Nitrification Failure and Bulking at an Oxidation Ditch

Work the mass balance.

• Mixed liquor solids: 0.9 MG x 8.34 lb/gal x 1,000,000 gal/MG = 7.51 Mlb of water; x 3,400 mg/L / 1,000,000 = 25,500 lb of MLSS in the ditch.
• Waste sludge: 0.090 MG x 8.34 = 0.751 Mlb; x 0.0065 = 4,880 lb/day.
• Effluent solids: 3.1 MGD x 8.34 = 25.85 Mlb; x 18/1,000,000 = 465 lb/day.
• SRT = 25,500 / (4,880 + 465) = 4.8 days.

At 13 degrees C a nitrifying population needs roughly 8-10 days minimum SRT, and a designer would want 12-15 days for reliability. At 4.8 days the nitrifiers are being washed out faster than they can grow. That is the primary cause.

Two things make it worse:

• Dissolved oxygen at 0.8-1.1 mg/L in the ditch. Nitrification needs about 2 mg/L; below that the rate falls off and the nitrifiers lose the competition for oxygen to the heterotrophs eating the septage.
• Alkalinity. Nitrification consumes about 7.14 mg of alkalinity per mg of ammonia-nitrogen oxidized. To nitrify 26 mg/L of ammonia takes about 186 mg/L of alkalinity, and the influent only carries 130 mg/L. Denitrification in the anoxic zone recovers roughly half of that, which is exactly why the ditch sits at pH 6.7 instead of crashing - but there is no margin left, and pH below about 6.8 slows nitrification further.

Low SRT to start, low DO and low alkalinity pushing it down, and septage raising the ammonia load. Every arrow points the same way.

Cutting wasting is directionally right; adding chlorine is the wrong tool.

Cutting wasting: Reducing wasting raises SRT, which is exactly what the nitrifiers need. But it also raises MLSS, and the clarifier blanket is already rising with an SVI of 240. A blanket that climbs faster than the return sludge can keep up will put solids over the weir and turn a 6 mg/L CBOD problem into a TSS and CBOD violation. So reduce wasting in steps and watch the blanket and effluent TSS daily rather than shutting wasting off.

Adding chlorine: Breakpoint chlorination takes roughly 7.6 mg of chlorine per mg of ammonia-N. Removing 5.8 mg/L of ammonia means about 44 mg/L of chlorine - at 3.1 MGD that is 3.1 x 8.34 x 44 = about 1,140 lb of chlorine per day, an enormous chemical cost, plus trihalomethanes in the effluent, chlorine residual toxicity in the receiving stream, and if any return stream carries it back to the ditch it will damage the biomass you are trying to save. It also does nothing about the SRT problem.

What I would actually do, in order:

1. Stop or cut the septage for 30 days. It is the newest load and the easiest to remove. That alone may recover most of the ammonia.
2. Restore DO to 2 mg/L - check diffuser fouling and blower condition before assuming the blowers are actually delivering design air.
3. Add alkalinity as caustic or soda ash to hold ditch pH at 7.0-7.2; without it the biology cannot use the DO or the SRT you give it.
4. Step wasting down toward a 10-12 day SRT, with daily blanket depth and effluent TSS checks.
5. Reduce infiltration on a capital timeline - the ditch is 24% over design flow and that is the root of the SRT.

Find the filament before you treat the filament.

An SVI of 240 with a rising blanket means filamentous organisms are competing successfully. The two most likely causes here are visible in the data:

• Low DO (0.8-1.1 mg/L) selects for filaments with a high oxygen affinity - Microthrix and type 1701 are the usual suspects.
• Septage brings sulfide, reduced sulfur and a large shot of readily degradable substrate in a small package. Sulfide from septic loads is a strong selector for type 021N and Thiothrix.

What I would do:

1. Microscope the mixed liquor and identify the filament if you can, or send a slide out. Chlorinating or dosing polymer without knowing the organism is a guess that often makes the blanket worse.
2. Fix the selector conditions first - get DO up and stop the septic load. This is the same fix that restores nitrification, which is convenient.
3. Protect the clarifier: adjust the return rate, increase polymer if you use it, and be prepared to haul solids rather than let them go over the weir. Taking the TSS limit seriously matters more than the SVI number.
4. Do not dump hypochlorite into the return line to knock back filaments while the nitrifiers are already at 4.8 days SRT. You will kill the slow-growing nitrifiers before you kill the filaments, and the ammonia problem gets worse.
5. Once DO, septage and alkalinity are under control, expect the SVI to drift back over a few weeks. Filament control is a weeks-long correction, not a shift-long one.