Dissolved oxygen and odour at a treatment plant: why the compressor runs but hydrogen sulfide stays

When a treatment plant starts to smell, the first instinct is to install a bigger compressor. Sometimes that helps, and sometimes the odour gets stronger, the effluent gets more turbid and the power bill goes up. The reason is that biology needs not air but dissolved oxygen in the right zone, and hydrogen sulfide forms precisely where oxygen does not reach.

Air in the pipe and oxygen in the water are different quantities

A compressor delivers air, a diffuser breaks it into bubbles, and while a bubble rises only part of its oxygen passes into the water. The rest escapes to the atmosphere. Microorganisms use only what has dissolved. So a humming compressor does not yet mean there is oxygen in the tank.

How much oxygen actually dissolves depends on many factors. Fine bubbles give up oxygen better than coarse ones. The deeper the diffuser, the longer the bubble stays in the water. Warm water holds less oxygen than cold: at saturation this is about 11 mg/L at 10 °C and about 8 mg/L at 25 °C, so in summer the same air flow leaves less margin. The more organic matter arrives with the wastewater and the more activated sludge is in the tank, the faster oxygen is consumed. Clogged diffusers and dead corners that bubbles do not reach reduce it further. The energy side — what excess air costs and how to cut it — is covered in detail in the article on a treatment plant's electricity use.

Where and how to measure oxygen

Dissolved oxygen is measured with a sensor — a fixed one in the tank or a portable meter. A single point says almost nothing. What makes sense is a profile: readings at the start and end of the aerobic zone, by the walls and in the corners, at different depths and at different times of day — at the morning peak and at night. If the middle of the tank shows 2 mg/L and a corner shows zero, anaerobic processes are already under way in that corner.

The reference values by zone are as follows. In the aerobic zone 1.5–3 mg/L is usually maintained. In the anoxic zone oxygen should be low, below roughly 0.5 mg/L, otherwise denitrification slows down. Why a scheme needs zones with different oxygen levels is covered in the article on anaerobic, anoxic and aerobic zones.

The readings themselves are interpreted together with what is seen at the outlet. Oxygen near zero with the compressor running means consumption exceeds supply: the tank is overloaded with organic matter, the diffusers are clogged or the air is escaping in coarse bubbles along one wall. Oxygen that drops at the morning peak and recovers at night shows that aeration is sufficient only for the average load. High oxygen with a turbid discharge points not to a lack of air but to a problem with the sludge or settling, and adding air is pointless here.

Where there is no oxygen at all, the meter reads zero and can no longer tell states apart. A second instrument helps here — an oxidation-reduction potential sensor (ORP, which shows how oxidising or reducing the environment is). As a rough guide, an anoxic zone with nitrate stays around zero millivolts, while clearly negative values, roughly below −200 mV, indicate conditions in which bacteria start reducing sulfate. Absolute values depend on the electrode, so it is more important to watch how the readings change over time than to compare them with other people's figures.

Where hydrogen sulfide comes from

Bacteria oxidise organic matter with whatever oxidant is at hand. While there is dissolved oxygen, aerobic bacteria do the work. When oxygen runs out, some bacteria switch to nitrate. When neither is present, sulfate-reducing bacteria take over: they use the sulfate that is always present in water and release sulfide. At neutral and acidic pH a noticeable share of sulfide exists as a dissolved gas — hydrogen sulfide — and it escapes into the air.

Sulfate reduction is favoured by the same conditions every time: a lot of organic matter, old accumulated sludge, long residence of wastewater without oxygen, dead zones, a grease crust on the surface and poor ventilation. The classic locations are old septic tanks, multi-chamber concrete tanks, sewage pumping stations with long standing time in the wet well and long pressure mains.

Hydrogen sulfide is dangerous not only because of the smell. In the gas space above the wastewater it turns into sulfuric acid and destroys concrete and metal. At high concentrations it is toxic and can no longer be smelled, so no one should go down into manholes or tanks without checking with a gas detector and without a safety line.

Why aerating an old tank can make the odour worse

If sludge, grease and sulfide have accumulated in a tank for years, suddenly switching on powerful aeration has the opposite effect. The bubbles strip dissolved hydrogen sulfide into the atmosphere — the odour above the hatch gets stronger. The air stirs up old sediment and the effluent turns turbid. Foam appears. Settling is disrupted and sludge leaves with the discharge. The first weeks after such a "repair" are often worse than before it.

The sequence that works is different. First the old sludge is pumped out and the real working volume is assessed. Then the aerated zone and the settling zone are separated, directed flow is arranged, for example with an airlift, and aeration is started gradually, giving the biomass time to grow. Converting existing tanks along these lines is the subject of SOLVIA Retrofit Module.

Where full aeration cannot be arranged, for example in a pressure main or a large pumping station, another technique is used: the environment is shifted from the sulfate-reducing regime to an anoxic one by dosing a nitrate-containing reagent. Bacteria prefer nitrate to sulfate, and hydrogen sulfide formation falls. The reagent is fed by a dosing station, and the dose is set from measurements, not by eye. For pumping stations this is combined with shorter standing time, regular removal of sediment and ventilation with treatment of the exhaust air.

Excess oxygen does harm too

The opposite mistake is no less common. Overly intense aeration breaks up activated sludge flocs, and they settle worse. Foam appears. If mixed liquor with high oxygen is returned to the anoxic zone, the oxygen slows denitrification there. And all of this costs electricity that brings no gain in quality.

Excess air often arises because one and the same air flow is used for two different tasks — oxygen supply and mixing. At night, when the load is low, little oxygen is needed, but mixing is still required. Separating these functions is the basis of the SmartAeration approach.

What next

Before replacing the compressor or adding diffusers, put together a picture of the oxygen regime:

• ✓daily and peak wastewater flow and whether there is a kitchen, laundry or production;
• ✓tank volumes, diffuser type and installation depth, compressor type;
• ✓dissolved oxygen readings at several points and at different times of day;
• ✓ORP readings in non-aerated zones, if there is a sensor;
• ✓sludge level and the date of the last pump-out;
• ✓where and when the odour is strongest: above the tank, at the outlet, at the pumping station;
• ✓signs at the outlet: turbidity, floc carry-over, foam.

With this data it becomes clear what is missing: oxygen in the right zone, time, separation of zones or removal of old sludge. Equipment for the stages before and after the biology is collected in the industrial wastewater treatment section.