Phenomena of Declined Settling Performance of Activated Sludge
The main phenomena observed when the settling performance of activated sludge declines include:
- An obvious increase in effluent COD and SS values, and a decrease in BOD removal efficiency.
- Fluctuating DO levels in the aeration tank, a decrease in sludge concentration, and an increase in SVI values.
- Microscopically, the flocs appear loose, with fewer protozoa and more filamentous bacteria.
- Floating sludge in the secondary clarifier effluent.
- Abnormal color and fragmentation of the activated sludge.
The decline in sludge settling performance is often accompanied by several of these phenomena. If this condition persists for a long time, the concentration of return sludge in the secondary clarifier will gradually decrease, the sludge blanket level will rise, and eventually, sludge bulking (flotation) in the secondary clarifier may occur.
Causes of Decline in Settling Performance of Activated Sludge
1. Poor Flocculation of Activated Sludge
In this case, the activated sludge under microscopic inspection primarily consists of swimming-type protozoa, such as Epistylis, Vorticella, and Trachelomonas. The cause of this phenomenon is that activated sludge contains many heterotrophic bacteria, which provide a food source for swimming-type protozoa. After 30 minutes of standing, the sludge remains dispersed with no distinct interface between the supernatant and settled sludge. This phenomenon can occur in both high and low BOD load conditions. In the case of a high BOD load, the wastewater entering the aeration tank leads to rapid bacterial growth under the influence of oxygen. As a result, the newly generated sludge grows rapidly, and during the transition from new to mature sludge, this phenomenon is inevitable. In low BOD load conditions, the sludge is in a starved state, causing selective pressure where only microorganisms suited to the environment survive, leading to sludge disintegration and a subsequent decrease in settling performance.
2. Decreased Density of Activated Sludge
(1) N2-Induced Decrease in Density
The reduction in density caused by nitrogen (N₂) mainly occurs during the nitrogen cycle, particularly through denitrification. The reaction is as follows:NO₃−+cBOD→(bacteria)NO₂−+CO₂+H₂O\text{NO₃}⁻ + \text{cBOD} \xrightarrow{\text{(bacteria)}} \text{NO₂}⁻ + \text{CO₂} + \text{H₂O}NO₃−+cBOD(bacteria)NO₂−+CO₂+H₂O NO₃−+cBOD→(bacteria)N₂↑+CO₂+H₂O\text{NO₃}⁻ + \text{cBOD} \xrightarrow{\text{(bacteria)}} \text{N₂}↑ + \text{CO₂} + \text{H₂O}NO₃−+cBOD(bacteria)N₂↑+CO₂+H₂ODenitrification’s final product is N₂, which can be adsorbed by the sludge. As the N₂ is released, the sludge adsorbing the N₂ rises with it, leading to sludge flotation.
(2) Sludge Density Reduction Due to Over-Aeration
Under high aeration conditions, the sludge undergoes intense agitation, which makes the microbial flocs more likely to absorb oxygen, causing the sludge to rise in the clarifier. The accelerated oxidation process in high aeration conditions leads to partial oxidation of the sludge, which manifests as a slight whitening of the sludge. This oxidation process also lowers the density of the sludge.
(3) Low Inorganic Matter Leading to Density Reduction
The content of inorganic matter can be evaluated by analyzing MLSS (Mixed Liquor Suspended Solids) and MLVSS (Mixed Liquor Volatile Suspended Solids). In general, municipal wastewater has an MLVSS/MLSS ratio of 65%-70%. If this ratio is higher, it indicates excessive organic components, which increases the likelihood of sludge washout.
Solutions for the Decline in Settling Performance of Activated Sludge
1. Solutions for Poor Flocculation-Induced Performance Decline
Gradually increase the BOD load as the water temperature rises. For municipal wastewater, the BOD load is generally maintained between 0.05–0.10 kg BOD/MLSS/m/d. Control the sludge age in winter to be >18 days, and in summer, to be >12 days.
2. Solutions for Low Inorganic Matter-Induced Performance Decline
In cases where inorganic matter is low, the addition of powdered carbon, diatomaceous earth, or lime to the sludge with a fixed sludge age can gradually increase the inorganic content, thus improving the settling properties of the sludge. The improvement in settling performance is a slow process and may take one year or longer, so it is important to combine emergency measures with routine operations.