(i) Flow-Through System

A flow-through system operates on the principle of extended aeration, a biological sewage treatment process. The system consists of three interconnected chambers. Raw sewage first enters a chamber where it may be passed through a screen or comminuted. Air is then pumped into this chamber via a diffuser, creating fine bubbles that promote the growth of aerobic bacteria. This process forms a biological sludge. The mixture then flows into a settling tank where the activated sludge settles to the bottom, leaving a clear effluent on top. This clear effluent is then passed to a third chamber, where it is disinfected with a chlorination process before being discharged into the sea. The settled sludge is recirculated back to the aeration chamber to continue the treatment process.

(ii) Collection/Holding/Transfer System

This system utilizes a vacuum to transport sewage. An eductor creates a vacuum that pulls waste from the toilets into a sewage holding tank. This system uses minimal water—around 1.2 liters per flush—since water is only used for rinsing the bowl, not for transportation. A pressure switch controls a centrifugal pump to maintain the vacuum in the lines. The water in the sewage tank acts as the driving force for the eductor. Float switches are used to manage the discharge from the holding tank to a sewage treatment plant or shore facilities while ensuring the vacuum is maintained.

(iii) Zero Discharge System

The zero discharge system is designed to minimize or eliminate the discharge of sewage by separating solids and liquids and reusing the liquid component. In an initial reception chamber, waste falls onto a moving perforated belt. The liquid passes through the belt into a treatment tank, while the solids are carried to a separate caustic treatment tank. The solids are then ground and transferred to a sullage or holding tank. The liquid effluent is treated with chlorine and caustic compounds, making it suitable to be recirculated and used as flushing fluid for the toilets. Seawater can be used for flushing when the vessel is at sea. A small holding tank (with a capacity of 21 liters per person per day) is used to store any excess, which is later pumped to shore facilities or discharged at sea in appropriate areas.

Sewage systems utilizing aerobic action are preferred because they use bacteria that thrive in the presence of oxygen. These bacteria break down organic matter to produce relatively harmless byproducts like water (H2​O) and carbon dioxide (CO2​), along with new bacterial cells. Conversely, anaerobic bacteria, which operate in the absence of oxygen, produce toxic and noxious gases during the decomposition process, including hydrogen sulfide (H2​S) and methane (CH4​), in addition to carbon dioxide. These gases pose safety hazards and produce foul odors, making aerobic systems the safer and more environmentally friendly option for sewage treatment.

The Meaning and Significance of BOD

Biochemical Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen consumed by microorganisms to break down organic matter in a water sample. It's typically measured over a 5-day period and is written as BOD5. The test involves incubating a diluted sewage sample at a specific temperature (usually 20°C) for 5 days and measuring the amount of oxygen absorbed.

The significance of BOD is twofold:

1. Environmental Impact: A high BOD value indicates a large amount of organic pollution in the effluent. If discharged into a waterway, this can lead to a drastic reduction in the water's dissolved oxygen content. This oxygen depletion can be lethal to aquatic life, such as fish and some plants, thereby disrupting the ecosystem.
2. Anaerobic Conditions: A high BOD also signifies conditions where aerobic bacteria are overwhelmed, allowing anaerobic bacteria to become dominant. As explained above, this leads to the production of noxious and toxic gases, causing foul smells and further damaging the water quality. A low BOD value, therefore, indicates that the effluent is stable and will not significantly harm the aquatic environment.