Sludge rising in the secondary clarifier: denitrification where nobody expects it
At a biological treatment plant the sludge settles properly in the clarifier, but an hour or two later dark mats appear on the surface and the effluent turns turbid. A common cause is not "bad sludge" or a lack of air, but denitrification that has started in the clarifier, where it should not happen. Below is how to recognise it and what is changed first at the plant.
What should happen in the secondary clarifier
Biological treatment relies on activated sludge, a community of bacteria that oxidises organic matter and ammonium in the aeration tank (the zone with air supply). From the aeration tank the mixed liquor flows to the secondary clarifier. There the sludge should settle calmly, the clarified water should leave to the outlet, and the settled sludge should return to the start of the process.
This flow is called return sludge. At small plants it is most often pumped by airlifts, pipes with air injected at the bottom: the bubbles lift a column of water together with the sludge without a pump. Biomass growth is removed separately as waste sludge.
A clarifier is a separation zone, not a reactor. The less biology happens in it, the more stable it is. How the zones inside a compact plant are arranged is covered in Deep biological treatment plant.
Why settled sludge rises
In the aeration tank nitrifying bacteria convert ammonium to nitrate. This is a normal and necessary step. Next, the nitrate should reach the anoxic zone, a section without air supply where other bacteria take oxygen from nitrate to oxidise organic matter, and nitrogen leaves to the atmosphere as a gas. This process is called denitrification. For nitrate to reach the anoxic zone, part of the flow from the end of the aeration tank is returned to its start. This is the nitrate recycle.
If the recycle is weak, nitrate travels on to the clarifier. There the settled sludge lies in a dense layer, the dissolved oxygen in it runs out quickly, and the bacteria keep breathing, now on nitrate. Nitrogen gas is released. The bubbles get trapped in the flocs, the flocs become lighter than water, and the sludge layer lifts off the bottom in mats. Part of the sludge leaves with the treated water, and the plant loses biomass.
Three conditions must coincide for this kind of rising: nitrate enters the clarifier, activated sludge is present in it, and the sludge stays there long enough without oxygen. Hence the paradox: the problem appears more often at plants where nitrification works well. The biology works, but the processes are distributed across the plant differently from the design intent.
The risk is higher in summer. In warm water bacteria work faster, and gas in the sludge layer is released sooner.
How to tell rising sludge from poor settling
A similar picture on the surface can have different causes, and they are dealt with differently.
With poor settling, the sludge settles slowly from the very start, the water above it is turbid, the flocs are small or loose. The causes are sought in the sludge structure: organic overload, filamentous bacteria, a toxic discharge, sludge that is too young or too old.
With denitrification rising, the sludge first settles normally with a sharp sludge line, and then, usually after one to three hours, the layer lifts. The floating clumps are dark, with visible bubbles. If such a clump is stirred, the gas escapes and the sludge sinks again.
The check is simple. A sample of mixed liquor from the end of the aeration tank is poured into a one-litre cylinder. After 30 minutes the volume of settled sludge is recorded; this is the usual settleability test. The sample is then left standing for another one to two hours. If sludge that settled normally rises with bubbles, the diagnosis is almost confirmed.
Persistent brown foam on the surface of the aeration tank is a third, separate problem linked to filamentous microorganisms. The measures in this article do not solve it.
What to check at the plant
Before changing anything, it is worth going through the list. Each item either confirms denitrification in the clarifier or points to another cause.
- ✓Nitrate at the aeration tank outlet and in the clarified water
- ✓Dissolved oxygen at the end of the aeration tank
- ✓Sludge blanket height in the secondary clarifier
- ✓Time after which the sludge rises in the cylinder
- ✓Return sludge flow relative to the inflow
- ✓Operation of each airlift: air supply, no blockages
- ✓Dead corners on the clarifier floor where sludge does not move
- ✓Nitrate recycle mode and flow
- ✓Date and volume of the last waste sludge removal
Nitrate and oxygen are measured with rapid tests or a portable meter; no laboratory is needed.
What is changed first
The logic is simple: get nitrate out of the clarifier and do not let sludge sit there.
Return sludge. If sludge lies on the bottom longer than necessary, the return sludge flow is increased. With airlifts this is done by adding air, cleaning, or replacing the pipe with a larger diameter. For many layouts the reference is a return of roughly 50–100 % of the inflow, but the exact value depends on the sludge concentration and the clarifier design. Too strong a return is also harmful: it stirs up the clarifier and overloads it hydraulically.
Nitrate recycle. Increasing it moves denitrification to the anoxic zone, where it belongs. The recycle is matched to the volume of that zone: if too much water with dissolved oxygen is returned to it, denitrification there slows down.
Dead zones. Dead corners on the floor, a poorly placed mixed liquor inlet, insufficient slope to the withdrawal point are places where sludge sits for hours. They are eliminated by moving the withdrawal point, adding an airlift or a scraper.
Waste sludge. If there is too much sludge in the system, the blanket in the clarifier grows, and the sludge residence time there increases by itself. Regular waste sludge removal lowers this risk. At larger volumes the sludge is thickened and dewatered; the equipment for this is in the sludge dewatering section.
Two common mistakes are simply adding air in the aeration tank or pumping the sludge out. More air strengthens nitrification, more nitrate is formed, and the rising may get worse. Pumping out gives relief for a few days, but if nitrate keeps entering the clarifier and the return stays weak, the problem comes back. Separate control of oxygen supply and mixing, which makes this kind of tuning easier, is described in the SmartAeration development. How large a share of plant energy goes to aeration is covered in Where treatment plant electricity goes.
In compact plants and SBR reactors (sequencing batch reactors, where aeration, settling and decanting take place in one tank in a cycle) the volumes are small and a failure shows up faster. There the same mechanism is controlled through the length of the unaerated phase and the settling time before decanting.
What next
If sludge is rising at your plant, start with observations. Run the cylinder test with a two-hour hold, measure nitrate and dissolved oxygen at the aeration tank outlet, check all airlifts and record the sludge blanket height in the clarifier. Data from a few days usually shows what the issue is: nitrate, hydraulics or the sludge itself.
If tuning alone is not enough and the plant needs modification, such as separating an anoxic zone, rebuilding the sludge return or turning an existing tank into managed biological treatment, see the Retrofit Module development. Equipment for industrial wastewater is collected in the industrial wastewater treatment section.