What Is Biological Contact Oxidation? A Detailed Explanation of the Sewage Treatment Process
Biological contact oxidation is a highly efficient water treatment process that uses biofilms attached to a carrier (commonly known as filler) to purify organic wastewater. It is a biofilm process with the characteristics of the activated sludge method, combining the advantages of both. Under biodegradable conditions, it has achieved good economic benefits whether applied to industrial wastewater, aquaculture wastewater or domestic sewage treatment. This process is widely used in sewage treatment systems across various industries because of its features of high efficiency, energy saving, small footprint, shock load resistance and convenient operation and management.
Biological treatment is the stage following physicochemical treatment, and also an important stage in the entire sewage treatment cycle. Here, harmful substances such as ammonia nitrogen, nitrite, nitrate and hydrogen sulfide are removed, playing a key role in the further treatment of water quality in subsequent processes.
At the end of the 19th century, Germany began to use biological contact oxidation for wastewater treatment, but limited by the industrial level at the time and the lack of suitable fillers, it was not widely applied. 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 researching the use of this method to treat municipal and industrial wastewater in the mid-1970s, and it has since been applied in production.
Biological contact oxidation is a biofilm process situated between the activated sludge method and the biofilter. Its characteristic is that fillers are installed in the tank, and aeration at the bottom of the tank oxygenates the sewage and keeps it flowing, ensuring sufficient contact between the sewage and the fillers in the tank, avoiding the defect of uneven contact between sewage and fillers in the biological contact oxidation tank. Its basic principle for purifying wastewater is the same as general biofilm methods: the biofilm adsorbs organic matter in the wastewater, and in the presence of oxygen, the organic matter is oxidized and decomposed by microorganisms, purifying the wastewater.
The oxygen required by microorganisms in this method is supplied by blower aeration. After the biofilm grows to a certain thickness, microorganisms on the filler wall will undergo anaerobic metabolism due to oxygen deficiency. The resulting gas and the scouring effect of aeration cause the biofilm to fall off and promote the growth of new biofilm. At this point, the detached biofilm flows out of the tank with the effluent.
The biofilm in the biological contact oxidation tank consists of zoogloea, filamentous bacteria, fungi, protozoa and metazoa. In the activated sludge method, filamentous bacteria are often a factor affecting normal biological purification; whereas in the biological contact oxidation tank, filamentous bacteria form a three-dimensional structure in the gaps of the filling, 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.
Biological contact oxidation is a kind of biofilm process that combines the advantages of both activated sludge and biofilm. Compared with the traditional activated sludge method and biofilter method, it has process advantages such as large specific surface area, high sludge concentration, long sludge age, high oxygen utilization rate, power consumption savings, low sludge production, low operating cost, easy operation and easy maintenance, and has been widely researched and applied at home and abroad. [2]
It has high purification efficiency, requires short treatment time, has strong adaptability to fluctuations in influent organic load, does not require sludge return, and has no sludge bulking problem, making operation and management convenient. The existing problem is mainly that the biofilm between fillers in the tank may sometimes become clogged, which remains to be improved. The research direction is to control aeration intensity for different influent loads to eliminate clogging; the second is to study reasonable oxidation tank shapes, dimensions and suitable filler materials.
1) Improve purification capacity by a staged method. The biochemical process is divided into two stages. First, organic matter is adsorbed onto the sludge or undergoes biosynthesis inside the cells, which is fast. The second stage of the biochemical process 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 rising velocity of the sedimentation tank is 6.5-7.5 m/h; the clarification zone retention time is 15 min.
3) The contact oxidation process can achieve the effect of 8 hours of the activated sludge process in only 0.5-1.0 h. 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 a large number of bubbles, so air flotation separation is appropriate.
The filler is the carrier of microorganisms. The choice of filler determines the size of the specific surface area available for biofilm growth and the amount of biofilm in the reactor. Under a certain hydraulic load and aeration intensity, it also determines the mass transfer conditions and oxygen utilization rate in the reactor, thus greatly affecting the process operation effect. A filler with good performance should have the following characteristics: the biofilm on the filler is evenly distributed, without obvious sludge accumulation or agglomeration; relatively high porosity, not clogged by the biofilm, and not easily stuck by oil in the water affecting treatment effect; high compressive strength, high salt and corrosion resistance; as high a specific surface area as possible and good hydrophilic performance; enable as much biofilm as possible to attach to the filler; good oxygen supply power effect to reduce operating costs; energy saving; low water flow resistance, strong chemical and biological stability; no leaching of harmful substances causing secondary pollution; form a uniform flow velocity between fillers; and be easy to transport and install.
Water temperature affects the biological contact oxidation process in two ways: first, it affects the catalytic reaction rate of biological enzymes; second, it affects the rate of diffusion of pollutants to microbial cells. The suitable range of water temperature in biological contact oxidation is 10-35 deg C. If the water temperature is too low, the activity of the biofilm is inhibited and the diffusion rate of reactants decreases, affecting the treatment effect. If the water temperature is too high, the effluent SS and BOD will increase; the temperature rise will also reduce dissolved oxygen, decrease the oxygen mass transfer rate, cause insufficient dissolved oxygen and sludge anoxic putrefaction, affecting the treatment effect. Therefore, high-temperature industrial wastewater such as printing and dyeing wastewater should be cooled before treatment.
As a microbial treatment process, pH value is an important environmental factor for biological contact oxidation. For most microorganisms, the most suitable pH value is around 7. For wastewater with pH that is too high or too low, pre-treatment to adjust pH should be considered, controlling the pH of the influent to the biological contact oxidation tank within 6.5-9.5. Villaverde S. et al. studied the effect of different pH values on the nitrification process in biological contact oxidation. The study showed that within the pH range of 5.0-9.0, each unit increase in pH increases nitrification efficiency by 13%, and the amount of nitrifying biofilm reaches a maximum at pH 8.2. [3]
The role of aeration in the biological contact oxidation tank is, first, to supply the oxygen required for biological oxidation, and second, to provide good flow turbulence in the reactor, facilitating full contact between pollutants, microorganisms and oxygen, ensuring mass transfer effect, while also achieving forced film stripping through disturbance of the water body, preventing filler sludge accumulation and maintaining biological activity. The dissolved oxygen in the biological contact oxidation tank should generally be maintained between 2.5-3.5 mg/L, with an air-water ratio of about (15-20):1. Insufficient dissolved oxygen reduces biofilm adhesion and causes it to fall off, increases water viscosity, decreases oxygen transfer efficiency, and then causes anoxia, forming a vicious cycle that worsens the treatment effect; an excessively high air-water ratio causes strong scouring of the biofilm, leading to massive biofilm shedding and affecting the treatment effect.
Suspended solids are an important influencing factor in biological contact oxidation treatment. Inorganic suspended solids and sand cannot be well intercepted and settled, which directly affects oxygenation and microbial growth. On the one hand, suspended solids settle on or adhere to the filler biofilm, hindering the mass transfer process between microorganisms and pollutants and dissolved oxygen in the water, reducing biofilm activity; on the other hand, the accumulation of suspended solids on the filler reduces the specific surface area of the filler, leading to a decline in biological treatment effect. Generally, inorganic suspended solids and sand in the sewage should be pre-treated before the sewage enters the contact oxidation tank.
Hydraulic retention time (HRT)
Hydraulic retention time is a crucial parameter of biological contact oxidation. Operating at a suitable hydraulic retention time can not only achieve the ideal treatment effect but also save infrastructure investment. For municipal domestic sewage, the retention time is generally selected as 0.8-1.2 h; for industrial wastewater, the difference is large. For example, for printing and dyeing wastewater, phenol-containing wastewater, etc., where COD is often around 500 mg/L, the retention time is generally 3.0-4.0 h; for slightly polluted source water, Tongji University research concluded that a retention time of 1.2-2.0 h is optimal [4].
The split-flow aeration device is on one side of the tank, with the filler installed on the other side. Relying on pump or air lifting, 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 partitioned compartment, with sufficient oxygen supply, which is beneficial to biofilm growth. The disadvantages are lower oxygen utilization rate and higher power consumption; because the hydraulic scouring effect is small, the aged biofilm is not easy to fall off, the metabolic cycle is longer, and the biofilm activity is smaller; at the same time, it may also cause filler clogging due to the difficulty of biofilm shedding.
The direct type is direct blower aeration at the bottom of the oxidation tank filler. The biofilm is directly and strongly disturbed by the upward airflow, 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, the biofilm thickness is controlled at about 1 mm.
Selecting appropriate fillers to increase the contact surface between biofilm and wastewater is an important measure to improve the biofilm's ability to purify wastewater. Honeycomb fillers are generally used. The honeycomb filler pore size should be selected according to factors such as wastewater quality (concentration of five-day biochemical oxygen demand BOD5, suspended solids, etc.), BOD load and oxygenation conditions. Under general conditions, with BOD5 concentration of 100-300 mg/L, a 32 mm pore size can be selected; for BOD5 of 50-100 mg/L, 15-20 mm can be selected; if below 50 mg/L, 10-15 mm pore size filler can be selected.
The filler should be light in weight, good in strength, strongly oxidation and corrosion resistant, and bring no new toxicity. More commonly used are glass cloth, plastic and other honeycomb fillers; in addition, ropes, synthetic fibers, zeolite, coke, etc. can also be used as fillers. Filler types include honeycomb, mesh, oblique corrugated plate, etc.
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 used, the wastewater retention time is 0.5-1.5 hours, and the oxygen consumption in the oxidation tank is about 1-3 mg/L. Because of the large biomass in the oxidation tank and high treatment load, the dissolved oxygen can be controlled higher; it is generally required that the residual dissolved oxygen in the oxidation tank effluent be 2-3 mg/L.
(1) The number of biological contact oxidation tanks or compartments should be no less than 2, and designed for simultaneous operation.
(2) The volume of the filler is calculated from the filler volume load and the average daily sewage volume. The filler volume load should generally be determined by test. When no test data is available, for domestic sewage or municipal sewage mainly composed of domestic sewage, the volume load is generally 1000-1500 g BOD5/(m3·d).
(3) The effective contact time of the 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 fillers are used, they should generally be installed in layers, each layer 1 m high, and the honeycomb pore size should be no less than 25 mm.
(5) The influent 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, with an air-water ratio of 15-20:1.
(7) To ensure uniform water and air distribution, each oxidation tank compartment area should generally not exceed 25 m2.
Specific data: specific surface area 300 m2/m3, filler length 1-2.5 m, diameter 150 mm, contact oxidation tank water depth can be 3-8 m, three-dimensional elastic filler design volume load can reach 2 kg/(m3·d) (general sewage), air-water ratio generally 15:1, operating dissolved oxygen content greater than 2 mg/L.
The suitable filler for biological contact oxidation is three-dimensional elastic filler. Compared with rigid honeycomb fillers, the three-dimensional elastic filler has large variable porosity and is not clogged; 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, fast film formation and low cost.
When using aerobic contact oxidation treatment, the influent BOD is less than 500 mg/L.
(Air-water ratio source: 1 cubic meter of air contains 20% oxygen, air density 1 kg/m3, each cubic meter of aeration contains about 0.1 kg of oxygen, air-water ratio 1.5 kg oxygen to water, BOD is 150 g oxygen/m3, utilization efficiency is 10%, which can meet the demand. The aeration flow should not be too large, otherwise it will impact the film formation on the filler.)
