Introduction to the Biological Contact Oxidation Process
Biological contact oxidation is a high-efficiency water treatment process that mainly uses the biofilm attached to a carrier (commonly called filler) to purify organic wastewater. It is a biofilm method with the characteristics of the activated sludge method, combining the advantages of both. Under biodegradable conditions, whether applied to industrial wastewater, aquaculture wastewater or domestic sewage, it has achieved good economic benefits. Because of its features of high efficiency and energy saving, small footprint, resistance to shock loads and convenient operation and management, this process is widely used in wastewater treatment systems of all industries.
Biological treatment is the step after physicochemical treatment and also an important link in the whole circulation process; here, harmful substances such as ammonia nitrogen, nitrous acid, nitrate and hydrogen sulfide are removed, playing a key role in the further treatment of water quality in subsequent processes.
(2) The volume of the filler is calculated from the filler volumetric load and the average daily sewage volume. The volumetric load of the filler should generally be determined through testing
At the end of the 19th century, Germany began to use the biological contact oxidation method for wastewater treatment, but limited by the industrial level at the time and the lack of suitable fillers, it was not widely used. By the 1970s, with the rapid development of the synthetic plastics industry and the advent of lightweight honeycomb fillers, Japan, the United States and others began to research and apply biological contact oxidation. China began to study the use of this method for treating municipal sewage and industrial wastewater in the mid-1970s and has already applied it in production.
Biological contact oxidation is a biofilm process between the activated sludge method and the biofilter. Its characteristic is that fillers are set in the tank and aeration at the bottom oxygenates the sewage and keeps it in a flowing state, so as to ensure sufficient contact between the sewage and the fillers in it and avoid the defect of uneven contact between sewage and fillers in the biological contact oxidation tank.
In this method, the oxygen needed by microorganisms is supplied by blower aeration [1]. After the biofilm grows to a certain thickness, the microorganisms on the filler wall undergo anaerobic metabolism due to oxygen deficiency; the resulting gas and the scouring effect of aeration cause the biofilm to slough off and promote the growth of a new biofilm, and the sloughed biofilm then flows out of the tank with the effluent.
(1) Improve purification capacity by the staging method. The biochemical process is divided into two stages. First, organic matter is adsorbed onto the sludge or undergoes biosynthesis inside the cells; this adsorption-synthesis is very fast. The second stage is mainly oxidation and is slower.
(2) Improve sedimentation tank efficiency by adding a contact layer. The biofilm in the sedimentation tank is removed by sedimentation, while fine suspended solids are intercepted by the filter layer; the sedimentation tank takes an upward flow velocity of 6.5-7.5 m/h and the clarification zone has a 15 min retention time.
(3) The contact oxidation process needs only 0.5-1.0 h to achieve the effect of 8 h of the activated sludge process. It mainly relies on the biofilm, dividing the oxidation tank into two stages and adding a contact layer to the sedimentation tank; the sludge separated from the contact oxidation tank contains many bubbles and is suitable for separation by flotation.
1. Because the filler has a large specific surface area and the oxygenation conditions in the tank are good, the amount of biological solids per unit volume in the tank is high; therefore, the biological contact oxidation tank has a high volumetric load.
2. Because there is a large amount of biological solids in the biological contact oxidation tank and the water flow is completely mixed, it has a strong adaptability to sudden changes in water quality and quantity.
3. The amount of excess sludge is small, there is no sludge bulking problem, and operation and management are simple.
Biological contact oxidation has the basic characteristics of the biofilm method but differs from the general biofilm method. First, the fillers for microorganisms to attach to are all immersed in the wastewater, so the biological contact oxidation tank is also called a submerged filter. Second, mechanical equipment is used to oxygenate the wastewater, unlike general biofilters that rely on natural ventilation for oxygen supply; it is equivalent to adding fillers for microorganisms to attach to in an aeration tank, and can also be called an aeration-circulation filter or contact aeration tank. Third, about 2-5% suspended activated sludge remains in the wastewater in the tank, which also purifies the wastewater. Therefore, biological contact oxidation is a biofilm method with the characteristics of the activated sludge method, combining the advantages of both.
The basic principle of purifying wastewater by biological contact oxidation is the same as that of the general biofilm method: the biofilm adsorbs the organic matter in the wastewater, and under aerobic conditions the organic matter is oxidized and decomposed by microorganisms, so that the wastewater is purified.
The biofilm in the biological contact oxidation tank is composed of zoogloea, filamentous bacteria, fungi, protozoa and metazoa. In the activated sludge method, filamentous bacteria are often a factor affecting normal biological purification; but in the biological contact oxidation tank, the filamentous bacteria form a three-dimensional structure in the filler voids, greatly increasing the contact surface between the biophase and the wastewater, and because filamentous bacteria have a strong oxidation capacity for most organic matter and greater adaptability to changes in water-quality load, they are a powerful factor in improving purification capacity.
In the split-flow type, the aeration device is on one side of the tank and the filler is on the other side; relying on the lifting action of a pump or air, the water flows in a cycle within the filler layer, supplying oxygen to the biofilm on the filler. The advantage of this method is that the wastewater is oxygenated in the compartment, with sufficient oxygen supply favorable to biofilm growth. The disadvantages are lower oxygen utilization and higher power consumption; because the hydraulic scouring effect is small, the aged biofilm does not slough off easily, the metabolism cycle is longer and the biofilm activity is lower; at the same time, clogging of the filler may occur because the biofilm does not slough off easily.
The direct type blows air for aeration directly at the bottom of the filler in the oxidation tank. The biofilm is directly and strongly disturbed by the rising air flow, renews faster and maintains higher activity; at the same time, with stable inlet load, the biofilm can maintain a certain thickness and is not prone to clogging. Generally, the biofilm thickness is appropriately controlled at about 1 mm.
Selecting appropriate filler to increase the contact surface between the biofilm and wastewater is an important measure to improve the biofilm's capacity to purify wastewater. Honeycomb fillers are generally used. The specific surface area of honeycomb fillers is as follows:
The pore size of the honeycomb filler must be selected according to the wastewater quality (BOD5, i.e. five-day biochemical oxygen demand, suspended solids concentration, etc.), BOD load, oxygenation conditions and other factors. Generally, when the BOD5 concentration is 100-300 mg/L, a pore size of 32 mm can be selected; when BOD5 is 50-100 mg/L, 15-20 mm can be selected; if below 50 mg/L, a filler with a pore size of 10-15 mm can be selected.
The filler should be light, strong, strongly resistant to oxidation and corrosion, and bring no new toxicity. Glass cloth and plastic honeycomb fillers are commonly used; in addition, rope, synthetic fiber, zeolite, coke, etc. can also be used as fillers. Filler types include honeycomb, mesh and inclined corrugated plate.
The BOD load of biological contact oxidation is related to the substrate concentration of the wastewater; for low-BOD-concentration (50-300 mg/L) wastewater, 2-5 kg (BOD5) per cubic metre of filler per day is used, the wastewater retention time is 0.5-1.5 h, and the oxygen consumption in the oxidation tank is about 1-3 mg/L. Because the oxidation tank has a large biomass and high treatment load, a higher dissolved oxygen level can be controlled; generally the residual dissolved oxygen in the oxidation tank effluent is required to be 2-3 mg/L.
High purification efficiency; short treatment time required; strong adaptability to variations in inlet organic load; no need for sludge return and no sludge bulking problem; convenient operation and management. The existing problem is mainly that the biofilm between the fillers in the tank sometimes becomes clogged, which remains to be improved. The research direction is to control the aeration intensity for different inlet loads to eliminate clogging; and second, to study reasonable oxidation tank shapes, sizes and suitable filler materials.
(1) The number of biological contact oxidation tanks or compartments should be no less than 2, and they should be designed to operate simultaneously.
If used in conjunction with the new JBM combined biological filler, it can accelerate the biological decomposition process and has the advantages of simple operation and management, low investment, high treatment effect and minimized footprint. [1]
determined by testing. When no test data are available, for domestic sewage or municipal sewage mainly consisting of domestic sewage, the volumetric load is generally taken as
1000～1500g BODs/(m³·d)。
(3) The effective contact time of sewage in the oxidation tank is generally 1.5-3.0 h.
(4) The total height of the filler layer is generally 3 m. When honeycomb filler is used, it should generally be installed in layers, each 1 m high.
The honeycomb pore size should not be less than 25 mm.
(5) The inlet BOD5 concentration should be controlled within the range of 150-300 mg/L.
(6) The dissolved oxygen content in the contact oxidation tank should generally be maintained between 2.5-3.5 mg/L, and the gas-water ratio is 15-20:1.
(7) To ensure uniform water and air distribution, the area of each oxidation tank cell should generally not exceed 25 m2.
At present, the most suitable filler for biological contact oxidation is the three-dimensional elastic filler. Compared with rigid honeycomb fillers, the three-dimensional elastic filler has large variable porosity and does not clog; compared with soft fillers, it has a long material life and does not stick or agglomerate.
Compared with semi-soft fillers, it has a large surface area, rapid biofilm attachment and low cost.
Specific data: specific surface area 300 m2/m3, filler length 1-2.5 m, diameter 150 mm.
The water depth of the contact oxidation tank can be 3-8 m; the design volumetric load of the three-dimensional elastic filler can reach 2 kg/(m3.d) (for general sewage); the gas-water ratio is generally taken as 15:1, and the dissolved oxygen content during operation is greater than 2 mg/L.
When using aerobic contact oxidation treatment, the inlet BOD should be less than 500 mg/L.
(Source of gas-water ratio: air contains 20% oxygen by volume, air density is 1 kg/m3, each cubic metre of aeration contains about 0.1 kg of oxygen; gas-water ratio 1.5 kg oxygen : water; BOD is 150 g oxygen/m3; utilization efficiency is 10%, which can meet the demand; the aeration flow rate should not be too large, otherwise it will impact film formation on the filler.)
