ABSTRACT:- final year project for civil engineering .In harbours wastewater can be produced on land –based harbours facilities (including restaurants, offices, shipyards and washing areas) or can be collected from different vessels (with different wastewater treatment plants onboard or without any kind of wastewater treatment plants onboard). To design wastewater treatment plants in harbours it is of extreme importance to have a very detailed analysis of the type of wastewater (prediction of quality and quantity) that can be found in the harbours (taking into account all the different sources previously mentioned). That is a crucial information to decide which type of WWTP can be effective (e.g., mechanical including chemical, or biological treatment).
Moreover, port authorities have to deal with two different groups of regulations for wastewater treatment: with legal requirements for effluent from wastewater treatment plants based on land and legal requirements for effluent from vessels (e.g. from WWTP onboard), which are stricter than the requirements for wastewater treatment on land. In this thesis different types of wastewater that can be found in harbours are characterized, as well as the problems that might be related with wastewater treatment of those effluents, considering the legal requirements and technical solutions. Onboard wastewater treatment is also studied.
Furthermore, a case study is presented in order to illustrate what typically may happen in a harbour, present the main problems and constraints they have regarding ships, as well as wastewater handling and disposal. In addition, the particular case of Lisbon Port has been analyzed in order to provide a solution for the wastewater treatment, including study a possible location for the wastewater treatment plant and the type of treatment that could be implemented.
A screen is a device with openings that is used for collecting (retaining) solids from influent wastewater. Openings on screen are usually uniform size and screen types are divided by the size of the openings. Two general types of screens are used in preliminary treatment of wastewater. Coarse screens have clear openings ranging from 6 – 150 mm and fine screens have clear openings less than 6 mm. In addition, there are micro screens with openings less than 50 μm which are used principally in removing fine solids from treated effluents.
The main role of screening is to remove coarse materials from influent wastewater in order to protect process equipment from damage, remove big solids that can reduce overall treatment process efficiency or remove coarse materials that can contaminate waterways.
Fine screens, besides the removal of waste materials, because of their openings size, can reduce parts of suspended solids from influent wastewater. Their effect of BOD5 reduction is 3 – 10 %, reduction in suspended solids 2 – 20 %, reduction of bacteria 10 – 20 % and reduction in the chemical oxygen demand is 5 – 10 %.
Coarse screens (bar rocks) can be hand – cleaned or mechanically cleaned. Hand – cleaned screens are usually used ahead of pumps in small wastewater pumping stations and sometimes ahead of small to medium sized wastewater treatment plants.
COARSE SOLIDS REDUCTION
Coarse solid reduction (Figure) is used as alternative to coarse bar screen or fine screens. The solids are cut up into the smaller parts and returned to the flow for subsequent removal by downstream treatment operation and processes. Smaller parts have more uniform size and particle size from 3 – 8 mm.
Most common used equipment for cutting up coarse solids is comminutors, macerators and grinders.
At the present, these processes are avoids because the separate solids are returned to water which increases foaming at the wastewater treatment plant.
Flow equalization is a process of collection influent wastewater into one big basin to equalize large fluctuations in the influent flow (quantity) during the day and achieve a constant or nearly constant flow rate. This process is very important for designing of wastewater treatment plant with industrial wastewater so in the harbour well designed equalizing basin is of great importance.
According to Metcalf & Eddy (2004) the principal benefits that are cited as deriving from application of flow equalization are:
MIXING AND FLOCCULATION
For a balanced operation of a wastewater treatment plant, the process of mixing is very important. Mixing as a unit operation is used in many phases of wastewater treatment. The most common use is for a complete mixing of two substances, blending of miscible liquids, continuous mixing of liquids suspension and flocculation of wastewater particles. Mixing devices are various. Usually, mixing devices are divided into horizontal gravity, vertical gravity and mechanical mixers (turbine and propeller mixers).
Flocculation is process of forming aggregates/flocs from finely divided particles and form chemically destabilized particles. Flocculation occurs immediately after mixing water with a coagulant. The smaller particles are formed into larger particles that can be easily removed from water by settling or filtration. The time required to obtain a flocculent size 0,5 – 0,6 mm is 10 – 30 minutes (Vukovi?, 1994).
Process of grit removal is accomplished in grit chambers which are designed to remove grit consisting of sand, gravel, cinders or other heavy solid materials that have subsiding velocities or specific gravities substantially greater than those of the organic putrescible solids in wastewater (Metcalf and Eddy, 2004). The purpose of grit chamber is protection of moving mechanical equipment from abrasion and accompanying abnormal wear, reduction of formation of heavy deposits in pipelines, channels and conduits and reducing the frequency of digester cleaning caused by excessive accumulations of grit. The three types of grit chambers usually used for grit removal are: horizontal – flow, rectangular or a square horizontal – flow and aerated or vortex type.
Gravity separation is the most widely used unit operation in wastewater treatment. Particles heavier than water are separated by gravitational settling. That process is called sedimentation. Sedimentation is used for the removal of grit, total suspended solids (TSS), removal of flocks in the activated – sludge settling basin and chemical flock removal. The primary purpose of gravity separation is clarification of effluent.
Primary sedimentation is used for removing from 40 to 70 percent of the suspended solids, from 25 to 40 percent of BOD5, reducing COD from 20 to 35 percent and reducing the amount of bacteria’s from 25 – 75 percent. The effect of purification in sedimentation tanks depends on the time of retention of wastewater in tanks. In addition, sedimentation tanks have also been used as storm water retention tanks. The most used sedimentation tanks nowadays are mechanically cleaned sedimentation tanks of standardized circular or rectangular design, but the selection of the type of sedimentation tank is based on the size of the installation, rules and regulations of local control authorities, local site condition etc.
Secondary sedimentation is most often used for clarifying of water, in which can be present sludge, after the end of biological wastewater treatment. Secondary sedimentation is often the last stage of the secondary treatment. The effect of purification in sedimentation tanks depends on the time of retention of wastewater in the tank. Secondary precipitators usually have circular layout.
Flotation is unit operation used to separate solid or liquid particles from wastewater by introducing fine gas (usually air) bubbles into the wastewater. With flotation suspended matter from wastewater is removed and bio solids are concentrated. The process of flotation is faster than the process of sedimentation because very small or light particles that settle slowly can be removed more completely and in a shorter time with greater gas bubbles which cause the small particle to rise to the surface. Efficiency of flotation with gas bubbles is up to 98 percent (Vukovi?, 1994).
Biological wastewater treatment is based on the activity of bacteria and other microorganisms that remove contaminants from wastewater by assimilating them. Microorganisms use soil organic matter as a food for the production of new cells (reproduction). As a byproduct of biological processes, gases and non – biodegradable parts are produced. Choosing the right system which will be used depends primarily on the amount of dissolved oxygen in the effluent and on condition in microorganism natural habitat but is also very important to understand various techniques available and evaluate them based on your requirements.
In addition when considering biological wastewater treatment for a particular application, it is important to understand the sources of the generated wastewater, typical wastewater composition, discharge requirements, events and practices within a facility that can affect the quantity and quality of the wastewater (Schultz, 2005). Consideration of these factors will allow maximizing the benefits of biological wastewater treatment plants. According to Schultz (2005) those benefits can include:
There are three basic categories of biological wastewater treatment:
The aerobic biological treatment involves contacting wastewater with microbes and oxygen in a reactor to optimize the growth and efficiency of the biomass. Therefore, aerobic treatment processes take place in presence of air and utilize those microorganism (also called aerobes), which use free oxygen to convert them in to carbon dioxide, water and biomass.
The anaerobic biological treatment processes take place in the absence of air by those microorganisms (also called anaerobes) which do not require air to assimilate organic impurities. The final products of organic assimilation in anaerobic treatment are methane and carbon dioxide gas and biomass (Mittal, 2011).
Movement of carbon and energy in aerobic (above) and anaerobic (below) wastewater treatment
The three individual types of biological treatment can be run together in combination or in sequence to offer better results of treatment. The key of an effective biological treatment is healthy biomass, sufficient quantity to handle maximum flows and the maximum organic load which can be found in wastewater treatment plants.
Therefore, there are different types of unit processes considering the way of maintenance of microorganisms in a biological wastewater treatment process.
Aeration systems are various so for the easiest understanding Table 16 was prepared where used systems and their applications are described. The systems used depend on the function to be performed, type and geometry of the reactor, and cost to install and operate the system.
AERATED LAGOONS (AERATED BASINS)
In aerated lagoons, oxygen is supplied mainly through mechanical aeration rather than by algal photosynthesis. Usually, three processes are used: diffused aeration, jet aeration and surface aeration.
Diffused aerators add air to wastewater, increasing dissolved oxygen content and supplying microorganism with oxygen necessary for aerobic biological treatment. Fine – bubble diffused aeration systems are available in various types including ceramic and membranous, and are highly efficient (Schultz, 2005).
The jet – aeration system is designed to provide required aeration as well as maintain suspension of biological solids, with the flexibility to either aerate or mix independently without the need for additional equipment. Air – flow rates in the system can be varied. When aeration requirements decrease and air is completely shut off, pumps provide the required mixing action to enhance process control and save energy. The subsurface discharge leads to smooth and quiet operation, with no misting, splashing or spray from the basin (Schultz, 2005).
High and low – speed floating aerators provide pumping action that transfers oxygen by breaking up the wastewater into a spray of droplets which provide efficient surface aeration. The large surface area of the spray allows oxygen to enter the wastewater from the atmosphere. At the same time, the oxygen – enriched water is dispersed and mixed, resulting in effective oxygen delivery. High and low – speed surface aerators offer excellent oxygen transfer and low operating cost. They are able to handle environmental extremes such as high temperatures (Schultz, 2005).
Another alternative for surface aeration is the use of horizontally mounted aeration discs or rotors (Figure 15). These disc or rotor aerators can be used in oxidation ditches known as looped, “race track” reactor configuration. They provide stable operation with resulting high – quality effluent. The aerators are above water for easy maintenance and energy efficient.
AERATION DISCS/ROTORS (EVOQUA WATER TECHNOLOGIES)
The area dimensions of lagoons are not generally critical, configured to appropriate dimension to fit available land area. Depths can range from 2 to 6 m, governed by required retention time, selected surface area, and the aeration device limits. Treating high influent biological oxygen demand (BOD) concentration is difficult because of the low microorganisms’ concentration. The design of an aerated lagoon involves most of the activated sludge design considerations governing (Celenca, 2000):
Activated sludge process is a common method of aerobic wastewater treatment. The purpose of the process is to reduce amounts of dissolved organic matter from wastewater, using microorganisms growing in aeration tanks. Microorganisms convert dissolved organic matter into their own biomass, oxidizing carbonaceous matter, oxidizing nitrogenous matter and removing phosphates. The formed semi – liquid material (a community of microorganisms grouped in flocs) is then separated. The treated wastewater runs over the edges of secondary clarifiers. A part of the settled sludge is being returned into aeration tanks, where it is mixed with "fresh" primary treated wastewater and the bio oxidation process goes on (Rech, 2008).
Activated sludge control is based on monitoring sludge blanket level SVI (Sludge Volume Index), MCRT (Mean Cell Residence Time), F/M (Food/Mass ratio). Some guidelines that can be applied to a conventional activated sludge plant are:
The basic indicator of normal plant operation is the quality of the plant effluent. Failure of plant efficiency may be due to either of the two most common problems encountered in the operation of an activated sludge plant, namely, rising sludge and bulking sludge. For an activated sludge process to achieve optimum plant efficiency the final clarification unit must effectively separate the biological solids from the mix liquor. If these solids are not separated properly and removed from the clarifier in a relatively short period of time, operating problems will result, causing an increased load on the receiving waters and a decline in plant efficiency. The most important function of the final clarifier is to maintain the wastewater quality produced by the preceding processes (Rech, 2008).
MEMBRANE BIOREACTORS (MBR)
Membrane bio-reactor systems (MBR) are the combination of a membrane process like microfiltration or ultrafiltration with a suspended growth bio-reactor. They are unique processes, which combine anoxic and aerobic biological treatments with an integrated membrane system that can be used with most suspended growth biological wastewater treatment systems. Before entering the MBR system, wastewater is screened in order to retain larger particles which can easily clog the membrane. Aeration within the aerobic reactor zone provides oxygen for biological respiration and maintains solids in suspension. To retain active biomass in the process, the MBR relies on submerged membranes rather than clarifiers, eliminating sludge settling issues which allows the biological process to operate longer than normal (according to Schultz, 2004, for MBR that period is 20 – 100 days) and to increase mixed liquor, suspends solids concentration for more effective removal of pollutants (Schultz, 2005). Proper maintenance (cleaning of membranes) is a key component to longevity of a membrane bio-reactor system.
MBR MAINTENANCE (HAZEN & SAWYER)
SEQUENCING BATCH REACTORS (SBR)
The sequencing batch reactor (SBR) is a variation of the conventional activated – sludge system where a clarifier is used to settle and cycle biomass back to an aeration basin. The SBR is a fill – and – draw, non – steady – state, activated – sludge process in which one or more reactor basins are filled with wastewater during a discrete time period and then operated in batch mode (Schultz, 2005). In a single reactor basins (Figure 19), the SBR accomplished equalization, aeration and clarification in a timed sequence. Depending on desired treatment objectives, the SBR can be operated in aerobic, anoxic or anaerobic conditions to encourage the growth of desirable microorganisms. One of the advantages of SBRs is good operability in winter, making them well suited for installations in colder climates. SBRs also take up little space because all of the treatment steps take place in a single reactor basin (Schultz, 2005).
SBR PROCESS PHASES
The term “pH” is used to indicate the degree of acidity of aqueous solutions. It is defined as:
pH = –log10[H+]
Where [H+] is the hydrogen ion concentration.
The pH adjustment process is also known as the process of neutralization and is related to the removal of excess acidity or alkalinity by treatment with a chemical of the opposite composition. All treated water, before being discharging into environment, will require neutralization. For pH adjustments there are number of chemicals that can be used and the choice will depend on the suitability of a given chemical or economic cost for using a certain chemical. In small wastewater treatment devices specialized for removing heavy metals from wastewater produced in washing areas in marines and harbours in the incoming wastewater (pH value around 7.0) a precise quantity of the reagent (acid – coagulant ) which lowers the pH value and thereby improves the deposition of particles containing heavy metals from wastewater is added. In the next step the water, with a reduced pH value, goes into the next tank where the next reagent (alkali) is added, which raises the pH value above 7.0 (ie. optimal for deposition of remaining metals in the water) which creates floes (flakes) and improves the process of purification. Water inside tanks (Figure 21) is mixed with mechanical mixers so the water quality in tanks is completely the same in every part of the tank. The whole process of adding reagents into tanks is fully automated and controlled from a single electrical cabinet, where the time and dosage of particular reagent added in certain container with wastewater is set, which makes the process easier, faster and cheaper. The numbers of different heavy metals, which can be found in the wastewaters from the washing area, with various curves of settling their hydroxide, selected pH values in each tank, for final processing, are present in the function of all heavy metals. Dosage of chemicals in quantities greater than optimal elevation can affect the output value of the COD, but also increase the prices of the device.
The definition of oxidation proposed by Weber (1972) as a process in which the oxidation state of a substance is increased is misleading, since the notion of oxidation state is only applicable to individual atoms within molecules, and not to “substance”. In wastewater treatment, chemical oxidation involves the use of oxidizing agents such as ozone (O3), hydrogen peroxide (H2O2), permanganate (MnO4), chloride dioxide (ClO2), chlorine (Cl2) or (HOCl) and oxygen (O2) to bring about change in the chemical composition of a compound or a group of compounds. Chemical oxidation, in wastewater treatment, is used for reducing the bacterial and viral content, removing ammonia, control of odors, reducing the concentration of residual organics, improving the treatability of non-bio-degradable organic compounds, BOD reduction, grease removal etc.
According to Eilbeck & Mattock (1987) reduction is conversely defined as the removal of oxygen or other electronegative elements by adding of hydrogen or electro – positive elements or, more generally, as a gain of electrons. Oxidation and reduction are complementary processes and chemical oxidation is impossible without a chemical reduction. Common reductases are sulphur dioxide (SO2) and its salts, sodium dithionite (Na2S2O4), ferrous iron (Fe2), metals (iron, zinc, aluminum ), hydrazine (N2H4), sodium borohydride (NaBH4), hydrogen peroxide (H2O2) etc.
CHEMICAL PRECIPITATION AND FLOCCULATION
Chemical precipitation converts soluble metallic ions and certain anions to insoluble forms, which precipitate from solution. Usually chemical precipitation is used to remove metal compounds from wastewater. The precipitated metals are subsequently removed from wastewater stream by liquid filtration or clarification. Chemical precipitation is a two – step process. In the first step, precipitants are mixed with the wastewater by mechanical devices such as mixers. The detention time depends on the type of wastewater which is being treated, the treatment chemicals used and the desired effluent quality. In the second step, the precipitated metals are removed from the wastewater by processes of filtration or clarification. Chemicals that are usually for chemical precipitation are sodium hydroxide, soda ash, sodium sulfide…
Flocculation is the stirring or agitation of chemically – treated water to induce coagulation. More specifically flocculation is the agglomeration of the destabilized particles by chemical joining and bridging. Flocculation enhances sedimentation of filtration treatment system performance by increasing particle size which results in increased settling rates and filter capture rates.
An emulsion is a disperse system in which both the disperse phase and his dispersion medium are liquids, the two being essentially immiscible. Emulsions all have hydrophilic and hydrophobic character, and generally are stabile only in the presence of a third substance, an emulsifier. Emulsifiers are divided into classes differentiated according to their chemical nature. According to Eilbeck & Mattock (1987) classes are as follow:
CHARACTERIZATION OF THE WASTEWATER TREATMENT ONBOARD
Sewage can be treated with three principal methods: mechanical, chemical and biological. According to Koboevi? & Kurtela the sewage treatment is usually a combination of the three principal methods, such as mechanical – chemical, mechanical – biological and chemical – biological. The treatment of sewage includes the following stages:
The term “marine sanitation device” (MSD) means equipment for installation onboard a vessel which is designed to receive, retain, treat or discharge sewage, and any process to treat such sewage (US environmental protection agency, 2008).
Waste water treatment for harbours is a very multifaceted question. The wastewater is of various kinds, it has different characteristics, and different quantities. Since we are talking about wastewater from the harbours, the main characterization of the wastewater from harbors is that
1. Wastewater that is produces on land in the facilities of the harbour.
2. The wastewater that is produced when the ship is in the sea and the wastewater is sent into the vessels.
On land, the harbour facilities produce wastewater for example; offices, canteens, etc. in shipyard, washing areas for vessels, etc. On the sea wastewater is generally black (sewage) or gray water from ships but there can also be wastewater from washing the holding tanks or from washing transport tank from different types of cargos.
Usually, harbour now a day’s don’t have the waste water treatment plants installed in them. Generally, trucks or barges are provided to transport wastewater from the vessels towards the wastewater treatment plants outside the harbours. This leads to high prices of disposal and transport off course.
The better solution to this problem is that the wastewater treatment plants should be constructed inside the harbour in order to prevent the potential environmental problems that can occur during the transport of the wastewater to the wastewater treatment plant outside the harbour or a problem can occur during the discharge of the wastewater from the vessels into the sea without the procedure of treating the wastewater.
Types of treatment in WWTP inside harbours depends on the quality of incoming wastewater, but because of possible high concentrations of chloride or high concentrations of heavy metals in the wastewater, it is safer to design and construct chemical treatment plants rather than biological (in order to avoid the complete destruction of biomass (bacteria) by toxic compounds that is necessary for wastewater treatment). WWTP designed in such a way that can accept wastewater from narrow coastal areas of cities which cannot be treated in the municipal biological WWTP because of higher concentration of chlorides, as well as wastewater that can be toxic (wastewater from washing holding tanks with a toxic cargo from cargo ships).
As an example, the case of the Port of Lisbon was studied, where different types of ships were visited and information about wastewater treatment on-board was collected. Problems that the port authority has to deal with during the process of accepting wastewater from ships (pumping), transportation to the WWTP and the whole process of organizing transport of wastewater was highlighted. In addition, for the Port of Lisbon a possible solution was given – a conception design of two smaller wastewater treatment plants – that can be constructed inside the harbour, one on each side of the Tagus River. For the south part of the Lisbon port the preferred choice would be a physical – chemical wastewater treatment plant with special emphasis on removing oil, grease and heavy metals from wastewater and on the northern part of the Tagus River the wastewater treatment plant could be a physical – biological, which should be designed to reduce the chlorides to required levels (processes of desalination, demineralization, coagulation, precipitation, electro dialysis, reverse osmosis etc.) or can be a physical – chemical WWTP just like the one on the south side of the river.
Therefore, we can conclude by saying that the disposal of the wastewater from the harbours is a very complex procedure which should be done in a short amount of time not making any room for mistakes to happen. It is very difficult to conclude that any of the type of WWTP is better than the other. Every WWTP plant got its pros & cons. The choice of which WWTP is to be construct depend on the society, the discharge requirements, quality and quantity of wastewater, environmental sensibility, and the most important, costs that port authorities are ready to incur. Construction of wastewater treatment plants is expensive and a very delicate project but can solve many problems that port authorities have to deal with during the transportation of wastewater from harbours to a possibly very distant WWTP and can prevent possible environmental disasters. A wastewater treatment plant inside the harbour area are basic necessities for them which are very distant from the open sea or for the harbour which spend a bundle of money on the transportation of the wastewater to the industrial WWTP, which are very far from the harbours.
|Doc||Complete Document IN MS -Word about the topic|