Safety Precautions for the Startup Operation of a Wastewater Treatment Plant
1. Purpose
The sludge retention time (SRT) is a core control parameter in the activated sludge system, directly determining microbial population structure, pollutant removal efficiency, and sludge characteristics. In nitrogen and phosphorus removal processes (such as A²/O, oxidation ditches, SBR, etc.), nitrifying bacteria (long generation time) and polyphosphate-accumulating bacteria (short generation time) inherently conflict with SRT requirements. In practical operation, rapid SRT adjustments, inaccurate measurements, or parameter mismatches can easily lead to issues like effluent exceedances, sludge abnormalities, and increased energy consumption.
II. Core Principles of Sludge Age Adjustment
• Definition: The ratio of the total amount of active sludge in the system to the daily residual sludge discharge (d), representing the average microbial retention time.
• Control logic: Increase sludge discharge → shorten SRT; reduce sludge discharge → extend SRT.
• Process Adaptation Range
◦ COD removal only: 5–10 d
Nitrification and denitrification: ≥10 days (≥15 days in winter)
Synergistic nitrogen and phosphorus removal: 12–20 days
Extended aeration/MBR: 20–30 d
3. Common Issues During Sludge Age Adjustment
(1) Issues Caused by SRT Too Short (Excessive Sludge Removal)
Nitrification collapse and ammonia nitrogen exceeding standards
Nitrifying bacteria have a long generation time (10–20 days). When the SRT is less than the minimum generation time, nitrifying bacteria are continuously washed out, leading to a sharp decline in ammonia nitrogen removal efficiency and rapid exceedance of effluent NH₃–N limits.
2. The sludge is immature and has poor settling properties
Microbial proliferation is too rapid, floc structure is loose, SV30 is low, SVI is abnormal, and fine flocs are carried over in the effluent from the secondary sedimentation tank, leading to an increase in SS.
3. Sharp increase in sludge production and rising disposal costs
High F/M operation results in high sludge yield, high moisture content, and difficulty in dewatering, leading to increased sludge dewatering frequency and chemical dosage.
4. Weak system resistance to impact
Insufficient biomass, low tolerance to load, toxicity, and temperature fluctuations, leading to significant water quality variations.
(2) Issues arising from excessive SRT (insufficient sludge discharge)
Sludge aging and floating sludge in secondary sedimentation tanks
Excessive endogenous respiration leads to sludge mineralization and flocculant fragmentation, resulting in rapid settling but prone to disintegration, with turbid effluent and超标 SS and turbidity.
2. Phosphorus removal efficiency decline
Phosphate-accumulating bacteria rely on a short SRT to remove phosphorus by sludge discharge; an excessively long SRT leads to phosphorus accumulation within the system and subsequent re-release, resulting in TP exceeding the standard.
3. Increasing energy consumption and operating costs
High MLSS leads to reduced oxygen transfer efficiency, forcing an increase in aeration volume; sludge aging results in higher energy consumption for reflux and mixing.
4. Filamentous bulking / Sludge bulking risk
Low load and long sludge age can easily induce excessive filamentous bacteria growth, leading to increased SVI and failure in sludge-water separation.
5. Accumulation of Soluble Microbial Products (SMP)
Endogenous metabolites are released, leading to increased COD and color levels in the effluent, and a rise in the load of advanced treatment.
(3) Systemic issues arising from improper adjustment operations
1. Excessive range adjustment, system oscillation
A significant one-time increase or decrease in sludge removal caused a sudden SRT change, leading to drastic shifts in microbial community structure and synchronous fluctuations in effluent indicators (NH₃–N, TN, TP, SS).
2. SRT Calculation and Measurement Distortion
Inaccurate MLSS monitoring, uncalibrated sludge discharge flow, and failure to account for sludge in the return/secondary settling tank result in a significant deviation between the nominal SRT and the actual SRT.
3. Lack of temperature adaptation
The nitration rate significantly decreases at low temperatures, and the SRT is not extended according to the seasons, leading to frequent ammonia nitrogen exceedances in winter.
4. Synergistic Contradiction in Denitrification and Phosphorus Removal
In a single-sludge system, maintaining nitrification results in a longer SRT and poorer phosphorus removal, while prioritizing phosphorus removal leads to a shorter SRT and insufficient nitrification, creating a trade-off between the two.
5. Equipment and Operation & Maintenance Shortcomings
Sludge pump blockage, valve internal leakage, and sludge accumulation in the discharge pipe lead to unstable sludge discharge, causing passive drift in SRT.
(4) Secondary operational issues
• DO Imbalance: Long SRT with high MLSS prone to localized hypoxia; short SRT susceptible to excessive aeration.
• Intensified carbon source competition: Unreasonable SRT exacerbates the competition for carbon sources between denitrification and phosphorus removal, leading to simultaneous exceedance of TN and TP.
• Microscopic examination indicates abnormal biological indicators: short SRT is dominated by free bacteria/small flagellates; long SRT shows aging indicators such as rotifers and epifiltrum.
