THEHAZARDSAND REGULATI ONSREGARDI NG THESEWAGESYSTEMS
Raw 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 dioide using up !ygen in the process. "hese compounds and Carbon dioide promote plant life which returns oygen to the water
When the quantity of !ygen becomes so depleted that the aerobic bacteria can no longer function# anaerobic or bacteria not requiring !ygen to function will take over. "he breakdown of the sludge is then associated with the same process of decay with foul smelling and dangerous gasses being produced. "herefore the principal means of sludge conditioning on board is that of aerobic action.
Regulations.
$nne I% of &$R'!( )*+), -I&! regulates the disposal of waste from ships internationally. In addition certain countries have their own national and regional controls.
In general this means that untreated sewage can only be dumped outside /0 miles offshore# and treated disinfected waste outside 1 miles.
$merican legislation defines three types of sewage treatment units.
Type 1
$ device capable of discharging effluent having no floating solids and a coliform count of less than /222 per /22ml of effluent.
Type II
$ device capable of discharging effluent with suspended solids not in ecess of /32mg+litre and a coliform count of less than 022 per /22ml
Type III
$ device to prevent the discharge overboard of treated or untreated waste.
%entilation systems are to be kept independent of other vents $ log is to be kept of any discharge overboard from a holding tank
AEROBI C( BI OLOGI CAL)TREATMENTPLANT( FLOW THR OUGH SYSTEM)
PRI NCI PLE
4iological system require a steady and relatively constant flow of solid sewage so the bacteria can eist in sufficient quantity to maintain effluent discharge at the correct quality. sludge build up is a possible problem although etended residence in the aeration chamber greatly reduces the amount. 5or eample# sewage with ,26 solid waste is reduced to 026 of its original weight after /0 hours in the aeration tank.
"he process of aerobicity strips oygen from the water and creates more water# carbon dioide and bacteria.
OPERATI ON
"he "rident 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. "he aeration chamber is where the main biological action takes place. Here air blowers mounted on the outside of the unit oygenate and stir the effluent and bacteria mi via a series of pipes and no77les. "he 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. $ second transfer pipe scum8s the surface of the settling tank and returns it back to the aeration chamber. "his returned sludge contains the bacteria to digest the incoming sewage. "hus the importance of this floc return can be seen
9ffluent passing over from this chamber should be clean and ready for disinfecting in the chlorinating chamber. "he level in this chamber is controlled by a pump and float switch arrangement. typical chlorine levels at discharge is 3ppm.
%alves are fitted to the aeration and primary chambers to allow them to be pumped out and back flushed as necessary.
"he 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. "here are special 8feeds8 which promote the reestablishment of these colonies.
PHYSI CALCHEMI CALSEWAGESYSTEM
"his is based on the separation of the liquid element from the sewage flow. "his is disinfected in a 36 chlorine for *2 minutes to kill off coliform bacteria and then discharged overboard in full &$R'!( compliance.
!ne problem with this system is the required space# !nly 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.
"here 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.
ELECTROCATALYTI COXI DATI ON
VACUUM SEWAGESYSTEMS
ADVANTAGESAND DI SADVANTAGES
%ery little flushing water is required and the volume of sewage dealt with can be much reduced with the downsi7ing of relevant equipment and cost saving."his has made them very popular for passenger vessels.(loyds regulations state that the capacity of a sewage system for flushing water with conventional plant is //3 litres+ person+ day and /3 litres for vacuum systems. "he main disadvantage is blockage due to drying and crystalisation of urea. !ver a period of time this can be so severe as to completely close the pipes. Chemicals are on the market which can be added in very small doses which help remove and prevent this deposits but there success is not guaranteed. In the event of vacuum failure a method must be in place to prevent dangerous gasses passing back into the accommodation
OPERATI ONALASPECTS
!nly approved toilet cleaning agents should be used# the use of ecessive 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. "he dangers of these gasses should be eplained to all crew.
Suspended solids
"he quantity of solid waste in the effluent is weighed. $fter drying on an asbestos mat filter element.
BI OLOGI CALOXYGEN DEMAND ( B. O. D. )
$erobic bacteria use !ygen in the process of breaking down the sewage. $t the end of the process the action of the bacteria reduces and so does the !ygen demand. "he effectiveness of a sewage treatment plant may be gauged by taking a one litre sample and incubating it for 3 days at 02oC. "he amount of !ygen consumed in milligrams per litre or ppm is termed the 4.!.;.
COLI FORM COUNT
It is possible that the effluent contain bacteria and viruses ha7ardous to health if it has not been properly treated at the final stage. $n indication of this is a count of the Coliform bacteria which are found in the intestine.
$ coliform count in a /22ml sample incubated for 1, hrs at *3oC. $nother test at the same temperature but over a 01 hour period produces a colony of bacteria.