Detailed Explanation of Wastewater Treatment Process: Biological Contact Oxidation
The biological contact oxidation method is a highly efficient water-treatment process that mainly uses the biofilm attached to a carrier (commonly called filler) to purify organic wastewater. As a biofilm process with the characteristics of the activated sludge method, it combines the advantages of both. Under biodegradable conditions, whether applied to industrial wastewater, aquaculture wastewater, or domestic sewage, it achieves good economic benefits. Because of its high efficiency, energy saving, small footprint, shock-load resistance, and easy operation and management, it is widely used in wastewater treatment systems across various industries.
Biological treatment is the stage after physicochemical treatment and also a key link in the whole circulation process, where harmful substances such as ammonia nitrogen, nitrite, nitrate, and hydrogen sulfide are removed, playing a crucial role in the further treatment of water quality in subsequent stages.
If used together with JBM new-type combined biological filler, it can accelerate the biological decomposition process and offers advantages such as simple operation and management, low investment, high treatment efficiency, and minimized footprint. [1]
At the end of the 19th century, Germany began to use biological contact oxidation for wastewater treatment, but limited by the industrial level of the time and the lack of suitable fillers, it was not widely applied. By the 1970s, the rapid development of the synthetic plastics industry and the advent of lightweight honeycomb fillers led Japan, the United States, and others to start researching and applying the method. China began researching its use for municipal sewage and industrial wastewater in the mid-1970s and has since applied it in production.
Biological contact oxidation is a biofilm process between the activated sludge method and the biofilter. Its feature is that fillers are set in the tank, and bottom aeration oxygenates the sewage while keeping it in a flowing state, ensuring full contact between the sewage and the fillers and avoiding the defect of uneven sewage-filler contact in the 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 new biofilm, and the sloughed biofilm then flows out of the tank with the effluent.
1 Use the staged method to improve purification capacity. The biochemical process is divided into two stages. First, organics are adsorbed onto the sludge or undergo biosynthesis inside cells; this adsorption-synthesis is 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 uses an upward flow velocity of 6.5-7.5 m/h, with a 15-min retention in the clarification zone.
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 sections 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 good oxygenation conditions, 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 of the large amount of biological solids and complete mixing of the water flow, it has 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.
The biological contact oxidation method has the basic characteristics of a biofilm process, but differs from general biofilm processes. First, the fillers for microbial attachment are entirely immersed in the wastewater, so the contact oxidation tank is also called a submerged filter. Second, it uses mechanical equipment to oxygenate the wastewater, unlike ordinary biofilters that rely on natural ventilation; it is equivalent to adding microbial-attachment fillers into an aeration tank, and can also be called an aerated circulating filter or contact aeration tank. Third, about 2-5% suspended activated sludge also exists in the tank wastewater and also purifies it. Therefore, biological contact oxidation is a biofilm process 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 general biofilm processes: the biofilm adsorbs organics in the wastewater, and under aerobic conditions the organics are oxidized and decomposed by microorganisms, purifying the wastewater.
The biofilm in the contact oxidation tank is composed of zoogloea, filamentous bacteria, fungi, protozoa, and metazoa. In the activated sludge method, filamentous bacteria are often a factor impairing normal biological purification; but in the contact oxidation tank, filamentous bacteria form a three-dimensional structure in the filler voids, greatly increasing the contact surface between the biomass and the wastewater, and because they have strong oxidation ability for most organics and good adaptability to load changes, they are a powerful factor in improving purification capacity.
When using aerobic contact oxidation, the influent BOD should be less than 500 mg/L.
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 upward air flow, renews quickly, and maintains high activity; meanwhile, with stable influent load, the biofilm can maintain a certain thickness and is not prone to clogging. Generally, a biofilm thickness of about 1 mm is appropriate.
Selecting appropriate fillers to increase the contact surface between biofilm and wastewater is an important measure to improve the biofilm's purification ability. Generally, honeycomb fillers are used. The specific surface area of honeycomb fillers is as follows:
The pore size of honeycomb fillers must be selected based on wastewater quality (BOD5, i.e., five-day biochemical oxygen demand, and suspended solids concentration), BOD load, oxygenation conditions, etc. Generally, when BOD5 concentration is 100-300 mg/L, a 32 mm pore size can be selected; when BOD5 is 50-100 mg/L, 15-20 mm; if below 50 mg/L, 10-15 mm pore size fillers can be selected.
Fillers should be light, strong, highly resistant to oxidation and corrosion, and introduce no new toxicity. Commonly used are glass-fiber and plastic honeycomb fillers; in addition, ropes, synthetic fibers, zeolite, coke, etc. can also be used. 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 meter of filler per day is adopted; 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 of the large biomass and high treatment load, a higher dissolved oxygen can be controlled; generally the remaining dissolved oxygen in the effluent is required to be 2-3 mg/L.
High purification efficiency; short treatment time; strong adaptability to variations in influent organic load; no sludge return needed and no sludge bulking problem; easy operation and management. The existing problem is that the biofilm between fillers in the tank sometimes clogs and needs improvement. Research directions are to control aeration intensity for different influent loads to eliminate clogging, and to study reasonable oxidation tank types, shapes, dimensions, and suitable filler materials.
(1) The number of biological contact oxidation tanks or compartments should be no less than 2, designed to operate simultaneously.
(2) The filler volume is calculated from the filler volumetric load and the average daily sewage volume. The filler volumetric load should generally be determined by test
verification. When no test data are available, for domestic sewage or municipal sewage dominated by domestic sewage, the volumetric load is generally
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 filler-layer height is generally 3 m. When honeycomb fillers are used, they should generally be installed in layers, each 1 m high,
and the honeycomb pore size should not be less than 25 mm.
(5) The influent BOD5 concentration should be controlled within 150-300 mg/L.
(6) The dissolved oxygen content in the contact oxidation tank should generally be maintained at 2.5-3.5 mg/L, with an air-water ratio of 15-20:1.
(7) To ensure uniform water and air distribution, the area of each oxidation tank compartment should generally not exceed 25 m2.
Currently, the most suitable filler for biological contact oxidation is the three-dimensional elastic filler. Compared with rigid honeycomb fillers, it has greater pore variability and does not clog; compared with soft fillers, its material has a longer life and does not adhere or agglomerate;
compared with semi-soft fillers, it has a larger surface area, faster biofilm formation, and lower 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 reach 3-8 m; the designed volumetric load of three-dimensional elastic filler can reach 2 kg/(m3.d) (general sewage); the air-water ratio is generally 15:1, and the operating dissolved oxygen content is greater than 2 mg/L.
In the split-flow type, the aeration device is on one side of the tank and the filler on the other; relying on pump or air lifting, water circulates within the filler layer to supply oxygen to the biofilm. Its advantage is that the wastewater is oxygenated in compartments, with sufficient oxygen supply favorable to biofilm growth. Its disadvantages are lower oxygen utilization and higher power consumption; because of weaker hydraulic scouring, aged biofilm does not slough off easily, the metabolic cycle is longer, and biofilm activity is lower; meanwhile, the difficulty of biofilm sloughing can also cause filler clogging.
(Source of air-water ratio: 1 m3 of air contains 20% oxygen, air density 1 kg/m3, so each cubic meter of aeration contains about 0.1 kg oxygen; the air-water ratio is 1.5 kg oxygen : water, BOD is 150 g oxygen/m3, utilization efficiency 10%, which can meet demand; the aeration flow should not be too large, otherwise it will shock the biofilm formation on the fillers.)
