5+Sewage+systems船用生活污水处理系统.doc

The Hazards and regulations regarding the Sewage SystemsRaw sewage discharged into restricted waters will eventually overwhelm the self purification ability of the limited quantity of water. In a closed dock the effect can be seen in a black sludgy water which when disturbed gives off an unpleasant smell possibly Hydrogen Sulphide. When the quantity of sludge is reasonable aerobic bacteria digest the sewage breaking it down to simple compounds and Carbon dioxide using up Oxygen in the process. These compounds and Carbon dioxide promote plant life which returns oxygen to the water. When the quantity of Oxygen becomes so depleted that the aerobic bacteria can no longer function, anaerobic or bacteria not requiring Oxygen to function will take over. The breakdown of the sludge is then associated with the same process of decay with foul smelling and dangerous gasses being produced. Therefore the principal means of sludge conditioning on board is that of aerobic action, Types of sewage disposal There are four main types of sewage disposal systems fitted to ships; a. Discharge from the toilet bowl into a common drain leading to overboard via storm valves b. As above except common drain leads to a storage tank with or without aeration. Contents discharged ashore or at sea when appropriate. c. Sewage treatment systems with sewage being collected and treated to produce an effluent suitable to discharge without effect on environment. d. Vacuum collecting system where the drains are kept at a slightly negative absolute pressure , on flushing water, sewage and air are drawn into the drains being led to a collecting or treatment tank which is kept at atmospheric pressure. Aerobic and anaerobic bacterial actionWhen the sewage enters the drainage system it is acted upon by aerobic bacteria and is broken down, during this process the naturally occurring Aerobic Bacteria strip the water of oxygen and produce; more water, Carbon Dioxide, and more bacteria. If, however, there is insufficient oxygen for these bacteria then alternative bacteria dominates. These Anaerobic Bacteria produce Hydrogen Sulphide, Methane and Ammonia. These gasses are either highly toxic or flammable or both. In particular Hydrogen Sulphide is toxic to humans in concentrations down to 10ppm and its flammable vapours are heavier than air so may build up in lethal pockets in enclosed spaces. Safety ParametersThe generation by anaerobic bacteria these toxic and flammable gasses is present in all types of systems to some degree. The possibility of anaerobic action within a sewage treatment plant should be reduced as far as possible. Should these gasses be generated and allowed to enter the accommodation could lead to disaster. The following are some methods which may help to reduce the risks; The fitting of proper ventilation in toilet spaces and the fitting of water traps can only be seen as secondary measures to reducing the risk. The primary concern is to eliminate the possibility of generating the gasses in the first place. Where sewage is stored in tanks for discharge, some method of maintaining an adequate level of oxygen in the water must be in place. Examples of these may be by direct air injection or by air entraining into the liquid whilst pumping through a nozzle. Where active aeration is not fitted then the contents of the storage tank should be changed within a maximum of a 24 hour period unless some other means of treatment is used. The conditions in the tank should be closely monitored Where aerobic treatment plants are used then manufacturers operating instructions should be closely adhered to. A system of maintenance should be in place. Maintenance of Aerobic treatment units.1. Thorough , regular cleaning and inspection with particular attention being paid to areas behind internal division plates 2. Checks on alarms and trips 3. Checks on aeration equipment 4. Checks on transfer systems in the tanks It is recommended that a low air pressure switch rather than a motor failed alarm be fitted to the air blower motor hence eliminating the danger of the fan belts snapping and going undetected. Tank Ventilation arrangements.Ventilation pipes should be in good condition and free from obstructions. They should be of a size to minimise pressure drop and ensure good gas clearance. They should be self draining to prevent blockage by water. Any flame gauzes or other fittings should be checked for cleanliness. Toilets, showers, washbasins, etc.The condition of drainage pipes should be checked regularly, as should the operation of the water seal or other fitted arrangements to prevent the back flow of gasses. Accommodation ventilation arrangementsThe ventilation should be sufficient to ensure proper balance allowing each compartment to be correctly supplied. The ventilation system should be correctly maintained and checked for cleanliness. Air extraction is of vital importance and the cleanliness of grills should be checked, the opening under doors should not be blocked, vent louvers should be correctly position to ensure all spaces are properly vented. The forced ventilation equipment should be regularly checked and maintained. Operational aspectsOnly approved toilet cleaning agents should be used, the use of excessive quantities of bleach should be avoided as this may kill the bacteria. . Complaints of foul or musty smells should be dealt with immediately as these may indicate anaerobic action. The dangers of these gasses should be explained to all crew. Suspended solidsThe quantity of solid waste in the effluent is weighed. After drying on an asbestos mat filter element. Biological Oxygen demand (B.O.D.)Aerobic bacteria use Oxygen in the process of breaking down the sewage. At the end of the process the action of the bacteria reduces and so does the Oxygen demand. The effectiveness of a sewage treatment plant may be gauged by taking a one litre sample and incubating it for 5 days at 20oC. The amount of Oxygen consumed in milligrams per litre or ppm is termed the B.O.D. Coliform countIt is possible that the effluent contain bacteria and viruses hazardous to health if it has not been properly treated at the final stage. An indication of this is a count of the Coliform bacteria which are found in the intestine. A coliform count in a 100ml sample incubated for 48 hrs at 35oC. Another test at the same temperature but over a 24 hour period produces a colony of bacteria. Regulations 1. Subject to the provision that discharge is not required for the safety of the vessel, the discharge of sewage into the sea is prohibited, except when: a. the ship is discharging comminuted and disinfected sewage using a system approved by the Administration in accordance with regulation at a distance of more than four nautical miles from the nearest land, or sewage which is not comminuted or disinfected at a distance of more than 12 nautical miles from the nearest land, provided that in any case, the sewage that has been stored in holding tanks shall not be discharged instantaneously but at a moderate rate when the ship is en route and proceeding at not less than 4 knots; the rate of discharge shall be approved by the Administration based upon standards developed by the Organization; or b. the ship has in operation an approved sewage treatment plant which has been certified by the Administration to meet the operational requirements referred to in regulation of this Annex, and i. the test results of the plant are laid down in the ships International Sewage Pollution Prevention Certificate (1973); ii. additionally, the effluent shall not produce visible floating solids in, nor cause discoloration of, the surrounding water; or c. the ship is situated in the waters under the jurisdiction of a State and is discharging sewage in accordance with such less stringent requirements as may be imposed by such State. 2. When the sewage is mixed with wastes or waste water having different discharge requirements, the more stringent requirements shall apply. Sewage systemsRegulations. Legislation preventing the discharge of untreated waste overboard has been in place for some time with a requirement that it should be retrofitted where not already in use. American legislation defines three types of sewage treatment units. Type 1 A device capable of discharging effluent having no floating solids and a coliform count of less than 1000 per 100ml of effluent. Type II A device capable of discharging effluent with suspended solids not in excess of 150mg/litre and a coliform count of less than 200 per 100ml Type III A device to prevent the discharge overboard of treated or untreated waste. Ventilation systems are to be kept independent of other vents A log is to be kept of any discharge overboard from a holding tank Aerobic (Biological) Treatment plant (Flow through system)PrincipleBiological system require a steady and relatively constant flow of solid sewage so the bacteria can exist in sufficient quantity to maintain effluent discharge at the correct quality. sludge build up is a possible problem although extended residence in the aeration chamber greatly reduces the amount. For example, sewage with 80% solid waste is reduced to 20% of its original weight after 12 hours in the aeration tank. The process of aerobicity strips oxygen from the water and creates more water, carbon dioxide and bacteria. OperationThe Trident sewage treatment unit shown above consists of three chambers.Sewage enters the aeration chamber via a coarse mesh filter where large solids are broken down. The aeration chamber is where the main biological action takes place. Here air blowers mounted on the outside of the unit oxygenate and stir the effluent and bacteria mix via a series of pipes and nozzles. The sewage remains in this aeration tank for some time. Incoming sewage displaces some effluent of the settling tank (or hopper) where under inactive conditions biological floc, activated sludge and bacteria, settle out and is returned to the aeration chamber via air lift pumps also driven by the blowers. A second transfer pipe scums the surface of the settling tank and returns it back to the aeration chamber. This returned sludge contains the bacteria to digest the incoming sewage. Thus the importance of this floc return can be seen Authors note:This is a common question in orals Effluent passing over from this chamber should be clean and ready for disinfecting in the chlorinating chamber. The level in this chamber is controlled by a pump and float switch arrangement. typical chlorine levels at discharge is 5ppm. Valves are fitted to the aeration and primary chambers to allow them to be pumped out and back flushed as necessary. The bacteria are susceptible to water conditions including temperature and the presence of toilet cleaning agents. In this way the system is fitted with by-pass valves so passing contaminated water overboard. Should the bacteria be killed it takes some time before a new colony forms. There are special feeds which promote the reestablishment of these colonies. Physical-Chemical Sewage systemThis is based on the separation of the liquid element from the sewage flow. This is disinfected in a 5% chlorine for 30 minutes to kill off coliform bacteria and then discharged overboard in full MARPOL compliance. One problem with this system is the required space, Only a finite amount of space can be set aside for the storage of the solid part of the waste which can only be discharged in port or outside territorial waters when allowed. If these facilities are unavailable the system become inoperative. There is also the need to carry quantities of Calcium Hypochlorite for conversion to Sodium HypoChlorite for the disinfection of sewage flow. Calcium Hypochlorite requires very careful handling. Electrocatalytic OxidationSewage is collected, macerated and passed through a electrolytic cell. Electrolysis produces Sodium Hypochlorite which is used to oxidise organic material before discharge. Alternately dosing by chlorine may be used. The effluent passes on through to a settling tank were the oxidation process is completed These type of plants can be 50% smaller than biological types, this and the fact that pass through times are extremely short-typically 30 minutes compared to the several hours of the biological unit- are the main advantages of this system.The discharge contains no solids and is totally free of coliform bacteria. A disadvantage of this system is due to the short exposure time in the oxidiser relatively high levels of chlorine are required to ensure destruction of the coliform bacteria. It is possible that this chlorine level can be present to some degree in the discharge. Dechlorination plant may be fitted Vacuum sewage systemsShown is a simple layout for a vacuum sewage system. OperationLiquid flows from the aeration tank of an aerobic sewage tank to a coarse impeller centrifugal pump. This delivers the liquid under pressure via an eductor and back to the tank. The eductor reduces the pressure in the sewage system pipework to a set point after which the pump is stopped. When the pressure in the pipework rises above a set value it is res