Effluent Quality
1. Excess Organic Matter
The factors that mainly affect the treatment efficiency of organic matter include:
(1) Nutrients
In general, nutrients such as nitrogen and phosphorus in wastewater are sufficient for microbial needs, and often in excess. However, when the proportion of industrial wastewater is relatively high, the carbon-nitrogen-phosphorus ratio should be checked to ensure it meets the standard of 100:5:1.
● If nitrogen is deficient, ammonium salts are usually added.
● If phosphorus is deficient, phosphoric acid or phosphates are usually added.
(2) pH
The pH of wastewater is usually neutral, ranging from 6.5 to 7.5. A slight decrease in pH may be caused by anaerobic fermentation in the sewage pipeline. Significant pH drops during the rainy season are often due to urban acid rain, especially in combined sewer systems.
A sudden and large change in pH, whether an increase or decrease, is usually caused by the large discharge of industrial wastewater. Adjusting the pH of wastewater usually involves adding sodium hydroxide or sulfuric acid, but this significantly increases treatment costs.
(3) Oils and Grease
When the content of oily substances in wastewater is high, the aeration efficiency of the aeration equipment will decrease. Without increasing aeration, the treatment efficiency will drop, but increasing aeration inevitably raises operating costs.
High oil content also reduces the settling performance of activated sludge, and in severe cases can cause sludge bulking, leading to suspended solids (SS) in the effluent exceeding standards. For influent with high oil content, oil removal equipment should be added in the pretreatment stage.
(4) Temperature
Temperature has a wide range of effects on the activated sludge process.
● First, it affects microbial activity. In winter, if no control measures are taken, the treatment efficiency will decrease.
● Second, it affects the separation performance in secondary sedimentation tanks; for example, temperature changes can cause density currents and short-circuiting; low temperatures increase sludge viscosity and reduce settling performance.
● Third, temperature affects aeration efficiency. In summer, higher temperatures lower dissolved oxygen saturation, making oxygen transfer more difficult and reducing aeration efficiency. It also decreases air density, so to maintain the same air supply, the air volume must be increased.
2.TP (Total Phosphorus) Exceeding Standards
Biological phosphorus removal relies on polyphosphate-accumulating organisms (PAOs) releasing phosphorus under anaerobic conditions and absorbing excess phosphorus under aerobic conditions. Phosphorus is removed by discharging phosphorus-rich excess sludge. Causes for effluent TP exceeding standards include:
(1) Temperature
Temperature affects phosphorus removal less obviously than biological nitrogen removal. Within a certain range, biological phosphorus removal operates successfully despite moderate temperature changes. Experiments show that phosphorus removal is preferable at temperatures above 10°C, since PAOs grow slower at low temperatures.
(2) pH Value
Between pH 6.5 and 8.0, phosphorus content and uptake rate of polyphosphate microorganisms remain stable. When pH drops below 6.5, phosphorus uptake declines sharply. Sudden pH drops cause rapid increases in phosphorus concentration in both aerobic and anaerobic zones; the greater the pH drop, the more phosphorus is released. This release is not a physiological or biochemical response of PAOs but a purely chemical "acid dissolution" effect. Larger anaerobic phosphorus release due to pH drop results in lower aerobic phosphorus uptake, indicating the release is destructive and ineffective. Slight phosphorus uptake occurs when pH increases.
(3) Dissolved Oxygen (DO)
Each mg of molecular oxygen can consume 1.14 mg of biodegradable COD, inhibiting PAO growth and hampering phosphorus removal. The anaerobic zone should maintain low DO to favor acid fermentation by anaerobes, promoting phosphorus release by PAOs, and to reduce consumption of biodegradable organic matter, enabling PAOs to synthesize more PHB. Conversely, the aerobic zone requires higher DO to support PAOs in degrading stored PHB to obtain energy for absorbing dissolved phosphate from sewage and synthesizing intracellular polyphosphate. DO should be controlled below 0.3 mg/L in anaerobic zones and above 2 mg/L in aerobic zones to ensure effective anaerobic phosphorus release and aerobic uptake.
(4) Nitrate Nitrogen in Anaerobic Tank
Nitrate nitrogen in the anaerobic zone consumes organic substrates, inhibiting PAOs' phosphorus release and thus affecting phosphorus uptake under aerobic conditions. Also, nitrate nitrogen is used by denitrifying bacteria as electron acceptors for denitrification, which interferes with fermentation processes producing acids needed for PAO phosphorus metabolism, suppressing PAO phosphorus release, uptake, and PHB synthesis. Each mg of nitrate nitrogen consumes 2.86 mg of biodegradable COD, suppressing anaerobic phosphorus release. Typically, nitrate nitrogen is controlled below 1.5 mg/L.
(5) Sludge Age
Phosphorus removal is mainly achieved by discharging excess sludge; thus, the amount of excess sludge determines removal efficiency. Sludge age directly affects sludge discharge volume and phosphorus uptake. Lower sludge age improves phosphorus removal by increasing excess sludge discharge and system phosphorus removal, reducing phosphorus in secondary sedimentation effluent. However, biological nitrogen and phosphorus removal requires sufficient sludge age for nitrifying and denitrifying bacteria growth, often making phosphorus removal unsatisfactory. Generally, sludge age in phosphorus removal systems is controlled between 3.5 and 7 days.
(6) COD/TP Ratio
In biological phosphorus removal, the type and amount of organic substrates in the anaerobic stage, and the ratio of nutrients needed by microbes to phosphorus in sewage, critically affect removal efficiency. Different substrates induce varying phosphorus release and uptake. Low molecular weight, easily degradable organics (e.g., volatile fatty acids) are readily used by PAOs to release stored polyphosphate and induce phosphorus release strongly. High molecular weight, hard-to-degrade organics induce weaker phosphorus release. The more complete the phosphorus release anaerobically, the greater the phosphorus uptake aerobically. PAOs use energy from anaerobic phosphorus release to absorb low molecular organics for survival under anaerobic conditions. Hence, sufficient organic matter (COD/TP > 15) is essential for PAO survival and ideal phosphorus removal.
(7) Readily Biodegradable COD (RBCOD)
Studies show that substrates like acetic, propionic, and formic acid lead to high phosphorus release rates, which depend on activated sludge concentration and microbial composition, not substrate concentration. Such phosphorus release follows zero-order kinetics. Other organics must be converted to these small molecules before PAOs can metabolize them.
(8) Glycogen
Glycogen is a large branched polysaccharide composed of glucose units and serves as intracellular energy storage. In PAOs, glycogen forms in aerobic environments, storing energy metabolized under anaerobic conditions to produce NADH (a precursor for PHA synthesis), providing metabolic energy. Excessive aeration or over-oxidation reduces glycogen in PAOs, causing NADH deficiency in anaerobic conditions and poor phosphorus removal.
(9) Hydraulic Retention Time (HRT)
In well-operated municipal biological nitrogen and phosphorus removal systems, phosphorus release and uptake typically require 1.5–2.5 hours and 2.0–3.0 hours, respectively. Phosphorus release is somewhat more critical; thus, anaerobic HRT is closely monitored. Too short anaerobic HRT prevents sufficient phosphorus release and organic matter breakdown to low fatty acids; too long increases cost and side effects. Phosphorus release and uptake are interrelated: sufficient anaerobic release improves aerobic uptake and vice versa, creating a positive cycle. Operational data indicate suitable HRTs as 1h15m–1h45m anaerobic and 2h–3h10m aerobic.
(10) Return Ratio (R)
In A/O (anaerobic/aerobic) processes, it is critical to maintain enough dissolved oxygen in the activated sludge returning from the aeration tank to the secondary sedimentation tank to prevent anaerobic phosphorus release in the latter. Without fast sludge removal, thick sludge layers cause anaerobic phosphorus release despite high DO. Thus, return ratios should not be too low, ensuring quick sludge discharge from sedimentation tanks. Excessively high return ratios increase energy consumption and reduce sludge retention time in the aeration tank, impairing BOD5 and phosphorus removal. Optimal return ratios range between 50% and 70%.
3.Mechanical and Electrical Equipment
Stable operation of sewage and sludge treatment depends on reliable mechanical and electrical equipment, which also impacts plant energy consumption.
(1) Bar Screen Machine
The first step in treatment, prone to faults that can halt sewage inflow. Common issues:
Jamming due to bearing wear or mechanical failure. Requires regular lubrication and inspection.
Blockage by fibers, plastic bags causing reduced flow and overflow. Requires technical upgrades or manual cleaning.
(2) Lift Pumps
Mostly submersible pumps. Pump impeller and seal ring gaps can be clogged by debris, reducing sealing and efficiency, causing motor failure. Regular inspection, pump rotation, and enhanced bar screen operation recommended.
Variable inflow and collection system design require pumps arranged in gradients with fixed-speed and variable-speed pumps to handle fluctuations efficiently.
(3) Blowers
Key and energy-intensive equipment. Parameters include airflow, pressure, power consumption, and noise. Centrifugal blowers commonly used with advantages over Roots blowers in efficiency, lifespan, noise, and stability. Variable frequency control and multiple blower configurations optimize energy use.
Regular maintenance of oil coolers, filters, and ensuring proper oil quality is necessary to prevent emulsification and overheating.
(4) Aeration Heads
Mostly microporous membranes (disc, dome, plate, tube types). Clogging and rubber aging reduce oxygen transfer efficiency. Regular cleaning with formic acid or high-pressure air is needed, with safety precautions. Drain valves should be regularly opened to remove condensate. Severely clogged or damaged diffusers should be replaced.
(5) Sludge Removal Equipment
Some processes lack secondary sedimentation tanks (e.g., SBR, UNITANK), causing sludge layer funneling and insufficient sludge discharge, increasing energy and chemical consumption. Intermittent or multi-point sludge discharge recommended. Regular maintenance of scraper and suction devices in sedimentation tanks is necessary.
(6) Dewatering Machines
Two main types: centrifuge and belt filter press.
4.Centrifuge:
Consider sludge concentration, feed rate, speed differential, polymer dosage on cake solids, filtrate SS, and recovery.
Larger speed differential shortens sludge retention, raising moisture content and filtrate solids.
Smaller differential improves separation but risks clogging.
Adjust polymer dosage and feed rate to optimize.
Common issues: alarms due to inadequate washing, bearing overheating from lubrication blockage, motor alarms from frequency converter, and sludge not discharged due to small sludge flocs especially during rainy seasons. Adjust operational parameters to mitigate.
Belt Filter Press:
Sludge compressed and sheared between two belts passing over rollers to remove water.
Operational and maintenance points include uniform sludge distribution, soft scrapers, nozzle cleaning systems, automatic belt tracking, and interlock protections.
Common issues: belt slippage, belt deviation, clogging, and cake solids decline mostly due to overloading, improper tension, damaged rollers, and excess polymer. Regular adjustment and cleaning are essential.
Monitoring Instruments
High impurity and harsh environment cause frequent measurement errors or damage to online analyzers, impacting control and automation.
Proper water sample pretreatment units and analyzers matched to concentration ranges are necessary. Large equipment should have control systems compatible with plant automation to reduce communication costs.
Maintenance procedures include planned spare parts, regular calibration, cleaning, and replacement of consumables.
Lightning protection is crucial for outdoor devices due to frequent lightning strikes at sewage plants. Lack of protection leads to high repair costs and operational risks.