A sewage treatment plant is meticulously engineered to process and purify raw sewage through a series of intricate steps, that encompass fragmentation, filtration, settling, controlled aerobic decomposition, and chemical treatment.
When it comes to human waste, the conventional notion used to be to dispose of it into sewers and leave it for governmental authorities to handle. Considering that the average person generates more than 90 liters of sewage daily, it’s natural to ponder how sewage is effectively managed and treated on ships.
In today’s environmentally conscious era, discharging untreated human waste at sea is no longer an option, given the limited storage capacity on board. Instead, sewage is subjected to a comprehensive treatment process before it is safely released. This systematic breakdown of sewage into smaller components, achieved through a combination of biological and chemical methods, is referred to as sewage treatment.
The infrastructure responsible for executing sewage treatment is aptly named a sewage treatment plant. On a ship, a sewage treatment plant comprises essential components like a screen filter, primary chamber, aeration chamber, demisters, blowers, settlement tank, and chlorination chamber. Collectively, these elements work in harmony to treat and discharge waste in compliance with Marpol Regulation IV.
But Why All This Effort? Why Not Let It Decompose Naturally? Ah, the bygone days before Marpol Regulation 4, when we could simply release untreated human waste into the sea. But why was this practice discontinued? Without the presence of a sewage treatment plant, discharging waste into open water causes it to attract aerobic bacteria and decompose spontaneously.
Not only does this process deplete the essential oxygen from the water, but it also poses a substantial risk of triggering health epidemics if performed near ports. Furthermore, not all components of sewage are biodegradable or break down at the same rate, adding complexity to the situation. Often, the presence of traces of nitrates, phosphates, and other organic matter found in untreated sewage can lead to contamination of the surrounding waters.
To safeguard the marine environment and prevent the spread of potential epidemics, the international maritime community introduced Marpol Regulation 4. This regulation, established on September 27, 2003, specifically addresses the prevention of pollution resulting from the discharge of sewage at sea.
Consequently, to evade hefty fines and safeguard the reputation of both the ship and the company, all vessels are equipped with dedicated sewage treatment plants.
Design and Construction of Sewage Treatment Plants
Sewage treatment plants are meticulously designed and constructed to cater to the specific needs of various facilities, whether they are industrial, maritime, or urban sewer systems. The treatment of sewage can generally be categorized into two major methods: chemical treatment and biological treatment, although some modern plants employ hybrid models. Let’s delve into the design characteristics of these prominent sewage treatment plant types with an in-depth discussion of each.
1) Chemical Sewage Treatment Plant
Chemical sewage treatment plants comprise preliminary treatment, primary treatment, and secondary treatment chambers. The preliminary chamber is equipped with coarse and fine mesh screens to function as filters, preventing large solid particles from entering the system. In many designs, it is positioned at the top of the primary chamber, featuring a flow measurement device that records and filters wastewater inlets simultaneously.
The primary chamber closely resembles an in-house septic tank, serving as a massive storage unit for all the waste flushed from toilets, wash basins, and bathrooms. Float switches within the chamber control the sewage level, and it also functions as a sedimentation tank, allowing solid particles to settle at the bottom. This enables the storage of a large volume of solid waste, which can later be discharged to designated authorities offshore.
The secondary chamber is responsible for chlorinating the non-solid water waste and storing it for further chemical treatment. This involves the addition of a 5% chlorine solution to eliminate bacteria within a 30-minute timeframe. Additional chemical treatment is employed to eliminate odors and restore the water to a clear appearance.
The treated water can then be discharged into the sea, directed to shore facilities, or used for toilet flushing. De-chlorination is a necessary step before discharging into the sea, involving the removal or reduction of chlorine compounds. This is achieved through de-chlorination solutions such as ammonia, sulfur compounds, and activated carbon. Solid waste is collected in a sludge tank for later disposal at shore collection facilities.
2) Biological Sewage Treatment Plant
In contrast to chemical sewage treatment plants, biological sewage treatment plants utilize a combination of aerobic and anaerobic bacteria to break down sewage into simpler forms and facilitate decomposition. The process employed in biological sewage treatment plants, using aerobic bacteria, is referred to as aeration.
These plants comprise six major components: the fine mesh filter, primary chamber, aeration chamber, demister, settlement chamber, and air blowers. The fine mesh filter is positioned at the inlet to the primary chamber, effectively filtering out unwanted solids and debris, preventing them from entering the system. Additionally, the mesh screen aids in breaking down solid particles of organic waste (sewage) into smaller, more manageable particles.
The primary chamber serves as the collection and holding tank for raw sewage. Equipped with level sensors and float switches, it detects sewage levels and prevents overflow. Within this chamber, the heaviest particles settle to the bottom, while the remaining sewage flows into the aeration chamber.
The aeration chamber is where the transformative process takes place. It functions as a hybrid bioreactor with air blowers that create optimal conditions for the reproduction and growth of aerobic bacteria.
After undergoing bacterial action, the sewage proceeds to the settling tank, which features a series of separating channels with sloped slides. Here, the heavier sludge and other sewage particles are separated from the lighter water. Subsequently, the treated water is discharged overboard, while the remaining sludge undergoes further processing.
Design of Sewage Plants Used On Board Ships
Sewage treatment plants on board ships are meticulously designed to comply with stringent rules and regulations governing the quality of water that can be discharged. These shipboard treatment plants typically employ a hybrid approach, with biological processes serving as the primary method of treatment and chemical processes as a secondary treatment.
The fundamental principle of these plants, decomposing raw sewage, is primarily managed by the biological units, while the disinfection of bacteria involved in the sewage treatment process before final discharge is handled by chemical units.
Much like biological sewage treatment plants, these shipboard systems consist of several key components, including a primary chamber, aeration chamber, and settling chamber. However, they also incorporate additional chambers for chemical treatment, such as chlorination and activated carbon, to ensure the highest water quality standards are met and maintained throughout the treatment process. This hybrid approach ensures that the sewage is efficiently broken down and thoroughly disinfected before being discharged into the marine environment, thereby minimizing any adverse impact on the surrounding ecosystems and complying with international regulations.
1) Primary Chamber
The primary chamber in a sewage treatment plant is the initial stage where raw wastewater from toilets, washbasins, and bathrooms is collected. This wastewater typically contains about 0.1% solid waste by weight. The primary chamber receives sewage through specialized macerator pumps, which break down human waste into a slurry through blending and grinding techniques.
To further refine the wastewater, it passes through a series of coarse screen meshes, functioning as filters to remove unwanted solids like metal scraps, plastics, and raw toilet paper. Afterward, the primary chamber stores the raw sewage for a period before transferring it to the aeration chamber.
2) Aeration Chamber
Aeration chambers play a crucial role in sewage treatment by providing an environment suitable for aerobic bacteria. These bacteria require oxygen, warmth, and a source of food, which is readily available in the form of raw sewage. However, oxygen levels are usually insufficient. To address this, aeration chambers are equipped with air blowers, typically in a redundant configuration with one in operation and the other on standby.
The air blowers facilitate the formation of air bubbles when air is pumped into the aeration chamber through controlled air diffusers. This process increases the surface area available for aerobic bacteria to thrive, reproduce, and efficiently break down the sewage. Proper ventilation and exhaust systems in the chamber allow for the removal of NH3 and CO2 produced during the treatment process.
3) Settling Chamber
Following biological treatment, the wastewater flows into the settling chamber, where the heavier solid particles settle due to gravity. To enhance this settling process and mitigate the disruptive effects of sewage flow, wastewater enters the chamber from the bottom and exits from the top into the next chamber.
The settling chamber often includes sloping slides, one after another, to boost the separation efficiency of the settling tanks or chamber. Some modern designs incorporate air-driven ejector pumps, which return a portion (typically 1/4) of the sludge back to the aeration chamber. This recirculation helps maintain an adequate population of bacteria for efficient treatment.
4) Activated Carbon Addition
Activated carbon is intentionally introduced into the biologically treated sewage to eliminate foul odors and undesirable colors. Activated carbon has a high adsorption capacity for organic molecules responsible for odor and coloration. In many designs, activated carbon beds are placed immediately after the settling chamber, allowing wastewater to undergo treatment before progressing to the next chamber in the process. This addition of activated carbon significantly improves the overall quality of the treated wastewater.