Biological & Chemical Sewage treatment plant

1. Sketch of Biological STP.
2.  Principle of Biological STP.
3. System construction.
4. Operation of Biological STP. 
5. B.O.D.
6. MARPOL Regulations
7. Maintenance
8. Cleaning and coating repair

  Sketch of Biological Sewage treatment plant.

Principle of Biological Sewage treatment plant. 

The Sewage Treatment Plant purifies sewage based on the activated sludge process. Activated sludge is a complicated substance mainly consisting of aerobic bacteria that attract not only organic and inorganic suspended solids in sewage water but also protozoa, metazoa and algae; and boasts strong absorptive power and positively degrades organic matters. When sewage water containing activated sludge is allowed to stand, the aerobic bacteria having adsorbed the impurities flocculate, forming large flocs that readily settle.

When sewage is fully in contact with activated sludge while a sufficient amount of oxygen is supplied into it, almost all the impurities, both organic and inorganic, in the sewage are adsorbed to the activated sludge.

The impurities adsorbed to the activated sludge serve as a nutrient for the microbes that are a major constituent of the activated sludge. A part of adsorbed impurities is decomposed into safe substances such as water and carbon dioxide by oxidation, and another part of it is converted into cells of the activated sludge. The activated sludge process has various variations to cope with a variety of applications. The Sewage Treatment Plant adopts "Total Oxidation Process". With this process, the activated sludge remains in contact with the sewage for a very long period. As a result of a very long contact period, the amount of impurities (nutrient for activated sludge) per a unit amount of activated sludge decreases, and, consequently, the microbes remain in a starved state. Then, the microbes begin metabolism using even their own cytoplasm.

Thus, they consume most of their own cytoplasm that they previously synthesized. Because of this, the increase in the amount of sludge is low even after prolonged operation, and the servicing and maintenance with the Plant are very easy. To sum up, the total oxidation process is an ideal process for a compact sewage treatment system. After activated sludge is allowed to remain in contact with sewage for a sufficiently long period, and if the mixture is allowed to stand in a clarification compartment, the activated sludge immediately forms large flocs that readily settle and are easily separated from the supernatant.

The settled and separated activated sludge is returned from a clarification compartment to an aeration compartment in order to maintain the adequate concentration of activated sludge in the aeration compartment.

System construction of Biological Sewage treatment plant. 

 

System Constitution

The Sewage Treatment Plant comprises three compartments: - an aeration compartment, a clarification compartment and a chlorination compartment.

The influent sewage enters the aeration compartment where it is supplied with air and is digested and decomposed by the activated sludge.

After being thoroughly purified in the aeration compartment, the sewage flows into the clarification compartment where the activated sludge is allowed to settle, and then the clean supernatant is thoroughly sterilized in the chlorination compartment and is discharged overboard.

Aeration compartment

In the aeration compartment, the activated sludge absorbs dissolved oxygen in the influent sewage and decomposes the sewage, which primarily consists of carbon, hydrogen, nitrogen and sulfur into carbon dioxide, water and new bacterial cells.

The generated carbon dioxide is released through the gas vent, and the generated water, together with the bacterial cells, after a residence time of approximately 24 hours, flows into the clarification compartment.

The aeration air is supplied by the air compressor and is bubbled into the aeration compartment via the diffusers situated at the bottom of the aeration compartment. For an easy maintenance work, the diffusers can be taken out through a manhole situated on a side of the tank.

Aeration supplies the oxygen that is needed for survival of the bacteria and constantly stirs the contents in the aeration tank.

Clarification compartment

The mixture liquid flowing into the clarification compartment is separated into the supernatant and activated sludge through sedimentation. The supernatant flows into the chlorination compartment through an overflow tube. The settled activated sludge is returned from the bottom of the clarification compartment to the aeration compartment by an airlift pump. The return line comprises a length of transparent vinyl hose so that the returning activated sludge can be visually monitored.

The clarification compartment is a hopper type. For this reason, the settled activated sludge is quickly drawn into the airlift pump rather than accumulating onto the bottom.

A skimmer is provided at the upper middle of the clarification compartment, and the scum floating on the liquid is returned to the aeration compartment by an airlift pump in a manner similar to that with the settled activated sludge.

Chlorination compartment

The supernatant overflowing from the clarification compartment dissolves chlorination tablets in the chlorinator and then flows into the chlorination compartment where it is sterilized.

On the side of the tank, the two float-type level switches (float switches) are provided to be able to control the operation of the discharge pump. Also, the high-level alarm float switch is provided which triggers a fault alarm if the liquid level has exceeded the "high" level. The chlorination compartment is provided with an emergency overflow tube: as a result, the treated water does not overflow the tank.

Chlorinator

In the chlorinator, the influent supernatant passes and dissolves the packed chlorination tablets in the cylinder to produce chlorine solute. The chlorinator consists of a rectangular chlorinator body and three tablet-feed cylinders. A cutout is provided at the bottom of each cylinder that allows the supernatant to flow in and come into contact with the tablets the cylinder contains so that a necessary amount of chlorine is dissolved in the supernatant.

A control weir is provided at the outlet of the chlorinator so that when the flow rate of influent supernatant increases and the liquid level rises, the supernatant contacts a maximum possible number of tablets. As a result, an appropriate amount of chlorine is added to the supernatant in accordance with a flow rate of influent supernatant.

Discharge pump

The Plant is equipped with one or two discharge pumps. The pumps are a volute, non-clogging type, and are directly coupled to motors. In normal operation mode, the pumps automatically discharge treated water. When the inside of the tank is rinsed, they can be manually operated to discharge activated sludge from the compartments.

Air compressor

The Plant is equipped with one or two air compressors. With the two-compressor configuration, one air compressor unit always remains running while the other unit remains to be a spare unit.

Float switches

The chlorination compartment is provided with three float switches-the high-level, low-level, and high-level alarm switches.

Control panel

The control panel houses the starters for the discharge pump(s) and air compressor(s). The discharge pump(s) and air compressor(s) operate in a manner described below:

When the selector switch on the panel is in the "AUTO" position, the discharge pump can automatically start running: when the liquid level in the chlorination compartment reaches the high-level float switch, the discharge pump starts running to discharge the treated water; when the liquid level drops to the low-level float switch, the discharge pump stops running.

For the manual operation mode, the selector switch must be set to the "MANUAL" position. Then, when the "START" push button is pressed, the discharge pump starts running, and when the "STOP" push button is pressed, the discharge pump stops running.

If the selector switch is set to the "START" position, the air compressor starts running.

In the case of the two-compressor configuration, the selector switch may be set to "NO. 1" or "NO. 2" position: the selected compressor will start running. If, by any reason, the liquid level exceeds the high level or the currently running compressor stops, the fault alarm signal is triggered.

Operation of Biological Sewage treatment plant.  

Preparation for Operation

Before operating the Plant, make sure that the Plant satisfies the following conditions:

• 1.    Check that the controls and associated components and mounts are free from any damages and looseness. Also check that the air compressor(s) and discharge pump(s) are fully operative.
• 2.    Check that the wiring connections are correct. (For the details for electrical wiring, see the wiring diagram provided on the back face of the control panel cover.)
• 3.    Check that the air compressor has not been stuck. (Manually operate the compressor by hand.)
• 4.    Check that the running direction of the air compressor(s) and discharge pump(s) is correct.

Operating Procedure

1. Turn "ON" the power switch, and set the discharge pump selector switch to the "AUTO" position. 
2. Close the valves 2 and 4, and open the valves 1 and 3. Allow the tank to be filled with water until the water passes the chlorinator.  
3. Close the valves 1 and 3, and open the valves 2 and 5.  Open the lid of the tablet-feed cylinder, and fill the cylinder with the tablets. Usually, fill one out of three cylinders with the tablet.  
4. Fully open all the valves in the air feed line. If the Plant is equipped with two air compressors, only close the outlet valve of the spare air compressor. Fully open the air feed plug cock of the scum return line, and set the compressor selector switch to the "START" position. If the Plant has two air compressors, select the air compressor to be used. Then, check that the reading on the pressure gage falls within the range of .01 to .05 Mpa. (If the compressor does not provide a discharge pressure, it is running in the reverse direction. If this situation occurs, do not run it for 3 seconds or longer. Correctly reconnect its motor wirings.
5.  Check that the fluid is flowing in the sludge return vinyl hose situated at the upper portion of the exterior of the aeration compartment. Note there are two vinyl hoses on the exterior of the aeration compartment. The hose situated toward the middle of the tank is for returning scum that floats in the clarification compartment, and the liquid flows in this hose only when sewage flows into the Sewage Treatment Plant. The flow rate of the air for returning sludge has been factory-adjusted with a plug cock: Do not alter the setting of the plug cock. If the setting has been erroneously changed, readjust the opening of the plug cock so that the liquid level inside the sludge return vinyl hose is approximately 1/2 the inside diameter of the hose.  
6. Now, the Plant is ready for operation. Open the sewage inlet valve to allow the sewage to flow into the Plant.

Measurement of the Residual Chlorine Content

For measurement of the residual chlorine content in the treated water, a measuring instrument is provided for the Plant. Sample water is obtained with the sampling cock on the chlorination compartment. For the measuring method, refer to the instruction manual included with the measuring instrument.

If the reading of residual chlorine content is less than 1 ppm, use two or three tablet feed cylinders.

Precautions for Operation

The Sewage Treatment Plant treats sewage using biological sludge. Therefore, strictly observe the following precautions. 

1. Immediately after starting operation of the Plant, it may not perform as designed before aerobic bacteria breed (this period usually lasts for 10 days to 1 month, though greatly varying, depending on the operating conditions).
2. As the bacteria increasingly bred, the water quality of the treated water becomes better. Therefore, keep the Plant operating. (This also applies when the inside of the Plant is cleaned and the operation is resumed.)
3. If the operation of the Plant is suspended, oxygen and nutrients are not supplied to the aerobic bacteria, and, as a result, a part or whole of the bacteria will die, and the Plant will not perform as designed. Therefore, even when the ship is outside the restricted waters, keep the Plant running. However, chlorination is not needed outside the restricted waters.
4. After the Plant has been stopped by any reason, be sure to clean the inside before resuming operation. It is recommended that no chemicals such as a cleaning agent for the lavatory flow into the Plant. Such chemicals can destroy the aerobic bacteria, causing the Plant to fail to perform as designed.
5. In the event of prolonged power failure, prohibit the use of lavatories or take a necessary measure to prevent sewage from flowing into the Plant.
6. Provide litter boxes in the lavatories to avoid pieces of cloth, paper towel or plastic sheets from being throw into the toilets.
7. If the flushing water for the toilets is allowed to flow away constantly, an excessive amount of sewage will flow into the Plant, deteriorating the performance of the Plant.
8. Be sure to flush water only when the lavatory has been used. After sterilization with chlorine, discharge the treated water below the waterline.

Precautions for Handling and Storing the Calcium Hypochlorite Tablets  (Chlorination Tablets)

The tablets are a powerful oxidizing agent. For this reason, strictly observe the following instructions:

1. The tablets, when in contact with exposed fire, heat, acid, alkali, grease, oil, rug, paint, thinner, dust, detergent, vinegar, charcoal, sulfur or other combustible materials or chemicals, can cause explosion or combustion.
2. Do not allow the tablets to come into direct contact with any of these items or mix them with these items. Avoid storing the tablets together with any of these items in a same location.Keep the tablets away from open fire such as that of a cigarette. Otherwise, a fire accident can occur.
3. Handle the tablets with dry, clean gloves or a metal utensil. Use of dirty gloves or utensil can cause fire.
4. Do not directly touch the tablets with wet hand.If a fire accident occurs, extinguish with a lot of water. Also, cool down the area at and around the fire source.
5. Be absolutely careful since as a result of a fire-fighting effort, the tablets or debris of the tablet container can be scattered or gas can occur.
6. Carry the tablet container very carefully so that water or a foreign matter does not enter the container. Also, be very careful not to damage the container by dropping it or exerting a large impact so as to prevent the tablets from being scattered.
7. Keep the tablets contained in the tightly sealed container to avoid humidity. Store the container in a cool, dark place (40°C or lower) such that it does not fall. Keep the container out of reach of children.
8. Do not fill the tablets in another type of container.
9. Do not return spilled or contaminated tablets to the tablet container. Consume all of such tablets.Wash away residual tablets with a plenty of water.
10. Do not discard a used tablet container together with the tablets remaining in it. Flush the container with a plenty of water, and only then dispose it in a safe place. 
11. First aid:- 
    In case of eye contact rinse the eyes with a plenty of water for at least 15 minutes, and immediately seek a medical attention with an oculist.  
    In case of contact with body and clothing, immediately wash away with a plenty of water. 
    If rash occurs on the skin, seek a medical attention. If swallowed, immediately drink the white of a raw egg, milk or rice gruel, and then take an emetic (a spoonful of mustard in a glass of water), and immediately seek a medical attention with a physician.
    

(a) What do you meant by B.O.D. What is the significance of B.O.D.

1. B.O.D is the amount if dissolved oxygen required by the bacteria at the end of treatment process.

2. The BOD should be very less and stable in the effluent that has been treated and ready to be pumped overboard by and approved STP.

3. Biochemical oxygen demand is the amount of oxygen needed by aerobic biological organism to break down organic material present in a given water sample at certain temperature over a specific time period.

4. A maximum of 50ppm of BOD in a sample of sludge in 5days of incubation at 20 deg cel. is normal.

5. The test is used 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 as it occurs. 

6. A high BOD in the effluent discharging overboard will result in the depletion of the dissolved oxygen in the sea water harming the marine life.

(b) What are the discharge criteria according to MARPOL Regulations? 

Marpol chapter 3, regulation 11:- Discharge of sewage

Discharge of the sewage into the sea is prohibited except when 

1. The ship is discharging comminuted and disinfected sewage, through a system approved by administration and at more than 3N-m from the nearest coast.

The sewage which is not comminuted  or disinfected shall be discharge at a distance more than 12N-m.

2. Sewage stored in holding tank should not be discharge instantaneously. Rate of discharge should be approved by administration.

3. Vessel should be en-route at a speed not less than 4knots.

4. Approved sewage treatment plant, test results are noted in ISPP ( international sewage pollution prevention) certificate and effulant should not produce visible floating solid or cause discoloration of the surrounding water.

5. Ship operating under jurisdiction of a state, discharge sewage in accordance with the less stringent requirement imposed by the state.

(c) What routine maintenance is carried out on such Sewage treatment plant?

Maintenance:-

Regular inspection:-

	Area to be inspected	Check points	Procedure
Every day	Compressor outlet pressure gauge	The pressure is in the range of 0.01-0.05Mpa	If the pressure is too low, check the compressor and piping for leakage. If the pressure is too high, check the diffuser tube and piping for blockage, and clean as necessary.
	Sludge return and scum return line.	Sludge and scum are being returned. ( note that the returning scum is not always flowing in the hose. It flows only when sewage flows into the plant.	If either sludge or scum is not being returned, correct according to the instructions.
	Chlorination tablets in the chlorinator	The height of tablets in the cylinder needs to be at least 1/4th the height of cylinder.	Remove the top cover, and fully fill the cylinder with the tablets.
Every weak	Residual chlorine content in treated water	The residual chlorine concentration must fall in the range of 1 to 5 ppm,	If the residual chlorine concentration exceeds 5ppm, use one cylinder only; if the concentration is lower than 1ppm, use two or three cylinders.
Every month	Air compressor (only when the plant has two compressors)	Use the spare air compressor instead of currently active compressor.	Repair the compressor if it has failed.
	Chlorinator	There must be no blockage in the cylinder.	Remove the chlorinator and clean it.
	Discharge pump	Check the mechanical seal for leakage.	If leakage rate is 0.5cc/min or greater, make correction as per manufacturer’s instructions
Every 2 to 3 months	Aeration compartment	Discharging of excess sludge	For 30 minutes, prohibit sewage from flowing into the plant and keep the plant inactive. Then, discharge approximately 75% of the mixture liquid in the aeration compartment by running the discharge pump. This operation must be performed outside the restricted waters. After discharging fill the tank with water, and restart the plant
		Check the interior screen for fouling.	If the screen has been fouled up or clogged, clean it with freshwater or clean seawater.
	Clarification compartment	Check the inside for fouling.	If the inside is fouled up, clean with fresh water or seawater.
Every 6 months	Gear oil on air compressor	Check for insufficient or deterioration of the oil	Replenish the gear oil as needed. Replenish the deteriorated oil with fresh oil.
	Bearing grease on air compressor	Check the grease for deterioration.	Replenish the gear oil as needed.
Every year	Tank, discharge pump, air compressor	Inspect the inside of the tank, as well as the components on the accessories.	Discharge the wastes, and clean the inside. Then inspect each area. If a problem is found, repair or replace.
Every 3 year	Inside of the tank	Damaged coating layer inside the tank.	Repair the damaged area with the coating.

Cleaning and Coating repair

I. Cleaning of the chlorinator

• The calcium hypochlorite tablets are dissolved in the treated water within the chlorinator. As a result, the residue of the tablets can remain in the chlorinator and deposits on the inside surface of the chlorinator. Therefore, rinse to clean the inside of the chlorinator at regular intervals. 
• Before cleaning with an acid, thoroughly remove the sterilizing tablets and their debris. Otherwise, chlorine gas will occur and pose an extreme hazard. Clean the chlorinator at a rinsing station where a plenty of water is available. Do not use water of which temperature exceeds 65"C (150°F).
• When removing the chlorinator, very carefully handle it as it is made of PVC resin and is relatively vulnerable to a shock or Impact.

(1) Transfer the chlorinator removed from the Plant to a place convenient for cleaning it with water. Wash away the tablet residue with clean water.

(2) With sponge or a brush, thoroughly remove the tablet residue adhering to the inside wall of the chlorinator.

(3) Rinse with water until the inside of the chlorinator is fully clean.

(4) When the cleaning work is complete, refill the calcium hypochlorite tablets.

II. Cleaning of the inside of the Plant

• Perform the cleaning work outside the restricted waters.
• During the cleaning work, keep the air compressor running. If the air compressor is not running during the cleaning work, activated sludge flows back in the sludge return line etc., possibly leading to blockage.
• Complete the cleaning work at least 10 days before the ship enters a restricted water where untreated sewage must not be discharged overboard.(It takes at least 10 days before an appropriate activated sludge concentration is restored after the cleaning work.)
• When discharging the mixture liquid in the Plant, be careful not to dry-run the discharge pump whenever possible.

(1) Close all the valves on the sewage inlet line. If the line lacks any valve, prohibit use of all the sewage sources.

(2) Tum OFF the discharge pump switch situated on the control panel.

(3) Open the valves 2, 3, 4 and 5.

(4) Check that the valve 1 is in the closed position.

(5) Set the discharge pump switch to the "MANUAL" position, and press the "START push button to run the discharge pump to empty out the Plant.

(6) When the Plant has been emptied, stop the pump by pressing the "STOP push button.

(7) Thoroughly clean the inside of the tank by using the water from the seawater feed line, or the nearest seawater or freshwater service hose coupling, and discharge the water by running the discharge pump. Insufficient cleaning can lead to corrosion within the tank, blockage of the piping and valves as well as sticking of the pump

(8) Stop the air compressor.

(9) Check for damages and fouling in the Plant, and repair as necessary.

During the inspection work within the Plant, provide positive ventilation. Otherwise, an accident can occur from shortage of oxygen.

III. Repair of the Coating inside the Tank

• If an area of damaged coating is found during a cleaning work inside the tank, be sure to repair the damaged area.
• During the coating work within the tank, provide positive ventilation and wear a breathing apparatus. Otherwise, an accident can occur from shortage of oxygen.Inspection of the Discharge Pump and Air Compressor.
• The inspection work necessary for the discharge pump and air compressor consists only of replenishment and/or replacement of oil and grease. If disassembly of the discharge pump or air compressor is necessary.

Sewage Plant
There are three main types of sewage plants biological, chemical and electro-catalytic oxidation. In the early 70s mainly biological plants were in use these worked on the principle of steady break down of sewage by using aerobic bacteria. The problems here are the regular de-sludging required, which is labour intensive.
Since then, two newer 'systems developed, which are physical-chemical and electro-catalytic oxidation.  Physical-Chemical 'treatment consists of separation of the liquid elements from sewage flow, and bathing this in Chlorine for about 30 minutes to kill the coliform bacteria.
This system complies with-MARPOL regulations. Around 5% Chlorine is used. The limitation with this equipment is that it cannot hold a large capacity, thus it is not suitable for extended port stay. There is also a requirement to carry calcium hypochlorite for the disinfection of the sewage.
The electro-catalytic Oxidation process works on a different principle. It uses electrolysis of sea water to produce sodium hypochlorite, which oxidises the organic material of the sewage. The advantage here is an almost 50% saving in size, as compared to a biological plant, since it is compact. The sewage is passed through a macerator, which breaks it down into, minute particles less than 1.5 mm, and is passes through an electrolytic cell, where the oxidation takes place.
After passage through the cell, the-effluent flows to a settling tank for completion of the oxidation and final discharge overboard.