TST visual overview board
This main board summarises the sewage treatment route and the key indicators used to judge whether the plant is healthy. Use it as the entry point, then expand on the engineering, biology and regulation using the notes below.
Sewage treatment system route
In an oral answer, explain the route in sequence: collection, transfer, screening or maceration, aeration / biological stage, clarification or membrane separation, disinfection, sludge handling and final discharge or retention. Separate the hydraulic path from the biological purpose of each stage.
Aerobic degradation process
Aerobic degradation is the controlled biological oxidation of sewage by microorganisms in the presence of dissolved oxygen. The aim is to reduce biodegradable organic matter, stabilise the sewage, reduce odour, reduce BOD and prepare the liquor for clarification and disinfection.
Detailed aerobic degradation
Aerobic treatment on board is usually based on suspended-growth biomass, fixed media, or compact packaged systems combining aeration and solids separation. In all cases the bacteria require a continuous oxygen supply and a suitable environment to metabolise sewage organics. If oxygen is lost, the biology shifts toward septic / anaerobic conditions, odour and H₂S can develop, and effluent quality deteriorates.
Biological objective
Reduce soluble and suspended biodegradable organics so the final effluent has lower oxygen demand and lower pollution potential. This is why BOD falls when the plant is healthy.
What bacteria need
Dissolved oxygen, substrate, nutrients, acceptable pH, acceptable salinity range, moderate temperature, mixing and time. Toxic cleaning chemicals or overload can suppress the biomass.
Biomass growth
Part of the sewage load is oxidised for energy and part is converted into new cell mass. That is why excess biological solids / sludge are produced and must be controlled.
When performance falls
Signs include odour, blackening, scum, poor settling, higher suspended solids, higher fecal count due to poor process stability, and rising BOD / COD in the final effluent.
General aerobic oxidation
Organic matter + O₂ + nutrients → CO₂ + H₂O + new biomass + energyThis is the broad process description used in marine wastewater treatment: oxygen is consumed while organics are stabilised and microorganisms reproduce.
Example using glucose
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energyA simple model for complete aerobic oxidation of a biodegradable organic substrate.
Cell synthesis / biomass formation
Organic substrate + O₂ + NH₃ + nutrients → C₅H₇NO₂ + CO₂ + H₂OC₅H₇NO₂ is commonly used as an approximate empirical formula for active biomass in activated-sludge style calculations.
Endogenous respiration
C₅H₇NO₂ + 5O₂ → 5CO₂ + NH₃ + 2H₂O + energyWhen food becomes limited, microorganisms oxidise their own cell mass. This affects sludge age and treatment stability.
Nitrification – stage 1
NH₄⁺ + 1.5O₂ → NO₂⁻ + 2H⁺ + H₂OAmmonium is first oxidised to nitrite by nitrifying bacteria under aerobic conditions.
Nitrification – stage 2
NO₂⁻ + 0.5O₂ → NO₃⁻Nitrite is then oxidised to nitrate. Combined overall nitrification is often shown as NH₄⁺ + 2O₂ → NO₃⁻ + 2H⁺ + H₂O.
First-order substrate removal
dL/dt = −kL Lₜ = L₀e−ktA simplified way to describe biodegradation of remaining organic load with time, where L is the biodegradable load and k is the reaction constant.
BOD exertion with time
BODₜ = L₀(1 − e−kt)Useful revision formula showing how exerted BOD approaches the ultimate oxygen demand over time.
Engineering detail – what the examiner wants to hear
Main plant elements
Typical items are collection / holding space, feed pump, macerator or screen, aeration chamber, air blower or compressor, bacterial zone / media, clarification or membrane stage, disinfection system, effluent sample point and sludge withdrawal line.
Operating controls
Monitor blower condition, air flow, liquor mixing, foam, tank level, pH trend, odour, dosing condition, sludge build-up, sample quality and any abnormal carryover into the discharge side.
Common faults
Blower failure, blocked diffusers, pump failure, membrane fouling, hydraulic overload, detergent / disinfectant shock, poor sludge handling, poor disinfection or stagnant pockets causing septic conditions.
Sludge handling
Biomass growth produces excess sludge. If it is not removed or managed correctly, solids carryover rises, treatment efficiency falls and odour plus H₂S risk increase.
MEPC / MARPOL effluent and discharge requirements
For a type-approved sewage treatment plant, the 2012 Guidelines in MEPC.227(64) set the core effluent standards. For untreated sewage stored in holding tanks, MEPC.157(55) gives the moderate-rate discharge formula. The Baltic Sea special-area passenger-ship regime comes from MEPC.200(62) and later IMO implementation dates.
Important MEPC milestones
Annex IV in force / revised Annex
MARPOL Annex IV entered into force on 27 September 2003. A revised Annex IV entered into force on 1 August 2005 and applies to relevant international-voyage ships.
Resolution MEPC.157(55)
Adopted 13 October 2006. Gives the standards for the moderate rate of discharge of untreated sewage from holding tanks.
Resolution MEPC.200(62)
Introduced Special Area sewage provisions and designated the Baltic Sea as the Annex IV special area.
Resolution MEPC.227(64)
Adopted 5 October 2012. The 2012 Guidelines superseded MEPC.159(55) for sewage treatment plant approval and performance tests.
Passenger ship implementation
According to IMO, Baltic Sea special-area sewage rules took effect on 1 June 2019 for new passenger ships, 1 June 2021 for most existing passenger ships and 1 June 2023 for certain remaining categories.
Condition interpretation and failure response
H₂S, septic conditions and practical engineering actions
Why H₂S appears
Hydrogen sulphide forms when sewage becomes septic and anaerobic bacteria dominate, especially in stagnant pockets, dead legs or when aeration has failed for a prolonged time.
Why it matters
H₂S is toxic, promotes corrosion and indicates the biological process is unhealthy. It often accompanies poor odour control and likely deterioration in effluent quality.
Response approach
Stop unsafe entry, ventilate as appropriate, gas test, restore aeration if safe, isolate faults, check discharge isolation and retain sewage if quality is in doubt.