Anaerobic, anoxic and aerobic zones: how biological wastewater treatment works
Biological treatment is often pictured as a tank with a compressor: blow in air and the wastewater is clean. In reality a plant relies on alternating conditions: some zones have no oxygen, some have limited oxygen and some have enough, and each zone does its own part of the job. Let us look at what each zone is responsible for and why the loss of any one of them shows up in the effluent.
Why a tank with air is not enough
Wastewater is treated by microorganisms. They break down organic matter, convert nitrogen compounds and form activated sludge flocs and biofilm on surfaces. They work only under certain conditions: enough contact time with the wastewater, suitable temperature and pH, no toxic shocks, retention of biomass inside the plant and regular removal of the surplus. How residence time relates to tank volume is covered in detail in the article on hydraulic retention time.
An ordinary septic tank does only part of this work. It holds back solids and partly digests them without oxygen, but dissolved organic matter and ammonium nitrogen pass through it almost unchanged. A septic tank can therefore be the first stage of a system, but on its own it is not a biological treatment plant.
The opposite extreme is one large aerated tank with no settling zone. Organic matter is oxidised in it, but the activated sludge leaves with the water, biomass is lost and the effluent is turbid.
Three oxygen regimes
The difference between zones is easiest to explain through what bacteria "breathe". To oxidise organic matter they need an oxidant, and they choose the one that yields the most energy. Dissolved oxygen is used first. When it runs out, some bacteria switch to the oxygen bound in nitrate. When nitrate runs out too, sulfate comes into play and organic matter starts to ferment. This sequence explains both the benefit of an anoxic zone and the danger of an uncontrolled anaerobic one: reducing sulfate produces hydrogen sulfide, which smells of rotten eggs.
| Zone | Dissolved oxygen | What happens in it | Sign that the zone is not working |
|---|---|---|---|
| Anaerobic | practically none, no nitrate either | hydrolysis and fermentation of organic matter; in schemes with biological phosphorus removal, release of phosphate by the sludge | hydrogen sulfide odour, black sludge if the zone is uncontrolled |
| Anoxic | usually below 0.5 mg/L, nitrate present | denitrification: nitrate is reduced to nitrogen gas using organic matter in the wastewater | high nitrate nitrogen in the effluent, rising sludge in the secondary clarifier |
| Aerobic | usually 1.5–3 mg/L | oxidation of organic matter, nitrification — conversion of ammonium to nitrate | ammonium and turbidity in the effluent, odour above the tank |
| Settling | not aerated | separation of sludge flocs from treated water | floc carry-over, turbid discharge |
An anaerobic zone is not a defect in itself. In a designed scheme it has a task, and anoxic and aerobic stages follow it. The trouble begins when the whole system becomes anaerobic: old concrete tanks without aeration, years of accumulated sludge, a grease crust, wastewater standing without oxygen for weeks. Then fermentation and sulfate reduction run in every tank at once, there are no boundaries between zones, and processes combine at random. In that case the task of an upgrade is not to "add air everywhere" but to give the system back its sequence: where oxygen is absent on purpose, where it is limited and where there is enough.
How the zones are linked
A common flow-through scheme places the anoxic zone ahead of the aerobic one. At first glance this looks odd: nitrate is formed in the aerobic zone but used earlier along the flow. The logic lies in organic matter. Denitrifying bacteria need organic matter, and the incoming wastewater has the most of it. So part of the nitrified mixed liquor from the end of the aerobic zone is returned by pump or airlift to the start of the anoxic zone. The flow of this internal recirculation is often 2–4 times the inflow.
The second recirculation returns sludge from the settling zone to the start of the scheme. It keeps the working biomass inside the plant. The third flow goes outward: surplus sludge is withdrawn regularly, otherwise it ages, takes up volume and itself becomes a source of odour.
Denitrification is useful not only for removing nitrogen. It returns to the process part of the oxygen bound in nitrate and part of the alkalinity consumed by nitrification. This reduces the air demand — more on the share of aeration in energy costs in the article on a treatment plant's electricity use. If the scheme has no anoxic zone, denitrification may still start — in the secondary clarifier. Nitrogen bubbles lift the sludge to the surface and it leaves with the discharge. This case is covered in the article on rising sludge in the secondary clarifier.
Zones in space and in time: flow-through, SBR, MBR, biofilm
In a flow-through scheme the zones are separated by baffles and the wastewater passes through them in sequence. In an SBR — a sequencing batch reactor — the same conditions follow one another in time in a single tank: filling, aeration with pauses (the pauses act as anoxic periods), settling, decanting of treated water and withdrawal of surplus sludge. This scheme suits uneven inflow but needs automation and well-tuned phase durations: aeration that is too short does not have time to oxidise the organic matter, and aeration during settling gives a turbid decant. An example of a pneumatic SBR is SOLVIA PneumoSBR.
In an MBR — a membrane bioreactor — membranes replace the settling zone and separate treated water from the sludge. This makes it possible to hold more biomass in a smaller volume and obtain consistently clear effluent. The price is the requirements for pretreatment, membrane cleaning and aeration of the membrane unit.
A separate way to retain biomass is biofilm on a carrier. Microorganisms attach to the carrier surface and are not washed out at peak inflow. A gradient forms inside the film: an aerobic layer on the outside and an oxygen-free layer deeper in, so different processes partly run within the same carrier.
What throws biology off balance
Typical causes of upset repeat from site to site. Kitchen grease forms crusts and films that hinder oxygen transfer. Shock loads after events or equipment washing overload the sludge with organic matter. Detergents and disinfectants in large doses suppress bacteria. Industrial wastewater — from dairy, cheese and meat processing — brings a load a domestic plant is not designed for. Cold water slows nitrification. Old accumulated sludge creates anaerobic pockets even in an aerated tank.
That is why kitchens and production sites put a grease trap, an equalisation tank and, at high load, flotation ahead of the biology. Simply adding air to an old tank without pumping out the sludge and separating the zones usually stirs up the sludge and strengthens the odour. Converting existing tanks into a controlled scheme is the subject of SOLVIA Retrofit Module.
What next
To find out which zones your system lacks, collect the basic data: type of site and number of users, daily and peak flow, whether there is a kitchen, laundry or production, the existing scheme with tank volumes, whether there is aeration and a settling zone, the date sludge was last pumped out and where treated water is discharged. If you have analyses — BOD, COD, ammonium, nitrate, suspended solids at inlet and outlet — compare them with the last column of the table: it will point to the zone that is underperforming. Discharge requirements are set by the water utility or local regulator, and they are specific to each site in its discharge permit.
Equipment for the stages before and after the biology — flotation units, lamella clarifiers, presses — is collected in the industrial wastewater treatment section, and equipment for handling surplus sludge in the sludge dewatering section.