WASTE  MANAGEMENT  :

The discharge of sewage in territorial waters must comply with certain limits laid down by the local or national authorities. This will usually be based on one or more of three factors, namely, the bio-chemical oxygen demand, (BOD) suspended solids content, e-coliform count.

Bio - chemical Oxygen demand - It is measured by incubating one litre sample of sewage at 20°c diluted in sufficient well oxygenated water, the amount of oxygen absorbed over a five day period is measured. The test is used in this context to evaluate the effectiveness of treatment as it measures the total amount of oxygen taken up as final and complete breakdown of organic matter by aerobic bacteria in the effluent occurs. The admissible Value changes from country to country, for U. K., this value should be within 20 mg/lit.    B.O.D of raw sewage

is 300 - 600 mg\lit I.M.O recommended 50mg\lit

if the effluent is overloaded with oxygen absorbing matter, the oxygen quantity of water in which it is discharged, will be less and fish and plant life cannot be supported. Secondly a class of bacteria which can live without oxygen will predominate and this bacteria produce hydrogen sulphide with its pungent smell,

CO₂ and methane.

Suspended Solid Content- Suspended solids may form silting problem over a course of time. It is measured by filtering a sample through a pre-weighed asbestos pad which is then dried and re-weighed. In U. K. it is usual to find a suspended solid limit of 30 mg/lit.  I.M.O recommended value 50mg\lit     

e - Coliform Count - It is a family of bacteria found in the human intestine. They can be quantified in the laboratory test, the result indicative of the amount of human waste present in a particular sewage sample. Coliform count of 1000/100ml is usually accepted from ship's discharge. I.M.O recommended value  250\100ml

SEWAGE  TREATMENT  :

Processes : (1) Biological, (2) Physical chemical and (3) Electro-Catalytic Oxidation.

Biological Technology :

The sewage, in this process, is fed directly-to an aeration tank, where air from fine bubble diffusers raises the dissolved oxygen level and coalesces with the waste products. The bacteria thrives in this environment and devours the organic substance, converting it into water, carbon dioxide and new bacteria cells.

This mixed liquid is then displaced into a settling tank by further incoming sewage, where the biological sludge consolidates under inactive conditions. It is then withdrawn for recycling to the aeration tank, and it is this recycled sludge which contains the bacteria that digests the incoming sewage.

Retention periods of twelve hours or more in the aeration tank significantly reduces the volume of organic sludge.

Clean effluent is taken from the settling tank by a skimmer system, and transferred to a collection tank, where a good dose of chlorine kills any residual organisms. The chlorine levels at discharge usually amount to no more than*5ppm.

Biological technology requires a steady and relatively constant intake of solidified sewage so that aerobic bacteria can feed on it in order to keep the discharge within regulatory limits. This process of aerobicity strips oxygen from the water, producing more water, carbon dioxide and new bacteria.

Moreover, biological systems need to operate day in-day out, so if the vessel in which this type of system is installed doesn't operate continuously, then this type of system may prove a wrong choice.

An extended aeration process, however, is said to minimise the build up of biological sludge in the treatment tank by retaining it for such a period as to allow the bacteria to digest the incoming organic waste.

Physical - Chemical Treatment :

Physical-chemical treatment of sewage; on the other hand, is based on the separation of the liquid element from the sewage flow, by bathing it in chlorine for thirty minutes to kill off coliform bacteria before discharging it overboard in full compliance with MARPOL regulations.

Using electrolysis of seawater to produce sodium hypochlorite to oxidise organic material before discharge (or by dosing with chlorine), this type of treatment plant, which, by the way can be up to 50% smaller than biological plants, collects

the sewage and directs it through a macerator which breaks it down into minute particles. The sewage is then passed through an electrolytic cell where the oxidation process takes place, and after chlorination.

After its passage through the cell, the effluent flows under its own pressure to a settling tank for completion of the oxidation process and direct discharge overboard.

There are obvious attractions to this type of system; its size is one of them, and it can also be switched on or off as needed.

According to Mr R Holubowicz of Marine Venture Ltd. the 'pass-through' process takes less than thirty minutes from the time the sewage enters the unit to its oxidation and discharge overboard, whether the vessel is in port, in regulated waters, or high seas. The discharge contains no solids, is totally free of coliform bacteria, with the only evidence of treatment being a 'bleachy' smell at the point of discharge.

Electro-Catalytic oxidation Process :

It operates on demand, and its surge tank operates as a reservoir to store both black and grey water, so as soon as enough wastewater is received to reach a high level start switch, sewage treatment begins. The macerator, after pumping wastewater out of the surge tank, then grinds and reduces the solidified sewage to 1.5mm before sending it through an electrolytic cell.

The electrolytic cell is at the heart of the matter, for it is during this process that the sodium chloride in the sewater is used to produce the sodium hypochlorite needed to disinfect the wastewater. The sewage and seawater, which is passed through various charged cells, react, causing the breakdown of organic molecules and the continued production of total bacteria kill.

Aerobic (Biological) Treatment plant (Flow through system)

Principle

Biological 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.

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 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 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 Oxidation

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

COMPREHENSIVE WATER   MANAGEMENT   (CWM)

A Canadian Company CWM Ltd has devised a plant, which provides a cost effective solution for total wastewater management with 0 ppm effluent. It also recycles water for use in non-potable water applications.

This unique system does oil-water separation, liquid solid separation and water disinfection technologies into a single common system.

The plant is said to be capable of treating wastewater from a combination of sources which include, but are not limited to : black and grey waters from toilets, diswashers, sinks, organic and inorganic matter from food preparation waste, chemical operations, ballast water and oily water from ship's bilges or oil spills. The proposed system is said to be able to remove pollutants, disinfect water, and separate and disinfect any solids. All liquid and solid by-products can be recycled for reuse or can be safely released into the environment.

The plant is a series of interconnected modules which process mixed efflucent streams. Processing occurs in the :

•               Oil - water separation module

•               Regrind module

.      Dissolved air floatation (DAF) module

•               Disinfection/oxidation/filtration module

•               Sludge management module.
WASTE WATER  TREATMENT

Oily water is collected in a gravity separation tank, where the water layer is fed to the regrind tank and the oil layer is fed into the heated oil tank. The combination of temperature and injection of a chemical emulsion breaker drives off any residual water. The oil is passed through a centrifuge, which further removes water from the oil and removes solid matter.

The resulting 'clean oil' has been tested by an independent laboratory to determine its suitability for use as an alternative fuel source.

Oily water, sewage and wastewater collected from all .other source are introduced into the regrind module. Mixing and particle size reduction is achieved in the tank by macerating the influent, which produces a fine slurry which is pumped to the dissolved air flotation (DAF) module.

DAF technology is used to separate solids from water, where flocculation of the suspended solids is promoted through a combination of pH adjustment and addition of polymers and coagulants. Solids are floated to the top of the DAF cell and' transferred to the sludge management module. Clear water is sent to the disinfection oxidation and filtration module for further processing.

Concentrated slurry is delivered to the sludge concentrator, the first element of the sludge module. The sludge is transferred to the filter press where about .30-40% of the water is squeezed out by coalescing plates. The water is passed on to the disinfection module and the filter cake is transferred to the dehydrator where it is dried. The heating process then produces a sterile, anhydrous powder that can be safely discharged to the environment or removed to landfill.

Water enters the disinfection module and passes through a filtration system that removes any remaining suspended solids, which were not removed during the DAF process. Disinfection of the water is achieved through hydrogen peroxide enhanced ionization. Ozone and hydrogen peroxide are injected into the fluid stream, producing a highly oxidative environment. This combination of ozone and hydroxyl radicals successfully reacts with any organic and inorganic materials present in the wastewater. Mixing and contact time is promoted in the ozone shock tank where the oxidation process eliminates all odours and produces water which is suitable for non-potable applications.

An oxidation reduction potential meter samples and measures the residual ozone content in the effluent stream and can automatically adjust the amount of ozone added to the system. If the reading is below the acceptable limit, the water will be recirculated through the ozone injection system until it can be safely discharged. The effluent will then pass through carbon filters which act as a final polish and remove any residual ozone. Disinfected water can be safely discharged or can be reused in various applications, as required. Additionally, the water can be fed to a reverse osmosis or distillation plant to satisfy potable water requirements.

Because the plant is modular, individual components can be altered in size and capacity to respond to various processing requirements and space limitations.

The recycle and reuse features of the system benefit the ship-owner through money and time savings, and potential legal liability is reduced through the system's zero effluent discharge philosophy. The option to reuse water for such things as toilet flushing, laundry, cleaning or even drinking, will reduce the need to carry, or produce freshwater.

The oil/water separation process removes the oil from the water and produces an alternative fuel suitable for reuse on the ship. The technology has built-in pH, turbidity, oil content and ORP monitors that initiate recirculation of the treated water if water reuse quality standards are not met.

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