Environmental Water Treatment Knowledge: The Meaning and Role of Aerobic Treatment
Aerobic treatment mainly relies on the biochemical action of aerobic bacteria and facultative anaerobic bacteria to accomplish the treatment process. Its mechanism is a method of degrading organic matter, with aerobic microorganisms as the main agents, on the premise that free oxygen is supplied. [1]
The reaction rate is relatively fast and the required reaction time is short; moreover, the process produces essentially no offensive odour and is fairly sanitary, making it suitable for wastewater with a BOD5 concentration below 600 mg/L.
Aerobic biochemical treatment includes the activated sludge process and the biofilm process.
1. Dissolved oxygen: the dissolved oxygen in wastewater should be between 0.3 and 2 mg/L, at which both aerobic bacteria and facultative bacteria can carry out aerobic respiration.
4. Evaluation indicators for the performance and quantity of activated sludge
3. Temperature: a water temperature between 20 deg C and 40 deg C is most suitable.
Nutrients essential for microbial growth: the six essential nutrient elements required for microbial growth: carbon, nitrogen, energy, growth factors (vitamins), inorganic salts (potassium, calcium, magnesium, iron, etc.) and water.
4. Toxic substances: most heavy metals, such as zinc, copper, lead and chromium, are toxic and unfavourable to the survival of microorganisms. However, if the concentration of toxic substances is gradually raised, it is possible, to a certain extent, for them to adapt to the new environment and thereby improve treatment efficiency.
5. Influent organic concentration: the influent BOD5 concentration is generally between 100 and 600 mg/L.
6. Biodegradability of wastewater: the biodegradability of wastewater is generally expressed by the BOD5/COD ratio. When BOD5/COD > 0.5, biological treatment is clearly effective; when BOD5/COD < 0.3, biological treatment is unsuitable.
1. Biological indicators of activated sludge
(1) Excessive aeration easily causes sludge ageing, and the sludge turns greyish white; at this time amoebae and rotifers appear in large numbers.
(2) When dissolved oxygen is insufficient, low-oxygen-tolerant organisms proliferate; among the bacteria, the white Beggiatoa sulphur bacteria dominate, and among the protozoa, genera such as Trichoda appear.
(3) When the organic concentration of the sewage is extremely low, metazoans such as rotifers dominate.
(4) Under shock loading or influx of toxic substances, the number of Aspidisca drops sharply.
Compared with bacteria, protozoa are larger individuals and can be evaluated under a microscope within a short time, offering the advantages of speed and simplicity; when the biology changes in an unfavourable direction, measures can be taken promptly to prevent further deterioration of the system.
2. The purification reaction process of activated sludge
Using the organic pollutants contained in the wastewater as a culture medium, activated sludge is cultivated continuously under conditions of dissolved oxygen, and its adsorption-flocculation and oxidation-decomposition actions are then used to purify the organic pollutants in the wastewater. This includes three stages:
(1) Adsorption stage: the pollutants in the sewage are adsorbed and attached by the flocs formed by the activated sludge microorganisms during contact with them.
(2) Oxidation stage: under aerobic conditions, the microorganisms utilise part of the adsorbed and ingested organic matter as nutrients to synthesise cell material, while another part of the organic matter is catabolised and releases energy.
(3) Floc formation and coagulation-sedimentation stage: the biomass synthesised in the oxidation stage flocculates into flocs, which are separated from the water by gravity sedimentation, thereby purifying the water.
3. The proliferation law of activated sludge
The proliferation of microorganisms in activated sludge is the inevitable result of the reactions occurring in the aeration tank and the degradation of organic matter, and the result of microbial proliferation is the proliferation of the activated sludge.
2. pH value: for aerobic treatment, the pH should be between 6 and 9.
Well-developed activated sludge appears as yellow-brown flocculent granular matter, also called biological flocs. The solid matter content of activated sludge accounts for less than 1%. The solid matter consists of several components, namely:
(1) Living cells (Ma);
(2) Residues of microbial endogenous metabolism (Me);
(3) Non-biodegradable organic matter carried in with the raw wastewater (Mi); (4) inorganic matter carried in with the raw wastewater and attached to the activated sludge (Mii).
(1) Raw water quality: readily biodegradable organic matter gives good wastewater treatment performance; a high content of readily biodegradable organic carbon together with low pH readily causes sludge bulking; toxic and harmful substances affect treatment performance; and industrial wastewater lacking in nitrogen and phosphorus also hinders the growth and reproduction of microorganisms.
(2) Process parameters: select appropriate organic loading and activated sludge concentration; adding chlorine to the return sludge can effectively suppress bulking caused by filamentous bacteria.
(3) Environmental conditions: temperature, including air and water temperature, affects the metabolism of microorganisms.
6. Main components of the activated sludge system:
(1) Aeration tank: the main body of the reaction, where organic matter is degraded and microorganisms proliferate.
(2) Secondary settling tank: separates sludge from water to guarantee effluent quality; concentrates the sludge to ensure sludge return and maintain the sludge concentration in the aeration tank.
(3) Return system: maintains the sludge concentration in the aeration tank; changing the return ratio can adjust the operating conditions of the aeration tank.
(4) Excess sludge: one of the routes for removing organic matter; maintains the stable operation of the system.
(5) Oxygen supply system: provides dissolved oxygen to the microorganisms.
Among these, the activated sludge process includes: plug-flow activated sludge process, completely mixed activated sludge process, step aeration activated sludge process, adsorptive-regenerative activated sludge process, extended aeration activated sludge process, deep-shaft aeration activated sludge process, pure-oxygen aeration activated sludge process, oxidation ditch activated sludge process, and sequencing batch reactor (SBR) activated sludge process.
The principle of the biofilm process is that the biofilm first adsorbs the organic matter in the attached water layer, which is decomposed by the aerobic bacteria in the aerobic layer, and then enters the anaerobic layer for anaerobic decomposition, while the flowing water layer washes away the aged biofilm to allow new biofilm to grow; this cycle repeats to achieve the purpose of purifying the sewage.
(1) Diversified microbial species: (no intense agitation, long sludge age)
Autotrophic and heterotrophic bacteria; fast- and slow-proliferating; algae, protozoa, metazoans; wide-ranging types, abundant genera and species, and long food chains.
(2) Long biological food chain: bacteria - algae - protozoa - metazoans; the long food chain makes the sludge yield of the biofilm process one-quarter lower than that of the activated sludge process;
(3) It can harbour microorganisms with long generation times: nitrifying and denitrifying bacteria; the biofilm process has the function of denitrification and nitrogen removal.
(4) In process design it is divided into multiple stages (levels), forming different distributions of dominant microbial genera, which is favourable to the full exertion of the metabolic functions of the microorganisms in the biofilm process.
(1) The biofilm has a relatively strong resistance to shock from water-quality fluctuations.
(2) Diversified biological phases; the combined action of various microorganisms is favourable to the degradation of macromolecular and refractory substances.
(3) The amount of excess sludge from the biofilm process is less than that from the activated sludge process.
(4) It can withstand a higher organic load.
(5) It has the advantages of no sludge-bulking phenomenon, convenient operation and management, and low power consumption.
Disadvantages: the one-off investment in the packing and its support structure is relatively large, and the packing is prone to clogging, etc.
(1) Wastewater enters the biofilm reactor after passing through the primary sedimentation tank.
(2) After the organic matter is removed by biological oxidation in the biofilm reactor, the effluent passes through the secondary sedimentation tank.
(3) The role of the primary sedimentation tank is to remove suspended solids from the wastewater in advance, preventing the biofilm reactor from being blocked by large matter.
(4) The role of the secondary sedimentation tank is to remove the biofilm that has sloughed off the packing into the wastewater.
(5) The moisture content of the biofilm is lower than that of activated sludge, the sludge settling velocity is greater, and the secondary sedimentation tank can be smaller in volume.
(6) Because the organisms grow attached, there is no need to return seed sludge, and in general there is no secondary-settling-tank sludge return in biofiltration. However, in order to dilute the raw wastewater and ensure scouring of the filter-media layer, high-rate filters and tower biofilters often use effluent recirculation.
5. The biofilm process includes: biofilter, biological rotating disc (bio-disc), biological contact oxidation, and biological fluidised-bed process. [1]
