During the operation of a municipal wastewater treatment plant, operators directly observe sensory indicators in the treatment process to determine if the influent is normal, if the various structures are operating normally, and if the treatment effect is stable. An experienced operator can often make a rough judgment based on observation, allowing for quicker adjustments to operating conditions. The main sensory indicators are as follows.

1. Color

In a municipal wastewater treatment plant, the color of relatively fresh influent is usually fecal yellow. If the influent is black and has a particularly strong odor, the wastewater is relatively stale and may have accumulated in the pipes for too long. The color of the mixed liquor in the aeration tank should be chocolate-like.

Color can also serve as an indicator of sludge health. A healthy aerobic activated sludge should be a chocolate-brown color. Dark black sludge typically indicates insufficient aeration, with the sludge in an anaerobic (i.e., decaying) state. Some abnormal colors in the aeration tank may also indicate the presence of certain colored substances (e.g., chemical dye wastewater) entering the treatment plant.

2. Odor

In addition to the normal fecal odor, wastewater treatment plants sometimes have a rotten egg smell near the sump, caused by a small amount of hydrogen sulfide gas produced by the decay of wastewater in the pipes. Odor can also indicate whether the wastewater treatment plant is operating normally.

A normal wastewater treatment plant should not produce unpleasant odors. A well-collected mixed liquor sample from the aeration tank should have a slight musty odor. Once the sludge odor turns into a putrid odor, the sludge color appears very dark, and the sludge will also emit a rotten egg-like smell (hydrogen sulfide smell). If there are other pungent and unbearable odors, it indicates that industrial wastewater has entered.

3. Foam

Foam can be divided into two types: chemical foam and biological foam. Chemical foam is formed by detergents in wastewater under aeration mixing and stripping. In the early stages of activated sludge cultivation, there is more chemical foam, sometimes piling up into white foam mountains several meters high on the surface of the aeration tank.

In daily operation, if white wave-like foam is found in the aeration tank, the amount of excess sludge discharged should be reduced. Dark black foam indicates sludge aging, and the amount of excess sludge discharged should be increased. Biological foam is brown and can also accumulate very high on the aeration tank and enter the secondary sedimentation tank to flow away with the water. This may be caused by the bacterium Cano, usually due to a large amount of oil and grease in the influent, such as hotel wastewater.

4. Bubbles

The appearance of bubbles in the secondary sedimentation tank indicates that the sludge retention time in the tank is too long, and the sludge return rate should be increased. If the sludge layer in the sedimentation tank is too thick, the bottom sludge will be in an anaerobic state, producing gases such as hydrogen sulfide, methane, and carbon dioxide.

These gases escape from the water surface in the form of bubbles. When the bubbles rise, they cause the flocs to rise with the bubbles and flow out with the effluent from the sedimentation tank, resulting in a decrease in effluent water quality.

5. Water Temperature

Water temperature is closely related to the treatment efficiency of the aeration tank. The water temperature of a wastewater treatment plant changes slowly with the seasons and remains almost unchanged throughout the day. If significant changes are observed within a day, check for the entry of industrial cooling water. When the temperature in the aeration tank is below 8℃, the BOD5 removal rate is often below 80%.

6. Water Flow State

Observing the water flow state in the aeration tank can determine the short-circuiting situation. Short-circuiting refers to wastewater flowing directly from the inlet to the outlet, resulting in an effective retention time lower than the design value and reduced treatment efficiency.

Sometimes, the short-circuiting of wastewater flow can be identified by observing the flow of foam, suspended solids, and floating matter in the tank. Installing appropriate baffles can solve this problem.

7. Aeration Amount

Observe whether the aeration amount in the aeration tank is uniform, whether the aeration amount stirs up all the sludge, whether there are any dead zones, and in terms of mechanical aeration, observe the size of the waves. The size of the waves indicates whether the immersion depth of the aerator is suitable.

Low dissolved oxygen concentration in the aeration tank also indicates that the blade immersion depth is not suitable, and the immersion depth of the blades should be observed to achieve optimal oxygenation efficiency.

8. Effluent Observation

Normally, the effluent from a wastewater treatment plant is highly transparent, with few suspended particles, a slightly yellowish color, and no odor. In summer, there are often a large number of water fleas (commonly known as fish insects) in the secondary sedimentation tank, and at this time, the effluent BOD5 may be around 3-5 mg/L.

Experienced operators can roughly judge the effluent water quality with the naked eye. If the effluent transparency suddenly deteriorates and contains a large amount of suspended solids, immediately check whether the sludge discharge is timely, whether the sludge discharge pipe is blocked, or whether the impact of peak flow on the secondary sedimentation tank is too large.

9. Sludge Discharge Observation

First, observe whether the activated sludge in the sludge outflow well of the secondary sedimentation tank flows out continuously and at a certain concentration. If wastewater is found to flow out during sludge discharge, adjustments should be made to the degree of gate valve opening and the control of sludge discharge time. Observe the sludge thickening tank to see if a large amount of sludge is carried out during skimming.

10. Observation of Various Flow Rates

Make full use of metering equipment or the relationship between water level and flow rate to firmly grasp the influent volume, return flow, sludge discharge volume, and air pressure and their changes during the observation period.

11. Touch Inspection

Touching is an important means of checking the operation of a wastewater treatment plant. If the surface temperature of water pumps, blowers, and motors feels hotter than usual, they should be further inspected to prevent major accidents.