Detailed Explanation of Wastewater Treatment Processes: Biological Contact Oxidation
The biological contact oxidation process is a high-efficiency water treatment process that mainly uses the biofilm attached to a carrier (commonly called packing media) to purify organic wastewater. As a biofilm method with the characteristics of the activated sludge process, it combines the advantages of both the activated sludge and biofilm methods. Under biodegradable conditions, whether applied to industrial wastewater, aquaculture wastewater, or domestic sewage, it achieves good economic benefits. Because of its features - high efficiency and energy saving, small footprint, resistance to shock loads, and convenient operation and management - it is widely used in wastewater treatment systems across all industries.
Biological treatment is the step after physicochemical treatment and also an important link in the entire cycle. Here, harmful substances such as ammonia nitrogen, nitrous acid, nitrate, and hydrogen sulfide are all removed, playing a key role in the further treatment of water quality in subsequent steps.
If used together with the new JBM combined biological packing, it can accelerate the biological decomposition process and offers advantages such as simple operation and management, low investment, high treatment effect, and minimal land occupation. [1]
At the end of the 19th century, Germany began to apply biological contact oxidation to wastewater treatment, but limited by the industrial level at the time and the lack of suitable packing, it was not widely used. By the 1970s, with the rapid development of the synthetic plastics industry and the advent of lightweight honeycomb packing, Japan, the United States, and others began to research and apply biological contact oxidation. China began researching the use of this method for municipal sewage and industrial wastewater in the mid-1970s and has already applied it in production.
The biological contact oxidation process is a biofilm process situated between the activated sludge process and the biofilter. Its characteristic is that packing is installed in the tank, and aeration at the tank bottom oxygenates the sewage and keeps it in a flowing state, so as to ensure full contact between the sewage and the packing in it, avoiding the defect of uneven contact between sewage and packing in the biological contact oxidation tank.
In this process, the oxygen required by microorganisms is supplied by blower aeration [1]. After the biofilm grows to a certain thickness, the microorganisms on the packing wall undergo anaerobic metabolism due to oxygen deficiency; the gases produced and the scouring effect of aeration cause the biofilm to slough off and promote the growth of a new biofilm. At this point, the sloughed biofilm flows out of the tank with the effluent.
1) Improve purification capacity by the staged method. The biochemical process is divided into two stages. First, organic matter is adsorbed onto the sludge or exists inside cells for biosynthesis; this adsorption-synthesis speed is fast. The second-stage biochemical process is mainly oxidation and is slower.
2) Improve sedimentation tank efficiency by adding a contact layer. For the biofilm in the sedimentation tank, sedimentation is adopted, while for fine suspended solids, filtration-layer interception is adopted; the sedimentation tank uses an up-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. Relying mainly on the biofilm, the oxidation tank is divided into two sections, a contact layer is added to the sedimentation tank, and the sludge separated from the contact oxidation tank contains a large number of bubbles, so air flotation is suitable for separation.
1. Because the packing has a large specific surface area and the tank has good oxygen 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 the biological contact oxidation tank contains a large amount of biological solids 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.
The biological contact oxidation process has the basic characteristics of the biofilm method, but differs from the general biofilm method. First, the packing for microorganisms to attach to is completely immersed in the wastewater, so the biological contact oxidation tank is also called a submerged filter. Second, it uses mechanical equipment to oxygenate the wastewater, unlike general biofilters that rely on natural ventilation for oxygen supply; it is equivalent to adding microorganism-attaching packing to an aeration tank, and can also be called an aeration-circulation filter or contact aeration tank. Third, about 2-5% suspended activated sludge still exists in the tank's wastewater, which also purifies the wastewater. Therefore, the biological contact oxidation process is a biofilm method with the characteristics of the activated sludge process, combining the advantages of both.
The basic principle of purifying wastewater by the biological contact oxidation process 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 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 process, filamentous bacteria are often a factor affecting normal biological purification; but in the biological contact oxidation tank, filamentous bacteria form a three-dimensional structure in the packing voids, greatly increasing the contact surface between the biological phase and the wastewater. At the same time, 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 aeration device, the aeration unit is on one side of the tank and the packing is on the other; relying on the lifting action of a pump or air, water circulates within the packing layer to supply oxygen to the biofilm on the packing. The advantage of this method is that the wastewater is oxygenated in compartments, with sufficient oxygen supply favorable to biofilm growth. The disadvantage is lower oxygen utilization and higher power consumption; because the hydraulic scouring effect is small, aged biofilm does not slough off easily, the metabolic cycle is longer, and biofilm activity is lower; at the same time, the packing can become clogged because the biofilm does not slough off easily.
The direct type blows air for aeration directly at the bottom of the packing in the oxidation tank. The biofilm is directly and strongly disturbed by the rising airflow, renews faster, and maintains higher activity; at the same time, with stable influent load, the biofilm can maintain a certain thickness and is not prone to clogging. Generally, the biofilm thickness is preferably controlled at about 1 mm.
Selecting appropriate packing to increase the contact surface between the biofilm and the wastewater is an important measure to improve the biofilm's ability to purify wastewater. Honeycomb packing is generally used.
The specific surface area of honeycomb packing is, for example:
The packing should be light, strong, highly resistant to oxidative corrosion, and bring no new toxicity. Those used more often are glass cloth, plastic, and other honeycomb packings; in addition, ropes, synthetic fibers, zeolite, coke, etc. can also be used as packing. Packing forms include honeycomb, mesh, and inclined corrugated plate.
The BOD load of the biological contact oxidation process is related to the substrate concentration of the wastewater. For low-BOD-concentration (50-300 mg/L) wastewater, the packing is loaded at 2-5 kg BOD per cubic meter per day (BODsquare), 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, the dissolved oxygen can be controlled higher; 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 influent organic load; no need for sludge return and no sludge bulking problem; convenient operation and management. The main existing problem is that the biofilm between the packing in the tank sometimes clogs and 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 types, shapes, dimensions, and suitable packing materials.
(1) The number of biological contact oxidation tanks or compartments should be no less than 2, and they should be designed to work simultaneously.
(2) The volume of packing is calculated from the packing volumetric load and the average daily sewage volume. The packing volumetric load should generally be determined through
testing. When no test data are available, for domestic sewage or municipal sewage mainly consisting of domestic sewage, the volumetric load generally adopts
10001500g BODs/(m³·d)
compared with semi-soft packing, it has a larger surface area, rapid film formation, and low cost.
(4) The total height of the packing layer is generally 3 m. When honeycomb packing is used, it should generally be installed in layers, each 1 m high,
with the honeycomb pore size not 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, the area of each oxidation tank compartment should generally not exceed 25 m2.
At present, the most suitable packing for the biological contact oxidation process is the three-dimensional elastic packing. Compared with rigid honeycomb packing, the three-dimensional elastic packing has large variable porosity and does not clog; compared with soft packing, its material has a long service life and does not adhere or form clumps;
(3) The effective contact time of sewage in the oxidation tank is generally 1.5-3.0 h.
Specific data: specific surface area 300 m2/m3, packing length 1-2.5 m, diameter 150 mm,
the water depth of the contact oxidation tank can reach 3-8 m; the design volumetric load of three-dimensional elastic packing can reach 2 kg/(m3.d) (general sewage); the air-water ratio is generally taken as 15:1; during operation the dissolved oxygen content is greater than 2 mg/L.
When using aerobic contact oxidation treatment, the influent BOD is less than 500 mg/L.
(Source of the air-water ratio: atmospheric air contains 20% oxygen, air density is 1 kg/m3, 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 is 10%, which can meet the demand; the aeration flow should not be too large, otherwise it will impact film formation on the packing.)
