok, M.R 1971)
Nitrification is a process where the dissolved ammonia is removed by oxidizing it to nitrate which occurs during the activated sludge process. As a high concentration of ammonia is toxic to marine life, they have to be removed from the wastewater via the nitrification process. Nitrification can be divided into two steps, the oxidation of ammonia into nitrate by Nitrosomomonas and the oxidation of nitrite to nitrate by Nitrobacter. As the nitrifying organisms are chemoautotrophs, they use carbon dioxide as their source of growth and for cell maintenance.
2 NH 3 + 2 CO 2 + 3 O 2 + Nitrosomonas → 2 NO 2 - + 2 H 2 O + 2 H +
( ammonia ) ( nitrite ion)
2 NO 2 - + 2 CO 2 + O 2 + Nitrobacter → 2 NO 3 -
( nitrite ion) ( nitrate )
The remaining solid particles flocculate to form larger and heavier particles that settle down more easily based on the biological reaction. Then, this mixture of wastewater and solid particles are pumped into a second clarifier or sedimentation tank where the solid particles are separated from the wastewater similar to the process in the primary sedimentation tank, where the resulting sludge is referred to as the activated sludge. Activated sludge is a biological material, brown in colour consisting of mainly saprotrophic bacteria that is produced by the activated sludge process which affects the purification process. In poorly managed activated sludge, a range of mucilaginous filamentous bacteria including Sphaetotilusnatanscan develop. These bacteria produce sludge that does not settle easily and therefore a possibility that a sludge blanket decanting over the weirs in the sedimentation tank that will severely contaminate the final effluent quality will form.
A portion of the solid is recycled back into the surface aerated basins to be re-used in the nitrification process as the microorganisms in the sludge are still active. Excess sludge which eventually accumulates beyond what is recycled is called Waste Activated Sludge and then removed from the treatment process to maintain the ratio of the biomass to food supply in the balance (F/L balance). As a whole, the aerobic process is preferred as it is more stable, reliable and a clearer process understanding.
The final stage of the wastewater treatment is the tertiary treatment. This tertiary treatment is considered the advanced treatment stage of wastewater treatment. The purpose of this stage is to raise the quality of effluent before it is discharged into the receiving environment including the ocean, rivers or lakes. Various different methods can be used to undergo this tertiary treatment, however it is been found that the most cost-effective and environmental friendly method is lagooning followed by the Ultra-Violet disinfection. The wastewater may also be treated by chlorine but high chlorine content will harm the aquatic life that receives the water. A chlorine-neutralising chemical is often added before the stream is discharged in situations where required. However, if very high quality effluent is needed, an additional step, namely the polishing process that use sand or gravel filters and wetlands is carried out as the water from the treatment process are not safe enough to be consumed as there is still bacteria present. Below is a picture of a UV channel used in wastewater tertiary treatment.
Source : City of Idaho, 2007
The sewage is then flowed into a series of large man made lagoons which are highly aerobic following the secondary treatment. Colonisation by algae and zooplankton is often encouraged due to their aerobic nature. The algae that grow in the lagoons captures trace amounts of organic nutrients and compounds in the wastewater which are then grazed by the zooplankton. The remaining algae and plankton settle to the bottom hence binding the nutrients in the sediment. Other microorganisms which are present in the sewage assist in reducing if not removing the harmful pathogens in water.
Furthermore, the sewage left in these lagoons are exposed to the ultra violet radiation from the sun coupled with the grazing zooplankton, which creates a far from ideal environment for the bacteria which results in a great reduction their quantity. Where greater intensity of UV radiation or where there is insufficient, the sewage may be transferred into ponds that generate the UV radiation with ultra violet light bulbs for further disinfection. This disinfection eliminates pathogens and cist and is very use friendly as well as operates at a low cost. How it works is that the UV radiation actually damages the genetic structure of the bacteria, viruses and other pathogens hence inhibiting them to reproduce. A key advantage is that no chemicals are added to the sewage when the UV light method is used, hence no adverse effect on organisms that later consume the water. (http://wastewater-treatment.org)
Generally lagoons need large spaces to operate however they do not need as much money and time as the traditional tertiary treatment wastewater procedures. The lagoons have also been found to be an important habitat for the birds. After the lagooning process, the treated effluent is ready to be released back into the environment and regarded as safe. Below are pictures of lagoons, the first one being an aerial view and the second one a close up.
Source : City of Idaho, 2007
Source : City of Idaho, 2007
Disadvantage s of the process
First and foremost, to operate a wastewater treatment plant, a significantly large amount of energy is needed. In most communities, they are often regarded as the largest energy consumer. As previously mentioned, energy is generated by the biogas produced onsite, however in most situations, this only accounts for approximately half the plants energy requirements. This is due to the fact that the wastewater treatment plants are consistently operating to keep up with the ever increasing inflow of sewage. This consumption of external energy, other than the one being produced in the plant leads to high operational costs and also, more importantly affects the environment if the energy is sourced from fossil fuel.
As the primary focus of the wastewater treatment is to remove contaminants from water, and eventually recycled into drinking water, recent studies show that the presence of certain contaminants including hormones and synthetic material can have an adverse impact even at minimal levels on the natural biota and for some cases, humans. For even processes that remove 99% of microorganisms, the final effluent declared as safe to drink may contain about 50 000 microorganisms. (Environmental Protection Agency United States, 2002) This is a threat when the receiving water is used for activities such as swimming or shellfish harvesting which need to be carried out in an environment with as little microorganisms as possible. BOD levels also affect the environment as although they are greatly reduced throughout the whole process the levels leaving the plant in most cases are high enough to damage the quality of the receiving environment.
In the preliminary treatment, the main disadvantage highlighted is the high cost of the screening due to high labour and maintenance costs, as well as high maintenance cost of the machinery. In the primary treatment, the accumulation of sludge that is disposed off onto landfills that will eventually get full and hence taking up a larger area. In the secondary treatment however, there are quite a few limitations. Firstly, in the anaerobic digestion process, the bi-product requires substantial wet biomass handling and disposal. If this waste was to be disposed of in a landfill, often they would break down anaerobically, releasing methane into the atmosphere where methane is about twenty times more potent than carbon dioxide as a greenhouse gas, hence significant adverse effects on the environment. (ABC News, 2008) For the aerobic digestion however, the operating costs are marginally greater due to the additional costs needed to add oxygen. Skilled manpower is also needed for the operation and maintenance of this process. Finally, in the tertiary treatment, the major limitation is the lagooning process which depends highly on the climate condition that affects the toxicity of municipal wastewater and effects in the receiving environment which includes dissolved oxygen content in sewage, temperature of wastewater as well as efficiency of microbial processes. Hence, the effluent produced is of different qualities. For the UV disinfection, frequent maintenance and replacements incur additional costs. Besides that, not all organisms are actually affected by the UV radiation. (http://wastewater-treatment.org)
The wastewater treatment process manages to remove almost all of the organic chemicals and metals present in the wastewater, however due to environmental degradation, the contaminants should not be discharged in large quantities as the contaminants may be toxic and stay in the environment for long durations. The contaminants will accumulate in the living tissue and be passed up along the food chain.
Suggested Improvements to the Process
General Improvements to the Process Overall
Alternative sources of energy, especially renewable energy should be explored as wastewater treatment plants generally consume large amounts of energy. The generation of energy from the produced biogas during the process is insufficient to run the plant thought it helps reduce the environment consequences and overall operational costs. A named alternative to be considered is the use of hydroelectric power. The flow of water before the final effluent is released into the receiving environment could be used to generate electricity by the use of turbines. However, the capital and maintenance costs of the plant would increase. The volume of wastewater entering the treatment plant can be reduced by reducing the pipes diameter or by a inserting a valve is important as this smaller flow of influent leads to improved treatment, longer system life and a lower chance of overflowing. Overall, the quality of effluent will be increased with the reduction of influent flow as the waste will remain in the system longer; therefore more time is provided for settling, decomposition and aeration. Unfortunately, the volume of wastewater is largely dependent on the amount of water used in the community. Therefore campaigns and awareness should be carried out to help reduce the inflow of sewage by conserving the use of water.
By their nature, processes involved in the wastewater treatment, primarily from the anaerobic digestion process generate odour. Odour is actually one of the biggest concerns of the operators of the wastewater treatment as well as the general public.
One way to overcome the release of odour into the surroundings is to capture the gas resulting from the anaerobic process and treat the trapped gasses. Examples of some treatment systems include activated charcoal bed systems, chemical scrubbers (often using hypochlorite solution), a compost pile type bio-filter and UV radiation treatment. The captured air may also be treated by pumping it through soil where the odorous compounds are absorbed into the soil particles and destroyed by naturally-occurring soil bacteria. (Wastewater treatment technologies)
Other methods may include the addition of ferrous chloride to the wastewater collection system to reduce the release of hydrogen sulphide gas. Ion generators may also be installed onsite to help reduce the odours.
Anaerobic and specific aerobic microbial processes are temperature sensitive, and generally if the temperature is reduced the rate of reactions also decreases. Therefore, the climate conditions affect the quality of the final effluent through the lagooning process and open air ponds. As the biogas produced is used to generate power usually by combustion, the heat generated from this can be used to regulate the temperature in the lagoons. This is a cost-efficient way to curb climate conditions with minimal environment impacts.
When the rate of the Nitrification process increases with the use of the ringlace fixed film system, the rate of reaction also increases. Ringlace is material developed in the 1980's by the Japanese and consists of a rope like material of high surface area and chemical composition conductive to bacterial attachment and growth. This system has been proven to increase the nitrification rates by 25% when the operation temperature is less than 1°C. Apart from being cost effective due to its minimal installation and maintenance costs, the ringlace system also positively affects the BOD reduction and hinders algae growth. (Richard, M)
Activated Sludge Process
For the effective removal of organic matter, the activated sludge process requires sufficient oxygen supply and thorough mixing. The rate of which the microorganisms decompose can be increased tremendously if the aeration systems in the sludge tanks design be improvised as to provide a higher input of oxygen into the sewage. A key advantage to this design improvement would be the decreased amount of ammonia discharged into the environment as final effluent.
Bio -solid Processing and Disposal
The sludge that accumulates from the wastewater treatment processes has to be dealt with and can be done in a number of ways. As these bio-solids are highly toxic, they require intensive treatment before it is ready for disposal. For now, the conventional means of bio-solid treatment is sufficient as in to be disposed off in landfills. However, in time to come, the space requirements to accommodate these solids are not practical.
Worthless sludge can be converted into marketable bio-solids through a process called Thermal Drying. The volume and mass of the solids are greatly reduced by evaporating the majority of their water content by the thermal dryers. To assist in forming larger aggregates of solids and releasing of water, chemical coagulants are usually used. Higher temperatures produce higher quality of bio-solids that can be sold as fertilizer. The product is easily handled, stored and transported. The main advantage of this process is that it can provide extra revenue to the plant. To the environment, it reduces odours resulting from the decomposition of the sludge. (Viessman, W Jr, 2010) A picture of the final product of the thermal drying is as below:
(Source : StibbeManagement, 2006)
On the other hand, thermal oxidation is an efficient process that converts bio-solids into an energy source, producing carbon dioxide, water and ash. The process occurs in a fluidized bed reactor that is highly energy efficient as it can be self-sustaining without auxiliary fuel when the combustion air is preheated to high temperatures. Heated air, gas, steam, water or oil which can be converted into electricity is recovered from these reactors. The advantages of this process include its low life-cycle cost, its ability to destroy all volatile solids and pathogens, minimising odour and offsets the energy consumption of the plant. Another approach involves treatment with lime (calcium oxide), which kills pathogens due to its high alkaline content. The heat generated from this reaction also helps in producing a drier final product.
The waste sludge may also be treated by a means of anaerobic digestion which is similar to the anaerobic digestion which occurs in the water treatment process. In the anaerobic digestion of bio-solids, the waste activated sludge and primary sludge are mixed together without the presence of air. The digestion takes place in two steps and involves two distinct groups of bacteria. In the first step, acid-forming bacteria convert complex organic wastes (proteins, carbohydrates, lipids) into organic fatty acids. The second step is where bacteria convert these organic acids into methane, carbon dioxide and other trace gasses. As before, the methane produced may be used to generate energy by a means of combustion. This process stabilises a majority of the organic waste in the sludge thus allowing the bio-solids to be utilised as a soil conditioner. The stabilised bio-solids contain nitrogen, phosphorus and potassium which are beneficial to plant growth. Application of these bio-solids in agriculture has lead to increased crop production. (Bio-energy from wastewater treatment)
The bioprocess involved in the steps of the wastewater treatment process namely the preliminary treatment which includes screening, grit removal the primary treatment which involves the primary sedimentation process, the secondary treatment which consists of the anaerobic and aerobic digestion and the secondary sedimentation and finally the tertiary treatment which is made up of the lagooning and the ultraviolet disinfection has been outlined in this report. The improvements as well as the key advantages were also discussed in the report. Improvements to the processes that would increase the quality of water discharged into the environment as well as the revenue of the wastewater company were focused on. Wastewater treatment is essential to ensure the preservation of our water and marine life and to the environment as a whole.
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