Aerobic Wastewater treatment

Nitrogen

Too much nitrogen in water causes algae blooms → kills fish → pollutes rivers. Needs to be removed

Step 1: NITRIFICATION

An aerobic process where ammonium (NH₄⁺) is first converted to nitrite (NO₂⁻), then to nitrate (NO₃⁻) by bacteria. It consumes oxygen, which is expressed as NOD (Nitrogenous Oxygen Demand) — the amount of oxygen required to nitrify the ammonium present in the wastewater.

5% of new biomass is created

Step 2: DENITRIFICATION

Removing nitrate (NO₃⁻) by converting it into harmless nitrogen gas (N₂)

• Anaerobic process — oxygen not needed

• Nitrate goes through intermediates: NO₂⁻ → NO → N₂O → N₂ (released into atmosphere)

• Requires organic carbon as energy source for bacteria

• Called NOE (Nitrogen Oxygen Equivalent) — represents the oxygen "saved" by using nitrate instead of O₂

Phosphorus in wastewater starts as Organic-P (bound in organic matter like food waste) and first breaks down into PO₄³⁻ (phosphate) (the dissolved form).

Two removal methods:

1. Biological — Acinetobacter bacteria absorb and store PO₄³⁻ into new biomass → removed with sludge

2. Chemical — add Fe³⁺ or Al³⁺ → reacts with PO₄³⁻ → forms solid precipitate (осадок) → removed with sludge

Volumetric Loading rate

Sludge Loading rate

X

Sedimented biomass concentration

Volumetric Loading Rate (Bv) — how much pollution enters per m³ of tank volume per day ~ 1 kg bCOD m⁻³ d⁻¹

Sludge Loading Rate (Bx) — how much pollution enters per kg of bacteria per day ~ 0.25 kg bCOD kgMLSS⁻¹ d⁻¹

X - Biomass concentration in aeration tank

Xr = Xw - Sedimented biomass concentration (bottom of settler)

Hydraulic Retention time (HRT)

Sludge retention time (SRT)

HRT = how long does water sit in the aeration tank, tells the volume needed given the flow = tank volume / flow of water coming

SRT = total biomass / biomass removed per day, How many days bacteria stay in the system.

Oxygen demand

Load

Oxygen capacity

Sludge production

Oxygen demand (OD;bCOD+NOD-NOE) - total oxygen demand in aeration tank.

!NOE is subtracted because denitrifying bacteria use nitrate as their oxygen source — so you need to pump less air into the tank.

OC =what your aerators/pumps must actually deliver, always install more aeration capacity than needed

For every kg of organic pollution eaten, bacteria produce 0.4 kg of new biomass (sludge).

Aeration methods

Energy cost = 0.2€ per kWh

Fine bubble diffusers - the most efficient

Biological wastewater treatment
with O2

COD removal + nitrification

X_H - heterotroph - removes organic pollution (COD)

X_A - autotroph - nitrification (H₄ → NO₃)

X _P - inert particulates - dead matter, waste

Biological wastewater treatment without O2

- denitrification

Savings in oxygenation costs

Heterotrophs eat organic matter (COD) using O₂ or nitrate as substitute. Autotrophs eat inorganic ammonium (NH₄⁺) always requiring O₂ to produce nitrate.

Expanded aeration

Much longer SRT 20-40 days: Microorganisms stay in the system much longer, so they consume not just the incoming organic matter but also their own cell mass → less waste in the end

Advantages:

Highly efficient BOD removal: more bacteria accumulated → higher bacteria-to-food ratio → more complete BOD removal

Limited sludge production

Disadvantages:

Suitable for small waste flows

Very low loading rate

Limited nutrient removal (N, P) - Bacteria use BOD for energy, but they only take up N and P when growing new cells. In extended aeration, bacteria are barely growing, so BOD is fully consumed but N and P pass through untreated.

Sequencing batch reactors

different processes happen in the same tank at different times

1. Fill — wastewater enters the tank

2. React — air is pumped in, bacteria break down the organic matter (like a normal aeration tank)

3. Settle — aeration stops, sludge sinks to the bottom, clean water rises to the top

4. Decant — the clean water at the top is removed through an outlet

5. Idle — tank waits until the next batch arrives, excess sludge can be wasted here

Advantages:

• Potential limited CAPEX (cheaper to build)

• Minimal footprint

• Operating flexibility and control - You can easily adjust the cycle just by changing the timing, without building anything new.

Disadvantages:

• Complexity and more labor-intensive maintenance

• Potential sludge discharge during draw phase - During decanting (step 4), if the outlet is placed incorrectly or sludge hasn't fully settled, sludge can accidentally get drawn out

Aerobic Granulation

Instead of flocs, bacteria form granules

Outer layer (red) → oxygen is available → heterotrophic bacteria break down COD, and nitrification happens

• Inner layer (blue) → no oxygen reaches here → denitrification and phosphate removal happen using stored COD

Advantages:

• High settling velocity

• High biomass retention - because they sink fast, bacteria stay in the tank rather than escaping with the effluent

• High loads possible - granules are so packed with bacteria that a small tank can treat a lot of wastewater

• Better withstands toxicants - toxic substances can only reach the outer layer, inner bacteria are protected

Disadvantage:

• Technically challenging to obtain aerobic flocs - getting bacteria to form granules instead of flocs is difficult and requires very precise conditions

Membrane bio-reactors (MBR)

MBR replaces the clarifier with a membrane filter inside the aeration tank → fewer tanks, cleaner water.

Advantages

• Long sludge age possible — in conventional systems, if you keep sludge too long it overflows with the water. In MBR the membrane physically holds all sludge back, so you can have a very long SRT without losing biomass → less sludge produced

• Almost complete disinfection — the membrane pores are so tiny that bacteria, viruses and pathogens physically cannot pass through → very clean effluent almost without extra disinfection steps

• No setling issues, high density sludge can be used — in conventional systems thick sludge settles poorly (bulking). In MBR settling doesn't matter at all since the membrane does the separation → you can have much more concentrated biomass (you can add more bacteria)

• Better system robustness — conventional systems can fail if sludge stops settling properly (bulking). MBR doesn't depend on settling at all → much more stable and reliable

• Limited footprint — no clarifier needed → smaller total system

Disadvantage

COST - Membranes are expensive to buy, maintain and replace. MBR costs roughly double in every category compared to conventional system