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2016년 3월 21일 월요일

Court orders EPA to revise ship ballast dumping regulations

TRAVERSE CITY, Mich. -A federal appeals court ordered the government Monday to rewrite its regulations on ballast water discharges from ships, one of the leading culprits in the spread of invasive species across U.S. waterways.Environmental groups contended in a lawsuit that an industry-wide permit issued by the U.S. Environmental Protection Agency two years ago wasn’t tough enough to prevent vessels from introducing additional harmful organisms such as zebra and quagga mussels, which have caused heavy economic and ecological damage in the Great Lakes and spread as far as the West Coast.
The 2nd U.S. Circuit Court of Appeals sided mostly with the environmentalists, saying the EPA erred in numerous ways, including settling for international limits on live organisms in ballast water when technology was available to meet tougher standards.The court also faulted the agency for failing to consider onshore treatment of ballast water, exempting vessels built before 2009 that operate only in the Great Lakes from the discharge limits, and requiring inadequate monitoring of discarded water to make sure it complies with the rules.
“This decision is welcome news for the millions of families, anglers, hunters, paddlers, beach-goers, and business owners, who have borne the brunt of damages from aquatic invasive species for far too long,” said Marc Smith, policy director for the National Wildlife Federation, one of the groups that had sued.
The EPA referred a request for comment to the U.S. Department of Justice, where spokesman Wyn Hornbuckle said the decision was under review.
Ships take on ballast water to maintain stability in rough seas, or as cargo is loaded and unloaded. Water sucked into ballast tanks while vessels are in port can habor fish, mussels, aquatic plants and other organisms, which are hauled to distant locations and released. More than 21 billion gallons of ballast water are dumped in the U.S. each year.
Some of the organisms have no natural predators in their new environments, allowing them to multiply rapidly, out-compete native species for food and habitat, spread disease and destabilize ecosystems.
Ballast was long exempt from regulation under the Clean Water Act, a policy the EPA abandoned in response to environmentalists’ lawsuits.
Its permit issued in 2013 put ceilings on the concentration of live organisms in ballast water using standards proposed by the International Maritime Organization and adopted by the U.S. Coast Guard in 2012. Additionally, it required transoceanic commercial vessels to exchange their ballast water 200 miles from the U.S. shoreline or rinse their tanks if empty, in hopes that the salty water would kill any freshwater organisms left behind.
In its 3-0 ruling Monday, a panel of the New York-based appeals court agreed with the National Resources Defense Council, Northwest Environmental Advocates, the Center for Biological Diversity and the National Wildlife Federation that the EPA “acted arbitrarily and capriciously” in crafting the permit.
Instead of adopting the international standards, the agency could have based its live-organism limits on what the best available technology could achieve, the judges said. Systems for killing creatures in ballast water have been devised using methods such as filtration, removing oxygen from the water, zapping it with ultraviolet light or adding chlorine.
The EPA’s Science Advisory Board found that while no system existed for completely sterilizing ballast water, those technologies potentially could be lethal enough to meet standards 10 times stronger than the international ones, the judges said.
On another issue, the EPA said it didn’t consider using onshore facilities such as sewage and drinking water treatment plants for cleansing ballast water because it knew of none capable of meeting the discharge standards. But the judges said the agency had “turned a blind eye” to the possibility and had discouraged the science board from exploring it.
The board’s report said the use of onshore facilities “appears to be technically feasible” and could have numerous advantages over shipboard systems, the judges said.
The appeals panel said the existing permit will remain in place until the EPA produces a new one. The panel set no deadline, but the existing permit expires in 2018

Mercy Ships selects Evac waste system

Waste technology expert Evac has been selected to supply its total waste management system to the world’s largest newbuild civilian hospital ship, the ‘Atlantic Mercy’.
Atlantic Mercy, being constructed by the China Shipbuilding Industry Corporation at the Tianjin Xingang Shipyard, will be owned and operated by the non-profit Mercy Ships, and used to provide medical care in the poorest parts of Africa.
Its waste management system will include 393 vacuum toilets, two vacuum units (type Evac OnlineMax 175), two sewage treatment plants (type Evac MBR 135K), one incinerator, a sludge handling system, a food waste vacuum collecting system, a converter for medical waste and a thermal steriliser for wastewater generated by the ship’s hospital area.
Vacuum toilets require only 1.2 litres of water per flush – six to seven times less than gravity toilets. This means water savings of 52 cubic metres per day for the ship which can carry 950 persons. Especially significant in Africa where pure water is a precious commodity.
Collected wastewater will be treated by two Evac MBR (Membrane Bioreactor) biological sewage treatment plants. The plant’s membranes work as mechanical barriers to any impurities and stop almost all bacteria and viruses.
The incinerator and equipment for dry waste handling will mean frequency of trips to shore for waste disposal will be decreased and environmental impact of waste disposal reduced. They will also reduce volume of dry waste negating the need for a large waste storage room and freeing up more room for medical equipment.
The package’s food waste vacuum collecting system is particularly beneficial in climates like Africa where temperatures can reach 45 degrees celsius. It requires a holding tank four times smaller than conventional systems and saves 3.3 cubic metres of water per day. Issues such as quick fermentation, smell, and contamination are virtually eliminated.
The vessel is scheduled for delivery in 2016.
Sorced by ekomeri.com

Basic Description of a Sewage Treatment Plant on Ship

Discharging of sewage in sea or territorial waters in banned as it can drastically affect the marine life. In case the sewage is to be discharged, first it has to be treated with the help of asewage treatment plant. STP is now mandatory on every ship, according to International legislature.
sewage-treatment-system
  • Introduction

    The first question you would probably ask is , “Why to use a ship sewage treatment plant if the waste is already biodegradable?”. The question definitely isn’t wrong but the problem is that not all sewage constituents are biodegradable and not all take the same time to break down. Also, untreated sewage has solid particles which take a lot of time to disintegrate. It is for this reason that a sewage treatment plant is used.
    Raw sewage in water needs oxygen to break down naturally. This sewage when disposed to the sea absorbs excessive oxygen and thus reduce the requisite amount of oxygenneeded by the fishes and marine plants. Bacterias present in the sewage produces hydrogen sulfide gas which produces acrid smell. Human waste has E. Coli bacterias which are found in the intestine. The amount of E.Coli bacteria in a water sample indicates the sewage content of that sample.
    Sewage treatment plants on ships are of two types:
    • Chemical sewage treatment plant
    • Biological sewage treatment plant
  • Chemical Sewage Treatment plant

    basic concept of sewage treatment systemA chemical treatment plant consists of a big storage tank which collects, treates and stores the sewage for discharging it to the sea or to a shore receiving facility. The sewage is first collected in a tank and the liquid content is reduced. This can be done by flushing water from wash basins and bathroom drains directly into the sea. The liquid from any other sources is treated chemically to get rid of the color and smell and then it is reused as flushing water in toilets. The chemicals that are used assist in the process of breaking the solid constituents and also in sterilization.
    A mechanical instrument known as comminutor is used to help breaking down of the solid particles to smaller ones. The liquid sewage remains at the top and the solid particles settle down, which are then discharged to a sullage tank. The liquid sewage is chemically treated and is used for toilet flushing purposes. The sewage from the sullage tank is discharged to the shore collecting facilities.
    It is important to supply adequate quantity of chemical dosages to prevent odour and corrosion due to high level of alkalinity.




  • Biological Sewage treatment plant

    Biological sewage treatment plant uses bacterias to facilitate the process of breaking down of solid constituents. It generates an oxygen rich atmosphere that aerobic bacterias utilizes to multiply and disintegrate the sewage waste to convert it into sludge. The treated sewage thus generated can be disposed off to any waters. The process that takes place inside the plant is known as aeration process.
    The whole plant is divided into three compartments namely,
    • aeration compartment
    • settling compartment
    • chlorine treatment compartment.
    basic concept of sewage treatment systemSewage enters the sewage treatment plant first through the aeration compartment. Aerobic bacterias digests the sewage waste and reduce it to small particles. A continuous supply of atmospheric oxygen is provided to increase the rate of digestion process.The disintegrated solid waste is then transferred to the settling compartment where the solid constituents settle down under the effect of gravity. The liquid at the top is then passed to the chlorine treatment compartment. In this compartment the liquid water is treated with chlorine and other chemicals to kill any surviving bacterias. Once done the water is then discharged into the sea. The process of chlorination is facilitated with the help of chlorine tablets. The sludge that settles down in the settling compartment is removed and stored in a storage tank to later discharge it to shore facilities or decontrolled areas.

Sewage From Ships in the BALTIC

In 2011 IMO designated the Baltic Sea as a “special area” for passenger ships in terms of MARPOL Annex IV (on sewage from ships). The coastal countries shall report to IMO (MEPC) that the sewage reception facilities in the Baltic Sea ports fulfill the criteria of adequacy, before the “special area” regulations will take effect on 1 January 2016, at the earliest.
The IMO decision to designate the Baltic Sea as an Annex IV “special area” in 2011 was based on a proposal by Baltic Sea coastal countries submitted in 201​0, developed as a follow up of a commitment included in the 2007 HELCOM Baltic Sea Action Plan (BSAP), as a measure to meet the country, and basin, specific nutrient pollution reduction goals.
The BSAP nutrient pollution goals are to be reached with measures taken within all relevant fields of human activity including agriculture, emissions to air from land and sea tr​affic as well as sewage, both from municipalities and industry facilities on land as well as from passenger ships.Baltic Sea Sewage Port Reception Facilities
In anticipation of the 2011 IMO decision the 2010 HELCOM Ministerial Meeting set up a Baltic Sea Cooperation Platform on sewage PRFs. The latter has during 2010-2013 involved the shipping industry, technology providers, ports and national authorities for discussions on outstanding issues around the improvement of sewage PRFs in the region. The outcomes have been reported to the HELCOM MARITIME Group where the competent national administrations of the Baltic Sea countries have provided their input.
By 2013 the work of this Cooperation Platform resulted in the document “HELCOM Interim Guidance on technical and operational aspects of sewage delivery to port reception facilities” which has been submitted to the HELCOM 2013 Ministerial Meeting for adoption. The Guidance outlines current best practices as well as outstanding issues in terms of PRF improvements.
In 2015, a report has been released to provide information on port reception facilities for sewage (PRFs) and their use by international cruise ships in the Baltic Sea area during 2014.
Cruise ships operating in the Baltic Sea, their length of sea voyages as well as frequency and duration of port visits are described in detail. Also the ports visited by cruise ships and the sewage facilities are covered in terms of facilities and traffic trends. The report is based on information from obligatory AIS (Automatic Identification System)​ position reports received from a comprehensive list of cruise ships operating in the region. It provides thus a nearly complete coverage of cruise ship movements during 2014.
Based on the analyses of ship movements, passenger capacity and port facilities, the new report helps also to clarify what the real needs of cruise traffic might be in terms of sewage management in the Baltic Sea cruise ports.

Sewage Treatment Plant on a Ship Explained

Discarding sewage produced onboard on a ship is one of the few tasks on a ship which should be taken utmost care of if one wants to same him and his shipping company from heavy fine. The sewage generated on the ship cannot be stored on the ship for a very long time and it for this reason it has to be discharged into the sea.
Though sewage can be discharged into the sea, we cannot discharge it directly overboard as there are some regulations regarding discharging of sewage that needs to be followed. Sewage on sea is generally the waste produced from toilets, urinals and WC scuppers. The rules say that the sewage can be discharged into the sea water only after it is treated and the distance of the ship is 4 nautical miles from the nearest land.
But if the sewage is not treated this can be discharged 12 nautical miles away from the nearest land. Also the discharged sewage should not produce any visible floating solids nor should it cause any discoloration of surrounding water.
Generally, ships prefer treating sewage before discharging to save themselves from any type of embarrassment. There are different methods of treating sewage available in the market, but the most common of them is the biological type for it occupies less space for holding tank, unlike those of the other methods. Moreover, the discharge generated from this plant is eco friendly. It is to not that each sewage treatment system installed onboard has to be certified by classification society and should perform as per their requirement and regulations. 
Working of a Biological Sewage Plant
The basic principle of the working of a biological treatment plant is decomposition of the raw sewage. This process is done by aerating the sewage chamber with fresh air. The aerobic bacteria survive on this fresh air and decompose the raw sewage which can be disposed off in the sea. Air is a very important criterion in the functioning of the biological sewage plant because if air is not present, it will lead to growth of anaerobic bacteria, which produces toxic gases that are hazardous to health.Also, after decomposition of the sewage with anaerobic bacteria, a dark black liquid causes discoloration of water which is not accepted for discharging. Thus in a biological sewage treatment plant the main aim is to maintain the flow of fresh air.
Chamber of Biological Sewage Plant
Division of Processes
The biological sewage plant is divides into three chambers:- 
Aeration chamber
 This chamber is fed with raw sewage which has been grinded to form small particles. The advantage of breaking sewage in small particles is that it increases the area and high number of bacteria can attack simultaneously to decompose the sewage. The sewage is decomposed into carbon dioxide, water and inorganic sewage. The air is forced through diffuser into the air chamber. The pressure of air flow also plays an important role in decomposition of the sewage. If pressure is kept high then the mixture of air and sewage will not take place properly and it will escape without doing any work required for decomposition. It is for this reason; controlled pressure is important inside the sewage treatment plant as this will help in proper mixing and decomposition by the agitation caused by air bubbles. Generally the pressure is kept around 0.3-0.4 bars. 
Settling tank
The mixture of liquid and sludge is passed to settling tank from the aeration chamber. In the settling tank the sludge settles at the bottom and clear liquid on the top. The sludge present at the bottom is not allowed to be kept inside the settling tank as this will lead to growth of anaerobic bacteria and foul gases will be produced.The sludge formed is recycled with the incoming sludge where it will mixes with the later and assist in the breakdown of sewage. 
Chlorination and Collection
In this chamber the clear liquid produced from the settling tank is over flown and the liquid is disinfected with the help of chlorine. This is done because of the presence of the e-coli bacteria present in the liquid. To reduce these bacteria to acceptable level chlorination is done. Moreover, to reduce the e-coli, the treated liquid is kept for a period of at least 60 minutes. In some plants disinfection is also done with the help of ultra violet radiation. The collected liquid is discharged to overboard or settling tank depending on the geological position of the ship. If the ship is in restricted or near coastline then the sewage will be discharged into the holding tank; otherwise, the sewage is discharged directly into the sea when high level is reached and is disposed automatically until low level switch activates.

The Hazards and regulations regarding the Sewage Systems

The Hazards and regulations regarding the Sewage Systems


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 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;
Discharge from the toilet bowl into a common drain leading to overboard via storm valves
As above except common drain leads to a storage tank with or without aeration. Contents discharged ashore or at sea when appropriate.
Sewage treatment systems with sewage being collected and treated to produce an effluent suitable to discharge without effect on environment.
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 action

Sewage Treatment Plant
SEWAGE TREATMENT PLANT of HOSEUNG Ent.Co.,Ltd
When 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 Parameters

The 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.

Thorough , regular cleaning and inspection with particular attention being paid to areas behind internal division plates.
Checks on alarms and trips
Checks on aeration equipment
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 gauze’s 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 arrangements

The 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 aspects

Only 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 solids

The 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 count

It 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

Annex IV of MARPOL 73/78 (IMO) 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 12 miles offshore, and treated disinfected waste outside 4 miles.
For further information see m-notice M.1548
  

Types of Sewage Treatment

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 Ⅰ  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)

biological sewage treatment system
sewage treatment system of HOSEUNG ent.Co.,LTD
SEWAGE TREATMENT SYSTEM of HOSEUNG Ent.Co.,Ltd















Principle
Biological system requires 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.
Operation
The 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 scum’s 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
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 system

This 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 becomes 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 Oxidation

Electrocatalytic Oxidation(sewage treatment system)
Sewage 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 systems

vacuumm sewage system

Operation
Liquid 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 restarted.
The pipework consists of a network of mainly pvc pipes connected into separate zones- typically by deck- and brought down to a common manifold via isolating valves. These valves allow work on sections of the system whilst still maintaining others in use.
The toilets are connect to the system via a vacuum operated foot valve. Vacuum timers are also fitted which allow measured quantities of flushing water to be applied.
Where toilets are connected in the same zone but exist at different heights non-returning valves may be fitted. In addition filter boxes may be fitted along with additional isolating valves to improve operation.

Advantages and disadvantages
Very little flushing water is required and the volume of sewage dealt with can be much reduced with the downsizing of relevant equipment and cost saving.This has made them very popular for passenger vessels.Lloyds regulations state that the capacity of a sewage system for flushing water with conventional plant is 115 litres/ person/ day and 15 litres for vacuum systems.
The main disadvantage is blockage due to drying and crystalisation of urea. Over 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 deposit but there success is not guarenteed.
In the event of vacuum failure a method must be in place to prevent dangerous gasses passing back into the accommodation.