Operator Resources

Winter Nitrification

• Keep your mixed liquor solids high
• Effluent alkalinity (total) must be above 100 mg/L

What can an operator do when the same causes or process upset and permit violation keep recurring?

by Edward L. Gillette, Jr.

In providing process troubleshooting services to all types of wastewater treatment plants (both industrial and municipal) across Pennsylvania, I find that the major causes of process upset and permit violation keep recurring.

First is the failure to waste enough sludge to keep the process balanced. An activated sludge treatment plant always produces more sludge than is required to balance the plant. In conventional detention time (time in the aeration tanks) processes, which is usually 12 hours or less, anywhere from 0.4 pounds to 0.6 pounds of solid waste must be wasted for each pound of Biochemical Oxygen Demand (BOD) fed to the plant. Each plant will have a different wasting number, so 0.4 to 0.6 is a good place to start. In long detention time processes, which can be defined as more than 24 hours, the solid production may be as low as 0.2 pounds of solids for every pound of BOD. The major point is that all plants need to waste solids virtually every day. There is no such thing as a plant that does not have to waste solids.

The second major cause of process upset and permit violation is aeration capacity. The bacteria we use in biological treatment require oxygen to eat the organic material in the wastewater. If a plant has an oxygen shortage, it usually occurs at peak loading periods, during the summer when wastewater is the hottest and it is most difficult to transfer oxygen.
The third major problem is lack of nutrients. For a biological wastewater treatment process to work, adequate amounts of the following two nutrients must be present: Nitrogen (N) in the form of ammonia (NH3) and Phosphorus (P) in the form of phosphate (PO4). The required ratio is 100 parts BOD to five parts Nitrogen to one part Phosphorus. If adequate Nitrogen is not present, it is my experience that a filament organism culture will form, causing effluent permit violations due to poor settling. If adequate Phosphorus is not present, high effluent solids will result from poor floc formation, usually known as pin floc. Nitrogen is easiest to add as Urea; Phosphorus is best added as phosphoric acid.

When a wastewater treatment plant is required to remove ammonia-nitrogen biologically, there are critical operation factors that must be recognized. These are:

1. The best pH for nitrification is 7.8 to 8.2 rather than the 6.5 to 7.5 operating range of most biological plants. pH adjustment to their optimum range will help the process.
2. There are three other criteria factors for nitrification that the operator must consider. They are alkalinity, temperature, and dissolved oxygen.

Alkalinity

The best-working definition of alkalinity is resistance to pH change. Alkalinity comes in two forms: Hydroxyl, which is present only above pH 8.6, and carbonate-bicarbonate, which is present between pH 4.3 and 8.6. The biochemical reaction that converts ammonia (NH3) to nitrate (NO3) consumes 7.2 parts alkalinity per part ammonia converted. To this requirement must be added 60 to 100mg/L carbonate-bicarbonate alkalinity to keep the clarifier effluent pH from decreasing below NDPES permit limits.

Assuming a domestic wastewater with up to 30 mg/L nitrogen available as ammonia or organic nitrogen (NH3 plus organic nitrogen = Total Kjedahl Nitrogen) after BOD removal, the alkalinity required is 7.2 x 30 + 100 = 316 mg/L alkalinity, which is measure as calcium carbonate (CaCO3). If this amount of alkalinity is not present during the startup, the nitrification process will not start. If the alkalinity declines below the required amount, the nitrification process will stop and clarifier effluent pH will decrease and may cause an effluent pH violation. We are fortunate in Pennsylvania that most of the heavily populated areas are hard water sources, which implies high alkalinity. If the required amount of alkalinity is not present, it must be addressed.

The conventional chemical feed for alkalinity addition tends to be caustic soda (NAOH, Sodium Hydroxide). If this chemical is used, alkalinity in the hydroxyl form is supplied. It was initially thought that none of this hydroxyl alkalinity was available for nitrification. Tests have been conducted, and it has been found that approximately 10% of this hydroxyl alkalinity converts to the required carbonate-bicarbonate alkalinity form. This 10% conversion means that 10 times the amount of caustic soda will have to be pumped into the system. It is far better to add lime or sodium bicarbonate (baking soda).

Remember, alkalinity must be tested analytically and added if sufficient amounts are not present, a step that must take place or the nitrification process will not work.

Temperature

Temperature of the wastewater and in the aeration tank is critical to nitrification. Temperatures below 55° F begin to inhibit nitrification; below 55° F, the process virtually stops. It will not proceed again until the temperature goes above 55° F and stays there for two weeks.

Dissolved Oxygen

Unlike BOD removal systems, which can operate at very low or zero DO, nitrification systems require a minimum of 2.0 mg/L DO (3.0 mg/L is better) or nitrification is inhibited.

Two of these items, alkalinity and DO, are operationally controlled and can mean success or failure of the process and plant. However, there isn't much an operator can do about the temperature.

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